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fibers mediate thermal hyperalgesia in a new model of causalgiform pain disorders

antagonist activity and phencyclidine-like behavioral effects of the pentadecapeptide, [Ser1]histogranin. *Pharmacol Biochem Behav,* Vol.50, No.1, 49-54. Siddall, P.; Xu, C.L. & Cousins, M. (1995). Allodynia following traumatic spinal cord injury

of the prevalence and characteristics of pain in the first 5 years following spinal

pain: peripheral mechanisms in inflammatory and neuropathic hyperalgesia.

**12**

*Canada*

**Contribution of Inflammation**

S. Echeverry, S.H. Lee, T. Lim and J. Zhang

 **to Chronic Pain Triggered by Nerve Injury**

An injury to a peripheral nerve can be a slight stretch, a compression or a severe laceration. Such damage usually leads to an acute phase response characterized by nociceptive pain, inflammation and restriction of normal function. However, in about 7%-18% of the general population, pain persists, even after the injury healing, producing a state of chronic neuropathic pain. This type of hypersensitivity is debilitating and refractory to the majority of available analgesics. It adversely affects quality of life and bears a substantial cost to society. In the last two decades, compelling evidence strongly suggested that, in addition to changes in neuronal system, pathogenesis of neuropathic pain involves the interaction between the immune system and the nervous system. Inflammatory process alters local homeostasis and impairs neuronal function. In this chapter, we primarily focus on the evidence with experimental animal models obtained from our laboratory and from literature to highlight inflammatory reaction along the pain pathway (from damaged nerve to the spinal cord), and the critical role of this reaction in the development and maintenance of neuropathic pain. We also discuss the current progress and challenges of translating the

Peripheral nerve injury triggers not only the activation of Schwann's cells and resident immune cells, but also the recruitment of circulating inflammatory cells. The two main cell types that enter the nerves after injury are neutrophils and macrophages. Neutrophils are the first immune cells migrating towards the injury site. Their numbers peak at 24 hours following peripheral nerve injury and decrease progressively after three days post-injury, but still remain significant for at least one week (Nadeau et al, 2011). Infiltrated neutrophils in injured nerves play an important role at the very early stages of neuropathic pain through the release of pro-inflammatory mediators, such as cytokines TNF-, IL-1 and IL-6 and reactive oxygen species, which are involved in regulating neuronal excitability (Schafers et al., 2003). In addition, neutrophils have a significant impact on subsequent macrophage infiltration to the injured nerves by secreting chemokines/cytokines MIP-1, MIP-1 and IL-

**1. Introduction**

**2. Peripheral nerve**

1 (Scapini et al., 2000).

inflammation related new targets into therapeutics.

**2.1 Infiltration of immune cells into injured peripheral nerves**

*The Alan Edwards Centre for Research on Pain, McGill University*

### **Contribution of Inflammation to Chronic Pain Triggered by Nerve Injury**

S. Echeverry, S.H. Lee, T. Lim and J. Zhang *The Alan Edwards Centre for Research on Pain, McGill University Canada*

#### **1. Introduction**

202 Basic Principles of Peripheral Nerve Disorders

Wu, G.; Ringkamp, M.; Hartke, T.V.; Murinson, B.B.; Campbell, J.N.; Griffin, J.W. & Meyer,

after injury to neighboring nerve fibers. *J Neurosci,* Vol.21, No.8, RC140. Xu, X.J.; Hao, J.X.; Aldskogius, H.; Seiger, A. & Wiesenfeld-Hallin, Z. (1992). Chronic pain-

for pain in patients with spinal cord injury. *Pain,* Vol.48, No.2, 279-290. Yezierski, R.P. & Park, S.H. (1993). The mechanosensitivity of spinal sensory neurons

Yoon, Y.W.; Na, H.S. & Chung, J.M. (1996). Contributions of injured and intact afferents to neuropathic pain in an experimental rat model. *Pain,* Vol.64, No.1, 27-36. Ziegler, E.A.; Magerl, W.; Meyer, R.A. & Treede, R.D. (1999). Secondary hyperalgesia to

Zochodne, D.W. & Max, M.B. (2003). An old acquaintance: opioids in neuropathic pain.

No.1, 115-119.

*Brain,* Vol.122 ( Pt 12), 2245-2257.

*Neurology,* Vol.60, No.6, 894-895.

R.A. (2001). Early onset of spontaneous activity in uninjured C-fiber nociceptors

related syndrome in rats after ischemic spinal cord lesion: a possible animal model

following intraspinal injections of quisqualic acid in the rat. *Neurosci Lett,* Vol.157,

punctate mechanical stimuli. Central sensitization to A-fibre nociceptor input.

An injury to a peripheral nerve can be a slight stretch, a compression or a severe laceration. Such damage usually leads to an acute phase response characterized by nociceptive pain, inflammation and restriction of normal function. However, in about 7%-18% of the general population, pain persists, even after the injury healing, producing a state of chronic neuropathic pain. This type of hypersensitivity is debilitating and refractory to the majority of available analgesics. It adversely affects quality of life and bears a substantial cost to society. In the last two decades, compelling evidence strongly suggested that, in addition to changes in neuronal system, pathogenesis of neuropathic pain involves the interaction between the immune system and the nervous system. Inflammatory process alters local homeostasis and impairs neuronal function. In this chapter, we primarily focus on the evidence with experimental animal models obtained from our laboratory and from literature to highlight inflammatory reaction along the pain pathway (from damaged nerve to the spinal cord), and the critical role of this reaction in the development and maintenance of neuropathic pain. We also discuss the current progress and challenges of translating the inflammation related new targets into therapeutics.

#### **2. Peripheral nerve**

#### **2.1 Infiltration of immune cells into injured peripheral nerves**

Peripheral nerve injury triggers not only the activation of Schwann's cells and resident immune cells, but also the recruitment of circulating inflammatory cells. The two main cell types that enter the nerves after injury are neutrophils and macrophages. Neutrophils are the first immune cells migrating towards the injury site. Their numbers peak at 24 hours following peripheral nerve injury and decrease progressively after three days post-injury, but still remain significant for at least one week (Nadeau et al, 2011). Infiltrated neutrophils in injured nerves play an important role at the very early stages of neuropathic pain through the release of pro-inflammatory mediators, such as cytokines TNF-, IL-1 and IL-6 and reactive oxygen species, which are involved in regulating neuronal excitability (Schafers et al., 2003). In addition, neutrophils have a significant impact on subsequent macrophage infiltration to the injured nerves by secreting chemokines/cytokines MIP-1, MIP-1 and IL-1 (Scapini et al., 2000).

Contribution of Inflammation to Chronic Pain Triggered by Nerve Injury 205

The dorsal root ganglia (DRG) are nodules containing cell bodies of afferent sensory neurons, which lie just outside of the spinal cord, in the intraforaminal spaces of the vertebral column. The nociceptive neurons of the DRG are classified into two groups depending on nerve fiber type: myelinated Aδ-fibers which are fast conducting, or unmyelinated C-fibers which are slow conducting. Like most neurons, sensory neurons in DRG are supported by a cast of other cells. In the DRG these supportive cells are satellite

SGCs encapsulate each neuronal cell body, and a basement lamina separates neighbouring engcapsulated neurons (Hanani, 2005). The SGCs in DRG perform many similar functions to the astrocytes of the CNS, as they perform many regulatory and immune functions. For example, SGCs regulate extracellular amounts of glutamate and aspartate (Duce & Keen, 1983).  In addition, like many other glia, SGCs communicate with each other using gap junctions Normal DRG are also home for a population of resident macrophages. Most of these resident macrophages contact the neuron-satellite cell complex, and there are a smaller number which reside perivascularly (Hanani, 2005). The DRG neurons are very well perfused, and the blood supply is much denser than in peripheral nerve or dorsal root. In fact, each DRG neuron soma is in close proximity to an extensive network of capillaries. The blood supply to the DRG is supplied by fenestrated capillaries which have no blood-nerve barrier, and the

Sensory ganglia do not have dendrites and chemical synapses, and thus each neuron within the ganglia was thought to be an independent communication pathway. However, this turns out not to be the case, and in fact, neighboring DRG neurons have the ability to crossexcite one another. Thus after peripheral nerve injury, cross-excitation is one possible mechanism by which sensory signals can be altered in time, space, and modality to result in neuropathic pain (Devor & Wall, 1990). Inflammation, which increases the excitability of DRG neurons, enhances cross-excitation. For example this mechanism is further influenced

Following peripheral nerve injury or inflammation, SGCs in DRG activate, and undergo proliferation (Lu et al., 1991). This process is thought to be mediated through release of ATP from damaged neurons, which stimulate purinergic receptors on SCGs (Filippov et al., 2004). Activation of SGCs results in increased glial fibrillary acidic protein (GFAP) expression and release of pro-inflammatory cytokines, for example, TNFα and IL-1β (Ohara et al., 2009). These cytokines released by activated SGCs have excitatory actions on nociceptive neurons (Takeda et al., 2009). In spinal nerve ligation (SNL) injured rats, GFAP expression peaks at one day after nerve injury, and is still present 10 days after ( Xie et al., 2009). Gap junctions in SGCs, are also increased after nerve injury (Jeon et al., 2009). These junctions seem to be involved with the generation and maintenance of neuropathic pain, as RNA interference targeting connexin 43, one of the major structural components of gap junctions in SGCs, prevents experimental neuropathic pain (Ohara et al., 2008). Gap junctions allow the redistribution of K+ ions though SGCs. K+ ion concentration regulation is another mechanism by which SCGs regulate the

glial cells (SGC), dendritic cells, macrophages, and endothelial cells.

**3. Dorsal Root Ganglia (DRG)**

vessels allow large molecules to pass easily.

excitability of DRG neurons (Takeda et al., 2009).

by interactions with SGCs.

Macrophages from peripheral circulation enter the injured nerve starting about 3 days and persist for several months post-injury (Frisen et al., 1993). Infiltrated macrophages exhibit different functional phenotypes in which they promote both injury and repair. The first important characteristic of macrophages is to be able to secrete pro-inflammatory mediators including TNF-, IL-1, IL-6, MIP-1, MIP-1 and MCP-1. These cytokine/chemokine expressing macrophages show high levels of MAC-1 and low levels of ED-1 antigens (personal unpublished data). Many of these immune molecules detected in damaged nerves at different time points post-injury are directly or indirectly involved in pain hypersensitivity. For instance, MCP-1 plays an important role in macrophage recruitment and the establishment of pain hypersensitivity. It has been observed that MCP-1 mRNA expression is markedly increased in the injured rat sciatic nerves in parallel with an increase of macrophage recruitment (Toews et al., 1998). However, the development of mechanical allodynia is totally abrogated in CCR2 (MCP-1 receptor) knockout mice while macrophage recruitment is attenuated (Abbadie et al., 2003). Furthermore, pro-inflammatory mediators including IL-1, TNF-, IL-6 and MIP-1 are directly implicated in pain hypersensitivity (Kiguchi et al., 2010; Sommer & Kress, 2004) through synergistic effects to amplify the inflammatory signals. It has been shown that direct injection of IL-1 and TNF- into peripheral nerves causes potent mechanical and thermal hyperalgesia in rats (Zelenka et al., 2005) whereas the application of neutralizing antibodies for IL-1 or IL-1 receptor antagonist effectively attenuates tactile allodynia and thermal hyperalgesia in animal models of neuropathic pain (Kawasaki et al., 2008). The use of siRNA against CCR1 and/or CCR5 (MIP-1 receptor) also effectively prevents the induction of tactile allodynia and thermal hyperalgesia through down-regulation of IL-1(Kiguchi et al,. Infiltrated macrophages also exhibit powerful phagocytic phenotypes. High level of ED1 antigen was found on the cell surface and membranes of cytoplasmic granules such as phagolysosomes, which indicates a close correlation with macrophage phagocytic activity (Damoiseaux et al., 1994). They engulf cellular debris such as injured Schwann cells and axotomised axons (Bruck, 1997) and promote repair process. However, whether and how these phagocytic macrophages contribute to the pain behaviour needs further investigation.

#### **2.2 Depletion of neutrophils or monocytes/macrophages impairs neuropathic pain behavior associated with nerve injury**

Systemic treatment of sciatic nerve injured mice with a monoclonal antibody against the Ly6G antigen specifically expressed by neutrophils demonstrated that depletion of neutrophils attenuates the development of neuropathic pain behaviour (Nadeau et al, 2011). Furthermore, using molecular and pharmacological approaches, partial or complete depletion of macrophages yield beneficial effects in alleviating nerve injury associated chronic pain. For instance, the depletion of circulating macrophages by liposomeencapsulated clodronate results in decreased macrophage recruitment to the injury site, alleviated thermal hyperalgesia and reduced degeneration of axons (Liu et al., 2000) as well as attenuates pain hypersensitivity in diabetic animals (Mert et al., 2009). In addition, Wallerian degeneration (WLD) in response to nerve injury is delayed in genetically deficient mice which delays macrophage recruitment (Araki et al., 2004). Hyperalgesia is also delayed in this type of mice (WLD mice) suggesting that macrophages contribute to the development of neuropathic pain following peripheral nerve injury (Myers et al., 1996).

#### **3. Dorsal Root Ganglia (DRG)**

204 Basic Principles of Peripheral Nerve Disorders

Macrophages from peripheral circulation enter the injured nerve starting about 3 days and persist for several months post-injury (Frisen et al., 1993). Infiltrated macrophages exhibit different functional phenotypes in which they promote both injury and repair. The first important characteristic of macrophages is to be able to secrete pro-inflammatory mediators including TNF-, IL-1, IL-6, MIP-1, MIP-1 and MCP-1. These cytokine/chemokine expressing macrophages show high levels of MAC-1 and low levels of ED-1 antigens (personal unpublished data). Many of these immune molecules detected in damaged nerves at different time points post-injury are directly or indirectly involved in pain hypersensitivity. For instance, MCP-1 plays an important role in macrophage recruitment and the establishment of pain hypersensitivity. It has been observed that MCP-1 mRNA expression is markedly increased in the injured rat sciatic nerves in parallel with an increase of macrophage recruitment (Toews et al., 1998). However, the development of mechanical allodynia is totally abrogated in CCR2 (MCP-1 receptor) knockout mice while macrophage recruitment is attenuated (Abbadie et al., 2003). Furthermore, pro-inflammatory mediators including IL-1, TNF-, IL-6 and MIP-1 are directly implicated in pain hypersensitivity (Kiguchi et al., 2010; Sommer & Kress, 2004) through synergistic effects to amplify the inflammatory signals. It has been shown that direct injection of IL-1 and TNF- into peripheral nerves causes potent mechanical and thermal hyperalgesia in rats (Zelenka et al., 2005) whereas the application of neutralizing antibodies for IL-1 or IL-1 receptor antagonist effectively attenuates tactile allodynia and thermal hyperalgesia in animal models of neuropathic pain (Kawasaki et al., 2008). The use of siRNA against CCR1 and/or CCR5 (MIP-1 receptor) also effectively prevents the induction of tactile allodynia and thermal hyperalgesia through down-regulation of IL-1(Kiguchi et al,. Infiltrated macrophages also exhibit powerful phagocytic phenotypes. High level of ED1 antigen was found on the cell surface and membranes of cytoplasmic granules such as phagolysosomes, which indicates a close correlation with macrophage phagocytic activity (Damoiseaux et al., 1994). They engulf cellular debris such as injured Schwann cells and axotomised axons (Bruck, 1997) and promote repair process. However, whether and how these phagocytic

macrophages contribute to the pain behaviour needs further investigation.

of neuropathic pain following peripheral nerve injury (Myers et al., 1996).

**behavior associated with nerve injury**

**2.2 Depletion of neutrophils or monocytes/macrophages impairs neuropathic pain**

Systemic treatment of sciatic nerve injured mice with a monoclonal antibody against the Ly6G antigen specifically expressed by neutrophils demonstrated that depletion of neutrophils attenuates the development of neuropathic pain behaviour (Nadeau et al, 2011). Furthermore, using molecular and pharmacological approaches, partial or complete depletion of macrophages yield beneficial effects in alleviating nerve injury associated chronic pain. For instance, the depletion of circulating macrophages by liposomeencapsulated clodronate results in decreased macrophage recruitment to the injury site, alleviated thermal hyperalgesia and reduced degeneration of axons (Liu et al., 2000) as well as attenuates pain hypersensitivity in diabetic animals (Mert et al., 2009). In addition, Wallerian degeneration (WLD) in response to nerve injury is delayed in genetically deficient mice which delays macrophage recruitment (Araki et al., 2004). Hyperalgesia is also delayed in this type of mice (WLD mice) suggesting that macrophages contribute to the development The dorsal root ganglia (DRG) are nodules containing cell bodies of afferent sensory neurons, which lie just outside of the spinal cord, in the intraforaminal spaces of the vertebral column. The nociceptive neurons of the DRG are classified into two groups depending on nerve fiber type: myelinated Aδ-fibers which are fast conducting, or unmyelinated C-fibers which are slow conducting. Like most neurons, sensory neurons in DRG are supported by a cast of other cells. In the DRG these supportive cells are satellite glial cells (SGC), dendritic cells, macrophages, and endothelial cells.

SGCs encapsulate each neuronal cell body, and a basement lamina separates neighbouring engcapsulated neurons (Hanani, 2005). The SGCs in DRG perform many similar functions to the astrocytes of the CNS, as they perform many regulatory and immune functions. For example, SGCs regulate extracellular amounts of glutamate and aspartate (Duce & Keen, 1983).  In addition, like many other glia, SGCs communicate with each other using gap junctions Normal DRG are also home for a population of resident macrophages. Most of these resident macrophages contact the neuron-satellite cell complex, and there are a smaller number which reside perivascularly (Hanani, 2005). The DRG neurons are very well perfused, and the blood supply is much denser than in peripheral nerve or dorsal root. In fact, each DRG neuron soma is in close proximity to an extensive network of capillaries. The blood supply to the DRG is supplied by fenestrated capillaries which have no blood-nerve barrier, and the vessels allow large molecules to pass easily.

Sensory ganglia do not have dendrites and chemical synapses, and thus each neuron within the ganglia was thought to be an independent communication pathway. However, this turns out not to be the case, and in fact, neighboring DRG neurons have the ability to crossexcite one another. Thus after peripheral nerve injury, cross-excitation is one possible mechanism by which sensory signals can be altered in time, space, and modality to result in neuropathic pain (Devor & Wall, 1990). Inflammation, which increases the excitability of DRG neurons, enhances cross-excitation. For example this mechanism is further influenced by interactions with SGCs.

Following peripheral nerve injury or inflammation, SGCs in DRG activate, and undergo proliferation (Lu et al., 1991). This process is thought to be mediated through release of ATP from damaged neurons, which stimulate purinergic receptors on SCGs (Filippov et al., 2004). Activation of SGCs results in increased glial fibrillary acidic protein (GFAP) expression and release of pro-inflammatory cytokines, for example, TNFα and IL-1β (Ohara et al., 2009). These cytokines released by activated SGCs have excitatory actions on nociceptive neurons (Takeda et al., 2009). In spinal nerve ligation (SNL) injured rats, GFAP expression peaks at one day after nerve injury, and is still present 10 days after ( Xie et al., 2009). Gap junctions in SGCs, are also increased after nerve injury (Jeon et al., 2009). These junctions seem to be involved with the generation and maintenance of neuropathic pain, as RNA interference targeting connexin 43, one of the major structural components of gap junctions in SGCs, prevents experimental neuropathic pain (Ohara et al., 2008). Gap junctions allow the redistribution of K+ ions though SGCs. K+ ion concentration regulation is another mechanism by which SCGs regulate the excitability of DRG neurons (Takeda et al., 2009).

Contribution of Inflammation to Chronic Pain Triggered by Nerve Injury 207

Glia represent, by far, the most abundant cells in the nervous system, largely outnumbering neurons. Astrocytes and microglia have been shown to undergo structural and functional modifications in the spinal cord in models of chronic pain (Colburn et al., 1999; Inoue & Tsuda, 2009; Moss et al., 2007). The data supporting a crucial role for these glial cells in the pathophysiology of neuropathic pain is ample (Gosselin et al., 2010). The long-lasting changes occurring in glia include structural alterations, cell proliferation, loss of neurotransmitter and release of proinflammatory mediators among other cellular processes. Several models of chronic pain investigated so far result in such glial phenotypic changes, essentially in the spinal cord (the first synaptic relay of the nociceptive pathway). Additionally, the involvement of glia in chronic pain has been remarkably validated by the prevention and reversal of behavioral manifestations of pain following treatments with molecules bearing glia-inhibitory properties (Hua et al., 2005; Milligan et al., 2003; Raghavendra et al., 2003; Tsuda et al., 2003). This suggests that glial alteration could be a crucial mechanism accounting for the persistence of hypersensitivity. In addition to resident glial cells, blood-borne monocytes and macrophages have the ability to infiltrate the spinal cord, where they proliferate and differentiate into activated microglia (Zhang et al., 2007). Activation of different glial/immune cells occurs along a complex temporal pattern. The contribution of each cell population to the modulation of nociceptive processing in pathological conditions follows a well-organized sequence of reciprocal communication between neurons and glia and among glial cells

**4.1 Activation of spinal glial cells following peripheral nerve injury**

themselves (Inoue & Tsuda, 2009; Vallejo et al,, 2010; Watkins & Maier, 2002).

development of neuropathic pain after peripheral nerve damage.

The events triggering astrocytic and microglial activation as well as the signals resulting from this activation producing pain hypersensitivity are currently under intense investigation. An increasing number of studies have proposed some mechanisms that seem to be consistently involved. Among them, the extracellular activating events, especially in neuropathic pain arising from peripheral damage, are very likely originating from sensory neuronal terminals reaching the spinal cord. Of special interest it's been the discovery of the critical role of several chemokines released by damaged neurons such as fractalkine (Clark et al., 2009; Milligan et al., 2004; Verge et al., 2004) and monocyte chemoatractant protein (MCP-1). The MCP-1 has raised a special interest in neuron-glia communication as its spatial profile of expression in the spinal cord dorsal horn matches that of activated microglia (Beggs & Salter, 2007; Thacker et al., 2009; Zhang & De Koninck, 2006). Peripheral nerve injury not only induces up-regulation of MCP-1 and its receptor CCR2 (Abbadie et al., 2003; Zhang & De Koninck, 2006) but also activation of spinal microglia (Zhang et al., 2007). Microgliosis in the spinal cord was prevented by spinal injection of MCP-1 neutralizing antibody or in mice lacking CCR2 (Zhang et al., 2007). Furthermore blockade of MCP-1/CCR2 signaling successfully prevents the infiltration of blood-borne monocytes into the spinal cord (Zhang et al., 2007). Taken together, these data suggest that spinal MCP-1/CCR2 signaling is critical for spinal microglial activation and the

Once activated, it is generally believed that the actions of glia in the context of neuropathic pain rely on their abilities to undergo drastic cellular changes and release of immune molecules including cytokines, chemokines and growth factors (Wieseler-Frank et al., 2005). The role of pro-inflammatory cytokines IL-6, IL-1 and TNF- has been extensively studied.

Peripheral nerve injury or inflammation also recruits resident macrophages of the DRG and circulating macrophages (Hu & McLachlan, 2002). Monocyte chemoattractant protein (MCP)-1 becomes upregulated at the DRG in injured neurons following nerve injury, and this has been found to be a critical event in the recruitment of macrophages (Jeon et al., 2009). The number of macrophages in the DRG peaks at 3 to 7 days after the trigger, and increased macrophages persist for weeks (Xie et al., 2006). The increase in macrophages at the DRG can occur even without any phagocytic events (Milligan & Watkins, 2009). The activated macrophages contribute to the persistence of neuropathic pain, and respond to numerous molecules, such as neurotransmitters, growth factors, and cytokines (Schreiber et al., 2002; Xie et al., 2006).

DRG neurons increase excitability in response to a number of inflammatory compounds, and can do so in a manner that sometimes does not require changes in protein expression. For instance, perfusion of DRG *in vitro* with TNFα results in an increase of A- and C-fiber discharge, as well as increased calcitonin gene related peptide (CGRP) release from nociceptor terminals at the spinal cord (Oprée & Kress, 2000). TNFα can act on TNFR1, one of the receptor subtypes for TNFα which is expressed exclusively by neurons (Li et al., 2004). The electrophysiological effects of TNFα on DRG neurons include a profound enhancement of tetrodotoxin (TTX)-resistant Na+ channels within a minute of application (Jin & Gereau, 2006), and an increase of vanilloid receptor 1 (TRPV1) function and expression (Hensellek et al., 2007). This increase in excitability of the DRG neurons by TNFα is thought to be mediated through p38 phosphorylation of TTX-resistant sodium channel subunits, and upregulation of TRPV1 protein expression (Jin & Gereau, 2006). Additionally other cytokines can also produce excitatory changes on DRG neurons, such as IL-1β and IL-6. DRG neurons can respond to IL-1β, sensitizing TRPV1 also (Obreja et al., 2002). Similarly, it seems IL-6 has effects on TRPV1 as well. IL-6 sensitizes the ion channel to heat, resulting in increased CGRP release from DRG neurons (Obreja et al., 2005).

Following injury to peripheral nerve, subsets of injured neurons will spontaneously ectopically discharge (Kim et al., 1993). This can result in a chronic sensitization of nociceptive neurons in both the PNS and CNS, resulting in pathological pain. While the exact mechanisms for the development of these changes are unknown, it is clear that inflammatory events at the DRG are critical (Miller et al., 2009). Such events at the DRG may include the cross-excitation of DRG neurons, activation of SGCs, recruitment and activation of macrophages, and the production of chemokines and cytokines.

#### **4. Spinal cord**

Central sensitization refers to the process through which a state of hyperexcitability is established in the central nervous system, leading to enhanced processing of nociceptive messages (Perl, 2007). Numerous mechanisms have been implicated in central sensitization, including alteration in glutamatergic neurotransmission/NMDA receptor-mediated hypersensitivity and loss of tonic inhibitory controls (disinhibition) (Basbaum et al., 2009). However, a remarkable series of findings in recent years has demonstrated a previously unrecognized role for inflammation in the initiation and maintenance of central sensitization. The involvement of non-neuronal cells, mainly astrocytes and microglia has been elucidated, to a degree that they appear now as reasonable future therapeutic targets.

Peripheral nerve injury or inflammation also recruits resident macrophages of the DRG and circulating macrophages (Hu & McLachlan, 2002). Monocyte chemoattractant protein (MCP)-1 becomes upregulated at the DRG in injured neurons following nerve injury, and this has been found to be a critical event in the recruitment of macrophages (Jeon et al., 2009). The number of macrophages in the DRG peaks at 3 to 7 days after the trigger, and increased macrophages persist for weeks (Xie et al., 2006). The increase in macrophages at the DRG can occur even without any phagocytic events (Milligan & Watkins, 2009). The activated macrophages contribute to the persistence of neuropathic pain, and respond to numerous molecules, such as neurotransmitters, growth factors, and cytokines (Schreiber et

DRG neurons increase excitability in response to a number of inflammatory compounds, and can do so in a manner that sometimes does not require changes in protein expression. For instance, perfusion of DRG *in vitro* with TNFα results in an increase of A- and C-fiber discharge, as well as increased calcitonin gene related peptide (CGRP) release from nociceptor terminals at the spinal cord (Oprée & Kress, 2000). TNFα can act on TNFR1, one of the receptor subtypes for TNFα which is expressed exclusively by neurons (Li et al., 2004). The electrophysiological effects of TNFα on DRG neurons include a profound enhancement of tetrodotoxin (TTX)-resistant Na+ channels within a minute of application (Jin & Gereau, 2006), and an increase of vanilloid receptor 1 (TRPV1) function and expression (Hensellek et al., 2007). This increase in excitability of the DRG neurons by TNFα is thought to be mediated through p38 phosphorylation of TTX-resistant sodium channel subunits, and upregulation of TRPV1 protein expression (Jin & Gereau, 2006). Additionally other cytokines can also produce excitatory changes on DRG neurons, such as IL-1β and IL-6. DRG neurons can respond to IL-1β, sensitizing TRPV1 also (Obreja et al., 2002). Similarly, it seems IL-6 has effects on TRPV1 as well. IL-6 sensitizes the ion channel to heat, resulting

Following injury to peripheral nerve, subsets of injured neurons will spontaneously ectopically discharge (Kim et al., 1993). This can result in a chronic sensitization of nociceptive neurons in both the PNS and CNS, resulting in pathological pain. While the exact mechanisms for the development of these changes are unknown, it is clear that inflammatory events at the DRG are critical (Miller et al., 2009). Such events at the DRG may include the cross-excitation of DRG neurons, activation of SGCs, recruitment and activation

Central sensitization refers to the process through which a state of hyperexcitability is established in the central nervous system, leading to enhanced processing of nociceptive messages (Perl, 2007). Numerous mechanisms have been implicated in central sensitization, including alteration in glutamatergic neurotransmission/NMDA receptor-mediated hypersensitivity and loss of tonic inhibitory controls (disinhibition) (Basbaum et al., 2009). However, a remarkable series of findings in recent years has demonstrated a previously unrecognized role for inflammation in the initiation and maintenance of central sensitization. The involvement of non-neuronal cells, mainly astrocytes and microglia has been elucidated, to a degree that they appear now as reasonable future therapeutic targets.

in increased CGRP release from DRG neurons (Obreja et al., 2005).

of macrophages, and the production of chemokines and cytokines.

al., 2002; Xie et al., 2006).

**4. Spinal cord**

#### **4.1 Activation of spinal glial cells following peripheral nerve injury**

Glia represent, by far, the most abundant cells in the nervous system, largely outnumbering neurons. Astrocytes and microglia have been shown to undergo structural and functional modifications in the spinal cord in models of chronic pain (Colburn et al., 1999; Inoue & Tsuda, 2009; Moss et al., 2007). The data supporting a crucial role for these glial cells in the pathophysiology of neuropathic pain is ample (Gosselin et al., 2010). The long-lasting changes occurring in glia include structural alterations, cell proliferation, loss of neurotransmitter and release of proinflammatory mediators among other cellular processes. Several models of chronic pain investigated so far result in such glial phenotypic changes, essentially in the spinal cord (the first synaptic relay of the nociceptive pathway). Additionally, the involvement of glia in chronic pain has been remarkably validated by the prevention and reversal of behavioral manifestations of pain following treatments with molecules bearing glia-inhibitory properties (Hua et al., 2005; Milligan et al., 2003; Raghavendra et al., 2003; Tsuda et al., 2003). This suggests that glial alteration could be a crucial mechanism accounting for the persistence of hypersensitivity. In addition to resident glial cells, blood-borne monocytes and macrophages have the ability to infiltrate the spinal cord, where they proliferate and differentiate into activated microglia (Zhang et al., 2007). Activation of different glial/immune cells occurs along a complex temporal pattern. The contribution of each cell population to the modulation of nociceptive processing in pathological conditions follows a well-organized sequence of reciprocal communication between neurons and glia and among glial cells themselves (Inoue & Tsuda, 2009; Vallejo et al,, 2010; Watkins & Maier, 2002).

The events triggering astrocytic and microglial activation as well as the signals resulting from this activation producing pain hypersensitivity are currently under intense investigation. An increasing number of studies have proposed some mechanisms that seem to be consistently involved. Among them, the extracellular activating events, especially in neuropathic pain arising from peripheral damage, are very likely originating from sensory neuronal terminals reaching the spinal cord. Of special interest it's been the discovery of the critical role of several chemokines released by damaged neurons such as fractalkine (Clark et al., 2009; Milligan et al., 2004; Verge et al., 2004) and monocyte chemoatractant protein (MCP-1). The MCP-1 has raised a special interest in neuron-glia communication as its spatial profile of expression in the spinal cord dorsal horn matches that of activated microglia (Beggs & Salter, 2007; Thacker et al., 2009; Zhang & De Koninck, 2006). Peripheral nerve injury not only induces up-regulation of MCP-1 and its receptor CCR2 (Abbadie et al., 2003; Zhang & De Koninck, 2006) but also activation of spinal microglia (Zhang et al., 2007). Microgliosis in the spinal cord was prevented by spinal injection of MCP-1 neutralizing antibody or in mice lacking CCR2 (Zhang et al., 2007). Furthermore blockade of MCP-1/CCR2 signaling successfully prevents the infiltration of blood-borne monocytes into the spinal cord (Zhang et al., 2007). Taken together, these data suggest that spinal MCP-1/CCR2 signaling is critical for spinal microglial activation and the development of neuropathic pain after peripheral nerve damage.

Once activated, it is generally believed that the actions of glia in the context of neuropathic pain rely on their abilities to undergo drastic cellular changes and release of immune molecules including cytokines, chemokines and growth factors (Wieseler-Frank et al., 2005). The role of pro-inflammatory cytokines IL-6, IL-1 and TNF- has been extensively studied.

Contribution of Inflammation to Chronic Pain Triggered by Nerve Injury 209

In addition to the fact that following nerve injury, many non-neuronal cells are activated and engaged in an inflammatory reaction, functional investigations using pharmacological or genetic approaches support the roles of spinal inflammation in chronic pain, especially the facilitation of neuropathic pain. Activated microglia release pain enhancing substances such as pro-inflammatory cytokines, nitric oxide, and prostaglandins, that excite spinal pain responsive neurons either directly or indirectly, and promote the release of other transmitters that can act on nociceptive neurons. Drugs that target glial activation have successfully controlled enhanced nociceptive states in animal models either by: inhibiting the synthesis of cytokines, blocking pro-inflammatory cytokine receptors or neutralizing endogenous cytokines, or disrupting pro-inflammatory cytokine signaling pathway with inhibitors of the p38MAP kinase. Anti-inflammatory molecules have been used to validate the concept in different nerve injury models associated with chronic pain. IL-10, secreted by monocytes and TH2 cells, reverses neuropathic pain behavior in animal studies ( Sloane et al., 2009; Soderquist et al., 2010). Enhancement of the TGF-1 pathway through administration of exogenous recombinant ligand (Echeverry et al., 2009) or through transgenic approaches prevents and reverses neuropathic pain induced by peripheral nerve injury (Tramullas et al., 2010). The importance of inflammation in chronic pain is also highlighted by the correlation between the pro- and anti-inflammatory balance and the outcomes of pain states in humans: Patients presenting painful neuropathies or CRPS show elevated levels pro-inflammatory cytokines (Davies et al., 2007) while patients with painless neuropathies have elevated levels of anti-inflammatory cytokines (Uceyler et al., 2007).

**4.3 Inhibition of the spinal inflammatory reaction can alleviate neuropathic pain**

**5. Current advances and challenges in knowledge translation**

In summary, following nerve injury, the communication between the immune and the nervous systems has been enhanced, which is observed not only at the sites of damage (peripheral nerve or DRGs), but also within the spinal cord. Such inflammatory reaction can be a critical underlying mechanism in generating neuropathic pain. The role of immune/glial cells in the development and the persistence of pain after nerve injury challenge conventional concepts that focus on neural activity being solely responsible for the changes that drive neuropathic pain. This shift in our understanding provides an incredible opportunity to progress to a new therapeutic approach that will be beneficial for the millions of people suffering of neuropathic pain. However, translation of such knowledge into clinical use for human has been largely underexplored. To date, very limited evidence is available for the roles of inflammation in persistent neuropathic pain states in human (McMahon & Malcangio, 2009). Only few clinical studies have tested immunosuppressive drugs or drugs interfering with glial functions for neuropathic pain (Scholz & Woolf, 2007). The effectiveness of some anti-inflammatory compounds, such as Anakinra (Kineret®) (recombinant human IL-1 receptor antagonist), Etanercept (Enbrel®) and Infliximab (Remicade®) (TNF inhibitors), was also limited to pain associated with rheumatoid arthritis and other inflammatory conditions. Considering the fact that current treatments for neuropathic pain offer only moderated pain relief with the severe side effects of sedation, tolerance and the risk of dependence, there are enormous needs to develop new therapeutics for which we think targeting one or multiple inflammatory pathways involved in the development of neuropathic pain can be a very exciting strategy. Large sizes, high quality randomized clinical trials with different anti-inflammatory molecules are needed to

#### **4.2 Peripheral nerve injury alters blood spinal cord barrier integrity**

The blood spinal cord barrier (BSCB) constitutes a physical/biochemical barrier between the central nervous system (CNS) and the systemic circulation, which serves to protect the microenvironment of the spinal cord. However, several studies (Beggs et al., 2010; Gordh et al., 2006) have reported an important increase in BCSB permeability after peripheral nerve injury. A recent observation from our laboratory revealed that peripheral nerve injury disrupted the integrity of the BSCB and inflammatory mediators are key regulators of BSCB function. MCP-1 released by damaged neurons is an endogenous trigger for the BSCB leakage. BSCB permeability can also be impaired by circulating IL-1. In contrast, anti-inflammatory cytokines TGF-1 and IL-10 were able to shut down the openings of the BSCB following nerve injury. The compromised BSCB allows penetration of both inflammatory molecules (e.g., cytokine IL-1) and immune cells (monocytes/macrophages, lymphocytes) into the spinal cord, participating in the central inflammatory response, a critical process for the development of neuropathic pain (Echeverry et al., 2011). In addition, endothelial cells have the capacity to secrete immune mediators, for example, IL-6 (Vallejo et al., 2010) and bear a series of surface molecules, such as chemokines, to modulate the entrance of immune cells which are known to have an impact on pain hypersensitivity. For a summary of the described kinetics see Fig 1.

Fig. 1. Neuronal MCP-1 released into the spinal cord not only activated its receptor CCR2 in endothelial cells (1) to impair the integrity of the BSCB, but also activated CCR2 in microglia (2) to trigger microglial activation. Microglial activation is not required for the increase of BSCB permeability (3), although due to the lack of a selective BBB/BSCB blocker without affecting the inflammatory response, it is not clear whether the opening of BSCB can promote microglial activation (4). Central inflammation resulted from the opening of BSCB and microglial activation was necessary for the development of neuropathic pain, since antiinflammatory cytokines IL-10 and TGF-1, which were able to shut-down the nerve injury induced BSCB disruption and microglial activation, attenuated neuropathic pain

The blood spinal cord barrier (BSCB) constitutes a physical/biochemical barrier between the central nervous system (CNS) and the systemic circulation, which serves to protect the microenvironment of the spinal cord. However, several studies (Beggs et al., 2010; Gordh et al., 2006) have reported an important increase in BCSB permeability after peripheral nerve injury. A recent observation from our laboratory revealed that peripheral nerve injury disrupted the integrity of the BSCB and inflammatory mediators are key regulators of BSCB function. MCP-1 released by damaged neurons is an endogenous trigger for the BSCB leakage. BSCB permeability can also be impaired by circulating IL-1. In contrast, anti-inflammatory cytokines TGF-1 and IL-10 were able to shut down the openings of the BSCB following nerve injury. The compromised BSCB allows penetration of both inflammatory molecules (e.g., cytokine IL-1) and immune cells (monocytes/macrophages, lymphocytes) into the spinal cord, participating in the central inflammatory response, a critical process for the development of neuropathic pain (Echeverry et al., 2011). In addition, endothelial cells have the capacity to secrete immune mediators, for example, IL-6 (Vallejo et al., 2010) and bear a series of surface molecules, such as chemokines, to modulate the entrance of immune cells which are known to have an impact on pain hypersensitivity. For a summary of the described kinetics see Fig 1.

Fig. 1. Neuronal MCP-1 released into the spinal cord not only activated its receptor CCR2 in endothelial cells (1) to impair the integrity of the BSCB, but also activated CCR2 in microglia (2) to trigger microglial activation. Microglial activation is not required for the increase of BSCB permeability (3), although due to the lack of a selective BBB/BSCB blocker without affecting the inflammatory response, it is not clear whether the opening of BSCB can promote microglial activation (4). Central inflammation resulted from the opening of BSCB and microglial activation was necessary for the development of neuropathic pain, since antiinflammatory cytokines IL-10 and TGF-1, which were able to shut-down the nerve injury

induced BSCB disruption and microglial activation, attenuated neuropathic pain

**4.2 Peripheral nerve injury alters blood spinal cord barrier integrity**

In addition to the fact that following nerve injury, many non-neuronal cells are activated and engaged in an inflammatory reaction, functional investigations using pharmacological or genetic approaches support the roles of spinal inflammation in chronic pain, especially the facilitation of neuropathic pain. Activated microglia release pain enhancing substances such as pro-inflammatory cytokines, nitric oxide, and prostaglandins, that excite spinal pain responsive neurons either directly or indirectly, and promote the release of other transmitters that can act on nociceptive neurons. Drugs that target glial activation have successfully controlled enhanced nociceptive states in animal models either by: inhibiting the synthesis of cytokines, blocking pro-inflammatory cytokine receptors or neutralizing endogenous cytokines, or disrupting pro-inflammatory cytokine signaling pathway with inhibitors of the p38MAP kinase. Anti-inflammatory molecules have been used to validate the concept in different nerve injury models associated with chronic pain. IL-10, secreted by monocytes and TH2 cells, reverses neuropathic pain behavior in animal studies ( Sloane et al., 2009; Soderquist et al., 2010). Enhancement of the TGF-1 pathway through administration of exogenous recombinant ligand (Echeverry et al., 2009) or through transgenic approaches prevents and reverses neuropathic pain induced by peripheral nerve injury (Tramullas et al., 2010). The importance of inflammation in chronic pain is also highlighted by the correlation between the pro- and anti-inflammatory balance and the outcomes of pain states in humans: Patients presenting painful neuropathies or CRPS show elevated levels pro-inflammatory cytokines (Davies et al., 2007) while patients with painless neuropathies have elevated levels of anti-inflammatory cytokines (Uceyler et al., 2007).

#### **5. Current advances and challenges in knowledge translation**

In summary, following nerve injury, the communication between the immune and the nervous systems has been enhanced, which is observed not only at the sites of damage (peripheral nerve or DRGs), but also within the spinal cord. Such inflammatory reaction can be a critical underlying mechanism in generating neuropathic pain. The role of immune/glial cells in the development and the persistence of pain after nerve injury challenge conventional concepts that focus on neural activity being solely responsible for the changes that drive neuropathic pain. This shift in our understanding provides an incredible opportunity to progress to a new therapeutic approach that will be beneficial for the millions of people suffering of neuropathic pain. However, translation of such knowledge into clinical use for human has been largely underexplored. To date, very limited evidence is available for the roles of inflammation in persistent neuropathic pain states in human (McMahon & Malcangio, 2009). Only few clinical studies have tested immunosuppressive drugs or drugs interfering with glial functions for neuropathic pain (Scholz & Woolf, 2007). The effectiveness of some anti-inflammatory compounds, such as Anakinra (Kineret®) (recombinant human IL-1 receptor antagonist), Etanercept (Enbrel®) and Infliximab (Remicade®) (TNF inhibitors), was also limited to pain associated with rheumatoid arthritis and other inflammatory conditions. Considering the fact that current treatments for neuropathic pain offer only moderated pain relief with the severe side effects of sedation, tolerance and the risk of dependence, there are enormous needs to develop new therapeutics for which we think targeting one or multiple inflammatory pathways involved in the development of neuropathic pain can be a very exciting strategy. Large sizes, high quality randomized clinical trials with different anti-inflammatory molecules are needed to

Contribution of Inflammation to Chronic Pain Triggered by Nerve Injury 211

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**13**

 *Spain*

Esther Uña Cidón

**Neuropathy Secondary to Chemotherapy:**

Cancer is still a major public health problem worldwide, being one of the leading causes of population mortality. In 2000, malignant tumours were responsible for 12 percent of the nearly 56 million deaths worldwide from all causes. In many countries, more than a quarter of deaths were attributable to cancer (Parkin et al., 2005). According to the World Cancer Report, which is the most comprehensive global examination of the disease to date, cancer rates are expected to further increase by 50% to 15 million new cases by 2020. Absolute deaths from cancer worldwide are projected to continue to rise to over 11 million by 2030 (Parkin et al., 2005). However, due to advances in multimodal treatment, screening and therefore early detection, as well as chemoprevention, the number of cancer survivors are also expected to increase in the near future (Parkin et al., 2005). This fact highlights the

Most patients diagnosed with cancer will receive chemotherapy over the course of the disease with a neoadjuvant, adjuvant, or palliative. Several major classes of chemotherapeutic agents can be grouped by their underlying mechanism of action (Hausheer et al., 2006). All these drugs can also have different profiles of toxicity according to their mechanism, some very serious. One of the most common and distressing adverse event is peripheral neuropathy whose incidence is now growing. In fact many chemotherapeutic agents belonging to different groups associate a dose-limiting chemotherapy-induced peripheral neuropathy (CIPN). The different properties of these agents are contributing to the pathogenesis of this toxicity, leading to variation in incidence, severity, kind of damage, etc. depending on the agent considered (Hausheer et al., 2006).

CIPN involves sensory and motor nerve damage or dysfunction with a prolonged course, which causes permanent damage in some long-term cancer survivors with the subsequent impairment in their QOL (Hausheer et al., 2006). CIPN affects not only QOL, since toxicity may have an impact on the planned therapy, and such alterations could have negative consequences for the outcome, especially if the patient´s malignancy is responding to treatment (Hausheer et al., 2006). Despite the increasing awareness of this potential toxicity and the improved knowledge of the pathogenic mechanisms, emerging evidence suggests that the incidence of CIPN is substantially under-reported in clinical trials, due mainly to

relevance of fostering and maintaining survivors´ quality of life (QOL).

**1. Introduction**

 **A Real Issue for Cancer Survivors**

*Oncology Department, Clinical University Hospital*

 *and Faculty of Medicine of Valladolid*

### **Neuropathy Secondary to Chemotherapy: A Real Issue for Cancer Survivors**

Esther Uña Cidón

*Oncology Department, Clinical University Hospital and Faculty of Medicine of Valladolid Spain*

#### **1. Introduction**

214 Basic Principles of Peripheral Nerve Disorders

Tramullas, M., Lantero, A., Díaz, A., Morchón, N., Merino, D., Villar, A., Buscher, D., et al.

Tsuda, M., Shigemoto-Mogami, Y., & Koizumi, S. (2003). P2X 4 receptors induced in spinal

Uçeyler, N., Eberle, T., Rolke, R., Birklein, F., & Sommer, C. (2007a). Differential expression

Uçeyler, N., Tscharke, A., & Sommer, C. (2007b). Early cytokine expression in mouse sciatic

Vallejo, R., Tilley, D. M., Vogel, L., & Benyamin, R. (2010). The role of glia and the immune

Verge, G.M., Milligan, E.D., Maier, S.F., Watkins, L.R., Naeve, G. S., & Foster, A. C. (2004).

*journal of neuroscience*, *20*(5), 1150-60. doi:10.1111/j.1460-9568.2004.03593.x Bruck, W. (1997). The role of macrophages in Wallerian degeneration. *Brain Pathology*, *7*(2),

Watkins, L.R., & Maier, S.F. (2002). Beyond neurons: evidence that immune and glial cells

Wieseler-Frank, J., Maier, S.F., & Watkins, L.R. (2005). Immune-to-brain communication

Xie, W., Strong, J. A., & Zhang, J. M. (2009). Early blockade of injured primary sensory

*Neuroscience*, *160*(4), 847-57. IBRO. doi:10.1016/j.neuroscience.2009.03.016 Xie, W.-R., Deng, H., Li, H., Bowen, T. L., Strong, J. A., & Zhang, J.-M. (2006). Robust

Zelenka, M., Schäfers, M., & Sommer, C. (2005). Intraneural injection of interleukin-1beta

Zhang, J., Shi, X.Q., Echeverry, S., Mogil, J. S., De Koninck, Y., & Rivest, S. (2007). Expression

neuropathic pain. *The Journal of Neuroscience*, *27*(45), 12396-406. doi:

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pain modulation. *The Journal of neuroscience*, *30*(4), 1502-11. doi:

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10.1523/JNEUROSCI.2584-09.2010

doi:10.1038/nature01859.1.

doi:10.1016/j.pain.2007.07.031

doi:10.1152/physrev.00011.2002

741-52.

4159.2006.03746.x

10.1523/JNEUROSCI.3016-07.2007

(2010). BAMBI (bone morphogenetic protein and activin membrane-bound inhibitor) reveals the involvement of the transforming growth factor-beta family in

microglia gate tactile allodynia after nerve injury. *Nature*, *424*(August), 1-6.

patterns of cytokines in complex regional pain syndrome. *Pain*, *132*(1-2), 195-205.

nerve after chronic constriction nerve injury depends on calpain. *Brain, behavior, and*

system in the development and maintenance of neuropathic pain. *Pain practice*,

Fractalkine (CX3CL1) and fractalkine receptor (CX3CR1) distribution in spinal cord and dorsal root ganglia under basal and neuropathic pain conditions. *The European*

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dynamically modulates pain: physiological and pathological consequences. *Brain,*

afferents reduces glial cell activation in two rat neuropathic pain models.

increase of cutaneous sensitivity, cytokine production and sympathetic sprouting in rats with localized inflammatory irritation of the spinal ganglia. *Neuroscience*,

and tumor necrosis factor-alpha into rat sciatic nerve at physiological doses induces signs of neuropathic pain. *Pain*, *116*(3), 257-63. doi:10.1016/j.pain.2005.04.018 Zhang, J., & De Koninck, Y. (2006). Spatial and temporal relationship between monocyte

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of CCR2 in both resident and bone marrow-derived microglia plays a critical role in

Cancer is still a major public health problem worldwide, being one of the leading causes of population mortality. In 2000, malignant tumours were responsible for 12 percent of the nearly 56 million deaths worldwide from all causes. In many countries, more than a quarter of deaths were attributable to cancer (Parkin et al., 2005). According to the World Cancer Report, which is the most comprehensive global examination of the disease to date, cancer rates are expected to further increase by 50% to 15 million new cases by 2020. Absolute deaths from cancer worldwide are projected to continue to rise to over 11 million by 2030 (Parkin et al., 2005). However, due to advances in multimodal treatment, screening and therefore early detection, as well as chemoprevention, the number of cancer survivors are also expected to increase in the near future (Parkin et al., 2005). This fact highlights the relevance of fostering and maintaining survivors´ quality of life (QOL).

Most patients diagnosed with cancer will receive chemotherapy over the course of the disease with a neoadjuvant, adjuvant, or palliative. Several major classes of chemotherapeutic agents can be grouped by their underlying mechanism of action (Hausheer et al., 2006). All these drugs can also have different profiles of toxicity according to their mechanism, some very serious. One of the most common and distressing adverse event is peripheral neuropathy whose incidence is now growing. In fact many chemotherapeutic agents belonging to different groups associate a dose-limiting chemotherapy-induced peripheral neuropathy (CIPN). The different properties of these agents are contributing to the pathogenesis of this toxicity, leading to variation in incidence, severity, kind of damage, etc. depending on the agent considered (Hausheer et al., 2006).

CIPN involves sensory and motor nerve damage or dysfunction with a prolonged course, which causes permanent damage in some long-term cancer survivors with the subsequent impairment in their QOL (Hausheer et al., 2006). CIPN affects not only QOL, since toxicity may have an impact on the planned therapy, and such alterations could have negative consequences for the outcome, especially if the patient´s malignancy is responding to treatment (Hausheer et al., 2006). Despite the increasing awareness of this potential toxicity and the improved knowledge of the pathogenic mechanisms, emerging evidence suggests that the incidence of CIPN is substantially under-reported in clinical trials, due mainly to

Neuropathy Secondary to Chemotherapy: A Real Issue for Cancer Survivors 217

Grisold, 2008). In addition, most cases will receive a combination of several chemotherapeutic compounds aiming to enhance the effectiveness of such treatment but with the disadvantage of inducing CIPN more often than individual agents (Cavaletti &

The clinical manifestations of CIPN are subjective and predominantly manifest as pure sensory symptoms, most frequently reported as progressive distal symmetrically distributed. The most important symptoms are numbness, tingling, "pins and needles", burning sensations, decreased or altered sensation, or increased sensitivity that may sometimes be painful in the feet and hands (Hilkens & Van den Bent, 1997; Lipton et al., 1989). The primary clinical goal in assessing these patients is to determine the presence of these complications after making a correct differential diagnosis and also to evaluate their severity because sometimes they can interfere with activities of daily living (ADL). These assessments are critical for making decisions related to the continuity, dose-intensity, or dose-density of the anticancer treatment (Cavaletti & Marmiroli, 2010). Although the most important spectrum of CIPN symptoms is sensory in nature, motor weakness may also be reported and when present is observed in patients with more persistent and severe sensory symptoms (American Society of Health-System Pharmacists, 2002). In fact, isolated motor weakness with the complete absence of sensory involvement has not been described, but if observed in a patient, consideration should be given to other conditions that may produce pure motor weakness, including steroid myopathy which is proximal, diabetic, or paraneoplastic motor neuropathy (Hausheer et al., 2006). The onset of CIPN is usually gradually progressive, although in some patients it can appear immediately after the administration of a neurotoxic drug or even during the infusion. This condition may thus pose difficulties for the clinician, especially to diagnose those patients with coexisting problems or disorders that involve the peripheral nervous system (Hausheer et al., 2006). No standard treatment currently exists for the prevention, mitigation, or management of

This chapter will focus on all these aspects of CIPN including the clinical manifestation, diagnosis, management, mechanisms underlying its onset, assessment, and current status of neuroprotection. The different agents used in anticancer treatments that might be related to

**Taxanes X Epothilones X Vinca alkaloids X**

Table 2. Classification of the most important agents reported in CIPN (Hilkens & Van den

**Sensory Sensory and motor**

Marmiroli, 2010).

CIPN (Hausheer et al., 2006).

toxicity will also be reviewed.

Bent, 1997; Lipton et al., 1989).

**Platinum agents X Bortezomib X Thalidomide X**

relevant limitations in the available grading scales that are commonly used for assessment (Cavaletti & Marmiroli, 2010).

Table 1. Mechanisms of anticancer action and most frequent clinical uses of neurotoxic agents (Cavaletti, G. & Marmiroli, 2006).

The challenges in diagnosing and assessing the extent of functional impairment in a reliable and reproducible manner is a paramount consideration for the clinician to make a clinical decision as well as in clinical trials involving the prospective evaluation of neurotoxic chemotherapy or interventions aimed at the prevention or mitigation of CIPN (Gándara et al., 1991). The drugs most commonly associated with CIPN are platinum analogues, antitubulins, the proteasome inhibitor bortezomib, and thalidomide, which are very commonly used in a wide range of solid and haematological malignancies (Windebank &

relevant limitations in the available grading scales that are commonly used for assessment

**Cisplatin** Alkylating, crosslink to DNA Lung cancer, Bladder cancer,

**Carboplatin** Alkylating, crosslink to DNA Ovarian cancer, Lung cancer

**Oxaliplatin** Alkylating, crosslink to DNA Colorectal cancer

microtubules

microtubules

microtubules

microtubules

microtubules

Immune-modulator

Table 1. Mechanisms of anticancer action and most frequent clinical uses of neurotoxic

The challenges in diagnosing and assessing the extent of functional impairment in a reliable and reproducible manner is a paramount consideration for the clinician to make a clinical decision as well as in clinical trials involving the prospective evaluation of neurotoxic chemotherapy or interventions aimed at the prevention or mitigation of CIPN (Gándara et al., 1991).

**Agent Mechanism of anticancer**

**Paclitaxel** Hyperpolymerisation of

**Docetaxel** Hyperpolymerisation of

**Ixabepilone** Hyperpolymerisation of

**Vincristine** Depolymerisation of

**Vinorelbine** Depolymerisation of

**Thalidomide** Antiangiogenic

agents (Cavaletti, G. & Marmiroli, 2006).

**action**

Grisold, 2008). In addition, most cases will receive a combination of several chemotherapeutic compounds aiming to enhance the effectiveness of such treatment but with the disadvantage of inducing CIPN more often than individual agents (Cavaletti & Marmiroli, 2010).

The clinical manifestations of CIPN are subjective and predominantly manifest as pure sensory symptoms, most frequently reported as progressive distal symmetrically distributed. The most important symptoms are numbness, tingling, "pins and needles", burning sensations, decreased or altered sensation, or increased sensitivity that may sometimes be painful in the feet and hands (Hilkens & Van den Bent, 1997; Lipton et al., 1989). The primary clinical goal in assessing these patients is to determine the presence of these complications after making a correct differential diagnosis and also to evaluate their severity because sometimes they can interfere with activities of daily living (ADL). These assessments are critical for making decisions related to the continuity, dose-intensity, or dose-density of the anticancer treatment (Cavaletti & Marmiroli, 2010). Although the most important spectrum of CIPN symptoms is sensory in nature, motor weakness may also be reported and when present is observed in patients with more persistent and severe sensory symptoms (American Society of Health-System Pharmacists, 2002). In fact, isolated motor weakness with the complete absence of sensory involvement has not been described, but if observed in a patient, consideration should be given to other conditions that may produce pure motor weakness, including steroid myopathy which is proximal, diabetic, or paraneoplastic motor neuropathy (Hausheer et al., 2006). The onset of CIPN is usually gradually progressive, although in some patients it can appear immediately after the administration of a neurotoxic drug or even during the infusion. This condition may thus pose difficulties for the clinician, especially to diagnose those patients with coexisting problems or disorders that involve the peripheral nervous system (Hausheer et al., 2006). No standard treatment currently exists for the prevention, mitigation, or management of CIPN (Hausheer et al., 2006).

This chapter will focus on all these aspects of CIPN including the clinical manifestation, diagnosis, management, mechanisms underlying its onset, assessment, and current status of neuroprotection. The different agents used in anticancer treatments that might be related to toxicity will also be reviewed.

Neuropathy Secondary to Chemotherapy: A Real Issue for Cancer Survivors 219

et al., 1999; Verdu et al., 1999). The level of hydrolysis of cisplatin and oxaliplatin differs from that of carboplatin, which is higher. This finding may contribute to the difference in the associated neurotoxic severity patterns (McWhinney et al., 2009). In this way, patients from clinical trials treated with these agents have shown that the severity of neurotoxicity is more commonly seen with cisplatin than with oxaliplatin and is much more severe than that

In the case of cisplatin, the differences in the degree of neurotoxicity could also be associated with the different plasma levels of the intermediary products of the aquation to the process of DNA adduct formation (McWhinney et al., 2009). Although these drugs cause the main cellular damage by the formation of these DNA adducts, additional complexes of covalently crosslinked platinum-DNA-protein have been proposed as another mechanism more specifically studied with cisplatin (Chvalova et al., 2007). These complexes lead to disruption of nuclear metabolism and spatial organisation of chromatin, and even inhibit DNA replication and repair (McWhinney et al., 2009). Another suggested underlying mechanism is the involvement of oxidative stress and mitochondrial dysfunction as a trigger of neuronal apoptosis. This process might be mediated by an increased activity of tumour protein p53 and mitochondrial release of cytochrome C (Cavaletti & Marmiroli, 2010).

Even though the exact mechanism of neuronal apoptosis is not fully understood, one proposal has suggested that the machinery of DNA repair is unable to repair the damaged DNA (Gill & Windebank, 1998). Polymorphisms in the DNA-repair genes, including genes for base-excision repair, nucleotide-excision repair, mismatch repair, and double-strand break repair pathways, cause the individuals to be less proficient in repairing carcinogen-induced damage (Cavaletti & Marmiroli, 2010). Moreover, platinum-DNA complexes (Zhu et al., 2005) interfere with the normal function of cellular proteins (i.e. binding or interactions with other proteins), and apoptosis has been observed in DG neurons following treatment with cisplatin, both *in vitro* and *in vivo*, and is correlated with increased platinum-DNA binding in these DG neurons (Cavaletti & Marmiroli, 2010). Finally, all these platinum compounds appear to affect the axons, myelin sheath, neuronal cell body, and the glial structures of the neurons. In support, levels of platinum are significantly higher in the DG than in the brain and spinal cord, which

Oxaliplatin and cisplatin differ in their severity of neurotoxicity to the DG, with cisplatin being more neurotoxic. In fact, cisplatin produces about three times more platinum- DNA adducts in the DG than do equimolar doses of oxaliplatin (McWhinney et al., 2009). Acute oxaliplatin neurotoxicity is thought to be caused by transient dysfunction of nodal, axonal, voltage-gated sodium channels, probably owing to an oxalate chelating effect on both calcium and magnesium ions that could interfere with the kinetics of sodium channels (McDonald et al., 2005; Ta et al., 2006). However, the small-conductance Ca(++)-activated K(+) channels, encoded by the SK1-3 genes, are also involved in membrane excitability, playing a role in after-hyperpolarisation at the motor-nerve terminal. Because the SK3 gene is characterised in Caucasians by a highly polymorphic CAG motif within exon 1, SK3 gene polymorphism may influence the development of acute nerve hyperexcitability in oxaliplatin-treated patients. The results of the study by Basso et al. (2011) have suggested that oxaliplatin neurotoxicity may be related to distribution of the polymorphic CAG motif of the SK3 gene, which might modulate nerve after-hyperpolarisation. The allele with 13-14

CAG repeats could mark patients susceptible to acute oxaliplatin neurotoxicity.

observed with carboplatin (McWhinney et al., 2009).

are protected by the blood-brain barrier (Hausheer et al., 2006).

#### **2. Pathogenic mechanisms and clinical features of CIPN**

The underlying pathogenic mechanisms for the development of CIPN have not been fully elucidated. In fact, a variety of mechanisms have been proposed. Different chemotherapeutic agents cause highly similar patterns and spectra of clinical manifestations, which include the length-dependent, symmetrical stocking-glove distribution with predominantly sensory symptoms subjectively reported by the patient (Gregg et al., 1992; McKeage et al., 2001; Verdu et al., 1989). The next sections will review the pathogenic mechanisms clinical features associated with every group of drugs or isolated agents previously mentioned in this chapter.

#### **3. Platinum analogues**

The profiles of general toxicity and neurotoxicity differ among the three platinum drugs (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). Although platinum-based chemotherapies (mainly cisplatin and carboplatin) are a mainstay for the treatment of most solid tumours, their clinical use is severely curtailed by dose-limiting nephro-, oto-, and neurotoxicities (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). Neurotoxicity induced by platinum agents is characterised by a dosedependent, cumulative, predominantly painful sensory neuropathy, presenting with symptoms in the distal extremities. These agents are most frequently associated with the development of axonopathy, but platinum-induced neuronopathy may also be observed, especially with higher doses of platinum (McWhinney et al., 2009). Due to the absence of therapies that either cure or prevent platinum-induced CIPN, especially with the advent of newer members of this class of anticancer agents, we need a better understanding of the underlying pathophysiology to develop more-effective therapies or even preventive measures.

Cisplatin, carboplatin, and oxaliplatin differ in solubility, chemical reactivity, oxygenated leaving groups, pharmacokinetics, and toxicology (McKeage, 1995). The leaving groups on each molecule are responsible for the differences in each drug's reactivity with nucleophiles, which is likely to contribute to their differences in toxic profiles (Hah et al., 2006; McWhinney et al., 2009). Despite of these differences, the primary mechanisms involved in their neurotoxicity are likely to be similar.

These agents can enter the DG and peripheral nerves, as opposed to the brain, mainly through passive diffusion, although current data indicates the presence of metal transporters that may also be involved in their entrance into the cell (Krarup-Hansen et al., 1999; Thompson et al., 1984). Current research indicates that after entering the DG cell, these agents will form an adduct with DNA (Cavaletti & Marmiroli et al., 2010). In fact, platinum agents undergo aquation to form a positively charged molecule that interacts with DNA to form crosslinks and finally DNA/platinum adducts (Cavaletti & Marmiroli et al., 2010). These DNA intrastrand adducts and interstrand crosslinks will alter the tertiary structure of DNA leading on the one hand to alterations in cell-cycle kinetics with postmitotic DG neurons trying to reenter the cell cycle and on the other hand to the promotion of apoptosis (Gill & Windebank, 1998).

The level of these compounds in DG neurons at a given cumulative dose has been significantly correlated with the degree of neurotoxicity (Cavaletti et al., 1992; Dzagnidze et al., 2007; Gregg et al., 1992; Meijer et al., 1999). The severity of neurotoxicity, however, involves other factors (Cavaletti et al., 1992; Dzagnidze et al., 2007; Gregg et al., 1992; Meijer

The underlying pathogenic mechanisms for the development of CIPN have not been fully elucidated. In fact, a variety of mechanisms have been proposed. Different chemotherapeutic agents cause highly similar patterns and spectra of clinical manifestations, which include the length-dependent, symmetrical stocking-glove distribution with predominantly sensory symptoms subjectively reported by the patient (Gregg et al., 1992; McKeage et al., 2001; Verdu et al., 1989). The next sections will review the pathogenic mechanisms clinical features associated with every group of drugs or isolated

The profiles of general toxicity and neurotoxicity differ among the three platinum drugs (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). Although platinum-based chemotherapies (mainly cisplatin and carboplatin) are a mainstay for the treatment of most solid tumours, their clinical use is severely curtailed by dose-limiting nephro-, oto-, and neurotoxicities (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). Neurotoxicity induced by platinum agents is characterised by a dosedependent, cumulative, predominantly painful sensory neuropathy, presenting with symptoms in the distal extremities. These agents are most frequently associated with the development of axonopathy, but platinum-induced neuronopathy may also be observed, especially with higher doses of platinum (McWhinney et al., 2009). Due to the absence of therapies that either cure or prevent platinum-induced CIPN, especially with the advent of newer members of this class of anticancer agents, we need a better understanding of the underlying pathophysiology to develop more-effective therapies or even preventive measures. Cisplatin, carboplatin, and oxaliplatin differ in solubility, chemical reactivity, oxygenated leaving groups, pharmacokinetics, and toxicology (McKeage, 1995). The leaving groups on each molecule are responsible for the differences in each drug's reactivity with nucleophiles, which is likely to contribute to their differences in toxic profiles (Hah et al., 2006; McWhinney et al., 2009). Despite of these differences, the primary mechanisms involved in

These agents can enter the DG and peripheral nerves, as opposed to the brain, mainly through passive diffusion, although current data indicates the presence of metal transporters that may also be involved in their entrance into the cell (Krarup-Hansen et al., 1999; Thompson et al., 1984). Current research indicates that after entering the DG cell, these agents will form an adduct with DNA (Cavaletti & Marmiroli et al., 2010). In fact, platinum agents undergo aquation to form a positively charged molecule that interacts with DNA to form crosslinks and finally DNA/platinum adducts (Cavaletti & Marmiroli et al., 2010). These DNA intrastrand adducts and interstrand crosslinks will alter the tertiary structure of DNA leading on the one hand to alterations in cell-cycle kinetics with postmitotic DG neurons trying to reenter the cell

cycle and on the other hand to the promotion of apoptosis (Gill & Windebank, 1998).

The level of these compounds in DG neurons at a given cumulative dose has been significantly correlated with the degree of neurotoxicity (Cavaletti et al., 1992; Dzagnidze et al., 2007; Gregg et al., 1992; Meijer et al., 1999). The severity of neurotoxicity, however, involves other factors (Cavaletti et al., 1992; Dzagnidze et al., 2007; Gregg et al., 1992; Meijer

**2. Pathogenic mechanisms and clinical features of CIPN**

agents previously mentioned in this chapter.

their neurotoxicity are likely to be similar.

**3. Platinum analogues**

et al., 1999; Verdu et al., 1999). The level of hydrolysis of cisplatin and oxaliplatin differs from that of carboplatin, which is higher. This finding may contribute to the difference in the associated neurotoxic severity patterns (McWhinney et al., 2009). In this way, patients from clinical trials treated with these agents have shown that the severity of neurotoxicity is more commonly seen with cisplatin than with oxaliplatin and is much more severe than that observed with carboplatin (McWhinney et al., 2009).

In the case of cisplatin, the differences in the degree of neurotoxicity could also be associated with the different plasma levels of the intermediary products of the aquation to the process of DNA adduct formation (McWhinney et al., 2009). Although these drugs cause the main cellular damage by the formation of these DNA adducts, additional complexes of covalently crosslinked platinum-DNA-protein have been proposed as another mechanism more specifically studied with cisplatin (Chvalova et al., 2007). These complexes lead to disruption of nuclear metabolism and spatial organisation of chromatin, and even inhibit DNA replication and repair (McWhinney et al., 2009). Another suggested underlying mechanism is the involvement of oxidative stress and mitochondrial dysfunction as a trigger of neuronal apoptosis. This process might be mediated by an increased activity of tumour protein p53 and mitochondrial release of cytochrome C (Cavaletti & Marmiroli, 2010).

Even though the exact mechanism of neuronal apoptosis is not fully understood, one proposal has suggested that the machinery of DNA repair is unable to repair the damaged DNA (Gill & Windebank, 1998). Polymorphisms in the DNA-repair genes, including genes for base-excision repair, nucleotide-excision repair, mismatch repair, and double-strand break repair pathways, cause the individuals to be less proficient in repairing carcinogen-induced damage (Cavaletti & Marmiroli, 2010). Moreover, platinum-DNA complexes (Zhu et al., 2005) interfere with the normal function of cellular proteins (i.e. binding or interactions with other proteins), and apoptosis has been observed in DG neurons following treatment with cisplatin, both *in vitro* and *in vivo*, and is correlated with increased platinum-DNA binding in these DG neurons (Cavaletti & Marmiroli, 2010). Finally, all these platinum compounds appear to affect the axons, myelin sheath, neuronal cell body, and the glial structures of the neurons. In support, levels of platinum are significantly higher in the DG than in the brain and spinal cord, which are protected by the blood-brain barrier (Hausheer et al., 2006).

Oxaliplatin and cisplatin differ in their severity of neurotoxicity to the DG, with cisplatin being more neurotoxic. In fact, cisplatin produces about three times more platinum- DNA adducts in the DG than do equimolar doses of oxaliplatin (McWhinney et al., 2009). Acute oxaliplatin neurotoxicity is thought to be caused by transient dysfunction of nodal, axonal, voltage-gated sodium channels, probably owing to an oxalate chelating effect on both calcium and magnesium ions that could interfere with the kinetics of sodium channels (McDonald et al., 2005; Ta et al., 2006). However, the small-conductance Ca(++)-activated K(+) channels, encoded by the SK1-3 genes, are also involved in membrane excitability, playing a role in after-hyperpolarisation at the motor-nerve terminal. Because the SK3 gene is characterised in Caucasians by a highly polymorphic CAG motif within exon 1, SK3 gene polymorphism may influence the development of acute nerve hyperexcitability in oxaliplatin-treated patients. The results of the study by Basso et al. (2011) have suggested that oxaliplatin neurotoxicity may be related to distribution of the polymorphic CAG motif of the SK3 gene, which might modulate nerve after-hyperpolarisation. The allele with 13-14 CAG repeats could mark patients susceptible to acute oxaliplatin neurotoxicity.

Neuropathy Secondary to Chemotherapy: A Real Issue for Cancer Survivors 221

The more recently developed oxaliplatin has provided a more dramatic reduction of nephro- and ototoxicity than its counterparts (McWhinney et al., 2009). It is structurally different from both cisplatin and carboplatin. The neurotoxicity of oxaliplatin, though, has been described as the most common and dose-limiting toxicity this agent can associate.

One type is an acute, transient, dose-related, and painful sensory peripheral neuropathy that is exacerbated by exposure to cold and is very frequent (Becouarn et al., 1998; Gilles-Amar et al., 1999; Grothey, 2003; Grothey & Schomoll, 2001). In fact, at least 90% of patients receiving oxaliplatin will develop symptoms during or soon after its administration, even from the first dose, but the symptoms typically solves within hours to days (Cassidy & Misset, 2002; Gamelin et al., 2002; Giacchetti et al., 2000; Maindrault-Goebel et al., 2001). This acute form consists of paraesthesia, dysaesthesia, and hypoaesthesia in hands, feet, perioral area, or throat and in many cases is precipitated by cold air, cold objects, or cold drinks. Pharyngolaryngeal dysaesthesia is reported by a small number of patients but is very disturbing. It is characterised by a subjective sensation of dyspnea and/or dysphagia accompanied by laryngospasm and stridor (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). The acute neurotoxicity may appear with notable signs of hyperexcitability similar to an acute myotonia (Hart et al., 1997; Saadati & Saif, 2009). This syndrome is an infrequent condition characterised by the inability to release gripped objects, muscle stiffness, slowed muscle relaxation, increased sweating, and less commonly, paraesthesia. The exact mechanism of myotonia is unknown. It has been postulated that either persistent sodiumchannel activity or decreased potassium conductance can be a mechanism for producing axonal hyperexcitability and repetitive discharges in human nerve cells (Hart et al., 1997; Saadati & Saif, 2009; Zielasek et al., 2000). Atypical acute neurotoxicities are sometimes described. One case combined acute motor and sensory hyperexcitability but affected only one hemibody contralateral to the arm of infusion (Uña, 2009). The patient had a history of previous brain ischemic transient attacks, which might have played a role in the atypical presentation of acute neurotoxicity. This asymmetry of symptoms may have been due to the subclinical neurological changes in neurons or axons after ischemia (Uña,

The second type of neurotoxicity, which is a more chronic, cumulative, sensory form, is persistent and has a gradual onset after multiple exposures to the drug. Although this type of neurotoxicity often decreases with drug discontinuation, it does not disappear (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). In fact, after stopping the administration of oxaliplatin, the chronic neurotoxicities improve in most patients within 4– 6 months and will completely disappear in approximately 40% of patients by 6–8 months. This chronic pattern of sensory neuropathy has been observed in about 50% of patients who received oxaliplatin with infusional 5-FU/LV in a clinical trial but generally appears in approximately 16-21% of patients (Krishnan et al., 2005). It is characterised by paraesthesias, dysaesthesias, and hypoaesthesias that can interfere with ADL (Cavaletti & Marmiroli, 2010;

Oxaliplatin can cause two different types of neurotoxicity.

**3.3 Oxaliplatin**

2009).

Hausheer et al., 2006; McWhinney et al., 2009).

The platinum drugs all share the 'coasting' phenomenon, which is an increase in the severity of symptoms for weeks, or even months, following the withdrawal of treatment (Cavaletti & Marmiroli, 2010).

Consequently, CIPN induced by platinum agents is a significant factor affecting the efficacy of the platinum drugs, as patients may experience either more negative side effects than benefits from this drug class or be forced to forego further therapy with an active agent.

#### **3.1 Cisplatin**

Cisplatin is a widely used chemotherapeutic agent with antitumour activity for a wide range of cancers. Along with its potent anticancer activity, though, cisplatin has also significant toxicities. Cisplatin-induced CIPN is the most common dose-limiting toxic effect. The reported incidence as a single agent ranges from 49-100% depending on the dose, schedule, treatment duration, and the drugs used previously. This toxicity is dose-related and generally appears after cumulative doses in excess of 300 mg/m2 (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009).

Cisplatin-induced CIPN is first characterised by painful paraesthesias and numbness in symmetrical stocking and glove distribution which typically occurs during the first few drug cycles and is thought to be produced by axonopathy. Sometimes this toxicity can be accompanied by progressively reduced or absent reflexes in the affected extremities. In several cases, loss of vibration sense, paraesthesia, and ataxia can be apparent after several cycles. Motor weakness and other central and autonomic symptoms such as Lhermitte´s sign, myelopathy, bilateral jaw pain, and urinary dysfunctions may also appear (Park et al., 2009).

In most patients, neurotoxicity occurs late along the course of treatment, but some patients report an earlier onset of symptoms after the administration of a single dose of cisplatin, especially with doses as high as 100 mg/m2 or more. The mechanism proposed for this last toxicity is neuronopathy (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009). In both cases, we must remember that all these symptoms can be progressive even after the end of treatment and in many cases are irreversible. Moreover, if cisplatin is used in combination with other drugs such as taxanes or vinca alkaloids, the appearance of neurotoxicity is more likely (Cavaletti & Marmiroli, 2010; Hausheer et al., 2006; McWhinney et al., 2009).

#### **3.2 Carboplatin**

Neurotoxicity secondary to carboplatin is less frequent (4–6%) (American Society of Health-System Pharmacists, n.d.) than that observed with cisplatin or the thirdgeneration platinum, oxaliplatin (15–60%), and is typically less severe (U.S. Food and Drug Administration, n.d.). Risk of CIPN induced by carboplatin increases in patients older than 65 years and in patients previously treated with cisplatin. Additionally, carboplatin does not cause the loss of hearing that is seen in the majority of patients treated with cisplatin (Chaney et al., 2005; Kartalou & Essigmann, 2001; Wang & Lippard, 2005). Carboplatin-induced CIPN is clinically indistinguishable from that induced by cisplatin, and although reported to be less severe, several studies suggest that the severity may be similar for both drugs.

#### **3.3 Oxaliplatin**

220 Basic Principles of Peripheral Nerve Disorders

The platinum drugs all share the 'coasting' phenomenon, which is an increase in the severity of symptoms for weeks, or even months, following the withdrawal of treatment (Cavaletti &

Consequently, CIPN induced by platinum agents is a significant factor affecting the efficacy of the platinum drugs, as patients may experience either more negative side effects than benefits from this drug class or be forced to forego further therapy with an active agent.

Cisplatin is a widely used chemotherapeutic agent with antitumour activity for a wide range of cancers. Along with its potent anticancer activity, though, cisplatin has also significant toxicities. Cisplatin-induced CIPN is the most common dose-limiting toxic effect. The reported incidence as a single agent ranges from 49-100% depending on the dose, schedule, treatment duration, and the drugs used previously. This toxicity is dose-related and generally appears after cumulative doses in excess of 300 mg/m2 (Cavaletti &