alzheimer 's disease ( ad ) is a pernicious neurodegenerative disease which is incurable with remedies developed up to date .
the number of patients increases every year worldwide and 5.2 million people in the united states are suffering from ad .
this number is expected to expand to 36 million to 115 million in worldwide around the year of 2050 and the estimated economic cost and suffering is increasing greatly ( 1 , 2 ) .
therefore , ad is seemingly insurmountable disease and the increasing numbers of patients produce diverse societal concerns in different aspects .
senile plaques of amyloid  ( a ) in the brain parenchyma have been regarded as not only the main pathological phenomena ( 3 , 4 ) but also the culprit of this disease according to amyloid cascade hypothesis based on molecular information found in ad study ( 5 ) .
abnormal production and accumulation of a in brain parenchyma result in ad pathologies through sequential events by aggregated forms of this protein and the amyloid plaque .
a is generated as a consequence of sequential cleavages of amyloid precursor protein ( app ) by - and -secretases ( 6 , 7 , 8) .
app is first cleaved by either - or -secretase , and then , the remaining remnants of c83 or c99 , respectively , are vulnerable to intramembrane proteolysis by -secretase .
amyloidogenic process of -secretase cleavage followed by -secretase produces aggregation - prone a which are in the center of ad etiology ( 4 , 9 ) .
disrupted synaptic plasticity , reduced dendritic spine density and memory impairment were proven in rodent model by extraction of a oligomers from human patients ( 10 ) .
along with a , microtubule - associated protein tau is another major factor of ad pathogenesis as a component of neurofibrillary tangles ( nfts ) ( 11 ) .
tau stabilizes microtubule protein and microtubule - associated processes in normal condition . during ad pathogenesis , tau becomes hyperphosphorylated , aggregated and finally accumulated as neurofibrillary tangles ( 12 ) .
tau hyperphosphorylation and nft formation is tightly related to the existence of excessive a and plaques , proving the tau pathology in ad ( 13 , 14 ) .
not only as axonal protein but also as regulator of dendritic function , tau plays a pivotal role , especially mediating early a toxicity during ad progress ( 15 ) .
many clinical trials aiming these two proteins have been performed whereas several lines of targets are still under investigations . up to date
, only a few ad medications have been proved as improving ad symptoms , but none of them modify disease progress or pathological cascades ( 16 ) .
researchers and clinicians suspect that the reason for many drug target candidates to fail in their clinical trials reside in the improper time of drug treatment , in fairly late stage of ad progression where irreversible damages have already occurred , including excessive a deposits , neuronal impairment , death and blood brain barrier ( bbb ) disruption ( 17 ) .
therefore , finding a diagnostic biomarker , especially for the early stages of ad pathology , is desperately needed for developing a valuable therapeutic target at the early stage and preventing progression of the disease .
diverse approaches for ad therapeutic strategy have arisen along with better understanding of cellular and molecular mechanism of ad pathogenesis . in fig .
1 and table 1 , we listed possible strategies of drug development target based on accumulated scientific findings . in this review , we summarize the recent status of some ad drug targets using different strategies among them from published reports and ongoing clinical studies . in each category
in spite of better understanding for molecular mechanism during ad pathogenesis , the available medications for ad up to date provide only symptomatic benefit , not regulate or delay the progression of disease pathology .
the us food and drug administration ( fda ) has approved only five medications for ad , including acetylcholinesterase inhibitors ( acheis : donepezil , rivastigmine , galantamine and tacrine ) and n - methyl - d - aspartate ( nmda ) receptor antagonist , such as memantine ( 18 , 19 , 20 , 21 ) .
. combined administration of these two medications accelerates symptomatic improvement , representing slow progress of cognitive and functional impairment and delayed time for nursing home admission ( 22 , 23 ) . besides
, several lines of regulators for neurotransmission were suggested for symptomatic therapies in ad , including neuronal nicotinic acetylcholine receptor activation ( abt-418 ) , gabab receptor antagonism ( sgs-742 ) and serotonergic modulation ( lu ae58054 ) .
however , they were insufficient to show significant efficacies of symptomatic improvement in clinical trials ( 24 , 25 , 26 , 27 ) .
recently , high affinity 5-hydroxytryptamine ( ht ) 6 receptor antagonist lu ae58054 ( [ 2-(6-fluoro-1h - indol-3-yl)-ethyl]-[3-(2,2,3,3,-tetrafluoropropoxy)-benzyl]-amine ) was reported to improve novel object recognition task in a rat model with cognitive impairment induced by phencyclidine ( 28 ) .
the combined treatment of lu ae58054 and donepezil is under phase iii clinical trial ( clinicaltrials.gov identifier : nct01955161 ) .
additionally , even drugs with uncertain mechanism were reported to be effective in symptomatic improvement and protection against neurotoxicity by a , including ethanolic extract of angelica gigas ( inb-176 ) and ginkgo biloba ( egb761 ) respectively , however , none of which showed successful effectiveness in their preclinical and clinical trials ( 29 , 30 , 31 ) .
abnormal a production and accumulation in brain parenchyma have been regarded as the central etiological hypothesis in ad pathogenesis ( 5 , 10 , 32 ) . therefore , the first line of strategy was inhibition of a generation processes to prevent or cure the disease .
the tight relevance of - , - and -secretases to a production made researchers to discover modulating drugs for these enzyme activities in order to reduce intracellular and extracellular a level . whereas effective -secretase activator was rarely identified , several types of -secretase inhibitors were discovered and tested , starting with first - generation potent inhibitor om99 - 2 , om00 - 3 ( 33 , 34 ) . since then , numerous reports and patents of -secretase inhibition were published , however , finding drug candidate with desirable potencies and efficacy has been fairly challenging ( 35 ) .
recently discovered mk-8931 ( merck ) is a promising -secretase inhibitor whose result of phase i clinical trial was released in april , 2012 .
mk-8931 is now under phase ii / iii trial which was initiated in 2012 ( clinicaltrials .
gamma - secretase plays the critical role in a generation , in charge of the rate - limiting cleavage of app into a. however , modulating this enzyme activity may cause diverse side effects because of its multiple cleavage actions on diverse substrates which are physiologically important , including notch receptor signaling . for this reason , modulating -secretase activity seems to be greatly complicated , requiring restricted substrate specificity for app to reduce a only , not affecting other substrate processing such as notch signaling ( 36 , 37 ) .
consequently , substrate specificity is the critical issue in the development of ad therapy using -secretase inhibition .
semagacestat ( ly450139 , eli lilly ) was a promising drug candidate targeting -secretase inhibition ( 38 ) , tested in two phase iii clinical trials . even though both trials finished with a disappointing result of insufficient efficacy it showed a breakthrough for possible utilization of -secretase modulation in ad therapeutic development .
mostly , a elicits its toxicity by aggregated forms ( 10 , 39 , 40 ) .
therefore , the inhibition of a aggregation is one of the most effective strategies in order to inhibit a toxicity .
curcumin and -sheet breaker such as rs-0406 were discovered to inhibit polymerization of a into oliogmer and fibril forms ( 41 , 42 ) .
compound d737 showed the most effective inhibition of a aggregation among a collection of 65,000 small molecule candidates and elicited increased lifespan in a drosophila melanogaster model of ad as well as reduction of a toxicity in cell culture system ( 43 ) .
indirect inhibition of a aggregation was suggested by metal hypothesis of ad ( 44 ) .
cupper / zinc ionophore , pbt2 , which target the copper and zinc ions that mediate a aggregation was proven to facilitate the aggregated a clearance in the cortex , to lower a level of cerebrospinal fluid ( csf ) and to restore the cognitive impairment in ad patients ( 44 , 45 , 46 ) .
pbt2 completed phase ii clinical trial ( clinicaltrials.gov identifier : nct00471211 ) and are now under phase ii clinical trial for huntington disease as well .
additional large - scale clinical tests and high throughput screening for candidates of a aggregation inhibitor are strongly encouraged in further investigation .
especially , pyroglutamyl modification in n - terminus of a is critical alteration because pyroglutamated a ( pglu - a ) species readily accumulated into senile plaque and vasculature deposit due to increased stability and aggregation velocity ( 47 , 48 , 49 ) .
glutaminyl cyclase ( qc ) was demonstrated as the main catalytic enzyme responsible for this pyroglutamyl modification of a and intracortical microinjection of qc inhibitor , pbd150 , significantly decreased pglu - a formation ( 50 , 51 ) .
since inflammation response and activation of phagocytic cells such as microglia and astrocytes had been appreciated as a pivotal contributor to ad pathogenesis , immune system became one of the most prominent targets in the aspect of ad therapeutic invention ( 52 ) .
cytokines and other neurotoxic adducts secreted by immune - related cells were suspected as possible mediators of neuronal degeneration and cell death ( 53 , 54 ) .
furthermore , data analysis using genome wide association study ( gwas ) supported this idea by proving that specific over - representation of genes related to immune pathway linked to ad risk ( 55 ) .
the protection effect of non - steroidal anti - inflammatory drugs ( nsaids ) , especially ibuprofen , against ad proved that the suppression of immune response should be beneficial in ad ( 56 ) .
many factors seemed to be tightly related to the protective effect of nsaids against ad , including age of cohort , apolipoprotein e ( apoe ) genotype , the duration of nsaids usage and nasaids types , showing significant effect in apoe 4 allele carrier ( 56 , 57 , 58 , 59 ) .
unfortunately , diverse clinical trials with different types of nsaids concluded not only beneficial effects but also insufficient efficacies and negative results ( 57 , 60 , 61 , 62 , 63 , 64 , 65 ) . narrowing down the target along with amyloid hypothesis , immunotherapy against a peptide attracted a great deal of attention because it is direct resolution of the seemingly main cause of pathogenesis and progression of disease in ad .
several strategies for a peptide immunotherapy has been tested , including passive immunization with monoclonal antibody against different regions of a42 as well as active immunization using synthetic a42 ( 66 ) . because a peptide , the major component of senile plaques in ad brain , is regarded as the critical contributor in ad pathogenesis , enhanced clearance of a via the administration of anti - a monoclonal antibody , including bapineuzumab and solanezumab ( passive vaccination ) , or a antigen with adjuvant such as an1792 ( active vaccination ) seemed fairly promising ( 67 , 68 ) .
preclinical trials for both active and passive immunotherapies against a represented diverse beneficial effects of ameliorated brain a burden , prevention of memory loss and improved cognitive function in different animal models of ad ( 67 , 69 ) .
preliminary test on human patients exerted promising outcomes of reduced plaque burden and cognitive benefit ( 70 , 71 ) , suggesting multiple mechanisms of actions including modulation of a equilibrium balance between the central nervous system and plasma ( 72 ) or improved peripheral clearance and sequestration of brain a ( 73 ) .
however , unavoidable side effects found in clinical trials hindered the further clinical development into ad therapeutic treatment on human , including meningoencephalitis , microhemorrhages and vasogenic edema ( 68 , 74 ) . also , clinical evaluation in human ad patients failed to replicate the identical results as in the animal ad model , showing unsynchronized phenomena between reduced a plaque and rescue of neurodegeneration during ad progression ( 75 ) .
immunotherapy is regarded as one of the most promising therapeutic strategies in ad and some immunotherapeutic drug candidates are still under clinical trials , including the first monoclonal antibody for a protofibril ( ban2401 , clinicaltrials.gov identifier : nct01767311 ) and immunoglobulins combined with albumin by means of diverse application methods ( clinicaltrials .
intracellular neurofibrillary tangle ( nft ) is another hallmark in ad pathogenesis , cytoskeletal inclusions consisted of hyperphosphorylated microtubule associated protein tau with paired helical filament structure ( 14 ) .
especially , modulating endogenous tau level in app - overexpressing mice halted a-induced behavioral deficit in spite of maintaining high a level , suggesting the relevance of tau during ad pathogenesis and implying the possibility for tau - targeting immunotherapy in ad ( 76 , 77 ) . moreover , tight relationship between a and tau pathologies during ad strengthened the rationale for tau - aiming therapeutic strategy in ad .
it was proven by that a immunotherapy reduced not only the extracellular a plaques but also intracellular a accumulation which resulted in the absence of early tau pathology ( 78 ) .
different mechanistic approaches of tau - targeting therapies were tried , including reducing tau level itself , preventing tau hyperphosphorylation and inhibiting the aggregation ( 26 ) .
many researches have focused on preventing hyperphosphorylation of tau , earlier event that cause detachment of tau protein from microtubule .
kinases responsible for tau phosphorylation ( glycogen synthase kinase , gsk-3 , cyclin - dependent kinase-5 , cdk5 and microtubule affinity - regulating kinase ) and phosphatase ( protein phosphatase 2a , pp2a ) are the possible targets to achieve tau - aiming therapeutics , altering tau phosphorylation by modulating activity of the enzymes ( 79 , 80 , 81 ) .
especially , gsk-3 inhibition is implicated in both a and tau pathway concomitantly and is greatly appreciated in ad therapeutic development .
gsk-3 inhibition by lithium , valproate , caffeine were also tested in preclinical and clinical studies and their efficacies were needed to be confirmed in further study because of inconsistent outcomes in different studies ( 82 , 83 , 84 , 85 ) .
azd 1080 ( astrazeneca ) and np-12/tideglusib ( noscria ) were the most promising gsk-3 inhibitor , however , azd 1080 was withdrawn from ad therapeutic development due to the nephrotoxic side effect in phase i clinical trials ( 68 , 86 , 87 ) . since then , np-12/tideglusib has been recognized as an effective gsk-3 inhibitor and completed not only pilot clinical study using small sample with the result of positive trends in mini - mental state examination ( mmse ) , alzheimer 's disease assessment scale - cognitive subscale ( adas - cog ) , global deterioration scale ( gds ) and global cortical atrophy ( gca ) ( 88 ) but also phase ii clinical trial ( clinicaltrials.gov identifier : nct01350362 ) .
in addition to tau phosphorylation , several agents were also suggested to prevent tau aggregation .
methylthioninium chloride ( methylene blue , mtc ) was the first tau aggregation inhibitor discovered and reduced version of mtc , trx0237 , is now in the process of phase iii clinical trial ( 89 ) ( clinicaltrials.gov identifier : nct01689246 ) .
also , diverse possible candidates were suggested as tau aggregation inhibitor , including anthraquinones , aminothienopyridazines , polyphenols and phenothiazines ( 68 , 90 , 91 , 92 ) .
these compounds , however , need more verification because they failed to show consistent efficacies in in vivo studies .
since type 2 diabetes mellitus ( dm2 ) was found to be related to ad , glucose metabolism has emerged as a new interest in ad research .
it was widely known that glucose metabolism and insulin signaling are impaired in ad brain ( 93 , 94 , 95 ) .
insulin - degrading enzyme ( ide ) was revealed to be responsible for a degradation as well , more efficiently intracellular a than extracellular form ( 96 , 97 , 98 ) .
even though ad and dm2 share ide as the key metabolic enzyme for their main etiological proteins , a for ad and insulin for dm2 , it is not enough to explain all the ad - mimic pathological phenomena found in diverse mechanism - driven diabetes mouse model with insulin resistance ( 98 , 99 , 100 ) .
insulin has effect on cerebral function per se and is also tightly involved in inflammation and oxidative stress , representing enhanced inflammatory response and markers of oxidative stress by hyperinsulinemia ( 101 ) . in other studies ,
increased autophagosome was suggested to accelerate the amyloidogenic app processing in insulin - resistant condition ( 102 ) .
thereby , insulin itself represented tight relevance to ad and became a new therapeutic target in ad .
metformin , a peripheral insulin sensitizer drug approved by the fda , was reported to sensitize brain insulin action and prevent ad - associated pathological alteration in in vitro ad model ( 103 ) .
also , other diverse insulin sensitizers are needed to be tested because they may have possibility to show valuable efficacy in ad as well .
msdc-0160 ( mtot modulator , metabolic solution development company , msdc ) was recently performed successful phase iia clinical trial for type 2 diabetes ( 104 ) .
msdc-0160 was demonstrated to elicit its insulin - sensitizing effect through newly discovered mitochondrial target of thiazolinedione ( mtot ) located in the mitochondrial inner membrane ( 104 , 105 ) .
mitochondria are one of the most devastated organelle in the process of ad development . during ad pathogenesis
, mitochondrial impairment occurs in various brain areas , representing not only morphological alteration but also physiological dysfunction ( 106 ) .
a was detected inside mitochondria compartment of ad mouse model expressing human mutant amyloid precursor proteins ( app ) , mostly within membrane and membrane - associated region ( 107 , 108 ) .
study using mitochondria - specific targeting a proved that mitochondrial accumulation of a induced not only morphological alteration but also physiological dysfunction which was fatal enough to induce neuronal apoptosis ( 109 , 110 ) .
other findings also showed morphological alteration of mitochondria and physiological dysfunction , especially electron transport chain through cytochrome c oxidase and increased oxidative stress in in vitro and in vivo ad - mimic system and patients ( 110 , 111 , 112 ) .
therefore , mitochondrial dysfunction by a is a critical contributor during ad pathogenesis , and in the same line of thought , mitochondrial protection becomes a new strategy in ad treatment . also , mitochondrial dysfunction and oxidative stress is more evidently the common linker between ad and abnormal glucose metabolism ( 113 , 114 ) .
therefore , mitochondrial recovery drug was proposed as a new concept for ad therapy and treatment of insulin - resistance .
dimebon ( pfizer ) , originally allergy - treating drug used in russia , was known to improve atp generation and energy metabolism in mitochondria and was tested its effectiveness in clinical trial for ad therapeutics ( 115 ) .
unexpectedly , phase iii trial performed with 598 patients ended up as failure with the lack of improvement .
this failure leads researcher to suspect the novel mitochondrial mechanism of action of dimebon , rather than promiscuous clinical effects , including inhibition of histamine h1 and serotonin receptors . in extended thoughts ,
other medicines with the effect of mitochondrial rejuvenation have been investigated as probable candidates for ad therapeutics .
piracetam is a nootropic drug and has effect on cognitive impairment during aging and dementia ( 116 ) .
the mechanism of action for piracetam was controversial , including effects on glutamate receptors , gaba - mimetic action and activation of calcium influx into neuronal cells ( 116 ) .
recently , mitochondrial relevance of this drug has been found to enhance the membrane fluidity in brain mitochondria and consequently improve membrane potential , atp generation and decrease apoptotic vulnerability in aging and ad model ( 117 ) .
these findings suggested the possibility that this drug exerted therapeutic effect through mitochondrial recovery in ad .
in addition to mitochondrial function itself , axonal transport of mitochondria through microtubule protein was observed to be impaired in ad ( 118 ) .
the acetylation status of -tubulin by histone deacetylation ( hdac6 ) is highly related to cargo transport along microtubules ( 119 ) . restored -tubulin acetylation by hdac6 inhibitor improves both anterograde and retrograde motility of mitochondria and , furthermore , rescued mitochondrial morphology in hippocampal neurons under ad - mimic condition of a-induced impairment of mitochondria function ( 120 ) .
these diverse mitochondria - targeting mechanisms of action are all likely to be probable therapeutic mechanism for ad treatment and deserve to be evaluated . besides the relevance to mitochondrial dysfunction
, disturbed glucose metabolism could be directly connected to a generation and accumulation by modulating the enzyme activity of a-generating enzyme through post - translational modification .
addition of o - linked -n - acetylglucosamine ( o - glcnac ) to protein is a glucose level - dependent post - translational modification and the specific inhibition of o - glcnacase ,
1,2-dideoxy-2-propyl--d - glucopyranoso-[2,1-d]-2-thiazoline ( nbutgt ) , ameliorated a generation by modulating nicastrin activity , a component of -secretase , through s708 site o - glcnacylation ( 121 ) .
different from other o - glcnacylation modulating drugs , such as o-(2-acetamido-2-deoxy - d - glucopyranosylidene ) amino n - phenyl carbamate ( pubnac ) and streptozotocin ( stz ) , nbutgt was specific to o - glcnacase and showed the lack of cellular toxicity nor insulin resistance ( 122 ) . additionally , o - glcnacase inhibitor was reported to reduce the tau phosphorylation and improve long - term potentiation ( ltp ) as well , which are possibly beneficial in ad ( 123 , 124 ) .
o - glcnacase inhibitor seems to be effective not only in a generation but also in memory impairment and taupathies , which is needed to be further verified in future clinical studies .
in addition to o - glcnacylation , diverse post - translational modification could be another valuable target for ad therapeutics , especially specific modification for ad - related proteins , including -secretase and -secretase .
recent findings suggest that a accumulation is fairly slow and time - consuming process , likely to require more than two decades ( 125 ) . during this long process , more than one physiological system seems to be linked each other to harmonize in order to induce pernicious ad pathology . taken information together ,
it is unlikely that a single remedy could cure ad because of its complexity and intricate relationship among the multitude of pathological components during pathogenesis .
for this reason , therapeutic targets for ad should include multiple strategies and combinational remedy , not single , for the maximum effectiveness and better consequences . also , not only direct therapeutic treatment for pathological intervention but also delaying this long pathogenic process would contribute to reduce the number of ad patients and increase prognostic benefit ( 26 ) .
up to date , researchers are desperate to find new ways for ad treatment and tune the drug candidates for the maximum efficacies .
for instance , in developing a synthesis modulator as ad therapeutic target , researchers has to consider diverse questions and concerns , including the right margins of decreased a production level , maintaining proper physiological level and off - target effects by influencing other substrates besides a. because it is hard to tune the enzyme activity within the right physiological catalytic range , the successful development of ad therapeutics with enzyme modulator is dependent upon its efficacy on aiming action , specificity and selectivity to target substrate .
also , as everyone agrees that the right timing of drug treatment is essential for the evaluation of efficacies for therapeutic targets , improvement in early diagnostic tool for ad have to be pursued along with ad therapeutic development .
 alzheimer 's disease ( ad ) is the most common form of dementia caused by neurodegenerative process and is tightly related to amyloid  ( a ) and neurofibrillary tangles . 
 the lack of early diagnostic biomarker and therapeutic remedy hinders the prevention of increasing population of ad patients every year . in spite of accumulated scientific information , 
 numerous clinical trials for candidate drug targets have failed to be preceded into therapeutic development , therefore , ad - related sufferers including patients and caregivers , are desperate to seek the solution . 
 also , effective ad intervention is desperately needed to reduce ad - related societal threats to public health . in this review , 
 we summarize various drug targets and strategies in recent preclinical studies and clinical trials for ad therapy : allopathic treatment , immunotherapy , a production / aggregation modulator , tau - targeting therapy and metabolic targeting . 
 some has already failed in their clinical trials and the others are still in various stages of investigations , both of which give us valuable information for future research in ad therapeutic development . 
INTRODUCTION
ALLOPATHIC TREATMENT FOR AD
A PRODUCTION/AGGREGATION MODULATOR
IMMUNOTHERAPY
TAU-TARGETING THERAPY
METABOLIC TARGETING
CONCLUSION