the history of the initial discovery and characterization of proprotein convertase subtilisin / kexin type 9 ( pcsk9 ) has been well described . the seminal discovery in 2003 by abifadel et al . linked mutations in the gene encoding pcsk9 with autosomal dominant hypercholesterolemia ( adh ) .
this finding uncovered a key new player in cholesterol homeostasis and set into motion intensive research in the field of pcsk9 biology .
the basic construct places this extracellular secretory protein as a central regulator of plasma low - density lipoprotein ( ldl)-cholesterol ( ldl - c ) concentration .
pcsk9 , by direct interaction with the hepatic ldl receptor ( ldlr ) , enhances its degradation by targeting it for destruction in the lysosome , disallowing its natural recycling loop .
the fundamental observation that pcsk9 is intrinsically linked to ldlr recycling and familial hypercholesterolemia ( fh ) provided the basis for pursuing it as a therapeutic target .
pcsk9 causes degradation of the ldlr , thus inhibiting pcsk9 prolongs the lifespan of the ldlr , which leads to drastic reductions in plasma ldl - c levels .
major findings over the last decade have revealed the following : 
 gain - of - function mutations in pcsk9 are a cause of adh.loss-of-function mutations in pcsk9 are associated with low ldl - c levels and markedly reduced cardiovascular risk.therapeutic antagonism of pcsk9 reduces ldl - c levels and , within the context of clinical trials , appears safe , efficacious , and well tolerated .
loss - of - function mutations in pcsk9 are associated with low ldl - c levels and markedly reduced cardiovascular risk .
therapeutic antagonism of pcsk9 reduces ldl - c levels and , within the context of clinical trials , appears safe , efficacious , and well tolerated . beyond the obvious association of pcsk9-induced ldlr degradation and hypercholesterolemia ,
emerging studies have uncovered other mechanisms by which pcsk9 may facilitate atherosclerosis development independent of its impact on lipids .
what follows is a survey of the literature that provides insight into the impact of pcsk9 on atherosclerosis via lipid and non - lipid pathways .
the classic teachings on lipoprotein metabolism place regulation of cholesterol squarely within the cell , with all the components available to sense and react to cholesterol depletion and excess .
the discovery and elucidation of pcsk9 has revolutionized our understanding of lipid metabolism and has introduced this secreted circulatory protein as a central actor in this process .
comprehensive reviews have described mechanisms by which pcsk9 impacts the ldlr and ultimately plasma ldl - c .
complete knockout of pcsk9 in mice results in severe hypocholesterolemia with reductions in ldl cholesterol of up to 80% and reductions in total cholesterol of up to 40% . despite the profound hypocholesterolemia ,
interestingly , mice that are engineered with liver specific pcsk9 knock down demonstrate a complete absence of plasma pcsk9 corroborating the notion that the liver , despite being only one of several tissues that synthesizes pcsk9 , is the primary source of its plasma levels .
this observation suggests that pcsk9 produced from extrahepatic tissues may have effects that are either exclusively intracellular , or extracellular but only paracrine , or both .
however , even in the complete absence of plasma pcsk9 , the liver specific pcsk9 knock down model only demonstrates an approximately 30% reduction in total plasma cholesterol levels , a significantly attenuated phenotype compared to complete knockout of pcsk9 .
this finding suggests that pcsk9 secreted from the liver exerts a dominant , but not complete , effect on plasma cholesterol levels , and provides impetus to study the role of extrahepatic pcsk9 in wholebody cholesterol homeostasis .
murine models incorporating a human pcsk9 transgene under an apoe promoter overexpress pcsk9 and are found to be healthy , with normal reproductive capacity , and display the anticipated severe hypercholesterolemia phenotype . as compared to ldlr knockout ( ko ) mice , which have a 15-fold increase in plasma ldl - c ,
this pcsk9 overexpression model exhibits a more modest ( 5-fold ) increase in plasma ldl - c despite the fact that , as in the ldlr ko model , there is no demonstrable ldlr in the liver .
this observation again highlights the importance of extrahepatic tissues in cholesterol metabolism and suggests the following : 
 extrahepatic ldlr plays a central role in ldl - c catabolismextrahepatic ldlr is not influenced by plasma pcsk9 extrahepatic ldlr plays a central role in ldl - c catabolism extrahepatic ldlr is not influenced by plasma pcsk9 as new data emerge , it has become evident that the impact of pcsk9 on lipids and atherosclerosis is transduced by mechanisms beyond its effect on plasma ldl clearance .
pcsk9 also targets receptors beyond the ldlr , including the vldlr , cd36 , apoer2 , and lrp1 .
the extent to which its action on these ldlr family members is significant for cholesterol homeostasis remains unclear , though there are several relevant observations relating to triglyceride - rich lipoprotein ( trl ) metabolism . given the importance of trl in the initiation and propagation of atherosclerosis , the impact of pcsk9 on these highly atherogenic particles may provide another mechanism by which therapeutic antagonism of pcsk9 may reduce the risk of atherosclerotic cardiovascular disease ( ascvd ) .
plasma pcsk9 concentration correlates to fasting triglyceride levels in males and females , though not in the obese .
individuals with the pcsk9 s127r gain - of - function mutation demonstrate increased levels of all apob100-containing lipoproteins , including vldl and remnants .
furthermore , therapeutic antibodies to pcsk9 reduce plasma triglycerides , though this effect appears to be more modest relative to the ldl - c reduction and as compared to the statin effect on tg vs. ldl - c .
since apob turnover is heavily modulated by ldlr - mediated reuptake of many apob - containing lipoproteins , therapeutic pcsk9 inhibition also impacts plasma trl concentrations .
a large proportion of newly synthesized apob is degraded within the secretory pathway by diversion to the autophagic compartment .
thus , the rate of apob secretion , and hence , vldl secretion , from the liver is determined by the proportion of apob that escapes post - translational degradation . in addition , reuptake of newly secreted lipoproteins also regulates the net output of apob .
demonstrated that the ldl receptor influences the posttranslational fate of apob by increasing presecretory apob degradation and mediating reuptake of nascent lipoprotein particles . in this context ,
pcsk9-mediated degradation of hepatic ldlr can cause hypertriglyceridemia by liberating apob from ldlr - induced intracellular catabolism as well as reducing uptake of nascent apob - containing lipoproteins ( eg , vldl ) .
indeed , several experimental models have demonstrated that pcsk9 directly increases hepatic production of apob - containing lipoproteins .
acute adenoviral - mediated overexpression of pcsk9 in fasting mice leads to profoundly increased hepatic production of apob and vldl - triglycerides .
similarly , chronic hepatic expression of pcsk9 leads to overproduction of vldl in transgenic mice .
murine expression of human pcsk9 led to increases in hepatic lipogenesis in both an apoe and ldlr - dependent manner .
shunting of trl from plasma to visceral adipose tissue in the absence of pcsk9 may be due to its effects on vldlr and cd36 , which are upregulated in the perigonadal fat depots of female pcsk9-knockout mice .
absence of pcsk9 leads to preservation of these two surface proteins which in turn facilitates fatty acid uptake into adipose tissue and causes adipocyte hypertrophy . despite these empiric observations ,
interestingly , a recent study in mice lacking ldlr demonstrated that pcsk9 did not affect the expression of genes and proteins involved in hepatic lipogenesis ( both srebp- and non - srebp - mediated ) , findings that differ from what was seen in the intestine .
the intestine is only second to the liver in production of pcsk9 and similarly has an impact on intestinally derived plasma trl , which are produced during meal absorption and contain apob48 as main structural protein .
mechanistic studies demonstrated that pcsk9 increases the production and secretion of intestinal trl by both transcriptional and post - transcriptional mechanisms involving both the sterol - regulatory element - binding proteins ( srebp ) and non - srebp pathways , with subsequent lipid accumulation in enterocytes . in murine knockout models ,
the absence of pcsk9 was associated with a reduction in apob48 secretion and attenuation of postprandial hypertriglyceridemia .
given the significant atherogenic impact of chylomicron remnants , this observation may represent additional therapeutic potential of pcsk9 inhibitors to reduce the risk of ascvd . beyond intestinal trl metabolism
, pcsk9 reduces cholesterol absorption by decreasing expression of npc1l1 , the main entry point of cholesterol into the enterocytes from the intestinal lumen .
furthermore , recent results suggest that pcsk9 increases the intestinal , not the hepatic , contribution to plasma cholesterol and triglyceride levels in the absence of ldlr .
lipoprotein(a ) [ lp(a ) ] is a low density lipoprotein ( ldl ) particle consisting of a molecule of apolipoprotein(a ) ( a variant protein with a highly heterogeneous number of plasminogen kringle 4 repeats ) covalently bound to apolipoprotein b. epidemiological and genetic data have consistently shown lp(a ) to be an independent risk factor for ascvd .
therapeutic monoclonal antibodies to pcsk9 have been found to reduce lp(a ) by about 2530% , irrespective of baseline levels by a yet unidentified mechanism , .
this observation is compelling , as lp(a ) is a highly atherogenic particle with no specific therapy to lower its plasma levels and thus has generated renewed interest in elucidating the molecular and cellular link between pcsk9 , ldlr , and lp(a ) metabolism .
while many have postulated that the observed lp(a ) lowering is due to extreme upregulation and clearance via the ldlr , this is difficult to reconcile in light of seminal work demonstrating that the ldlr is not a physiologically important route of lp(a ) catabolism in humans .
furthermore , a recent study demonstrated that the effect of pcsk9 on lp(a ) is related to reduced secretion rates , rather than its cellular uptake .
we previously demonstrated that plasma pcsk9 preferentially associates with lp(a ) particles in humans with elevated lp(a ) . in this context ,
injection of anti - pcsk9 mabs may lead to the formation of immune complexes on the surface of lp(a ) particles that may be cleared by mechanisms unrelated to normal lp(a ) clearance routes .
pcsk9 induced perturbations of lipid metabolism and ultimately plasma lipid levels are the most obvious mechanism that links pcsk9 to atherosclerosis .
a recent line of investigation has sought , however , to determine if pcsk9 itself has a primary role in atherosclerotic plaque development , independent of lipid changes .
results of such studies , summarized below , have made it clear that the proatherosclerotic action of pcsk9 is not entirely due to its impact on plasma concentrations of apob containing lipoproteins , at least in the mouse model .
several years ago , investigators demonstrated that pcsk9 is expressed in endothelial cells , vascular smooth muscles cells ( vsmc ) , and in other areas of the human atherosclerotic plaque .
furthermore , vsmc expression of pcsk9 leads to cleavage of ldlr at the surface of arterial macrophages .
additional study revealed that vsmc produce considerably more pcsk9 than do endothelial cells , and that expression is dependent on shear stress , with maximal secretion at the lowest levels of shear stress corresponding to the known correlation between shear and arterial branch points ( eg , bifurcations are vulnerable areas ) . in a pcsk9 murine model of vascular injury , placement of a non - occlusive collar on the carotid artery
was associated with reduced neointimal formation and atherosclerotic lesions with reduced vsmc and collagen , though without a difference in macrophage content .
an interesting facet of the association between inflammation , pcsk9 , and atherosclerosis relates to lox-1 , an important receptor for oxidized - ldl ( ox - ldl ) .
lox-1 expression is upregulated in the inflammatory state . in ldlr mice fed a high cholesterol diet , atherosclerosis and arterial wall inflammation are reduced with deletion of lox-1 .
interestingly , the crosstalk between pcsk9 expression and lox-1 in vsmc leads to a feed - forward loop where pcsk9 stimulates transcription of lox-1 and lox-1 activation stimulates pcsk9 expression .
whether pcsk9 inhibitors can inhibit atherogenesis by antagonizing lox-1 expression remains to be seen . in murine models of overexpression of either normal or gain - of - function pcsk9 ,
increased atherosclerotic plaque size was observed and explained by the increase in plasma levels of atherogenic lipoproteins , .
as might be anticipated , the extent of expression of pcsk9 in ldlr mice has little to no effect on plasma lipids and atherosclerosis .
interestingly , while the absence of pcsk9 in apoe mice has a negligible impact on plasma lipids and atherosclerosis , the overexpression of pcsk9 has negligible effects on plasma lipids but is associated with increased atherosclerotic lesion size . in a series of experiments ,
tavori et al . investigated the mechanisms of action of human pcsk9 on hepatic lipid and lipoprotein production on atherosclerosis using a previously developed line of transgenic mice expressing human pcsk9 within the normal physiologic range in human plasma ( 303000 ng / ml ) .
specifically , plasma lipids , hepatic lipogenesis , and atherosclerotic lesion size and composition were evaluated in these transgenic mice expressing human ( h ) pcsk9 ( hpcsk9 ) on a wild - type ( wt ) , ldlr , or apoe background .
plasma apob , cholesterol , and triglyceride levels increased in the wt mice as a result of decreased hepatic ldlr and increased hepatic lipid production .
these changes were mediated both transcriptionally and post - transcriptionally by pcsk9 in both an ldlr- and apoe - dependent manner .
although in mice expressing hpcsk9 but lacking either ldlr or apoe no significant changes in plasma lipids were seen , hpcsk9 expression increased aortic lesion size in the absence of apoe but not in the absence of ldlr . furthermore
, the atheromatous plaque in apoe mice was enriched in hpcsk9 and ly6c ( inflammatory ) monocytes .
these findings establish that pcsk9 significantly affects atherosclerotic lesion size and by mechanisms that are not exclusively related to systemic lipid changes .
pcsk9 promotes atherosclerosis directly by modulation of the entry of inflammatory monocytes into the artery wall by pcsk9-ldlr interaction in the plaque . as an extension ,
this supports the notion that therapeutic antagonism to pcsk9 may improve ascvd outcomes through both lipid and non - lipid pathways .
further validation of the role of pcsk9-induced plaque inflammation , independent of plasma lipid changes , comes from studies of apoe and ldlr mice expressing hpcsk9 from bone marrow - derived cells .
such a model allows investigation of the direct effect of pcsk9 in the atheroma without confounding by plasma lipoproteins . while these mice manifested similar lipid levels and atherosclerotic lesion size , plaque composition demonstrated an ldlrdependent increase in ly6c monocytes .
interestingly , these compositional changes are observed in the background of a very small macrophage contribution to plasma pcsk9 ( 0.51.5% of total plasma pcsk9 ) , supporting the notion that the plaque may be enriched by macrophage - derived pcsk9 rather than being bathed in plasma pcsk9 .
additionally , expression of both pro- and anti - inflammatory cytokines was significantly modulated by macrophage hpcsk9 in an ldlr - dependent fashion .
specifically , previous work has demonstrated that pro - inflammatory leukocytes modulate atherosclerotic lesion composition and are an important factor in progression of atherosclerosis , despite similar plasma cholesterol levels and lesion size .
therefore , based on new experimental observations , one can hypothesize that , even in the absence of changes in lipid levels , hpcsk9 directly contributes to atherogenesis by altering plaque morphology and increasing inflammatory ly6c monocyte infiltration and differentiation in the plaque .
it is tempting to speculate that the absence of ldlr should mitigate the negative effect of pcsk9 on inflammation since pcsk9 accumulation in tissues depends on its presence .
however , other ldlr family members , such as lrp1 , play a role in inflammation and atherogenesis and interact with pcsk9 ( fig .
1 ) . pcsk9 leads to degradation of lrp1 in macrophages which may , in part , explain the relationship of pcsk9 and atherosclerosis , as macrophage lrp1 deficiency is associated with increased atherosclerosis and plaque inflammation , .
sources and targets of intraplaque pcsk9 pcsk9 can be produced locally within atherosclerotic plaque by endothelial cells , vascular smooth muscle cells , and arterial macrophages .
intraplaque pcsk9 exerts paracrine effects on an array of foam cell receptors , including the ldl receptor , lrp1 , and cd36 .
while local production of pcsk9 within the plaque has the potential to modulate atherogenesis independent of plasma concentrations , its impact on each of these foam cell receptors in relationship to atherosclerosis remains unclear .
pcsk9 = proprotein convertase subtilisin kexin type 9 ; ldl = low - density lipoprotein ; ldlr = low - density lipoprotein receptor ; lrp1 = low density lipoprotein receptor - related protein 1 ; cd36= cluster of differentiation 36 ; vsmc = vascular smooth muscle cell
the experimental studies reviewed above provide mechanistic insight into the relationship of pcsk9 and atherosclerosis . as a first step in translation ,
a series of atherosclerosis imaging studies have been performed exploring the association between serum pcsk9 and atherosclerotic plaque burden .
chan et al performed carotid intima - media wall thickness ( cimt ) measurements in 295 asymptomatic subjects from the community .
they found a significant association between pcsk9 and cimt that was independent of traditional cardiovascular risk factors , including gender , hypertension , smoking , ldl - c , triglycerides , lp(a ) , obesity , and biomarkers of inflammation .
these findings are in agreement with smaller studies investigating the association of pcsk9 and cimt .
on the other hand , a sub - analysis of the fate study ( firefighters and their endothelium ) evaluated 1,527 middle - aged men free of vascular disease who underwent risk factor assessment , including advanced biomarkers , flow - mediated dilation , and cimt .
while pcsk9 concentration was associated with several traditional risk factors , there was no relationship between pcsk9 levels and measures of subclinical atherosclerosis .
computed tomography for quantification of coronary artery calcium ( cac ) has also been used as a tool to evaluate the association of pcsk9 and atherosclerosis .
alonso et al . studied a cohort of 161 genetically confirmed subjects with fh who had plasma measurements of pcsk9 and underwent cac scoring .
specifically , serum pcsk9 concentrations were significantly lower and higher in subjects without and with the highest absolute cac scores , respectively .
perhaps most interesting , after correction for potential confounders , including age , gender , body mass index , fasting glucose , statin therapy , tobacco use , hypertension , and lipid / lipoprotein parameters , only pcsk9 and apo(a ) remained significantly predictive of cac scores in this cohort of asymptomatic fh subjects .
utilized a novel approach to see if they could recapitulate experimental findings regarding pcsk9-induced inflammation and atherosclerosis by employing intravascular ultrasound virtual histology ( ivus - vh ) as a tool in humans .
they evaluated the association of serum pcsk9 levels with necrotic core within coronary atherosclerotic lesions in subjects with known cad confirmed at the time of angiography .
they found that pcsk9 concentrations were linearly associated with a higher necrotic core fraction in coronary plaque .
this observation remained significant in all subgroups , independent of ldl - c levels and use of statins .
evaluated the association of serum pcsk9 with progression of carotid plaque as part of the chinese multi - provincial cohort study .
the investigators found that among 643 participants from the general population free of cardiovascular disease at baseline , pcsk9 levels were associated with progression of atherosclerosis as reflected by total plaque area , independent of plasma ldl - c concentrations .
more recently , the glagov ( global assessment of plaque regression with a pcsk9 antibody as measured by intravascular ultrasound ) trial evaluated the effects of the pcsk9 inhibitor , evolocumab , on progression of coronary atherosclerosis in statin treated patients using seral ivus .
glagov is a multicenter , double - blinded , placebocontrolled trial , which randomized subjects with angiographic coronary artery disease on baseline statin therapy to receive monthly evolocumab ( n = 484 ) or placebo ( n = 484 ) .
ivus was performed at baseline and then at 76 weeks to assess changes in coronary atherosclerosis .
they found a greater decrease in percent atheroma volume ( 1% difference ) in the group that received evolocumab .
additionally , treatment with evolocumab was associated with plaque regression in a greater percentage of patients than placebo ( 64.3%vs 47.3% ; difference , 17.0% ; p < .001 ) .
this study demonstrates that pcsk9 inhibition with aggressive ldl - c lowering is associated with a modest degree of plaque regression .
whether this finding is related to the aggressive ldl - c lowering ( achieved mean , time - weighted ldl - c on evolocumab / statin was 36.6 mg / dl ) , other non - lipid anti - atherosclerotic effects of pcsk9 , or both , remains to be elucidated .
given the results from experimental and imaging studies that demonstrate an important relationship between pcsk9 and atherogenic lipids , inflammation , and atherosclerosis , there has been a natural interest in exploring pcsk9 as a biomarker of ascvd risk , both in the context of primary and secondary prevention .
as part of the fate study , 1,527 middle aged firefighters who were free of vascular disease at baseline were followed longitudinally for a mean of 7.2  1.7 years to evaluate the ability of pcsk9 concentrations to predict cardiovascular events . while pcsk9 correlated with ldl - c , insulin , and triglycerides , it did not demonstrate a significant relationship with cardiovascular events .
performed a nested case - control evaluation from a prospective cohort of 28,000 initially healthy american women ,  45 years old and not on statin therapy , who participated in the women 's health study .
the investigators measured plasma concentrations of pcsk9 at baseline among 358 cases ( mi , ischemic stroke , cv death ) and 358 age , smoking , and hormone replacement therapy matched controls ( remained free of disease during 17 years of follow - up ) . the median pcsk9 concentrations were not statistically different amongst cases vs control ( 304.4 s 299.7 ) , respectively .
as expected , there was a modest , positive association between pcsk9 level and apob and triglycerides . while baseline apob predicted the occurrence of first cardiovascular events , baseline levels of pcsk9 did not .
more recently , leander et al . enrolled 4,232 sixty - year old men and women living in stockholm county in a prospective cohort study examining the relationship between pcsk9 and future cardiovascular events ( composite primary outcome of fatal or non - fatal myocardial infarction , angina , chronic ischemic heart disease , sudden cardiac death , and fatal or non - fatal ischemic stroke ) . during the 15 years of follow - up
not surprisingly , there was a modest correlation between pcsk9 levels with ldl cholesterol ( r = 0.18 , p < 0.0001 ) and triglycerides ( r = 0.12 , p < 0.0001 ) .
they found improved cardiovascular disease event - free survival in subjects in the lowest quartile of pcsk9 and the highest hazard ratios for incident cardiovascular disease in those in the highest quartile of pcsk9 .
one particularly interesting sub - analysis demonstrated that subjects with discordant pcsk9 and ldl - c levels ( high pcsk9  low ldl - c ) had the highest future hazard of the primary outcome , even compared to those with high pcsk9  high ldl - c . additionally , the authors demonstrated improved discrimination ( modestly improved c - statistic ) and net reclassification with the incorporation of pcsk9 levels into their model ( though neither was statistically significant ) .
in particular , leander et al . used a customized elisa assay that found a much lower median pcsk9 levels than the ridker cohort ( 94.3 ng / ml vs 300 ng / ml , respectively ) .
it must be noted that leander et al . enrolled a representative sample of men and women from the general swedish population whereas ridker et al . studied a sub - cohort from a randomized controlled trial in women who were assigned to treatment with aspirin or vitamin e for prevention of cardiovascular disease . in the context of secondary prevention , werner et al
. tested whether fasting pcsk9 serum concentrations predict events in a large cohort of statin - treated patients with well - controlled ldl - c levels and angiographically documented stable coronary artery disease .
these subjects were followed prospectively for the primary composite outcome including cardiovascular death , acute coronary syndrome ( acs ) , or unplanned , symptom - induced coronary revascularization .
interestingly , while serum pcsk9 levels predicted the primary outcome in subjects on statin therapy and well controlled ldl - c levels , the association was lost when adjusting for fasting triglyceride levels .
perhaps this observation is not surprising when taking into account the influence of pcsk9 on other non - ldl atherogenic lipoprotein fractions .
studied 616 chinese subjects with stable coronary artery disease for 17 months to evaluate the association of pcsk9 levels with the composite primary outcome of cardiac death , stroke , myocardial infarction , revascularization , and unstable angina .
these subjects were not on lipid - lowering therapy prior to enrollment but received medical therapy without or with percutaneous coronary intervention after enrollment .
specifically , there were significantly higher pcsk9 levels observed in subjects with a syntax score in the highest vs the lowest tertile .
furthermore , at 17 months the event rate was higher in those with higher pcsk9 levels , but evident only among subjects treated medically , and not those who underwent coronary revascularization . the prognostic value of pcsk9 levels has also been studied in patients with acs .
the participants were followed up at 30 days and 1 year after the event for the primary endpoint of all - cause death .
they observed that pcsk9 concentrations in subjects with acs were associated with measures of inflammation , lipid - lowering therapy , and the clinical onset of acs .
clearly , on the basis of the various trials evaluating pcsk9 as a biomarker of ascvd risk , there are discrepant results from which no firm conclusions can be drawn . on that basis ,
a group of investigators performed a systematic review and meta - analysis of studies evaluating the relationship of pcsk9 levels with the risk of future cardiovascular events .
ultimately , 8 studies were included in their analysis , all published very recently , and with 12,081 subjects followed for a mean of 6.6 years .
the main finding from this analysis is that pcsk9 levels are modestly but significantly correlated with risk of total cardiovascular events . specifically , high vs. low pcsk9 levels
furthermore , an increase in pcsk9 levels by 1 sd corresponds to a 10% increase in total cardiovascular events .
based on the available data , acquisition of plasma pcsk9 levels can not be endorsed at this time for clinical use .
its association with ascvd events is modest and less robust than other commonly used and more validated biomarkers .
however , these analyses highlight the need for standardization and refinements in the methodologies to assay pcsk9 . currently , there are two commonly used methods to measure plasma pcsk9 , elisa ( common and scalable ) and mass spectrometry ( expensive and cumbersome ) .
importantly , recent lines of experimental evidence have demonstrated that pcsk9 is compartmentalized within the plasma , with approximately 40% of pcsk9 bound to ldl or lp(a ) particles and the remainder circulating freely , or at least not associated with lipoproteins .
given the stoichiometry between ldl and pcsk9 in plasma , approximately only 1 ldl particle in every 5001000 carries at least one molecule of pcsk9 .
ldl - bound pcsk9 appears to be the more biologically active from , with free plasma pcsk9 susceptible to furin cleavage , a form that is less biologically active .
currently available elisa techniques are not able to discriminate the various forms of plasma pcsk9 .
development of a practical and reproducible assay that can quantitate the different compartments of plasma pcsk9 may finally herald the birth of pcsk9 as a valuable clinical biomarker .
examination of experimental , imaging , and epidemiological studies demonstrates that pcsk9 is inextricably linked to atherosclerosis through both lipid and non - lipid pathways . one important culmination of this work will perhaps be reflected by the results of the clinical trials testing the therapeutic monoclonal antibodies to pcsk9 , the first of which is expected to be published in 2017 .
preliminary analyses suggest that randomized controlled trials are indeed likely to demonstrate that antagonism of pcsk9 is associated with a significant reduction in ascvd events .
these trials , of course , can not answer the question as to which and to what relative extent each of these mechanisms of pcsk9 inhibition thwarts atherosclerosis .
furthermore , the current clinical approach utilizing antibodies only inhibits plasma ( hepatic , secreted , extracellular ) pcsk9 .
novel agents in development have the ability to inhibit production ( eg , antisense oligonucleotides , silencing rna ) of pcsk9 and therefore may highlight the relative importance of intracellular pcsk9 on inflammation and atherosclerosis .
we are still at the dawn of the pcsk9 era , and the future is bright !
dr . fazio has received compensation for advisory activities from the following companies : aegerion , amarin , amgen , kowa , merck , sanofi .
shapiro has received compensation for advisory activities from the following companies : alexion , amgen , bracco , ge health care , synta pharmaceuticals .
 even though it is only a little over a decade from the discovery of proprotein convertase subtilisin / kexin type 9 ( pcsk9 ) as a plasma protein that associates with both high and low cholesterol syndromes , a rich body of knowledge has developed , and drugs inhibiting this target have been approved in many markets . while the majority of research in recent years has focused on the impact of therapeutic antagonism of this molecule , important lines of investigation have emerged characterizing its unique physiology as it relates to cholesterol metabolism and atherosclerosis . 
 the pcsk9 story is unfolding rapidly but is far from complete . 
 one chapter that is of particular interest is the possible direct link between pcsk9 and atherosclerosis . 
 this review specifically examines this relationship drawing from data produced from experimental models of plaque biology and inflammation , atherosclerosis imaging studies , and observational epidemiology . 
Introduction
PCSK9 and Lipoprotein Metabolism
PCSK9 and Inflammation
PCSK9 and Atherosclerosis Imaging
PCSK9 as a Biomarker of ASCVD Risk
Conclusion
Disclosures