Inhibitor Report for: Darapladib

PURPOSE: To investigate the potential of lipoprotein-associated phospholipase A2 inhibition as a novel mechanism to reduce edema and improve vision in center-involved diabetic macular edema (DME). DESIGN: Prospective, multicenter, randomized, double-masked, placebo-controlled phase IIa study. PARTICIPANTS: Fifty-four center-involved DME patients randomized 2:1 to receive darapladib (n = 36) or placebo (n = 18). METHODS: Darapladib 160 mg or placebo monotherapy was administered orally once daily for 3 months, and patients were followed up monthly for 4 months. MAIN OUTCOME MEASURES: Mean change from baseline in best-corrected visual acuity (BCVA) and the center subfield and center point of the study eye at month 3 as determined by spectral-domain optical coherence tomography. RESULTS: Five patients in the study received intravitreal anti-vascular endothelial growth factor rescue therapy before the day 90 assessment, 2 of 36 (6%) in the darapladib arm and 3 of 18 (17%) in the placebo arm. Administration of 160 mg darapladib for 3 months resulted in statistically significant mean improvements, from baseline to month 3, in BCVA of 4.1 Early Treatment Diabetic Retinopathy Study (ETDRS) letters (95% confidence interval [CI], 2.3-5.8) and of 57 mum in central subfield thickness (95% CI, -84 to -30) in the study eyes. An increase in BCVA of 1.7 ETDRS letters (95% CI, -1.0 to 4.4) and a decrease in center subfield thickness of 34 mum (95% CI, -75 to 6.8) for the placebo group were not significant. No ocular severe adverse events (SAEs) or SAEs considered related to darapladib were reported. One SAE of myocardial infarction, not considered related to darapladib, was reported, and 1 SAE of severe diarrhea was reported in a placebo patient, subsequently withdrawn from the study. Study eye ocular adverse events (AEs) and nonocular AEs were similar between treatment groups. CONCLUSIONS: Once-daily oral darapladib administered for 3 months demonstrated modest improvements in vision and macular edema that warrant additional investigation of this novel lipoprotein-associated phospholipase A2 inhibitory mechanism for the treatment of DME.

PURPOSE OF REVIEW: There is substantial data from over 50 000 patients that increased lipoprotein-associated phospholipase A2 (Lp-PLA2) mass or activity is associated with an increased risk of cardiac death, myocardial infarction, acute coronary syndromes and ischemic stroke. However, only recently have data emerged demonstrating a role of Lp-PLA2 in development of advanced coronary artery disease. Indeed, Lp-PLA2 may be an important link between lipid homeostasis and the vascular inflammatory response. RECENT FINDINGS: Lp-PLA2, also known as platelet-activating factor acetylhydrolase, rapidly cleaves oxidized phosphatidylcholine molecules produced during the oxidation of LDL and atherogenic lipoprotein Lp(a), generating the soluble proinflammatory and proapoptotic lipid mediators, lyso-phosphatidylcholine and oxidized nonesterified fatty acids. These proinflammatory lipids play an important role in the development of atherosclerotic necrotic cores, the substrate for acute unstable coronary disease by recruiting and activating leukocytes/macrophages, inducing apoptosis and impairing the subsequent removal of dead cells. Selective inhibition of Lp-PLA2 reduces development of necrotic cores and may result in stabilization of atherosclerotic plaques. SUMMARY: Recent data have shown that immune pathways play a major role in the development and progression of high-risk atherosclerosis, which leads to ischemic sudden death, myocardial infarction, acute coronary syndromes and ischemic strokes. Persistent and sustained macrophage apoptosis appears to play a major role in the resulting local inflammatory response in part by effects elicited by Lp-PLA2. Selective inhibition of Lp-PLA2 has been postulated to reduce necrotic core progression and the clinical sequelae of advanced, unstable atherosclerosis.

Modification of the pyrimidone 5-substituent in clinical candidate SB-435495 has given a series of inhibitors of recombinant lipoprotein-associated phospholipase A(2) with sub-nanomolar potency. Cyclopentyl fused derivative 21, SB-480848, showed an enhanced in vitro and in vivo profile versus SB-435495 and has been selected for progression to man.

Darapladib is one of the most potent Lp-PLA(2) (Lipoprotein-associated phospholipase A(2)) inhibitor with an IC(50) of 0.25 nM. We demonstrate that a crucial step of Darapladib synthesis was not correctly described in the literature, leading to the production of wrong regioisomers. Moreover we show that the inhibitory activity is directly linked to the position on N1 since compounds bearing alkylation on different sites have potentially less interaction within the active site of Lp-PLA(2).

Darapladib, a lipoprotein-associated phospholipase A2 (Lp-PLA2) inhibitor, failed to demonstrate efficacy for the primary endpoints in two large phase III cardiovascular outcomes trials, one in stable coronary heart disease patients (STABILITY) and one in acute coronary syndrome (SOLID-TIMI 52). No major safety signals were observed but tolerability issues of diarrhea and odor were common (up to 13%). We hypothesized that genetic variants associated with Lp-PLA2 activity may influence efficacy and tolerability and therefore performed a comprehensive pharmacogenetic analysis of both trials. We genotyped patients within the STABILITY and SOLID-TIMI 52 trials who provided a DNA sample and consent (n = 13,577 and 10,404 respectively, representing 86% and 82% of the trial participants) using genome-wide arrays with exome content and performed imputation using a 1000 Genomes reference panel. We investigated baseline and change from baseline in Lp-PLA2 activity, two efficacy endpoints (major coronary events and myocardial infarction) as well as tolerability parameters at genome-wide and candidate gene level using a meta-analytic approach. We replicated associations of published loci on baseline Lp-PLA2 activity (APOE, CELSR2, LPA, PLA2G7, LDLR and SCARB1) and identified three novel loci (TOMM5, FRMD5 and LPL) using the GWAS-significance threshold P<=5E-08. Review of the PLA2G7 gene (encoding Lp-PLA2) within these datasets identified V279F null allele carriers as well as three other rare exonic null alleles within various ethnic groups, however none of these variants nor any other loci associated with Lp-PLA2 activity at baseline were associated with any of the drug response endpoints. The analysis of darapladib efficacy endpoints, despite low power, identified six low frequency loci with main genotype effect (though with borderline imputation scores) and one common locus (minor allele frequency 0.24) with genotype by treatment interaction effect passing the GWAS-significance threshold. This locus conferred risk in placebo subjects, hazard ratio (HR) 1.22 with 95% confidence interval (CI) 1.11-1.33, but was protective in darapladib subjects, HR 0.79 (95% CI 0.71-0.88). No major loci for tolerability were found. Thus, genetic analysis confirmed and extended the influence of lipoprotein loci on Lp-PLA2 levels, identified some novel null alleles in the PLA2G7 gene, and only identified one potentially efficacious subgroup within these two large clinical trials.

BACKGROUND: Despite systemic exposure to risk factors, the circulatory system develops varying patterns of atherosclerosis for unclear reasons. In a porcine model, we investigated the relationship between site-specific lesion development and inflammatory pathways involved in the coronary arteries (CORs) and distal abdominal aortas (AAs). METHODS AND RESULTS: Diabetes mellitus (DM) and hypercholesterolemia (HC) were induced in 37 pigs with 3 healthy controls. Site-specific plaque development was studied by comparing plaque severity, macrophage infiltration, and inflammatory gene expression between CORs and AAs of 17 DM/HC pigs. To assess the role of lipoprotein-associated phospholipase A2 (Lp-PLA2) in plaque development, 20 DM/HC pigs were treated with the Lp-PLA2 inhibitor darapladib and compared with the 17 DM/HC untreated pigs. DM/HC caused site-specific differences in plaque severity. In the AAs, normalized plaque area was 4.4-fold higher (P<0.001) and there were more fibroatheromas (9 of the 17 animals had a fibroatheroma in the AA and not the COR, P=0.004), while normalized macrophage staining area was 1.5-fold higher (P=0.011) compared with CORs. DM/HC caused differential expression of 8 of 87 atherosclerotic genes studied, including 3 important in inflammation with higher expression in the CORs. Darapladib-induced attenuation of normalized plaque area was site-specific, as CORs responded 2.9-fold more than AAs (P=0.045). CONCLUSIONS: While plaque severity was worse in the AAs, inflammatory genes and inflammatory pathways that use Lp-PLA2 were more important in the CORs. Our results suggest fundamental differences in inflammation between vascular sites, an important finding for the development of novel anti-inflammatory therapeutics.

PURPOSE: To investigate the potential of lipoprotein-associated phospholipase A2 inhibition as a novel mechanism to reduce edema and improve vision in center-involved diabetic macular edema (DME). DESIGN: Prospective, multicenter, randomized, double-masked, placebo-controlled phase IIa study. PARTICIPANTS: Fifty-four center-involved DME patients randomized 2:1 to receive darapladib (n = 36) or placebo (n = 18). METHODS: Darapladib 160 mg or placebo monotherapy was administered orally once daily for 3 months, and patients were followed up monthly for 4 months. MAIN OUTCOME MEASURES: Mean change from baseline in best-corrected visual acuity (BCVA) and the center subfield and center point of the study eye at month 3 as determined by spectral-domain optical coherence tomography. RESULTS: Five patients in the study received intravitreal anti-vascular endothelial growth factor rescue therapy before the day 90 assessment, 2 of 36 (6%) in the darapladib arm and 3 of 18 (17%) in the placebo arm. Administration of 160 mg darapladib for 3 months resulted in statistically significant mean improvements, from baseline to month 3, in BCVA of 4.1 Early Treatment Diabetic Retinopathy Study (ETDRS) letters (95% confidence interval [CI], 2.3-5.8) and of 57 mum in central subfield thickness (95% CI, -84 to -30) in the study eyes. An increase in BCVA of 1.7 ETDRS letters (95% CI, -1.0 to 4.4) and a decrease in center subfield thickness of 34 mum (95% CI, -75 to 6.8) for the placebo group were not significant. No ocular severe adverse events (SAEs) or SAEs considered related to darapladib were reported. One SAE of myocardial infarction, not considered related to darapladib, was reported, and 1 SAE of severe diarrhea was reported in a placebo patient, subsequently withdrawn from the study. Study eye ocular adverse events (AEs) and nonocular AEs were similar between treatment groups. CONCLUSIONS: Once-daily oral darapladib administered for 3 months demonstrated modest improvements in vision and macular edema that warrant additional investigation of this novel lipoprotein-associated phospholipase A2 inhibitory mechanism for the treatment of DME.

Mounting ambiguity persists around the functional role of the plasma form of platelet-activating factor acetylhydrolase (PAF-AH). Because PAF-AH hydrolyzes PAF and related oxidized phospholipids, it is widely accepted as an anti-inflammatory enzyme. On the other hand, its actions can also generate lysophosphatidylcholine (lysoPC), a component of bioactive atherogenic oxidized LDL, thus allowing the enzyme to have proinflammatory capabilities. Presence of a canonical lysoPC receptor has been seriously questioned for a multitude of reasons. Animal models of inflammation show that elevating PAF-AH levels is beneficial and not deleterious and overexpression of PAF receptor (PAF-R) also augments inflammatory responses. Further, many Asian populations have a catalytically inert PAF-AH that appears to be a severity factor in a range of inflammatory disorders. Correlation found with elevated levels of PAF-AH and CVDs has led to the design of a specific PAF-AH inhibitor, darapladib. However, in a recently concluded phase III STABILITY clinical trial, use of darapladib did not yield promising results. Presence of structurally related multiple ligands for PAF-R with varied potency, existence of multi-molecular forms of PAF-AH, broad substrate specificity of the enzyme and continuous PAF production by the so called bi-cycle of PAF makes PAF more enigmatic. This review seeks to address the above concerns.

The secreted or plasma form of platelet-activating factor acetylhydrolase (PAF-AH), also known as lipoprotein-associated phospholipase A2 (Lp-PLA2) or phospholipase A2 group 7 (PLA2G7), is a member of the PLA2 superfamily of enzymes that circulates in blood in association with lipoproteins, and is found in atherosclerotic lesions. This enzyme was discovered in the early 1980s based on its ability to hydrolyze the pro-inflammatory glycerophospholipid PAF, and was thus proposed to have anti-inflammatory properties. In subsequent years, it was recognized that PAF-AH hydrolyzes glycerophospholipids containing short and/or oxidized functionalities at the sn-2 position, with no preference for the type of linkage at the sn-1 position, i.e., alkyl versus acyl. Substrate hydrolysis catalyzed by PAF-AH generates lysoPAF/lyso phosphatidylcholine (lysoPC) and short and/or oxidized fatty acids, many of which also have been reported to have pro-inflammatory and pro-oxidative activities. These observations fueled multiple investigations that led to controversial views regarding the role of PAF-AH in human physiology and disease. Notably, the hypothesis that PAF-AH might actively contribute to vascular inflammation during atherogenesis owing to its ability to generate pro-inflammatory substances led to the proposition that inhibition of the activity could offer vascular protection in addition to that afforded by cholesterol lowering agents. A number of reversible PAF-AH inhibitors were developed in the pharmaceutical industry and one of them, darapladib, has been extensively tested in vitro and in vivo . Moreover, GlaxoSmithKline sponsored three darapladib clinical trials that yielded relatively consistent results. In this issue of the Journal of Lipid Research, ( Marathe et al.) provide their views on the role of PAF-AH in inflammatory responses, with a focus on CVD. The authors make several key points and offer a seldom encountered perspective that takes into consideration the origin and wide range of substrates hydrolyzed by PAF-AH, the physiological meaning of studies involving one of the products of the reaction (lysoPAF/lysoPC), and the impact of receptors that recognize substrates and products on downstream signaling events. The authors present several lines of evidence arguing against a pro-atherogenic role for PAF-AH and its products, and suggest that elevated enzyme levels reflect a response to the pro-inflammatory/pro-oxidative stress that is typical of atherosclerosis. Their conclusions are timely and consistent with results from recent clinical trials in humans. In the Integrated Biomarker and Imaging Study 2 phase II trial involving patients with coronary heart disease, the PAF-AH inhibitor darapladib did not meet prespecified primary and secondary endpoints that included effects on coronary atheroma deformability, composition and size, CRP levels, and total atheroma volume. While darapladib administration inhibited necrotic core expansion, this conclusion was reached only by fine interpretation of imaging data. Nonetheless, the finding provided, in part, the basis to conduct two phase III trials. The recently published STABILITY (Stabilization of Atherosclerotic Plaque by Initiation of Darapladib Therapy) trial showed that darapladib did not affect the primary composite endpoint that included time to cardiovascular death, myocardial infarction, or stroke in patients with stable coronary heart disease. Similarly, results recently reported from SOLID-TIMI (Stabilization of Plaques using Darapladib-Thrombolysis in Myocardial Infarction) revealed no reduction in major coronary events when added to standard of care after an acute coronary syndrome. An important lesson learned from the outcome of studies using darapladib is related to the necessity of gathering rigorous scientific evidence supporting a solid rationale to justify launching clinical trials in humans. Arguably, this was not the case for darapladib. Mechanistically, the trials were largely based on laboratory studies that investigated pro-inflammatory functions of lysoPC and, to a lesser extent, oxidized fatty acids. Marathe et al. appropriately describe two major potential problems associated with interpretation of studies using exogenous lysoPC. First, trace amounts of PAF and/or related phospholipids have been shown to contaminate numerous commercial lysoPC preparations. This problem may have affected multiple studies in which contaminating PAF could have accounted for responses incorrectly ascribed to lysoPC . Marathe et al. also point out that lysoPC occurs naturally at very high concentrations in body fluids and atherosclerotic tissues. Its amphipathic nature and detergent-like properties can induce nonspecific cellular responses. In physiological settings, these effects are in large part prevented because lysoPC forms complexes with serum proteins, immunoglobulins, and plasma membranes. Thus, total lysoPAF/lysoPC concentrations do not relate to bioavailability and it is highly unlikely that the relatively small amounts of free lysoPAF/lysoPC generated by PAF-AH contribute to inflammatory responses in the vasculature. A second issue to reflect upon is the importance of interpreting correlative studies appropriately. Strong positive correlations between plasma PAF-AH and LDL cholesterol levels were established approximately 30 years ago. Plasma PAF-AH associates with cholesterol-containing LDL particles; as expected, enzyme levels decrease in response to statin treatment. Many prospective population-based studies confirmed tight links between increased plasma PAF-AH levels and increased cardiovascular risk (2834). Predictably, the strength of this association is highly reduced after adjustment for baseline concentrations of lipids and apolipoproteins, particularly apoB levels. These observations demonstrate that elevated circulating levels of PAF-AH (Lp-PLA2, the FDA-cleared diagnostic test is called PLAC) are associated with atherosclerosis. But such observations support neither causal nor protective roles for PAF-AH in the disease process. In contrast, the fact that partial inhibition of PAF-AH with darapladib did not prevent adverse events in coronary heart disease patients argues against active contribution of the enzyme to plaque vulnerability and major adverse cardiovascular complications such as heart attack, stroke, and death. A third consideration is that the relationship between PAF-AH, PAF, and PAF-like substrates and products generated to various extents in settings of inflammation and oxidant stress is incompletely understood. In this regard, Marathe et al. present a hypothetical model suggesting that the relative abundance of alkyl versus acyl PAF may determine whether PAF-AH has pro- or anti-inflammatory/atherogenic functions. While alkyl and acyl PAF and PAF-like lipids have been reported to be continuously generated, alkyl-linked species are functionally more potent owing to their higher affinity for the PAF receptor (PAF-R), a G protein-coupled receptor that transduces PAF signals. According to Marathe et al., acyl-linked PAF-like analogs are less potent and could potentially behave as relative PAF-R antagonists, although this remains to be shown experimentally. If this scenario is correct, hydrolysis of acyl-linked PAF-like lipids by PAF-AH could effectively decrease the levels of PAF-R antagonists, potentially increasing pro-inflammatory/atherogenic activities. This interesting and provocative model will require future pharmacologic and molecular studies, including targeted silencing of PAF biosynthetic and hydrolytic pathways. It is important to also consider, however, that PAF-AH substrates often elicit biological activity in PAF-R-independent manners. Moreover, it will be important to establish whether the truncated/oxidized sn-2 fatty acids released by PAF-AH also contribute to its physiologic function. Adding to the complexity is that in this process truncated oxidized phospholipids (OxPLs), which are potent pro-inflammatory molecules, are degraded by this enzymatic activity. While darapladib failed to reach all of its primary endpoints, the inhibitor showed some efficacy when administered in experimental animal models and humans. Indeed, darapladib and related compounds significantly decreased atherosclerotic coronary lesion development, reduced macrophage content in vascular lesions, and attenuated plaque inflammation in various animal models. In interpreting these studies, it is important to establish whether darapladib exerted biologic effects by inhibiting PAF-AH activity or by some other mechanism. Careful analysis of changes in lipid metabolites suggests the possibility that darapladib may have antioxidant and/or other off target effects. Although darapladib treatment was associated with reduced content of lysoPC in pig atherosclerotic lesions, it did not affect the levels of truncated OxPL species known to be metabolized by PAF-AH, and did not alter serum PAF levels in two murine models of atherosclerosis. Besides its effects on lipid metabolism, darapladib treatment decreased caspase-3 and caspase-8 activity in vivo), and a compound related to darapladib (SB222657) inhibited macrophage apoptosis induced by oxidized LDL in vitro. These observations raise the possibility that darapladib has activities in addition to its inhibitory effects on PAF-AH. It is thus conceivable that these effects may contribute to its in vivo activities. Regardless, darapladib has taught us that partial inhibition of PAF-AH does not appear to have major impact on vascular events and that the enzyme is unlikely to be the sought-after cholesterol-independent biomarker and target whose inhibition would further decrease morbidity and mortality of patients with vascular disease. The fact that darapladib-mediated partial inhibition of PAF-AH did not reduce coronary events does not, however, suggest that the future of anti-inflammatory heart drugs is dimmed, as recently suggested . The activity of PAF-AH has not been shown to lead to a net increase in pro-inflammatory lipid mediator levels in vascular settings. Regrettably, the enzyme is often referred to as a pro-inflammatory protein despite the fact that it has anti-inflammatory properties, as demonstrated in the original article describing cloning of the gene and characterization of the enzyme. Deciphering the physiologic roles of PAF-AH continues to be a challenge for investigators across academia and the private sector alike, and a number of issues remain to be resolved. For example, it is not clear whether PAF-AH functions in the circulation, in atherosclerotic plaques and other tissues, or both. A study using mice lacking PAF-AH expression suggested that the enzyme may not function in the circulation and that substrate transport to the intracellular compartment may be required before hydrolysis occurs. In addition, the relationship between circulating and tissue PAF-AH and OxPLs has not been critically evaluated, and this issue raises important questions regarding the impact of enzyme, substrate, and products in different biologic compartments. The article by Marathe et al. discusses some of these variables in the intricate biochemistry and biology of PAF-AH, PAF, and related lipids.

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2) or PLA(2)G7) binds to low-density lipoprotein (LDL) particles, where it is thought to hydrolyze oxidatively truncated phospholipids. Lp-PLA(2) has also been implicated as a pro-tumorigenic enzyme in human prostate cancer. Several inhibitors of Lp-PLA(2) have been described, including darapladib, which is currently in phase 3 clinical development for the treatment of atherosclerosis. The selectivity that darapladib and other Lp-PLA(2) inhibitors display across the larger serine hydrolase family has not, however, been reported. Here, we describe the use of both general and tailored activity-based probes for profiling Lp-PLA(2) and inhibitors of this enzyme in native biological systems. We show that both darapladib and a novel class of structurally distinct carbamate inhibitors inactivate Lp-PLA(2) in mouse tissues and human cell lines with high selectivity. Our findings thus identify both inhibitors and chemoproteomic probes that are suitable for investigating Lp-PLA(2) function in biological systems.

IMPORTANCE OF THE FIELD: Atherosclerosis is an inflammatory-immune mediated disease process. Plaque rupture is responsible for the clinical events of ischemic death, myocardial infarction, acute coronary syndromes and ischemic strokes. Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) seems to play a major role in the development of such high-risk lesions, in both the coronary and carotid arteries. Darapladib is a selective inhibitor of Lp-PLA(2). AREAS COVERED IN THIS REVIEW: An overview of darapladib by reviewing the studies (1990 - 2009) that have provided the rationale for the development of darapladib; and a discussion of its potential merit as a new therapeutic drug to target high-risk atherosclerosis. WHAT THE READER WILL GAIN: The reader should gain an understanding of the importance of inflammation during atherogenesis as well as of the biology of Lp-PLA(2) and its proatherogenic role. Additional insights will be gained into the role of selective inhibitors of Lp-PLA(2) as new therapeutic agents. TAKE HOME MESSAGE: Darapladib is a selective inhibitor of Lp-PLA(2) and represents a new class of therapeutic agents that target inflammation to treat high-risk atherosclerosis.

Darapladib, under development by GlaxoSmithKline plc, is a novel inhibitor of lipoprotein-associated phospholipase A2 (PLA2), an enzyme that may link lipid metabolism with inflammation, leading to the increased stability of atherosclerotic plaques present in the major arteries. Darapladib exhibits favorable pharmacokinetics, minimal predicted drug-drug interactions, sustained blood levels with once-daily oral dosing and limited inhibition of other PLA2 isozymes. Preclinical studies in diabetic-hypercholesterolemic swine (useful for the study of human atherosclerosis mechanisms) demonstrated that darapladib attenuated the progression of arterial plaques to a higher-risk phenotype by reducing the number of inflammatory macrophages within plaques and dampening T-cell responses. Two phase II clinical trials demonstrated sustained lipoprotein-associated PLA2 inhibition with daily oral dosing, and favorable effects on markers of inflammation and plaque stability. Phase III trials are ongoing to assess the safety and efficacy of darapladib in reducing adverse clinical events in patients with atherosclerosis.

PURPOSE OF REVIEW: There is substantial data from over 50 000 patients that increased lipoprotein-associated phospholipase A2 (Lp-PLA2) mass or activity is associated with an increased risk of cardiac death, myocardial infarction, acute coronary syndromes and ischemic stroke. However, only recently have data emerged demonstrating a role of Lp-PLA2 in development of advanced coronary artery disease. Indeed, Lp-PLA2 may be an important link between lipid homeostasis and the vascular inflammatory response. RECENT FINDINGS: Lp-PLA2, also known as platelet-activating factor acetylhydrolase, rapidly cleaves oxidized phosphatidylcholine molecules produced during the oxidation of LDL and atherogenic lipoprotein Lp(a), generating the soluble proinflammatory and proapoptotic lipid mediators, lyso-phosphatidylcholine and oxidized nonesterified fatty acids. These proinflammatory lipids play an important role in the development of atherosclerotic necrotic cores, the substrate for acute unstable coronary disease by recruiting and activating leukocytes/macrophages, inducing apoptosis and impairing the subsequent removal of dead cells. Selective inhibition of Lp-PLA2 reduces development of necrotic cores and may result in stabilization of atherosclerotic plaques. SUMMARY: Recent data have shown that immune pathways play a major role in the development and progression of high-risk atherosclerosis, which leads to ischemic sudden death, myocardial infarction, acute coronary syndromes and ischemic strokes. Persistent and sustained macrophage apoptosis appears to play a major role in the resulting local inflammatory response in part by effects elicited by Lp-PLA2. Selective inhibition of Lp-PLA2 has been postulated to reduce necrotic core progression and the clinical sequelae of advanced, unstable atherosclerosis.

OBJECTIVES: This study examined the effects of darapladib, a selective lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) inhibitor, on biomarkers of cardiovascular (CV) risk. BACKGROUND: Elevated Lp-PLA(2) levels are associated with an increased risk of CV events. METHODS: Coronary heart disease (CHD) and CHD-risk equivalent patients (n = 959) receiving atorvastatin (20 or 80 mg) were randomized to oral darapladib 40 mg, 80 mg, 160 mg, or placebo once daily for 12 weeks. Blood samples were analyzed for Lp-PLA(2) activity and other biomarkers. RESULTS: Baseline low-density lipoprotein cholesterol (LDL-C) was 67 +/- 22 mg/dl. Plasma Lp-PLA(2) was higher in older patients (>or=75 years), in men, in those taking atorvastatin 20 mg, at LDL-C >or=70 mg/dl or high-density lipoprotein cholesterol (HDL-C) <40 mg/dl, or in those with documented vascular disease (multivariate regression; p < 0.01). Darapladib 40, 80, and 160 mg inhibited Lp-PLA(2) activity by approximately 43%, 55%, and 66% compared with placebo (p < 0.001 weeks 4 and 12). Sustained dose-dependent inhibition was noted overall in both atorvastatin groups and at different baseline LDL-C (>or=70 vs. <70 mg/dl) and HDL-C (<40 vs. >or=40 mg/dl). At 12 weeks, darapladib 160 mg decreased interleukin (IL)-6 by 12.3% (95% confidence interval [CI] -22% to -1%; p = 0.028) and high-sensitivity C-reactive protein (hs-CRP) by 13.0% (95% CI -28% to +5%; p = 0.15) compared with placebo. The Lp-PLA(2) inhibition produced no detrimental effects on platelet biomarkers (P-selectin, CD40 ligand, urinary 11-dehydrothromboxane B(2)). No major safety concerns were noted. CONCLUSIONS: Darapladib produced sustained inhibition of plasma Lp-PLA(2) activity in patients receiving intensive atorvastatin therapy. Changes in IL-6 and hs-CRP after 12 weeks of darapladib 160 mg suggest a possible reduction in inflammatory burden. Further studies will determine whether Lp-PLA(2) inhibition is associated with favorable effects on CV events.

BACKGROUND: Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) is expressed abundantly in the necrotic core of coronary lesions, and products of its enzymatic activity may contribute to inflammation and cell death, rendering plaque vulnerable to rupture. METHODS AND RESULTS: This study compared the effects of 12 months of treatment with darapladib (an oral Lp-PLA(2) inhibitor, 160 mg daily) or placebo on coronary atheroma deformability (intravascular ultrasound palpography) and plasma high-sensitivity C-reactive protein in 330 patients with angiographically documented coronary disease. Secondary end points included changes in necrotic core size (intravascular ultrasound radiofrequency), atheroma size (intravascular ultrasound gray scale), and blood biomarkers. BACKGROUND: =0.37). In contrast, Lp-PLA(2) activity was inhibited by 59% with darapladib (P<0.001 versus placebo). After 12 months, there were no significant differences between groups in plaque deformability (P=0.22) or plasma high-sensitivity C-reactive protein (P=0.35). In the placebo-treated group, however, necrotic core volume increased significantly (4.5+/-17.9 mm(3); P=0.009), whereas darapladib halted this increase (-0.5+/-13.9 mm(3); P=0.71), resulting in a significant treatment difference of -5.2 mm(3) (P=0.012). These intraplaque compositional changes occurred without a significant treatment difference in total atheroma volume (P=0.95). CONCLUSIONS: Despite adherence to a high level of standard-of-care treatment, the necrotic core continued to expand among patients receiving placebo. In contrast, Lp-PLA(2) inhibition with darapladib prevented necrotic core expansion, a key determinant of plaque vulnerability. These findings suggest that Lp-PLA(2) inhibition may represent a novel therapeutic approach.

Modification of the pyrimidone 5-substituent in clinical candidate SB-435495 has given a series of inhibitors of recombinant lipoprotein-associated phospholipase A(2) with sub-nanomolar potency. Cyclopentyl fused derivative 21, SB-480848, showed an enhanced in vitro and in vivo profile versus SB-435495 and has been selected for progression to man.