Abstract-Over the last 10 to 15 years, animal and human observational studies have identified elevated levels of both proinflammatory secretory phospholipase A2-IIA and lipoprotein-associated phospholipase A2 as potential risk factors for coronary heart disease. However, Mendelian randomization, a genetic tool to test causality of a biomarker, and phase III randomized controlled trials of inhibitors of theses enzymes (varespladib and darapladib) converged to indicate that elevated levels are unlikely to be themselves causal of coronary heart disease and that inhibition had little or no clinical utility. The concordance of findings from Mendelian randomization and clinical trials suggests that for these 2 drugs, and for other novel biomarkers in future, validation of potential therapeutic targets by genetic studies (such as Mendelian randomization) before embarking on costly phase III randomized controlled trials could increase efficiency and offset the high risk of drug development, thereby facilitating discovery of new therapeutics and mitigating against the exuberant costs of drug development.
Arterioscler Thromb Vasc BiolNovember 2015 , all of which have been reported to be associated with atherothrombotic disease from animal and human observational studies. The predicted efficacy of their inhibition on the development of atherosclerosis and CHD has been tested using the genetic approach of MR (see Figure 1 for a description of MR and comparison to conventional RCT), and this, together with the outcome of the drug trials, is reviewed.
Secretory PLA2s and AtherogenesisThe sPLA2 enzymes are the largest group of this family of enzymes, representing around 30% of all known phospholipases. They are small calcium-dependent, disulphide-rich enzymes with molecular weights ranging between ≈14 and 16 kDa, with the exception of sPLA2-III, which has a molecular weight of ≈55 kDa. 10 The reason for the biological requirement for so many sPLA2s may arise because of their specificity for different phospholipid substrates, as well as their cellular and tissue-specific diversity. 13 The purported impact of sPLA2s on inflammatory diseases, such as atherosclerosis and rheumatoid arthritis, stems from their enzymatic action, generating proinflammatory biolipid mediators ( Figure 2B). Of the sPLA2s, sPLA2-IIA, 14 sPLA2-V, 15 and sPLA2-X 13 have been identified in atherosclerotic plaques by immunohistochemistry. The ability of sPLA2s to promote atherosclerosis is suggested to reflect their action on the phospholipid bilayer of low-density lipoprotein (LDL) particles, generating proatherogenic, small dense LDL with 50% less surface phospholipids. 16 This results in a conformational change in the apolipoprotein B on the LDL particle, exposing proteoglycan-binding sites. This leads to the aggregation and retention of the modified lipoprotein by proteoglycans in the intima, where they are prone to oxidation and subsequent uptake by macrophages, generating foam cells. 17 In the intima of the vessel wall, phospholipase hydrolysis conti...