Animal experiment:

Mice[2] Male homozygous LDLR-deficient mice (C57/Bl6 genetic background) are fed a high-fat diet consisting of 18% hydrogenated cocoa butter, 0.15% cholesterol, 7% casein, 7% sucrose, and 3% maltodextrin for 17 weeks, beginning at 6 weeks of age. Forty mice are divided into two groups (n=20 per group) randomly. One group receive Darapladib by gavage (50 mg/kg per day) once daily, while the other group receive the vehicle (saline). During the 6 weeks of treatment, all mice are housed in a room with a 12-h light/dark cycle and are allowed free access to a high-fat diet and water.

Plaque rupture is responsible for the clinical events of ischemic death, myocardial infarction, acute coronary syndromes and ischemic strokes. Lipoprotein-associated phospholipase A2 (Lp-PLA2) 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-PLA2.

In vitro: Darapladib potently inhibited Lp-PLA2 with an IC50 of 270 pM. A lack of selectivity against other secretory PLA2s postulated to play a role in atherogenesis had been demonstrated. The percentage inhibition achieved when 1 μM darapladib was evaluated against human secretory PLA2s IIA, V and X, was 0, 0 and 8.7%, respectively [1].

In vivo: Inhibition of lp-PLA2 by darapladib led to attenuation of inflammation in vivo and decreased plaque formation in ApoE-deficient mice, suggesting an anti-atherogenic role during the progression of atherosclerosis [2].

Clinical trial: Darapladib produced sustained inhibition of plasma Lp-PLA2 activity in patients receiving intensive atorvastatin therapy. IL-6 and hs-CRP changes after 12 weeks of darapladib 160 mg suggested a possible reduction in inflammatory burden [3].

References:
[1] Bui QT, Wilensky RL.  Darapladib. Expert Opin Investig Drugs. 2010;19(1):161-8.
[2] Wang WY, Zhang J, Wu WY, Li J, Ma YL, Chen WH, Yan H, Wang K, Xu WW, Shen JH, Wang YP.  Inhibition of lipoprotein-associated phospholipase A2 ameliorates inflammation and decreases atherosclerotic plaque formation in ApoE-deficient mice. PLoS One. 2011;6(8):e23425.
[3] Mohler ER 3rd, Ballantyne CM, Davidson MH, Hanefeld M, Ruilope LM, Johnson JL, Zalewski A; Darapladib Investigators.  The effect of darapladib on plasma lipoprotein-associated phospholipase A2 activity and cardiovascular biomarkers in patients with stable coronary heart disease or coronary heart disease risk equivalent: the results of a multicenter, randomized, double-blind, placebo-controlled study. J Am Coll Cardiol. 2008;51(17):1632-41.