Abstract 471: Systemic Chymase Inhibition Directs Atherosclerotic Plaques Towards a Stable Phenotype in ApoE Deficient Mice

Abstract

Activated mast cells have been identified at the site of rupture in human coronary artery plaques and appear to contribute considerably to plaque progression and stability. We and others have previously demonstrated that the mast cell constituents chymase and tryptase promote apoptosis of plaque cells. In this study, we aimed to investigate whether inhibition of mast cell chymase by a specific chymase inhibitor indeed has a beneficial effect on plaque stability. Preincubation of 48/80 activated MC/9 murine mast cells or freshly isolated peritoneal mast cells with chymase inhibitor RO5010226 – 000 – 004 (RO501; 1 μM) inhibited mast cell activation, as illustrated by a decreased β-hexosaminidase activity in the releasate (−41% compared to control MC/9 cells, *P=0.04, and −80% compared to control peritoneal mast cells, *P=0.02) as well as chymase release and activity (−71% and −65%, *P=0.04, respectively). Next, we addressed whether chymase inhibition also was effective in vivo. Atherosclerotic carotid artery lesions were induced in ApoE −/− mice by perivascular collar placement; during lesion development mast cells were activated by a DNP challenge once weekly for 4 weeks. Concomitantly, a subset of mice received the chymase inhibitor (50 mg/kg/day, n=14) as diet supplement, leading to continuous serum concentrations of ~2 μM or control diet (n=12). After 6 weeks, the advanced plaques were analyzed for size and stability. While plaque size did not differ, collagen content of the lesions was 2-fold enhanced in mice treated with the chymase inhibitor compared to controls (RO501: 1.4 ± 0.5% versus controls: 0.7 ± 0.2%). This was accompanied by a significant decrease in necrotic core size of the plaques (RO501: 52 ± 3% versus controls: 41 ± 4%, *P=0.04) as well as by an increased plaque cellularity (RO501: 2.6 ± 0.1*10 3 versus controls: 2.3 ± 0.1*10 3 cells/mm 2 tissue). In agreement with these data we did observe increased peritoneal leukocyte numbers in the RO501 treated mice (RO501: 4.2 ± 1.1*10 6 cells versus 2.2 ± 0.3*10 6 cells in controls, *P=0.04). In conclusion, our data suggest that chymase inhibition indeed results in enhanced plaque stability, identifying chymase inhibition as a new therapeutic approach in the prevention of acute coronary syndromes.