Pharmacological modulation of human platelet leukotriene C 4-synthase

Abstract

The aim of this study was to test if human platelet leukotriene C 4-Synthase (LTC 4-S) is pharmacologically different from cloned and expressed LTC 4-S and, in light of the significant homologies between 5-lipoxygenase activating protein (FLAP) and LTC 4-S, if different potencies of leukotriene synthesis inhibitors acting through binding with FLAP (FLAP inhibitors) reflect in different potencies as LTC 4-S inhibitors. Leukotriene C 4 (LTC 4) synthesis by washed human platelets supplemented with synthetic leukotriene A 4 (LTA 4) was studied in the absence and presence of two different, structurally unrelated FLAP inhibitors (MK-886 and BAY-X1005) as well as a direct 5-lipoxygenase inhibitor (zileuton). LTC 4 production was analyzed by RP-HPLC coupled to diode array detection. We report that human platelet LTC 4-S was inhibited by MK-886 and BAY-X1005 (IC 50 of 4.7 μM and 91.2 μM, respectively), but not by zileuton (inactive up to 300 μM); all 3 compounds were able to inhibit 5-lipoxygenase metabolite biosynthesis in intact human polymorphonuclear leukocytes (IC 50 of 0.044 μM, 0.85 μM, and 1.5 μM, respectively). Platelet LTCμ-S does not appear pharmacologically different from expression cloned LTC 4-S. LTC 4 -S inhibition by FLAP inhibitors is in agreement with the significant homology reported for expression-cloned LTC 4-S with FLAP. Furthermore, functional homology of the binding sites for inhibitors on LTC 4-S and FLAP is suggested by the conservation of the relative potencies of MK-886 and BAY-X1005 vs FLAP-dependent 5-lipoxygenase activity and LTC 4-S inhibition: MK-886 was 19.3-fold more potent than BAY-X1005 as FLAP inhibitor and 19.6-fold more potent than BAY-X1005 as LTC 4-S inhibitor.