Summary

The main structural feature of direct thrombin inhibitor LK-732responsible for the appropriate interaction at the thrombin activesite is a strong basic group.A possibility that a strong basicgroup of LK-732 might contribute to the mast cell degranulationeffect and consequent reduction of tracheal air flow (TAF) andfall of mean arterial blood pressure (MAP) in rats was investigatedin the present study. At doses up to 5 mg/kg (i.v.), LK-732did not cause significant changes of TAF and MAP. At 7 mg/kg(i.v.), a sudden reduction of TAF and a fall of MAP was observedwithin 5 min after LK-732 administration (75% mortality,p=0.007).A less basic direct thrombin inhibitor LK-658 (21 mg/kg, i.v.) did not significantly disturb TAF and MAP.A reduction ofTAF and a fall of MAP caused by LK-732 (7 mg/kg,i.v.) was a lomost completely abolished in rats with degranulated mast cells (0%mortality, p=0.008). LK-732 concentration-dependently degranulatedrat peritoneal mast cells in vitro (pEC 50=1.92±0.05µ M).A structure-activity relationship (SAR) study revealed thatthe terminal basic groups attached to the aromatic ring are responsiblefor the mast cell degranulation effect. A good correlationwas observed between mast cell degranulation and pKbof analogues of LK-732 (R2 =0.49), but not between mast cell degranulationand thrombin K i (R2 =0.23). LK-732-induced reductionofTAF, the fall of MAP and high mortality originate from LK-732-induced mast cell degranulation. As judged by the SARstudy, this effect could be overcome by reducing the basicity ofLK-732