The design, synthesis, and pharmacological evaluation of (S)-(-)-ethyl [6-[4-(morpholinoformimidoyl)benzamido]-3, 4-dihydro-2H-1-benzopyran-3-yl]acetate hydrochloride ((S)-4.HCl, MS-180), an orally active glycoprotein IIb-IIIa (GPIIb-IIIa) antagonist, are reported. Pharmacophore mapping of amidino and carboxyl groups of already known GPIIb-IIIa antagonists led to the synthesis of nine amidino acids containing 6,6-bicyclic ring skeletons (10a-i). Among them, the compounds 10a,c,e having an amide bond and 1,2,3,4-tetrahydronaphthalene or 3, 4-dihydro-2H-1-benzopyran skeleton showed marked inhibitions with IC50 values of 46-57 nM in human platelet aggregation assay in vitro, but low oral activities. N-Alkylation of the amidino group coupled with the ester prodrug approach afforded MS-180 ((S)-4.HCl), which generates in vivo the corresponding carboxylic acid (S)-3 as an active species. In vitro, (S)-3 inhibited ADP-induced aggregation of guinea pig, dog, and human platelets (IC50 = 110, 253, and 35 nM, respectively) and inhibited the binding of fibrinogen to immobilized GPIIb-IIIa of human platelets (IC50 = 0.12 nM). After oral administration of MS-180 ((S)-4.HCl) to fasted beagle dog, ex vivo inhibition of platelet aggregation was observed. The maximal inhibitions were observed 2-4 h after dosing with dose dependency (60% inhibition at a dose of 1 mg/kg, 85% at 3 mg/kg, and 100% at 10 mg/kg, respectively) and the extent of the inhibitions paralleled the plasma concentration of the active species (S)-3. On the basis of these studies, we selected MS-180 ((S)-4.HCl) as a candidate for clinical evaluation as a drug for the treatment and prevention of thrombosis in patients.