SM-20302, a synthetic inhibitor of the
fibrinogen receptor of platelets, has been shown to inhibit the platelet aggregation induced by various stimuli. In the present study, we performed ex vivo platelet aggregation studies by using heparinized platelet-rich plasma (PRP) as well as citrated PRP and compared the antiaggregatory activity with the in vivo antithrombotic efficacy of
SM-20302. The
oral administration of
SM-20302 (0.3-10 mg/kg) to guinea pigs completely inhibited the
ADP-induced ex vivo platelet aggregation in citrated PRP. In heparinized PRP,
SM-20302 (1-10 mg/kg) showed a dose-dependent inhibition of ex vivo platelet aggregation, and it exhibited complete inhibition at a dose of 3 and 10 mg/kg, respectively. The concentration of ionized
calcium in the citrated samples was approximately 35 times lower than that in heparinized samples. Chelation of ionized
calcium caused an enhancement of the antiaggregatory activity of
SM-20302 in guinea pig heparinized PRP in vitro. And addition of CaCl2 to citrated PRP reversed the enhancement.
Citrate therefore appeared to enhance the inhibitory activity of
SM-20302 by lowering the ionized
calcium levels. We also examined the in vivo efficacy of
SM-20302 in a photochemically induced femoral artery
thrombosis model in guinea pigs. The photochemical injury of the endothelium of femoral artery resulted in a progressive decline in the blood flow. The
oral administration of
SM-20302 (0.1-3 mg/kg) produced a dose-dependent maintenance of the femoral artery patency and significantly prolonged the time to occlusive
thrombus formation at a dose of 1 and 3 mg/kg, respectively. These results suggest that
SM-20302 may be an orally active
antithrombotic agent, and its in vivo antithrombotic efficacy appeared to correlate well with the ex vivo platelet inhibition in PRP prepared from heparinized blood but not in PRP anticoagulated with
citrate.