INHIBITION OF GUINEA-PIG PLATELET FUNCTION IN VIVO AND EX VIVO USING THE THROMBOXANE A2 ANTAGONISTS, AH23848 AND GR32191

The thromboxane A₂ antagonist, GR32191 (Lumley et al., this meeting) was tested as an inhibitor of platelet aggregation in the guinea-pig and compared with another Tx-antagonist, AH23848 (Brittain et al, 1985). Guinea-pigs were dosed with AH23848 or GR32191 at 0.01-1.0mg/kg. At intervals, blood was taken and PRP was prepared for ex vivo aggregation studies. Collagen concentrations causing half maximal aggregation (IC₅₀) were calculated for test and vehicle-dosed groups. Inhibition was expressed as a concentration ratio (IC₅₀ test/IC₅₀ vehicle). For in vivo studies, ¹¹¹In-labelled platelets (12μCi, 200μl) were injected into anaesthetised guinea-pigs and 24 hrs later oral doses of AH23848 or GR32191 (0.01-1.0mg/kg) or indomethacin (5mg/kg) were given. After one hour, blood was taken for platelet and radioactivity counting. The carotid artery was exposed under anaesthesia and a current of 2mA was applied for 60 sec. After 90 min, 1cm of the damaged and contralateral carotid vessels were removed for gamma-counting. Inhibition of accumulation of platelets on the injured artery was measured by comparison with the undamaged contralateral artery. Numbers of platelets deposited were calculated from the radioactivity of each section of artery and the radioactivity and platelet count in the blood. Oral doses of AH23848 or GR32191 inhibited ex vivo platelet aggregation induced by collagen. Maximum inhibition occurred one hour after dosing, and was still present at 6 hours for AH23848 (l.Omg/kg) and GR32191 (0.3mg/kg). GR32191 and AH23848 were active in vivo causing inhibition of platelet deposition at doses of 0.01-lmg/kg. The maximum inhibition of deposition was 58% for AH23848 (0.1mg/kg) and 63% for GR32191 (0.1mg/kg), with 50% inhibition at 0.02mg/kg for both. Indomethacin (5mg/kg p.o.) caused maximum inhibition of 58% at 5mg/kg p.o. suggesting that this represents the total thromboxane involvement in platelet deposition. GR32191 and AH23848 are thromboxane A₂ antagonists with antithrombotic activity after oral dosing to guinea-pigs.

Brittain R.T. et al Circulation, 72, 1208-1218, 1985.