EP 171: a high affinity thromboxane A2-mimetic, the actions of which are slowly reversed by receptor blockade

1. Replacement of the four-carbon omega-terminus in 9,11-endoxy-10a-homo prostaglandin H2 with a p-fluorophenoxy group produces a compound (EP 171) with very high agonist potency at TP-receptors. 2. On six isolated smooth muscle preparations EP 171 was 33-167 times more potent as a TP-receptor agonist than U-46619 (11,9-epoxymethano PGH2); EC50 values ranged from 45 to 138 pM. The actions of EP 171 were difficult to study because of their slow onset and offset. For example, on the guinea-pig trachea the time required for 50% reversal of EP 171-induced contractions during washout was about 3 h. 3. On the pig pulmonary artery, a more rapidly responding preparation, it was possible to show that the TP-receptor antagonist EP 092 blocked the contractile actions of EP 171 and U-46619 to similar extents: pA2 = 8.09 and 8.15 respectively. 4. EP 171 was also a very potent activator of human blood platelets, being about 90 times more potent than U-46619. Both shape change (0.1 nM) and aggregation (1 nM) were slow in onset, a profile not previously observed for a thromboxane A2-mimetic. 5. When potencies at TP-, EP1-(guinea-pig fundus) and FP-(dog iris sphincter) receptors were compared, EP 171 showed a higher specificity as a TP-receptor agonist than either STA2 or U-46619. These studies also showed that contrary to earlier reports, the guinea-pig fundus does contain TP-receptors mediating muscle contraction. However, the maximal response due to activation of TP-receptors was only about 35% of the PGE2 maximum. 6. Established responses to EP 171 were slowly reversed following addition of a high concentration of a TP-receptor antagonist (EP 092, GR 32191 or BM 13177). Faster reversals of three less potent 16-p-halophenoxy prostanoids and U-46619 were obtained. Half-times for offset (and onset) of agonist action appeared to correlate with potency rather than with lipophilicity. 7. Competition between the agonists and a radio iodinated PTA2 derivative ([125I]-PTA-OH) for binding to TP-receptors on intact human platelets was studied. IC50 values correlated well with aggregating potency, EP 171 having the lowest IC50 of 2.9 nM. The true Ki for EP 171 may be about 1 nM if both its racemic nature and reduction of initial free ligand concentration due to TP-receptor binding are taken into account. 8. It is concluded from a comparison of agonist potency rankings that subclassification of the TP-receptor is not warranted at this time. The factors that may be responsible for the slow kinetics of EP 171 action are discussed.