Oral administration of Lefradafiban maintains the potent platelet GP IIb/IIIa antagonism of Fradafiban during treatment of healthy subjects for 1 week without signs of loss of the antiplatelet activity.

1. In a randomized, double‐blind trial we compared the inhibition of the platelet‐vessel wall interactions in whole blood ex vivo. There were four groups of 24 healthy volunteers each of whom were treated orally for 3.5 days with either 200 mg dipyridamole (sustained release preparation), 25 mg acetylsalicylic acid, both drugs combined or placebo twice daily. 2. The mean area of all platelets/aggregates was reduced by 6.2% +/− 4.2% (+/− s.e. mean) by placebo (n = 23), 19.8% +/− 6.7% by dipyridamole (n = 22), 53.7% +/− 4.9% by acetylsalicylic acid (n = 23) and 71.4% +/− 3.7% by the combination of both drugs (n = 24), when compared with total inhibition of aggregation by EGTA. Thus, low‐ dose acetylsalicylic acid inhibited aggregation (P less than 0.001). 3. Dipyridamole reduced the size of platelet aggregates (P less than 0.01, two‐fold analysis of variance). The reduction was correlated with the individual dipyridamole plasma levels (P less than 0.05, analysis of covariance). The subgroup of large and very large thrombi being formed was also reduced by dipyridamole (P less than 0.05). 4. This ex vivo study demonstrates that dipyridamole alone inhibits formation of thrombi on subendothelial matrix and enhances the inhibitory effect of low dose acetylsalicylic acid in this model of thrombosis.

Two preparations of dipyridamole have been studied by oral administration to 11 normal volunteers. The plasma levels of dipyridamole and its glucuronide were determined simultaneously by high performance liquid chromatography. The instant form (I.F., 100 mg) was administered four times daily and the slow release preparation (SRP, 200 mg) twice daily, for 3 days. Multiple blood samples were collected on Days 1-4 to provide plasma for assay, and simultaneously, platelet rich plasma was prepared for ex vivo study of the effect of dipyridamole on platelet uptake of adenosine. The pharmacokinetics of absorption and distribution of dipyridamole were described using a two compartment model with lag time and prolonged absorption. Strong inhibition of the platelet adenosine uptake was observed at therapeutic plasma levels. The inhibition of platelet adenosine uptake may be related to some of the pharmacological properties of dipyridamole.

A series of omega-disubstituted alkenoic acid derivatives were designed and synthesized as antithrombotic inhibitors of thromboxane A2 synthetase and thromboxane A2 receptor antagonists. Hexenoic acid derivatives with a 3-pyridyl group and a 4-(2-benzenesulfonamidoethyl)phenyl substituent were found to be optimal with regard to the dual mode of action. The most potent compound, (E)-6-(4-(2-(((4-chlorophenyl)sulfonyl)amino)ethyl)phenyl)-6-(3-pyridyl) hex-5-enoic acid (36), inhibits thromboxane A2 synthetase in gel-filtered human platelets with an IC50 value of 4.5 +/- 0.5 nM (n = 4), whereas an inhibitory effect on cyclooxygenase is seen only at a much higher concentration (IC50: 240 microM). Radioligand-binding studies with [3H]SQ 29,548 in washed human platelets revealed that 36 blocks the prostaglandin H2/thromboxane A2 receptor with an IC50 of 19 +/- 5 nM (n = 5) and is therefore 85-fold more potent than another combined thromboxane A2 synthetase inhibitor/receptor antagonist, Ridogrel (4). Compound 36 inhibits the collagen-induced platelet aggregation in human platelet-rich plasma and whole blood with an EC50 of 1 microM (Ridogrel: 16 microM) and 100 nM, respectively, and was selected for further development.

A new series of omega-disubstituted alkenoic acid derivatives derived from samixogrel 5 were designed and synthesized as combined thromboxane A2 receptor antagonists/thromboxane A2 synthase inhibitors with improved solubility and reduced protein binding compared to 5. Hexenoic acid derivatives with a 3-pyridyl group and 3-(2-cyano-3-alkyl-guanidino)phenyl substituent were found to be optimal with regard to this dual mode of action. The most potent compound, E-6-(3-(2-cyano-3-tert-butyl-guanidino)phenyl)-6-(3-pyridyl)hex-5-eno ic acid, "terbogrel" 32 inhibits the thromboxane A2 synthase in human gel-filtered platelets with an IC50 value of 4.0 +/- 0.5 nM (n = 4). Radioligand binding studies with 3H-SQ 29,548 revealed that 32 blocks the thromboxane A2/endoperoxide receptor on washed human platelets with an IC50 of 11 +/- 6 nM (n = 2) and with an IC50 of 38 +/- 1 nM (n = 15) in platelet-rich plasma. Terbogrel inhibits the collagen-induced platelet aggregation in human platelet-rich plasma and whole blood with an IC50 of 310 +/- 18 nM (n = 8) and 52 +/- 20 nM (n = 6), respectively. This was shown to translate into a potent antithrombotic effect in vivo as demonstrated in studies using a model of arterial thrombosis in rabbits (ED50 = 0.19 +/- 0.07 mg/kg; n = 20). Thus, terbogrel is the first compound with a guanidino moiety demonstrating both a potent TXA2 synthase inhibition and a potent TXA2 receptor antagonism and has been selected for further clinical development.