Correlation Between the Anticoagulant and Antiplatelet Effect of D-Phe-Pro-Arg-H (Rg-2958).

Abstract

Anticoagulants usually have no antiplatelet effect and platelet function inhibitors do not interact with the coagulation factors. Since thrombin has a decisive role in thrombus formation (growth and stabilization), inhibitors of the effect of thrombin on platelets may be of special importance in developing a novel type of anticoagulant with antiplatelet properties.D-Phe-Pro-Arg-H /I/ designed and synthetized in our Institute was found to be a highly specific,reversible non-competitive inhibitorgOf thrombin,a specific platelet agonist. (K.= 1x10-8 M). /I/ was administered parenterally and orally to white New Zealand rabbits and to beagle dogs. The kinetics of action was recorded by measuring the WBCT, APTT, PT, TT,platelet count (PC) and platelet aggregation (PA). Optimum degree of anticoagulation was considered by the values proposed by Verstraete and Verwilghen. /I/ was shown to be a highly specific inhibitor of PA induced by thrombin.No direct interaction between the inhibitor and the platelet membrane could be detected. Aggregability of human platelets in citrated PRP and that of the gel-filtrated platelets induced by ADP or collagen did not change after incubation with /I/. The antiplatelet effect of /I/ was studied by ex vivo experiments where the inhibitor was the anticoagulant ( 30 ug/ml whole blood ) instead of citrate. Comparing the aggregability caused by several inducers in citrated human PRP with that of in /I/-PRP a significant difference was observed when epinephrine was the PA-inducer. /I/ acts via formation of an enzyme-inhibitor complex that inhibits the binding of thrombin on their receptor-sites at the platelet membrane. In vivo experiments showed a close correlation between TT and PA induced by thrombin. /I/ proved completely harmless to platelets and red blood cells. No significant change in PC could be detected.

Pharmacologic Aspects of the Development of Selective Synthetic Thrombin Inhibitors as Anticoagulants

“…D-Phe-Pro-Arg-H was shown to be an effective inhibitor of thrombin in vitro and in vivo. [11][12][13][14] Similarly, the BOC-derivative is a potent thrombin inhibitor. On storage of the D-Phe-Pro-Arg-H in solution, a loss of activity occurs, owing to cyclization.…”

“…According to data from the literature, the following compounds have been studied in more detail in vitro and in vivo: The arginine amide derivatives OM 189, OM 407, OM 805; [25][26][27][50][51][52][53] the benzamidine derivatives APPA, TAPAP, NAPAP; 6,29,33,54,55 the guanidinobenzoic ester POY; 42,44,45 the tripeptide derivatives D-Phe-Pro-Arg-H; 11,13,21,56 D-MePhe-Pro-Arg-H; 15,16 D-Phe-Pro-ArgCN; 22,57,58 and D-Phe-Pro-Arg-CH 2 Cl. 19,20,21…”

“…Since the anticoagulant action of selective thrombin inhibitors is based solely on their effect on the enzymatic activity of thrombin, its blockade not only prevents plasmatic coagulation processes, but also the activation of blood platelets. 11,[67][68][69][70][71][72][73][74][75] This is also true for tumor cellmediated aggregation of platelets. 76 Inhibition of thrombin-induced platelet reactions, however, requires higher inhibitor concentrations than are necessary for preventing the reaction of thrombin with its substrate fibrinogen.…”