Pharmacokinetics of three commercial antivenoms in patients envenomed by the Malayan pit viper, Calloselasma rhodostoma, in Thailand

The pharmacokinetics of 3 monospecific antivenoms were compared in patients envenomed by the Malayan pit viper, Calloselasma rhodostoma. There was a biphasic decline in serum concentrations following intravenous administration. The initial rapid decline was attributable to the formation of venom-antivenom complexes, as the fall in antivenom during this phase was positively correlated with the initial venom concentration (P = 0.045). The total apparent volume of distribution for each antivenom was 1.5-3 times larger than that of the central compartment, which suggests some tissue distribution in addition to complex formation. This was marked for antivenom from the Government Pharmaceutical Organization of Thailand which contained mostly F(ab)2 fragments. The terminal elimination half time was shorter for Twyford antivenom of caprine origin. Systemic clearance was lower for Thai Red Cross antivenom. In 8 of the 26 patients who experienced recurrence of non-clotting blood after initial response to antivenom, serial measurements of plasma venom and antivenom concentrations revealed that recurrence of venom antigenemia and non-clotting blood bore no direct relation to the elimination half-life of the antivenom used, but non-clotting blood recurred when serum antivenom levels fell below 10-20% of the total given. There is no substitute for close monitoring of envenomed patients so that indications for further antivenom can be detected promptly.