Absorption and Elimination of Viper Venom after Antivenom Administration GILLES RIVIÈRE, VALÉRIE CHOUMET, BERNARD SALIOU, MARCEL DEBRAY and CASSIAN BON

The mechanisms by which antivenom neutralizes the venom are still poorly understood. In the present work, we studied the effects of antivenom, constituted with either F(ab9)2 or Fab, on the processes of absorption and elimination of Vipera aspis venom in experimentally envenomed rabbits. We first concluded from this study that during the few hours after intramuscular injection, the venom rapidly disappeared from the site of injection but did not immediately reach the vascular system, suggesting that it is partly absorbed via the lymphatic circulation. Concerning the elimination process of the venom in the presence of antivenom, we observed that the elimination of F(ab9)2/venom complexes is slower than that of free venom in the absence of antivenom but faster than that of free F(ab9)2, suggesting that F(ab9)2/venom complexes are eliminated by phagocytosis. The Fab/venom complexes, on the other hand, are eliminated more slowly than free Fab. These complexes are not eliminated through the renal route in agreement with their high molecular weight. In addition, we observed that the treatment of envenomed rabbits with antivenom made of Fab, but not F(ab9)2, is responsible for an oliguria that could be responsible for clinical problems. Immunotherapy performed with specific Fab fragments is widely used to reverse digitalis (Smith et al., 1976) or colchicine (Baud et al., 1995) intoxications. These immunoglobulin fragments neutralize the toxicity of the toxic compounds by reducing their volume of distribution due to their ability to form immunocomplexes and by increasing their elimination through the renal route (Butler et al., 1977; Sabouraud et al., 1992). Immunotherapy is also widely used in the case of snake and scorpion envenomations (De Rezende et al., 1995; Thomas et al., 1996). In this case, most of the antivenoms are F(ab9)2 prepared from horses hyperimmunized against the concerned venoms (Theakston and Warrell, 1991). However, some authors recommend the use of immunoaffinity purified sheep Fab (Consroe et al., 1995; Rawat et al., 1994; Sjostrom et al., 1994), which proved to be responsible for a lower rate of side effects than horse F(ab9)2 (Hickey et al., 1991; Smith et al., 1992). It has been recently shown in clinical and experimental studies (Karlsonstiber et al., 1997; Rivière et al., 1997), that specific Fab induce a more transient neutralizing effect than F(ab9)2 on the toxicity of the venom mainly due to their shorter elimination half-life, t1⁄2 5 4.3 hr compared with that of F(ab9)2, t1⁄2 5 18 hr (Meyer et al., 1997). In agreement with the long duration of snake venom envenomations (Audebert et al., 1994), these results indicated that Fab fragments have to be reinjected after a few hours to maintain their neutralizing action, whereas a single injection of F(ab9)2 neutralizes the venom for a much longer time (up to several days). Thus, the efficacy of specific venom Fab in treating envenomation is lower than that of specific colchicine or digitalis Fab in treating intoxications. Moreover, the process of detoxification of the venom with specific Fab seems to be different than when Fab is directed against digitalis or colchicine. In the present report, we examine the mechanisms by which specific fragments of immunoglobulins [F(ab9)2 or Fab] detoxify the venom. We first determined the mechanism of absorption of the venom injected intramuscularly and the effect of the intravenous injection of antivenom on the venom absorption. We then analyzed the effects of antivenom on the elimination process of Vipera aspis venom. Finally, we examined the interest of the intramuscular route for the administration of antivenoms made of F(ab9)2 or Fab.