Does succinate enhance the antihypoxic effects of 2-ethylthiobenzimidazole?

Introduction: 2-Ethylthiobenzimidazole hydrobromide (2-ETBI∙HBr) has shown itself as a drug of the restoring therapy after injuries, infections, intoxications, surgeries, and also as actoprotective drug used to increase physical activity in the extreme conditions. 2-Ethylthiobenzimidazole succinate (2-ETBI•SUCN) was synthesized, which is expected to have similar properties. The aim of this study was to compare the antihypoxic effects of two 2-ethylthiobenzimidol compounds (2-ETBI•HBr and 2-ETBI•SUCN) in two models of normobaric hypoxia in mice – acute hypoxic hypoxia with hypercapnia and acute hypoxic hypoxia without hypercapnia in gas exchange camera. Materials and Methods: The experiments were performed on 86 white outbred male mice weighing 20-22 g. Several groups of 8-12 mice were formed. Two of them were served as controls, and the rest received one of the compounds (2-ETBI•HBr or 2-ETBI•SUCN) 50 or 100 mg/kg once intraperitoneally 1 hour before hypoxic exposure. The doses were chosen on the base of the described maximum of antihypoxic effect of 2-ethylthiobenzimidazole. We also took into account acute toxicity (LD50), which was 450±35 mg/kg for 2-ETBI•HBr and 520±30 mg/kg for 2-ETBI•SUCN. Acute normobaric hypoxic hypoxia with hypercapnia was reproduced by placing mice in individual glass chambers with a volume of 0.25 L. The lifespan of the animals was recorded in minutes. Acute normobaric hypoxic hypoxia without hypercapnia was created using a hermetic gas chamber for laboratory studies with automatic monitoring, control and maintenance of the hypoxic gas composition. Normobaric hypoxia in the chamber was modeled by reducing the oxygen concentration in the gas mixture from 21% to 2% by displacing it with nitrogen. Lifespan, survival at different oxygen concentrations, threshold oxygen concentration, and resistance to hypoxia were estimated. Results: In the model of acute hypoxia with hypercapnia (“cupping test”) both 2-ETBI•HBr and 2-ETBI•SUCN showed a moderate antihypoxic effect. It turned out that the dose of 50 mg/kg of both compounds was more effective. Under normobaric hypoxic hypoxia in the gas exchange chamber 2-ETBI∙HBr increased the lifespan by 58%, 2-ETBI•SUCN – by 46%, but statistical differences between these values were not noted. Both compounds increased the lifespan of mice at critical oxygen concentrations (6→2%) more than 2 times. At the same time, the tolerance to low oxygen concentrations increased from its concentration of 4.56% in the control to 2.97% after administration of 2-ETBI•HBr and to 3.23% after 2-ETBI•SUCN administration. Finally, both antihypoxants also increased the overall score of resistance to hypoxia in points more than 2 times. Conclusion: 2-Ethylthiobenzimidazole, regardless of the anionic part of the molecule, exhibits a pronounced antihypoxic effect in tow models of normobaric hypoxic hypoxia in mice. Its value for 2-ETBI•HBr and 2-ETBI•SUCN was similar in the model of normobaric hypoxia in a gas chamber. The minimum oxygen concentration at which mice survived was about 3%. In ”cupping hypoxia” with hypercapnia, both antihypoxants showed greater effectiveness at a dose of 50 mg/kg than 100 mg/kg. Thus, succinate does not make a significant contribution to the antihypoxic activity of the 2-ethylthiobenzimidazole.