Chronic Toxicity and Reversibility of Antifertility Effect of Immunization against Gonadotropin-Releasing Hormone in Male Rats and Rabbits

The chronic systemic toxicity of immunization with gonadotropin-releasing hormone, conjugated to tetanus toxoid (GnRH-TT), was investigated in male rats and rabbits in order to start Phase I clinical trials. Groups of rats and rabbits were immunized with GnRH-TT dissolved in aqueous adjuvant. The antigen was administered at weeks 0, 4, and 8, followed by boosters to maintain high antibody titers. At termination (8–9 months after first immunization), twenty rats and ten rabbits exhibiting the highest mean anti-GnRH titers and all the controls were selected for complete toxicological evaluation. In the rat study, a castrated control group was included for comparison with the immunized group. The hematological and serum chemistry parameters of immunized rats and rabbits were not affected in a significant manner. Most of the changes in serum chemistry of immunized rats were also found in castrated rats, indicating that the changes are most likely due to the withdrawal of androgenic support. The weights of the testes, epididymides, and sex accessory glands were lower in all immunized animals. There was significant atrophy of the germinal epithelium, which, however, sustained a population of Sertoli cells, spermatogonia, and pachytene spermatocytes. Other morphological changes in the prostate, seminal vesicles, pituitary, and mammary gland reflected the effect of androgen withdrawal. The decrease in the weight of liver, kidney, and heart seen in the immunized rats was also present in castrated rats and was not associated with any histopathological changes. The reversibility of immunization-induced infertility was investigated by mating the rats with normal females. Four months after the start of immunization, 9 out of 10 immunized rats were infertile whereas by nine months, all rats had regained fertility. Thus, it is concluded that immunization with GnRH-TT had no systemic toxicological effects in the adult male rats and rabbits for the period studied. The results also indicated that the GnRH-TT immunization had an antifertility effect in male rats. Fertility was restored following cessation of immunization and decline in anti-GnRH antibody titers.