Relevance of immuno-contraceptive vaccines for population control. I. Hormonal immunocontraception

Abstract

Human chorionic gonadotropin--subunit β (β-hCG) has been so far most extensively studied antigen for immuno-contraceptive properties. Studies of vaccination of non-human primates has been so far controversially reported since some antisera obtained from baboons immunized with β-hCG had shown cross-reactivity to other tissues. Despite these problems and concerns about the complete safety of this first generation vaccine, a decision has been made to proceed with a limited clinical trials with this vaccine. Results from these trials are encouraging and indicate that contraceptive antibody titers can be achieved with permissible adjuvants. Neutralization of LH and/or FSH by circulating antibodies may impede their action and interfere with the maturation of gametes. Gonadotropin releasing hormone can be also a suitable target for immunological attack. The human reproductive process entails numerous possible sites for immunological intervention aimed at controlling fertility. Currently under investigation are a vaccine based on gonadotropin releasing hormone (GnRH) interception, regulation of male fertility by immunointerception of follicle-stimulating hormone (FSH), vaccines based on the neutralization of human chorionic gonadotropin (hCG), and anti-hCG immunization. In Phase I clinical trials, passive transfer of anti-GnRH antibody has produced short-term infertility; long-term infertility has been achieved through active immunization against the decapeptide. In terms of FSH, the research has failed to demonstrate how long-term neutralization brings about male infertility. Before immunization against FSh is considered further as a means of immunocontraception, further studies are needed on the required levels of physiological FSH and testosterone for normal testicular functions to be maintained. 4 other Phase I clinical trials in this area involve vaccines based on the β subunit of hCG or its fragment. Preliminary results from these trials suggest that contraceptive antibody titers can be obtained with permissible adjuvants, especially tetanus toxoid.