A booster for contraceptive vaccines

Abstract

Because of limitations inherent in the various methods of contraception currently available researchers are hopeful about recently published reports from G. P. Talwar of India about a vaccine which successfully immunized women against human chorionic gonadotropin (hCG). Currently one other anti-hCG clinical trial is underway and other vaccines including some designed to suppress sperm production or disable sperm are being tested in primates. Anti-hCG vaccines are of special interest because the beta-hCG subunit sequence can be attached to a toxoid to induce immunity. In fact a vaccine being tested by the World Health Organization (WHO) is based on only a fragment of the beta subunit but Talwar claims that WHOs strategy is overly cautious. So far safety trials have confirmed Talwars approach. Both of these vaccines however require regular booster shots so researchers are trying to develop a slow-release intermuscular delivery system. The vaccines are also criticized as abortifacients because they are effective only after fertilization. For this reason other researchers are trying to prevent fertilization by coating the sperm or the egg with antibodies to prevent fusion. This research which often targets the zona pellucida which surrounds the egg cell depends upon identification of small peptides that do not provoke an autoimmune reaction. Another approach would be to vaccinate women against sperm. Initial tests of immunization of guinea pigs with a sperm protein (PH-20) have proved successful. So far only a form of the enzyme lactate dehydrogenase has been shown to depress fertility in humans. The efficacy of this contraceptive was only 78% however and the goal is 95% Talwars vaccine is effective in 80% of the women tested and the remaining 20% of the women may be producing a range of major histocompatibility complex (MHC) glycoproteins that fail to bind effectively to the foreign antigens. This situation may be corrected by replacing the toxoid carrier with peptides that bind to a wide range of different MHC glycoproteins. Talwar expects his research to extend until the end of the decade.