Contraceptive vaccine development

Recent advances in antigen definition and production have made the development of a contraceptive vaccine more attainable. Such a vaccine must evoke an immune response that blocks an indispensable step in the reproductive process. Vaccine research involves many approaches to fertility prevention. Vaccines are being developed that could interrupt fertility by inhibition of gonadotrophin release, the function of follicle-stimulating hormone or the effects of human chorionic gonadotrophin (hCG); alternatively, they may prevent fertilization by interfering with the transport of spermatozoa or with sperm-zona pellucida binding. The most advanced prototype is a vaccine based on antibodies to beta hCG. Such vaccines are being studied for clinical efficacy. Many hurdles remain in contraceptive vaccine development. Since the antigens are peptides or small proteins, the resultant immune response is usually moderate, and better adjuvants and delivery systems must be developed to enhance and maintain the immune response. Improvement of the mucosal immune response may be necessary for vaccines incorporating sperm antigens. Research on vaccines that control fertility has resulted in a fascinating base of scientific knowledge that, it is hoped, can be converted into products that will allow another option for individuals who wish to control their fertility.

A birth control vaccine is on the horizon for family planning

Vaccines for control of fertility are likely to have an important impact on family planning methods. They are designed to act by mobilization of an internal physiological process and do not require external medication on a continuous basis. A number of birth control vaccines are at different stages of development, the most advanced being a vaccine inducing antibodies against human chorionic gonadotrophin (hCG). This vaccine consists of a heterospecies dimer (HSD, beta hCG associated with alpha-subunit of ovine luteinizing hormone, beta hCG:alpha oLH) linked to tetanus toxoid (TT) or diphtheria toxoid (DT) as carriers. The vaccine has recently passed an important milestone; it has completed the first leg of phase II efficacy trials. Women of proven fertility leading active sexual life were protected from becoming pregnant at antibody titres > or = 50 ng of hCG bioneutralization capacity per ml. This vaccine has previously been demonstrated to be reversible in its effect. It is free from any notable side-effects on endocrine, cardiovascular and other body functions. Ovulation was not disturbed and menstrual regularity was maintained. A logistic disadvantage of the present vaccine is the requirement for multiple injections. This is expected to be overcome by encapsulation of the requisite doses of the vaccine in biodegradable microspheres, which could be given at a single contact point for sustained antibody titres lasting over a year. A live recombinant vaccine has also been made that elicits high anti-hCG titres in monkeys for nearly 2 years following primary immunization and a booster at 8-9 months.

Anti-sperm and anti-ovum vaccines: the selection of candidate antigens and the outcome of preclinical studies

A clear conceptual basis for anti-sperm and other modes of immunological birth control exists in spite of the limited basic research done on surface antigens of sperm or egg. In the normal physiological condition, the immune system does not respond to sperm, egg or fetus. Compared with anti-egg or anti-fetus immunocontraception, an anti-sperm vaccine has two theoretical advantages. First it would work in both males and females; second, it would not raise problems of autoimmunity in the female if a sperm-specific protein is used. However, for the development of such contraceptive vaccines the following scientific questions need to be resolved: (1) what surface protein to use as an antigen; (2) what adjuvant to use in humans for an anti-fertility vaccine; (3) how to maintain a high titre; (4) how the contraceptive effect will be terminated; and (5) what are the potential side-effects.

Anti-trophectoderm vaccines: rationale and methods used for antigen identification and selection

Trophoblast, forming a continuous interface at the maternal-fetal junction, is of considerable interest to biologists. An area of recent research is the search for trophoblast-specific antigens. It is believed that identification and characterization of these antigens may have many practical applications such as in development of contragestational vaccines. To be acceptable for humans such a vaccine will need to be effective before the completion of implantation and appearance of the primitive streak about 14 days after fertilization. This will not alter the menstrual cycle or the time of menses. Vaccines having a later effect resulting in termination of pregnancy after this time would be considered abortifacients. Although logistical and ethical considerations necessitated the use of post-implantation human trophoblast in WHO Birth Control Vaccine studies, the antigens isolated from such tissues would need to be expressed and be detectable on the surface of pre-implantation trophectoderm if they are to represent appropriate candidates for vaccine development. The criteria for identification and selection of these antigens will be reviewed.

Anti-LHRH and anti-pituitary gonadotropin vaccines: their development and clinical applications

Active immunization against hormones involved in the regulation of reproduction is a promising approach to immunocontraception. The hypothalamic peptide, LHRH, controls the synthesis and release of the pituitary gonadotropins, LH and FSH, which regulate gonadal steroidogenesis, sperm production, follicular development and ovulation. Immunizing female primates against LHRH or LH induces infertility, but also disrupts the menstrual cycle. Immunization against the beta subunit of the placental hormone, chorionic gonadotropin (hCG), or its fragment prevents pregnancy without interfering with menstrual cycles or ovulation. hCG vaccines have reached the stage of clinical trials. FSH and LHRH have been tested for immunocontraception in male primates. While active as well as passive immunization against FSH reduced spermatogenesis severely, azoospermia could not be achieved consistently. Immunization against LHRH effectively suppressed spermatogenesis in rats and rabbits. Normal sexual behaviour was maintained by concomitant androgen administration. Fertility was restored when antibody titres declined and no adverse effects were observed. A number of LHRH vaccine preparations are being tested in men in several countries, including the United States. Since the LHRH vaccine reduces serum testosterone levels the first clinical studies involve men with prostate cancer. These trials will be followed by immunization of normal men if the antibody response is sufficient and no adverse effects are observed.

Anti-hCG vaccines are in clinical trials

Two vaccines inducing antibodies against human chorionic gonadotropin (hCG) have completed Phase I clinical trials, indicating the reversibility and safety of these vaccines. One is currently in Phase II efficacy trials in women in three major centres in India. The available data suggest that the vaccine prevents pregnancy above antibody titres of 50 ng/ml hCG bioneutralization capacity.

Options for immunocontraception and issues to be addressed in the development of birth control vaccines

Although the number of family planning methods currently available to couples has never been greater, the range and type of options are still not adequate to meet the widely varying personal needs and demands of individuals worldwide. Birth control vaccines offer a number of theoretical attractions although the development, preclinical and clinical testing of such vaccines pose a number of unique problems requiring novel solutions. If the on-going studies in this area are successful, a valuable new family planning method may be available by the end of the current decade.

Vaccines against fertility

The first evidence for the efficacy of a birth control vaccine in humans is now available from the Phase II trials on the human chorionic gonadotrophin vaccine in India. Several sperm antigens have been identified as potential contraceptive immunogens and zona pellucida antigens have been reported that reversibly control fertility.

Vaccines for control of fertility and hormone dependent cancers

Two vaccines, namely one inducing antibodies against hCG and the other against GnRH, are now in clinical trials. The hCG vaccine has entered Phase II clinical trials in three centres in India after successfully completing Phase I clinical studies in several centres in India and in four countries abroad. The vaccine was found to be devoid of side-effects; its effect was reversible. The available data on 179 cycles indicate that the vaccine prevents pregnancy at antibody titres above 50 ng/ml. A genetically engineered version of the vaccine has also been approved for trials in human lung cancer patients of the type which make hCG. hCG is observed to be a growth factor for such tumours. The GnRH vaccine is usable in both males and females as the deca-peptide is common to both sexes. Following suitable experimental and toxicology studies, the vaccine is currently in Phase I/Phase II clinical trials in patients of prostate carcinoma. Where antibody GnRH antibodies were induced, the LH, FSH and testosterone levels declined. This was accompanied by a reduction in prostate specific antigen. Clinical improvement was observed in many cases. The vaccine has also entered Phase I clinical studies in postpartum women, with the objective to extend the lactational amenorrhoea and extend inter-child interval.

The WHO Task Force on Vaccines for Fertility Regulation. Its formation, objectives and research activities

Over the past 18 years, the WHO Task Force on Vaccines for Fertility Regulation has been supporting basic and clinical research on the development of birth control vaccines directed against the gametes or the preimplantation embryo. These studies have involved the use of advanced procedures in peptide chemistry, hybridoma technology and molecular genetics as well as the evaluation of a number of novel approaches in general vaccinology. As a result of this international, collaborative effort, a prototype anti-HCG vaccine is now undergoing clinical testing, raising the prospect that a totally new family planning method may be available before the end of the current decade.

Phase I clinical trial of a World Health Organisation birth control vaccine

A birth control vaccine incorporating a synthetic peptide antigen representing the aminoacid sequence 109-145 of the C-terminal region of the beta subunit of human chorionic gonadotropin (hCG-beta) was submitted to a phase 1 clinical trial. Thirty surgically sterilised female volunteers, divided into five equal groups for different vaccine doses, received two intramuscular injections six weeks apart. Over a six-month follow-up there were no important adverse reactions, and potentially contraceptive levels of antibodies to hCG developed in all subjects. In the highest vaccine dose group, the results gave promise of a contraceptive effect of six months' duration.