Monkeys given the vaccine produced sperm that could not fertilize eggs.© SPL

A male contraceptive jab has edged closer to reality, after a study showed that monkeys are rendered infertile by a vaccine aimed at their sperm.

In contrast to the wide range of contraceptives on hand for women, men wanting to curb their fertility can choose between only condoms or a vasectomy. As part of an effort to extend these options, researchers have been struggling for more than 20 years to work out how to vaccinate men temporarily so that the immune system hobbles their sperm.

Now Michael O'Rand of the University of North Carolina at Chapel Hill and his colleagues have met with some success. They injected nine monkeys every three weeks with a human protein called Eppin, which sits on the outer coat of sperm.

After the immunization, seven monkeys manufactured copious antibodies against the Eppin protein, and all of these monkeys proved infertile when mated with females. Two-thirds of a group injected with a placebo vaccine went on to father offspring.

The authors suggest that the antibodies bind to Eppin proteins and prevent sperm from working normally once they are ejaculated. They say that the approach might be adapted as a form of contraception for men. "I'm optimistic," says O'Rand.

"It's more positive data than I've ever seen, although there are many unanswered questions," comments Bernard Robaire, a specialist in male reproduction at McGill University in Montreal, Canada. It is still unclear, for example, whether the vaccine is fully reversible. Only five out of seven monkeys recovered their fertility once the injections were stopped.

Beyond condoms

Until now, attempts to create a vaccine against sperm have "universally failed", Robaire says. One problem is that antibodies against sperm proteins can affect other cells in the testes and cause inflammation. In the new study, published in Science¹, it is thought that the antibody is latching on to sperm further down the male reproductive tract, leaving the testes unharmed.

A second difficulty is that a male contraceptive has to be effectively 100% perfect in order to stop any sperm from escaping. This means that the body must make enough antibody to cripple all of the hundreds of millions of sperm in each ejaculate.

Most research into male contraceptives has therefore focused on shifting levels of male hormones to shut off sperm production completely; the pharmaceutical companies Schering and Organon are trialling a hormonal skin implant and injection. But hormones act all over the body and the treatments can therefore have unwanted side-effects.

A vaccine contraceptive might be advantageous if it takes less time than hormonal treatments to kick in, and leaves sperm production intact. "We need lots of things on the shelf," says reproductive biologist Barry Zirkin of Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. "There is no one-size-fits-all." 

• References

  1. O'Rand M. G., et al. Science, 306. 1189 - 1190 (2004). | Article |