Male Birth Control Drug ‘Stops Sperm in Their Tracks’ in Study of Mice

The drug served as a quick-acting, temporary contraceptive with no noticeable side effects, researchers say

A sperm cell under a microscope
The experimental treatment inhibits an enzyme that is needed for sperm to move. BSIP / UIG via Getty Images

Scientists are testing an experimental birth control that can stop sperm from swimming. Male mice injected with the drug quickly and temporarily became infertile, researchers reported Tuesday in the journal Nature.

The findings are proof of concept that a birth control pill for men taken before sex might be within reach, the paper’s authors write.

“This could be a game changer for a male contraceptive,” Wipawee “Joy” Winuthayanon, a reproductive health expert at the University of Missouri who did not contribute to the research, tells the Washington Post’s Carolyn Y. Johnson.

While several methods of birth control exist for women, men are offered only two: condoms and vasectomies. Researchers are currently investigating a variety of possible birth control mechanisms for men, but many are hormone-based, requiring months of treatment before they work. And after patients stop taking these treatments, months can pass before their fertility returns, per the paper.

The drug used in the new study is an experimental treatment for a rare eye disease. But it also inhibits a regulatory enzyme called soluble adenylyl cyclase (sAC), which sperm rely on to move. Men who cannot produce sAC are infertile, according to New Scientist’s Grace Wade. After all, sperm must be able to move to swim into the fallopian tube and reach an egg. By blocking sAC, the drug can make sperm immobile almost immediately.

“It’s pretty clear that this is an on-off switch for sperm,” Lonny Levin, a co-author of the study who is also a pharmacologist at Cornell University, tells Wired’s Emily Mullin. “We thought inhibiting this would be a great way to stop sperm in their tracks, prevent them from ever leaving the vagina and getting to the promised land to fertilize an egg.”

In the study, male mice received an injection of the drug, then mated with females within 2.5 hours after treatment. Among 52 pairings, not a single female mouse became pregnant. In a control group, the male mice impregnated females 30 percent of the time.

Over longer time frames, the treatment’s effectiveness waned. One of 45 male mice that received the drug impregnated a female when they mated between 30 minutes and 3.5 hours after the injection. Between one and nine hours after injection, five pregnancies resulted from 55 matings.

After 24 hours, the effects wore off, and the injected mice’s sperm moved normally again.

This means “we not only have an on-demand contraceptive, but one that is also rapidly reversible,” Melanie Balbach, lead author of the study and a pharmacologist at Cornell University, tells New Scientist.

2019 case study of two men without sAC enzymes found the only major health issue they had was an increased risk for kidney stones—so the researchers predicted the drug likely wouldn’t cause major health problems, per the Washington Post. Studies on some other forms of male contraceptives have been stopped because of unwanted side effects, which are similar to those caused by birth control methods that are available to women, notes Forbes’ Robert Hart.

To test the drug’s safety, the team used a pump to continuously infuse male and female mice with it for six weeks, per Wired. They didn’t notice any side effects and will now test the drug on rabbits.

The researchers hope to begin clinical trials in humans by 2025, per New Scientist. They’d like a future version of the drug to prevent pregnancy for 12 hours and be available as a pill, per the Washington Post.

The findings come at a time of heightened interest in contraceptives for men after the Supreme Court overturned Roe v. Wade last June.

“This has been a major step forward in the field,” Gunda Georg, a medicinal chemist at the University of Minnesota who didn’t contribute to the study, says to the Wall Street Journal’s Nidhi Subbaraman.

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