A compound that renders sperm immobile has shown promise as a reversible form of contraception in mice.

The choices of birth control for men are limited to using condoms or having a vasectomy. “We definitely need more options,” says at Baylor College of Medicine in Texas.

Now, he and his colleagues have identified a potential new target — a protein expressed by the gene STK33 that is found in high concentrations in the testes of mice and men.

“In the absence of the STK33 protein, mice and human sperm wind up having very abnormal sperm tails, which causes motility defects,” says Matzuk. Studies have also shown that due to having malformed sperm.

To identify a compound that could bind to and inhibit the activity of the STK33 protein, the team combed through a library containing billions of chemicals, looking for a suitable molecule. Eventually, they found a candidate called CDD-2807.

Next, the team injected six male mice with CDD-2807 twice a day for 21 days and another seven male mice once a day for the same amount of time. After those three weeks, they housed all the male mice with females, while continuing the treatment for around another 40 days.

Free newsletter

Sign up to Health Check

Sign up to newsletter

Only one mouse across both groups had impregnated a female by the end of the 40 days. That mouse only sired one pup, whereas mice litters typically have up to six pups, a sign that the treatment still reduced its fertility. No side effects were recorded. Three weeks after stopping the treatment, the researchers found that the mice’s infertility reversed and they were able to reproduce as normal.

Matzuk says they now hope to test an oral version of the treatment in primates.

at Cornell University in New York says the paper is “very solid”, but that a not-yet-approved gel called NES/T, containing synthetic progesterone and testosterone, may work faster than targeting STK33 in people. The same would probably also be true for a contraceptive that , which is involved in sperm production, he says.

A drug that temporarily paralyses sperm could become the first on-demand male birth control pill. In mice, the contraceptive was 100 per cent effective at preventing pregnancy for about 2 hours, with full fertility returning 24 hours later.

“This is, in the male contraceptive field, totally revolutionary,” says at Cornell University in New York. Most other prospective male contraceptives in clinical development are only effective after eight to 12 weeks, he says.

Previous research has shown that sperm require a protein called soluble adenylyl cyclase (sAC) to move, and that men who cannot produce sAC due to rare genetic mutations are infertile. So, Buck and his colleagues assessed whether a drug inhibiting sAC could be used as a male contraceptive. If sperm are immobile, they can’t travel up the vaginal tract to fertilise an egg.

The team assessed the movement of sperm collected from 17 male mice, eight of whom received the drug. In samples collected 2 hours after mice received the drug, only about 6 per cent of sperm were mobile on average compared with about 30 per cent in samples from control mice. The effect wore off after about 24 hours, “which means we not only have an on-demand contraceptive, but one that is also rapidly reversible”, says , also at Cornell University.

In another test, the researchers paired 52 male mice with females 30 minutes after giving the males the contraceptive drug. After 2 hours each pair had mated, but there were no resulting pregnancies, indicating the contraceptive was 100 per cent effective. The drug also didn’t cause noticeable side effects, even when mice received three times the standard dose of a comparable compound continuously for 42 days.

“What I like about the proposed contraceptive in this study is the on-demand option,” says at the KTH Royal Institute of Technology in Sweden. “It would act rapidly, temporarily and is efficient at the first dose.”

Buck and Balbach plan to refine the drug so that it lasts longer before testing it in humans. If all goes well, they hope to begin clinical trials by 2025.

“We need more [birth control] options, and men need an option so that the burden of contraception is not on females anymore,” says Balbach. “We’re very optimistic that once men take the inhibitor, it will have the same effect.”

Sign up to our free Health Check newsletter for a round-up of all the health and fitness news you need to know, every Saturday

Discreet contraceptive

A patch filled with tiny needles can inject up to 60 days’ worth of hormonal contraceptives. Women could apply the patch at home, making it a potentially hassle and pain-free alternative to injections or implants.

Mark Prausnitz at the Georgia Institute of Technology and his colleagues created the patch, which contains 100 microneedles, each around half a millimetre long and 0.01 millimetres wide at the tip. When the patch is applied, the needles come off the patch, break the skin and sit under the surface.

Each needle contains a small dose of the hormonal contraceptive levonorgestrel, which is slowly released as the microneedle degrades over a period of up to 60 days.

Normally, long-acting contraceptives need to be administered by professionals, but this could be self-administered, says Prausnitz. The patch could be used once a month, rather than every day like with the pill.

The team tested the patch on rats and found that more than 90 per cent of the microneedles detached after 50 seconds on average.

To see how well the patch would work with people, the team tested it on 10 women using a placebo patch that didn’t contain any levonorgestrel. The microneedles detached from the patch to enter the body just as well as they did in rats.

All the women said they would opt for the patch over a monthly injection of hormones, while only one would stick with a daily pill.

Chenjie Xu at Nanyang Technological University, Singapore, says the next step should be to test a version of the patch with a hormonal contraceptive in people. It is important to know if the work in rats translates to humans, he says.