Current male contraceptive options are slim. Withdrawal is a bad idea and shouldn't be relied on, and condoms have their flaws. A vasectomy is fine if you're done having kids, but it's essentially a permanent procedure and is difficult to reverse if you change your mind. A new contraceptive called Vasalgel could offer a long-lasting, reversible and convenient option. Tests on rhesus monkeys have now shown that it can effectively prevent pregnancy for over a year.

Research is still underway into male contraceptives that are long-acting and reversible. Sperm counts could be safely lowered using ultrasound or molecular compounds, or the little swimmers could be rendered inert by disrupting their response to female sex hormones.

None of these are commercially available yet, but Vasalgel may be one of the closest off the blocks. The technique involves injecting a polymer into the vas deferens, the tube that carries sperm out of the testicles. Once injected, this polymer, made of styrene-alt-maleic acid (SMA) dissolved in dimethyl sulfoxide, then forms a hydrogel that blocks sperm from passing through, although not completely: fluids can still make their way through the gel very slowly, which could prevent back-pressure complications that vasectomies can induce.

Blocking the passage of sperm is a different approach to that of Vasalgel's past. Back in 2011, when it was known as RISUG, the polymer was designed to allow sperm through but chemically incapacitate them on the way past, rendering them infertile.

To test out Vasalgel's sperm-blocking strength, the researchers injected the polymer into 16 adult male rhesus monkeys. After letting them recover for a week, each one rejoined a social group that included between three and nine breeding females, and the researchers let nature run its course. At a minimum, all the males were monitored for one breeding season, and almost half of them were studied over two. The Vasalgel didn't impede the males' desire to mate, but during that time none of the female monkeys fell pregnant.

Minor complications did occur. In one case, the team misplaced the Vasalgel into the vas deferens, and another monkey developed a sperm granuloma, a hardened leakage of sperm from the tube which can also occur as a result of vasectomy.

Since the technique proved successful overall in the monkey trial, as well as in previous tests in rabbits, the team are now preparing to begin human clinical trials of Vasalgel. Effectiveness will be tested first, but a longer term goal is to make the process reversible by flushing the obstruction out – something that's so far only been demonstrated in rabbits.

If it becomes a viable method of contraception, humans won't be the only ones to benefit from Vasalgel. Considering it works so well in monkeys, the technique could find use in zoos and breeding programs, where population control is important and vasectomy is currently the only male option – and it's not as easy as it is in humans.

"It is ideal to house monkeys together for their social welfare, but we also need to consider the fertility of these animals, which is typically high, and be able to make responsible decisions on population size," says Angela Colagross-Schouten, lead veterinarian on the project. "While vasectomy is a quick and relatively simple procedure in humans, in monkeys there can be additional complications, as it is inherently more complex. We were impressed that this alternative worked in every single monkey, even though this was our first time trying it. Hopefully, Vasalgel placement can be an option for other captive colonies, including zoos, that want to manage reproductive rates while allowing for social housing."

The research was published in the journal Basic and Clinical Andrology.

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