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An injectable male contraceptive that blocks sperm flow with a gel has been successful in monkey trials, scientists said today, bringing the prospect of an alternative form of birth control for humans closer.

The contraceptive called Vasalgel provided effective birth control in rhesus monkey groups for more than one year, according to researchers from California National Primate Research Center in the US.

With proof of efficacy in monkeys and rabbits, preparations are being made for the first clinical trial in humans, researchers said.

Male contraceptive options have not changed in over a century, and are currently limited to condoms and withdrawal (with high pregnancy rates in typical use), or vasectomy (meant to be permanent), they said.

The trial used Vasalgel in groups of rhesus macaques – confirming previous preclinical findings in rabbits on the efficacy of the new device and offering a new tool to colony managers.

“Vasalgel is a high molecular weight polymer that consists of styrene-alt-maleic acid (SMA) dissolved in dimethyl sulfoxide and could be the first long-acting, non-hormonal, potentially reversible male contraceptive to reach market,” the company behind the gel said.

The polymer forms a hydrogel after injection into the vas deferens, creating a blockage to the passage of sperm.

It is thought that fluids are able to pass slowly through the gel, reducing back-pressure on the epididymis (the sperm storage area) that has been noted after vasectomy.

The contraceptive effect of Vasalgel has been “reversed” in a rabbit by flushing the material out with a simple sodium bicarbonate solution, researchers said.

The purpose of the current study was to put Vasalgel to the ultimate test – preventing pregnancy, not just eliminating sperm – in larger animals more anatomically similar to humans, before human use. However, the contraceptive had benefits to the monkeys as well, they said.

Sixteen adult male rhesus monkeys received intravas injections of Vasalgel. After a one-week recovery, each male was eturned to outdoor group housing, which included three to nine intact, breeding females with a successful reproductive history.

The monkeys lived in social groups in a setting that is closer to a free-living environment than traditional research housing, following and exceeding European guidelines for space per monkey.

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