A gel squirted into the sperm ducts of monkeys has been effective at preventing pregnancy, said a study Tuesday which offered hope of a solution for men reluctant to go under the knife for family planning.
While several birth control options exist for women, the race is on for a non-surgical, long-term and reversible male contraceptive without the side effects of hormonal changes.
The only short-term solutions available today are condoms, which many people complain interfere with sex, and withdrawal before ejaculation, which comes with a high risk of pregnancy.
Longer term, the sole option is a vasectomy, which involves tying or cutting the sperm-conducting tubes called vas deferens. This prevents sperm from mixing with seminal fluid ejaculated during sex.
Vasectomies can be reversed in some cases, but the procedure is technically challenging and leads to low rates of fertility.
Researchers in the United States are developing a possible alternative, dubbed Vasalgel, which has proven effective in rabbits and now also in rhesus monkeys - more closely related and anatomically similar to humans.
Vasalgel is a polymer gel injected directly into the vas deferens, creating a blockage in the tube that transports sperm from the testes out through the penis.
In an experiment at the California National Primate Research Center, 16 adult male monkeys were treated. They were housed with females, and monitored for up to two years - covering at least one breeding season per animal.
"Treated males have had no conceptions since Vasalgel injections," the research team wrote in the journal Basic and Clinical Andrology.
Normally, the expected pregnancy rate among females housed with males would have been about 80 per cent.
"The presence of Vasalgel appears to be well tolerated and placement resulted in minimal complications," the researchers wrote.
One monkey of the 16 had symptoms of sperm granuloma, a buildup in the vas deferens which is a common complication in about 60 per cent of human vasectomies, they added.
Not yet tested in monkeys, the reversibility of the method was tested in earlier experiments in rabbits, when the gel was successfully flushed out with solution of sodium bicarbonate.
Preparations are underway for a clinical trial with Vasalgel in humans, said the Parsemus Foundation, a non-profit organisation funding the product's development.
The research has benefits for the monkeys as well, researchers added.
It is ideal to house captive rhesus monkeys in groups for their social welfare, but populations can quickly explode due to high fertility.
And vasectomy in monkeys is more complex than in humans, with many complications.
"We were impressed that this alternative worked in every single monkey, even though this was our first time trying it," said Angela Colagross-Schouten, the project's lead veterinarian.