(10-07) 09:49 PDT SYDNEY, Australia (AP) --
A practical male contraceptive could be a step closer after researchers said Tuesday that trials prevented pregnancies among 55 couples using it for 12 months.
The treatment stops sperm production by using implants of the male hormone testosterone under the skin and injections of the female hormone, progestin.
Experts said the results were a significant step forward that builds on many years of related research.
Previous studies have shown that a combination of testosterone and progestin can switch off sperm production in men. However, the latest study is the first to measure the effect on preventing pregnancy -- the ultimate goal.
The U.S. government-funded trial, sponsored by Virginia-based family planning organization CONRAD, successfully and reversibly turned off sperm production in the men who took part, said Rob McLachlan, director of clinical research at Prince Henry's Institute of Medical Research in Sydney, Australia.
CONRAD often funds medical studies outside the United States and works closely with the World Health Organization, which is interested in finding new contraceptive methods for the developing world.
"This is the first time the combination of long-acting testosterone and a progestin have been used in an effectiveness study," McLachlan said in a telephone interview.
The idea of using hormone treatment to turn off sperm production has been tested for about 30 years, but much of the earlier work focused on using testosterone alone.
About a decade ago, two World Health Organization studies demonstrated that sperm counts could be reversibly suppressed and pregnancies avoided using testosterone injections.
The idea is similar to that of the female contraceptive pill, which tricks the body into thinking it is pregnant. The testosterone injections trick men's bodies into thinking their testes have produced enough testosterone already, so the body switches off testosterone and sperm production in the testes.
Injections of too much testosterone can create problems, however, so much of the latest research has used a combination of testosterone and progestin to lessen the amount of testosterone needed, said Dr. Richard Anderson, a clinical scientist at the Human Reproduction Unit at Edinburgh University in Scotland who conducts similar research.
Progestin, which is found in men's bodies but only in very small amounts, does most of the sperm suppression; used alone, it would be equivalent to chemical castration, Anderson said.
"You have to add back a normal amount of testosterone to make the guys feel OK," said Anderson, who was not connected with the Australian study.
Tony Morrow, an endocrinologist at Sydney's Mona Vale hospital who also was not linked to the research, said that if the treatment ever becomes available, it might not be popular with some men "because it involves injections and testosterone implants."
The 55 couples in the Australian study made an 18-month commitment, McLachlan said.
In the study's first phase, men took testosterone and progestin for up to six months until their sperm count reached zero. Then the men had testosterone implants placed under their skin in minor surgeries and received progestin injections for the next 12 months, to keep their sperm suppressed.
The trial was unusual because it tested real-life birth control effectiveness, McLachlan said. "Once the sperm was suppressed, the couples used this as their (contraceptive) method. Other studies only monitor how far the sperm count falls."
All the male participants' sperm counts returned to their pretrial levels in six to 12 months after the trial, McLachlan said.
He said the contraceptive's cancer risk for men would likely be similar to that of women using traditional methods, such as the pill or contraceptive implants.
"There's no evidence for this problem at this point of time. Cardiovascular and prostate health are long-term safety issues that we'll need to keep a close eye on, but so far so good," he said.
Morrow said more research with larger groups was necessary to evaluate other possible side effects, such as sleep disorders and breathing difficulties.
McLachlan said it could take "several years" of more and longer tests before the contraceptive would be available.
The study will be published in the October edition of Journal of Clinical Endocrinology and Metabolism.
