By Ray Dunne
BBC News Online health staff
Does a drug that can protect against HIV already exist?
It is a question being asked by a growing number of scientists and doctors.
Tenofovir is being tested in three trials around the world
Tenofovir has been given to people with HIV since 2001. It is now routinely used in combination with other drugs to help people with the virus live longer.
In 1995, Gilead, the company behind the drug, tested it on monkeys. They found it protected healthy monkeys against the virus.
At the time, the company was concentrating its efforts on developing Tenofovir as a treatment for people who were already infected with HIV.
"There was a great need for anti-retroviral agents at the time," says James Rooney, its vice-president of clinical research.
"Five or six years ago, people were also much more hopeful of finding a vaccine.
"If a vaccine had become available then the need for this kind of approach would not have been as apparent."
The failure to find a vaccine has prompted calls for scientists to try something different.
Some doctors have pressed for a full-scale trial to see if Tenofovir can protect humans against HIV. Three such trials are now underway.
The Bill & Melinda Gates Foundation is giving $6.5m to one which will examine if a daily dose of Tenofovir can protect people in developing countries against the virus.
The trial, which is being spearheaded by Family Health International, will involve 2,000 volunteers in Cambodia, Ghana, Cameroon, Nigeria and Malawi.
The US government is funding two other trials. Its National Institute of Health is spending $2.1m to see if the drug can stop 960 Cambodian women, most of them sex workers, from contracting the virus.
The US Centers for Disease Control is spending a further $3.5m to see if it can prevent 400 gay and bisexual men in San Francisco and Atlanta from becoming HIV positive.
HIV infecting an immune cell
These are all phase II trials, which means they will test the safety of the drug.
"This drug has been shown to be safe for people who have HIV," says Dr Ward Cates of Family Health International (FHI).
"What we want to see is how safe it is for people who are not infected."
The FHI trial will run for two years initially. It will be extended if there are signs that the drug can offer protection.
"We will not know until the end of two years what the effect is," says Dr Cates.
"There is always the hope that it will offer protection. Otherwise we wouldn't bring it to this stage."
Tenofovir can slow down the spread of HIV in people who are already infected. It blocks a key protein in the virus, which enables it to infect and take over a healthy cell.
The hope is this action can be replicated in people who are newly infected with HIV before the virus takes hold - a so-called pre-exposure prophylaxis or PREP.
"There is plenty of science suggesting you can prevent diseases with agents used to treat diseases," says Dr Mike Youle, director of HIV research at London's Royal Free Hospital.
"We do it with malaria and we do it with other branches of medicine.
"Clearly, there needs to be adequate clinical trials prior to any suggestion that there could be routine provision of this drug."
Tenofovir is regarded as the best candidate for PREP around at the moment.
"It has been shown to be very effective in animals, in different species of monkeys and against different strains of the monkey Aids virus," says James Rooney.
It also has fewer side-effects and patients develop resistance to it more slowly compared with other anti-HIV drugs.
"It is one of our newer anti-retrovirals," says Dr Cates. "It has a good safety profile and a low resistance profile."
"It is the best candidate that is already licensed," says Dr Youle.
The decision to start trialling a drug like Tenofovir reflects growing disillusionment within the scientific community with efforts to find a cure or a vaccine for HIV.
A major vaccine trial ended in failure last year when scientists found no evidence to suggest it worked.
Another trial, involving 16,000 people in Thailand, has been slammed by experts as a waste of time. Scientists say there is no evidence to suggest it will work.
"There is increasing disillusionment with the development of vaccines," says Dr Youle.
"We have been getting nowhere. That's why we need to get back to first principles. This trial of Tenofovir provides us with an additional route."
All of these trials are at very early stages and it will be a number of years before there will be any indication if this drug or other anti-retrovirals could protect people against the disease.
But if it works, the prize could be immense. It could transform the lives of millions of people around the world.
"If this is found to be effective, it will be to HIV what oral contraception was to pregnancy," says Dr Cates.
"It will make it much more easier to protect against HIV."