A controversial Cambodian trial of tenofovir for pre-exposure prophylaxis (PREP) is unlikely to proceed after sex workers and the Cambodian government claimed the trial did not include adequate safeguards for the health and human rights of the participants.

The trial, co-sponsored by the US National Institute of Allergies and Infectious Diseases, the University of NSW and the Bill and Melinda Gates Foundation, was to have investigated whether the antiviral drug tenofovir, made by Gilead Sciences, could protect HIV-negative Cambodian sex workers from HIV infection.

Researchers had hoped that the trial results might have supported the use of tenofovir as a means of preventing infection in high-risk populations. Parallel trials in Africa are still going ahead, as is a study among highly sexually active gay men in the US.

Following protests by Cambodian sex workers at the International AIDS Conference in Bangkok in July, the Cambodian Prime Minister, Hun Sen, ordered a halt to the study. The sex workers were concerned that there had been inadequate provision made for their health, and had argued that participants should be provided with a 40-year insurance coverage for any unforeseen side effects of the treatment.

On August 3, Hun Sen said the researchers should do their research on animals, not Cambodians. In mid-August, the Cambodian health minister said the trial had been stopped.

Animal studies have been done which showed that tenofovir provided strong protection from infection in monkeys. The drug is also in widespread use among people with HIV.

With no likelihood of an HIV vaccine becoming available within the next few years, the study’s authors had hoped the research could have provided a stop-gap measure for those people at greatest risk of infection. Cambodia has one of the highest HIV infection rates in south-east Asia, an epidemic which is being fuelled by a booming sex industry.

The planned 960 participants in the trial would have received once-daily tenofovir or a placebo, with all participants being offered tenofovir for a two-year period if it was shown to be effective. Participants who contracted HIV were also to be given access to treatment.

The cancellation of the study raises many questions about the standard of care needed for similar studies, and will be the subject of a debate at the ASHM conference in Canberra in September.