Leaders of AIDS advocacy organizations joined researchers in expressing both optimism and caution over a study released last week showing that the use of a daily anti-retroviral pill significantly lowered the chance of becoming infected by HIV.

The study found that a sample of mostly gay men who were assigned to take a daily tablet of the widely used HIV medication Truvada experienced an average of 43.8 percent fewer HIV infections than participants who received a placebo pill.

Findings of the study were published Nov. 23 in the New England Journal of Medicine.

The study, which began in 2007 and included a total of 2,499 participants in the U.S., Brazil, Ecuador, Peru, South Africa and Thailand, was funded by the National Institutes of Health and the Bill & Melinda Gates Foundation. It was coordinated by the Gladstone Institutes, an arm of the University of California at San Francisco.

Participants were limited to mostly men who have sex with men and a smaller number of transgender women who have sex with men. Researchers conducting the study said the sample was chosen to represent a population group considered at high risk for contracting HIV.

“All study participants received a comprehensive package of prevention services designed to reduce their risk of HIV infection throughout the trial, including HIV testing, intensive safer sex counseling, condoms and treatment and care for sexually transmitted infections,” the Gladstone Institute said in a statement.

NIH official Dr. Anthony Fauci triggered a wave of optimism over the study findings when he told the New York Times last week that the Truvada pill was more than 90 percent effective in preventing HIV infections among a smaller number of study participants who faithfully adhered to the prescribed daily regimen.

The 43.8 percent reduction figure for HIV infections was based on findings from all 1,251 participants who were given the Truvada pill, including many who did not take the daily pill consistently, according to a detailed summary of the study released by the Gladstone Institutes.

The summary says 1,248 participants received a placebo pill as part of what Gladstone officials called a “double blind” study, in which both participants and employees dispensing the pills did not know who received the drug or the placebo.

In addition to interviewing participants to obtain their own accounts of whether they took the pills every day, researchers conducting the study administered regular blood tests of all participants and were able to confirm which ones took the Truvada pill as directed.

Dr. Kenneth Mayor, medical research director of the Fenway Institute in Boston, an HIV treatment facility that was one of two U.S. sites involved in the study, called the findings “a tremendous step forward” in the quest to lower the rate of HIV infection.

“For more than 15 years, the rate of new infections in the U.S. has been stuck at about 56,000 per year,” Mayor said in a statement. “There has been no downward movement in this number, which has meant that we desperately needed a new approach.

“It’s now possible that within the next few years, we can significantly reduce the number of new infections,” he said.

Other AIDS specialists, including Dr. Ray Martins, medical director of D.C.’s Whitman-Walker Clinic, expressed caution over the implications of the study and the widespread use of a prevention pill approach that researchers call “pre-exposure prophylaxis” or “PreP.”

Martins said the average reduction rate in the study for HIV infections of fewer than 44 percent was lower than he expected and shows a tendency of non-compliance by a large percentage of participants.

Similar to other AIDS specialists assessing the study, Martins expressed concern that a lack of adherence to a daily pill regimen of Truvada or other anti-retroviral medications could result in strains of the HIV virus that are resistant to Truvada or other HIV drugs.

The study showed that just a few participants developed resistant viral strains; they were believed to have become infected before they joined the study, with their infections undetectable at the time.

Martins called Truvada “the major backbone to almost all our current HIV regimens” for treating people with HIV and AIDS.

“So if we increase resistance to those meds and people start transmitting more resistant virus, this could be very bad for the HIV community as a whole,” he said. “To me, that was the most worrisome thing.”

He said Whitman-Walker has no immediate plans to dispense Truvada as a prevention pill for D.C.-area residents who might request it. However, he said the Clinic does prescribe Truvada to people who come to the Clinic for “post-exposure prophylaxis” – a short-term drug regimen used for people who seek treatment immediately after engaging in unsafe sex.

“I think more studies have to be done before we would recommend this as a kind of generalized use,” he said.

Martins said the Clinic would welcome the chance to participate in another study that might offer pre-exposure prophylaxis for patients at its sexually transmitted disease clinic program, who are considered to be among the highest risk group for HIV.

Longtime AIDS researcher and physician Dr. Joseph Sonnabend of San Francisco, where the other U.S. site for the prevention pill study was located, said the reduction rate for HIV infections was far too low to make a Truvada prevention pill useful for large populations.

“Daily Truvada reduced new HIV infections by only 44 percent,” he said in a statement. “This is useless, so how on earth can this be construed as a triumph?”

He said the far higher reduction rate for study participants who adhered to the daily pill regimen was meaningless in drawing conclusions for widespread use of a prevention pill “in real world conditions.”

Some AIDS advocacy organizations also expressed concern that the availability of an HIV prevention pill could result in less adherence to condom use and other safer sex practices.

Jose Zuniga, president of the International Association of Physicians in AIDS Care, called the study “an important first step in validating biomedical prevention of HIV in adults.” However, he pointed to a number of potential drawbacks to the use of Truvada as a prophylaxis in addition to the concern over viral resistance to the drug.

Possible side effects to the medication include kidney-related problems with the potential of forcing people to stop taking the medication, Zuniga noted. He said the average cost of $10,000 or more per year for the drug, which likely would not be picked up by health insurance, is also a major concern in considering its widespread use for prevention.

“The bottom line message is that condoms, clean syringes and behavioral interventions must remain our first line of defense against HIV transmission,” Zuniga said.

The study’s organizers at the Gladstone Institutes say the findings show participants increased rather than curtailed safer sex practices during their involvement in the study.

“In fact, self-reported HIV risk behavior decreased among participants in both arms of the study and condom use increased,” Gladstone said in its summary statement.

Among the groups expressing optimism over the study’s findings is Project Inform, a San Francisco-based advocacy organization for the development of effective HIV treatment and prevention options.

“A study showing that a daily pill reduces HIV risk in gay men is reason for great hope,” the group said in a statement. “U.S. agencies should waste no time in assessing the ability of pre-exposure prophylaxis to slow the HIV/AIDS epidemic.”

The group’s four-page analysis of the study is available here.