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(WB) For the past several years, PrEP, the powerful Pre-Exposure Prophylaxis regimen credited with preventing the spread of HIV, has been championed as a silver bullet that allows men who have sex with men (MSM) to avoid seroconverting while still engaging in high-risk sexual activities.

But now, as three more cases of seroconversion of those on PrEP have come to light, the community is concerned that a drug-resistant strain of HIV is preventing Truvada from doing its job.

The Melbourne, Australia publication The Age reports that a local man tested positive for HIV at a Melbourne clinic while participating in a trial of PrEP.

On Monday, a spokesman at The Alfred confirmed researchers were "reviewing the clinical details of a man who has tested positive for HIV while being a registered participant of the Victorian PrEPX study."

Doctors are quick to note that hundreds of thousands of people use PrEP across the world successfully, with only two confirmed cases of a drug-resistant strain of the virus arising among those taking the medication correctly --- one in Toronto and one in New York, both in 2016. This is the first case of HIV resistance to PrEP in Australia.

Doherty Institute director and infectious diseases physician, University of Melbourne Professor Sharon Lewin said all people who start HIV treatment have a test for their own resistance to the drug before they begin the medication.

She said once researchers at The Alfred have determined whether the man had been correctly and consistently taking PrEP, the HIV virus itself will be analyzed for resistance.

"In cases of someone testing HIV positive while taking PrEP, we would first take a detailed history to understand how PrEP is being taken and potential exposures," she said, adding, "I don't think there is any reason for alarm here. PrEP works and any rare cases of failure need to be investigated fully so we fully understand why this can very rarely happen."

"We would then look at the genetic code of the virus as this can tell us whether it is a drug-resistant strain, or a non-resistant strain. We would also look quite closely at drug levels in blood and the immune response to the virus, when the anti-body test became positive."

An Amsterdam man's PrEP failed earlier this year but is still being investigated. POZ Magazine reports that this Dutch man's case has puzzled experts, as he has contracted a non-drug-resistant strain of HIV while adhering well to his PrEP regimen. 

Researchers can only speculate on how he contracted HIV but theorize that his "remarkably high" number of partners and condomless sex may have contributed. He also reported using recreational drugs during sex, including crystal meth, cocaine, GHB, mephedrone, and ketamine.

However, the 50-year-old MSM began taking PrEP as part of the Amsterdam PrEP study AMPrEP and tested HIV-negative upon enrollment. He was adhering well to the daily regimen, which was confirmed by dried blood spot testing. But after eight months on PrEP, he developed symptoms of a fever and painful urination and difficulty urinating, known as dysuria. A test taken at that time indicated he likely had very recently contracted HIV. 

His case is unique because unlike the other men who seroconverted on PrEP, the drug-resistance tests indicated he contracted a wild-type virus, with no detectable resistance mutations to Truvada or other ARVs.

The study head Elske Hoornenborg of the Public Health Service Amsterdam theorized that the man might have repeatedly been exposed to HIV, which remained in a localized infection in his rectum until a possible drop in Truvada concentration gave the HIV a window to thrive.

Hoornenborg told POZ at CROI that, "Internationally, tens of thousands of people are using PrEP. And we see that PrEP offers a high degree of protection against HIV. It's not 100 percent. So we'll have to acknowledge that. It also doesn't protect against STDs. So we think it's clever to combine different prevention methods -- PrEP being one of them, condoms and other risk-reducing behaviors being other ones."