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MONTREAL – Men who have sex with men appeared to avoid contracting gonorrhea, chlamydia, and syphilis by taking a single dose of the antibiotic doxycycline after engaging in condomless sexual activity, researchers said here.

Among the 327 participants taking pre-exposure prophylaxis (PrEP) to prevent human immunodeficiency virus (HIV), new sexually transmitted diseases (STDs) occurred in 32% of those in the control arm and in 11% of those in the doxycycline arm of the DOXY/PEP study, reported Annie Luetkemeyer, MD, of University of California San Francisco.

At a press conference at the , Luetkemeyer said the difference represented a reduction of STDs by 66% per quarter (P<0.001).

Similarly, she said, of 174 people living with HIV, new STDs occurred in 30.5% of the control arm vs 12% in the doxycycline arm for a 62% reduction per quarter (P<0.001).

"This is really fascinating work on sexually-transmitted disease prevention. This is a game-changing sexually-transmitted infection prevention strategy," said Sharon Lewin, MD, president-elect of the International AIDS Society, and infectious disease specialist at the University of Melbourne, Australia.

"We look forward to hearing about the implementation of this very simple intervention," said Lewin, the moderator of the press conference at which the DOXY/PEP study was presented.

"Our study," Luetkemeyer said, "demonstrated that a single dose of doxycycline taken after condomless sex is a safe and highly effective strategy to reduce sexually transmitted disease infections in populations heavily impacted by the ongoing sexually-transmitted disease epidemic."

Luetkemeyer cited the ongoing epidemic of infections such as syphilis, gonorrhea, and chlamydia in the U.S. and in many other parts of the world.

"Men who have sex with men and transgender women are disproportionately impacted by this epidemic, and in particular people living with HIV and those who are taking HIV pre-exposure prophylaxis," she said. "The oral antibiotic doxycycline has shown promises of potential strategy to reduce sexually-transmitted diseases when taken after condomless sex."

She and her research associates investigated the effectiveness of doxycycline as post-exposure prophylaxis (PEP) in a randomized open-label clinical research project enrolling participants from HIV sexual health clinics in Seattle and San Francisco.

Eligible participants had to be male sex at birth, living with HIV or taking HIV PrEP, and have a history of bacterial STDs and condomless sex with a male partner, both in the year preceding enrollment. Median number of sex partners in the past 3 months was nine.

DOXY/PEP participants were randomized 2:1 to take 200 mg of doxycycline as soon as possible within 24 hours after condomless sex or they were randomized to standard of care.

All of the STD endpoints were reviewed by a blinded endpoint committee.

The control arm of our study was stopped early in May 2022 when a planned interim analysis determined that the doxycycline group had substantially less STDs, both in those living with HIV and in those taking HIV PrEP.

Participants reported taking doxycycline 87% of the time after having condomless sex. About half the participants took fewer than 10 doses per month; 30% took 10-20 doses per month; and 16% took more than 20 doses per month, Luetkemeyer said.

"We saw no serious or grade 2 or higher adverse events attributed to doxycycline and a majority of patients said taking doxycycline was acceptable or very acceptable," she said.

The researchers noted that available culture data of those who had gonorrhea infections during the study demonstrated a relatively low 20% rate of tetracycline resistance -- a proxy for doxycycline resistance -- which may in part account for effectiveness of doxycycline/PEP to reduce gonorrhea infections in this study.

"However, larger studies and population-based surveillance of those taking doxycycline/PEP are needed to understand if doxycycline use could drop by development of tetracycline resistance to gonorrhea," Luetkemeyer said. "It is important to note that doxycycline is not used to treat active gonorrhea infections. Analyses of impact of intermittent doxycycline use on antibiotic resistance are ongoing."

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