Can Daily Doxycycline Prevent STIs?
A pilot study looked at offering this form of pre-exposure prophylaxis (PrEP) to a select population to see if it could be effective in preventing sexually transmitted infections (STIs).
In recent years, doxycycline post-exposure prophylaxis (Doxy-PEP) has been studied in specific populations.
This significant, influential work has led to the Centers for Disease Control and Prevention (CDC) providing guidelines around STI prevention. Specifically, the federal agency recommends, “that providers offer persons in this group [including men who have sex with men (MSM), transgender women, and those who may be at risk for antibacterial sexually transmitted infections] a prescription for doxy PEP to be self-administered within 72 hours after having oral, vaginal, or anal sex. The recommended dose of doxy PEP is 200mg and should not exceed a maximum dose of 200 mg every 24 hours.”1
Another novel approach that is being studied is daily doxycycline or doxy-PrEP to prevent STIs. Troy Grennan, MD, MSc, FRCPC, physician lead, HIV/STI Program, British Columbia Center for Disease Control (BCCDC), and clinical associate professor, Infectious Diseases, University of British Columbia has been a leading researcher in this area, and at this week’s International AIDS Society 2024 conference, he is presenting his pilot study in a poster examining doxy-PrEP.
Grennan and his coinvestigators looked at a subset population to decipher if they could prevent STIs by offering daily doxycycline to participants. They performed a double-blind pilot study involving MSM living with HIV with previous syphilis infection who were randomized 1:1 to receive 48 weeks of daily doxycycline 100mg orally vs placebo in Toronto and Vancouver, Canada.2
A total of 52 MSM were enrolled with a median age of 43 years (interquartile range [IQR], 38-54), from January 2020 to January 2023. Forty-one participants (78.8%) completed the study protocol. 2
The results were impressive. “We observed a reduction of 79%, 92%, and 68% in syphilis, chlamydia, and gonorrhea, respectively, in the doxycycline arm relative to placebo,” Grennan and investigators wrote in the poster. “There were no between-arm differences in drug adherence (pill count) or sexual behaviors (number of partners, condomless sex acts) at any time point.” 2
“We saw an overall 80% reduction in bacterial STI,” Grennan said in an interview with Contagion. “I wouldn't say the takeaway from that is we should go out and offer this as an additional option for everyone,” Grennan said. “Right now, I think there's still a bit of work we need to do, but it's looking very promising. And, it at least justifies further work in this area.”
Antimicrobial Resistance Concerns
One of the specific concerns in looking at these studies is whether it could lead to resistance of doxycycline, a well-established therapy that can be used for STIs, and has a favorable safety profile.
“That is definitely the top issue that we all need to be thinking about," Grennan said, referring to resistance.
“I think the comparison that you bring up so doxy-PrEP vs doxy-PEP, I think there's unknowns everywhere still, so I don't even think we're at the point where we can talk about comparing them,” Grennan said. “I think we just have to consider this to be a still unresolved issue for doxycycline use period for STI prevention. So I think even in the large studies that have been done for doxy-PEP, the numbers that they have in terms of samples and the analysis they've done so far have not been huge…So we're still waiting for more data, and I think there's a lot of work ongoing…I think the question around resistance is really one that's relevant to both, and I don't think we're at a point where we can consider them yet.”
Comparing Doxy-PrEP and Doxy-PEP
As there are differences between pre and post exposure prophylaxis, the next step is to study how they compare against one another in terms of STI prevention. Grennan and coinvestigators are looking to recruit 560 participants in a non-inferiority trial. They have been recruiting for approximately a year and he offered a glimpse of potential early data being reported for that study.
“We're hoping to have the recruitment completed by next summer, and the follow up is about a year, so we should have data about a year out from that,” Grennan said. “So, let's say maybe summer 2026 we should start having a better idea of what data we will have.”
