STI rates in high-risk populations curbed with doxycycline PrEP

Pre-exposure prophylaxis (PrEP) with the antibiotic doxycycline helps to prevent sexually transmitted infections (STIs) in high-risk populations, including female sex workers and HIV-positive men who have sex with men (MSM), according to two small studies.

Female sex workers

In a retrospective study involving 40 cisgender female sex workers (median age of 29 years) in Japan, the overall STI incidence rate dropped from 232.3 to 79.2 per 100 person-years following initiation of doxycycline PrEP at 100 mg/day (incidence rate ratio [IRR], 0.33, 95 percent confidence interval [CI], 0.13–0.84; p=0.020), reported lead author Dr Seitaro Abe, an infectious disease specialist at the National Center for Global Health and Medicine in Tokyo, Japan.

There was a marginally significant reduction in chlamydia (from 159.2 to 57.2 per 100 person-years; IRR, 0.35, 95 percent CI, 0.12–1.03; p=0.056) but no substantial change in gonorrhoea (from 55.9 to 22.0 per 100 person-years; p=0.136). No new cases of syphilis infections occurred (from 17.2 to 0 per 100 person-years). [AIDS 2024, abstract OAC0803]

“Despite the increased testing frequency, the positivity rate of STIs considerably decreased, suggesting that doxycycline PrEP is effective in reducing STIs,” Abe noted.

More importantly, he added, doxycycline had no notable effects on the vaginal flora. Vaginal smear results from 27 women showed that the incidence of bacterial vaginosis did not significantly increase (from 105.2 to 137.7 per 100 person-years; IRR, 1.19, 95 percent CI, 0.72–1.94; p=0.499). The same was true for vulvovaginal candidiasis (from 52.6 to 71.8 per 100 person-years; IRR, 1.52, 95 percent CI, 0.62–3.70; p=0.358).

“PrEP doxycycline was well-tolerated, and participant satisfaction was high,” Abe said.

Based on follow-up interview summaries from 22 participants, 72.7 percent strictly adhered to PrEP doxycycline, with no missed doses, and 72.7 percent reported reduced anxiety about acquiring STIs. Condom use remained constant in 95.4 percent of participants.

Nausea and vomiting (22.7 percent) and genital itching (13.6 percent) were the most common adverse events (AEs). There were no reports of severe doxycycline-related AEs.

“This study provides evidence that doxycycline PrEP can reduce STIs among cisgender female sex workers, a group with limited clinical data,” Abe said.

He acknowledged several limitations to the study, including its short observational period, small sample size, focus on PrEP (not postexposure prophylaxis [PEP]), and the inability to assess drug resistance.

MSM living with HIV

In a pilot randomized controlled trial (DISCO) that involved 52 gay, bi, and other MSM who were HIV-positive (median age 43 years, 59.6 percent White) in Canada, doxycycline PrEP likewise yielded significant reductions in bacterial STIs at week 48 compared with placebo (incidence rate, 23.71 vs 119.44 per 100 person-years; rate ratio [RR], 0.20, 95 percent CI, 0.08–0.51; p<0.001), reported lead study author Dr Troy Grennan, physician lead for the HIV/STI program at the British Columbia Centre for Disease Control in Vancouver, Canada.

Specifically, syphilis decreased by 79 percent (incidence rate, 3.95 vs 19.26 per 100 person-years; RR, 0.21, 95 percent CI, 0.04–0.97; p=0.04), chlamydia by 92 percent (incidence rate, 3.97 vs 50.09 per 100 person-years; RR, 0.08, 95 percent CI, 0.01–0.49; p=0.01), and gonorrhoea by 68 percent (incidence rate, 15.88 vs 50.09 per 100 person-years; RR, 0.32, 95 percent CI, 0.12–0.86; p=0.02). [AIDS 2024, abstract LB11]

Drug adherence by pill count and sexual behaviours (number of partners and condomless sexual act) at any time point did not significantly differ between the doxycycline and placebo groups, Grennan noted.

In terms of safety, most AEs were mild (78.4 percent), and the frequency of AEs related to the study drug was similar in the two treatment groups. In the doxycycline group, one drug-related discontinuation (worsening gastroesophageal reflux) and one serious AE (mpox-related proctitis; unexpected and unrelated to drug) were documented.

The development of doxycycline resistance in S. aureus isolates at week 48 was comparable between the doxycycline and control groups (3 of 19 vs 2 of 19; p=0.57), according to Grennan.

“These pilot findings support the ongoing evaluation of doxycycline PrEP vs PEP in our ongoing DISCO trial,” he said.