MSM continue to experience high rates of sexually transmitted infections (STI). Use of HIV pre-exposure prophylaxis (PrEP) significantly reduces risk of HIV infection, and a similar strategy using daily doxycycline may serve as STI PrEP. We undertook a pilot study to determine STI outcomes of HIV-negative MSM on dual HIV/STI PrEP.
HIV-negative MSM with prior diagnosis of syphilis received 48 weeks of tenofovir DF 300mg-emtricitabine 200mg daily and were randomized 1:1 to receive either immediate daily doxycycline 100mg, or deferred doxycycline beginning 24 weeks later in an open-label pilot study of Dual Daily HIV and Syphilis PrEP (The DuDHS Study) in Vancouver, Canada. Participants underwent screening for STI every 3 months, with Staphylococcus aureus nares cultures collected to evaluate tetracycline/doxycycline resistance by Kirby Bauer testing. STI rates were compared between those on dual PrEP vs. HIV PrEP alone over the initial 24 weeks using Fishers Exact test.
Fifty-two MSM were randomized with median age of 34 years (interquartile range [IQR], 29 – 43). Overall, 55.8% self-reported prior gonorrhea and chlamydia infection. Chlamydia infection occurred only in the deferred arm during the first 24 weeks (n=10) (rate 0 vs. 81.63/100 PY, p = 0.001), subsequently no infections occurred in either arm. No individuals in the immediate arm, and one individual in the deferred arm developed syphilis infection during the first 24 weeks (rate 0 vs. 8.16/100 PY, p = 0.98) with no infections seen thereafter in either arm. By 24 weeks, n= 4 in the immediate arm and n=7 in the deferred arm tested positive for gonorrhea (rate 31.37 vs. 57.14/100 PY, p= 0.505), and only one additional infection was seen in each arm for 24 – 48 weeks. In a logistic model receipt of doxycycline was associated with reduced probability of any STI (OR 0.18, 95% CI 0.05 – 0.68) during the first 24 weeks. Tetracycline resistance was seen in 1/3 S. aureus isolates at 24 weeks and 3/6 isolates at 48 weeks in the immediate arm and in 1/2 isolates after six months of doxycycline use in the deferred arm.
STI PrEP using daily doxycycline demonstrated decreased rates of chlamydia infection while impact on syphilis could not be ascertained. Tetracycline resistance amongst nasal carriage of S.aureus was observed over the study duration. Further evaluation of potential benefits and antimicrobial resistance in a larger study may be warranted.