Background Syphilis infections increase spread of HIV. This pilot study investigated the feasibility of conducting a large, randomised trial to determine whether prophylactic doxycycline can reduce incident syphilis among HIV+ men who have sex with men having two or more syphilis infections since HIV diagnosis.

Methods Thirty subjects from the L.A. Gay & Lesbian Center were randomised to doxycycline 100 mg daily (Doxy) or Contingency Management (CM). Behavioral and biomedical measurements were taken at baseline, 12, 24, 36 and 48 weeks. Binomial Random Effects Logistic Regression Models assessed changes in behavioural measurements and analysed STI contraction at follow-up visits. Fisher Exact Tests compared dropout rates. Given small sample size, an alpha level of 0.1 was chosen.

Results Study retention did not differ significantly between groups: 12 and 11 subjects in the Doxy and CM groups, respectively, completed the 48 week study (Fisher Exact p-value = 1.0). On intent-to-treat basis 12 (80%), 11 (73.3%) and 11 (73.3%) subjects had detectable doxycycline levels at 12, 24, and 36 weeks, respectively. New STIs for Doxy vs. CM groups during 9 month on-drug study portion were: syphilis 2 vs. 6, Neisseria gonorrhoea 1 vs. 2, Chlamydia trachomatis 1 vs.3. CM subjects reported more condom use than Doxy subjects (p = 0.09), however CM group subjects had 3.47 greater odds (p-value = 0.037; CI: 1.09 – 11.03) for a new STI compared to Doxy group subjects. Behavioral outcomes for drug use (p = 0.91), anonymous partners (p = 0.45), and number of regular partners (p = 0.14) were not significantly different.

Conclusion No safety concerns were associated with daily administration of doxycycline. High retention and drug adherence rates suggest this high risk population will participate in an effectiveness study using doxycycline to reduce syphilis incidence. Efficacy of doxycycline in decreasing incidence of STIs in this pilot study further justifies a definitive study.