Approximately 10 million syphilis infections were diagnosed in 2008 according to the World Health Organization (WHO), many of which occurred in men who have sex with men or individuals coinfected with human immunodeficiency virus (HIV) [1]. The association of syphilis and HIV may be causal, as syphilis can facilitate HIV acquisition [2–8] via mucosal disruption, ulceration [9], or inflammation [10] and HIV transmission by increasing HIV RNA in blood and genital secretions [11]. Alternatively, the association could be due to increased sexual risk behaviors or participation in networks with high prevalence of HIV. Observational evidence from retrospective analyses suggests an association between syphilis and HIV acquisition [12], but few studies have assessed this association among a closed, prospective cohort of MSM with almost complete follow-up. The Preexposure Prophylaxis Initiative (iPrEx) trial [13] and others [14–16] demonstrated the efficacy of once-daily oral emtricitabine/tenofovir disoproxil fumarate (FTC/TDF) in preventing HIV acquisition. The iPrEx study prospectively followed a diverse cohort of men and transgender women who have sex with men (MSM) with frequent sexually transmitted infection (STI) and HIV testing and offers a unique opportunity to assess the relationship between syphilis and HIV acquisition in the context of FTC/TDF preexposure prophylaxis (PrEP) use. Syphilis provides an informative marker of heightened HIV risk and may be an important consideration in assessing the need for PrEP. It is also critical to determine whether syphilis decreases PrEP efficacy, which could result from compromising mucosal barriers, increasing HIV inoculum size [17], or decreasing PrEP adherence. In the present study, we determined rates and correlates of prevalent and incident syphilis in the iPrEx study, the extent to which incident syphilis was associated with HIV acquisition, and whether that association varied by treatment group or by level of adherence (as measured by detected drug) among participants in the active arm. METHODS Participants and Specimens The iPrEx study enrolled 2499 HIV-seronegative MSM to evaluate the safety and efficacy of once-daily oral FTC/TDF for HIV prevention [13]. Study visits were scheduled every 4 weeks after enrollment. The details of the primary study and visits have been described elsewhere [13]. In a subset of participants sampled from the active arm including longitudinal samples from HIV-positive cases with site-matched controls, longitudinal samples from participants enrolled in a bone mineral density substudy, and cross-sectional samples of participants at weeks 8 and 24, plasma was tested for the presence of FTC and TDF, and peripheral blood mononuclear cells (PBMCs) were tested for FTC triphosphate (FTC-TP) and tenofovir diphosphate (TFV-DP), as described elsewhere [18]. Diagnosis and treatment of symptomatic STIs occurred at every visit. At screening and at 24-week intervals during follow-up, participants were screened for asymptomatic urethritis, syphilis, and antibodies to herpes simplex virus type 2 (HSV-2), and they were examined for genital warts and ulcers. In addition, participants could present for an interim visit on an as-needed basis following a possible exposure, onset of new symptoms, or STI treatment. Partners of participants with a curable STI were offered evaluation and treatment. Syphilis diagnoses were based on standard algorithms according to local guidelines or developed by normative bodies [19] and included an initial nonspecific test—either a rapid plasma reagin (RPR) test (bioMérieux, Marcy l'Etoile, France; Becton, Dickinson and Company, Franklin Lakes, New Jersey; Laborclin, Sao Paolo, Brazil; WAMA Diagnostica, Sao Paolo, Brazil) or Venereal Disease Research Laboratory (VDRL) test (Standard Diagnostics, Kyonggi-do, South Korea) performed at local sites. Specimens with a newly positive RPR or VDRL result were sent for confirmatory testing using fluorescent treponemal antibody-absorption (FTA-ABS) (Biokit, Barcelona, Spain; Fujirebio Inc, Tokyo, Japan; Zeus Scientific, Somerville, New Jersey; bioMérieux; WAMA Diagnostica). Participants diagnosed with prevalent and incident syphilis (see definitions below) were treated in accordance with local guidelines, or if not available, according to the 2006 sexually transmitted disease guidelines of the Centers for Disease Control and Prevention (CDC) [19]. Evaluation and treatment were managed by the local treating physician under the supervision of the site investigator. Each incident case was also reviewed by a studywide clinical monitoring committee to ensure that appropriate guidance for clinical management was provided. Definitions At screening, we examined the number of participants with a positive RPR result and a positive confirmatory test, which was defined as syphilis prevalence. Patients newly diagnosed with syphilis were treated and their serological response was subsequently monitored. At follow-up, incident syphilis was defined as (1) a change in serology from negative to positive with confirmation by FTA-ABS, or (2) at least a 4-fold increase in serological titer from prior test with documentation of prior treatment appropriate for the stage of syphilis. Person-time was calculated for participants with incident syphilis as time between enrollment and first episode and for HIV as the time between enrollment and first evidence of HIV infection. For participants without syphilis, person-time was calculated as years between enrollment and the date of last syphilis test. Those with confirmed syphilis at screening without clear documentation of prior effective treatment were treated and were excluded from the analysis of syphilis incidence, unless clear documentation of prior effective treatment was available. Detected drug was defined as the detection of FTC or TDF in plasma or FTC-TP or TFV-DP in PBMCs regardless of level [18]. Statistical Methods Baseline characteristics were compared by an unequal-variance t test for continuous variables and the Fisher exact test for categorical variables. Syphilis incidence was calculated as described above and the results were stratified by site, age, condomless insertive or condomless receptive anal intercourse in the past 3 months, reported STI in the past 6 months, HSV-2 positivity, and syphilis prevalence at screening. Predictors of incident syphilis were modeled using a stratified Cox proportional hazards approach. HIV seroconversion rate was calculated as incident cases divided by person-years of observation and was stratified by incident syphilis. Hazard ratios for the effect of incident syphilis on HIV acquisition were calculated using a Cox proportional hazards model with incident syphilis as a time-dependent covariate stratified by study site. A bivariate model was evaluated for the effect of timing of incident syphilis on HIV seroconversion. A multivariate model that evaluated the association of incident syphilis and HIV seroconversion included study site, age, race, HSV-2 acquisition (as a time-dependent covariate), randomization group, condomless sex, and number of partners at screening and at follow-up; and a history of an STI, syphilis, HSV-2, HIV-positive partners, and circumcision status at screening. An interaction hypothesis was prespecified and tested for detected drug. RESULTS Baseline Characteristics, Predictors of Syphilis, and Syphilis Incidence Of 2499 individuals, 360 (14.4%) had a positive RPR result at screening, of whom 333 (92.5%) had a positive confirmatory test, which did not differ between the arms (FTC/TDF vs placebo, P = .81). Fourteen had a false-positive RPR result and 13 had no confirmatory testing performed. Individuals with prevalent syphilis infection at screening were less educated, were older, and reported having more sexual partners and more episodes of condomless anal intercourse in the previous 3 months. The overall syphilis incidence during the trial was 7.3 cases per 100 person-years, but varied by site, age, condomless insertive anal intercourse in the past 3 months, condomless receptive anal intercourse in the past 3 months, reported STI in the past 6 months, HSV-2 positivity, and syphilis prevalence at screening (Table 1). There was no difference in syphilis incidence between the study arms (7.8 cases per 100 person-years for FTC/TDF vs 6.8 cases per 100 person-years for placebo, P = .304). Table 1. Predictors of Onset of Incident Syphilis Predictor Total No. Incident Syphilis (n = 279) P Value No. Person-years Ratea Overall 2499 279 3814 7.3 Site 17 622 88 1021 8.6 Condomless insertive anal intercourse at screening 1 partner 443 69 1270 5.4 Condomless receptive anal intercourse at screening 1 partner 671 161 1763 9.1 Anal intercourse .223 Insertive only 645 20 336 6.0 Receptive 1485 47 623 7.5 No condomless intercourse 369 38 446 8.5 Reported STI in 6 months before screening 90 days from HIV seroconversion), giving an HR of 3.8 (95% CI, 1.9–7.6; P 90 days. These associations remained significant but did not differ from each other significantly in a multivariate model that included study site, race, randomization group, syphilis at screening, HSV-2 at screening, HSV-2 acquisition (as a time-dependent covariate), condomless sex (at screening and at follow-up), number of partners at screening and at follow-up, history of an STI at screening, HIV-positive partners at screening, age, and circumcision status (HR for syphilis ≤90 days, 3.4 [95% CI, 1.7–7.0; P = .001] vs >90 days, 2.1 [95% CI, 1.0–4.2; P = .035]; P = .30 for the interaction). A similar association of incident syphilis on HIV acquisition was seen among participants without syphilis at screening and also within each study arm. Figure 1. Human immunodeficiency virus (HIV) incidence according to incident syphilis and treatment arm. Abbreviation: FTC/TDF, emtricitabine/tenofovir. Evaluation of Interactions of Incident Syphilis, FTC/TDF, and HIV Acquisition There was no evidence for interaction of syphilis on FTC/TDF's efficacy. In the modified intent-to-treat analysis (mITT), FTC/TDF efficacy was 40% (95% CI, 11%–59%) in the absence of incident syphilis (70 placebo, 40 FTC/TDF), whereas mITT efficacy was 47% (95% CI, 5 sex partners (PAF = 13%) [39]. Clinicians should prioritize PrEP for individuals at highest risk, including those infected with syphilis.
