A Phase 4 interventional study of Doxycycline and Placebo in Syphilis and Sexually Transmitted Infections, sponsored by British Columbia Centre for Disease Control. Completed at 1 site in Canada. Open to male participants aged 18 Years and older. Per ClinicalTrials.gov, last updated 2025-02-12.
Sponsored by British Columbia Centre for Disease Control · Phase 4, Interventional, and Prevention
Syphilis is a sexually transmitted infection (STI) disproportionately affecting gay, bisexual and other men who have sex with men (gbMSM), with the potential for significant sequelae - particularly in those who are Human Immunodeficiency Virus (HIV)-positive. Rising rates of this STI have prompted a search for novel prevention solutions. A recent pilot study of daily doxycycline prophylaxis demonstrated promise as a novel STI prevention tool. This innovative approach to STI prevention has solid clinical precedent, both from the HIV pre-exposure prophylaxis (PrEP) literature, as well as doxycycline's use as prophylaxis for other infections. The overarching goal of this project is to determine whether the daily use of doxycycline is an efficacious and acceptable intervention for syphilis prevention in high-risk, HIV-positive gbMSM.
Syphilis disproportionately impacts gbMSM:
Syphilis remains an important cause of morbidity in gbMSM. Up to 75% of early syphilis infections occur in gbMSM - a population with syphilis rates nearly 10-fold higher than in those who are HIV negative. In Canada's urban centres, 2/3 of syphilis cases are in HIV-positive gbMSM, and re-infection remains high. HIV co-infection with syphilis has important implications for HIV management. Syphilis may increase HIV viral load levels in virologically suppressed individuals, and genital ulcers increase the risk of HIV transmission and acquisition.
The era of novel, biomedical prevention technologies:
Significant advancements have shaped the past two decades of HIV care. Antiretroviral therapy (ART) has substantially reduced HIV-associated morbidity and mortality and extended life expectancy to near-normal levels. With reduced morbidity and mortality associated with HIV, gbMSM may also be changing their sexual behaviours. Seroadaptive strategies, such as serosorting (selecting sexual partners of the same serostatus) and seropositioning (selecting a sexual position to minimize the risk of HIV transmission) are common among gbMSM. While this may be mitigating the risk of HIV transmission or acquisition, there is evidence to suggest that decreasing condom use may contribute to a rise in syphilis and other bacterial STIs in high-risk populations. Further, new evidence has emerged demonstrating the efficacy of HIV PrEP in preventing HIV acquisition. The combination use of tenofovir/emtricitabine was shown in the Pre-exposure Prophylaxis Initiative (iPREX) trial to reduce the risk of HIV acquisition by 44% in HIV-negative gbMSM when taken daily as PrEP. Among those with detectable drug levels, the risk reduction was > 90%.
A role for PrEP in other STIs? The exciting developments in HIV prevention have encouraged investigators to expand this strategy to the prevention of other STIs, a modality previously shown to be acceptable to gbMSM. A recent pilot study randomized participants to receive either the antimicrobial doxycycline daily or a monetary incentive to remain STI-free throughout the study period. After 48 weeks, those receiving doxycycline were significantly less likely to be diagnosed with any STI (odds ratio: 0.27; 95% confidence interval: 0.09-0.83). Though there was also a trend toward benefit for doxycycline in specifically preventing syphilis, significance was not achieved for this outcome.
The first large-scale syphilis PrEP study:
Given the rising rates of syphilis and its disproportionate impact on HIV-positive gbMSM, there is an urgent need to replicate this study on a broader scale. This proposed study will contribute to this field by being - to the investigators' knowledge - the first large-scale, methodologically rigorous trial of syphilis PrEP, and has the potential to provide a completely novel and innovative tool to the syphilis prevention armamentarium. It will provide insight on whether daily doxycycline is an efficacious PrEP intervention for the prevention of syphilis and other bacterial STIs, as well as shedding light on issues relating to its feasibility as a prevention tool, including its safety, tolerability, how well individuals adhere to it, and its impact on the development of antimicrobial resistance. These outcomes are directly in line with the planned outputs of this study, and have the potential to drastically shift how people think about and manage syphilis and other STI prevention in gbMSM.
Primary objective
To assess feasibility of using daily doxycycline PrEP, as defined by:
a. Evaluation of feasibility of recruitment for a larger study i. Proportion of participants approached for study who are eligible and agree to participate.
b. Adherence to 24 and 48 weeks of study drug (doxycycline or placebo) i. Determine proportion of individuals with >95% adherence to study drug over 24 and 48 weeks ii. Proportion of individuals in the doxycycline arm with therapeutic doxycycline plasma level at each study visit.
c. Tolerability of doxycycline i. Comparison of grade 3 or 4 adverse events in those receiving doxycycline vs. placebo ii. Comparison of the proportion of individuals with adverse event-related discontinuation of study drug in each arm
2.2 Secondary objectives
To evaluate antimicrobial resistance over time.
a. Change in proportion of participants with evidence of tetracycline class resistance in common flora, namely Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae from baseline to 24 and 48 weeks.
To describe frequency of other STIs diagnosed in study participants over the study period.
Exploratory objectives will include:
424 studies on the registry are indexed under Sexually Transmitted Diseases; 71 are open to participants now.
This study's enrollment of 52 is below the median of 470 across 346 interventional studies indexed under Sexually Transmitted Diseases.
Browse Sexually Transmitted Diseases studies →British Columbia Centre for Disease Control is the lead sponsor of 6 studies on the registry; none are open to participants now.
Counted across the registry records on this site, refreshed daily.
The inclusion criteria for this study are as follows:
The exclusion criteria for this study are as follows:
Participants in this intervention group will receive doxycycline 100mg orally daily, which is available as a 100mg capsule. This single daily dose was chosen to maximize adherence, given the common use of once-daily Human Immunodeficiency Virus (HIV) pre-exposure prophylaxis (PrEP), as well as its efficacy as once-daily prophylaxis against malaria and its utility as dosing as infrequent as once weekly for another spirochete infection, leptospirosis.
Drug: Doxycycline
Participants in this control group will receive a placebo capsule identical in appearance, taste, and size to the capsule provided to the intervention group.
Other: Placebo
doxycycline 100mg orally daily
placebo capsule identical in appearance, taste, and size to the capsule provided to the intervention group
To assess the proportion of participants who are eligible and consent to participate amongst those approached.
To determine the feasibility of daily syphilis PrEP
Time frame: 15 months
To assess the proportion of participants reporting > 95% adherence to study drug, according to self-report and pill counts.
To determine the feasibility of daily syphilis PrEP
Time frame: 15 months
To assess the proportion of individuals with therapeutic doxycycline drug level (defined at ≥ 1000 ng/mL) at each study time point.
To determine the feasibility of daily syphilis PrEP
Time frame: 15 months
To assess the proportion of individuals reporting grade 3 or 4 adverse events in each study arm.
To determine the feasibility of daily syphilis PrEP
Time frame: 15 months
Proportion of individuals with adverse event-related study drug discontinuation in each study arm.
To determine the feasibility of daily syphilis PrEP
Time frame: 15 months
To assess the proportion of individuals with evidence of tetracycline class resistance in common flora (Staphylococcus aureus, Streptococcus pyogene and Streptococcus pneumoniae)
To evaluate tetracycline class resistance in common flora (i.e. syphilis, gonorrhea, or chlamydia)
Time frame: At 6 and 12 months
To assess the changes in sexual risk behaviour will be calculated by comparing risk parameters (i.e. number partners, number of anal sexual encounters that were condomless) from pre-PrEP initiation to the study period.
Change in sexual behaviour will also be compared between study arms.
Time frame: 15 months
To assess the incidence of early syphilis infection
To evaluate syphilis and other STI incidence over the study period.
Time frame: 15 months
To assess the incidence of gonorrhea or chlamydia infection
To evaluate syphilis and other STI incidence over the study period.
Time frame: 15 months
To assess changes in the bacterial populations found in the rectal microbiome.
Exploratory outcome: to assess % changes of each bacterial genus in rectal microbiome
Time frame: 15 months
Plan to share: No
This study is completed, as verified in Feb 2025. You cannot join it, but the record below documents what was studied.
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Sexually Transmitted Diseases→
British Columbia Centre for Disease Control