CTN 329: Doxycycline Intervention for bacterial STI ChemoprOphylaxis (DISCO)

About the Study

Doxycycline is a commonly used and widely accepted antibiotic for acne treatment and malaria prophylaxis. Four small studies have indicated that doxycycline may also be beneficial in preventing bacterial STIs, and could be used as STI pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP). However, definitive data about the acceptability, safety, and efficacy of doxycycline use for STI prevention is needed.

CTN 329 is the first methodologically rigorous randomized controlled trial investigating the use of doxycycline in the prevention of bacterial STIs (syphilis, gonorrhea, or chlamydia) among gay, bisexual, and other men who have sex with men (gbMSM) and transgender women.

About the Disease

Canada has seen an increase in bacterial STIs of more than 160 per cent over the last decade. In Canada, syphilis rates are at a record high, with two-thirds of cases in urban centres identified among gbMSM living with HIV. Gonorrhea and chlamydia also disproportionately affect gbMSM. With high reinfection rates, gbMSM are at risk of developing complications such as antimicrobial resistance, meaning treatments may become less effective.

In addition, people living with HIV can be at a higher risk for more serious STI complications. For example, people coinfected with HIV and syphilis are more likely to progress to neurosyphilis, where the syphilis bacteria infect the brain and spinal cord, damaging the nervous system. There is also evidence that being infected with a bacterial STI could increase risk of HIV transmission.

Given the significant rise in STI cases and the potential development of complications, novel STI prevention strategies are needed.

Study Approach

A total of 560 participants will be recruited from nine clinical sites in Vancouver, Calgary, Toronto, Hamilton, Ottawa, and Montreal. The participants will be separated into two arms: the STI PrEP arm will take 100mg doxycycline daily and the STI PEP arm will take 200mg doxycycline within 24–72 hours after an at-risk sexual encounter (condomless or oral sex).

Once recruited, each participant will receive their allocated intervention for 48 weeks and be followed for 60 weeks. They will need to attend study visits at baseline and weeks 4, 12, 24, 36, 48, and 60. These visits will include an adverse event assessment, routine bloodwork, STI screening, adherence assessment, evaluation of antimicrobial resistance, and questionnaires on sexual risk behaviour and substance use.