Are we doxy-PrEPared?

A report on the debate moderated by Lewis Marshall, "Doxycycline should be provided for syphilis prophylaxis in those at high risk" 

Prof Lewis Marshall was a lively and entertaining referee for the match between Prof Basil Donovan (in the blue corner) and Prof Kit Fairley (in the red), on the subject of “syphilaxis” or “doxy-PrEP” – the use of daily doxycycline as pre-exposure prophylaxis against syphilis infection in high-risk individuals. 

Prof Donovan made the argument for providing this intervention, citing evidence for doxycycline post-exposure prophylaxis from IPERGAY, showing a hazard ratio of infection of 0.27 in those using “doxy-PEP”. He dismissed concerns about antimicrobial resistance, explaining that this is unlikely to develop in chlamydia and syphilis given their slow rate of reproduction. Widespread resistance to doxycycline in Neisseria gonorrhoea has already developed. 

The ‘for’ argument acknowledged the unknown effects on the gut microbiome, but cited multiple other medical indications for long-term doxycycline use, including acne vulgaris as well as malaria prophylaxis. 

Prof Fairley made a strong case for restricting doxycycline use as syphilaxis to clinical trials until further research is conducted. He cited modelled data which shows that even for the highest risk individuals seen in HIV PrEP trials, ten years of daily doxycycline would need to be taken by an individual to prevent a single syphilis infection. For lower-risk individuals, the financial cost per case averted is several thousands of Australian dollars. Antimicrobial resistance was another clear concern. 

These arguments were rebutted by Prof Donovan who claimed the models used were out of date and not reflective of current sexual practices. Agricultural use of doxycycline was said to far outweigh the risks associated with human use. 

The crowd seemed to remain cautious of doxy-PrEP, with a slight majority falling on the ‘Red’ side of the fence at the end of the debate. Nevertheless, certain communities are already vocally demanding access to doxy-PrEP, both in clinics and in the streets – with posters advising HIV PrEP users to “look up doxycycline 100mg a day” sighted in Australian cities. It’s very possible that clinicians could come across clients who are already taking syphilaxis (with or without a prescription) in the near future, and research to inform the subsequent discussion is urgently needed. 

“Syphilaxis” (Impact of the Daily Doxycycline Pre-exposure Prophylaxis (PrEP) on the Incidence of Syphilis, Gonorrhoea and Chlamydia) begins recruiting in NSW this year. 


    Author bio: David is a Sexual Health Registrar at Sydney Sexual Health Centre