Taking antibiotic after sex cuts STIs by two-thirds, 'DoxyPEP' study finds - aidsmap
Doxycycline study was stopped early due to its high efficacy, International AIDS Conference hears
A US study that provided gay and bisexual men and transgender women with the antibiotic doxycycline to take after condomless sex was stopped a year early because of its high efficacy, the 24th International AIDS Conference in Montreal was told yesterday.
The DoxyPEP study compared the incidence of gonorrhoea, chlamydia and syphilis in 374 people who were given doxycycline to take after sex with the incidence in 180 people who were not provided with the drug. It fond that doxycycline cut the risk of STIs by two-thirds.
The study was originally going to end in May 2023 but was stopped exactly a year early last May. At the time the researchers made it clear that the study was stopped because of positive findings, but the exact data were not released until today.
The study involved 360 HIV-negative people using HIV pre-exposure prophylaxis (PrEP) and 194 HIV-positive people, aged over 17 and born male, attending two clinic sites in Seattle and San Francisco. The other criterion for the study was that they had to have had at least one of the three STIs in the last year.
An interim review of 13 May 2022 found that efficacy was so high it would be unethical to continue with the control arm and all participants were given doxycycline.
Efficacy in the HIV-negative participants was 66%; there were 65 STI infections detected in the control arm and 47 taking DoxyPEP, despite there being twice as many people in the DoxyPEP arm. This equated to an incidence, per every three months of follow-up, of 29.5% in the control arm and 9.6% in the DoxyPEP arm.
Efficacy in the participants with HIV was a bit lower, at 62%, but still significant. There were 30 STI infections in the control arm and 31 in the DoxyPEP arm, equating to a three-month incidence rate of 27.8% in the control arm and 11.7% in the DoxyPEP arm.
Doxycycline was chosen as it is cheap and has a relatively long half-life but is not often used to treat STIs, except for people with chlamydia or syphilis who are allergic to the more commonly used drugs (azithromycin and penicillin, respectively). It is not advised for gonorrhoea treatment.
It has also been used before as a daily prophylactic (preventative) drug for other diseases, most notably malaria, where it is commonly used by travellers going to endemic areas.
This was an open-label study, meaning people in the control arm were not given placebo pills. Participants in the intervention arm were told to take a single dose of doxycycline as soon as possible and no more than 72 hours after condomless sex.
Until now, it has not been certain whether doxycycline would work as post-exposure prophylaxis (taking it after sex – PEP). A couple of previous studies have been done, a small one in the US from 2015, and a French study from 2017. But both of these used daily STI pre-exposure prophylaxis (taking it before sex – PrEP). Furthermore, while both studies cut the incidence of syphilis, the US study had no significant effect on chlamydia and neither had an effect on gonorrhoea. In the latter case, this was ascribed to the fact that there are many strains of gonorrhoea resistant to doxycycline.
So it was excellent news that the incidence of all three STIs fell in the new study, in both HIV-negative and HIV-positive people. Efficacy against chlamydia was high, at 79% in HIV-negative people and 70% in people with HIV. Though more moderate, efficacy was still significant against gonorrhoea, at 59% in people without HIV and 57% in people with HIV. In the case of syphilis, efficacy was 76% in HIV-negative people and 42% in people with HIV, but the number of infections was small (three versus two in the DoxyPEP and control arms in HIV-positive people), so this lower efficacy in people with HIV may not be significant.
Resistance tests showed that about 20% of gonorrhoea infections had some resistance to drugs of the tetracycline class.
Adherence to doxycycline was good, with 87% of incidents of condomless sex protected by it. More than half of participants (54%) took fewer than ten PEP doses a month, 39% took 10-20 doses, and 16% took over 20 doses (so almost daily). There were no serious adverse events.
The efficacy of DoxyPEP now opens up the possibility of its more widespread use as STI prophylaxis. DoxyPEP implies taking the drug less often than for DoxyPrEP, both saving money and reducing concerns about the over-use of antibiotics leading to resistance and side effects.
However, other studies are still underway. An Australian study is looking at daily DoxyPrEP and a Canadian study is comparing doxycycline PEP and PrEP. A French DoxyPEP study adds in Bexsero, a meningitis vaccine that also has modest efficacy (in the 33-40% range) against gonorrhoea, although this study excludes men with HIV because they might have different responses to the vaccine.
Professor Annie Luetkemeyer, Director of of Medicine and Infectious Diseases at the University of California San Francisco, was one of the principal investigators on the study. She said: "We need larger studies to know if DoxyPEP could drive antibiotic resistance in gonorrhoea, or whether there might be other untoward results."
But Professor Sharon Lewin, President-elect of the International AIDS Society, hailed DoxyPEP as a "remarkably simple" intervention that could make a big difference to the health of people at risk of STIs.
References
Luetkemeyer A et al. Doxycycline post-exposure prophylaxis for STI prevention among MSM and transgender women on HIV PrEP or living with HIV: high efficacy to reduce incident STIs in a randomised trial. 24th International AIDS Conference, Montreal, abstract no OALBX0103, 2022.
View the abstract on the conference website.
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