An old antibiotic finds new life as STD prevention pill

The antibiotic doxycycline is being repurposed as a 'morning after' pill to help prevent three common STIs. — AFP

The United States is set to roll out a powerful new weapon in the long fight against sexually-transmitted infections (STIs): a decades-old antibiotic repurposed as a preventative pill.

DoxyPEP, or doxycycline used as a post-exposure prophylaxis, has been found to significantly cut the risk of chlamydia, gonorrhoea and syphilis when used after condomless sex.

The US Center for Disease Control and Prevention (CDC), which is developing national guidance for clinicians in the US, will need to weigh the need to contain record high rates of STIs impacting millions of Americans against potentially giving rise to more antibiotic-resistant strains.

"Innovation and creativity matter in public health, and more tools are desperately needed," senior US CDC official Dr Jonathan Mermin said.

But the recommendations, set for publication this summer (2023), will remain narrow in scope.

They will likely target only the most at-risk groups of gay men and transgender women with histories of prior infection.

Two-thirds reduction

Reported cases of the three bacterial infections grew to 2.5 million in the US in 2021, following about a decade of growth.

Several issues are behind the rise: fewer people are using condoms since the advent of PrEP – daily pills that significantly reduce chances of contracting HIV (human immunodeficiency virus).

And people who are on PrEP are recommended to undergo screening every three months, likely increasing the identification of infections.

Then there is the basic epidemiological fact that the greater the number of people infected, the more they can further infect.

Researchers have found DoxyPEP efficacious in three of four trials.

"What we found was there was about a two-thirds reduction in sexually-transmitted infection every three months," Professor Dr Annie Luetkemeyer, who co-led a US trial, said.

The physician-scientist at the University of California, San Francisco, recruited some 500 people in San Francisco and Seattle among communities of men who have sex with men and transgender women.

Efficacy was greatest for chlamydia and syphilis, both of which were reduced by about 80%, while for gonorrhoea it was about 55%.

There were few side effects.

Antibiotic resistance

Broadening access to doxycycline has prompted concerns about causing antibiotic resistance, particularly in gonorrhoea, whose causative bacteria is fast mutating.

But early research hasn't shown cause for alarm.

Prof Dr Connie Celum of the University of Washington, who co-led the US study, said that researchers tested gonorrhoea samples from breakthrough infections in the DoxyPEP group and compared them to the group who didn't receive the pill.

Though they found the rate of resistant gonorrhoea slightly higher in the DoxyPEP group, she says the finding could simply mean the pill is less effective against already resistant strains, rather than causing that resistance.

DoxyPEP could even boost responsible antibiotic stewardship – cutting the incidence of infections, thus also cutting need for antibiotic treatment.

If it slashed gonorrhoea cases by some 50%, it could reduce the number of people requiring antibiotic treatment with the current frontline treatment drug, ceftriaxone, which doctors are eager to preserve against antibiotic resistance.

Longer term study is required, on both impacts on STIs, as well as "bystander" bacteria such as Staphylococcus aureus, which live inside people's noses, but sometimes cause serious infections.

For Prof Dr Luetkemeyer, DoxyPEP won't be "the answer" to the STI epidemic, and there is considerable interest in the development of a gonorrhoea vaccine.

"But I'm optimistic... I think this is an additional tool," she said. – AFP Relaxnews