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Doxycycline post‑exposure prophylaxis for sexually transmitted infections in MSM and transgender women on antiretroviral drug combination

Verified siteRegistered drugNo placebo
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What is this trial about?

A plain-language summary of the goals, design and what participants do

This trial looks at men who have sex with men and transgender women who are at risk for sexually transmitted infections. It also examines the presence of methicillin-resistant Staphylococcus aureus (a type of bacteria that no longer responds to many common antibiotics) in the nose and changes in the gut microbiota (the community of bacteria that live in the intestines). Participants may be using pre-exposure prophylaxis (PrEP) (a daily pill taken to prevent HIV infection) or be living with HIV. The study medication is a daily oral tablet containing doxycycline, referred to in the protocol as doxyPEP, which is an antibiotic used to try to prevent infections after possible exposure. The trial also looks at the development of antibiotic resistance (when bacteria change so medicines no longer work).

The purpose is to see how taking this antibiotic affects the rate of resistant nasal bacteria, the frequency of sexually transmitted infections, and the composition of gut bacteria over one year. Participants will be asked to take the study tablet for about 12 months, attend a few clinic visits for simple nose swabs (samples taken from the inside of the nose) and stool samples, and answer short questionnaires about health and sexual activity. The study will compare the outcomes with those of participants who do not receive the antibiotic.

The research process

The trial runs in 4 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Baseline assessment and enrollment

    You will attend an initial visit where study staff will confirm your eligibility and obtain written consent.

    A nasal swab will be taken to check for colonization by staphylococcus aureus, a stool sample will be collected to analyze the gut microbiota, and testing for sexually transmitted infections (such as syphilis, chlamydia, gonorrhea) will be performed.

  2. Step 2

    Randomization and start of study medication

    You will be randomly assigned to receive either doxycycline as post‑exposure prophylaxis or a comparator regimen.

    If assigned to doxycycline, you will be instructed to take a single oral dose of 200 mg within 72 hours after each sexual encounter, for the duration of the study (up to 12 months).

  3. Step 3

    Regular follow‑up visits

    You will attend follow‑up visits at scheduled intervals, such as month 3, month 6, and month 12.

    At each visit a repeat nasal swab and stool sample will be taken to monitor changes in nasal colonization by doxycycline‑resistant staphylococcus aureus and in the gut microbiota.

    Testing for new sexually transmitted infections will also be performed, and any infections will be recorded.

  4. Step 4

    Final assessment

    At the month 12 visit the study will be concluded for you.

    Final samples will be collected, and the overall incidence of nasal colonization, resistance patterns, and sexually transmitted infections during the study period will be evaluated.

Who can join the trial?

3 criteria

  • Adult patients (18 years or older) who are MSM (men who have sex with men) or transgender women, and who are using PrEP (pre‑exposure prophylaxis, a daily medicine to prevent HIV) or are living with HIV.
  • Must have at least two risk factors for sexually transmitted infections (STIs). The possible risk factors are: having more than 10 different sexual partners in the past year; having anal sex without a condom at least once in the past year; using drugs that are linked to unprotected sex at least once in the past year; or taking post‑exposure prophylaxis (medicine taken after possible exposure to prevent infection) more than once in the past year.
  • Must sign an informed consent form, which is a written agreement to join the study after understanding what it involves.

Who cannot join the trial?

4 criteria

  • Being younger than 18 years old.
  • Taking ongoing medicines that are not HIV medicines (antiretroviral therapy) and also using antibiotics for a long time for another health condition.
  • For people using PrEP (a daily pill to prevent HIV) who have been on it for less than 3 months, or for people living with HIV who either have a measurable amount of virus in their blood (detectable viral load) or have been on HIV medicines (antiretroviral therapy) for less than 3 months.
  • Being involved in another research study that uses drugs that fight germs (antimicrobials).
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Investigated drugs

  • Doxycycline

    is the test medication in this study. It is given as a pill after a possible exposure to a sexually transmitted infection. The goal is to see if taking doxycycline can lower the chance of getting infections like chlamydia, gonorrhea, or syphilis, and to examine how it might affect the bacteria living in the gut.

  • Juluca

    is a combination pill that contains two HIV medicines, rilpivirine and dolutegravir. In the trial it is used as a standard HIV treatment that some participants are already taking, allowing researchers to compare outcomes while participants also receive the doxycycline prophylaxis.

  • Triumeq

    contains three HIV drugs: abacavir, lamivudine, and dolutegravir. It serves as another background HIV therapy for participants, helping ensure that any effects seen are due to the doxycycline and not changes in HIV medication.

  • Dovato

    combines lamivudine and dolutegravir in one tablet. It is listed as a comparator HIV regimen that participants may be using during the study.

  • Odefsey

    is a fixed‑dose tablet with emtricitabine, tenofovir alafenamide, and rilpivirine. It is included as a common HIV treatment among the study population.

  • Biktarvy

    provides emtricitabine, tenofovir alafenamide, and bictegravir in a single pill. It is another standard HIV medication used by some participants in the trial.

What is already known about the treatment

  • Juluca 50 mg/25 mg film-coated tablets

    This tablet is taken by mouth and comes as a film‑coated pill. It is an approved, widely used medicine for treating HIV infection. The pill combines rilpivirine, which blocks a viral enzyme needed to make new virus, and dolutegravir, which stops another viral enzyme that inserts viral DNA into human cells; together they belong to the classes of non‑nucleoside reverse transcriptase inhibitors and integrase inhibitors. It is prescribed for adults with HIV to keep the virus under control.

  • Triumeq 50 mg/600 mg/300 mg film-coated tablets

    This oral tablet contains three drugs in one pill and is taken once daily. It is an approved HIV treatment that includes abacavir, lamivudine, and dolutegravir. Abacavir and lamivudine are nucleoside reverse transcriptase inhibitors that stop the virus from copying its genetic material, while dolutegravir is an integrase inhibitor that prevents the virus from inserting its DNA into the host cell; the combination belongs to the nucleoside reverse transcriptase inhibitor and integrase inhibitor classes. It is used to treat adults living with HIV.

  • Dovato 50 mg/300 mg film-coated tablets

    This medication is an oral, film‑coated tablet taken once a day. It is an approved fixed‑dose combination for HIV infection that contains lamivudine and dolutegravir. Lamivudine blocks a viral enzyme needed for copying the virus’s genetic code (a nucleoside reverse transcriptase inhibitor) and dolutegravir blocks the integrase enzyme that inserts viral DNA into human cells (an integrase inhibitor). It is prescribed for adults with HIV to keep the virus suppressed.

  • DOXYCYCLINE

    This product is an oral tablet that also contains ambroxol hydrochloride, and it is taken by mouth as a pill. Doxycycline is an approved antibiotic that belongs to the tetracycline class and is used to treat a wide range of bacterial infections. It works by attaching to bacterial ribosomes and stopping the bacteria from making proteins they need to grow, while ambroxol helps thin mucus in the throat and lungs. In this trial it is being studied as a post‑exposure preventive pill to reduce sexually transmitted infections.

  • Odefsey 200 mg/25 mg/25 mg film-coated tablets

    This tablet is taken orally once daily and combines three drugs in a single film‑coated pill. It is an approved HIV medication that includes emtricitabine, tenofovir alafenamide, and rilpivirine. Emtricitabine and tenofovir alafenamide are nucleoside reverse transcriptase inhibitors that stop the virus from copying its DNA, while rilpivirine is a non‑nucleoside reverse transcriptase inhibitor that blocks the same enzyme in a different way; the combination belongs to the nucleoside and non‑nucleoside reverse transcriptase inhibitor classes. It is used to treat adults living with HIV.

  • Biktarvy 30 mg/120 mg/15 mg film-coated tablets

    This oral, film‑coated tablet is taken once daily and contains three HIV drugs. It is an approved, widely used treatment that combines emtricitabine, tenofovir alafenamide, and bictegravir. Emtricitabine and tenofovir alafenamide are nucleoside reverse transcriptase inhibitors that block the virus from copying its genetic material, and bictegravir is an integrase inhibitor that prevents the virus from inserting its DNA into human cells; the product belongs to the nucleoside reverse transcriptase inhibitor and integrase inhibitor classes. It is prescribed for adults with HIV infection.

Investigated diseases

  • Methicillin-resistant Staphylococcus aureus colonization

    This condition refers to the presence of MRSA bacteria in the nasal passages without causing obvious illness. The bacteria can remain on the skin or in the nose for months, often spreading to other body sites. When the skin barrier is broken, the colonizing bacteria may cause localized skin infections. Repeated exposure to antibiotics can increase the proportion of resistant strains. Over time, the colonization can become more widespread within a community.

  • Syphilis

    Syphilis is an infection caused by the bacterium Treponema pallidum. It typically begins with a painless sore at the site of entry, then may develop a rash and other systemic signs. If untreated, the infection can enter a latent phase that may last years. Later stages can involve the heart, nervous system, and other organs. The disease progresses in a stepwise manner from early to late manifestations.

  • Chlamydia trachomatis infection

    This infection is caused by the bacterium Chlamydia trachomatis and often produces no symptoms. When symptoms appear, they may include urethral discharge or cervical irritation. The infection can ascend to the upper genital tract, leading to pelvic inflammatory disease. Repeated infections can cause scarring of reproductive tissues. Over time, chronic inflammation may develop if the organism persists.

  • Neisseria gonorrhoeae infection

    Gonorrhea results from infection with the bacterium Neisseria gonorrhoeae. It commonly affects the urethra, cervix, rectum, or throat, causing discharge or discomfort. The bacteria can spread through the bloodstream to joints and other sites. Persistent infection may lead to chronic inflammation of affected tissues. The condition can evolve from localized symptoms to more widespread involvement.

Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2025-525150-19-01Protocol codeMiDoxyR-PEPEstimated enrolment392 patientsSponsorInstituto De Investigacion Marques De Valdecilla

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