Mecillinam against genital Chlamydia trachomatis infection: a small-scale proof-of-concept study shows a low cure rate.

Objectives: Mecillinam is highly active in vitro against Chlamydia spp. We aimed to determine whether mecillinam should be evaluated further as treatment for genital Chlamydia trachomatis infection.

Patients And Methods: The study was conducted at an open-access clinic for sexually transmitted infections in Oslo, Norway. We planned to include 50 patients. Participants were asymptomatic, heterosexual male patients with a first-void urine sample found to be positive for C. trachomatis by PCR. Treatment consisted of 400 mg of pivmecillinam hydrochloride three times a day for 7 days. A test-of-cure sample, a medication diary and a questionnaire were returned by the participants, and they were used to evaluate treatment outcome, compliance, risk of reinfection and theoretical percentage of time above MIC (t/MIC %). The study was registered in Eudra-CT (no. 2013-002379-179) and clinicaltrals.gov (NCT02083276).

Results: The study was discontinued after including 20 patients, due to a high failure rate. Only two of the 17 participants who delivered a test-of-cure sample were cured. Three participants reported condomless sex before the follow-up sample. When the average or most favourable pharmacokinetics (PK)/pharmacodynamics (PD) reported from other studies were applied in a theoretical model, the estimated t/MIC % was above 50% for all of the 15 participants returning a medication diary. Using the least favourable PK/PD, no participant had t/MIC % of >36%. The mean dose interval was 8 h 36 min (standard deviation 3 h 12 min).

Conclusions: A low cure rate combined with uncertainty about intracellular availability and attained t/MIC % makes mecillinam an unattractive candidate for further evaluation as treatment for genital C. trachomatis infection.