This study aims to build on previous work characterising the PK of penicillin-V to explore the potential impact of probenecid on PK-PD target attainment. Achievement of the aims of this study would provide data to support the design of experimental studies exploring the clinical impact of probenecid on treatment outcomes and also provide a rationale for exploration of probenecid's effects on a larger number of beta-lactam antibiotics.

Hypothesis: Addition of probenecid to oral phenoxymethylpenicillin (penicillin-V) has a clinically relevant effect on pharmacokinetic-pharmacodynamic (PK-PD) target attainment.

Participants will be screened and consented to attend Imperial College Clinical Research Facility (CRF) at Hammersmith Hospital on two study visits, at least 7 days apart. For one visit (randomised), participants will be required to take penicillin-V only. For their other visit, they will take penicillin-V plus probenecid at standard recommended dose. Prior to the study visits, participants may be required to have taken 36-hours of penicillin +/- probenecid, documenting this in a dosing diary. On arrival at the CRF, the participant will take an observed dose of penicillin +/- probenecid. They will undergo blood draw via needle phlebotomy or a cannula (participant choice) at 45 and 180 minutes post the observed. Samples will be spun down and frozen at -80oC. They will subsequently be analysed using an in-house HPLC-MS/MS methodology to determine total and free-unbound drug concentration.

For analysis, data from this study will be pooled with rich PK data from a prior study that assessed plasma concertation of penicillin-V in healthy volunteers. Pmetrics in R will be used to model the data looking to explore the effect of probenecid on clearance of free-penicillin-V. Probability of target attainment for streptococci species will also be estimated to evaluate the potential clinical impact of the addition of probenecid to routine penicillin-V use. Rich PK data for intravenous benzylpenicillin will be used to estimate PK-PD target attainment and PTAs for intravenous formulations, allowing direct comparison of oral and IV regimes.

Inclusion Criteria:

• Adult healthy volunteers (>18 years old).
• Previously taken penicillin-based antibiotics without allergic response.
• Estimated Glomerular Filtration Rate (eGFR) > 90.

Exclusion Criteria:

• Lacking capacity to consent.
• Documented allergy to penicillin, other beta-lactam antibiotics, or probenecid.
• History of G6PD Deficiency.
• Known blood dyscrasias.
• Anaemia (Hb < 12g/dL female, 13g/dL males).
• Abnormal liver function (ALT, AST, ALP or bilirubin > ULN).
• eGFR < 90.
• Pregnant or likely to become pregnant during study period.
• Breastfeeding women.
• Symptoms consistent with active infection.
• History of gout or uric acid kidney stones.
• Taking regular medication that may interact with probenecid including, but not limited to methotrexate, lorazepam, acetaminophen, oral hypoglycaemic medication, sulfa containing drugs, non-steroidal anti-inflammatory drugs.
• History of evidence of any medical, neurological, or psychological condition that would expose the subject to an undue risk of a significant adverse event or interfere with study assessments during the course of the trial as determined by the clinical judgement of the investigator.
• Recent involvement in other research (within prior 3 months).

Data will be shared upon reasonable request with appropriate permissions in place.

The protocol will be available from commencement of study. The clinical study report will be published within 12 months of completion of the study.

Access via the study principle investigator or online publications.

One participant withdrawn after enrolment in Penicillin 750mg QDS THEN Penicillin 750mg QDS plus probenecid 500mg QDS

Data collected over 3 weeks from participant enrolment to final visit completion per participant.

Results have been reported as per our protocol. Results are not reported per arm/group as dose dependent adverse events were not assessed. Assessment of adverse events aimed to compare penicillin alone against penicillin with probenecid, as is presented in this table.