Efficacy of alternative fidaxomicin dosing regimens for treatment of simulated Clostridium difficile infection in an in vitro human gut model

Background: Fidaxomicin treatment reduces the risk of recurrent Clostridium difficile infection (CDI) compared with vancomycin. Extending duration of fidaxomicin therapy may further reduce recurrence. We compared the efficacy of four extended fidaxomicin regimens in an in vitro model of CDI. Methods: Four gut models were primed with human faeces, spiked with C. difficile spores (PCR ribotype 027) and clindamycin instilled (33.9 mg/L, four-times daily, 7 days) to induce simulated CDI. Four extended fidaxomicin treatment regimens were evaluated: model 1, 20 days, 200 mg/L twice daily; model 2, 5 days 200 mg/L twice daily, 5 days rest, 5 days 200 mg/L twice daily; model 3, 5 days 200 mg/L twice daily, 5 days rest, 10 days 200 mg/L once daily; and model 4, 5 days 200 mg/L twice daily, 20 days 200 mg/L once every other day. C. difficile populations, toxin, gut microbiota and antimicrobial levels were monitored daily. Results: All fidaxomicin regimens successfully resolved simulated CDI without recurrence. Five days of fidaxomicin instillationwas barely sufficient to resolve CDI (models 2-4). A second pulse or tapered dosing further reduced C. difficile and toxin detection. All regimens were sparing of microbiota, affecting only enterococci and bifidobacteria. Pulsed or tapered regimens allowed greater bifidobacteria recovery than the extended (20 day) regimen. Bioactive fidaxomicin persisted throughout the experiment in all models at concentrations inhibitory to C. difficile. Conclusions: Pulsed or tapered fidaxomicin regimens may enhance suppression of C. difficile whilst allowing microbiota recovery; clinical studies are required to ascertain the potential of this approach in further reducing recurrent CDI.