Fecal pharmacokinetics/pharmacodynamics characteristics of fidaxomicin and vancomycin against Clostridioides difficile infection elucidated by in vivo feces-based infectious evaluation models

Objectives: The pharmacokinetics (PK)/pharmacodynamics (PD; PK/PD) characteristics of fidaxomicin (FDX) and vancomycin (VCM) against Clostridioides difficile infection (CDI) are yet to be elucidated because of the lack of an established PK/PD analysis method for intestinal infections and unabsorbed oral drugs. Here, we developed a feces-based PK/PD analysis method and determined the fecal PK/PD index, with target values of FDX and VCM against CDI. Methods: The antimicrobial susceptibility, time-kill curves, and post-antibiotic effects (PAEs) of FDX and VCM against C. difficile were determined in vitro. The optimal fecal PK/PD indices, with target values, were determined from the results of PK and PD studies involving 5-week-old female C57BL/6J mice infected with C. difficile ATCC (R) 43255. The minimum inhibitory concentration (MIC) breakpoints for C. difficile were estimated based on clinical data concerning fecal antibiotic concentrations in patients with CDI. Results: FDX and VCM inhibited C. difficile growth via time-dependent antibacterial activity and exerted PAEs. In the CDI mouse model experiments, the changes in C. difficile load and clinical cures (72-hour survival rates and clinical sickness score grading) were most highly correlated with the ratio of area under the fecal drug concentration-time curve to MIC (AUC0_,infinity/MIC). The target AUC0_,infinity/MIC values of FDX and VCM for 3 log10 reduction in C. difficile load was 13,173 and 8,308, respectively. The MIC breakpoints of FDX and VCM for C. difficile was estimated to be 1.0 and 2.0 mg/ mL, respectively. Conclusions: The developed in vivo feces-based PK/PD analysis method elucidated the optimal fecal PK/ PD index, with target values of FDX and VCM against CDI. Sho Tashiro, Clin Microbiol Infect 2023;29:616 (c) 2022 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.