Model-Based Dose-Exposure-Response Assessment for Lead and Backup Spectinamide in a Mouse Model of Tuberculosis
Authors:   ,   University of Tennessee Health Science Center, Wagh Santosh 
Published: April 2021

Despite decades of research, tuberculosis remains the oldest pathogen-based disease that is the leading cause of death from a single infectious agent. Among many anti-tubercular therapies under investigation, the semisynthetic compounds spectinamides are a promising novel class of anti-tuberculosis agents. One such lead candidate, spectinamide 1810, and backup spectinamide 1599 have demonstrated excellent efficacy, safety, and drug-like properties in various in vitro and in vivo assessments. The dose-ranging and dose fractionation studies were designed to characterize the dose-exposure-response relationship for lead and backup spectinamide in a mouse model of Mycobacterium tuberculosis infection. In this current study, we used 26 and 23 combinations of dose level and dosing frequency for the lead and backup spectinamide, respectively. The dedicated pharmacokinetic studies with a collection of series of blood samples were conducted in healthy animals. Population pharmacokinetic analysis was performed using non-linear mixed effect modeling to estimate pharmacokinetic parameters in healthy animals. The Bayesian principles were applied for reliable pharmacokinetic estimation in infected animals by using informed priors obtained from healthy animals. The individual pharmacokinetic parameters were obtained for infected animals through post-hoc estimation and subsequently used for pharmacokinetic/-pharmacodynamic (PK/PD) indices and mechanism-based PK/PD modeling. The obtained data on spectinamides’ plasma concentrations and counts of colony-forming units were analyzed using a PK/PD approach as well as classical anti-infective PK/PD indices. The population pharmacokinetic analysis results suggest that there is no difference in the pharmacokinetic parameters of lead and backup spectinamide in infected animals as compared to healthy animals. The PK/PD index analysis showed that the efficacy of spectinamide 1810 is largely driven by concentration (Cmax/MIC) and exposure (AUC/MIC) rather than a threshold minimum inhibitory level (T>MIC). Although similar results were obtained for spectinamide 1599 in previously performed in vitro experiments, in the present in vivo studies, spectinamide 1599 did not demonstrate the expected correlation between efficacy and PK/PD indices. Therefore, we could not identify major drivers for the efficacy of this compound. Additionally, a novel mechanism-based PK/PD model with consideration to post-antibiotic effect could adequately describe the exposure-response relationship for lead and backup spectinamide. This supports the idea that the in vitro observed post-antibiotic effect of these spectinamides can translate to the in vivo situation, as well. Altogether we suggest, the obtained results and pharmacometric model for the exposure-response relationship of lead and backup spectinamides provide a rational basis for dose selection for future efficacy studies of these compounds against Mycobacterium tuberculosis in mice and other animal species.