Abstract
Introduction: ZSP1601 is a novel pan-phosphodiesterase inhibitor developed in China specifically for the treatment of nonalcoholic fatty liver disease (NAFLD).

Aim: The aim is to develop a population pharmacokinetic (pop PK) model for ZSP1601 by integrating data from two clinical studies. This undertaking aims to deepen our understanding of the clinical factors that influence ZSP1601 exposure while simultaneously investigating exposure-response (ER) relationships related to efficacy and safety. The goal is to guide formulating optimal dosage strategies in the subsequent phases of clinical trials.

Methods: Analysis of pooled concentration-time data from 95 subjects, with 2647 observations from two clinical trials involving healthy volunteers and NAFLD patients, employed a nonlinear mixed-effects modeling approach to characterize ZSP1601 pharmacokinetics. Covariate impact on ZSP1601 pharmacokinetics was investigated, and relationships between ZSP1601 exposure, efficacy and safety endpoints were explored.

Results: A two-compartment model featuring sequential zero-order then first-order absorption and first-order elimination effectively described ZSP1601's pharmacokinetic profile. Covariate analyses identified body weight as a statistically significant factor affecting drug central volume, while FED (food consumption) influenced absorption rate constant and duration. The Sigmoid Emax model aptly captured exposure-response relationships for ALT (alanine aminotransferase), AST (aspartate aminotransferase), and LFC (liver fat content) percentage changes relative to baseline and ZSP1601 exposure levels (AUCss) on the 29th day. ZSP1601 exposure levels (Cmax1) exhibited a significant exposure-response relationship with headaches (p < 0.001).

Conclusion: The PopPK model and ER analysis, based on available data, comprehensively characterizes ZSP1601's pharmacokinetic, safety and efficacy profile, aiding informed decisions regarding dosage selection for the drug's complete developmental trajectory. The exposure-response (ER) analysis yields quantitative insights into the optimal balance of efficacy and safety within different dosage regimens for patient administration. In light of these findings, the dose regimen of 100 mg administered twice daily is proposed for subsequent clinical investigations.