Population Pharmacokinetic and Pharmacodynamic Modeling of Epacadostat in Patients With Advanced Solid Malignancies

Epacadostat (EPA, INCB024360) is a selective inhibitor of the enzyme indoleamine 2,3-dioxygenase 1 (IDO1) and is being developed as an orally active immunotherapy to treat advanced malignancies. In the first clinical study investigating the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of EPA in oncology patients, increasing doses of EPA ranging from 50 mg once daily to 700 mg twice daily were administered as a monotherapy to 52 subjects with advanced solid tumors. The EPA plasma concentration-time profiles were adequately described by a population PK model comprised of the first-order kinetics of oral absorption with 2-compartment distribution and constant clearance from the central compartment. Body weight was the only significant covariant to influence EPA PK. Determination of EPA's on-target potency, ie, its half-maximal inhibitory concentration (IC₅₀ ) against IDO1, is important for dose selection but complicated by the bioconversion of tryptophan (TRP) to kynurenine (KYN) catalyzed by both IDO1 and TRP 2,3-dioxygenase (TDO). In vitro and ex vivo, the IC₅₀ was estimated following the selective induction of IDO1, rendering the TDO activity relatively insignificant; however, it was desirable to determine the in vivo IC₅₀ without inducing an IDO1 abundance. A mechanistic population PD model was developed based on time-matched EPA, TRP, and KYN plasma concentrations in 44 oncology patients, and EPA in vivo IC₅₀ was estimated to be ∼70 nM, consistent with the ex vivo value independently determined. The model suggests that ∼60% and 40% of TRP→KYN bioconversion was mediated by IDO1 and TDO, respectively, in the cancer patients at baseline. For this study population of limited numbers of subjects, neither age nor sex was a significant covariate for EPA PK or PD.