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EGFR inhibitors, such as gefitinib, are examples of targeted anticancer drugs whose drug sensitivity is related to gene mutations that adds a pharmacogenetic [PG] dimension to any pharmacokinetic [PK] and pharmacodynamic [PD] analysis. The goal of this project was to characterize the PK/PD properties of gefitinib in tumors and then apply these results to design rational drug design regimens, and provide a foundation for future studies with EGFR inhibitors. Progressions of in vitro and in vivo studies were completed to understand the PK and PD behavior of gefitinib. In vitro cytotoxicity assays were first conducted to confirm the gefitinib sensitivity differences in a pair of human glioblastoma cell lines, LN229-wild-type EGFR and LN229-EGFRvIII mutant, an EGFR inhibitor-sensitizing mutation. Subsequent in vitro PD studies identified phosphorylated-ERK1/2 (pERK) as a common PD marker for both cell lines. To describe the most salient features of drug disposition and dynamics in the tumor, groups of mice bearing either subcutaneous LN229-wild-type EGFR or LN229-EGFRvIII mutant tumors were administered gefitinib at doses of 10 mg/kg intravenously (IV), 50 mg/kg intraarterially (IA) and 150 mg/kg orally (PO). In each group, gefitinib plasma and tumor concentrations were quantitated, as were tumoral pERK. Hybrid physiologically-based PK/PD models were developed for each tumor type, which consisted of a forcing function describing the plasma drug concentration-profile, a tumor compartment depicting drug disposition in the tumor, and a mechanistic target-response PD model characterizing pERK in the tumor. Gefitinib showed analogous PK properties in each tumor type, yet different PD characteristics consistent with the EGFR status of the tumors. Using the PK/PD model for each tumor type, simulations were done to define multiple-dose regimens for gefitinib that yielded equivalent PD profiles of pERK in each tumor type. Based on the designed PK/PD equivalent dosing regimens for each tumor type, gefitinib 150 mg/kg PO qd × 15 days and 65 mg/kg PO qd × 15 days multiple-dose studies were conducted in wild-type EGFR and EGFRvIII mutant tumor groups, respectively. In each tumor group, gefitinib plasma and tumor concentrations were measured on both day 1 and day 15, as were tumoral amounts of pERK. Different from single-dose model simulations, gefitinib showed nonlinear PK property in the wild-type tumor due to the down-regulation of membrane transporter ABCG2. Moreover, acquired resistance of tumoral pERK inhibition was observed in both tumor types. Nevertheless, gefitinib had an analogous growth suppression action in both tumor groups, supporting the equivalent PD dosing strategy. Overall, single-dose gefitinib PK/PD investigations in a pair of genetically distinct glioblastomas facilitated the development of hybrid physiologically-based PK/PD models for each tumor type, and further introduced a novel concept of PK/PD equivalent dosing regimens which could be applied in novel drug development paradigms. Preliminary multiple-dose gefitinib studies revealed more complex PK/PD characteristics that needed to be further explored. / Pharmaceutics

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