Glioblastomas are highly vascular brain tumors that are characterized as heterogeneous, comprised of an anatomically and functionally irregular blood brain barrier (BBB) that contributes to variable drug distribution and possibly associated pharmacodynamics (PD) responses. Standard pharmacokinetic (PK) approaches that are based on whole tumor homogenates and accordingly averaged drug concentrations are limited in their ability to depict regional variations in drug concentrations, and could lead to faulty assessments of drug distribution and activity. Given the paucity of quantitative information on intratumoral PK/PD variability of anticancer drugs, the goal of this project was to characterize the regional PK and PD properties of gefitinib, an EGFR inhibitor, in brain tumors, in the context of biological characteristics, and use the information to develop mechanistic PK/PD models that may be valuable to understand why some anticancer drugs are inactive. Towards this end, in vitro cytotoxicity assays and pilot in vivo studies were first conducted and identified U87VIII mutant cell line as a gefitinib sensitive orthotopic brain tumor model with suitable growth characteristics to allow for immunohistochemical (IHC) analysis of tumor biological characteristics for further studies. Subsequently, in vitro PD studies identified phosphorylated-ERK1/2 (pERK) as a PD marker for this cell line. Thereafter, to set up a framework to obtain intratumoral PK/PD information in vivo, a novel tumor sectioning protocol was devised and sensitive and robust PK (LC-MS/MS) and PD (Meso Scale Discovery, MSD, electrochemiluminescence-based assay) methods were developed to enable the use of minimal amounts of tumor samples to assess the intratumoral PK and PD characteristics of gefitinib. Mice bearing orthotopic U87VIII mutant tumors were administered gefitinib at doses of 150 mg/kg and 50 mg/kg orally (p.o.), followed by collection of plasma and tumor samples at various time points, based on a serial sacrifice study design. Serial tumor sections were obtained in four distinct regions according to the aforementioned protocol for PK, PD and IHC measurements. An IHC index called microvessel pericyte index (MPI), a measure of BBB integrity, reflected the variability in gefitinib brain tumor concentrations, and was used to bin the data to generate three intratumoral PK/PD data sets. These data sets were then used to develop a set of hybrid physiologically based PK/PD models accounting for variable BBB permeability within the tumor. Each model consisted of a forcing function describing plasma concentration profile, a tissue compartment to represent the drug disposition within the tumor, and target-response compartments for the PD model. The intratumoral variation in the PKs of gefitinib was accurately described by the MPI classifications and ranged about 2-fold, and was responsible for the associated PD variability. In summary, using a novel tumor sectioning protocol and sensitive analytical methods in an orthotopic glioblastoma (GBM) model, the intratumoral variability of gefitinib PK/PD could be binned according to BBB integrity and enabled the development of a mechanistic hybrid physiologically based PK/PD models, which provides a means to assess the influence of tumor heterogeneity on drug response. / Pharmaceutical Sciences