Abstract 1663: A tyrosine kinase inhibitor enhances topotecan penetration of orthotopic glioma xenografts
Authors: Carcaboso Angel M. , Elmeliegy Mohamed A. , Shen Jun , Juel Stephen J. , Zhang Ziwei M. , Calabrese Christopher , Tracey Lorraine , Waters Christopher M. , Stewart Clinton F. 
Published: February 2011

Gefitinib, an epidermal growth factor receptor tyrosine kinase inhibitor, modulates the ABC transporters P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP), and increases brain parenchymal extracellular fluid (ECF) accumulation of topotecan, a substrate of Pgp and BCRP. The effect of modulating these transporters on topotecan penetration in brain tumors has not been thoroughly studied. Thus, we performed intracerebral microdialysis on mice bearing intracranial human gliomas (U87 and MT330 models) to assess topotecan tumor ECF (tECF) penetration, and the effect of gefitinib on topotecan tECF penetration and intratumor topotecan distribution. Following topotecan (4 mg/kg) intravenous injections, drug penetration (Ptumor, calculated as the tECF/plasma ratio of topotecan exposures) of U87 was 0.96 ± 0.25, n=7, compared with that of contralateral brain (Pcontralateral, 0.42 ± 0.11, n=5; P=0.001). In MT330 tumors, Ptumor (0.78 ± 0.26, n=6) and Pcontralateral (0.42 ± 0.11, n=5) also differed significantly (P=0.013). The increased intratumor drug penetration in these models may be explained by compromised blood brain barrier integrity, as confirmed by contrast-enhancement magnetic resonance imaging. Because both tumor models showed profuse expression of BCRP (as assessed by western blot and immunohistochemistry), we tested the effect of oral gefitinib (200 mg/kg) on topotecan efflux by tumor cells. We used the U87 model and a steady-state drug administration approach (based on continuous release subcutaneous pumps) to characterize, by microdialysis, the effect of gefitinib on topotecan Ptumor. At equivalent plasma topotecan exposures (30 ng/mL), Ptumor (calculated as the tECF/plasma ratio of topotecan steady-sate concentrations) decreased significantly with gefitinib administration (from 0.72 ± 0.35, n=7, in absence of gefitinib, to 0.21 ± 0.03, n=3, with gefitinib; P=0.017). To evaluate whether the decrease in tECF drug levels and Ptumor values was due to increased intracellular penetration, we used a separate cohort of animals to determine topotecan levels in whole tumor homogenates and calculated the volume of distribution of unbound topotecan in tumor, Vu,tumor (ml/g tumor). Vu,tumor was significantly higher in groups receiving gefitinib (6.01 ± 2.61 mL/g, n=6; versus 1.96 ± 0.92 mL/g without gefitinib, n=8; P=0.016), which implies that gefitinib administration leads to a greater proportion of intracellular topotecan in the brain tumor. To support these results, we performed additional studies in vitro and showed significantly increased topotecan accumulation in cells co-cultured with gefitinib (P&lt;0.05 in U87 and MT330 models). Our results provide crucial insights into the role that transporters play in CNS drug penetration and provide a better understanding of the effect of the co-administration of transporter modulators on anticancer drug distribution within a tumor.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1663.