Tyrosine kinase inhibitor gefitinib enhances topotecan penetration of gliomas

Gefitinib, an epidermal growth factor receptor tyrosine kinase inhibitor, increases brain parenchymal extracellular fluid (ECF) accumulation of topotecan, a substrate of the ATP-binding cassette (ABC) transporters P-glycoprotein (Pgp/MDR-1) and breast cancer resistance protein (BCRP/ABCG2). The effect of modulating these transporters on topotecan penetration in gliomas has not been thoroughly studied. Thus, we performed intracerebral microdialysis on mice bearing orthotopic human gliomas (U87 and MT330) and assessed topotecan tumor ECF (tECF) penetration and the effect of gefitinib on topotecan tECF penetration and intratumor topotecan distribution. We found that topotecan penetration (P_tumor) of U87 was 0.96 ± 0.25 (n = 7) compared with that of contralateral brain (P_{contralateral}, 0.42 ± 0.11, n = 5; P = 0.001). In MT330 tumors, P_tumor (0.78 ± 0.26, n = 6) and P_{contralateral} (0.42 ± 0.11, n = 5) also differed significantly (P = 0.013). Because both tumor models had disrupted blood-brain barriers and similar P_tumor values, we used U87 and a steady-state drug administration approach to characterize the effect of gefitinib on topotecan P_tumor. At equivalent plasma topotecan exposures, we found that P_tumor after gefitinib administration was lower. In a separate cohort of animals, we determined the volume of distribution of unbound topotecan in tumor (V_u,tumor) and found that it was significantly higher in groups receiving gefitinib, implying that gefitinib administration leads to a greater proportion of intracellular topotecan. Our results provide crucial insights into the role that transporters play in central nervous system drug penetration and provide a better understanding of the effect of coadministration of transporter modulators on anticancer drug distribution within a tumor.