Measuring and Modifying Temozolomide Delivery in Brain TUmours

INTRODUCTION: Malignant brain tumours constitute a fatal diagnosis with 5-year survival rates of less than 5%. Temozolomide (TMZ) is the central chemotherapy agent used in the treatment of malignant brain tumours; however, despite aggressive treatment approaches outcomes are still poor with many patients developing chemoresistance. P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) are the main efflux transporters implicated in the development of chemoresistance. Furthermore, P-gp and BCRP appear to work closely together as anticancer efflux pumps. Interestingly, the relationship between TMZ and P-gp and BCRP remains poorly understood. In this work we aim to evaluate the roles of P-gp and BCRP in restricting TMZ delivery into the brain and evaluate the effects of P-gp and BCRP inhibition on the delivery of TMZ in a glioma model in mice. METHODS: TMZ drug transport experiments were performed in a porcine derived and validated in-vitro blood-brain barrier model. In-vivo Positron Emission Tomography (PET) imaging studies with [11C]TMZ in wild type mice and in mice lacking P-gp, BCRP or both transporters. Similar PET studies with [11C]TMZ were conducted in wild type mice with and without the chemical inhibition of P-gp and BCRP. Finally, a human glioma model in mice was derived by intracranial injection of human U87 glioma cells in thirty-five athymic female mice. Animals were treated with TMZ alone (50 mg/kg), Tariquidar alone (TQD; a potent P-gp and BCRP inhibitor – 40 mg/kg) or combination therapy (TMZ and TQD). Control animals received vehicle solution only. Response to treatment was evaluated by performing volumetric tumour measurement using Magnetic Resonance Imaging (MRI). RESULTS: In-vitro transport experiments showed a trend for TMZ to be transported P-gp and BCRP. PET scans confirmed higher delivery [11C]TMZ in mice lacking P-gp, BCRP and both transporters. Similar results were noted in wild type animals following the chemical inhibition of P-gp and BCRP. In U87 glioma model experiments; both TMZ alone and combination therapy resulted in a significant glioma response when compared with the control and the TQD alone treatment groups. Moreover, combination therapy resulted in a significantly superior response when compared with TMZ alone treatment. CONCLUSION: Our combined in-vitro and in-vivo data confirms TMZ as a weak substrate for both P-gp and BCRP at the blood-brain barrier. Furthermore, P-gp and BCRP inhibition may be a suitable strategy for modifying TMZ delivery in brain tumours that warrants further investigations.