Clinical and Functional Evidence That Drug Transporter Expression Dictates the Response to Imatinib in Chronic Myeloid Leukaemia.

We have previously shown that imatinib is transported into cells by hOCT1. More recently, we and others have presented data that hOCT1 expression may affect the clinical response to imatinib. Here we extend these observations in two main ways: 1) We have assessed the expression of hOCT1, MDR1 (ABCB1), MRP1 and ABCG2 (BCRP) in a cohort of 70 CML patients, of which 36 achieved complete cytogenetic response (CCR), 4 partial response and 30 no cytogenetic response (NCR). Peripheral blood was taken at baseline and after 4 weeks and 3 months of imatinib treatment. hOCT1, MDR1, MRP1 and ABCG2 gene expression levels were assessed by using real-time quantitative RT-PCR on a LightCycler. Patients were allocated to high-expression and low-expression groups using the median expression as the cut-off level. With a median follow-up of 22 months (from the start of imatinib treatment), patients with high-expression of hOCT1 at baseline had better progression free and overall survival (both p=0.005, log rank test) and a higher probability of achieving CCR (p=0.001, log rank test). MDR1, MRP1 and ABCG2 expression levels at the start of treatment had no correlation with either survival or the achievement of CCR. In addition, amongst responding patients there was a trend for patients with higher MDR1 expression to have a higher incidence of later relapse, typically with the emergence of mutations in the BCR-ABL kinase domain. In 3 of 4 patients who have lost their initial CCR, the MDR1 expression levels were greater than 10% at 3 months and all 4 cases developed BCR-ABL kinase mutation. In contrast, only 4 of 28 patients who had sustained their CCR had greater than 10% of MDR1 expression and none of them had BCR-ABL kinase mutation. 2) In order to further explore the pivotal role of hOCT1 in imatinib uptake into CML cells, we have established stably transfected KCL22 CML cells with high hOCT1 expression. In comparison to untransfected and mock transfected controls, high hOCT1 expressing cells show increased uptake of the fluorescent cation 4-(4-(dimethyl-amino)styryl)-N-methylpyridinium iodide (ASP + ), ¹⁴C-TEA and ¹⁴C-radiolabelled imatinib (p=0.0002). This enhanced uptake was blocked by the hOCT1 inhibitors amantidine (p=0.0003) and verapamil (p=0.0001). Taken together, the data demonstrate that the baseline expression of hOCT1 (but not MRP1 or ABCG2) is important in dictating the clinical response to imatinib, by means of increased cellular imatinib delivery. In addition, patients with high MDR1 expression may have a higher probability of loss of imatinib response and of kinase domain mutations, and may benefit from closer surveillance of BCR-ABL transcripts and mutations.