Unlike Imatinib, Dasatinib Uptake into Chronic Myeloid Leukaemia Cells Is Independent of hOCT1 Expression.

In chronic myeloid leukaemia (CML), resistance to imatinib is an important clinical issue. We have previously shown that imatinib uptake into CML cells and cell lines is dependent on the uptake transporter human Organic Cation Transporter 1 (hOCT1; SLCA22). Furthermore, in clinical samples, low hOCT1 expression is an important mechanism of imatinib resistance (Wang L et al, Clinical Pharmacology and Therapeutics; epub June 13^th 2007). Dasatinib is a second generation novel tyrosine kinase inhibitor that is effective in many imatinib-resistant patients. Its mechanism of transport into and out of CML cells has not been previously investigated. We hypothesised that dasatinib might be transported differently to imatinib, which might account for its favourable effects in imatinib-resistant patients. The CML cell line KCL22 was selected for transfection work as it has a particularly low basal level of hOCT1 expression. KCL22 cells were transfected with pcDNA3-hOCT1 plasmid (kind gift of D Gründemann, Germany) and stable lines were selected with high hOCT1 expression. The uptake of radiolabelled dasatinib (kind gift from Bristol Myers Squibb) was greater in high-hOCT1 expressing cells than in mock transfected cells (p=0.0149, n=3). However, prazosin and amantadine, both inhibitors of hOCT transport, did not decrease dasatinib uptake into mock transfected KCL22 cells, in sharp contrast to the block on imatinib uptake seen with both these inhibitors. Dasatinib decreased the level of phosphorylated CRKL (surrogate marker for BCR-ABL) by 49.9% in mock transfected KCL22 cells and 40.3% in high-hOCT1 expressing cells. The distribution coefficient (logD) of dasatinib between 1-octanol and HEPES medium was 2.05, compared with 0.81 for imatinib, demonstrating that dasatinib is more lipophilic. The efflux of dasatinib was investigated in confluent monolayers of Madin-Darby canine kidney (MDCKII) cells on a semipermeable membrane. These cells stably express ABCB1 (MDR1) on their apical but not their basal aspect. Both dasatinib and imatinib were transported from the basal to the apical layer, indicating ABCB1 transporter-mediated efflux of both drugs (p=0.001, p<0.0001, respectively). Addition of the ABCB1 inhibitor PSC833 blocked transport of both drugs (p=0.0013, p<0.0001 respectively). Taken together, the data are consistent with the view that dasatinib, unlike imatinib, may achieve adequate intracellular levels and BCR-ABL suppression even in cells with low or blocked hOCT1 function. However, in cells with high hOCT1 expression, dasatinib uptake may be augmented. Efflux of dasatinib and imatinib appear similar and via ABCB1. Dasatinib may be useful therapeutically in patients with imatinib resistance related to low hOCT1 expression.