We have previously shown that imatinib is a substrate for both the efflux transporter P-glycoprotein (ABCB1; MDR-1 gene product) and the influx transporter human Organic Cation Transporter 1 (hOCT1; SLCA22). High hOCT1 and low ABCB1 expression levels in chronic myeloid leukaemia (CML) patients correlate with improved clinical outcome (

Wang L. et al. Clinical Pharmacology and Therapeutics; epub June 13th
). The second generation tyrosine kinase inhibitor nilotinib is 30-fold more potent than imatinib and is effective in imatinib-resistant patients. However, its cellular transport has not been characterised. The CML cell line KCL22 was used as it has low basal hOCT1 expression. Drug transport studies of 14C-radiolabelled nilotinib (kind gift from Novartis) were performed using high expressing hOCT1 transfected KCL22 cells, with or without OCT1 and ABCB1 inhibitors. Further vectorial ABCB1 efflux studies were performed on confluent monolayers of ABCB1 transfected Type II Madin Darby canine kidney (MDCKII) cells. Nilotinib efflux was not modulated in the presence of the ABCB1 inhibitors PSC-833 (10μM), tariquidar (500nM) and verapamil (500μM), and no evidence of transport via ABCB1 was seen in the transfected ABCB1 MDCKII cells. Nilotinib influx was not modulated in hOCT1 over-expressing cells, or in the presence of OCT1 inhibitors prazosin (100μM) and amantadine (500μM). Nilotinib uptake was also not modulated by the hOCT1 substrate tetraethylamine (TEA) (5μM), however nilotinib inhibited TEA uptake. The distribution coefficient (logD) of nilotinib was 2.1, compared with 0.81 for imatinib, demonstrating high lipophilicity for nilotinib. Flow cytometry revealed greater suppression of phospho-CrKL (a surrogate marker for BCR-ABL blockade) with nilotinib after 1 hour when compared with imatinib, regardless of hOCT1 expression, indicating that nilotinib is effective at BCR-ABL suppression even at low hOCT1 levels, in contrast to imatinib. Nilotinib was not a substrate for the ABCB1 and hOCT1 transporters; however it may be a potential inhibitor of hOCT1. Therefore the level of expression of ABCB1 and hOCT1 in CML is unlikely to be an indicator of the clinical outcome for patients receiving nilotinib, in contrast to what we have shown for imatinib. Nilotinib may therefore be effective in patients who have low hOCT1 expression before start of therapy.

Author notes

Disclosure:Research Funding: Funding received from Novartis and the Leukaemia Research Fund (LRF).