Enhancing the Functional Activity of the OCT-1 Influx Pump May Overcome the Negative Impact of Low OCT-1 Activity in Imatinib Treated CML Patients

Abstract

The human organic cation transporter-1 (hOCT-1) is the major active influx protein responsible for the transport of imatinib into blood cells 1,2. The functional activity of the OCT-1 protein is defined as the intracellular uptake and retention (IUR) of 14-C labelled imatinib into patient pre therapy mononuclear blood cells over a two hour period, which is inhibited by OCT-1 inhibitors such as prazosin or procainamide. The level of OCT-1 activity is a key determinant of the interpatient variation observed in intrinsic sensitivity to imatinib induced kinase inhibition (IC50imatinib3). We have previously demonstrated that a significantly greater proportion of de-novo CML patients with high functional activity of OCT-1, achieve a major molecular response (MMR: 3 log reduction in BCR-ABL mRNA from standardised baseline) when treated with imatinib, than patients with low OCT-1 Activity 4. We have also identified a link between dose and OCT-1 Activity, demonstrating that the negative impact of low OCT-1 Activity could be overcome to a variable extent by imatinib dose increase. However, not all patients can dose increase, largely because of tolerability issues. While the transport of second-generation ABL-kinase inhibitors (nilotinib and dasatinib5) is not OCT-1 mediated, the long term effect of these drugs is not yet known. Hence, we sought to identify strategies to increase OCT-1-mediated imatinib uptake. We queried the drug gene expression signatures in version 1 of the Connectivity Map (CMAP; Lamb J, Nat. Rev. Cancer 7; 54–60, 2007: http://www.broad.mit.edu/cmap) with 3 transporters including OCT-1. This identified the Rho kinase inhibitor fasudil and COX-2 inhibitor / celecoxib analogue LM1685 as potential up-regulators of OCT-1 mRNA. The impact of these drugs on OCT-1 mRNA expression and IC50imatinib (fasudil alone to date) has been analysed in two bcr-abl positive cell lines (K562 and KU812). The effect of these two candidate OCT-1 enhancers on OCT-1-mediated imatinib uptake was also assessed in 10 newly diagnosed chronic phase CML patients, previously demonstrated to have low OCT-1 Activity (4 with no demonstrable OCT-1 Activity), using the IUR assay.Table 1: Assessing the effects of fasudil and LM1685 on the intracellular transport of imatinib. These data demonstrate a statistically significant increase in OCT-1 Activity with LM1685, and show a strong trend towards significance with fasudil. Importantly, we show that patients with no demonstrable OCT-1 Activity (0ng/200,000 cells) have detectable Activity in the presence of both fasudil (Range 1.5 to 2ng/200,000 cells) and LM1685 (Range 1.5 to 4.5 ng/200,000 cells). We have previously demonstrated that patients with no detectable OCT-1 Activity universally fail to achieve imatinib therapeutic response milestones (imatinib failure), whereas 54% of patients with low, but detectable OCT-1 Activity achieve these milestones4. The ability to enhance the functional activity of the OCT-1 protein may therefore be of significant clinical relevance in this group. In addition we demonstrate an increase in imatinib IUR which, along with the increase in OCT-1 Activity, is likely associated with increased OCT-1 mRNA levels. Significantly, in the two CML cell lines tested we show a marked reduction in the IC50imatinib indicating that the observed increase in IUR and OCT-1 Activity translates to an increase in the kinase inhibitory activity of imatinib. Preliminary analysis in one patient analysed to date also indicates a reduction in IC50 from 0.48 to 0.35μM in the presence of fasudil. In the clinical scenario the use of such OCT-1 enhancers may improve the response of some imatinib treated patients to both standard and increased dose imatinib. Importantly, these findings validate the use of resources such as C-MAP to identify candidate drugs that may mediate desired changes in the levels of key proteins resulting in improved response to therapy.Fasudil (10μM) %increase from controlLM1685 (1μM) % increase from controlIUR of imatinibOCT-1 ActivityIC50imatinibIUR of imatinibOCT-1 ActivityK56276% (n=5)163% (n=5)51% reduction (n=3)41% (n=2)122% (n=2)KU81210.7% (n=2)75% (n=2)15% reduction (n=2)NANACML patients n=108%89%9%114%p value>0.050.08>0.050.03