Low Baseline Intrinsic Sensitivity to Imatinib (High IC50) in CML Patients Is Due to Reduced Oct-1 Mediated Influx. Intrinsic Sensitivity to AMN107 Does Not Correlate with That of Imatinib and Uptake of AMN107 Is Not Oct-1 Mediated.

We have recently demonstrated that there is variation in the intrinsic sensitivity of de novo CML patients to imatinib. Patients with low intrinsic sensitivity to imatinib, have a lower probability of achieving major molecular response by 12 months. Intrinsic sensitivity was determined pre-therapy using western blot, with the IC50^imatinib defined as the dose of drug required in vitro to reduce phosphorylation of the adaptor protein Crkl (p-Crkl) by 50%. In the current study we show that the observed variation in IC50^imatinib between patients can be explained by the intracellular concentration of imatinib. In 16 de novo CML patients the IC50^imatinib strongly correlated (R² = −0.6, p = 0.002) with the intracellular concentration of ¹⁴C-labelled imatinib after 2 hours exposure at 37°C. In keeping with the notion that the organic cation transporter Oct-1 is responsible for imatinib influx, the addition of Prazosin, an inhibitor of Oct-1, to ¹⁴C imatinib experiments greatly reduced the inter-patient variation in intracellular concentration.

We have now demonstrated that IC50 values for the novel, more selective Bcr-Abl inhibitor AMN107 are on average 22-fold lower than those observed for imatinib and there is no correlation between the IC50 values for imatinib and AMN107 (R² = 0.375, p>0.05). There was also less inter-patient variation in AMN107 IC50 (range 0.07uM) when compared to imatinib (range 3.1uM). In contrast to imatinib, there is no correlation between IC50^AMN107 and the intracellular concentration of drug achieved in ¹⁴C-uptake studies (R² = 0.337, p>0.05). Further to this the addition of prazosin had no effect on influx of AMN107 suggesting that Oct-1 may not be a rate limiting determinant of intracellular drug concentration. In conclusion, variable Oct-1 mediated influx of imatinib may explain the heterogeneous IC50^imatinib values we have observed in de novo CML patients. Oct-1 mediated influx may therefore be a key determinant of molecular response to imatinib. The causes of inter-patient variability in Oct-1 mediated influx requires further investigation. By contrast, poor Oct-1 mediated uptake is unlikely to have any impact on response to AMN107. This may be of relevance to patients with low intrinsic sensitivity to imatinib.