The Functional Activity of the OCT-1 Protein Is Predictive of Molecular Response and Survival in CP-CML Patients Treated with Imatinib: A 5 Year Update of the TIDEL Trial.

The major active influx protein for imatinib into target BCR-ABL positive cells is the organic cation transporter OCT-1. We have previously demonstrated that the functional activity of the OCT-1 protein (OCT-1 activity) is predictive of molecular response in TIDEL (trial of imatinib 600 mg/day with selective dose intensification in untreated CP-CML) The OCT-1 activity (OA) is measured in mononuclear cells from untreated CML patients by calculating the intracellular concentration of 14-C imatinib less the intracellular concentration in the presence of OCT-1 inhibition. To address the question of whether OA is predicting only the rate of response, we now investigate the impact of OA on response and progression at 5 years. There is a significant difference in the achievement of MMR (p=0.007) and CMR by 60 months (p=0.032) (Table 1). Six patients developed kinase domain mutations over the course of this study. 5/6 had low OA. Significantly, for the first time addressing Event Free Survival (events defined as loss of CHR, MCR or CCR, progression to AP or BC or change of therapy due to unsatisfactory efficacy), we demonstrate that more patients with high OA are event free at 5 years when compared to patients with low OA (Table 1).

To determine whether the detrimental effect of low OA on survival was more significant in those patients with OA in the lowest quartile (Q1) we compared the response of Q1 patients to all other patients (Table 2). These data demonstrate importantly, that patients in Q1 have significantly poorer outcomes, than the remainder of the patient cohort.

In previous analyses we have shown that the effects of a low OA can be partially overcome by higher imatinib doses. Limiting the analyses to those patients receiving <600mg average daily dose over the first 12 months there was a significant difference in the achievement of MMR (low OA (n=11) 27%: high OA (n=12) 92% p=0.021) and EFS (36% vs 75% p=0.03). In patients receiving ≥600 mg there was no significant difference between the groups, reinforcing the importance of dose.

In 45 patients we examined the expression of OCT-1 mRNA for prediction of MMR, CMR, EFS and mutation development. Dividing the patients into low and high OCT-1 expression about the median we found that the level of mRNA is not predictive of MMR (low–60% vs high 78 p=0.241) CMR (low–45% vs high 55 p=0.456) EFS (low–55% vs high 70 p=0.315) or mutation development (low–18% vs high 14% p=0.666). These data indicate that the level of OCT-1 mRNA is not sufficiently discriminating to predict response and progression.

While our previous studies demonstrated that OA could predict the rate of decline in BCR-ABL over the first 12-24 months, this update demonstrates for the first time, that this assay can identify nearly all patients (>80%) who fail to achieve MMR in the long term. Most importantly OA is also strongly predictive of resistance and progression events. Functional assessment of OCT-1 Activity provides prognostic information that is more discriminating than assaying the level of OCT-1 mRNA. This long term study reinforces the notion that OA is an important predictive variable in CP-CML patients treated with IM. It provides further evidence that OA is a critical variable to consider in future trials of imatinib and a key factor to enable individualization of imatinib dose to optimize the long term outcome for CML patients.

White:Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Research Funding. Manley:Novartis: Employment. Hughes:Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.