OCT-1 Activity in CML CD34+ Cells Is Not Predictive of Molecular Response to Imatinib Treatment in CP-CML Patients, Despite the Strong Predictive Value of MNC OCT-1 Activity.
Authors: Engler Jane R , Frede Amity , Saunders Verity A , Zannettino Andrew Christopher William , White Deborah L , Hughes Timothy P. 
Published: October 2019

Abstract
Abstract 2189
Poster Board II-166

Introduction.
The organic cation transporter 1 (OCT-1) is the major active influx transporter for imatinib in PB mononuclear cells (MNC) from CML patients. We have previously demonstrated that the functional activity of the OCT-1 protein (OCT-1 Activity, OA) in MNC from chronic phase (CP) CML patients is highly variable (mean: 7.2 ng/200,000 cells, standard deviation: 6.4). We also demonstrated that the MNC OA is significantly related to a patient's sensitivity to imatinib-induced kinase inhibition, and their subsequent molecular response. In addition, we have recently demonstrated that OA is predictive of event free and transformation free survival to 5 years (White ASH submitted 2009). This is of a greater importance for those patients on lower doses of imatinib (below 600mg). For these patients who have a low MNC OA, 82% failed to achieve a major molecular response (MMR) by 18 months of imatinib treatment, as opposed to 17% for patients with a high MNC OA (p=0.022). Recently, we have shown that the OA in CML CD34+ cells is significantly lower than that found in mature CML cells (mean CD34+: 3.9, CD34-: 11.6 ng/200,000 cells, p<0.001, n=36), however interpatient variation in CD34+ OA still remains (standard deviation: 2.65). Given the importance of low MNC OA as a prognostic indicator of poor molecular response, we sought to ascertain whether a patient's MNC OA was related to the OA in their CD34+ cells, and as such could be used as a surrogate indicator for efficient targeting of the primitive population. To do this, we firstly looked for a correlation between the OA in a patient's MNC and their CD34+ cells. Secondly, we assessed the predictive value of CD34+ OA for MMR in imatinib treated patients.

Methods.
MNC and CD34+ cells were isolated from imatinib naïve, newly diagnosed CP-CML patients using density gradient centrifugation and magnetic cell sorting, respectively. OA was determined using [14C]-labeled imatinib and the OCT-1 inhibitor prazosin. OA is calculated as the difference between the uptake and retention of imatinib in the absence and presence of prazosin. Statistical analyses were performed using the Mann-Whitney Rank Sum or Student's t-test, correlations were performed using the Spearman-Rank Order.

Results.
In 34 CML patients no correlation was observed between the OA in their MNC and the OA in their CD34+ cells (R=0.09, p=0.577). Furthermore, when these patients were divided as having high or low OA in their MNC (about a previously described median of 7.2 ng/200,000 cells), no difference was seen between the OA in their corresponding CD34+ cells (Table 1).
Finally, in 19 patients where 12 month response data was available, patients were grouped according to the achievement or not of MMR by 12 months. MNC OA was found to be significantly associated with the achievement of MMR (Table 2). This suggests that patients with a high OA in their MNC achieve better molecular responses, which is in agreement with our previous findings. However, assessment of CD34+ OA failed to demonstrate a relationship between OA and the achievement of MMR (Table 2). Therefore, a low OA in patient's CD34+ cells (unlike MNC) does not appear to be associated with poor molecular responses.

Conclusion.
No relationship was identified between the OA measured in a patient's MNC and that in their CD34+ cells. Furthermore, CD34+ OA is not predictive of a patient's molecular response to imatinib treatment. This is in contrast to MNC OA, which is predictive of event free and transformation free survival to 5 years of imatinib treatment. This data suggests that the predictive value of the MNC OA may primarily reflect the effective targeting and subsequent eradication of mature CML cells. And that kinase inhibition in these mature cells (as opposed to CD34+ progenitor cells) may be the key determinant of MMR achievement in CP-CML.
Supported by Novartis Oncology, Clinical Development, TOPS Correlative Studies Network

Disclosures:
Zannettino: Novartis Pharmaceuticals: Research Funding. White:Novartis Pharmaceuticals: Honoraria, Research Funding. Hughes:Novartis Pharmaceuticals: Honoraria, Research Funding, Speakers Bureau.