CD117 (c-kit) Expression On CD34+ Cells Participates In The Cytogenetic Response To Imatinib In CML Patients In First Chronic Phase

Chronic myeloid leukemia (CML) biology seemed to be perfectly explored especially at the beginning of tyrosine kinase inhibitors era (TKI). Later years with imatinib and second generation TKI showed variety of resistance mechanisms and it became obvious bcr-abl chimeric gene is not the only enemy to fight. Some studies assumed the decreased rate of programmed cell death (apoptotic) to be the primary mechanism by which BCR-ABL effects expansion of the leukemic clone in CML. Other studies showed the important role of patients adherence in achieving best possible response for imatinib. Imatinib plasma level was recognized as a useful tool for adherence evaluation. In this study we evaluate the expression of apoptotic marker, Annexin V in CD34+CD117+ cells of CML patients in first chronic phase treated with imatinib and try to find the correlation between this expression and cytogenetic response or imatinib plasma level.

The group of 54 CML patients (F/M = 30/24, median age, 50.5) in first chronic phase treated with imatinib (400 mg daily) was analyzed. The median time of treatment was 17 months (min. 12 months, max. 24 months. Only patients with the conventional translocation (Philadelphia chromosome) without additional chromosomal aberrations or clonal evolution during the treatment period were included in the study. Patients were categorized according to ELN criteria for cytogenetic response: complete cytogenetic response (CCyR) vs. no cytogenetic response (NCyR, defined as everything less than CCyR), and for molecular response: major molecular response (MMR) vs. no molecular response (NMR).

In the cohort of 54 patients, 39 achieved CCyR in a median time of 12 months. Among these patients, 30 achieved MMR within the same time. Bone marrow CD34 positive cells were assessed for expression of CD117 and Annexin V in all groups of patients (CCyR with MMR, CCyR with NMR, and NCyR). The mean percentage of CD34+CD117+ cells was significantly higher in the NCyR group (7.67±5.62) in comparison with the CCyR group (2.27±1,78; p=0.002). The difference between MMR and NMR subgroups was not significant. While analyzing the CD34+CD117+ population, we found a significantly higher percentage of apoptotic cells (Annexin V positive) in the CCyR group (4.65±4.55) than in the NCyR group (1.67±1.22; p=0.004). Once again this difference was not significant between MMR and NMR subgroups. Serum imatinib levels were quantified in both CCyR and NCyR groups. We found higher values in the CCyR group (1244 μg/l±599) than in the NCyR group (1192 μg/l±593) but this difference was insignificant.

It was recently reported that c-kit must be inhibited to allow apoptosis of CML cells. Our results also correspond with these data. Not only was a lower percentage of CD34+CD117+ cells found in the CCyR group, but the fraction of these cells that were apoptotic was significantly greater compared with the NCyR group. Although other studies have indicated that trough plasma concentration of imatinib reflects clinical response in chronic phase of CML, we did not observe this. Our results showed higher imatinib levels in CCyR, but these data were insignificant. In conclusion, our results indicate that to achieve optimal treatment response in CML patients, c-kit kinase inhibition may be a requirement for successful proapoptotic activity of imatinib.

No relevant conflicts of interest to declare.