Residual Peripheral Blood CD26(+) Leukemic Stem Cells in Chronic Myeloid Leukemia Patients During TKI Therapy and During Treatment-Free Remission

Chronic myeloid leukemia (CML) patients in sustained ``deep molecular response{''} may stop TKI treatment without disease recurrence; however, half of them lose molecular response shortly after TKI withdrawing. Well-defined eligibility criteria to predict a safe discontinuation up-front are still missing. Relapse is probably due to residual quiescent TKI-resistant leukemic stem cells (LSCs) supposedly transcriptionally low/silent and not easily detectable by BCR-ABL1 qRT-PCR. Bone marrow Ph+ CML CD34(+)/CD38(-) LSCs were found to specifically co-express CD26 (dipeptidylpeptidase-IV). We explored feasibility of detecting and quantifying CD26(+) LSCs by flow cytometry in peripheral blood (PB). Over 400 CML patients (at diagnosis and during/after therapy) entered this cross-sectional study in which CD26 expression was evaluated by a standardized multiparametric flow cytometry analysis on PB CD45(+)/CD34(+)/CD38(-) stem cell population. All 120 CP-CML patients at diagnosis showed measurable PB CD26(+) LSCs (median 19.20/mu L, range 0.27-698.6). PB CD26(+) LSCs were also detectable in 169/236 (71.6\%) CP-CML patients in first-line TKI treatment (median 0.014 cells/mu L; range 0.0012-0.66) and in 74/112 (66\%), additional patients studied on treatment-free remission (TFR) (median 0.015/mu L; range 0.006-0.76). Notably, no correlation between BCR-ABL/ABL(IS) ratio and number of residual LSCs was found both in patients on or off TKIs. This is the first evidence that ``circulating{''} CML LSCs persist in the majority of CML patients in molecular response while on TKI treatment and even after TKI discontinuation. Prospective studies evaluating the dynamics of PB CD26(+) LSCs during TKI treatment and the role of a "stem cell response" threshold to achieve and maintain TFR are ongoing.