Data from Evaluation of Residual Disease and TKI Duration Are Critical Predictive Factors for Molecular Recurrence after Stopping Imatinib First-line in Chronic Phase CML Patients

Purpose:Tyrosine kinase inhibitor (TKI) discontinuation is an emerging goal in chronic myelogenous leukemia (CML) management and several studies have demonstrated the feasibility of safely stopping imatinib. A sustained deep molecular response on long-term TKI is critical prior to attempting treatment-free remission. Reproducible results from several studies reported recently, failed to identify robust and reproducible predictive factors for the selection of the best candidates for successful TKI cessation.Patients and Methods:We conducted a prospective national phase II study evaluating the cessation of imatinib after at least 2 years of MR4.5 obtained on imatinib first-line in patients with chronic phase CML.Results:A total of 218 patients with de novo chronic phase CML were involved in the study. The median follow-up after imatinib cessation was 23.5 (1–64) months, 2 patients died from unrelated causes, and 107 experienced a confirmed increase in BCR-ABL1 levels defined as molecular recurrence. The molecular recurrence-free survival was 52% [95% confidence interval (CI), 45%–59%] at 6 months, and 50% (95% CI, 43%–57%) at 24 months. Droplet digital PCR (ddPCR) was used to evaluate more accurately low levels of BCR-ABL1 in 175 of 218 patients at imatinib cessation. To apply positive BCR-ABL1/ABL1 ratios on the international scale (IS), a conversion factor was calculated for ddPCR and the significant cut-off point was established at 0.0023%IS. In a multivariate analysis, the duration of TKI (≥74.8 months) and ddPCR (≥0.0023%IS) were the two identified predictive factors of molecular recurrence, with P = 0.0366 (HR, 0.635; 95% CI, 0.415–0.972] and P = 0.008 (HR, 0.556; 95% CI, 0.360–0.858), respectively.Conclusions:We conclude that the duration of TKI and residual leukemic cell load as determined by ddPCR are key factors for predicting successful treatment-free remission for patients with de novo chronic phase CML.See related commentary by Yan et al., p. 6561