Updated 18-Month Results from Dasfree: A Study Evaluating Dasatinib Discontinuation in Patients (Pts) with Chronic Myeloid Leukemia in Chronic Phase (CML-CP) and Deep Molecular Response (DMR)

Introduction: Tyrosine kinase inhibitor (TKI) discontinuation is being investigated in pts with CML-CP with sustained DMR (defined here as MR^4.5 or BCR-ABL1 ≤ 0.0032% on the International Scale [IS]), with the goal of treatment-free remission (TFR). Successful TFR has been reported previously for pts enrolled in DASFREE (CA180-406/NCT01850004), which showed that 48% of CML-CP pts with DMR for ≥ 1 year were able to stop dasatinib and maintain major molecular response (MMR) 12 months after discontinuation. Here we present updated results from pts followed for a minimum of 18 months, in order to understand the durability of TFR beyond 12 months.

Methods: DASFREE is a phase 2, open-label, single-arm study in adult pts with CML-CP on dasatinib for ≥ 2 years as 1st-line or subsequent therapy. Eligible pts had dasatinib-induced DMR (MR^4.5) confirmed at a local lab for ≥ 1 year prior to enrollment, with a 1-log reduction in BCR-ABL1 from baseline within 3-6.5 months of starting dasatinib. MR^4.5 was confirmed at a central lab twice within 3 months prior to dasatinib discontinuation (screening phase). BCR-ABL1 was monitored centrally after discontinuation every month in the 1st year, then every 3 months. Pts resumed dasatinib at their previous dose if MMR was lost. The primary endpoint is the rate of MMR 12 months after dasatinib discontinuation. Secondary endpoints include BCR-ABL1 kinetics, molecular relapse-free survival (MRFS; no loss of MMR), relapse-free survival (RFS; no loss of MMR, complete cytogenetic response, or complete hematologic response, or progression to accelerated/blast phase [AP/BP] CML), rate of transformation to AP/BP, progression-free survival, and overall survival. Exploratory analyses include frequency of adverse events (AEs) after discontinuation and during dasatinib treatment, and MMR after reinitiating dasatinib.

Results: In total, 84 pts enrolled between February 2014 and June 2016 discontinued dasatinib; all had

≥ 18 months of follow-up after discontinuation at the time of this analysis. Pt characteristics were previously reported (the majority [64%] had low Sokal scores; no pt had prior interferon; 37 pts were on 1st-line dasatinib, 47 on subsequent lines of dasatinib). At 18 months after discontinuation, the RFS rate was 48% (95% CI 37-58) in all pts (Figure), 54% (95% CI 38, 70) in 1st-line pts, and 42% (95% CI 28, 57) in pts who received subsequent-line therapy. With longer follow-up, 1 additional pt lost MMR at 18 months following discontinuation. Of the 45 pts who lost MMR and restarted treatment, 44 regained MMR (1 pt discontinued after only 1 molecular assessment) in a median of 2 months (range 1-4) and 42 regained MR^4.5 in a median of 3 months (range 2-18). Analyses of baseline pt characteristics revealed that for the 40 pts who did not lose MMR after discontinuation, 15 (37.5%) were able to maintain MR^4.5. Additionally, the median time in prior MR^4.5 was 28 months (range 13-116) for all pts, and was similar for 1st-line pts who maintained (27 months [range 13-56]) or lost MMR (27 months [range 15-68]) at 12 months. With longer follow-up, AEs (any cause) identified were consistent with previous reports and were found to be similar on and off treatment: 8 (10%) pts off treatment and 8 (18%) pts on treatment experienced grade 3/4 AEs of any cause after restarting dasatinib (4.4% were drug related). No transformation events or deaths occurred. Of the 13 reported withdrawal events occurring in 8 (9.5%) pts, 10 were resolved (5 off treatment, 5 resolved after restarting treatment due to loss of MMR) after a median of 5 months (range 1-12) after onset. One pt discontinued after restarting dasatinib due to malignancy unrelated to treatment. In addition to efficacy and safety data, multivariate analyses evaluating prognostic factors for MMR will be presented.

Conclusions: Additional follow-up of pts enrolled in DASFREE revealed that TFR remained durable at 18 months after discontinuing dasatinib. AEs reported here were consistent with the known safety profile of dasatinib, and withdrawal was well tolerated. Collectively, this trial, the largest dasatinib discontinuation trial to date, continues to support the feasibility and practicality of TFR in pts with CML-CP in DMR treated with dasatinib in the 1st line and beyond.

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