Dasatinib or Nilotinib Discontinuation in Chronic Phase (CP)-Chronic Myeloid Leukemia (CML) Patients (pts) with Durably Undetectable BCR-ABL Transcripts: Interim Analysis of the STOP 2G-TKI Study with a Minimum Follow-up of 12 Months – on Behalf of the French CML Group Filmc

Background: Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL have revolutionized the prognosis of pts suffering from CML but these drugs are considered as non-definitively curative and current recommendation is to treat pts during their entire lifespan. However, prospective trials such as STIM, TWISTER and EUROSKI suggest that imatinib may be successfully stopped in pts with deep and sustained molecular responses. Here, we report on the feasibility of second generation TKIs discontinuation in the setting of the French STOP 2G-TKI study.

Methods: Adult CP-CML pts on dasatinib or nilotinib first line or after imatinib without prior allogeneic transplantation or progression to advanced phase CML were proposed TKI discontinuation when presenting: (1) b2a2 or b3a2 BCR-ABL transcripts subtype, 2) TKI treatment duration for at least 36 months, (3) CMR4.5 achieved and maintained for at least 24 months. The primary objective was treatment-free survival without loss of major molecular response (MMR). After TKI discontinuation, BCR-ABL transcripts were monitored monthly during the first 12 months, every 3 months during the 2^ndyear and every 3 to 6 months thereafter. Molecular relapse was defined by MMR loss on a single occasion and triggered TKI reintroduction. Data as of August 1, 2014 are reported in pts with at least 12 months of follow-up (n=52) and median follow-up was 32 months (12-56).

Results: Median age was 60 years (34-81) and 61.5% of pts were female. Sokal risk group was low in 58%, intermediate in 23%, high in 13% and unknown in 6%. 2G-TKIs were given after imatinib intolerance in 67% of pts, suboptimal response or resistance to imatinib in 23% and upfront in 10%. Median duration of CML, TKI treatment, 2G-TKI treatment and CMR4.5 was 83 months (36-218), 78 months (36-136), 39 months (19-72) and 28 months (24-64), respectively. Twenty four pts lost MMR after a median time of 4 months (1-38) at last follow-up. Importantly, no loss of CHR or progression to advanced phase CML was observed. The 12- and 24-month probabilities of treatment-free survival without MMR loss were 61.4% (95% CI, 48.1-74.6) and 57% (95% CI, 43.3-70.6), respectively. The majority of relapses occurred within 6 months and in a landmark analysis, pts who were still in MMR without therapy at 6 months had 12- and 24-month probabilities of treatment-free survival without MMR loss of 91.2% (95% CI, 81.6-100) and 84.7% (95% CI, 72.2-97.1), respectively. All pts but 1 who lost MMR restarted 2G-TKI treatment and regained MMR after a median time of 3 months (1-8). Pts in MMR without any therapy (n=28) displayed varying patterns of spontaneous molecular response including stable CMR4.5 in 7 and fluctuations between CMR4.5 and MR4.5, CMR4.5 and MR4, CMR4.5 and MMR in 9, 4 and 4 pts, respectively. Gender, age, prior interferon exposure, 2G-TKI type, treatment duration and duration of CMR4.5 were not found to have any impact on outcome. By contrast, prior history of suboptimal response or resistance to imatinib was associated with a significantly lower chance of successful treatment discontinuation, with a 12-month probability of treatment-free survival without MMR loss of 41.7% (95% CI; 13.8%-69.6%), compared to 67.3% (95% CI, 52.6%-81.8%) in other patients (p=0.04).

Conclusions: 2G-TKI could be safely and successfully discontinued in CP-CML pts with long-lasting undetectable BCR-ABL transcripts, especially in those without prior history of suboptimal response or resistance. Most of molecular relapses had an early onset and all were sensitive to 2G-TKI resumption. The recurrence of low levels of detectable residual disease below MMR after 2G-TKI withdrawal did not automatically herald CML relapse and did not preclude the possibility to remain treatment-free.