The Canadian Tyrosine Kinase Inhibitor Discontinuation Trial with Imatinib Discontinuation As a First Attempt and with Dasatinib Discontinuation As a Second Attempt of Treatment-Free Remission: Results of 4 Years of Follow-up

Introduction: The Canadian trial entitled "Treatment Free Remission Accomplished By Dasatinib" (BMS CA180543, NCT#02268370) is ongoing since Jan 2015, and has completed accrual of 131 patients. The study was designed to determine if using dasatinib (DA) can lead to a successful treatment-free remission (TFR) after failing a first attempt of TKI discontinuation following imatinib (IM) treatment. The preliminary results (ASH 2018) indicate: 1) The 6-month molecular relapse-free survival (mRFS) rate is estimated as 58.0%; 2) DA re-treatment is feasible and safe, with achievement of excellent rates of MMR and MR4; 3) The estimated TFR2 rate after DA discontinuation was 21.5±8.5% at 6 months [7.9-39.5%]). Herein, we report the 4-year follow-up results with updated TFR2 after second TFR attempt following DA discontinuation.

Methods and materials: This prospective clinical trial has 3 phases: 1) IM discontinuation phase, 2) DA rechallenge phase, 3) DA discontinuation phase. Molecular relapse is defined as an increase in BCR-ABL transcript level above MR4.0 on 2 consecutive occasions, or an increase in BCR-ABL transcript level above MR3.0 on a single occasion. 100mg daily of DA is started if molecular relapse is confirmed and is discontinued 12 months after achieving MR4 following a 2^nd TFR attempt.

Results: As of Jun 25, 2019, 58 (44.3%) of 131 enrolled patients experienced molecular relapse after IM discontinuation with a mRFS rate of 59.1% [50.1-67.0%] and 56.8% [47.8-64.8%] at 6 and 12 months, respectively. TFR using loss of MMR as an event was 69.8% at 6/12 months. Of the 58 patients who lost response, 53 patients (91.4%) lost response within 6 months after IM discontinuation: 7 (10.1%) lost response within 2 months, 20 (34.5%) within 3 months, 14 (24.1%) within 4 months, 9 (15.5%) within 5 months, and 3 (5.2%) within 5-6 months. Beyond 6 months, 5 patients (15.5%) lost response within 7, 8, 10, 20, 21 months, respectively. Only two patients experienced late relapse occuring 15 months after IM discontinuation.

54 patients started DA, of whom 49 patients (90.7%) achieved MR4.5 on DA. Median time to MMR, MR4 and MR4.5 was 0.94, 1.95, and 2.48, respectively. The incidence of MMR, MR4 and MR4.5 at 3 months was 99.0% (86.3-99.0%), 91.5% (78.4-96.7%), and 76.6% (60.9-86.0%), respectively.

32/49 patients receiving DA attained MR4.5, and discontinued DA for a 2^nd TFR attempt (TFR2). 25/32 (78.1%) of these patients lost molecular response at a median of 3.67 months after DA discontinuation. The estimated TFR2 after DA discontinuation was 18.5% at 6 months [6.8-34.7%], TFR2 using loss of MMR as a definition of molecular relapse was 20.4% [7.6-37.4%], while TFR2 using two consecutive losses of MR4 was 25.4% [9.4%-45.2%]. Two patients continued to attain deep molecular response at MR4.2 and undetectable level (equivalent to MR5.5) beyond 18 months after DA discontinuation.

At last follow-up of Jun 25, 2019, 30 patients are still being monitored on trial on IM discontinuation (n=20), DA rechallenge (n=4) or DA discontinuation phases (n=6). With a median follow-up duration of 36 months, risk factor analyses were performed using Cox's proportional hazard regression model suggesting a strong correlation of mRFS with total duration of IM treatment prior to IM discontinuation (p<0.001, HR 0.864), MR4 duration (p<0.001, HR 0.867), but not with time to achieve MR4 (p=0.216). When patients attained MR4 or deeper for longer than 10.59 years, their mRFS was 91.8% [71.1-97.9%] at 12 months. The group who attained MR4 duration of 6.93 years or longer had 40.3% reduced risk of mRFS (HR 0.597 [0.449-0.794], p<0.001] compared to those with MR4 duration less than 6.93 years. The group who attained 10.59 years of MR4 duration or longer had a 65.5% reduced risk of mRFS (HR 0.345 [0.171-0.699], p=0.003] compared to those with MR4 duration less than 10.59 years.

Conclusion: The 4-year follow-up results suggests that DA rechallenge after failing a first IM discontinuation attempt for TFR was safe, feasible and well tolerated. It was effective in most cases rapidly regained at least MR4. Based on the two cases who successfully discontinued DA more than 18 months after DA consolidation following achievement of deep molecular response, second generation TKI therapy after imatinib discontinuation failure is a feasible option. Further follow-up is strongly warranted in order to reach a clear conclusion on this issue.