Background: Asciminib is the first BCR::ABL1 inhibitor to specifically target the ABL Myristoyl Pocket (STAMP). In the ASCEMBL primary analysis, asciminib had superior efficacy and better safety/tolerability than BOS in pts with CML-CP after ≥2 prior TKIs. After a median follow-up of 2.3 years (16.5 months’ additional follow-up since primary analysis), we report efficacy and safety results (cutoff: October 6, 2021). Methods: Eligible pts were adults with CML-CP after ≥2 prior TKIs, with intolerance or lack of efficacy per 2013 European LeukemiaNet. Pts were randomized 2:1 to asciminib 40 mg twice daily or BOS 500 mg once daily, stratified by major cytogenetic response (MCyR) status (Ph+ metaphases ≤35%) at baseline. The key secondary endpoint was major molecular response (MMR) rate at wk 96. Results: 233 pts were randomized to asciminib (n=157) or BOS (n=76). At cutoff, treatment was ongoing in 84 (53.5%) and 15 (19.7%) pts, respectively; the most common reason for discontinuation was lack of efficacy in 38 (24.2%) and 27 (35.5%) pts, respectively. MMR rate at wk 96 (per ITT) was higher on asciminib (37.6%) than BOS (15.8%). The difference, adjusting for baseline MCyR, was 21.7% (95% CI, 10.5%-33.0%; 2-sided P=.001). More pts on asciminib than BOS, respectively, had BCR::ABL1^IS≤1% (45.1% vs 19.4%) at wk 96. The probability of maintaining MMR and BCR::ABL1^IS≤1% for ≥72 wk was 96.7% (95% CI, 87.4%-99.2%) and 94.6% (95% CI, 86.2%-97.9%), respectively, on asciminib and 92.9% (95% CI, 59.1%-99.0%) and 95.0% (95% CI, 69.5%-99.3%), respectively, on BOS. Median duration of exposure was 103.1 (range, 0.1-201.1) wk on asciminib and 30.5 (range, 1.0-188.3) wk on BOS. Despite the longer duration of asciminib exposure, safety/tolerability of asciminib continued to be better than that of BOS (Table). No new on-treatment deaths were reported since the primary analysis. Most frequent (>10%) grade ≥3 adverse events (AEs) on asciminib vs BOS were thrombocytopenia (22.4%, 9.2%), neutropenia (18.6%, 14.5%), diarrhea (0%, 10.5%), and increased alanine aminotransferase (0.6%, 14.5%). Conclusions: After >2 years of follow-up, asciminib continued to show superior efficacy and better safety/tolerability vs BOS. Responses were durable, with more pts on asciminib in MMR. Additionally, more pts on asciminib had BCR::ABL1^IS≤1%, a milestone response in later lines associated with improved survival. These results continue to support the use of asciminib as a new CML therapy, with the potential to transform standard of care. Clinical trial information: NCT03106779.

^a 1 pt developed cytopenia after randomization and was not treated per investigator’s decision.