Background: There is a need for new treatment options for pts with CML who are intolerant/resistant to currently available BCR-ABL1 ATP-binding site targeted TKIs. Asciminib is a potent and specific BCR-ABL1 inhibitor with a novel allosteric mechanism of action targeting the ABL1 myristoyl pocket. This results in a mutation-driven resistance profile different from that of ATP binding-site TKIs, providing potential for both monotherapy and combination therapy with ATP-binding-site TKIs. In a phase 1 study (NCT02081378), asciminib showed clinical activity and good safety/tolerability in CML pts with resistance/intolerance to ≥2 TKIs and in pts with the T315I mutation. The recommended dose for asciminib monotherapy in CML pts without the T315I mutation was established as 40 mg BID. An ongoing phase 3 study (NCT03106779) is evaluating asciminib monotherapy vs bosutinib in pts with CML who have been treated with ≥2 prior ATP-binding-site TKIs. Methods: Eligible pts are adults with CML-CP who previously received ≥2 TKIs, with intolerance or failure to the most recent TKI. Treatment failure is defined per 2013 European LeukemiaNet (ELN) recommendations. Pts harboring T315I or V299L mutations are excluded. Pts are randomized 2:1 – stratified by baseline cytogenetic response status – to receive asciminib 40 mg BID or bosutinib 500 mg QD (planned enrollment: N = 222). Primary and key secondary objectives are to compare the rate of major molecular response (BCR-ABL1^IS≤0.1%) with asciminib vs bosutinib at 24 and 96 weeks, respectively. In a recent protocol amendment, the baseline BCR-ABL1^IS threshold for enrollment was lowered from ≥1% to > 0.1% for pts with intolerance to the most recent TKI. This change was implemented to align with clinical practice and satisfy the treatment need to avoid waiting for an increase in BCR-ABL1^IS levels to ≥1%. In addition, pts with documented bosutinib treatment failure as per ELN recommendations may switch to receive asciminib therapy at any time, as such pts may have limited treatment options outside of this study. This study is ongoing with 149 participating study sites in 30 countries. Clinical trial information: NCT03106779