Study to determine the dose and safety of asciminib in pediatric patients with chronic myeloid leukemia.

Prior treatment

• Known presence of the T315I mutation prior to study entry or of a BCR: ABL mutation with known resistance to study treatment any time prior to study entry
• Known second chronic phase of CML after previous progression to AP/BC.
• Previous treatment with a hematopoietic stem-cell transplantation.
• Patient planning to undergo allogeneic hematopoietic stem cell transplantation.

Other illness

• Cardiac or cardiac repolarization abnormality.

• Male or female participants: a. Pediatric formulation group: ≥1 and less than 18 years of age at study entry. b. Adult formulation group: ≥14 and less than 18 years of age and body weight of ≥ 40 kg at study entry.
• Participants with Ph+ CML-CP must meet all of the following laboratory values at the screening visit. In the case where bone marrow blast and promyelocyte counts are available, these will be accepted if done locally within 56 days prior to the screening visit, to avoid unnecessary repetition of this test. a. <15% blasts in peripheral blood and bone marrow b. < 30% combined blasts plus promyelocytes in peripheral blood and bone marrow c. < 20% basophils in the peripheral blood d. Neutrophils ≥ 1.5 x 10^9/L (or white blood cell (WBC) ≥ 3 x 10^9/L if neutrophils are not available) and platelet count ≥ 100 x 10^9/L e. No evidence of extramedullary leukemic involvement, with the exception of hepatosplenomegaly
• Prior treatment with a minimum of one TKI.
• Failure or intolerance to the most recent TKI therapy at the time of screening.
• Evidence of typical BCR-ABL fusion gene (BCR-ABL1) transcript [e14a2 and/or e13a2] at the time of screening which are amenable to standardized real time quantitative polymerase chain reaction (RQ-PCR) quantification.