Chronic Myelogenous Leukemia

BCR-ABL1 oncogene, the Philadelphia chromosome, and the TKI revolution that transformed a fatal disease into a chronic manageable condition

Key Points

• CML is defined by the Philadelphia chromosome t(9;22)(q34;q11.2), producing the BCR-ABL1 fusion oncoprotein with constitutive tyrosine kinase activity.
• TKI therapy (imatinib, dasatinib, nilotinib, bosutinib, ponatinib) has transformed CML from a disease with median survival of 3–5 years to near-normal life expectancy for most patients in chronic phase.
• Response milestones are time-dependent: complete hematologic response by 3 months, major cytogenetic response (≤35% Ph+ metaphases) by 6 months, and complete molecular response (BCR-ABL1 ≤0.1% on IS) by 12–18 months.
• Resistance is most commonly driven by BCR-ABL1 kinase domain mutations; T315I ("gatekeeper" mutation) is resistant to all first- and second-generation TKIs but sensitive to ponatinib and asciminib.
• Asciminib (STAMP inhibitor targeting the ABL myristoyl pocket) is FDA approved for CML with T315I mutation or failure of ≥2 prior TKIs.
• Treatment-free remission (TFR) — discontinuing TKI therapy in patients with sustained deep molecular response (MR4–MR4.5) — is achievable in ~40–50% of patients attempting discontinuation.
• Allogeneic SCT is now reserved for blast phase CML and rare cases of TKI-refractory accelerated phase disease.

Epidemiology & Pathobiology

CML accounts for approximately 9,280 new cases and 1,220 deaths in the United States in 2024, representing ~15% of all leukemia cases. The median age at diagnosis is 64 years, with a slight male predominance. CML can occur at any age but is rare in children ( 95% of cases. This reciprocal translocation fuses the BCR gene on chromosome 22 with the ABL1 gene on chromosome 9, creating the BCR-ABL1 fusion oncogene. BCR-ABL1 encodes a 210 kDa constitutively active tyrosine kinase (p210BCR-ABL) that: • Activates RAS/MAPK, PI3K/AKT, JAK/STAT5 survival pathways • Inhibits apoptosis via BCL-XL and…

Clinical Phases & Presentation

Chronic phase (CP-CML): • The vast majority (~90%) of CML is diagnosed in chronic phase • Often asymptomatic — discovered incidentally on CBC showing leukocytosis • Symptomatic presentation: Fatigue, night sweats, weight loss, left upper quadrant fullness/early satiety from massive splenomegaly (the most characteristic finding — present in ~50–70%) • Leukocytosis: Characteristic "left-shifted" differential with neutrophils, metamyelocytes, myelocytes, promyelocytes, and blasts ( 10,000/μL or platelets <100,000/μL unresponsive to therapy • Increasing splenomegaly unresponsive to therapy •…

Diagnosis & Monitoring

Diagnosis: CBC with differential (leukocytosis with complete myeloid left shift), bone marrow biopsy (hypercellular with granulocytic hyperplasia; 95%), FISH for BCR-ABL1, and quantitative RT-PCR for BCR-ABL1 (establishes baseline for monitoring on the International Scale — IS). Molecular monitoring — the cornerstone of CML management: • BCR-ABL1 quantitative PCR on peripheral blood (IS) every 3 months for 2 years, then every 3–6 months • Response definitions (ELN 2020): – Complete hematologic response (CHR): WBC 1% at 3 months, >1–10% at 6 months, >0.1–1% at 12 months • Failure: BCR-ABL1…

Treatment: TKI Selection & Response Milestones

First-line TKI options for newly diagnosed CP-CML: • Imatinib (Gleevec, 400 mg daily): First-generation TKI; IRIS trial landmark; 10-year OS ~83–84% on imatinib; most common long-term side effects are edema, fatigue, and muscle cramps; generic availability reduces cost significantly. • Dasatinib (Sprycel, 100 mg daily): Second-generation TKI; ENESTnd-equivalent trial DASISION — significantly higher MMR (46% vs. 28%) and CCyR at 12 months vs. imatinib; faster deep molecular responses; more patients eligible for TFR attempt; key toxicities: pleural effusion (~28%), pulmonary arterial…

Treatment-Free Remission & Advanced Phase CML

Treatment-free remission (TFR) — stopping TKI in sustained deep molecular remission: • Eligibility criteria (ELN/NCCN): ≥3 years of TKI therapy, ≥2 years of sustained MR4.5 (BCR-ABL1 ≤0.0032% IS), stable disease, no prior blast or accelerated phase, access to frequent molecular monitoring. • TFR success rates: ~40–50% maintain TFR at 2 years after stopping; the majority of molecular relapses occur within the first 6 months. • Molecular relapse: BCR-ABL1 loss of MMR; restart same TKI → virtually all patients re-achieve MMR; no known oncologic harm from relapse. • Second TFR attempt: ~40% of…