## First-Line Treatment of CML in Chronic Phase **Key Point:** Imatinib mesylate is the standard first-line tyrosine kinase inhibitor (TKI) for BCR-ABL1-positive CML in chronic phase, achieving complete cytogenetic response in >80% of patients. ### Mechanism of Action Imatinib is a selective BCR-ABL1 tyrosine kinase inhibitor that blocks the constitutive kinase activity driving myeloid proliferation in CML. ### Dosing and Response - Standard dose: 400 mg daily for chronic phase CML - Leads to complete hematologic response in 95% and complete cytogenetic response in 80–90% at 12 months - Monitoring: BCR-ABL1 transcript levels by qPCR at 3, 6, and 12 months ### Why Imatinib Over Other Agents | Feature | Imatinib | Hydroxyurea | Busulfan | IFN-α | |---------|----------|-------------|----------|-------| | Mechanism | BCR-ABL1 TKI | Ribonucleotide reductase inhibitor | Alkylating agent | Immune modulation | | Cytogenetic response | 80–90% | <10% | <5% | 20–30% | | Survival benefit | Yes (landmark) | Cytoreduction only | Cytoreduction only | Modest | | Tolerability | Excellent | Moderate | Poor (cumulative toxicity) | Poor (flu-like) | | Current role | First-line | Cytoreduction in TKI-intolerant | Rarely used | Historical | **High-Yield:** Imatinib revolutionized CML management post-2001 and is the gold standard; second-generation TKIs (dasatinib, nilotinib, bosutinib) are reserved for imatinib resistance or intolerance. **Clinical Pearl:** Patients with imatinib-resistant CML (defined as failure to achieve milestones: CHR by 3 months, minor CyR by 6 months, complete CyR by 12 months) should be screened for BCR-ABL1 kinase domain mutations and switched to second-generation TKI. **Mnemonic:** **TKI-CML** = Tyrosine Kinase Inhibitor is the gold standard for CML.
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