## First-Line Tyrosine Kinase Inhibitor in CML **Key Point:** Imatinib mesylate is the gold-standard first-line treatment for newly diagnosed chronic-phase CML. It is a selective inhibitor of BCR-ABL tyrosine kinase, the constitutively active fusion protein encoded by the Philadelphia chromosome. **High-Yield:** Imatinib achieves complete cytogenetic response (CCyR) in >80% of chronic-phase CML patients and has transformed CML from a fatal disease to a manageable chronic condition with near-normal life expectancy. ### Mechanism & Rationale The BCR-ABL fusion protein drives uncontrolled proliferation in CML. Imatinib binds to the ATP-binding pocket of ABL kinase, blocking phosphorylation and downstream signaling (RAS/MAPK and PI3K/AKT pathways). This selective targeting minimizes off-target toxicity compared to non-selective kinase inhibitors. ### Dosing & Efficacy - **Chronic phase CML:** 400 mg daily orally - **Accelerated phase/blast crisis:** 600 mg daily - **Time to CCyR:** 12–18 months in most patients - **5-year overall survival:** >90% in chronic phase **Clinical Pearl:** Imatinib is well-tolerated with a favorable side-effect profile (fluid retention, GI upset, rash, muscle cramps). Regular BCR-ABL transcript monitoring (PCR) guides dose adjustments and detects resistance mutations early. ### Second-Generation TKIs (Dasatinib, Nilotinib, Ponatinib) These are reserved for: - Imatinib resistance or intolerance - Accelerated phase or blast crisis - Specific BCR-ABL kinase domain mutations (e.g., T315I for ponatinib) Dasatinib and nilotinib are more potent and have faster kinetics than imatinib but are NOT first-line due to higher cost and no proven survival advantage in chronic phase. **Mnemonic:** **IMATINIB First** — Imatinib is the **I**nitial choice, **M**anaged orally, **A**chieving **T**argeted **I**nhibition of **N**ew **I**ncident **B**CR-ABL.
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