Which tyrosine kinase inhibitor is the standard first-line therapy for chronic myeloid leukemia in chronic phase and acts by competitive inhibition of the ATP-binding site of BCR-ABL?

Explanation

## Mechanism of Action **Key Point:** Imatinib mesylate is a selective tyrosine kinase inhibitor that competitively binds to the ATP-binding pocket of the BCR-ABL fusion protein, preventing autophosphorylation and downstream signaling. ## Clinical Role in CML **High-Yield:** Imatinib is the gold standard first-line therapy for chronic myeloid leukemia (CML) in chronic phase, achieving complete cytogenetic response in >90% of patients when started early. ## Comparison with Other TKIs | TKI | Generation | Mechanism | Indication | Notes | | --- | --- | --- | --- | --- | | Imatinib | 1st | Competitive ATP-site inhibitor | CML CP (1st-line), Ph+ ALL | Standard of care; oral bioavailability ~98% | | Dasatinib | 2nd | Competitive; broader kinase spectrum | CML AP/BC, imatinib resistance | More potent; faster response | | Nilotinib | 2nd | Competitive; selective for BCR-ABL | CML CP/AP, imatinib resistance | Potent; requires fasting | | Ponatinib | 3rd | Binds inactive conformation | CML with T315I mutation | Reserved for resistant/intolerant cases | ## Why Imatinib is First-Line 1. Excellent efficacy and safety profile in CML-CP 2. Oral administration with good GI absorption 3. Well-tolerated with manageable side effects (edema, GI upset) 4. Cost-effective compared to second/third-generation TKIs 5. Decades of clinical data supporting long-term outcomes **Clinical Pearl:** Resistance to imatinib may develop through BCR-ABL kinase domain mutations (most common), BCR-ABL amplification, or clonal evolution. Second-generation TKIs (dasatinib, nilotinib) are used for imatinib-resistant disease, except T315I mutation, which requires ponatinib. [cite:Harrison 21e Ch 104]