## Acute Promyelocytic Leukemia (APL) and t(15;17) **Key Point:** The translocation t(15;17) is pathognomonic for acute promyelocytic leukemia (APL, AML-M3), which fuses PML and RARA genes, producing an abnormal retinoic acid receptor. **High-Yield:** ATRA (all-trans retinoic acid) is the cornerstone of APL therapy. It induces differentiation of abnormal promyelocytes into mature granulocytes, leading to resolution of coagulopathy and achievement of complete remission in >90% of patients. **Clinical Pearl:** Modern APL treatment combines ATRA with arsenic trioxide (ATO), which targets the PML-RARA fusion protein and has synergistic activity. This combination achieves cure rates >95% with minimal chemotherapy-related toxicity. ## Mechanism of Action | Agent | Mechanism | Role in APL | |-------|-----------|-------------| | ATRA | Binds RARA, induces differentiation | Differentiates abnormal promyelocytes | | Arsenic trioxide | Degrades PML-RARA fusion protein | Induces apoptosis of leukemic cells | | Imatinib | BCR-ABL tyrosine kinase inhibitor | For CML, not APL | | Hydroxyurea | Ribonucleotide reductase inhibitor | Cytoreductive, not curative for APL | **Warning:** ATRA monotherapy alone is now considered suboptimal; the combination with ATO is the current standard of care per modern APL protocols. **Tip:** Remember that APL is a medical emergency due to severe coagulopathy (DIC). ATRA + ATO achieves remission rapidly while managing bleeding risk.
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