## Acute Promyelocytic Leukemia (APL) — t(15;17) Management **Key Point:** The t(15;17) translocation in AML defines acute promyelocytic leukemia (APL), a distinct subtype with a unique and highly effective treatment paradigm that differs fundamentally from standard AML chemotherapy. ### Pathophysiology of t(15;17) The translocation produces a PML-RARA fusion protein that blocks terminal differentiation of promyelocytes. ATRA (all-trans retinoic acid) binds to the RARA moiety and causes degradation of the fusion protein, restoring differentiation and apoptosis. ### ATRA + Arsenic Trioxide Regimen 1. **ATRA** — induces differentiation of leukemic promyelocytes; does not cause myelosuppression 2. **Arsenic trioxide (ATO)** — induces apoptosis and further degrades PML-RARA fusion protein 3. Combined approach achieves **cure rates >90%** in pediatric APL 4. Dramatically reduces early hemorrhagic deaths (APL is associated with severe coagulopathy and DIC) **High-Yield:** APL with t(15;17) is the **only AML subtype where chemotherapy-free induction is standard**. ATRA + ATO alone (without cytarabine or daunorubicin) is the preferred induction regimen. **Clinical Pearl:** APL patients are at extreme risk for fatal hemorrhage during early induction due to severe thrombocytopenia and release of procoagulants from abnormal promyelocytes. ATRA + ATO rapidly reduces this risk by inducing differentiation and apoptosis without prolonged myelosuppression. ### Differentiation Syndrome (ATRA Syndrome) - Occurs in 5–30% of APL patients during induction - Presents with fever, dyspnea, pulmonary infiltrates, weight gain, hypotension - Managed with dexamethasone; may require temporary ATRA discontinuation [cite:Harrison 21e Ch 101]
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