Acute Promyelocytic Leukemia (APL)

Biology: t(15;17) PML-RARA

• Defining lesion: t(15;17)(q24.1;q21.2) PML::RARA. The retinoic acid receptor alpha (RARA) gene on 17q fuses with the promyelocytic leukemia (PML) gene on 15q, producing the PML-RARA fusion oncoprotein.
• Mechanism: PML-RARA silences genes required for promyelocyte differentiation, causing a differentiation arrest at the promyelocyte stage with increased proliferation. This is the FAB M3 subtype.
• Morphology / immunophenotype: hypergranular promyelocytes with Auer rods (needle-shaped cytoplasmic crystalline inclusions of fused lysosomes/granules), bilobed nuclei; strong MPO. Flow: HLA-DR negative, CD34 negative, CD33 positive, MPO positive (loss of the blast antigens CD34 and HLA-DR is characteristic).

Presentation: coagulopathy is an emergency

• DIC / coagulopathy is the hallmark and the leading cause of early death (hemorrhage). APL causes a combined consumptive coagulopathy and hyperfibrinolysis.
• Start ATRA immediately (STAT) on any clinical suspicion, even before genetic confirmation, to avoid catastrophic DIC. If FISH later returns negative but clinical suspicion remains high, continue ATRA and confirm with RT-PCR or another molecular assay (cryptic PML-RARA can be FISH-negative); discontinue ATRA only once the integrated genetic/diagnostic workup excludes APL.
• Aggressive blood product support:
  • Cryoprecipitate to keep fibrinogen >150 mg/dL.
  • Platelet transfusion to a goal above 30 to 50K.
  • Correct coagulopathy with fresh frozen plasma as needed; monitor fibrinogen, PT/PTT, and platelets closely.
• Illustrative case: a 35-year-old man with acute bleeding, DIC (fibrinogen 75), WBC 5,000; flow consistent with AML, CD33 positive, CD34 and HLA-DR negative; Auer rods and bilobed nuclei.

Sanz risk stratification

• Low and intermediate risk are grouped together as low-to-intermediate risk (WBC ≤10K) for treatment selection. High risk (WBC >10K) is managed with a chemotherapy-containing regimen.

Treatment: low-to-intermediate risk (ATRA + ATO)

• Standard of care is ATRA + arsenic trioxide (ATO), chemotherapy-free, for low-to-intermediate-risk APL (WBC ≤10K).
• Lo-Coco F et al. NEJM 2013 (ATRA+ATO vs ATRA+chemotherapy in low-to-intermediate risk):
  • ATRA/ATO arm: ATRA plus ATO daily until CR, followed by ATO 5 days/week (4 weeks on, 4 weeks off) for 4 courses and ATRA 2 weeks on / 2 weeks off for 7 courses (total ~28 weeks).
  • CR was 100% with ATRA/ATO vs 95% with ATRA/chemotherapy (P = 0.12).
  • Two-year event-free survival 97% (ATRA/ATO) vs 86% (ATRA/chemo) (P <0.001 for noninferiority, P = 0.02 for superiority); overall survival also better with ATRA/ATO (P = 0.02).
  • ATRA/ATO had less hematologic toxicity and fewer infections but more hepatic toxicity.
• Consolidation and maintenance follow the initial protocol; for low-to-intermediate risk, ATRA + ATO is at least noninferior and may be superior to ATRA plus chemotherapy and is the new standard.

Treatment: high risk (ATRA + ATO + chemotherapy)

• High-risk APL (WBC >10K) requires the addition of cytotoxic chemotherapy (anthracycline and/or gemtuzumab ozogamicin) to control the high leukemic burden. Representative induction regimens:
  • ATRA + ATO + gemtuzumab ozogamicin (GO): ATRA 45 mg/m2 in divided doses + arsenic trioxide (0.3 mg/kg IV days 1 to 5 of week 1, then 0.25 mg/kg twice weekly weeks 2 to 8) + GO 6 mg/m2 day 1 (an alternative uses ATO 0.15 mg/kg/day with GO 9 mg/m2 day 1).
  • ATRA + ATO + idarubicin: ATRA 45 mg/m2 (days 1 to 36, divided) + age-adjusted idarubicin 6 to 12 mg/m2 on days 2, 4, 6, 8 + ATO 0.15 mg/kg (days 9 to 36).
  • ATRA + 7+3 (ATO-free alternative): ATRA 45 mg/m2 divided + daunorubicin (50 mg/m2 x 4 days or 60 mg/m2 x 3 days) + cytarabine 200 mg/m2 x 7 days.
  • ATRA + idarubicin (AIDA-type) (ATO-free alternative): ATRA 45 mg/m2 divided + idarubicin 12 mg/m2 on days 2, 4, 6, 8.

Drug notes: ATRA and ATO

• ATRA (all-trans retinoic acid): usually well tolerated. A distinctive neurologic toxicity is pseudotumor cerebri (PTC) / idiopathic intracranial hypertension, mechanistically similar to vitamin A toxicity (retinoids increase CSF production and may impair CSF absorption at the arachnoid villi). Manage with acetazolamide, topiramate, therapeutic lumbar puncture, and diuretics; CSF shunting for progressive visual loss. ATRA is often held and can be rechallenged (same or lower dose) once PTC improves.
• ATO (arsenic trioxide): binds the PML moiety of PML-RARA, leading to degradation of the fusion protein with partial differentiation and apoptosis of leukemic promyelocytes; synergizes with ATRA (which binds the RARA moiety). ATO blocks potassium currents and causes QTc prolongation (risk of ventricular tachyarrhythmia); electrolyte deficiencies worsen this. Monitor ECGs and electrolytes closely and replete magnesium and potassium.

Differentiation syndrome (APL differentiation syndrome)

• Incidence: up to 25% of patients treated with ATRA and/or ATO. Onset as early as day 1 to as late as the second month of induction (most in the first 1 to 2 weeks).
• Mechanism: differentiating promyelocytes release inflammatory cytokines (IL-1, IL-6, IL-8, TNF-alpha, CCL2) and granule contents, driving capillary leak, pulmonary edema, and fluid retention.
• Features: fever, rising WBC, dyspnea, hypoxemia, pleural/pericardial effusions, weight gain, hypotension, acute kidney injury; CXR shows diffuse interstitial infiltrates. DDx: infection/sepsis, VTE, heart failure, drug allergy, AKI.
• Management: start dexamethasone 10 mg IV every 12 hours immediately; continue until signs and symptoms resolve (minimum 3 days), then taper per clinical course. It is usually readily reversible. Continue ATRA and add steroids for mild-to-moderate disease; interrupt/hold ATRA (or ATO) only if there is progression or severe symptoms.
• Prophylaxis: for high-risk disease (WBC >10K), prophylactic corticosteroids (e.g., prednisone 0.5 mg/kg/day or dexamethasone) are commonly recommended (NCCN), although evidence is limited.

Monitoring and relapse

• MRD by PCR for PML-RARA: Post-induction PML-RARA PCR is often positive and should not change treatment. Assess molecular remission after consolidation. A positive result after consolidation, or suspected molecular recurrence in follow-up, warrants a confirmatory marrow (repeat in ~2 weeks) before declaring molecular persistence/relapse and starting salvage.
• Relapse: re-treat with ATO plus ATRA (definitively ATO if ATO-naive), chemotherapy, or single-agent gemtuzumab ozogamicin. On achieving a second remission (CR2): if PCR negative and transplant-eligible, autologous HCT (or ATO x 6 cycles if not a transplant candidate); if PCR positive, proceed to allogeneic transplant. Perform CNS prophylaxis when a second morphologic remission is achieved.
• Timing nuance: for relapse >6 months after an ATRA+ATO-based first line, give an arsenic-based regimen +/- ATRA +/- anthracycline or GO.

High yield (APL)

• t(15;17) PML-RARA; FAB M3; HLA-DR and CD34 negative, CD33 and MPO positive, Auer rods.
• DIC is a medical emergency: start ATRA STAT on suspicion and support aggressively (cryoprecipitate for fibrinogen <150, platelets to 30 to 50K).
• Sanz risk by WBC and platelets: low-to-intermediate (WBC ≤10K) vs high (WBC >10K).
• Low-to-intermediate risk: chemo-free ATRA + ATO (Lo-Coco NEJM 2013). High risk: add chemotherapy (anthracycline and/or GO).
• Differentiation syndrome: dexamethasone immediately; hold ATRA/ATO only if severe. ATO prolongs QTc; ATRA can cause pseudotumor cerebri.