BCR::ABL1 tyrosine kinase inhibitors

Mechanism & Class Overview

• Class mechanism: Inhibit BCR-ABL fusion protein tyrosine kinase activity by blocking the ATP-binding site of BCR-ABL (p210/p190); asciminib instead binds the ABL myristoyl pocket allosterically; variability in spectrum (off-target kinases like c-KIT, PDGFR, SRC).
• Resistance: BCR-ABL kinase domain point mutations (a small set account for the majority; T315I resists imatinib, dasatinib, nilotinib, and bosutinib but not ponatinib (ATP-site TKI active vs T315I) or asciminib (allosteric)), ↑ BCR-ABL expression, P170 (efflux pump) overexpression, c-KIT mutation, clonal evolution.
• Class toxicities: All BCR-ABL TKIs cause myelosuppression (anemia, thrombocytopenia, neutropenia) and rash. Dasatinib, nilotinib, and bosutinib need an acidic gastric pH for absorption, so PPIs reduce absorption and should be avoided; antacids/H2 blockers may be time-separated per drug (dasatinib antacid ≥2h; nilotinib H2 10h before/2h after); imatinib, ponatinib, and asciminib are not meaningfully affected.

1st Generation TKIs

Imatinib (Gleevec)

• Mechanism: Inhibits BCR-ABL, PDGFR, c-KIT.
• Indications: Ph+ ALL, CML, MDS/MPD with PDGFR rearrangement, GIST.
• Drug interactions: CYP3A4 inhibitors/inducers (ketoconazole, itraconazole, erythromycin, clarithromycin); warfarin. Reduce dose 25% only for severe hepatic impairment (no initial adjustment for mild/moderate).
• Toxicities: Myalgias, rash, fluid retention, N/V, periorbital edema; fluid retention can lower LVEF toward CHF (check baseline TTE).

2nd Generation TKIs

Dasatinib (Sprycel)

• Mechanism: Inhibits BCR-ABL, SRC family, c-KIT, EPHA2, PDGFRβ (~300× more potent vs imatinib).
• Indications: CML (100 to 140 mg), Ph+ refractory ALL.
• Drug interactions: CYP3A4 inhibitors/inducers; PPIs/H2 blockers ↓ drug level (needs acidic pH).
• Toxicities: Pulmonary arterial hypertension, pleural effusion (fluid retention), platelet inhibition, QT prolongation, aspirin-like effect.

Nilotinib (Tasigna)

• Mechanism: Inhibits BCR-ABL, PDGFR, c-KIT (~30× more potent vs imatinib).
• Indications: CML.
• Drug interactions: CYP3A4 inhibitors/inducers; PPIs/H2 ↓ absorption.
• Toxicities: QT prolongation, ↑ LFTs (hepatotoxicity), rash, ↑ lipase, ↓ phosphate (hypophosphatemia), and peripheral arterial disease (PAD, a vascular occlusive signal).

Bosutinib (Bosulif)

• Mechanism: ATP-competitive dual SRC/ABL inhibitor with minimal activity against c-KIT and PDGFR (distinct off-target profile from imatinib, dasatinib, nilotinib).
• Indications: Ph+ CML (newly diagnosed CP; resistant/intolerant CP/AP/BP).
• Toxicities: Diarrhea, GI upset (D/V, abdominal pain), rash, fatigue, ↑ LFTs.

3rd Generation TKIs (ponatinib; STAMP inhibitor asciminib)

Ponatinib (Iclusig)

• Mechanism: Pan-TKI; inhibits ABL and T315I-mutated ABL, plus VEGFR, PDGFR, FGFR, SRC family, c-KIT, RET, TIE2, FLT3.
• Indications: CML: CP resistant/intolerant to ≥2 prior TKIs, or AP/BP with no other TKI indicated, or T315I+ (any phase), Ph+ ALL (newly diagnosed with chemo, or T315I+/no other TKI indicated).
• Drug interactions: strong CYP3A4 inhibitors (reduce dose); avoid strong CYP3A4 inducers. PPIs have no clinically meaningful effect.
• Toxicities: Black box warning: arterial occlusive events (MI, stroke, PAD), venous thrombosis, CHF; hepatotoxicity; pancreatitis with ↑ lipase/amylase.

Asciminib (Scemblix)

• Mechanism: STAMP (Specifically Targeting the ABL Myristoyl Pocket) allosteric BCR-ABL1 inhibitor. Binds the myristoyl pocket, distinct from ATP-site TKIs, and overcomes many kinase-domain mutations including T315I.
• Indications: previously treated Ph+ CML-CP (original 2021 accelerated approval was after ≥2 prior TKIs; current label no longer requires 2 prior TKIs); T315I+ CP-CML (FDA 2021); 1L CP-CML (FDA Oct 29, 2024) based on ASC4FIRST (Hughes NEJM 2024): asciminib vs investigator's choice TKI (imatinib or a 2G-TKI) in newly diagnosed CP-CML; MMR at 48 wk 67.7% vs 49.0% (p<0.001); superior to imatinib and with non-inferior/superior tolerability vs 2G-TKIs.
• Toxicities: lower pleural-effusion/edema signal vs dasatinib, but carries its own CV toxicity warning (arterial thrombotic/ischemic events, cardiac failure); no head-to-head vs ponatinib; thrombocytopenia, neutropenia, headache, pancreatic enzyme elevations (monitor lipase/amylase).

Ponatinib updates (2024)

• PhALLCON (Jabbour JAMA 2024): ponatinib + chemo vs imatinib + chemo in 1L Ph+ ALL. MRD-negative CR 34.4% vs 16.7% at end of induction. FDA Mar 19, 2024 accelerated approval for newly diagnosed Ph+ ALL in combination with chemo.
• OPTIC dose-optimization (Cortes Blood 2021, mature 2024): start 45 mg daily then reduce to 15 mg once BCR-ABL1 ≤1%, which lowers the arterial-occlusive event rate while preserving efficacy.

Non-TKI option after TKI failure

Omacetaxine mepesuccinate (Synribo; discontinued in the US)

• Mechanism: Semisynthetic homoharringtonine (HHT), a Cephalotaxus plant alkaloid. Not a TKI: it inhibits protein synthesis by binding the A-site cleft of the peptidyl-transferase center of the large ribosomal subunit and blocking the initial protein elongation step (so it is independent of the BCR-ABL mutation status).
• Indications: CP or AP CML with resistance and/or intolerance to ≥2 TKIs.
• Toxicities: Severe/fatal myelosuppression (thrombocytopenia, neutropenia, anemia), fatal CNS bleeds, severe GI bleeds, ↑ glucose, fetal harm.