A 58-year-old man with a 20-year history of chronic myeloid leukemia (CML) in chronic phase presents with a blast crisis. Peripheral blood smear shows 85% blasts. He has been off imatinib for 6 months due to financial constraints. What is the drug of choice for blast crisis CML?

Explanation

## CML Blast Crisis — Second-Generation TKI Selection ### Clinical Context The patient presents with: - CML in blast crisis (>30% blasts in peripheral blood or bone marrow) - Prior imatinib exposure (6 months off therapy) - High risk of imatinib resistance due to BCR-ABL mutations ### Mechanism of Resistance in Blast Crisis **Key Point:** Blast crisis CML frequently develops point mutations in the BCR-ABL kinase domain, especially the **T315I mutation**, which confers resistance to first-generation TKIs (imatinib). Second-generation TKIs (dasatinib, nilotinib, bosutinib) have broader kinase inhibition profiles and overcome most resistance mutations except T315I. ### TKI Comparison Table | TKI | Generation | BCR-ABL Potency | T315I Resistant? | Blast Crisis Role | Onset | |-----|-----------|-----------------|------------------|-------------------|-------| | **Imatinib** | 1st | Standard | Yes (resistant) | Not preferred in BC | Slow | | **Dasatinib** | 2nd | 325× more potent | Yes (resistant) | **First-line for BC** | Rapid | | **Nilotinib** | 2nd | 30× more potent | Yes (resistant) | Alternative to dasatinib | Moderate | | **Bosutinib** | 2nd | Moderate potency | Yes (resistant) | Third-line option | Moderate | ### Why Dasatinib is First-Line in Blast Crisis ```mermaid flowchart TD A[CML Blast Crisis<br/>Prior imatinib exposure]:::outcome --> B[Assess BCR-ABL mutation status]:::decision B -->|T315I or unknown| C[Dasatinib 140 mg daily]:::action B -->|No T315I mutation| D[Dasatinib or nilotinib]:::action C --> E[Rapid cytoreduction<br/>within 2-4 weeks]:::outcome D --> E E --> F[Consolidate with<br/>allogeneic HSCT]:::action F --> G[Long-term remission]:::outcome ``` **High-Yield:** Dasatinib is preferred in blast crisis because: 1. **Highest potency** against BCR-ABL (325-fold more potent than imatinib) 2. **Rapid onset** — cytoreduction within 2–4 weeks 3. **Broad spectrum** — covers most kinase domain mutations except T315I 4. **Established efficacy** — FDA-approved for CML blast crisis 5. **Better CNS penetration** — important if CNS involvement suspected **Clinical Pearl:** Blast crisis CML is a medical emergency requiring immediate TKI escalation plus consideration of allogeneic hematopoietic stem cell transplantation (HSCT) for long-term cure. Dasatinib achieves hematologic remission in ~50% of blast crisis patients. ### Role of Other Agents - **Imatinib:** Inadequate for blast crisis; too slow and likely resistant after prior exposure. - **Nilotinib:** Alternative second-generation TKI with similar efficacy to dasatinib but slightly slower onset; reserved if dasatinib contraindicated. - **Hydroxyurea:** Cytoreductive agent only; does NOT target BCR-ABL and is insufficient monotherapy for blast crisis.