A 58-year-old man with chronic myeloid leukemia (CML) in chronic phase is found to have a BCR-ABL1 transcript on initial diagnosis. After 2 years of imatinib therapy, he develops resistance. Which investigation is most appropriate to detect the mechanism of resistance and guide salvage therapy?

Explanation

## Detecting Acquired Resistance in CML **Key Point:** RT-PCR with sequencing of the BCR-ABL1 kinase domain is the gold standard for identifying point mutations that confer tyrosine kinase inhibitor (TKI) resistance, enabling selection of second-line TKIs. ### Mechanism of TKI Resistance in CML 1. **BCR-ABL1 kinase domain mutations** (~50% of resistance cases) - T315I (most common, confers resistance to imatinib, dasatinib, nilotinib) - Other mutations: M244V, G250E, F359V - Detected by RT-PCR + Sanger or NGS sequencing 2. **Non-mutational resistance** (~50%) - BCR-ABL1 overexpression - Clonal evolution (loss of Ph chromosome, acquisition of secondary mutations) - Reduced drug bioavailability ### Investigation Comparison | Investigation | Detects | Clinical Use | |---|---|---| | **RT-PCR + sequencing** | BCR-ABL1 kinase mutations | Identifies specific mutation; guides TKI selection (ponatinib for T315I) | | **Flow cytometry** | Blast percentage, immunophenotype | Detects blast crisis, not mutation-based resistance | | **Bone marrow aspirate** | Morphology, blast count | Assesses disease phase; does not identify molecular mechanism | | **Conventional cytogenetics** | Ph chromosome, clonal evolution | Detects loss of Ph or secondary chromosomal changes; not point mutations | **High-Yield:** - **T315I mutation** = "gatekeeper" mutation; confers pan-TKI resistance; only ponatinib is effective - **Dasatinib/nilotinib resistance mutations** = ponatinib or second-generation TKI with activity against specific mutation - RT-PCR is quantitative and can also monitor BCR-ABL1 transcript levels (>1% on international scale = warning for resistance) **Mnemonic: TIKI — T315I is the "gatekeeper" mutation in BCR-ABL1 kinase domain; ponatinib overcomes it.** **Clinical Pearl:** Patients with imatinib resistance should undergo RT-PCR sequencing BEFORE switching to second-line TKI. If T315I is present, ponatinib is the only approved option; other mutations may respond to dasatinib or nilotinib.