BCR::ABL1 kinase domain mutation testing and clinical outcome in a nationwide chronic myeloid leukemia patient population.

OBJECTIVES: Acquired missense mutations in the BCR::ABL1 kinase domain (KD) may cause tyrosine kinase inhibitor (TKI) treatment failure. Based on mutation-specific in vitro derived IC50-values, alternative TKI may be selected. We assessed clinical practice of BCR::ABL1 KD mutation testing, clinical response in relation to IC50-values, and clinical outcome of tested patients. METHODS: Patients from six Dutch CML reference centers and a national registry were included once a mutational analysis was performed. Reasons for testing were categorized as suboptimal TKI response, and primary or secondary TKI resistance. RESULTS: Four hundred twenty analyses were performed in 275 patients. Sixty-nine patients harbored at least one mutation. Most analyses were performed because of suboptimal TKI response but with low mutation incidence (4%), while most mutations were found in primary and secondary resistant patients (21% and 51%, respectively). Harboring a BCR::ABL1 mutation was associated with inferior overall survival (HR 3.2 [95% CI, 1.7-6.1; p < .001]). Clinically observed responses to TKI usually corresponded with the predicted TKI sensitivity based on the IC50-values, but a high IC50-value did not preclude a good clinical response per se. CONCLUSIONS: We recommend BCR::ABL1 KD mutation testing in particular in the context of primary or secondary resistance. IC50-values can direct the TKI choice for CML patients, but clinical efficacy can be seen despite adverse in vitro resistance.

Digital PCR for BCR-ABL1 Quantification in CML: Current Applications in Clinical Practice.

Molecular monitoring of the BCR-ABL1 transcript for patients with chronic phase chronic myeloid leukemia (CML) has become increasingly demanding. Real-time quantitative PCR (qPCR) is the routinely used method, but has limitations in quantification accuracy due to its inherent technical variation. Treatment recommendations rely on specific BCR-ABL1 values set at timed response milestones, making precise measurement of BCR-ABL1 a requisite. Furthermore, the sensitivity of qPCR may be insufficient to reliably quantify low levels of residual BCR-ABL1 in patients in deep molecular response (DMR) who could qualify for an attempt to discontinue Tyrosine Kinase Inhibitor (TKI) therapy. We reviewed the current use of digital PCR (dPCR) as a promising alternative for response monitoring in CML. dPCR offers an absolute BCR-ABL1 quantification at various disease levels with remarkable precision and a clinical sensitivity reaching down to at least MR5.0. Moreover, dPCR has been validated in multiple studies as prognostic marker for successful TKI treatment discontinuation, while this could not be achieved using classical qPCR. dPCR may thus prospectively be the preferred method to reliably identify patients achieving treatment milestones after initiation of TKI therapy as well as for the selection and timing for TKI discontinuation.