ABL Kinase Inhibitor Therapy for CML: Baseline Assessments and Response Monitoring

Abstract

For chronic phase chronic myeloid leukemia (CML) patients treated with imatinib, the essential pretherapy assessments include bone marrow morphology and cytogenetics as well as a baseline real-time quantitative PCR (RQ-PCR) for BCR-ABL. Baseline assessments of clinical relevance include Sokal and Hasford prognostic scores. Several other baseline assays are likely to be predictive of response, including IC50 imatinib , organic cation transporter-1 (OCT-1) mRNA level, and gene expression profiles, but further confirmation is required. RQ-PCR assays of blood at least every 3 months once patients have commenced imatinib is recommended. This will facilitate early identification of suboptimal responders who may benefit from higher doses of imatinib or alternative therapy, and identify at an early stage patients with acquired resistance. Management of the latter group can be further guided by the findings from cytogenetics and BCR-ABL kinase domain mutation screening. Bone marrow cytogenetics is indicated at least every 6 months until the patient achieves major molecular response. RQ-PCR is only clinically useful if it is conducted under a rigorous quality control regimen so that fluctuations in the BCR-ABL level can be confidently attributed to a biological cause rather than assay variation. To further improve the clinical value of RQ-PCR monitoring, expression of results on an international scale is needed. This will facilitate a more uniform and rational approach to management of suboptimal response and loss of response.

Comparison of mutated ABL1 and JAK2 as oncogenes and drug targets in myeloproliferative disorders

“…The levels of residual BCR-ABL1 transcripts detected by quantitative reverse transcriptase PCR (RT-PCR) at certain time points are excellent predictors of achievement and durability of CCRs and favorable clinical course. 160,161 The kinetics of response in terms of speed and depth are highly variable probably due to the heterogeneity of leukemia, as well as the variation in different pharmacokinetic and pharmacodynamic background of individual patients. Persistent BCR-ABL1 positivity in the majority of patients even after years of treatment indicates that imatinib does not cure CML.…”

An In-house Method for Molecular Monitoring of BCR-ABL

“…To prepare the calibrators the plasmid copy number μL -1 volume was determined using the following equation (in brief, the molecular weight of the DNA template can be determined by multiplying the number of base pairs (bp) by the weight of 1 mol of a bp estimated to be 650 g; using 6.022 x 10 23 [Avogadro's number] molecules mol -1 , the number of molecules of the template can be calculated by first converting to ng [multiplying 1 x 10 9 ] and then multiplying by the quantity of template) [14]: copy number μL -1 = [quantity (ng μL -1 ) x (6.022 x 10 23 )]/[plasmid length (bp) x 1.10 9 x 650] Calibrators containing 10 2 , 10 3 , 10 4 , 10 5 , and 10 6 gene copies were prepared for both the BCR-ABL and ABL genes. These calibrators were used as templates in RQ-PCR reactions to generate standard curves.…”

“…Prior to the use of tyrosine kinase inhibitors for the treatment of CML a molecular response was rarely achieved and it was therefore not feasible to routinely monitor BCR-ABL transcript levels, with the exception of patients that underwent bone marrow transplantation [1][2][3][4][5][6]. Currently, tyrosine kinase inhibitors are the preferred first-line therapy for CML and based on IRIS trial data a 3-log reduction in BCR-ABL transcript levels after 12 months correlates with progression-free survival [1][2][3][4][5][6].…”

“…Since the late 1980's the percentage of Ph-positive cells in bone marrow based on cytogenetic analysis has been the gold standard for monitoring the response to therapy [1,2,6]. Prior to the use of tyrosine kinase inhibitors for the treatment of CML a molecular response was rarely achieved and it was therefore not feasible to routinely monitor BCR-ABL transcript levels, with the exception of patients that underwent bone marrow transplantation [1][2][3][4][5][6].…”

“…In turn, there has been an expected increase in the number of centers that provide RQ-PCR results for BCR-ABL mRNA. The rapid increase of these centers revealed that there were significant variations between individual laboratories in reporting RQ-PCR data [5][6][7][8][9][10][11][12]. The International Scale (IS) has been adopted for reporting BCR-ABL values based on the application of laboratory-specific conversion factors that are derived using patient samples [6][7][8][9][10][11][12].…”