Response to Imatinib Mesylate In CML Patients as Assessed by Quantitative Real Time PCR Can Predict the Probability for Resistance Mutations In the BCR-ABL1 Kinase Domain.
Authors: Schnittger Susanne , Alpermann Tamara , Kern Wolfgang , Haferlach Torsten , Haferlach Claudia 
Published: October 2019

Abstract
Abstract 3420
The introduction of imatinib into the treatment of patients with chronic myeloid leukemia (CML) resulted in a seven year event free survival of 81% (IRIS trial). Quantitative BCR-ABL1 monitoring to control for therapy response has been recommended every three months. Major molecular response (MMR) was defined as reduction of the %BCR-ABL1/ABL1 to ≤0.1 after the first year of treatment (European LeukemiaNet, ELN, Baccarani et al., JCO 2009). However, a small proportion of patients in chronic phase experience refractoriness to kinase inhibitor treatment. As a major cause of resistance mutations within the BCR-ABL1 kinase domain have been described. Therefore, analysis for mutations has been recommended in patients not responding to therapy (primary refractoriness) as well as in those with refractory disease after initial response (secondary resistance). While approximately 50% of patients with secondary resistance harbour mutations, mutations in cases with primary resistance are less frequent. We speculated that the incidence of mutations is inversely related to the quality of molecular response. Therefore, we here compared for the first time the frequency of BCR-ABL1 mutations with molecular response patterns on imiatinib treatment. We investigated a cohort of 88 CML patients that experienced refractoriness and in whom frequent quantitative BCR-ABL1/ABL1 assessments (at least every three months) had been performed (n=854 in total, median number/patient: n=7, range: 3–29). Based on the response pattern two different groups were defined: 1) Patients with molecular relapses after a MMR (secondary resistance). In this group at least at one time point a % BCR-ABL1/ABL1 ≤0.1 was reached. 2) Patients who did not achieve a MMR and had increasing BCR-ABL1 levels (including both primary and secondary resistance). Prerequisite to enter the latter group was the availability of a sample at diagnosis and follow-up samples before occurrence of relapse analyzed at intervals of three months or shorter within the first year. Based on these criteria 55 patients were assigned to one of these two groups. 12/55 pts qualified for group 1. Best MMR in these cases was between 0.001 and 0.097 % BCR-ABL1/ABL1 (median: 0.024). In total 43/55 pts qualified for group 2. The lowest levels of % BCR-ABL1/ABL1 in this group ranged from 0.25 to 74.83 (median: 7.333), 24/43 cases from group 2 had a minor response ranging from 0.25 to 10.0% BCR-ABL1/ABL1 (secondary resistance) and 19/43 revealed nearly no responses (>10 % BCR-ABL1/ABL1). Mutation analyses was performed by Sanger sequencing 97–600 days (median: 289 days) after best detectable response in group 1 and 95–641 days (median: 333 days) after best detectable response in group 2. In 8 of 12 patients (66.7%) from group 1 BCR-ABL1 mutations were detected while this was true in only 6 out of 43 (14%) patients from group 2 (p<0.001). In group 1 six pts revealed one mutation each (V299L, E255V, E279K, T315I, M351T, or F359V) and two pts revealed two mutations (M248V + T315I and T315I + F359C, respectively). In group 2 four patients had one mutation (E255V, D276G, T315I or L387M) and two patients showed two different mutations (M244V + T315I and T315I + F359C, respectively). In more detail, only 2/19 patients (10.5%) with primary resistance revealed mutations and 4/24 (16.7%) with minor response did so (maximum response of 0.23, 0.53, 1.12 and 2.15 % BCR-ABL1/ABL1, respectively) (no significant frequency differences between these two subgroups of group 2). Thus, the kind of mutations and the pattern of mutations within the respective patient groups did not differ between both groups whereas the frequency of mutations was significantly higher in group 1. This data suggests that the level and depth of molecular response is associated with the probability for mutations leading to imatinib resistance. It can be speculated that BCR-ABL1 tyrosine kinase mutations are a typical resistance mechanism in pts relapsing after MMR whereas mainly other yet unknown mechanisms are involved in CML patients never achieving MMR before resistance to imatinib.

Disclosures:
Schnittger: MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Alpermann:MLL Munich Leukemia Laboratory: Employment. Kern:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership.