Association of Interphase Fluorescence in Situ Hybridization (IP-FISH) Response Criteria with Conventional Metaphase Cytogenetics in CML Patients On Therapy.
Authors: Schenk Thomas , Müller Martin C , Fabarius Alice , Erben Philipp , Ernst Thomas , Haferlach Claudia , Hochhaus Andreas 
Published: October 2019

Abstract
Abstract 4707
The Philadelphia (Ph) chromosome and its molecular equivalent, the BCR-ABL fusion gene, represent the pathogenetic cause and a useful marker for diagnosis and follow up monitoring of chronic myeloid leukemia (CML). Cytogenetic analysis of bone marrow metaphases (Cy) has been established as the standard method. In contrast, interphase fluorescence in situ hybridization (IP-FISH) has been increasingly applied in many studies due to recent optimization of the technique but is not represented in current treatment guidelines. We therefore sought to define IP-FISH response criteria which correspond best with complete (CCyR) and major cytogenetic responses (MCyR). In order to quantitatively compare results of both methods 1,749 consecutive non selected bone marrow samples from 748 CML patients at different stages of CML were analyzed in parallel with Cy and IP-FISH. 5 patients with Ph negative/BCR-ABL positive CML were excluded from the analysis. 643 patients in chronic phase (CP) were analyzed during imatinib based therapy, ten patients received interferon alpha. 74 patients at different stages of the disease received 2nd generation tyrosine kinase inhibitors: nilotinib, n=18 (CP, n=13; accelerated phase, AP, n=2; blast crisis, BC, n=3); dasatinib, n=56 (CP, n=41; AP, n=4; BC, n=11). 21 patients received no therapy or the therapy was not evaluable. The correlation between Ph positive metaphases and the proportion of FISH positive interphase cells was determined using the Spearman's rank correlation coefficient. The chi-square test was used to compare IP-FISH and Cy data. The optimally separating threshold value between Cy and IP-FISH was chosen as cut-off point. Cy and IP-FISH data correlated well (r=0.89; p<0.0001). The following cut-off values were defined: '30% IP-FISH positivity was found to correspond best with MCyR ('35% Ph+ metaphases); <6% IP-FISH positivity was concordant with CCyR (0% Ph+ metaphases). Of 1,163 samples of patients in CCyR, 99.1% showed a percentage of <6% IP-FISH positive cells. 82 of 1,163 samples (7.0%) with 0% Ph+ metaphases by Cy were IP-FISH positive (median 3%, range, 1-21% positive interphases). IP-FISH showed false negative results in 10 of 1,090 samples (0.9%) with a median of 8% Ph+ metaphases (range, 4-40%). Using these IP-FISH cut-off points, the diagnostic specificity for the definition of CCyR was 93.8% for all patients and 93.7% for CP pts only and for the definition of MCyR 89.4% for all patients and 88.4% for CP patients only, respectively. In conclusion, BCR-ABL FISH data derived from bone marrow interphase cells are comparable with metaphase cytogenetics but the cut-off points differ. IP-FISH might be used instead of Cy in order to assess the achievement of response milestones in CML patients during therapy. The prognostic value of IP-FISH data, however, should be analyzed in prospective controlled trials.

Disclosures:
No relevant conflicts of interest to declare.