PF414 DNA‐BASED BCR‐ABL1 ANALYSIS PROVIDES A “TRAFFIC LIGHT” STRATIFICATION MODEL FOR CHRONIC MYELOID LEUKEMIA WITH IMPLICATIONS FOR TREATMENT FREE REMISSION

Abstract

Background:Treatment free remission (TFR) or molecular relapse free survival (MRFS) have become important therapeutic aims for patients with chronic myeloid leukemia (CML). This aim can be realised through achievement of a sustained deep molecular response (DMR).Aims:In our previous work (Machova Polakova K, EHA 2018) we showed that a BCR‐ABL1 DNA‐based approach for residual disease monitoring may be superior to mRNA analyses in a significant number of peripheral blood samples. In this collaborative EUTOS study we aimed to assess whether DNA PCR provided better prediction of TFR.Methods:Consecutive samples before and after TKI cessation were available for 54 CML patients, of whom 16 were enrolled in the EURO‐SKI study. Thirty‐five patients, who fulfilled criteria but could not be involved in the EURO‐SKI trial, gave consent for an intermittent regimen (INTReg) of TKI administration every second month. The MRFS rate of INTReg patients was comparable to EURO‐SKI and other TKI stopping trials in later time points (39% at 20 months and 32% at 70 months after TKI cessation). Patient‐specific genomic BCR‐ABL1 fusions were characterized by next generation sequencing and measurable residual disease was compared using DNA and mRNA based digital droplet (dd) PCR. The sensitivity of standard DNA and mRNA analysis of peripheral blood samples were comparable, varying between MR4‐MR5.Results:DNA and mRNA BCR‐ABL1 levels from consecutive analysis were available for 33/54 patients (altogether 945 samples; median 30 samples per patient; range 11–44). Analysis of these 33 cases enabled a “traffic light” stratification according to BCR‐ABL1 DNA and mRNA status before TKI cessation (Figure 1); “Green” group ‐ continually double negative (n = 4), “Yellow” group ‐ continually DNApos/RNAneg (n = 13) and “Red” group ‐ double positive (n = 16). All 4 patients from the Green group sustained in MRFS with continuous double negative status. Two of these patients gave consent for bone marrow (BM) aspiration at the time of TKI cessation. Both BM samples were double negative when analysed by ddPCR with enhanced sensitivity of MR6‐MR6.5 by performing multiple replicates. Molecular relapse (loss of MMR) occurred in 12/16 patients of the Red group. In the Yellow group, 8/13 patients survive without molecular relapse after TKI cessation (median 39.8 months; range 19.1–82.2); in 5/8 patients, the BCR‐ABL1 expression became detectable, however, only at DMR levels, i.e. MR4 or lower. BCR‐ABL1 expression remained undetectable in 3/8 patients after TKI cessation (median 27.3 months; range 19.1–53.6). BM was collected in 2/3 patients at the time of TKI cessation; in both cases, DNA assays detected residual CML cells despite the absence of BCR‐ABL1 expression.Summary/Conclusion:BCR‐ABL1 DNA analysis allows clinically relevant stratification of CML patients in DMR before complete or intermittent treatment cessation. Our approach identifies truly BCR‐ABL1 negative patients that are likely to benefit from TKI cessation. A biologically interesting Yellow group represents patients with detectable CML cells but with undetectable BCR‐ABL1 expression on TKI therapy. Although BCR‐ABL1 expression became detectable after TKI cessation in many of these cases, a high proportion (62%) achieved MRFS. Molecular relapse occurred in a high proportion (76%) of double positive patients. Currently we are validating the “traffic light” stratification scheme on the remaining 21 patients from our cohort.imageSupport: EUTOS2018, MZCR 00023736, AZV 16–30186A