Measurable Residual Disease Monitoring for Philadelphia Positive Acute Lymphoblastic Leukemia (Ph+ALL) in the Setting of the Gimema ALL2820 Trial

Introduction. Measurable residual disease (MRD) negativity represents the primary endpoint in the management of adult Philadelphia-positive acute lymphoblastic leukemia (Ph+ ALL) patients. MRD assays must be highly sensitive and specific. Droplet digital PCR (ddPCR) and next-generation sequencing (NGS) can overcome some limitations of standard methodologies. In order to evaluate the best marker for MRD monitoring, in this study we performed: i) MRD evaluation by BCR::ABL1 fusion transcript and immunoglobulin/T-cell receptor clonal gene rearrangements (IG/TR); ii) a comparison of the MRD concordance rate between the two markers; iii) a correlation with biologic features and clinical outcome.

Methods. A total of 167 samples from 111 adults enrolled in the ongoing phase III GIMEMA ALL2820 trial were collected. Samples derived from cases from both the experimental and the control arm, based respectively on ponatinib followed by blinatumomab and on a combination of imatinib and conventional chemotherapy. Established time points were day +70 (end of induction) and day +133 (after 2 cycles of blinatumomab or after 6 cycles of chemotherapy, respectively). At diagnosis, patients underwent a screening for the identification of the predominant IG/TR rearrangement by standard PCR and/or by NGS approach [LymphoTrack assay for IGH (FR1/2/3) and IGK] and the IKZF1^plus signature by Multiplex Ligation-dependent Probe Amplification (MLPA). MRD monitoring was evaluated by RQ-PCR of BCR::ABL1 and, for translational research purposes, IG/TR monitoring was evaluated by ddPCR. The MRD concordance rate was evaluated comparing the results obtained between the 2 markers.

Results. Overall, 97/111 (87.4%) cases were evaluable for IG/TR MRD monitoring. At day +70, the overall concordance rate between BCR::ABL1 and IG/TR was 46.4%; 48/97 (49.5%) cases were BCR::ABL1^pos; 20 were concordantly IG/TR^pos (41.7%), 1 was IG/TR^{positive not quantifiable} and 27 were IG/TR^neg; 34/97 (35%) cases were BCR::ABL1^neg, 25 of which were concordantly IG/TR^neg (73.5%), 5 were IG/TR^pos and 4 were IG/TR^{positive not quantifiable}; finally, 15/97 (15.5%) cases were BCR::ABL1^{positive not quantifiable}, 5 of which were IG/TR^pos and 10 were IG/TR^neg. The concordance rate was similar between IKZF1^plus vs IKZF1 WT/IKZF1 loss (48.4% vs 44%), and p190 vs p210-p190/p210 (47% vs 42%) cases. At day +133, 70 patients were studied and the overall concordance rate was 41.4%: 28/70 (40%) cases were BCR::ABL1^pos, 4 of which were concordantly IG/TR^pos (14.3%), 2 were IG/TR^{positive not quantifiable} and 22 were IG/TR^neg; 27/70 (38.6%) cases were BCR::ABL1^neg, 25 of which were concordantly IG/TR^neg (92.6%) and 2 were IG/TR^pos; finally, 15/70 (21.4%) cases were BCR::ABL1^{positive not quantifiable} and IG/TR^neg. A lower concordance was observed in the IKZF1^plus (24%) vs IKZF1 WT/IKZF1 loss (51%) and in the p210-p190/p210 (27.3%) compared to p190 (48%) cases. Five patients experienced a hematologic relapse. Dual monitoring was feasible in 4/5 cases (in 1 material was lacking) and, in addition to the evaluation at day +70 and +133, a retrospective backtracking was carried out at a previous time-point: 1 was concordantly BCR::ABL1^pos (0.26 BCR::ABL1/ABL1 x 100) and IG/TR^pos (4.0E^-04), 1 was BCR::ABL1^pos (7.12 BCR::ABL1/ABL1 x 100) and IG/TR^neg, while the other 2 cases were both BCR::ABL1^neg but IG/TR^pos at 4.0E^-05 and 2.0E^-04, respectively, suggesting the presence at the onset of non Ph+ subclone.

Conclusions. IG/TR MRD monitoring was feasible in 87.4% of Ph+ ALL cases indicating that a fraction of patients is not suitable for this strategy. Overall, the concordance rate between BCR::ABL1 and IG/TR is limited, in line with the literature. While some groups reported a higher predictive prognostic power of IG/TR monitoring, our findings do not confirm these data, also in view of the very low rate of relapses so far observed. Nevertheless, a double-hit strategy may be informative for MRD monitoring and possibly for the distinction between typical/lymphoid Ph+ ALL vs multilineage/CML-like Ph+ ALL.