P063 | DIGITAL DROPLET PCR FOR MINIMAL RESIDUAL DISEASE MONITORING IN PHILADELPHIA NEGATIVE ACUTE LYMPHOBLASTIC LEUKEMIA: A REPORT FROM THE PROSPECTIVE NORTHERN ITALY LEUKEMIA GROUP (NILG) ALL 10/07 STUDY

Background Real time quantitative PCR (RQ-PCR) with patient-specific probes is the standard of care in minimal residual disease (MRD) monitoring in Philadelphia negative (Ph-) ALL in Europe. However, the need for a standardization curve and a significant quota of positive non-quantifiable (PNQ) results limit this technique, posing a significant clinical challenge. Digital droplet PCR (ddPCR) is a third-generation PCR that allows absolute quantification of MRD, thus enabling more precise MRD quantification. Methods In the NILG ALL 10/07 study, risk stratification was based on biological and clinical features (standard risk SR, high risk HR, very high-risk VHR). SR-HR MRD negative/PNQ patients (pts) at specific timepoints (TP) were assigned to chemotherapy alone; VHR pts and MRD positive pts were assigned to HSCT consolidation. We performed ddPCR on adult Ph- B-ALL samples from the NILG ALL 10/07 study with RQ-PCR MRD levels <10-4 or PNQ using IG/ TCR gene rearrangements as molecular markers at TP2 (w10), TP3 (w16), TP4 (w22), and compared the outcomes with RQ-PCR positive pts at TP2 in the NILG ALL 10/07 study. Results Forty-three pts were included in the study, and ddPCR was performed on 104 samples: 42 samples from TP2, 31 from TP3, and 31 from TP4. Accordance rate between RQ-PCR and ddPCR was 74%. Among RQ-PCR PNQ samples, ddPCR was able to quantify MRD in 72.2% (13/18), classifying 5 samples as ddPCRneg, 8 as ddPCR-pos, and 5 as ddPCR PNQ. In total, 16 samples (15.4%) from 15 pts (34.9%) were classified as ddPCRpos. As per outcome, relapse free survival at 5 years was 68% among RQ-PCRneg;ddPCRneg pts, 47% among RQ-PCRneg;ddPCRpos pts, 32% among RQ-PCRpos pts (RQ-PCRneg;ddPCRneg vs RQ-PCRneg;ddPCRpos p= 0.03; RQ-PCRneg;ddPCRpos vs RQ-PCRpos p= 0.69). Overall survival at 5 years was 75% among RQ-PCRneg;ddPCRneg pts, 53% among RQ-PCRneg;ddPCRpos pts and 32% among RQ-PCRpos pts (RQ-PCRneg;ddPCRneg vs RQ-PCRneg;ddPCRpos p= 0.09; RQ-PCRneg;ddPCRpos vs RQ-PCRpos p= 0.17). We observed a numerically longer median time to relapse among ddPCRpos pts compared to RQ-PCRpos pts (12 mo vs 6 mo, p=0.08). Conclusions In our study, ddPCR had a higher specificity than RQ-PCR in MRD monitoring in Ph- B-ALL, identifying a subset of pts with increased relapse rate and reduced survival that may have benefited from early HSCT intensification. In the era of novel immunotherapy, improving MRD monitoring techniques in ALL is crucial to optimize patient management and minimize relapse risk.