Minimal Residual Disease Monitoring by Two Sensitive Techniques May Contribute to Improved Outcome of Allogeneic Transplantation for Ph(-) Acute Lymphoblastic Leukemia

Monitoring of Minimal Residual Disease (MRD) in Ph(-) Acute Lymphoblastic Leukemia (ALL) patients post allogeneic Hematopoetic Cell transplantation (allo-HCT) defines patients in risk for imminent relapse who further benefit from immunotherapeutic interventions. Six-color Flow Cytometry (FC) and unmanipulated bone marrow chimerism with STR-PCR by fragment analysis (STR-PCR) have been easily applied. We evaluated the prognostic value of these techniques in monitoring MRD for Ph(-) ALL patients post allo-HCT. Thirty patients aged 30 (13-58) years who underwent allo-HCT for B- (14) or T- (16) ALL mainly after myeloablative conditioning (28/30) were evaluated. Fifteen out of 30 transplants were matched unrelated, 13 sibling and two alternative ones. Monitoring of MRD with both methods was available for all patients, performed on days +30, +60, +90 and every 3 months for 2 years. Median follow-up was 26 (2-76) months and 176 samples were analysed. Overall, only 4/30 patients relapsed. FC and STR-PCR showed concordant results in 133/176 samples (77%). Discordant results were as follows: i) 7 patients in 11 samples had positive FC-MRD and complete donor chimerism in STR-PCR (FC+/ STR-). According to our protocol, immunosupression was reduced promptly and only one patient relapsed. Interestingly, in this cohort, subsequently six patients experienced low level mixed chimerism (LLMC), reflecting the increased sensitivity of both methods, ii) 15 patients in 32 samples >+3 months post-alloHCT, experienced mixed chimerism with negative FC-MRD(FC-/STR+) and no intervention was applied due to chimerism >=97.5%. Five out of 15 patients had previously been FC+/STR-, as described before. None of the remaining 10 FC-/STR+ patients relapsed. Progression Free Survival (PFS) and Overall Survival (OS) did not differ in the two groups compared to the entire cohort. Similarly, no differences were found in PFS and OS when each method was assessed separately. However, patients experiencing concurrently FC and STR-PCR positive MRD showed a trend for shorter PFS and OS [mean PFS: 43 months(95% CI: 21-65) vs 53(21-65), p=ns and mean OS: 50 months (95% CI: 27-74) vs 60 (54-65), p=0.08, medians not reached, for FC+/STR+ vs other, respectively]. In conclusion: i) positive MRD by FC is frequently followed by low level mixed chimerism (LLMC) by STR-PCR, reflecting the high sensitivity of both methods and leading to prompt therapeutic interventions ii) concurrent detection of MRD by FC and STR-PCR indicates a worse prognosis. In this cohort of patients, 6-color FC was not superior to STR-PCR in terms of DFS and OS. This could be explained by the low incidence of relapse in our population following our immunotherapeutic interventions. Larger scale studies are warranted to evaluate the prognostic value of each method and answer the arising question of the benefit from the concurrent diagnostic approach.