Immunomodulatory Interventions Based on Minimal Residual Disease before and after Transplantation for Childhood Acute Lymphoblastic Leukemia

INTRODUCTION. Pre- and post-transplant MRD strongly affects transplantation outcome in ALL. To which extent any MRD-driven intervention may lower its impact is still to be assessed.

PATIENTS AND METHODS. One-hundred-and-seventeen children and adolescents who underwent allogeneic transplantation for ALL in remission (period 2001-2015) had been assessed for MRD by RQ-PCR before and at 1,3,6,9 and 12 months after transplantation.

Four MRD-driven interventions were prospectively planned throughout the years. According to pre-transplant MRD ³1x10^-4, chemotherapy intensification before transplant and early (2nd month) immunosuppression discontinuation after transplant, if no GVHD, were planned, since 2007; according to any level of post-transplant MRD, sudden immunosuppression discontinuation, since 2003, and donor lymphocyte infusions, since 2010, for MRD levels above 1x10-3, and, since 2013, for any level, were planned, if no GVHD.

RESULTS. Five-year-EFS and CIR were 75.6% (SE5.0) and 13.8% (SE4.1), respectively, for patients with pre-transplant MRD<1x10^-4 (65%), versus 50.4% (SE 7.9) (p-value<0.001) and 44.3% (SE 7.8) (p-value<0.001), respectively, for those with MRD³1x10^-4 (35%). Pre-transplant MRD³1x10^-4 was associated with a 4.8-fold risk of relapse (CI 2.18-10.51; p-value <0.001) and a 3.2-fold risk of any event (CI 1.64-6.10; p-value <0.001), compared with patients with MRD negative or <1x10^-4.

Patients who experienced any post-transplant MRD positivity did not necessarily relapse (5-year-EFS 53.8%, SE 7.7), but had a 3.7-fold risk of failure (CI 1.37-9.90; p-value 0.01) if any level was first detected after 6 months or later; a positivity detected in the first 100 days was not significantly associated with outcome, after adjusting for pre-transplant MRD and disease phase. Pre-transplant MRD positivity remained highly significantly associated with lower EFS in patients experiencing any MRD positivity in the first 3 months post-transplant (HR 2.8, CI 1.33-5.71; p-value 0.006), but no significant association could be detected in those experiencing their first post-transplant positivity after 6 months or later.

The presence of any acute GVHD was significantly associated with a lower risk of failure, both in patients with pre-transplant MRD positivity (HR 0.20; CI 0.12-0.49; p-value <0.001) and with early post-transplant first MRD positivity, (HR 0.27; CI 0.12-0.60; p-value 0.001), but not in those with late post-transplant first MRD positivity.

Patients who received pre-transplant additional chemotherapy, in the attempt to reduce MRD, had a non significant 2-fold reduction of the risk of failure (hazard-ratio 0.41, 95% CI 0.14-1.22, p-value=0.11).

Out of the 41 patients with pre-transplant MRD positivity, of the 39 patients evaluable in the second month, immunosuppression could be tapered early in 12 of the 14 patients who had no GVHD; 6 of the 12 who could taper immunosuppression are in long-term remission, compared with 0 of the 2 who did not taper and 15 of the 25 who could not taper due to ongoing GVHD.

Of the 45 patients who experienced any post-transplant MRD positivity, 10 patients received DLI, 7 of whom are in long-term remission, compared with 18 of the 35 who did not receive them.

All those patients achieving a MRD level 5x10^-4 ultimately relapsed, regardless of immunosuppression discontinuation or donor-lymphocyte-infusion.

In conclusion the risk of relapse was strongly associated with pre-transplant MRD level and relapse did not necessarily occur after post-transplant MRD positivity, especially if detected early and at levels <1x10^-4. Reduction of MRD burden before transplant by additional chemotherapy and post transplant immunomodulation might reduce the risk of relapse associated with high pre-transplant MRD. Immunosuppression discontinuation and DLI might revert the increased risk of relapse associated with post-transplant MRD positivization and improve ultimate outcome.