Incidence and Kinetics Of Full Donor Chimerism and The Impact Of Minimal Residual Disease Status Following Outpatient Non-Myeloablative Allogeneic Stem Cell Transplantation For Myeloma

Abstract Background Chimerism analysis is routinely performed following allogeneic haemopoietic stem cell transplantation (allo HSCT) as a means of monitoring donor engraftment and relapse risk, although its role in the management of allografted myeloma patients is controversial. Multiparameter flow cytometry (MFC) assessment of minimal residual disease has been demonstrated to predict outcome following myeloblative autologous stem cell transplantation (ASCT) in multiple myeloma (MM). Relapse and disease progression is the principle cause of allograft failure and the detection of minimal residual disease (MRD) following tandem autologous/non-myeloablative(NMA) allo HSCT may potentially identify patients at risk of early relapse and provide a platform for early post-allograft therapy designed to further cytoreduce or enhance alloreactive T cell function and graft versus myeloma effect. Aim To evaluate the impact of MRD assessment performed at 3 months post-allo HSCT on PFS and OS and document the incidence and kinetics of full donor chimerism (FDC) in a cohort of myeloma patients undergoing tandem ASCT/outpatient non-myeloablative (NMA) allogeneic HSCT for MM. Methods Retrospective analysis of 33 MM patients referred to our centre for tandem ASCT/outpatient NMA HSCT between May 2008 and December 2012. Patients were transplanted as part of their front line management (upfront) (n=18) if they had one or more high risk disease features (poor prognosis cytogenetics/FISH abnormalities, elevated LDH, stage III disease or less than a PR following a novel agent-containing induction regimen) or as a ‘deferred’ procedure (n=15), if relapsing/progressing after an initial ASCT and maintenance/consolidation. Allo HSCT was undertaken as an ambulatory procedure and conditioned with oral fludarabine 48mg/m2 on days -4 to -2 and 2Gy TBI on day 0 followed by donor stem cell infusion, with a target CD34 dose of 2 x 106/kg and T cell dose of 3 x 108/kg. Sequential peripheral blood monitoring of CD3+ and CD33+ donor-specific chimerism was performed on days 30, 60, 90, 180 and 365 following allo HSCT. Bone marrow samples were collected for MRD analysis using MFC incorporating a CD56/CD19/CD45 panel following CD38/CD138 gating on plasma cells. MRD assessment was performed every 3 months during the first 12 months, 6 monthly during the second year and annually thereafter. Results The median age for the entire cohort (M 19, F 14) was 52 years (range 39-65) and the analysis performed after a median follow up of 719 days (50-1733). All patients were transplanted using matched sibling (20/33) or unrelated donors (13/33). Non-relapse mortality was 0% at 1 year and 6% overall. At the time of analysis, 26 (79%) patients were alive including 15 (45%) in CR. The median PFS for all patients was 2.8 years and although the median OS for the entire cohort has not been reached, the estimated probability of overall survival at 4 years is 67% (95% CI: 38-85%). 29/33 (88%) patients had MRD assessments performed at least once post-allo HSCT and 4 patients progressed within 3 months prior to MRD evaluation. At the first 3 month assessment, 11/29 (38%) were MRD +, 12/29 (41%) were MRD – , results were unavailable in 3/29 (10.5%) and 3 (10.5%) patients had morphological evidence of disease. Patients achieving MRD negativity at 3 months had superior PFS in comparison to those who remained MRD positive (median 2.81 years v 1.02 years; p=0.01) but this has yet to translate into an OS advantage. The MRD + patients transplanted as a deferred procedure (n=6) had a median PFS of less than 6 months (0.47 years), suggesting this subset of patients have a particularly poor outcome, although overall numbers are insufficient to directly compare outcome according to MRD status and transplant timing. 30 (91%) patients achieved FDC (16 and 14 in the upfront and deferred groups respectively) with 2 patients not assessed. The median time to FDC for the entire cohort was 180 days (range 30-730) and there was no significant difference according to transplant timing or relationship between FDC and MRD status, GVHD, PFS or PS. Conclusion The majority of patients undergoing outpatient-based tandem ASCT/NMA allo HSCT for myeloma achieve FDC. MRD status as assessed by MFC at 3 months post-allo HSCT appears to predict PFS and may be a useful early trigger for additional post-allograft therapy, particularly in patients with high risk disease or those transplanted later in their disease course. Disclosures: Spencer: Celgene: Honoraria, Membership on an entity’s Board of Directors or advisory committees.