397: Minimal Residual Disease and Chimerism in CML Patients Receiving Allogeneic Transplants after Myeloablative or Reduced Conditioning

It is not known whether reduced conditioning is associated with prolonged period of detectable disease after SCT in patients with CML. We have therefore compared BCR-ABL transcript levels in patients receiving a conventional myeloablative conditioning with those receiving reduced conditioning. We also made chimerism studies in both patient groups. Myeloablative Conditioning (MYC): 31 patients, 19 matched unrelated donors (MUD) and 12 sibling donors (Sib). Median age was 42 years (10–61). Conditioning with Bu+Cy (n = 31). Twenty of the MUD patients were also given ATG. Seven patients have died (6 GVHD, 1 Relapse). Median follow-up time: 56 months (10–99). Reduced Intensity Conditioning (RIC): 24 patients, 13 MUD and 11 Sib. Median age was 55 years (11–64). Conditioning with Flu+Bu+ATG. Seven patients have died (3 Relapses, 1 GVHD, 3 other reasons). Median follow-up time: 43 months (18–100). Results: There was no statistical significant difference in overall survival and relapse free survival between both groups. 5/29 patients in the MYC group relapsed as compared to 7/23 patients in the RIC group. Only one patient in the RIC group rejected the graft. The incidence of severe acute GVHD was significantly higher in the MYC group (48%) as compared to the RIC group (13%), p = 0.004. Interestingly, the incidence of chronic GVHD was slightly higher in the RIC group, 51% vs. 37% (p = 0.11). During the first 3 months, MRD levels in the RIC group was in median one log of magnitude higher than in the MYC group (p = 0.01). After that, no significant differences in the MRD incidence and MRD level were found between the groups. Median time to complete donor T-cell chimerism was 34 days (14–285) in the MYC group as compared to 60 days (24–245) in the RIC group (p = 0.04). Also, complete myeloid cell engraftment was delayed in the RIC group, median 44 days (14–165) as compared to 25 days (14–270) in the MYC group (p = 0.02). Conclusion: Despite higher BCR-ABL levels during the early post-transplant period and higher incidence of mixed chimerism, nonmyeloablative transplantation for CML patients may induce molecular remission in the majority of the patients.