Reduced Intensity Versus Conventional Myeloablative Conditioning (RIC vs. MAC) Allogeneic Stem Cell Transplantation (allo-SCT) for Patients with Acute Lymphoblastic Leukemia (ALL): A Survey from the Acute Leukemia Working Party of EBMT

The exact role of RIC allo-SCT for adult patients with ALL is still under considerable debate. While the use of such so-called nonmyeloablative or RIC regimens has emerged as an attractive modality to decrease transplant-related mortality, toxicity might represent only one aspect of the problem, since ALL encompasses a group of chemosensitive diseases, raising concerns that significant reduction of the intensity of the preparative regimen may have a negative impact on long-term leukemic control. In this multicenter retrospective study, the outcomes of 601 adult (age at transplantation >45 y.) patients with ALL who underwent transplantation in complete remission (CR) with an HLA–identical sibling donor, were analyzed according to 2 types of conditioning: RIC in 97 patients, and standard MAC (or high-dose) in 504 patients. Both groups were comparable in terms of gender, CR status (CR1 and CR2), interval from diagnosis to allo-SCT, and recipient/donor CMV serostatus. Patients in the RIC groups were older (median 56 y. vs. 50y in the MAC group; P<0.0001), Most of the patients in the MAC group received high dose TBI (80%), while the majority of the RIC regimens included either low-dose TBI or were ATG+chemotherapy-based regimens. The majority of patients (88%) from the RIC group received a PBSC graft. In the MAC group, the stem cell source consisted of bone marrow in 42% of patients. With a median follow-up of 13 months (range, 1–127), the incidences of grade II-IV and grade III-IV acute GVHD were: 35%, 14%, and 28%, 10% in the MAC and RIC groups respectively (P=NS). The cumulative incidence of non-relapse mortality at 2 years (NRM) was 32% (MAC) vs. 22% (RIC) (P=0.04). The cumulative incidence of relapse at 2 years was 30% (MAC) vs. 42% (RIC) (P=0.0007). However, the latter differences did not translate into any significant difference in term of leukemia-free survival (LFS) at 2 years: 38% (MAC) vs. 37% (RIC) (P=0.42). In multivariate analysis for LFS, the status at transplant was the only factor associated with an improved LFS (p<0.0001, RR=0.55, 95%CI, 0.42–0.72). The results of this retrospective registry based study suggest that RIC regimens may reduce NRM rate after allo-SCT for adult ALL when compared to standard MAC regimens, but with a higher risk of disease relapse and no impact on LFS. The latter represent promising findings, since patients who received RIC are likely to have serious comorbidities, which led the transplantation center to choose RIC, and surely most of these patients would not have received a standard allo-SCT in most institutions. Therefore, RIC allo-SCT for adult ALL (>45 y.) may represent a valid therapeutic option when a conventional standard conditioning is not possible, warranting further prospective investigations.