Hematopoietic Stem Cell Transplantation (HSCT) from Alternative Donors for Acute Leukemia at High-Risk of Relapse. The Haploidentical Option.

Despite advances in chemotherapy for acute leukemia, survival is poor when patients have unfavourable prognostic features at diagnosis, when they do not achieve CR after the first induction cycle and when they are in second or later remission. In these circumstances an allogeneic HSCT is preferred. The chance of finding a matched unrelated donor depends on the HLA diversity and although molecular analysis achieves closer matches it reduces the probability of finding a donor. Furthermore, many patients relapse while waiting for transplant. Transplantation of HSCs from a one-haplotype mismatched family member offers an immediate source of HSCs to almost all leukemia patients who urgently need an allogeneic transplantation because of the high-risk of leukemia relapse and who do not have a matched, either related or unrelated, avaible donor. Over the past decade, our group has shown the two major obstacles to mismatched transplants, that is severe acute GVHD in T-cell-replete transplants and graft rejection in T-cell-depleted transplants, can be overcome by infusing a megadose of extensively T-cell-depleted HSCs after an immuno-myelo-ablative conditioning regimen. Since our first reports (Aversa et al. Blood 1994 and NEJM 1998), the main modifications to our original approach were: a) in October 1995, fludarabine was substituted for cyclophosphamide in our TBI-based conditioning regimen; b) peripheral blood cells were positively selected by using initially the Ceprate device and then, since January 1999, the Clinimacs instrument which ensures a 4.5 log T-cell depletion in a one-step procedure with no E-rosetting; c) in the 138 patients transplanted since January 1999 post-transplant G-CSF administration was stopped so as to improve immune recovery.

The patient population included 90 AML and 48 ALL, median age 28 years (range 9–62), 40 (29%) in bad-risk CR I, 43 (31%) in second or later CR and 55 (40%) in relapse at transplant. Primary full-donor engraftment was achieved in 125/134 evaluable patients (93%); 8 patients engrafted after second transplants. Overall engraftment was achieved in 133 patients (96%). Without any post-transplant immunosuppressive prophylaxis, grade II-IV acute GvHD occurred in 7/133 evaluable patients and 5/106 developed chronic GvHD. Cumulative incidence (C.I. 95%) of non-leukemia mortality was 36% (19%–53%) and 40% (19%–66%) for patients who were respectively in CR or in relapse at transplant. 38/51 deaths were infection-related. Disease status was the major risk factor for relapse and EFS. Cumulative incidence of leukemia relapse was 27% (12%–45%) and 60% (30%–80%), p=0.006, for ALL patients in CR and relapse respectively; 17% (8%–29%) and 46% (29%–61%), p=0.0001, for AML in CR and relapse respectively. ALL and AML patients transplanted in relapse have, respectively, a 6% and 13% probability of surviving event-free. For those transplanted in remission, EFS is respectively 38% and 50% for ALL and AML patients in any CR at transplant. These results indicate the mismatched transplant should be offered to high-risk acute leukemia patients without a HLA-identical donor not as a last resort, but as a viable option in the early stages of the disease.