Post-Transplant Cyclophosphamide Haplo-HSCT Revised: Peripheral Blood Stem Cell Graft and Sirolimus To Enhance Immune Reconstitution and Graft Versus Leukemia Effect In Patients With Active Leukemia

Achieving a cure for relapsed leukemia remains a challenge. Hematopoietic stem cell transplantation (HSCT) from an HLA-haploidentical relative provides a potentially curative treatment for such patients in urgent need. Based on previous studies on the feasibility and low toxicity profile of haploidentical HSCT with high-dose post-transplant Cyclophosphamide (PT-Cy), we planned to intensify this protocol for patients with active disease by the use of: 1) myeloablative conditioning, 2) peripheral blood stem cells (PBSCs) graft instead of bone marrow (BM), and 3) sirolimus instead of tacrolimus, for its direct anti-leukemic effect.

Here we report the results on 18 patients treated between November 2012 and July 2013. The median age was 59 years (range, 30–78). Diagnoses included AML (10 refractory, 3 CR1, and 2 CR>1), ALL (2 refractory), and 1 CML in lymphoid blast crisis. The majority (78%) of patients were not in remission at the time of HSCT. Consistently, 13 (72%) patients scored high/very high accordingly to the CIBMTR disease score (Todisco et al., Leukemia 2013). Fourteen (78%) patients had a Sorror comorbidity score ≥ 3. Conditioning consisted of treosulfan (14 g/m2/day) on days –6 to –4, fludarabine (30 mg/m2/day) on days –6 to –2, and melphalan (70 mg/m2/day) on days –2 and –1, followed by T-repleted peripheral blood stem cells (PBSCs). Postgrafting immunosuppression consisted of PT-Cy (50 mg/kg/day) on days 3 and 4, followed by mycophenolate mofetil for 30 days, and sirolimus for 3 months.

PBSC grafts contained an infused median TNC/kg of 10.5 e8 (range 5.5–20.7 e8), CD3/kg 18.4 e7 (range 10.5–40.9 e7) and a CD34/kg of 7.01 e6 (range 4.3–8.02 e6). All patients but 1 engrafted (94%), with a median time to neutrophil and platelet recovery of 18 days (range 14–26) and 19 days (range 9–51) respectively. Notably, the patient with primary graft failure had a HLA-DP mismatch in HvG direction. No patient experienced secondary graft failure. Post-HSCT recovery of lymphocyte subsets was broad and fast: the median CD3 count at day 30 after HSCT was 642/µl and exceeded 1000/µl by day 60, with a median value of CD4 T cell counts of 329/µl at day 30; NK cells reached a median value of 192/µl at day 30 and remained stable thereafter, while B cells steadily increased from a median value of 2/µl at day 30 to 48/µl at day 180. Circulating T cells comprised both naïve and memory subsets, with a recovery of CD31+ recent thymic emigrants (RTEs) starting from day 30. All patients had a significantly higher proportion of circulating RTEs at day 30, 90 and 180 compared to their pre-HSCT levels, suggesting an improvement in their thymic function after HSCT. Peri-engraftment syndrome (PES) occurred in 4 patients with a median time to onset of 15 days and was promptly resolved with short-course steroids. Interestingly, the percentage of circulating Ki-67+, dividing, effector memory CD8 T cells at day 8 post HSCT was able to accurately predict the occurrence of PES. Grade I/II skin acute GvHD occurred in 6 patients, while 4 patients experienced grade III/IV acute GvHD (22%). Acute GvHD were accompanied by a rise in circulating Ki-67+ CD8 T cells, while response to first line therapy resulted in a drop in Ki-67 expression. Chronic GVHD occurred in 2 of the 7 patients at risk, but only one patient required systemic therapy. Of the 16 patients at high risk, 7 had CMV reactivations. No patient had EBV-related PTLD. With a median follow-up of 100 days (range 15–260), 14 (78%) out of the 18 patients are alive. Only two patients died of non-relapse mortality (NRM, 11%). All patients in CR at time of transplant are alive and disease-free, while 8 (57%) out of 14 patients with active disease at transplant are alive and leukemia-free. In 4 patients tumor-specific T cells against PRAME or WT1 were detectable both in the peripheral blood and BM up to 180 days after HSCT, suggesting that PT-Cy did not hamper the GvL effect.

Myeloablative haploidentical HSCT with PBSCs, PT-Cy and sirolimus is feasible and might be a valid option for patients with refractory/active leukemia. If confirmed in a larger cohort of patients and with a longer follow-up, these results suggest that the acceptable rates of GvHD and NRM as well as the favorable immune reconstitution profile open the way for combining it with novel immunomodulatory or cellular therapies to improve leukemia-free survival.

Bonini:MolMed SpA: Consultancy.