Non-Myeloablative Haploidentical Transplantation of Unmanipulated, G-CSF Mobilized Peripheral Blood Stem Cells with High Dose Post-Transplant Cyclophosphamide: Fast Engraftment, No Evidence of Graft Rejection and Low Incidence of Graft-Versus-Host-Disease

Haploidentical bone marrow transplantation using non-myeloablative conditioning and high-dose, post-transplant cyclophosphamide (Cy) has been shown to be a feasible approach for patients lacking an HLA identical donor with acceptable rates of acute and chronic Graft-versus-Host-Disease (GvHD); in the attempt to reduce the associated risk of graft failure and relapse incidence we decided to apply the same conditioning regimen using unmanipulated G-CSF mobilized peripheral blood stem cells (PBSC). From July 2010 to January 2014, 20 patients affected by high-risk hematological malignancies, median age 56 years (range 19-70) (2 Acute Myeloid Leukemia, 5 Acute Lymphoblastic Leukemia, 5 Non-Hodgkin Lymphoma, 3 Multiple Myeloma, 2 Myelodysplastic Syndrome, 3 Hodgkin Lymphoma), with no available HLA identical donor (neither related or unrelated) were enrolled in a prospective clinical protocol at our institution. Disease status at transplant was the following: 10 complete remission (6 patients in 1°CR, 3 patients in 2°CR, 1 patient in 3°CR), 4 partial response (PR), 3 stable disease (SD), 2 progressive disease (PD), and a patient with myelodysplastic syndrome who received the transplant as upfront therapy. Conditioning regimen consisted of cyclophosphamide, fludarabine and TBI followed by infusion of haploidentical unmunipulated mobilized PBSC; post-transplant immunosuppression consisted of high-dose Cy on days +3 and +4, tacrolimus and micofenolate mofetile from day +5.

A median of 5.5x10⁶ (range 3.8-13.7) CD34+ cells/kg was infused with a median of 2.8x10⁸(range 0.3-5.4) CD3+ cells/kg in 19/20 evaluable patient: 1 patient was unable to receive the complete conditioning regimen due to rapid worsening of her clinical condition and therefore she was taken out of the study protocol. Patients readily engrafted with a median time to absolute neutrophil count ≥500/µL of 17.5 days (range 14-28) in 17/19 patients and a median time to platelet ≥20,000/µL of 21 days (range 11-53) in 18/19 evaluable patients. One patient died of infection and progressive disease before engraftment, another patient showed myeloid engraftment but never recovered platelet counts and prematurely died of cytomegalovirus (CMV) disease.

At a median follow-up of 486 days (12-1297), median OS has not been reached with 13 patients being alive, 8 in CR, 3 in PR and 2 in PD. Causes of death included progressive disease in 3 patients and infections in the remaining 3, with a cumulative transplant related mortality of 15%. CMV reactivation occurred in 12 of 16 patients at risk, at a median time of 34 days (range 22-55) after transplant, resolving with preemptive therapy in 11 patients and causing a fatal infection in 1 heavily pretreated patient.

Acute GvHD (aGvHD) occurred in 1 patient, with 1-year cumulative incidence of grade I/II aGvHD of 5.6% (95% CI:8.0-33.4%) and no incidence of grade III/IV GvHD. Mild chronic GvHD (cGvHD) was observed in 2 patients with 1-year cumulative incidence of cGvHD of 8.3% (95% CI:1.2-46.1%). Achievement of mixed donor chimerism was rapid: 16/18 evaluable patients showed a CD3+ chimerism >50% by day +28. At day + 84, in 14/16 evaluable patients CD3+ cells chimerism was >50%, in 6 of them it was >95%. Modulation and withdrawal of immunosuppression increased donor CD3+ chimerism in both of the two remaining patients to 67% and 74% at day +360 and +180, respectively. Notably, no graft failure was observed.

Our data show that non-myeloablative haploidentical peripheral blood stem cell transplantation with high-dose post-transplant Cy is a feasible and safe approach for patients lacking an HLA identical donor. The use of unmanipulated, G-CSF mobilized PBSC with the infusion of a greater number of CD3+ cells allows a rapid and sustained engraftment, reduces the risk of graft failure and does not appear to increase the risk of GvHD. Although the incidence of major infection was low, CMV reactivation remains a critical issues and further studies are needed to clarify recovery of CMV immunity and to establish better prophylaxis therapy.