Ex Vivo Nicotinamide-Expanded (NAM-Expanded) Unrelated Cord Bloodtransplantation (UCB) for Refractory Severe Aplastic Anemia Results in Rapid Engraftment and Expedites Immune Recovery

Abstract

Introduction Allogeneic hematopoietic stem cell transplantation using UCB (UCBT) is an alternative approach for severe aplastic anemia (SAA) patients (pts) lacking an HLA matched donor. However, UCBT is associated with delayed engraftment, high graft failure and mortality rates. Ex-vivo expanded UCB using nicotinamide (NAM) can engraft in NOD/SCID mice, and in pts with hematological malignancies in pilot studies. Here we investigated whether NAM-expanded UCBT would accelerate engraftment and immune reconstitution in refractory SAA pts. Methods Eligible SAA pts with severe neutropenia who failed immunosuppressive therapy underwent a NAM-expanded UCBT at a single center in a phase II trial. Pts were conditioned with cyclophosphamide (60 mg/kg × 2), horse ATG (40 mg/kg × 4), fludarabine (25 mg/m2 × 5) and 200cGy of TBI. GVHD prophylaxis included tacrolimus and MMF. Cohort 1 is designed to transplant six pts with a single NAM-expanded unit combined with 3 × 106 CD34+ cells/kg from a haploidentical donor as a stem cell backup. Once adequate cord engraftment is established, Cohort 2 will transplant a NAM-expanded unit alone. Engraftment and immune recovery were assessed and compared with a conventional UCBT combined with haploCD34+ cells using the same conditioning and GVHD prophylaxis. Results From 2017 to 2018, two SAA pts (22 years male and 45 years female, pre-transplant ANC ≤300/uL, failed ATG/CSA/Eltrombopag) successfully underwent a single 6/8 HLA-matched NAM-expanded UCBT combined with haplo CD34+ cells. UCB units before expansion contained a median total nucleated cell (TNC) dose of 2.8 × 107/kg and 1.7 × 105 CD34+ cells /kg. At transplant, NAM-expanded units contained a median 6.0 × 107 TNCs/kg and 96.4 × 105 CD34+ cells /kg, representing a median TNC and CD34+ cell expansion of 2-fold and 52-fold, respectively. At 14 and 7 months post-transplant, both pts were GVHD-free and survived with stable engraftment and transfusion independence (Figure 1). The median time to neutrophil and platelet recovery was only 6.5 days (range 6-7) and 35.5 days (31-40), with >95% cord donor myeloid and T-cell chimerism occurring at days 6.5 and 23.5, respectively. Immune recovery was brisk (Figure 2): absolute CD4+ > 200 cells/μL occurred at 17 and 60 days; at day 100, mean CD4+ cells were 382/μL and IGA 92 mg/dL. Compared to 16 SAA pts who received a single unexpanded UCBT with haplo CD34+ cells at our institute from 2013 to 2016, engraftment and immune recovery were superior in SAA pts with NAM-expanded UCBT (P Conclusion These results show for the first time that NAM-expanded UCB results in rapid cord engraftment, sustained hematopoiesis and accelerated immune recovery in treatment refractory, neutropenic SAA pts. The high numbers of CD34+ progenitor cells in NAM-expanded grafts could potentially overcome graft failure which limits conventional UCBT for SAA.