Rapid Engraftment and Immune Recovery in Treatment Refractory Severe Aplastic Anemia Patients Undergoing Ex Vivo Nicotinamide-Expanded (NAM-Expanded) Unrelated Cord Blood Transplantation
Authors: Childs Richard W. , Vo Phuong T. , Aue Georg , Wilder Jennifer , Wells Brian , Clara Joseph , Shalabi Reem , Prince Patricia , Stroncek David F. , Reger Robert , Flegel Willy A. , Adams Sharon , Gunn Kristen , Worthy Tatyana , Cook Lisa , Tian Xin 
Published: October 2019

Abstract
Introduction. Allogeneic hematopoietic stem cell transplantation using UCB is an alternative approach for pts (pts) with hematological malignancies lacking an HLA matched donor. However, for pts with severe aplastic anemia (SAA), UCB transplants are associated with delayed engraftment, high graft failure rates and poor survival. Ex-vivo expanded UCB using nicotinamide (NAM) can engraft in NOD/SCID mice, and in pilot studies in pts with hematological malignancies, results in rapid engraftment and durable hematopoiesis. Here we investigated a novel transplantation approach using NAM-expanded UCB in refractory SAA pts who lacked an HLA matched donor, hypothesizing this regimen would accelerate engraftment and immune reconstitution compared to conventional UCB transplants.
PTS AND Methods. Eligible pts had SAA with severe neutropenia (ANC<1000) unresponsive to immunosuppressive therapy (IST) underwent a NAM-expanded UCB-transplant at a single center in a phase II trial (NCT03173937). Pts were conditioned with cyclophosphamide (60 mg/kg x 2), horse ATG (40 mg/kg x 4), fludarabine (25 mg/m2 x5) and 200cGy of TBI. GVHD prophylaxis included tacrolimus and MMF. Cohort I is designed to transplant six pts with a single NAM-expanded unit combined with 3 x 106 CD34+ cells/kg from a haploidentical donor as a backup stem cell source. Once adequate cord engraftment is established in Cohort 1, the study will proceed to Cohort 2 to transplant a NAM-expanded unit alone without haplo-CD34+ cells. Engraftment, chimerism, and immune recovery were assessed and compared with SAA pts who received a conventional non-expanded UCB transplant combined with haplo-CD34+ cells using the identical conditioning and GVHD prophylaxis.
Results. From 2017 to 2018, two SAA pts (22 years male and 45 years female, pre-transplant ANC ≤300/uL, and had failed ATG/CSA/Eltrombopag) were successfully transplanted with a single ≥ 6/8 HLA-matched NAM-expanded UCB unit combined with haplo CD34+ cells from a relative. The UCB units before expansion contained a median total nucleated cell (TNC) dose of 2.8 x 107/kg and 1.7 x 105 CD34+ cells/kg. At transplant, the cultured NAM-expanded units contained a median 6.0 x 107 TNCs/kg and 96.4 x105 CD34+ cells/kg, representing a median post TNC and CD34+ cell expansion of 2-fold and 52-fold, respectively. At 12 months and 5 months post-transplant, both pts survive with stable engraftment, transfusion independence, and without acute or chronic GVHD. The median time to neutrophil recovery (ANC > 500/ μL) was only 6.5 days (range 6-7), and platelet recovery was 35.5 days (31-40); chimerism studies showed that both pts achieved >95% cord donor myeloid chimerism and T-cell chimerism at a median 6.5 (6-7) and 23.5 days (21-26) respectively . Immune recovery in both pts receiving NAM-expanded UCB was brisk (Figure 1): absolute CD4+ count > 200 cells/μL occurred at 17 and 60 days; at day 100, median CD4+ numbers was 382/μL and median IGA was 92 mg/dL. In comparison to 16 SAA pts transplanted sequentially at our institute from 2013-2016 using a single unexpanded CBU combined with haplo CD34+ cells, median cord graft doses were 3.6 x 107 TNCs/kg and 1.2 x105 CD34+ cells/kg; b) median time to ANC and platelet recovery was 10 and 51 days; c) median time to >95% cord donor myeloid chimerism was 63 days; d) at day 100, only 3/16 (19%) unexpanded UCB recipients had CD4+ count > 200 cells/μL, and the median CD4+ number was only 74 cells/μL and the median IGA was only 31 mg/dL. This first in human transplant trial suggests neutrophil engraftment, platelet recovery, and post-transplant immune recovery are superior inSAA pts transplanted with NAM expanded UCB compared to conventional nonexpanded UCB (all P<0.05, Figure 1).
Conclusion. These encouraging preliminary 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 transplanted CD34+ progenitor cells in NAM-expanded grafts could potentially overcome graft failure associated with conventional UCB transplantation for SAA, obviating the need for co-transplanting haplo CD34+ cells as a stem cell back-up.

Disclosures
No relevant conflicts of interest to declare.