Novel Conditioning Regimen for Haploidentical Hematopoietic Cell Transplant for Severe Aplastic Anemia in Children

Abstract

New hematopoietic cell transplant (HCT) approaches are urgently needed for patients with severe aplastic anemia (SAA) who lack an HLA-identical donor. Haploidentical HCT with post transplant cyclophosphamide (PTCy) represent a potential universal available option for almost all children with SAA. We present a novel conditioning regimen for haploidentical HCT in children with SAA in a center where horse ATG is not available. Conditioning regimen consists of rabbit ATG 2.5 mg/kg/day from day -9 to -7, Fludarabine 30 mg/kg/day from day -6 to -2, Cyclophosphamide 14.5 mg/kg/day from day -6 to -5, Thiotepa 5 mg/kg/day from day -4 to -3, and 4 Gy TBI on day -1 in in two fractions. GvHD prophylaxis consist of PTCy 50 mg/kg/day on days +3 and +4 along with Cyclosporine A and Mycophenolate mofetil (MMF) starting on day +5. Four consecutive children with SAA referred to our center for haploidentical HCT starting in 2018. Median age at HCT was 9 years (5-16) with 3 males and 1 female. All patients were heavily transfused with both blood and platelets prior to referral for HCT. Two patients had strong and one had weak positive anti-HLA antibodies (DSAs) and received desensitization with IVIG, Rituximab and plasmapheresis. One patient received buffy coat infusion on day -1 due to persistent strong DSAs despite desensitization. Median CD34+ dose received was 12 x 10e6, and median CD3+ dose was 29 x 10e6. Donors were all same blood group to patients. All patients successfully engrafted neutrophils at median of 13 days (12-14). Platelets engraftment in 3/4 patients at median of 7 days (5-10). All patients received peripheral blood as stem cell source. Three of four patients survived and doing well at last follow up. One patient had toxic death on day +38 due to chemotherapy related toxicity causing multi-organ failure. Chimerism analysis was full donor in all four patients at median follow up time of 11 months (2-12). Patients were sent home at median of 24 days post HCT. None developed grade 2-4 acute GvHD nor chronic GvHD. Acute GvHD of skin grade 1 stage 1 developed in 2 patient and managed with topical steroids. Viral reactivations consisted of CMV viremia and BK hemorrhagic cystitis in all patients, and have all resolved. No post transplant autoimmune complications. Haploidentical HCT with PTCy represents a quick and first line approach in heavily transfused children with SAA. Although yet limited number of patients, this regimen is feasible and appears to be safe. A great advantage of this regimen is the rapid engraftment of both neutrophils and platelets. Moreover, although using peripheral blood as a stem cell source, there was no severe acute or chronic GvHD. Disclosures No relevant conflicts of interest to declare.