Ibrutinib for Pure Red Cell Aplasia after Allogeneic Hematopoietic Stem Cell Transplant with Major ABO Incompatibility.

Since Human leukocyte antigen (HLA) and ABO blood group system are inherited independently and with up to 50% of allogeneic hematopoietic cell transplantations (HCT) are performed with donor-recipient ABO incompatibility, recipients are at increased risk for acute and delayed hemolytic reaction and delayed RBC precursors engraftment; i.e. pure RBC aplasia (PRCA). PRCA is a sever consequence of major and bi-directional ABO mismatch after alloHCT, leading to frequent transfusions, iron overload and secondary complications. Risk factors for PRCA after Major ABO incompatibility include; stem cell source, conditioning intensity, and titer of iso-agglutinins. Treatment is mostly supportive with transfusions and growth factors as well as withdrawing immunosuppressive therapy (IS). Targeting recipient immune system with high-dose steroids, DLI, and rituximab have been reported with variable results. Here we report a case series of a novel method of targeting recipient iso-agglutinins-producing B cells by Ibrutinib. We report 3 cases of PRCA refractory to conventional therapy who responded to ibrutinib, a BTK inhibitor targeting recipient B cells to allow engraftment of donor RBCs. Patient, transplant and disease characteristic including prior therapies, time to transfusion independence, and switch of blood group (BG) are summarized in Table 1. First patient's day 30 post-transplant bone marrow biopsy (BMBx) showed complete remission (CR) with full donor chimera, but BG remained O. He developed severe anemia requiring transfusions and Day 100 BMBx showed CR with markedly decreased number of erythroid progenitors. He did not respond to IS withdrawal, prednisone, rituximab or bortezomib. But BMBx 4 weeks after starting ibrutinib showed trilineage hematopoiesis. Second patient remained RBC transfusion dependent post-HCT with Day 100 biopsy showing PRCA and full donor chimera. BG remained O. He developed severe GVHD of skin and eyes after DLI and His PRCA was refractory to multiple lines of therapies but responded to ibrutinib. Third patient underwent HCT for refractory AML with MDS; Anti-A isoagglutinin level pre-HCT was high and patient underwent plasma exchange. Day 30 BMBx showed CR and full donor chimera. Day +73 BMBx showed CR with near absent erythropoiesis. PRCA was refractory to rituximab and responded to ibrutinib. All three patients with PRCA refractory to multiple lines of therapy responded to ibrutinib with median time to transfusion independence of 4 weeks and median time to BG switch of 6 weeks. All patients tolerated therapy well. Our experience at City of Hope shows that ibrutinib could offer a safe and efficacious treatment option for cases of refractory PRCA. Prospective studies are needed to assess if early therapy with ibrutinib could reduce the morbidity resulting from frequent transfusions, and GVHD complications. [ABSTRACT FROM AUTHOR]