Extracorporeal Photopheresis in Addition to Pentostatin in Conditioning for Canine Hematopoietic Cell Transplantation: Role in Engraftment

Extracorporeal photopheresis (ECP) and the purine analog pentostatin exert potent immunomodulatory effects but have not been evaluated for their ability to enhance engraftment of hematopoietic stem cells. We evaluated in a canine model of dog leukocyte antigen (DLA)-identical hematopoietic cell transplantation (HCT) whether ECP in combination with pentostatin could enhance engraftment using a nonmyeloablative regimen consisting of 100 cGy total body irradiation (TBI) and postgrafting immunosuppression with mycophenolate mofetil (MMF) and cyclosporine (CSP). We have shown previously that with 100 cGy TBI alone as conditioning, all of six dogs rejected their graft 2–12 weeks after HCT. With the addition of pentostatin to 100 cGy TBI, 6 of 10 dogs rejected their graft. We now tested the additional use of ECP alone (n=2) or ECP plus 3–6 doses of pentostatin (n=7) before 100 cGy TBI and HCT. Eight out of 9 dogs rejected their grafts within 6–11 weeks after HCT. Compared to data without ECP, we failed to demonstrate a positive impact of the use of either ECP or pentostatin for prevention of rejection.