A novel spleen tyrosine kinase (SYK) P-site inhibitor, 1,4-Bis (9-O dihydroquinidinyl) phthalazine/hydroquinidine 1,4-phathalazinediyl diether (C-61), (but not vehicle) markedly enhanced H₂O₂-induced apoptosis of primary leukemia cells from each of five relapsed B-lineage acute lymphoblastic leukemia (ALL) patients, as measured by in vitro TUNEL assays. A highly radiation-resistant subclone of the murine B-lineage leukemia cell line BCL-1 was next used to investigate the in vivo radiosensitizing effects of C-61. C-61 enhanced the antileukemia potency of 7 Gy total-body irradiation (TBI) in the context of syngeneic bone marrow transplantation (BMT) at 20% of its nonobservable adverse effect level (NOAEL) that does not exhibit detectable single-agent activity against BCL-1 leukemia in vivo. Based on this preclinical proof-of-principle study, we hypothesize that the incorporation of C-61 into the pretransplant TBI regimens of patients with recurrent or high-risk B-lineage acute lymphoblastic leukemia (ALL) will help overcome the radiochemotherapy resistance of their leukemia cells and thereby improve their treatment response and survival outcome after BMT.