Myeloablative radioimmunotherapy with I-131-BC8 (anti-CD45 antibody) for acute myeloid leukemia: Effect of spleen size and uptake on bone marrow absorbed dose

1663 Objectives: The challenging and fatal nature of advanced AML and refractory myelodysplastic syndrome result in poor cure rates with current management techniques. Iodine-131-BC8 antibody is a novel anti-CD45 myeloablative radioimmunotherapy agent for treating these patients. Bone marrow is a target tissue. We investigated the effect of spleen size and relative splenic uptake (%ID/g) of radiolabeled antibody on the absorbed dose to bone marrow. Methods: We reviewed biokinetic data on 108 patients who received tracer infusions of 150-370 MBq I-131-BC8 antibody (0.5 mg/kg) followed by serial gamma camera imaging and whole body NaI probe counting prior to RIT. Iliac crest bone marrow biopsies were obtained at 24 h to calculate percent administered activity (%ID/g) in marrow and to normalize the marrow time-activity curves from direct counting. Patient-specific organ absorbed doses were calculated with correction for organ volumes from CT scans. We compared the calculated red marrow doses with spleen size and splenic uptake to see whether the spleen would act as a sink for radiolabeled antibody and reduce the dose to the marrow. Results: Spleen sizes ranged from 71-830 g (mean 301±153), and initial antibody uptakes in spleen ranged from 0.0181-0.169 %ID/g (mean 0.0751±0.0301). Calculated marrow doses ranged from 2.52-28.2 cGy/mCi (mean 7.21±3.56). Total marrow absorbed doses ranged from 428-3731 cGy (mean 1550±785). No negative or positive correlation was found between marrow dose and either spleen size or splenic uptake of radiolabeled antibody. Conclusions: Radioimmunotherapy with I-131-BC8-antibody delivered substantial radiation doses to hematopoietic tissues in patients treated for advanced AML and high-risk MDS. Neither spleen size nor relative uptake of I-131-BC8-antibody in the spleen had any significant effect on the dose delivered to red marrow. Research Support (if any): Supported by NIH P01-144991, LSS SCOR Grant 7040, and DOE AC06-76RL0-1830.