Anti-CD45 Ab Pretargeted Radioimmunotherapy Using An Alpha Emitting Radionuclide (213Bi) Delivers Selective Radiation to Human Myeloid Leukemias in a Mouse Xenograft Model and Results in High Rates of Complete Remission and Long Term Survival.

Abstract 1035

Poster Board I-57

Little change has been seen over the past two decades in the progression-free survival of patients with Acute Myeloid Leukemia (AML) undergoing allogeneic hematopoietic cell transplantation (HCT). Efforts to decrease the risk of relapse after HCT have included increasing the intensity of the preparative regimen. Our group has focused on targeted radioimmunotherapy using an anti-CD45 antibody (Ab) as part of the preparative regimen prior to HCT. Despite the promise of this approach, recurrent malignancy remains a problem, particularly for patients with high-risk disease. More recently we have explored a pretargeted RIT (PRIT) strategy to augment the anti-tumor efficacy of the transplant preparative regimen while diminishing overall toxicity. These studies have employed an anti-CD45 Ab conjugated to streptavidin (SA) followed by a biotinylated, N-acetylgalactosamine-containing clearing agent (CA) to remove circulating Ab-SA conjugate from the blood and then with radiobiotin using beta-emitting radionuclides that have relatively low energy transfer characteristics and long path lengths that may result in suboptimal killing of leukemia cells and normal organ toxicity due to cross-fire from malignant cells. Alpha-emitting radionuclides exhibit very high cytotoxicity coupled with a short path length, potentially increasing the therapeutic index and making them an attractive alternative to beta-emitting radionuclides. Therefore, we have now employed PRIT using an anti-CD45 Ab-SA conjugate, CA, and biotin labeled with an alpha-emitting radionuclide (213Bi) in mice with human erythroid leukemia (HEL) xenografts. Results of biodistributions of radioactivity demonstrated excellent localization of 213Bi-biotin to tumors with minimal uptake into normal organs due to elimination of non-specific radiation exposure from blood-borne radiolabeled Ab. After 10 minutes, 4.5 ± 1.1% of the injected dose of 213Bi was delivered per gram of tumor (% ID/g). Imaging using a novel alpha camera demonstrated uniform radionuclide distribution within tumor tissue at 10 minutes after 213Bi-biotin injection. Estimated radiation-absorbed doses delivered to HEL xenografts delivered 3.4-and 2.1-fold more radiation to tumor than to liver and lungs, respectively. These target-to-non-target ratios of absorbed radiation obtained using PRIT with 213Bi were similar to those observed using a beta-emitting (90Y) radionuclide in the same animal model. Based on these encouraging results, we conducted therapy experiments in a minimal disease xenograft model using a single dose of 213Bi-biotin given 24 hours after anti-CD45 Ab-SA conjugate. In an initial attempt to compare 90Y and 213Bi, we gave equal μCi doses of each radionuclide. Eighty percent of mice treated with anti-CD45 Ab-SA conjugate followed by 800 μCi of 213Bi-biotin survived leukemia-free for >100 days with minimal toxicity. By comparison, only 20% of mice treated with PRIT anti-CD45 Ab-SA conjugate followed by 800 μCi 90Y-biotin exhibited long term leukemia-free survival. While we acknowledge that equal μCi doses may not necessarily result in equivalent doses delivered to normal tissue, these data suggest that anti-CD45 PRIT using an alpha-emitting radionuclide may be highly effective and minimally toxic for the treatment of myeloid leukemias.