Pretargeted radioimmunotherapy (PRIT) has demonstrated remarkable efficacy targeting tumor antigens, but immunogenicity and endogenous biotin blocking may limit clinical translation. We describe a new PRIT approach for the treatment of multiple myeloma (MM) and other B-cell malignancies, for which we developed an anti-CD38-bispecific fusion protein that eliminates endogenous biotin interference and immunogenic elements. In murine xenograft models of MM and non-Hodgkin lymphoma (NHL), the CD38-bispecific construct demonstrated excellent blood clearance and tumor targeting. Dosimetry calculations showed a tumor-absorbed dose of 43.8 Gy per millicurie injected dose of Y, with tumor-to-normal organ dose ratios of 7:1 for liver and 15:1 for lung and kidney. In therapy studies, CD38-bispecific PRIT resulted in 100% complete remissions by day 12 in MM and NHL xenograft models, ultimately curing 80% of mice at optimal doses. In direct comparisons, efficacy of the CD38 bispecific proved equal or superior to streptavidin (SA)-biotin-based CD38-SA PRIT. Each approach cured at least 75% of mice at the highest radiation dose tested (1200 µCi), whereas at 600- and 1000-µCi doses, the bispecific outperformed the SA approach, curing 35% more mice overall ( < .004). The high efficacy of bispecific PRIT, combined with its reduced risk of immunogenicity and endogenous biotin interference, make the CD38 bispecific an attractive candidate for clinical translation. Critically, CD38 PRIT may benefit patients with unresponsive, high-risk disease because refractory disease typically retains radiation sensitivity. We posit that PRIT might not only prolong survival, but possibly cure MM and treatment-refractory NHL patients.

α-Emitting radionuclides deposit a large amount of energy within a few cell diameters and may be particularly effective for radioimmunotherapy targeting minimal residual disease (MRD). To evaluate this hypothesis, (211)At-labeled 1F5 monoclonal antibody (mAb) (anti-CD20) was studied in both bulky lymphoma tumor xenograft and MRD animal models. Superior treatment responses to (211)At-labeled 1F5 mAb were evident in the MRD setting. Lymphoma xenograft tumor-bearing animals treated with doses of up to 48 µCi of (211)At-labeled anti-CD20 mAb ([(211)At]1F5-B10) experienced modest responses (0% cures but two- to threefold prolongation of survival compared with negative controls). In contrast, 70% of animals in the MRD lymphoma model demonstrated complete eradication of disease when treated with (211)At-B10-1F5 at a radiation dose that was less than one-third (15 µCi) of the highest dose given to xenograft animals. Tumor progression among untreated control animals in both models was uniformly lethal. After 130 days, no significant renal or hepatic toxicity was observed in the cured animals receiving 15 µCi of [(211)At]1F5-B10. These findings suggest that α-emitters are highly efficacious in MRD settings, where isolated cells and small tumor clusters prevail.

Purpose: Alpha-emitting radionuclides exhibit a potential advantage for cancer treatments because they release large amounts of ionizing energy over a few cell diameters (50-80 µm), causing localized, irreparable double-strand DNA breaks that lead to cell death. Radioimmunotherapy (RIT) approaches using monoclonal antibodies labeled with α emitters may thus inactivate targeted cells with minimal radiation damage to surrounding tissues. Tools are needed to visualize and quantify the radioactivity distribution and absorbed doses to targeted and nontargeted cells for accurate dosimetry of all treatment regimens utilizing α particles, including RIT and others (e.g., Ra-223), especially for organs and tumors with heterogeneous radionuclide distributions. The aim of this study was to evaluate and characterize a novel single-particle digital autoradiography imager, the ionizing-radiation quantum imaging detector (iQID) camera, for use in α-RIT experiments. Methods: The iQID camera is a scintillator-based radiation detection system that images and identifies charged-particle and gamma-ray/x-ray emissions spatially and temporally on an eventby-event basis. It employs CCD-CMOS cameras and high-performance computing hardware for real-time imaging and activity quantification of tissue sections, approaching cellular resolutions. In this work, the authors evaluated its characteristics for α-particle imaging, including measurements of intrinsic detector spatial resolutions and background count rates at various detector configurations and quantification of activity distributions. The technique was assessed for quantitative imaging of astatine-211 ( 211 At) activity distributions in cryosections of murine and canine tissue samples. Results: The highest spatial resolution was measured at ∼20 µm full width at half maximum and the α-particle background was measured at a rate as low as (2.6 ± 0.5) × 10 −4 cpm/cm 2 (40 mm diameter detector area). Simultaneous imaging of multiple tissue sections was performed using a large-area iQID configuration (ø 11.5 cm). Estimation of the 211 At activity distribution was demonstrated at mBq/µg-levels. Conclusions: Single-particle digital autoradiography of α emitters has advantages over traditional film-based autoradiographic techniques that use phosphor screens, in terms of spatial resolution, sensitivity, and activity quantification capability. The system features and characterization results presented in this study show that the iQID is a promising technology for microdosimetry, because it provides necessary information for interpreting alpha-RIT outcomes and for predicting the therapeutic efficacy of cell-targeted approaches using α emitters. C 2015 American Association of Physicists in Medicine. [http://dx

The vast majority of patients with plasma cell neoplasms die of progressive disease despite high response rates to novel agents. Malignant plasma cells are very radiosensitive, but the potential role of radioimmunotherapy (RIT) in the management of plasmacytomas and multiple myeloma (MM) has undergone only limited evaluation. Furthermore, CD38 has not been explored as a RIT target despite its uniform high expression on plasma cell malignancies. In this report, both conventional RIT (directly radiolabeled antibody) and streptavidin-biotin pretargeted RIT (PRIT) directed against the CD38 antigen, were assessed as approaches to deliver radiation doses sufficient for MM cell eradication. PRIT demonstrated biodistributions that were markedly superior to conventional RIT. Tumor-to-blood ratios as high as 638:1 were seen 24hr after PRIT, while ratios never exceeded 1:1 with conventional RIT. 90Yttrium absorbed dose estimates demonstrated excellent target-to-normal organ ratios (6:1 for the kidney, lung, liver; 10:1 for the whole body). Objective remissions were observed within 7 days in 100% of the mice treated with doses ranging from 800 µCi to 1200 µCi of anti-CD38 pretargeted 90Y-DOTA-biotin, including 100% complete remissions (no detectable tumor in treated mice compared to tumors that were 2982±2834% of initial tumor volume in control animals) by day 23. Furthermore, 100% of animals bearing NCI-H929 multiple myeloma tumor xenografts treated with 800 µCi of anti-CD38 pretargeted 90Y-DOTA-biotin achieved long-term myeloma-free survival (>70 days) compared to none (0%) of the control animals.

Key Points• Astatination of anti-CD45antibody via a closodecaborate compound yields a stable conjugate that targets radiation to hematologic organs.• 211 At-anti-CD45 radioimmunotherapy combined with bone marrow transplantation prolongs survival in a disseminated murine leukemia model.Despite aggressive chemotherapy combined with hematopoietic stem cell transplantation (HSCT), many patients with acute myeloid leukemia (AML) relapse. Radioimmunotherapy (RIT) using monoclonal antibodies labeled with b-emitting radionuclides has been explored to reduce relapse. b emitters are limited by lower energies and nonspecific cytotoxicity from longer path lengths compared with a emitters such as 211 At, which has a higher energy profile and shorter path length. We evaluated the efficacy and toxicity of anti-CD45 RIT using 211 At in a disseminated murine AML model. Biodistribution studies in leukemic SJL/J mice showed excellent localization of 211 At-anti-murine CD45 mAb (30F11) to marrow and spleen within 24 hours (18% and 79% injected dose per gram of tissue [ID/g], respectively), with lower kidney and lung uptake (8.4% and 14% ID/g, respectively). In syngeneic HSCT studies, 211 At-B10-30F11 RIT improved the median survival of leukemic mice in a dose-dependent fashion (123, 101, 61, and 37 days given 24, 20, 12, and 0 mCi, respectively). This approach had minimal toxicity with nadir white blood cell counts >2.7 K/mL 2 weeks after HSCT and recovery by 4 weeks. These data suggest that 211 At-anti-CD45 RIT in conjunction with HSCT may be a promising therapeutic option for AML. (Blood. 2013;121(18):3759-3767) IntroductionAcute myeloid leukemia (AML) is an aggressive malignancy with few treatments producing prolonged remissions in high-risk patients. Hematopoietic stem cell transplantation (HSCT) may offer the best chance for a cure, but it has been associated with high rates of treatment-related mortality and relapse. Investigators have escalated chemotherapy and/or radiation doses to decrease relapse, but this strategy has been associated with substantial toxicity yielding no significant improvement in overall survival. 1 Monoclonal antibodies (mAbs) targeting hematologic-specific antigens have been used in radioimmunotherapy (RIT) studies as a means to deliver higher radiation doses prior to HSCT. [2][3][4][5][6] One such target is CD45, a cell surface antigen highly expressed on hematologic tissues (;200 000 binding sites per cell) with minimal expression on nonhematologic tissues. 7,8 CD45 is not extensively internalized after mAb binding, 9,10 further making anti-CD45 RIT a viable approach for therapy of high-risk AML. In particular, anti-CD45 mAb coupled to 131 I has been shown to deliver an average twofold to threefold higher radiation-absorbed dose to spleen and bone marrow than to nonleukemic normal organs, and it can be safely administered to high-risk patients with acute leukemia or myelodysplastic syndrome in conjunction with standard high-dose chemotherapy and 12Gy totalbody irradiation. 5,11 Favorable results ...