Pretargeted  Emitting Radioimmunotherapy Using 213 Bi 1,4,7,10- Tetraazacyclododecane-N,N,N,N-Tetraacetic Acid-Biotin Zhengsheng Yao, 1 Meili Zhang, 3 Kayhan Garmestani, 3 Donald B. Axworthy, 6 Robert W. Mallett, 6 Alan R. Fritzberg, 6 Lou J. Theodore, 6 Paul S. Plascjak, 2 William C. Eckelman, 2 Thomas A. Waldmann, 3 Ira Pastan, 4 Chang H. Paik, 1 Martin W. Brechbiel, 5 and Jorge A. Carrasquillo 1 1 Nuclear Medicine and 2 Positron Emission Tomography, 2 Department of the Warren G. Magnuson Clinical Center, 3 Metabolism Branch, 4 Laboratory of Molecular Biology, and 5 Radiation Oncology Branch of the National Cancer Institute, NIH, Bethesda, Maryland, and 6 NeoRx Corporation, Seattle, Washington ABSTRACT Purpose: The use of an  emitter for radioimmuno- therapy has potential advantages compared with  emitters. When administered systemically optimal targeting of intact antibodies requires >24 h, therefore limiting the use of short- lived  emitters. This study investigated the biodistribution of bismuth-labeled biotin in A431 tumor-bearing mice pretar- geted with antibody B3-streptavidin (B3-SA) and examined the therapeutic efficacy of the  emitter, 213 Bi-labeled biotin. Experimental Design: Biotinidase-resistant 7,10-tetra- azacyclododecane-N,N,N,N-tetraacetic acid (DOTA)-bio- tin was radiolabeled with 205,206 Bi or 213 Bi. Treatment of tumor-bearing mice began by administration of B3-SA (400 g) to target the tumor sites for 24 h. Then, an agent containing biotin and galactose groups was used to clear the conjugate from the circulation. Four h later, bismuth-radio- labeled DOTA-biotin was given, and biodistribution or ther- apy was evaluated. Dose escalation treatment from 3.7–74 MBq was performed, and the effects on tumors of different sizes were investigated. Tumor growth, complete blood cell counts, toxicity, and survival were monitored. Results: Radiolabeled biotin cleared rapidly. Rapid tu- mor uptake resulted in much higher tumor:nontumor tar- geting ratios than achieved with the directly labeled mono- clonal antibody. Dose escalation revealed that 74 MBq caused acute death of mice, whereas 0.37–37 MBq doses inhibited tumor growth and prolonged survival signifi- cantly. Evidence of mild hematological toxicity was noted. At therapeutically effective doses renal toxicity was ob- served. Conclusions: 213 Bi-DOTA-biotin, directed by the Pre- target method to tumor-targeted B3-SA, showed a therapeu- tic effect, although the therapeutic index was low. The source of the toxicity was most likely related to the renal toxicity. INTRODUCTION Clinical studies using  - particle-emitting radiolabeled monoclonal antibodies (MoAbs) for the radioimmunotherapy of lymphomas have shown promising results (1– 4) and have re- sulted recently in Food and Drug Administration approval of ibritumomab tiuxetan, the first radiolabeled antibody approved for radioimmunotherapy (5, 6). In contrast, numerous studies performed with MoAbs directed against various epithelial tu- mors have only rarely shown partial or complete remissions (7–10). This lack of antitumor effect is thought to be due to the limited radiation dose delivered together with the lower radio- sensitivity of most epithelial tumors when compared with lym- phomas. However, in the case of lymphoma, when higher doses were delivered in the setting of myeloablation and bone marrow rescue, a higher incidence of complete remissions and longer duration of remissions are observed (11). Approaches are needed that result in the delivery of higher tumor concentrations of radiolabeled MoAb with relative sparing of normal tissues. Compared with  - emitters,  emitters have certain theo- retical advantages, higher linear energy transfer with DNA damage that is difficult to repair, reduced nonspecific irradiation to normal tissues around the target cells because of their shorter path lengths (m), and hypoxia-insensitive cytotoxicity. Bis- muth radioisotopes ( 212 Bi and 213 Bi) and 211 At have been used in preclinical trials, and some clinical applications are currently being explored (12–14). Because of its short half-life (46 min), 213 Bi will be limited to systems with rapid delivery and target- ing. Rapid delivery can be obtained when targeting circulating cells or cellular targets that are rapidly accessible, such as in the endothelial vasculature, spleen, and bone marrow, or when local delivery can be performed (14 –16). In most clinical situations, optimal targeting of both hematological and epithelial malig- nancies with intact antibodies requires 24 h. Therefore, pre- targeting approaches, in which delivery of the large molecular weight antibody and the small molecule radionuclide are uncou- pled from each other, appear to be reasonable alternatives for tumor targeting with short-lived radionuclides. Intact immunoglobulins have limited accessibility into solid tumors because of their size (17). In addition, because of increased pressure gradients in tumors, the kinetics of delivery are slow (18). Furthermore, there is often heterogeneity in the distribution of intact IgG because of binding site barriers (19). Several approaches have been tried to overcome these problems, including utilization of small molecular weight fragments that Received 9/5/03; revised 1/12/04; accepted 1/19/04. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Requests for reprints: J. Carrasquillo, Department of Nuclear Medi- cine, 10 Center Drive MSC-1180, Bethesda, MD 20892-1180. Phone: (301) 496-6455; Fax: (301) 402-4085; E-mail: jcarrasquillo@cc. NIH.gov. 3137 Vol. 10, 3137–3146, May 1, 2004 Clinical Cancer Research Cancer Research. on September 11, 2018. © 2004 American Association for clincancerres.aacrjournals.org Downloaded from