Donald B. Axworthy

Pretargeted α Emitting Radioimmunotherapy Using 213Bi 1,4,7,10-Tetraazacyclododecane-N,N′,N″,N‴-Tetraacetic Acid-Biotin

Purpose: The use of an α emitter for radioimmunotherapy has potential advantages compared with β emitters. When administered systemically optimal targeting of intact antibodies requires >24 h, therefore limiting the use of short-lived α emitters. This study investigated the biodistribution of bismuth-labeled biotin in A431 tumor-bearing mice pretargeted with antibody B3-streptavidin (B3-SA) and examined the therapeutic efficacy of the α emitter, 213Bi-labeled biotin. Experimental Design: Biotinidase-resistant 7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA)-biotin was radiolabeled with 205,206Bi or 213Bi. Treatment of tumor-bearing mice began by administration of B3-SA (400 μg) 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-radiolabeled DOTA-biotin was given, and biodistribution or therapy 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 tumor uptake resulted in much higher tumor:nontumor targeting ratios than achieved with the directly labeled monoclonal antibody. Dose escalation revealed that 74 MBq caused acute death of mice, whereas 0.37–37 MBq doses inhibited tumor growth and prolonged survival significantly. Evidence of mild hematological toxicity was noted. At therapeutically effective doses renal toxicity was observed. Conclusions:213Bi-DOTA-biotin, directed by the Pretarget method to tumor-targeted B3-SA, showed a therapeutic effect, although the therapeutic index was low. The source of the toxicity was most likely related to the renal toxicity.

Intraperitoneal Pretarget Radioimmunotherapy with CC49 Fusion Protein

Purpose: This study examined a pretarget radioimmunotherapy strategy for treatment of an i.p. tumor model (LS174T). Experimental Design: The strategy used regional administration (i.p.) of a novel targeting molecule composed of four CC49 anti–tumor-associated glycoprotein 72 (TAG-72) single-chain antibodies linked to streptavidin as a fusion protein (CC49 fusion protein); 24 hours later, a synthetic clearing agent was administered i.v. to produce hepatic clearance of unbound CC49 fusion protein/synthetic clearing agent complexes. Four hours later, a low molecular weight radiolabeled reagent composed of biotin conjugated to the chelating agent 7,10-tetra-azacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA) complexed with 111In-, 90Y-, or 177Lu-DOTA-biotin was injected. Results: Radiolocalization to tumor sites was superior with i.p. administration of radiolabeled DOTA-biotin as compared with i.v. administration. Imaging and biodistribution studies showed excellent tumor localization of radioactivity with 111In- or 177Lu-DOTA-biotin. Tumor localization of 111In-DOTA-biotin was 43% ID/g and 44% ID/g at 4 and 24 hours with the highest normal tissue localization in the kidney with 6% ID/g at 48 and 72 hours. Therapy studies with 90Y-DOTA-biotin at doses of 400 to 600 μCi or 177Lu-DOTA-biotin at doses of 600 to 800 μCi produced significant prolongation of survival compared with controls (P = 0.03 and P < 0.01). Conclusions: Pretarget radioimmunotherapy using regional administration of CC49 fusion protein and i.p. 90Y- or 177Lu-DOTA-biotin represents a successful therapeutic strategy in the LS174T i.p. tumor model and this strategy may be applicable to human trials in patients with i.p. ovarian cancer.