Radioimmunotherapy of Nude Mice Bearing a Human Interleukin 2 Receptor α-expressing Lymphoma Utilizing the α-emitting Radionuclide-conjugated Monoclonal Antibody 212Bi-anti-Tac

Abstract The efficacy, specificity, and toxicity of bismuth ( 212 Bi) α particle-mediated radioimmunotherapy was evaluated in nude mice bearing a murine lymphoma transfected with the human CD25 [human Tac; interleukin 2 receptor α (IL-2Rα)] gene. The therapeutic agent used was the tumor-specific humanized monoclonal antibody anti-Tac conjugated to 212 Bi. The human IL-2Rα-expressing cell line was produced by transfecting the gene encoding human Tac into the murine plasmacytoma cell line SP2/0. The resulting cell line, SP2/Tac, expressed approximately 18,000 human IL-2Rα molecules/cell. Following s.c. or i.p. injection of 2 × 10 6 SP2/Tac cells into nude mice, rapidly growing tumors developed in all animals after a mean of 10 and 13 days, respectively. The bifunctional chelate cyclohexyldiethylenetriaminepentaacetic acid was used to couple 212 Bi to the humanized anti-Tac monoclonal antibody. This immunoconjugate was shown to be stable in vivo . Specifically, in pharmacokinetic studies in nude mice, the blood clearance patterns of i.v. administered 205/206 Bi-anti-Tac and coinjected 125 I-anti-Tac were comparable. The toxicity and therapeutic efficacy of 212 Bi-anti-Tac were evaluated in nude mouse ascites or solid tumor models wherein SP2/Tac cells were administered either i.p. or s.c., respectively. The i.p. administration of 212 Bi-anti-Tac, 3 days following i.p. tumor inoculation, led to a dose-dependent, significant prolongation of tumor-free survival. Doses of 150 or 200 µCi prevented tumor occurrence in 75% (95% confidence interval, 41–93%) of the animals. In the second model, i.v. treatment with 212 Bi-anti-Tac 3 days following s.c. tumor inoculation also resulted in a prolongation of the period before tumor development. However, prevention of tumor occurrence decreased to 30% (95% confidence interval, 11–60%). In both the i.p. and s.c. tumor trials, 212 Bi-anti-Tac was significantly more effective for i.p. ( P 2 = 0.0128 50/100 µCi 212 Bi-anti-Tac versus 50/100 µCi Mik β; P 2 = 0.0142 150/200 µCi anti-Tac versus 150/200 µCi Mik β) and for s.c. tumors ( P 2 = 0.0018 100 µCi anti-Tac versus 100 µCi Mik β; P 2 = 0.0042 200 µCi anti-Tac versus 200 µCi Mik β1) than the control antibody Mik β1 coupled to 212 Bi at comparable dose levels. In contrast to the efficacy observed in the adjuvant setting, therapy of large, established s.c. SP-2/Tac-expressing tumors with i.v. administered 212 Bi-anti-Tac (at doses up to 200 µCi/animal) failed to induce tumor regression. Pharmacokinetic and tissue distribution studies of radiolabeled anti-Tac in this particular therapeutic situation provided an explanation for this observation. Only 5–6% of the injected dose of radiolabeled antibody was present per g of tumor at 2 h following injection at a time when 75% of the administered 212 Bi radioactivity had decayed. Furthermore, at this time point, there was no greater uptake of Bi-anti-Tac into Tac-expressing tumors than was observed with Tac-nonexpressing variants of SP2/0. Finally, the specific antibody 205/206 Bi-anti-Tac was not enriched in the tumor when compared to the irrelevant monoclonal antibody 205/206 Bi- Mik β1. Although specific enrichment of radiolabeled Bi-anti-Tac was not seen at 2 h, such enrichment in the tumor was observed at 5 and 24 h postinjection with up to 15.6% injected dose present per g of tumor. The dose-limiting acute toxicity following i.v. administration of 212 Bi-anti-Tac was bone marrow suppression, which was observed at doses above 200 µCi. In summary, 212 Bi-anti-Tac as a complete antibody may be of only limited value in the therapy of bulky solid tumors due to the short physical half-life of 212 Bi and the time required to achieve a useful tumor:normal tissue ratio of the radionuclide following administration of the radiolabeled antibody. However, this radionuclide may be useful in select situations such as adjuvant or intracavitary therapy, strategies that target the vascular endothelial cells of tumors, or in the treatment of leukemias.