title: Therapeutic efficacy and dose-limiting toxicity of Auger-electron vs. beta emitters in radioimmunotherapy with internalizing antibodies: evaluation of 125I- vs. 131I-labeled CO17-1A in a human colorectal cancer model.
abstract: Recent clinical results suggest that higher anti-tumor efficacy may be achieved with internalizing monoclonal antibodies (MAbs) at lower toxicity when labeled with Auger-electron, as compared to conventional beta-emitters. The aim of our study was to compare the toxicity and anti-tumor efficacy of the 125I-labeled internalizing MAb, CO17-1A, with its 131I-labeled form in a human colon cancer model in nude mice. Biodistribution studies were performed in nude mice bearing s.c. human colon cancer xenografts. For therapy, the mice were injected either with unlabeled 125I- or 131I-labeled C017-1A at equitoxic doses. Control groups were left untreated, were given a radiolabeled isotype-matched irrelevant antibody or a tumor-specific, but noninternalizing antibody. The maximum tolerated activities (MTD) of 131I-and 125I-CO17-1A without artificial support were 300 microCi and 3 mCi, respectively. Myelotoxicity was dose-limiting; bone marrow transplantation allowed for an increase of the MTD to 400 microCi of 131I-17-1A, whereas the MTD of 125I-17-1A with bone marrow support had not been reached at 5 mCi. Whereas no significant therapeutic effects were seen with unlabeled C017-1A, tumor growth was retarded with 131I-CO17-1A. With the 125I-label, however, therapeutic results were clearly superior. In contrast, no significant difference was observed in the therapeutic efficacy of the 131I- vs. 125I-labeled, noninternalizing antibodies. Our data indicate a superiority of Auger-electron emitters, such as 125I, as compared to therapy with conventional beta-emitters with internalizing antibodies. The lower toxicity of Auger emitters may be due to the short path length of their low-energy electrons, which can reach the nuclear DNA only if the antibody is internalized (as is the case in antigen-expressing tumor tissue, but not in the stem cells of the red marrow).
authors.affiliation: Department of Nuclear Medicine, Georg-August-University, Göttingen, Germany. tmbehr@WisLAN1.med.uni-goettingen.de
authors.forename: T M
authors.initials: TM
authors.lastname: Behr
authors.forename: G
authors.initials: G
authors.lastname: Sgouros
authors.forename: V
authors.initials: V
authors.lastname: Vougiokas
authors.forename: S
authors.initials: S
authors.lastname: Memtsoudis
authors.forename: S
authors.initials: S
authors.lastname: Gratz
authors.forename: H
authors.initials: H
authors.lastname: Schmidberger
authors.forename: R D
authors.initials: RD
authors.lastname: Blumenthal
authors.forename: D M
authors.initials: DM
authors.lastname: Goldenberg
authors.forename: W
authors.initials: W
authors.lastname: Becker
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000627:therapeutic use / Q000633:toxicity; D001610:Beta Particles / Q000627:therapeutic use*; D015179:Colorectal Neoplasms / Q000378:metabolism / Q000532:radiotherapy*; D004307:Dose-Response Relationship, Radiation; D004583:Electrons / Q000627:therapeutic use*; D005260:Female; D006801:Humans; D000922:Immunotoxins / Q000493:pharmacokinetics / Q000627:therapeutic use* / Q000633:toxicity; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D016499:Radioimmunotherapy / Q000379:methods*; D012680:Sensitivity and Specificity; D014018:Tissue Distribution; D014183:Transplantation, Heterologous; D014407:Tumor Cells, Cultured
chemical_list: D000911:Antibodies, Monoclonal; D000922:Immunotoxins; D007457:Iodine Radioisotopes