title: Accumulation of a recombinant immunotoxin in a tumor in vivo: fewer than 1000 molecules per cell are sufficient for complete responses.
abstract: Recombinant immunotoxins have been shown to cure human tumor xenografts in mice, but their biodistribution to both tumors and normal organs has not been reported. Anti-Tac(Fv)-PE38 is a single-chain recombinant immunotoxin composed of the variable heavy and light domains of the anti-Tac monoclonal antibody that reacts with the primate interleukin 2 (IL2) receptor alpha subunit (IL2R alpha or CD25) fused to a truncated form of Pseudomonas exotoxin (PE). 125I-labeled anti-Tac(Fv)-PE38 was given i.v. to immunodeficient mice each bearing two A431 tumors, one that expresses IL2R alpha (ATAC-4) and one that does not (A431, parental). A single i.v. dose of 4 microg/mouse caused complete regression of the IL2R alpha + tumor. At 6 h, over 6% of the injected dose/g was found in the ATAC-4 tumor, and 2% was in the A431 tumor. Uptake in the ATAC-4 tumor was higher than in any other tissue. Sections of tumor examined by autoradiography indicated that anti-Tac(Fv)-PE38 was distributed throughout the entire tumor, with some portions having higher uptake than others. By subtracting uptake in tumors without receptor (A431) from uptake in receptor-containing tumors (ATAC-4), we calculated that at least 400 molecules/cell specifically bound to IL2R alpha-positive tumor cells at 90 min and 750 molecules/cell bound at 360 min. This is similar to the 400-870 molecules/cell required for >99.9% killing of ATAC-4 cells growing as a monolayer. The results show that solid tumors in mice can be eradicated like cells in tissue culture, and that delivery of less than 1000 molecules/cell is sufficient to cause complete tumor regressions.
authors.affiliation: Laboratory of Molecular Biology, Division of Cancer Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: R J
authors.initials: RJ
authors.lastname: Kreitman
authors.forename: I
authors.initials: I
authors.lastname: Pastan
mesh_terms: D036002:ADP Ribose Transferases; D000818:Animals; D000911:Antibodies, Monoclonal / Q000493:pharmacokinetics / Q000627:therapeutic use; D001427:Bacterial Toxins; D005098:Exotoxins / Q000493:pharmacokinetics* / Q000627:therapeutic use; D006801:Humans; D000922:Immunotoxins / Q000493:pharmacokinetics* / Q000627:therapeutic use*; D051379:Mice; D008819:Mice, Nude; D009374:Neoplasms, Experimental / Q000188:drug therapy* / Q000276:immunology / Q000473:pathology; D015375:Receptors, Interleukin-2 / Q000276:immunology*; D011993:Recombinant Fusion Proteins / Q000493:pharmacokinetics / Q000627:therapeutic use; D037521:Virulence Factors; D000097668:Pseudomonas aeruginosa Exotoxin A
chemical_list: D036002:ADP Ribose Transferases; D000911:Antibodies, Monoclonal; D001427:Bacterial Toxins; D005098:Exotoxins; D000922:Immunotoxins; D015375:Receptors, Interleukin-2; D011993:Recombinant Fusion Proteins; D037521:Virulence Factors; D000097668:Pseudomonas aeruginosa Exotoxin A