Accumulation of a recombinant immunotoxin in a tumor in vivo: fewer than 1000 molecules per cell are sufficient for complete responses.

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.