Human leukemia cell binding and killing by anti-CD5 radioimmunotoxins

We have synthesized a reagent for antibody directed cell targeting composed of the monoclonal antibody (MoAb) T101 linked to the potent toxin ricin. The immunotoxin (IT) was subsequently radiolabeled by a cyclic anhydride procedure with ⁹⁰Yttrium (⁹⁰Y) to construct a radioimmunotoxin (RIT) that may have potential for cancer therapy. We evaluated the reagent for selectivity in binding and protein synthesis inhibition (PSI) assays. The RIT selectively bound antigen positive leukemia T-cell lines, with minimal binding to antigen negative control lines. The IT inhibited 87% or greater protein synthesis activity at 1 μg ml and exhibited an IC₅₀ (the dose inhibiting 50% activity) of 0.18 ± 0.08 μg ml in the presence of lactose. RIT and nonlabeled IT showed comparable degrees of PSI at 1 μg ml and 10 μg ml, suggesting that labeling had little overall effect on the activity of the immunoconjugate. However, indirect evidence showed that the galactose binding site of ricin was inhibited 10-fold by its exposure to ⁹⁰Y. Control RIT were minimally inhibitory. IT labeled with ¹³¹Iodine (¹³¹I) by an iodine monochloride technique also retained its capability to selectively inhibit protein synthesis. When RIT were tested for potency in a clonogenic assay against human leukemia T-cell lines, they inhibited 3.61 logs of tumor cell growth at μg mlml. This did not represent an improvement over the log elimination with radiolabeled antibody alone, which showed 4.19 log elimination of tumor cells. Our observation that the ⁹⁰Y-labeled RIT and labeled antibody can selectively eliminate about four logs of tumor cells in an in vitro clonogenic assay is unique. The ability of RIT to kill several logs of tumor cells in vitro renders RIT interesting anti-tumor reagents.