In vivo studies of whole ricin monoclonal antibody immunoconjugates for the treatment of murine tumours.

Ricin is a highly potent toxin which binds to cells via galactose binding sites on the B chain; the toxicity is manifest by the A chain and most studies with immunotoxins have used ricin A chain-antibody conjugates. We have previously described a method for the coupling of whole ricin to monoclonal antibody (MoAb) so that the galactose binding sites on the B chain are blocked; these conjugates are not non-specifically toxic. Whole ricin-MoAb conjugates were examined in vivo for their stability, clearance rates and toxicity, and these were compared with modified ricin-MoAb conjugates produced by periodate treatment of ricin. First, the biological half-life of whole ricin or modified ricin was determined and indicated that while ricin was rapidly cleared from the circulation (t 1/2 = 4.5 +/- 0.5 min) compared with modified ricin (t 1/2 = 17 +/- 1 min), covalent linkage of native ricin to MoAb increased the blood circulation time of the toxin (t 1/2 = 22 +/- 1 min). Immunotoxins synthesized with the Ly-2.1 MoAb using native or modified (deglycosylated) ricin were compared in vivo. In two different models anti-Ly-2.1 immunotoxins could be shown to be effective in vivo: (i) intraperitoneal tumours, thymomas grown in the peritoneal cavity could be completely eradicated by ricin-MoAb; (ii) subcutaneous tumours, mice with tumours approximately 0.75 cm in diameter received intravenous doses of the whole ricin-MoAb and a substantial reduction in tumour size was achieved. Thus whole ricin-antibody conjugates made with the galactose binding site blocked combine the advantage of high potency with high specificity (which was previously lacking in intact ricin conjugates) and can be successfully used in vivo to treat tumours.