Letter to the Editor Demonstration of the Hepatic Uptake of Radiolabelled Immunotoxins Using Gamma Scintigraphy

A RANGE of immunotoxins based on the combination ofmonoclonal antibodies and plant-derived toxins have recently been advocated as agents for the selective killing of tumours and metastatic deposits [l, 21. We have recently monitored the biodistribution of an immunotoxiin based on a conjugate of ricin toxin A chain and the monoclonal antibody 791T/ 36 by gamma scintigraphy. The 791T/36 antibody is known to recognize a 72,000 Dalton glycoprotein antigen associated with a range of human tumours [3]. The immunotoxin comprised immunoaffinity purified ricin toxin A chain (RTA) linked by a disulphide bridge to the intact monoclonal antibody 791T/36 (IgGSb). Radioiodination was conducted by reacting sulphydryl-protected RTA with [‘311]sodium iodide at 37-55 MBq/mg RTA using Iodogen at 0.1 mg/ml protein. After removing the excess unreacted iodide, the protecting group was removed by reacting the RTA with 50 mM dithiothreitol. The resulting [1311]RTA-SH was then conjugated to monoclonal antibody 791Tl36 using N-succinimidyl-2-(2-pyridyldithio)-propionate as previously described [3]. Nude mice (OLAC U.K. Ltd, Oxon, U.K.) were anaesthetized using ether and placed prone