Antitumour activity of a sterically blocked ricin immunotoxin on a human colorectal adenocarcinoma grafted subcutaneously in nude mice

Abstract

We prepared a ricin-antibody conjugate, lacking the ability to bind the galactosidic residues of Sepharose 6B, a so-called blocked immunotoxin. The monoclonal antibody AR-3 was cross-linked to ricin through a thioether bond. Further studies showed that the immunoconjugate suppressed the tumour growth of HT-29 cells in intraperitoneally grafted nude mice, without showing any undesirable ricin toxicity. In this work, to demonstrate the therapeutic activity of the AR-3-ricin conjugate injected into mice bearing subcutaneous tumour, we first evalauted its pharmacokinetic behaviour and biodistribution. The behaviour of the immunoconjugate injected intravenously was almost intermediate between that of the antibody and ricin. Moreover, when the immunotoxin was intravenously administered to nude mice bearing subcutaneous tumour, no therapeutic effects appeared, in accordance with the relatively low permeability of the immunotoxin from the blood to the skin. In contrast, peritumoral treatment produced a strong reduction of the neoplastic nodules without substantial regrowth of the malignant cells. This result was also achieved when the immunotoxin treatment was performed on a well-established tumour. This finding was strictly related to the specifcity of the immunoconjugate, since the analogous treatment with an irrelevant immunotoxin showed therapeutic failure.

Use of antibodies and immunoconjugates for the therapy of more accessible cancers

“…Hepatic toxicity has frequently limited the use of toxin immunoconjugates [47]. Most plant toxins are composed of 2 parts, one responsible for cytotoxicity and another component responsible for binding to cells, which was responsible for severe toxicity in the very early work with immunotoxin conjugates [48,49]. Even with the cell-binding component removed, toxin conjugates continued to cause severe hepatic toxicity.…”

Role of Cross-Linking Agents in Determining the Biochemical and Pharmacokinetic Properties of Mgr6−Clavin Immunotoxins

“…Several investigators used a reduced size IT Fab linked to the toxic domain of diphteria toxin or Pseudomonas exotoxin to allow sufficient tissue permeation; however, this approach was not efficient in the treatment of solid tumors due to faster increased in vivo clearance (30). We and others have used whole ITs (mAb-RIP) with high potency and good pharmacokinetic properties (6,10,31,32).…”