IN-VITRO AND IN-VIVO ANTITUMOR-ACTIVITY OF ZENECA ZD0490, A RECOMBINANT RICIN A-CHAIN IMMUNOTOXIN FOR THE TREATMENT OF COLORECTAL-CANCER

ZENECA ZD0490 is a recombinant ricin A-chain-containing immunotoxin th at recognizes an antigen that is expressed on approximately 65% of col orectal tumors. The antigen CA242 is recognized by a mouse monoclonal antibody designated C242. C242 antibody was conjugated to recombinant ricin A-chain via a methyl-hindered disulfide linker which confers in vivo stability. ZD0490 was extremely potent against colorectal cell li nes CoLo201 and CoLo205, which express the CA242 antigen. ZD0490 activ ity was determined iii vitro by both protein synthesis inhibition (50% inhibitory concentrations of 1-20 ng/ml after 24-h exposure) and clon ogenic assay (76-95% cell kill after 24-h exposure to a 50% inhibitory concentration for protein synthesis inhibition; >99.99% cell kill at 1000 ng/ml). This in vitro activity was translated to in vivo efficacy where single dose i.v. administration of 2.5 mg/kg of ZD0490 was suff icient to induce substantial growth delays of both CoLo201 and CoLo205 s.c. tumors in nude mice. This growth delay equates to between 40 and 60% inhibition of tumor protein synthesis as quantified by an in vivo [C-14]leucine incorporation assay. Using this technique, it was shown that protein synthesis inhibition persisted for at least 96 h after a single dose of ZD0490. Administration of the same total dose given as daily doses over 5 days did not alter the antitumor efficacy of ZD049 0 in either the growth delay or the protein synthesis inhibition assay s. The in vitro and in vivo activity of ZD0490 detailed in this paper show that this novel immunotoxin is worthy of clinical evaluation, whi ch is currently under way in the United Kingdom.