PREPARATION OF A TRIVALENT ANTIGEN-BINDING CONSTRUCT USING POLYOXIME CHEMISTRY - IMPROVED BIODISTRIBUTION AND POTENTIAL FOR THERAPEUTIC APPLICATION

In an attempt to improve the pharmacokinetic behavior of an antitumor radioimmunoconjugate, we have prepared a trivalent antigen-binding con struct formed from three Fab' fragments derived from the parent murine monoclonal antibody (MAb) 35 directed against the carcinoembryonic an tigen. The construct was generated by a novel approach using polyoxime chemistry. This approach leads to a homogeneous construct, as judged by SDS-PAGE and by mass spectrometry, which was found to retain full i mmunoreactivity. A comparison of the monovalent, divalent, and trivale nt F(ab')(n) materials in vitro revealed the expected trend of increas ing association constant with increasing valency. The in vivo biodistr ibution of the I-125-labeled trivalent construct was studied in xenogr aft-bearing nude mice. Absolute tumor accumulation seen with the triva lent construct (10.8% injected dose/g) was lower than that seen with t he intact MAb35 (15.2% injected dose/g). This finding and the more rap id loss of activity from tumor are presumably the consequence of the q uicker blood clearance of the trivalent material. However, the constru ct showed tumor:blood ratios up to 10-fold higher than those seen for the parent antibody, and ratios of tumor:normal tissue accumulation we re generally greatly improved. These improvements were achieved despit e only modest reduction in maximum tumor accumulation when compared to the parent MAb35, and this augurs well for an improved potential for this novel construct as an agent for radioimmunotherapy and radioimmun oscintigraphy.