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 construct formed from three Fab' fragments derived from the parent murine monoclonal antibody (MAb) 35 directed against the carcinoembryonic antigen. 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 immunoreactivity. A comparison of the monovalent, divalent, and trivalent F(ab')n materials in vitro revealed the expected trend of increasing association constant with increasing valency. The in vivo biodistribution of the 125I-labeled trivalent construct was studied in xenograft-bearing nude mice. Absolute tumor accumulation seen with the trivalent construct (10.8% injected dose/g) was lower than that seen with the intact MAb35 (15.2% injected dose/g). This finding and the more rapid loss of activity from tumor are presumably the consequence of the quicker blood clearance of the trivalent material. However, the construct showed tumor:blood ratios up to 10-fold higher than those seen for the parent antibody, and ratios of tumor:normal tissue accumulation were generally greatly improved. These improvements were achieved despite 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 radioimmunoscintigraphy.