Parameters affecting tumor-specific delivery of anti-CD5 antibody-ricin A chain immunotoxins in vivo.

The tumor localization of various anti-CD5 immunotoxins (ITs) was studied in vivo in BALB/c nude mice bearing human ascitic Ichikawa tumor cells. Plasma and ascitic clearance of IT as well as the number of intact IT molecules taken up per tumor cell was measured with a highly sensitive immunoenzymometric assay. This assay allowed the quantification of as few as 400 molecules per cell, either bound to cell surface antigens or internalized. Tumor cell localization of an i.v.-administered IT composed of a whole IgG linked by a disulfide bond to native ricin A chain (RTA) was extremely small. The highest binding level of this IT to ascitic cells did not exceed 1,600 molecules per cell, whereas up to 25,000 molecules per cell were quantified after i.p. injection. To improve IT localization after i.v. injection, both the size of the monoclonal antibody moiety and glycosylation of the ricin A chain were examined. Two major additive improvements were obtained by (a) Fc removal and (b) partial deglycosylation of RTA. The localization of F(ab')2-deglycosylated RTA IT was 5.5 times greater than that of IgG-RTA IT, resulting from reduced plasma clearance and better extravasation of the conjugate. However, the IT targeting observed in vivo after i.v. administration was unexpectedly lower than the theoretical number of bound IT molecules, calculated from the concentration of soluble IT molecules in the ascitic fluid. This discrepancy was found to be related to a dramatic decrease in the antigen binding capacity of the antibody moiety in ascitic fluid.