Pharmacokinetics and antitumor activity of a bivalent disulfide-stabilizedFv immunotoxin with improved antigen binding to erbB2

We have generated a stable bivalent Fv molecule [(dsFv)(2)] of the anti-erb B2 monoclonal antibody e23 in which the V-H and V-L domains of the Fv are l inked to each other by a disulfide bond and the two Pvs are connected by a 15-amino acid Linker (T. K. Bera et al.t, J. Mol. Biol., 281: 475-483, 1998 ). The e23 (dsFv)(2) molecule is linked to a truncated form of Pseudomonas exotoxin (PE38) to generate a bivalent disulfide-stabilized immunotoxin e23 (dsFv)(2)-PE38. Compared to the monovalent immunotoxin, the (dsFv)(2) immu notoxin showed greatly Increased cytotoxicity to four cancer cell lines exp ressing low levels of erbB2 but not to four other cell lines with high erbB 2 expression. e23 (dsFv)(2)-PE38 was administered i.v. to mice, and its hal f-life was determined. The t(1/2)alpha and t(1/2)beta were 20 and 325 min, respectively, whereas the corresponding values for the monovalent dsFv immu notoxin were shorter, 6 and 52 min. The antitumor activities of the monoval ent and bivalent immunotoxin were compared using mice bearing A431 tumors. Despite the fact that e23 (dsFv)(2)-PE38 was 13-fold more active than e23 d sFv-PE38 on A431 cells in cell culture, its antitumor activity in mice was <2-fold that of the monovalent immunotoxin. These data show that a large in crease in avidity does not always lead to an increase in cytotoxic activity . Furthermore, in one of the cases in which cytotoxic activity in vitro was greatly enhanced, there was only a small increase in antitumor activity.