Mono and bivalent binding of a scFv and covalent diabody to murine laminin-1 using radioiodinated proteins and SPR measurements: Effects on tissue retention in vivo

Phage display techniques identified a scFv, 15-9, which binds to murine laminin-1 and accumulated selectively in tumors. In this study, a covalent diabody was constructed by changing the amino acid residues at positions VH44 and VL100 to cysteine residues so that the diabody form could be stabilized via a disulfide bond. The covalent diabody was expressed in Pichia pastoris and purified by affinity chromatography. The binding properties were measured by surface plasmon resonance and solid phase binding of ¹²⁵I diabody and scFv. Data from the plasmon resonance method yielded calculated K_Ds of 4.4 × 10^{- 10} M for the covalent diabody and 9.9 × 10^{- 8} M for the scFv. K_Ds calculated from solid phase binding of radioiodinated proteins were 1.7-2.1 × 10^{- 10} M and 2.1-2.4 × 10^{- 8} M respectively. The rate of dissociation of ¹²⁵I scFv from solid phase laminin was independent of laminin concentration; however, the dissociation of the ¹²⁵I diabody was dependent both on the concentration of laminin and on the concentration of the diabody. Specifically, high concentrations of laminin yielded very slow rates of diabody dissociation indicating that bivalent attachments had formed. When higher amounts of diabody were used that essentially saturated the laminin sites with univalent binding, the dissociation rate was similar to that for the scFv indicating univalent binding. Biodistribution studies in tumor-bearing SCID mice showed that the covalent diabody improved the ratio of tumor/muscle 2 fold over that obtained with the scFv, although the absolute amount of protein bound to the tumor site was not significantly different for the two forms. The data also showed that retention of the diabody in the tumor and kidney, sites where laminin is present in high concentration, was much longer compared to that of scFv. These data are consistent with the hypothesis that both scFv and diabody forms bind to available laminin in vivo with similar association kinetics, but that in situations of high target concentration, the diabody can bind bivalently and is thus retained at the binding site much longer than the scFv.