FIGURE 2.

Fusion of scFv and Fn3 binding domains to rGel leads to enhanced cytotoxicity specific for antigen-positive cells. A, using the WST-1 assay, the cytotoxicity of soluble rGel was tested on all four cell lines used in the study. Across all cell lines, rGel showed an IC₅₀ of ∼500 nm. B, antigen-negative cells (HT-1080) were treated with the four different immunotoxins that displayed roughly equivalent potency to soluble toxin. C, immunotoxins were also tested for cytotoxicity on the double-positive, low-antigen density HT-29 cell line. Surprisingly, none of the immunotoxins showed enhanced cytotoxicity compared with the IC₅₀ of rGel. D, on high antigen-expressing cells (HT-1080(CEA) and A431), significantly greater potency was observed for the immunotoxins compared with the soluble toxin. Against cells expressing their respective antigen targets, they had IC₅₀ values of 250 pm for 3ErGel, 1.5 nm for FErGel, 8 nm for C7rGel, and 30 nm for E4rGel.