Recombinant F(ab') C242-Pseudomonas exotoxin, but not the whole antibody-based immunotoxin, causes regression of a human colorectal tumor xenograft.

We have previously made a Mr 195,000 immunotoxin (IT) composed of mAb C242 coupled to Pseudomonas exotoxin A. This IT inhibited growth but did not cause regression of a human colorectal cancer xenograft growing in nude mice (W. Debinski et al., J. Clin. Invest., 90: 405-411, 1992). Since smaller proteins penetrate into tissues and tumors better than larger proteins, we have made a smaller recombinant (r) IT to overcome the hypothesized poor tumor penetration of the Mr 195,000 conjugate. This was accomplished by making a C242rF(ab')-based Mr 86,000 IT. To make rF(ab')-IT, the Fd and kappa chains of mAb C242 were cloned, and kappa was fused at the gene level to PE38QQR, a mutant form of Pseudomonas exotoxin. Both C242Fd and C242kappa-PE38QQR were expressed in a bacterial expression system, and large amounts of the C242Fd attached via a disulfide bond to the C242kappa-PE38QQR were obtained. The C242rF(ab')-IT covalently linked heterodimer has a 50% inhibitory concentration of 0.2-2.0 ng/ml (2-20 pm, respectively) on human colon adenocarcinoma cell lines that express the C242 antigen. When injected into mice bearing Colo205 tumors, the C242rF(ab')-PE38QQR caused an immediate regression of the tumors, while the C242-PE38QQR conjugate had only a growth inhibitory effect. In addition, several cures were obtained. Our results indicate that rIT C242F(ab')-PE38QQR is a much more potent antitumor agent than an IgG conjugate.