ANTIGEN-SPECIFICITY AND TUMOR TARGETING EFFICIENCY OF A HUMAN CARCINOEMBRYONIC ANTIGEN-SPECIFIC SCFV AND AFFINITY-MATURED DERIVATIVES

We have examined the biological properties of CEA6, a human carcinoemb ryonic antigen (CEA)-specific single-chain Fv (scFv) isolated by phage display, and five related clones derived by affinity maturation and s elected for improved off-rate (K-off). All clones bind strongly and sp ecifically to CEA-positive human rumours by immunocytochemistry and sh ow negligible cross-reactivity with normal colon. Flow cytometry of sc Fv on human liver cells indicates a shift in fine epitope specificity resulting from mutagenesis. All monomeric scFv have been radioiodinate d, retaining effectively full binding activity. A single intravenous i njection into nude mice bearing human colon tumour xenografts confirms tumour targeting in all cases. As reported in other studies, the kidn ey is the main route of elimination of scFv at early time points. Tumo ur binding of the parental antibody CEA6 consistently gives the highes t rumour-blood ratios at 24 h (mean 16:1), Clone TO6D11, which has a s evenfold reduced K-off, relative to CEA6, showed no difference in tumo ur uptake at 24 h but persisted at the tumour site for longer than CEA 6, This study demonstrates a possible correlation between binding affi nity and tumour residence time when examined in this model.