Minibody: A novel engineered anti-carcinoembryonic antigen antibody fragment (single-chain Fv-CH3) which exhibits rapid, high-level targeting of xenografts.

A novel engineered antibody fragment (VL-VH-CH3, or "minibody") with bivalent binding to carcinoembryonic antigen (CEA) was produced by genetic fusion of a T84.66 (anti-CEA) single-chain antibody (scFv) to the human IgG1 CH3 domain. Two designs for the connecting peptide were evaluated. In the T84.66/212 LD minibody, a two-amino acid linker (generated by fusion of restriction sites) was used to join VH and CH3. In the T84.66/212 Flex minibody, the human IgG1 hinge plus an additional 10 residues were used as the connecting peptide. Size exclusion chromatography of purified minibodies demonstrated that both proteins had assembled into Mr80,000 dimers as expected. Furthermore, analysis by SDS-PAGE under nonreducing conditions was consistent with disulfide bond formation in the hinge of the T84.66 Flex minibody. Purified minibodies retained high affinity for CEA (KA, 2 x 10(9) M(-1)) and demonstrated bivalent binding to antigen. Tumor targeting properties were evaluated in vivo using athymic mice bearing LS174T human colon carcinoma xenografts. 123I-labeled T84.66 minibodies demonstrated rapid, high tumor uptake, reaching 17% injected dose/gram (%ID/g) for the LD minibody and 33%ID/g for the Flex minibody at 6 h following injection. Radioiodinated minibody also cleared rapidly from the circulation, yielding high tumor:blood uptake ratios: 44.5 at 24 h for the LD minibody and 64.9 at 48 h for the Flex minibody. Rapid localization by the T84.66/212 Flex minibody allowed imaging of xenografts at 4 and 19 h after administration.