MINIBODY - A NOVEL ENGINEERED ANTICARCINOEMBRYONIC ANTIGEN-ANTIBODY FRAGMENT (SINGLE-CHAIN FV-C(H)3) WHICH EXHIBITS RAPID, HIGH-LEVEL TARGETING OF XENOGRAFTS

A novel engineered antibody fragment (V-L-V-H-C(H)3, or ''minibody'') with bivalent binding to carcinoembryonic antigen (CEA) was produced b y genetic fusion of a T84.66 (anti-CFA) single-chain antibody (scFv) t o the human IgG1 C(H)3 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 V-H and C (H)3. In the T84.66/212 Flex minibody, the human IgG1 hinge plus an ad ditional 10 residues were used as the connecting peptide. Size exclusi on chromatography of purified minibodies demonstrated that both protei ns had assembled into M(r)80,000 dimers as expected. Furthermore, anal ysis by SDS-PAGE under nonreducing conditions was consistent with disu lfide bond formation in the hinge of the T84.66 Flex minibody. Purifie d minibodies retained high affinity for CEA (K-A, 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 c arcinoma xenografts. I-123-labeled T84.66 minibodies demonstrated rapi d, 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 injecti on. Radioiodinated minibody also cleared rapidly from the circulation, yielding high tumor:blood uptake ratios: 44.5 at 24 h for the LD mini body 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.