CHIMERIC (MOUSE-HUMAN) ANTI-COLON CANCER ANTIBODY C30.6 INHIBITS THE GROWTH OF HUMAN COLORECTAL-CANCER XENOGRAFTS IN SCID SCID MICE/

The mouse monoclonal antibody, m30.6 (IgG2b), detects an antigenic det erminant expressed predominantly on the surface of colorectal adenocar cinoma cells and has been shown previously to be a potentially useful therapeutic and diagnostic reagent for human colon cancer. We report t he production and characterization of a mouse/human chimeric antibody, c30.6, with potent iii vitro and in vivo antitumor activity. The gene s encoding the variable domains for heavy and light chains mere amplif ied by thermal cycling using degenerate oligonucleotide primers comple mentary to conserved immunoglobulin framework sequences. The gene segm ents were sequenced, subcloned into eukaryotic expression vectors cont aining human constant region genes (IgG1 and kappa), and cotransfected into nonsecreting Sp2/0 mouse myeloma cells. There were significant d ifferences in the biological activities of the murine and chimeric ant ibodies. The i.p. administration of c30.6 but not of m30.6 produced a marked growth inhibition of s.c. 30.6(+) COLO 205 tumors in scid/scid mice (similar to 40% reduction in tumor size, measured 21 days after t umor inoculation). Reduced tumor growth was not due to altered binding characteristics of c30.6 because: (a) the chimeric antibody was shown by flow cytometry to bind exclusively to cell lines that expressed th e 30.6 determinant; (b) c30.6 was able to completely inhibit the bindi ng of m30.6 on 30.6(+) cells; and (c) the affinity of binding of the t wo antibodies was the same (K-a, similar to 1.50 x 10(8)). Up to 15% o f the total injected antibody dose/g tissue was localized in 30.6(+) t umors at 24 h, similar to 13% was present in the tumors at 48 h, and s imilar to 10% was present at 72 h. Furthermore, c30.6 demonstrated a s horter circulating half-life (53 h; m30.6, 72 h) when given i.p. to C5 7BL6 x BALB/c F-1 mice. Unlike m30.6, c30.6 was also strongly active i n antibody-dependent cell-mediated cytotoxicity against a range of 30. 6(+) tumor target cells in vitro. Up to 80% specific Cr-51 release was achieved using either freshly isolated human peripheral blood mononuc lear cells or 2-day-old interleukin 2-stimulated human peripheral bloo d mononuclear cells as effecters. The enhanced antitumor activity of c 30.6 suggests that it might be a useful immunotherapeutic reagent for colorectal carcinoma.