Antibody-directed enzyme prodrug therapy (ADEPT).

A conjugate of the bacterial enzyme carboxypeptidase G2 and the F(ab)(2) fragment of the anti-CEA monoclonal antibody A5B7 was directed in vitro at the human colon tumour cell line LS174T and the human non-small cell lung line COR-L23. Indirect immunofluorescence microscopy was used to show that the conjugate bound to LS174T cells but not to COR-L23. The cytotoxicity generated by addition of a phenol mustard prodrug to each cell line after pre-incubation with conjugate was found to be significantly greater for LS174T cells (IC50=0.24 mu M) than COR-L23 cells (IC50=108 mu M). However, for a 1:1 mixture of these cells an IC50, of 3.4 mu M was obtained. These data show that phenol mustard released by a localised conjugate can exert a bystander effect on neighbouring cells to which the conjugate does not bind.