Antibody-directed enzyme prodrug therapy (ADEPT)

Antibody-directed enzyme prodrug therapy (ADEPT) has been studied in a human ovarian carcinoma xenograft grown subcutaneously in nude mice. Radioimmunoassay of supernatants obtained from tumor homogenates showed these to contain carcinoembryonic antigen (CEA). Biodistribution studies with¹²⁵I-labeled monoclonal anti-CEA antibody, A5B7, and its F(ab′)₂ fragment showed localization in these xenografts. The AB57-F(ab′)₂ fragment conjugated to a bacterial enzyme, carboxypeptidase G2 (CPG2), and, radiolabeled with¹²⁵iodine, also localized in the xenografts. The radiolabeled conjugate cleared from blood faster than the antibody alone. The percentage of injected dose per gram in tumor at 24 h postinjection was about fivefold lower than antibody alone. Tumor-to-blood ratio at 72 h after injection of the radiolabeled conjugate was 7 and the tumor-to-normal tissue ratios at this time point ranged from 20 (liver) to 75 (colon). A three-phase ADEPT antitumor study was carried out in which A5B7-F(ab′)₂-CPG2 was allowed to localize and was followed by accelerated inactivation/clearance of blood CPG2 by a galactosylated anti-CPG2 antibody (SB43gal). A benzoic acid mustard-derived prodrug was injected 24 h after the conjugate, which led to growth delay in this tumor compared to the control untreated group. Further antitumor studies in this model are in progress.