TUMOR-SELECTIVE PRODRUG ACTIVATION BY FUSION PROTEIN-MEDIATED CATALYSIS

A two component system, consisting of a fusion protein and an appropri ate prodrug, suited to perform selective tumor therapy in vivo is pres ented. The fusion protein, due to its humanized carcinoembryonic antig en-specific variable region, specifically binds to carcinoembryonic an tigen-expressing tumors and has an enzymatic activity comparable to th at of human beta-glucuronidase. The prodrug is a nontoxic glucuronide- spacer derivative of doxorubicin decomposing to doxorubicin by enzymat ic deglucuronidation. In vivo studies in nude mice bearing human carci noembryonic antigen-expressing tumor xenografts revealed that 7 days a fter injection of 20 mg/kg fusion protein a high specificity ratio (>1 00:1) was obtained between tumor and plasma or tumor and normal tissue s. Injection of 250 mg/kg of prodrug at day 7 resulted in tumor therap eutic effects superior to those of conventional chemotherapy without a ny detectable toxicity. These superior therapeutic effects which were observed using established human tumor xenografts can be explained by the approximately 4-12-fold higher doxorubicin concentrations found in tumors of mice treated with fusion protein and prodrug than in those treated with the maximal tolerable dose of drug alone. The nondetectab le toxicity in the animals treated with fusion protein and prodrug is probably caused by up to 5-fold lower drug concentrations in normal ti ssues compared to the animals treated with doxorubicin. Thus, a more t umor-selective therapy, resulting in stronger therapeutic effects and reduced toxicity seems to be possible by the appropriate use of the hu manized nontoxic fusion protein and the nontoxic prodrug.