ELUCIDATION OF THE MECHANISM ENABLING TUMOR SELECTIVE PRODRUG MONOTHERAPY

Elucidation of the mechanism enabling tumor selective PMT in vivo with appropriate glucuronyl-spacer-doxorubicin prodrugs, such as HMR 1826, is important for the design of clinical studies, as well as for the d evelopment of more selective drugs. Enzyme histochemistry, immunohisto chemistry, and the terminal deoxytransferase technique were applied us ing human cryopreserved cancer tissues, normal human, monkey, and mous e tissues, and human tumor xenografts to examine mechanisms underlying the selectivity of successful PMT with HMR 1826. It could unambiguous ly be shown by enzyme histochemistry that necrotic areas in human canc ers are the sites in which lysosomal beta-glucuronidase is liberated e xtracellularly in high local concentrations. The cells responsible for the liberation of the enzyme are mainly acute and chronic inflammator y cells, as shown by IHC. Furthermore, it could be demonstrated that b eta-glucuronidase liberated in necrotic areas of tumors can activate H MR 1826, resulting in increased doxorubicin deposition in human tumor xenografts or in human lung cancers subjected to extracorporal perfusi on, compared to chemotherapy with doxorubicin, Additionally, the doxor ubicin load to normal tissues was significantly reduced compared to ch emotherapy with doxorubicin. Surprisingly, the increased doxorubicin d eposition in tumors also resulted in strong antitumor effects also in cancers resistant to maximum tolerated doses of systemic doxorubicin. Finally, toxicity studies in mice and monkeys revealed an excellent to lerability of HMR 1876, up to a dose of 3 g/m(2) (monkeys), These data suggest that HMR 1826 is a promising candidate for clinical developme nt.