A novel doxorubicin-glucuronide prodrug DOX-GA3 for tumour-selective chemotherapy: Distribution and efficacy in experimental human ovarian cancer

The doxorubicin (DOX) prodrug N-[4-doxorubicin-N-carbonyl (oxymethyl) phenyl] O-β-glucuronyl carbamate (DOX-GA3) was synthesised for specific activation by human β-glucuronidase, which is released in necrotic areas of tumour lesions. This novel prodrug was completety activated to the parent drug by human β-glucuronidase with V_max = 25.0 μmol min^-1mg^-1 and K_m = 1100 μM. The pharmacokinetics and distribution of DOX-GA3 in nude mice bearing human ovarian cancer xenografts (OVCAR-3) were determined and compared with DOX. Administration of DOX at 8 mg kg^-1 i.v. (maximum tolerated dose, MTD) to OVCAR-3-bearing mice resulted in a peak plasma concentration of the drug of 16.4 μM (t = 1 min). A 7.6-times lower peak plasma concentration of DOX was measured after injection of DOX-GA3 at 250 mg kg^-1 i.v. (50% of MTD). In normal tissues the prodrug showed peak DOX concentrations that were up to 5-fold (heart) lower than those found after DOX administration. DOX-GA3 activation by β-glucuronidase in the tumour yielded an almost 5-fold higher DOX peak concentration of 9.57 nmol g^-1 (P <0.05) than the peak concentration of only 2.14 nmol g^-1 observed after DOX. As a consequence, the area under the curve of DOX calculated in tumour tissue after DOX-GA3 (13.1 μmol min^-1 g^-1) was 10-fold higher than after DOX (1.31 μmol min^-1 g^-1). The anti-tumour effects of DOX-GA3 and DOX were compared at equitoxic doses in OVCAR-3 xenografts at a mean tumour size of 125 mm³. The prodrug given i.v. at 500 mg kg^-1 weekly × 2 resulted in a maximum tumour growth inhibition of 87%, while the standard treatment with DOX at a dose of 8 mg kg^-1 i.v. weekly × 2 resulted in a maximum tumour growth inhibition of only 56%. Treatment with DOX-GA3 was also given to mice with larger tumours containing more necrosis. For tumours with a mean size of 400 mm³ the specific growth delay by DOX-GA3 increased from 2.7 to 3.9. Our data indicate that DOX-GA3 is more effective than DOX and suggest that the prodrug will be specifically advantageous for treatment of advanced disease.