Characterization of the major metabolite of the novel topoisomerase I inhibitor NU/ICRF 505.

NU/ICRF 505 is a tyrosine conjugate of anthraquinone modified at the C terminus of the amino acid as an ethyl ester and it stabilizes topoisomerase I (topo I)-cleavable complexes. It is active in vitro against a panel of human cell lines, including drug-resistant variants, and possesses in vivo antitumour activity. NU/ICRF 505 was rapidly metabolized in nude mice to a product which represented the sole detectable form of the drug present in plasma and a chemosensitive human xenograft (HT-29 colon cancer). The metabolite (codenamed NU/ICRF 505/M) was purified, characterized by mass spectrometry and UV-visible spectroscopy, and shown to be the free amino acid produced by hydrolysis of the ethyl ester bond. NU/ICRF 505/M stabilized topo I-cleavable complexes in assays with human enzyme and was equipotent to the parent drug. Nonetheless, the metabolite was inactive in vitro against a panel of human tumour cell lines (including HT-29) and was not significantly taken up into cells in drug-uptake studies. Levels of the metabolite measured in the HT-29 xenograft after administration of a therapeutic dose of NU/ICRF 505 (25 mg/kg i.p.) remained above 1 microM for 6 h, and exceeded 10 microM at 10 min and 2 h. These data suggest that NU/ICRF 505 is a prodrug in nude mice for its topo-active metabolite NU/ICRF 505/M which accumulates in the tumour.