CHARACTERIZATION OF THE MAJOR METABOLITE OF THE NOVEL TOPOISOMERASE-IINHIBITOR 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 topois omerase I (topo I)-cleavable complexes. It is active in vitro against a panel of human cell lines, including drug-resistant variants, and po ssesses in vivo antitumour activity. NU/ICRF 505 was rapidly metaboliz ed in nude mice to a product which represented the sole detectable for m of the drug present in plasma and a chemosensitive human xenograft ( HT-29 colon cancer). The metabolite (codenamed NU/ICRF 505/M) was puri fied, characterized by mass spectrometry and UV-visible spectroscopy, and shown to be the free amino acid produced by hydrolysis of the ethy l ester bond. NU/ICRF 505/M stabilized topo I-cleavable complexes in a ssays with human enzyme and was equipotent to the parent drug. Nonethe less, the metabolite was inactive in vitro against a panel of human tu mour cell lines (including HT-29) and was not significantly taken up i nto 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 mu M for 6 h, and exceeded 1 0 mu M at 10 min and 2 h. These data suggest that NU/ICRF 505 is a pro drug in nude mice for its topo-active metabolite NU/ICRF 505/M which a ccumulates in the tumour.