Metabolism and biliary excretion of the novel anticancer agent 10-hydroxycamptothecin in the isolated perfused rat liver

10-Hydroxycamptothecin (HCPT), a natural analog of the alkaloid camptothecin (CPT), is a promising anticancer agent currently undergoing preclinical trials. Though HCPT is less toxic and more active in various human cancer cell lines and in animal tumor models than the clinically approved CPT-analog topotecan, little is known about its biotransformation products and their route of elimination. To investigate the metabolism and biliary excretion, livers of male Wistar rats were perfused with HCPT (5 μM). Bile and perfusate samples were collected for 60 min and quantified by reversed-phase high-performance liquid chromatography (HPLC). Besides HCPT, three metabolites, namely HCPT glucuronide (M1), hydroxyHCPT glucuronide (M2), and hydroxyHCPT (M3) could be identified by enzymatic hydrolysis with β-glucuronidase and mass spectroscopy. Biliary secretion of HCPT and M1-M3 reached a peak secretion of 1532±124, 75±16, 5.8±1.6 and 2.1±0.5 pmoles/g liver.min, respectively, after 25 min. The total amount of HCPT and M1-M3 excreted into bile during the time of perfusion (60 min) was low and represented a mean of 9.9±3.2%, 0.44±0.17%, 0.041±0.010%, and 0.022±0.004% of the initial HCPT dose, respectively. In the perfusate, besides HCPT M1 and M2 but not M3 could be detected (maximum concentrations after about 20 min: 3248±210, 16.8±2.8 and 1.0±0.4 pmoles/g liver.min, respectively). The cumulative efflux of HCPT and M1 and M2 into the perfusate was 21.1±3.9%, 0.145±0.036% and 0.018±0.004% of the initial dose, respectively, indicating a preferable non-biliary secretion for HCPT and a predominant biliary elimination for conjugated HCPT biotransformation products. In conclusion, HCPT is biotransformed in a rat liver model to three metabolites, mainly excreted into bile, which may be of clinical relevance during cancer therapy.