Identification of the metabolites of roxithromycin in humans

The semisynthetic antibiotic roxithromycin (RXM) exists in an (E)-configuration. Metabolites of RXM in the bile of four cholecystectomy patients with T-tube drainage and in the urine and plasma of four healthy volunteers after single oral doses of 150 mg of RXM were investigated. A total of 15 metabolites were found in bile, urine, and plasma by HPLC with ion trap mass spectrometric and electrochemical detection. These metabolites were identified as descladinose derivative of RXM (M1), erythromycin-oxime (M2), N-, O-, and N,O-di-demethylated derivatives of RXM (M3, M4, and M6), and N-mono- and N-di-demethylated derivatives of erythromycin-oxime (M5 and M7), as well as the (Z)-isomers (M8-M15) of RXM and metabolites M1 to M7, respectively. Structures of six major metabolites (M1-M4, M8, and M10) were established by chromatographic and mass spectrometric determination and comparison with synthesized standards. The stability of RXM and the six synthesized substances was investigated to exclude artifact products. These results, together with previous findings, suggest that biotransformation pathways elucidated for RXM include: 1) isomerization of RXM derivatives, from E-isomer to Z-isomer; 2) O-demethylation; 3) N-demethylation; 4) hydrolysis of the cladinose moiety; and 5) dealkylation of the oxime ether side chain. Secondary metabolism via these pathways was also evidenced. The O-demethylation and isomerization of RXM derivatives represent two novel biotransformation pathways recovered for RXM.