Isolation and structure characterization of related impurities in etimicin sulfate by LC/ESI-MSn and NMR

Etimicin sulfate is a semi-synthetic aminoglycoside which is modified from gentamicin C1a, used as an antibiotic effective both to Gram-positive and Gram-negative bacteria infections. Ten impurities were detected in the bulk drug by LC-ELSD and LC-ESI-MSn. Weakly acidic cation exchange resin, CM-sephadex and silica gel column chromatography were used for the isolation of six impurities. Based on LC/ESI-MSn and NMR analysis, related impurities were characterized as gentamicin C1a (1), 1-N-ethyl garamine (2), 2″-N-ethyl gentamicin C1a, or 6″-N-ethyl gentamicin C1a (3), 1-N-ethyl-3′-N-demethyl gentamicin C2b (4), 3-N-ethyl gentamicin C1a (5), 6″-deamino-6″-hydroxyl etimicin (6), 1,3-N, N-diethyl garamine (7), 1-N-ethyl gentamicin C2b (8), 2″ or 6″-N-ethyl etimicin (9), 1,3-N, N-diethyl gentamicin C1a (10). 10 is a new aminoglycoside, the 1H and 13C NMR data of compounds 5-7, 10 are first reported in the paper. The possible mechanism for the formation of impurities is also discussed.