Erythromycin EP Impurity B

Although erythromycin A contains five hydroxyl groups, regioselective methylation at the C-6 hydroxyl group was achieved to the extent of 90% when a 9-O-substituted erythromycin A 9-oxime was employed as substrate. The methylation and its selectivity are dependent on an O-protecting group at the 9-oxime, solvent, base, and methylating reagent. In particular, the use of a polar aprotic solvent is indispensable for the methylation. Among the 9-oxime derivatives, 2'-O,3'-N-bis(benzyloxycarbonyl)-N-demethylerythromycin A 9-[O-(2-chlorobenzyl)oxime] was the most important intermediate for the synthesis of clarithromycin (6-O-methylerythromycin A).

Current compendial methods of assay for the analysis of erythromycin and its related substances involve the use of microbiological techniques. These techniques are non-selective and tedious, thus there is a need for the development of highly specific, quantitative analytical methods. Erythromycin was analysed in a 50 mM phosphate buffer (pH 7.5) and run at an applied voltage of 20 kV. Detection sensitivity was enhanced by using a wavelength of 200 nm and selecting an injection solvent of lower conductivity than the electrolyte: acetonitrile-water (20:80, v/v). In order to facilitate the separation of erythromycin and its related substances, the organic solvent ethanol (35%, v/v) was incorporated into a modified 150 mM phosphate buffer (pH 7.5) and run at an applied voltage of 30 kV. Resolution of all the compounds was achieved in approximately 45 min. The methods described are accurate and precise and thus suitable for the quantitative determination of erythromycin and the related substances, erythromycin C, anhydroerythromycin and N-demethylerythromycin A.

A simple, isocratic liquid chromatographic (LC) method using volatile mobile phase constituents for the identification of related substances in erythromycin samples is described. For method development, evaporative light scattering detection (ELSD) was used. An XTerra RP18 column was used at 70 degrees C with a mobile phase consisting of acetonitrile-isopropanol-0.2M ammonium acetate pH 7.0-water (165:105:50:680). Mass spectral data were acquired on an ion trap mass spectrometer equipped with an electrospray interface operated in the positive ion mode. First, a library was created using MS/MS and MS(n) spectra of reference substances available in the laboratory. Using these reference spectra as interpretative templates, eight novel related substances in erythromycin samples were identified: N-demethylerythromycin E, erythromycin E N-oxide, anhydroerythromycin C, N-demethylerythromycin B, anhydro-N-demethylerythromycin A, pseudoerythromycin E enol ether, EF lacking the neutral sugar and EA lacking the neutral sugar.