NEOMYCIN - MICROBIOLOGICAL ASSAY OR LIQUID-CHROMATOGRAPHY

In a multicentre study involving six laboratories, a microbiological a ssay was performed on three neomycin samples containing respectively, 0.12, 2.1 and 11% (m/m) of neomycin C, as well on a pure neomycin C sa mple. The potency was determined according to the European Pharmacopoe ia method but using a neomycin B base standard. The relative standard deviations between laboratories (RSD) on the potencies varied from 4.8 to 50%, depending on the sample examined. The RSD increased with the neomycin C content of the samples and the highest RSD values were obse rved for the pure neomycin C sample. The activity of neomycin C relati ve to neomycin B was found to be 62% by diffusion (RSD: 41%) and 56% b y turbidimetry (RSD: 50%). This confirmed that the presence of neomyci n C in a neomycin sample influences the reproducibility of the microbi ological assay. To estimate the influence of this effect on official s tandards, their composition was verified by liquid chromatography. The neomycin C base content of the standards varied between 0.4 and 5.8% (m/m). Based on the results obtained and on formerly published reports discussing problems encountered with microbiological assay of neomyci n, it is proposed to introduce liquid chromatography in official monog raphs to replace microbiological assay. (C) 1998 Elsevier Science B.V. All rights reserved.