Pharmaceutical formulations can be held for long periods in warehouses before they are sold or prescribed, and they can be stored for weeks and months in people's homes once they have been opened. For some active ingredients, long-term storage is fine but others may be susceptible to degradation either by heat, or, once opened, by light. The threat of microorganisms is also ever present and this is often tackled by adding antifungal preservatives to the formulation.

The additives and the drug will have been tested for safety during the clinical approval process but the toxicity of a particular preparation might also depend on any impurities present. Ideally, the composition of a formulation could be examined in one step that would identify and measure the drug, any impurities and the preservatives, rather than requiring different procedures for each component.

Scientists in Brazil have designed such a method for commercial formulations of econazole nitrate, known chemically as 1-{[2-(4-chlorophenyl)methoxy]-2-(2,4-dichlorophenyl)ethyl}-1H-imidazole moninitrate. It is a broad spectrum antifungal agent that is widely used for the topical treatment of skin infections. The preparation was marketed as a cream and contained econazole nitrate and the common preservatives methylparaben and ethylparaben. It might also contain degradation products of the active ingredient and two are known: 4-chlorobenzyl alcohol and alpha-(2,4-dichlorophenyl)-1H-imidazole-ethanol.

Angel Arturo Gaona-Galdos and colleagues from the Department of Pharmacy at the University of Sao Paulo used HPLC with a diode array detector. The conditions were optimised using standard solutions of the five compounds (drug, degradants, preservatives) plus miconazole nitrate as an internal standard. The solutions were separated on a reversed-phase C₁₈ column and, after much adjustment, the mobile phase chosen was a gradient of methanol in water.

The compounds were all clearly separated with good peak shapes within 14 minutes. The detection limits for econazole nitrate, methylparaben and propylparaben were 4.71, 2.35, and 0.57 µg/mL, respectively and those of the two degradation products were each 0.35 µg/mL. The calibration curves showed good linearity and the recoveries from spiked solutions were 97.9-102.3% with intra-day and inter-day precisions of less than 2.2%.

The team also varied the column temperature from 25 °C and altered the detection wavelength but this only affected measured values by about 1.8%, so the method was deemed to be robust.

The method was applied to the commercial preparation containing 1% (by weight) of econazole nitrate. The cream was extracted with aqueous methanol, spiked with the internal standard and filtered before analysis.

Three chromatograms for a placebo containing only methylparaben and propylparaben, a standard solution, and the cream extract were superimposed. This comparison revealed that no other compounds from the cream interfered in the analysis. The commercial cream was found to contain both parabens but no econazole nitrate decomposition products were detected.

The new procedure will be useful for monitoring the composition of econazole nitrate formulations and studying their stability under different storage conditions. It permits the detection of preservatives and degradation products in one run with a rapid analysis time and good accuracy and precision.

Related links:

• Talanta 2008, 77, 673-678: "Simultaneous determination of econazole nitrate, main impurities and preservatives in cream formulation by high performance liquid chromatography"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.