Characterization and analysis of polymorphs by near-infrared spectrometry

The polymorphic purity of drug is of high pharmaceutical interest as it often dictates its bioavailability. In this work, we developed a rapid, efficient method for the characterization and determination of azithromycin polymorphs using near-infrared (NIR) spectrometry. The drug is characterized by comparison with a NIR spectral library that permits one to determine whether the amount of crystalline form contained in an amorphous azithromycin sample exceeds allowed levels. While the crystalline form is a hydrate, the amorphous form is anhydrous; however, the absorption of a small amount of moisture by the drug reduces the spectral differences between the two forms and hinders the establishment of an accurate calibration model. In this work, we determined the crystalline form by using a partial least-squares regression model (PLS1) for calibration and examined the influence of factors such as spectral treatment, wavelength range and moisture content on the results. The high correlation between the spectra for the two forms enabled the development of a PLS2 model for determining both species jointly. The proposed method was validated with a view to its subsequent use in the analytical control of azithromycin. © 2003 Elsevier B.V. All rights reserved.