Quantification of clarithromycin polymorphs in presence of tablet excipients

Excipients can cause a considerable challenge when developing a solid form of an active pharmaceutical ingredient (API). The aim of this present study was to analyze the polymorphs of clarithromycin (CAM) mixed with excipients using powder X-ray diffraction (PXRD). Polymorphic Form I (CAM-1), Form II (CAM-2) and an amorphous phase of CAM were characterized using thermal and crystallographic methods. CAM-1 and CAM-2 were monotropically related, with CAM-2 being the stable form. PXRD instrument related parameters were optimized for the characterization of CAM polymorphic forms using a variety of excipients. Calibration curves for CAM-1 and CAM-2 mixed with excipients were also prepared. Analytical methods based on the differences in the diffraction patterns of CAM-1, CAM-2 and the excipients were developed. Sodium methyl paraben, sodium propyl paraben, microcrystalline cellulose and magnesium stearate were crystalline showing characteristic diffraction patterns. Starch, croscarmellose sodium, talc and sodium starch glycolate were semicrystalline in nature, while colloidal silicon dioxide was amorphous. A diffraction peak at 8.7° 2θ provided a quantification of CAM-2 when mixed with excipients. The analytical method was evaluated and validated for accuracy, precision, inter- and intra-day variation, variability due to sample repacking and instrument reproducibility. The method for quantification of CAM-2 in the range of 80 to 100% w/w was linear with R2 = 0.998. Relative standard deviation (RSD), due to sample repacking, was 2.77% indicating good homogeneity of mixing of the samples. RSD due to assay errors was 1.66%. PXRD analysis of the commercial tablet showed the CAM-2 as a major polymorph being 98% of the overall content of the API. CAM-1 was found to be present as an impurity at trace levels shown by peaks at 2θ values of 5.2° and 6.7°. This method provides a method for characterization of the polymorphic forms of CAM in the presence of commonly used excipients. It could be a useful tool for monitoring solid form behavior during product development and stability studies.