Near Infrared Spectroscopy of Magnesium Stearate Hydrates and Multivariate Calibration of Pseudopolymorph Composition

Elucidating the Variability of Magnesium Stearate and the Correlations With Its Spectroscopic Features

The objective of this study is to investigate the variability in physiochemical and spectral properties of commercially available vegetable-grade magnesium stearate (MgSt) samples and to assess the correlation between physiochemical properties and near-infrared and Raman spectroscopic features to determine if fast spectral measurements could be used for physical and chemical evaluation. Thirteen MgSt samples of 9 manufacturer grades were obtained from 3 suppliers. The chemical composition was examined using gas chromatography and loss on drying. The physical characteristics were examined on 3 levels: solid state, particle, and bulk level. Comparing the largest to the smallest test values of 13 samples, the variation of the properties ranged from 7% to 335%, with majority of them varying by more than 100% of the smallest value. The samples could be categorized into 4 groups based on solid state properties (1) monohydrate, (2) dihydrate, (3) mixture of monohydrate and dihydrate, and (4) anhydrous form. Scanning electron microscopy images revealed 2 morphological types: thin, flat, and plate-like crystal habit versus irregular crystal habit. The overall variability was mapped using Principal Component Analysis. The greatest variation was due to different manufacturers and perhaps manufacturing methods and starting materials. Based on correlations to physiochemical properties of MgSt, near-infrared and Raman spectra showed potential as a rapid technique for evaluating the differences in excipient properties.

Effect of Magnesium Stearate Mono- and Dihydrate Dispersibilities on Physical Properties of Tablets

Magnesium stearate (MgSt), an essential lubricant in the manufacturing of tablets, is available in several hydrate forms with different qualities that affect the physical properties of tablets. This study examined MgSt mono- and dihydrates, and their effects on tablet dissolution, disintegration, and hardness. These effects were examined in terms of surface free energy and dispersibility. Dissolution, disintegration, and hardness were evaluated for tablets manufactured from powder mixtures of each MgSt hydrate form and other components, including ethenzamide as an active ingredient, using different mixing times. The surface energy was evaluated for MgSt mono- or dihydrate powder mixtures with a surface tensiometer. For dispersibility, the adhesion states of MgSt hydrates to other components were visually observed via near-infrared (NIR) chemical imaging. The dispersion behavior of MgSt hydrates was examined by quantitative evaluation of skewness and kurtosis of histograms, based on NIR images, and domain size estimated from their binary images. It was found that changes in those parameters related to dispersibility and dissolution differed between MgSt hydrates. This suggests that the quantitative determination of dispersibility of MgSt using NIR chemical imaging is a useful methodology for improving the understanding of tablet manufacturing blending processes.

Application of Multivariate Strategies to the Classification of Pharmaceutical Excipient Manufacturers Based on Near-Infrared (NIR) Spectra

Using partial least square discriminate analysis (PLSDA), we studied the spectroscopic differences between the commonly used filler-binder microcrystalline cellulose (MCC) from five manufactures. These samples had subtle differences in the chemical and physical properties, which are often the cause of differences in excipient performance. Studying these differences allowed us to build and validate a model to classify five manufacturers of MCC using near-infrared (NIR) spectra. The sample training set includes 39 MCC samples collected from five manufactures with regions spanning the United States of America, Japan, Taiwan, Germany, and Brazil. The samples from individual manufacturers include diverse grades that differ in moisture content, particle size, and bulk density. Optimized pretreatment methods were identified as standard normal variate normalization, followed by Savitzky-Golay second derivative, mean centering, and orthogonal signal correction. The model was optimized with cross-validation and validated with an independent sample set comprising nine samples collected from those five manufacturers. The results showed that none of the samples in the independent validation set was misclassified. The score and loading plots revealed that the differences in content of oxidized cellulose group, water content and states, hydrogen bonding, and degree of polymerization of the MCC samples are responsible for the class differentiation. Permutation test demonstrated that the outcome of the PLSDA model was significantly different from that of the randomly generated model. The advantages and limitations of the method in this type of application were discussed.

Raman chemical mapping of magnesium stearate delivered by a punch-face lubrication system on the surface of placebo and active tablets

Raman chemical mapping was used to determine the distribution of magnesium stearate, a lubricant, on the surface of tablets. The lubrication was carried out via a punch-face lubrication system with different spraying rates applied on placebo and active-containing tablets. Principal component analysis was used for decomposing the matrix of Raman mapping spectra. Some of the loadings associated with minuscule variation in the data significantly overlap with the Raman spectrum of magnesium stearate in placebo tablets and allow for imaging the domains of magnesium stearate via corresponding scores. Despite the negligible variation accounted for by respective principal components, the score images seem reliable as demonstrated through thresholding the one-dimensional representation and the spectra of the hot pixels that show a weak but perceivable magnesium stearate band at 1295 cm(-1). The same approach was applied on the active formulation, but no magnesium stearate was identified, presumably due to overwhelming concentration and spectral contribution of the active pharmaceutical ingredient.

Quantitative analysis of pseudopolymorphic transformation of imidafenacin by application of a novel combination of near-infrared spectroscopy and a humidity-controlled 96-well plate

OBJECTIVES

This study was conducted to test whether our novel combination (i.e. microanalysis) of near-infrared (NIR) spectroscopy and humidity-controlled 96-well plate, reported in a previous issue of this journal, can be successfully applied to quantitative evaluation of the pseudopolymorphic transformation of imidafenacin.

METHODS

Sample powders of a drug compound were placed in a humidity-controlled 96-well plate containing various saturated salt solutions, and stored at 35°C. NIR spectra were collected using a Fourier transform-NIR spectrometer in combination with a fiber-optic probe. The actual hydrate contents of samples were determined by using thermal gravimetric analysis (TGA). A series of sets of NIR spectra and TGA data were used to establish a calibration model with which to predict the contents of monohydrate by partial least-squares regression (PLS).

KEY FINDINGS

The PLS calibration model analysis showed that the plots of NIR predicted values to the actual values gave a straight line with correlation coefficients of 0.9868 and 0.9936, respectively, for the 96-well plate and glass-bottle uses. The model using the 96-well plate was therefore able to predict the transformation of imidafenacin in one form to another form as quantitatively as the conventional model using glass bottles.

CONCLUSION

The present study confirmed that our microanalysis quantitatively predicts the pseudopolymorphic transformation of our tested drug in small amounts and suggests that this particular method is a simple and convenient one that may be efficiently applied to polymorphic and pseudopolymorphic studies of a hygroscopic drug candidate, at even earlier development stages than the conventional method using glass bottles.