Determination of the relative amounts of three crystal forms of a benzimidazole drug in complex finished formulations by FT-Raman spectroscopy

A 5% (m/m) premix for animal use was quantitatively characterized for the polymorph composition of its benzimidazole drug substance. Raman spectra of reference samples (pure polymorphs A, B and C in lactose at a concentration of 5%, m/m) were compared with the spectra of benzimidazole samples with a known polymorph composition and with the spectra of uncharacterized premixes. The raw intensities of 78 selected wavenumbers were vector-normalized and application of stepwise linear regression models estimated the relative quantities of the benzimidazole-drug polymorphs A, B and C in the different samples. Modelling results of the samples with known polymorph composition were in compliance with the expected concentrations, validating the proposed methodology. The benzimidazole drug substance in the premixes was predominantly polymorph B. Although statistically not significant, some traces of polymorph A could not be ruled out. Similar analyses were performed to evaluate the solid-state stability of the benzimidazole drug substance in another drug formulation, i.e. a suspension-emulsion. Suspension-emulsions originally determined as containing polymorph B benzimidazole drug substance were stored for 12 months at 25 °C/60%RH. FT-Raman spectroscopy revealed that no polymorph transformations occurred during this storage.