Material properties evaluated through product rheology are also an important determinant of evaporation dynamics (endothermic evaporative cooling) and the uniform distribution of an active pharmaceutical ingredient within the product, as required for therapeutic equivalence. Accordingly, the rheological properties of these products are regularly assessed in post-manufacturing quality assurance. Rheological tests are now a key component in product-specific guidances and are especially pertinent in characterizing generic drug products as being pharmaceutically equivalent to a Reference Listed Drug (RLD) product because they meet the necessary critical quality attributes. In generic product development, rheology plays a critical role in ensuring that product quality and stability are maintained. Their rheology is best characterized by changes in flow and deformation under varying shear conditions. Furthermore, depending on the excipients and manufacturing processes used, their microstructure, resulting stability, and rheological behavior can vary significantly. Creams, gels, and ointments are the most used forms and have a semisolid consistency, but vary in their microstructure and composition. Topically administered pharmaceutical drug products are available in solution, suspension, lotion, cream, gel, ointment, paste, and powder dosage forms. In conclusion, these analyses not only ensure quality and stability, but also enable the microstructure to be characterized as being flexible (gels) or inelastic (creams). However, the transition of G′ > G″ to G″ > G′ during the continual strain increment was more rapid for the creams, elucidating a relatively brittle deformation, whereas these transitions in gels were more prolonged, consistent with a gradual disentanglement of the polymer network. All dose forms showed viscoelastic solid behavior having a storage modulus (G′) higher than the loss modulus (G″) in the linear viscoelastic region. C3 exhibited a smaller linear viscoelastic region and lower η 0 (2547 Pa∙s) and τ 0 (2 Pa), consistent with lotion-like behavior. All products, other than C3, met the Critical Quality Attribute criteria for high zero-shear viscosity ( η 0) of 2.6 × 10 4 to 1.5 × 10 5 Pa∙s and yield stress ( τ 0) of 55 to 277 Pa. Three creams (C1 and C3 as oil-in-water and C2 as water-in-oil emulsions), and two gels (G1 and G2 carbomer-based) were characterized using the dynamic range of controlled shear in steady-state flow and oscillatory modes. Rheological characteristics and shear response have potential implication in defining the pharmaceutical equivalence, therapeutic equivalence, and perceptive equivalence of commercial topical products.
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