ELUCIDATION OF HOT MELT WAX COATING SYSTEMS
Hot melt coating methods are an interesting alternative to commonly used aqueous polymeric film coating methods. A critical evaluation of development of hot melt coating methods for sustained drug release and protection of moisture labile active ingredient applications has been performed. A detailed examination of the interaction of water with excipient films of pharmaceutical significance is reported. As water is an important detrimental variable in performance of pharmaceutical dosage forms, the description of a convenient method to characterize the interaction of water with excipient films is very useful. The widespread use of coated pharmaceutical products for a variety of applications accentuates the need to obtain a fundamental understanding of water excipient film systems during development. A simple and rapid method employing the powerful new technique of dynamic vapor sorption was developed to understand the interaction of water with two excipient films (glyceryl behenate/lipidic and a formulation of polyvinyl alcohol/polymeric) of pharmaceutical significance. This method allows estimation of adsorption (k1) and desorption (k2) microrate constants. It was found that k1 varied exponentially with water activity for the polymeric film showing predictive utility of such a relationship. The dramatic influence that film thickness and temperature may have on water sorption of films has been demonstrated. Results from application of theoretical models and thermodynamic analysis were not always consistent. Established models are useful for quantitative analysis, whereas thermodynamic analysis allows qualitative insights into water-excipient film interaction. Young & Nelson model was successful in accurately modeling the hysteresis phenomena but did not offer any information on the mechanistic details. Computation of isoteric heats of adsorption confirmed hydrogen bonding as the reason for hysteresis and allowed determination of number of hydrogen bonds per water molecule. Comparison of weighted relative partial molal free energies of excipient film and water showed that assumption of adsorbent to be an inert participant in the sorption process as being incorrect for both excipients at all temperatures. In conclusion, (a) hot melt coating methods are an interesting alternative to aqueous coating techniques (b) dynamic vapor sorption is very useful in investigation of water-excipient film interactions, and ( c) it is necessary to apply both theoretical models and thermodynamic concepts for complete characterization of water interaction with excipient films.