Tissue-on-tissue lubricity studies of saliva substitutes as-supplied, mixed with and compared with human saliva from control and xerostomia patients
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Relief is sought from "dry mouth" symptoms in increasingly older populations experiencing side effects from xerostomic drugs, multi-drug interactions, and cancer therapies. This investigation applied a new tissue-on-tissue lubricity test procedure, which has shown clinical correlations leading to successful formulations for relieving "dry eye" symptoms, to commercial saliva substitutes as-formulated and after admixture with pre-applied aliquots of unstimulated saliva from ten normal control vs ten xerostomia patients in a protocol approved by a Human Subjects Institutional Review Board. From 31 identified commercial products, 19 were available for laboratory testing of their intrinsic capabilities to reduce Coefficients of Friction (CoF) of saline-wetted articulating pericardium couples, sustain those CoF reductions over time, and demonstrate substantivity of the friction-reduction effects after further dilution by physiologic buffer, as related to their pH values, surface tensions, and functional ingredients characterized by Multiple Attenuated Internal Reflection InfraRed (MAIR-IR) spectroscopy. The most effective constituents were rinse-resistant natural polysaccharides such as linseed extracts and xanthan gum, at neutral pH, while formulations at both low and high pH or based on synthetic carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, as well as natural esters and glycerin, were generally not lubricious or were less substantive (except for undisclosed silicone components in some products). Intrinsic lubricities of saliva from both patient groups were excellent, reducing CoF values from above 0.4 to about 0.1 in all cases, and saliva aliquots sustained these CoF reductions after additions of 4 different saliva substitutes selected to examine their admixture effects on continuing lubricity and substantivity. It was demonstrated that small amounts of natural saliva can convey their superior lubricity to saliva substitutes not very lubricious alone. Based on these results and comparative MAIR-IR spectra showing similar carbohydrate-to-protein ratios, in spite of lower pH values for xerostomia saliva specimens, xerostomic saliva was proven to be deficient mainly in amount secreted, and not in composition or function as a superior tissue lubricant. Unstimulated saliva from daily samples of a pre-menopausal female control subject did show pH and MAIR-IR spectral variations suggesting a possible confounding effect of cyclic hormonal influences on these lubricious secretions. Studies with experimental formulations showed results superior to those of natural saliva and all saliva substitutes tested, providing guidance for further efforts aimed at producing xerostomia-relief formulations of sufficient potency and substantivity to be safely and effectively delivered by intra-oral prosthetic devices.