University of Groningen
Salivary lubrication and xerostomia
Vinke, Jeroen
DOI:
10.33612/diss.133408765
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Publication date: 2020
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Citation for published version (APA):
Vinke, J. (2020). Salivary lubrication and xerostomia. University of Groningen. https://doi.org/10.33612/diss.133408765
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Advances in medical research have resulted in successful treatment and/or management of many diseases. Either the disease itself or its treatment, can be accompanied by unfavourable conditions that might have an impact on quality of life. A reduced salivary flow (hyposalivation) and a sensation of oral dryness (xerostomia) are amongst these unfavourable conditions. Saliva substitutes are used to ease the oral dryness related symptoms, but unfortunately the current saliva substitutes do not possess the properties that are needed to effectively relief oral dryness (Chapter 1). There is a need for an ex vivo
model to test the lubricating properties of human saliva as well as to objectively test the potential lubricating efficacy of existing and new saliva substitutes. Therefore, the aim of the study described in this thesis was to develop and test a new method for quantifying oral lubrication on biological relevant tissues ex vivo.
The development of this ex vivo friction assay simulating dry mouth conditions is described in Chapter 2. The ex vivo friction system facilitates objective comparison of saliva substitutes. A reciprocating sliding
tongue-enamel system was developed and compared with a PDMS (polydimethylsiloxane)-PDMS friction system. The tongue-enamel system, but not the PDMS-PDMS model, showed that high containing saliva (unstimulated and submandibular/sublingual saliva) gave a higher Relief than mucin-poor lubricants (water, parotid saliva, Dentaid Xeros). The measured Relief correlated well (r = 0.97) with in vivo mouth feel. It was concluded that the developed tongue-enamel friction system mimicked dry
mouth conditions and relief. The system was considered to be well suited to test agents meant to lubricate desiccated oral surfaces, as well as saliva from different sources.
Sjögren’s patients and patients that have been treated with radiotherapy in the head and neck region against tumours, develop xerostomia in a different way possibly due to different lubricating abilities of patients’ saliva. Therefore, in Chapter 3, lubricating properties (Relief and Relief period) of chewing
stimulated whole saliva from healthy controls (n=22) were compared with those of saliva from patients suffering from primary Sjögren’s syndrome (n=37) and patients undergoing head and neck radiotherapy (n=34). In the latter patients, saliva samples were collected prior to and six months after radiotherapy. All participants completed the xerostomia inventory questionnaire to score dry mouth sensation. Lubrication was measured using an ex vivo tongue-enamel friction system. MUC5b levels of the saliva
samples were measured by an enzyme-linked immunosorbent assay. Total protein concentration in saliva was measured using a bicinchoninic acid assay. Twenty-four head and neck radiotherapy patients returned for sampling six months after radiotherapy. Dropouts were due to being deceased (n=4), too limited saliva secretion (n=2), tumour recurrence (n=2), or other reasons (n=2). Stimulated salivary flow rate did not correlate with dry mouth sensation. Relief of either Sjögren’s patients’ saliva and post-irradiation patients’ saliva was similar compared with healthy controls, but saliva from post-irradiation patients lubricated significantly better than saliva from Sjögren’s patients. The Relief period was similar between the three groups. The Relief and Relief period were higher for saliva samples post-irradiation compared with pre-irradiation. MUC5b and total protein concentrations were comparable in all groups. MUC5b and total protein output, calculated by multiplying concentration with flow rate, were significantly lower in patient subjected to radiotherapy compared to saliva from healthy controls and patients pre-irradiation. MUC5b concentrations positively correlated with lubricating properties of post-irradiation patients’ saliva. The lubricating properties of patient saliva was not any worse than healthy controls. A lower flow rate leads to lower availability of saliva in the oral cavity and decreases the overall output of protein and MUC5b, which might result in an insufficient replenishing of the mucosal salivary film. An insufficient replenishing might underlie the sensation of a dry mouth and loss of oral function.
Dry mouth patients depend on the use of saliva substitutes to relieve dry mouth. The efficacy of saliva substitutes, however, were never quantitatively tested in an ex vivo dry mouth simulating system.
Therefore, in the study described in Chapter 4, different saliva substitutes were assessed for their efficacy
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to lubricate the oral cavity. In addition, the measured oral lubrication was related to the ability of saliva substitutes to adsorb on and change the structure of an initially existing salivary conditioning film (SCF). Quartz crystal microbalance with dissipation was used to study the capability of saliva substitutes to interact with natural SCF and the ability to change the secondary SCF (S-SCF). The tongue-enamel friction system mimicking xerostomic conditions was used to assess the Relief and Relief Period expected from these substitutes under set circumstances. Saliva Orthana spray, Biotène spray and Gum Hydral gel had an immediate effect on a SCF, increasing its structural softness. BioXtra gel, Biotène gel, Gum Hydral gel and Glandosane spray changed the S-SCF by increasing salivary protein adsorption, while others showed no sign of interaction. With respect to Relief only one out of the sixteen saliva substitutes tested (Gum Hydral gel) performed better than water. Overall, Relief period correlated well to structural softness change, whereas a correlation was seen between Relief and mass adsorption. It was concluded that the majority of saliva substitutes did not adsorb on the SCF, and thus did not enhance lubrication. Only saliva substitutes containing carrageenan, carboxymethylcellulose, pig gastric mucin, xanthan gum and carbomer performed better in enhancing oral lubrication than water. In other words, objective assessment of the lubricating potential of saliva substitutes can help clinicians and patients to make a better choice of which saliva substitute might be effective for that patient. The knowledge provided in this study can also help the industry to develop saliva substitutes with a greater efficacy.
As shown in Chapter 4, most saliva substitutes lack efficacy which might be due to the presence of exogenous molecules with limited lubrication properties. In Chapter 5, we investigated the influence of
recombinant supercharged polypeptides (SUPs) on lubrication properties when added to saliva. SUPs are presumed to have the potential to improve salivary lubrication by enhancing functionality of endogenously available salivary proteins, which is in sharp contrast to administration of exogenous lubrication enhancers. This novel approach is based on establishing a layered architecture enabled by electrostatic bond formation to stabilize and produce robust SCFs in vitro. We first determined the optimal
molecular weight of SUPs to achieve the best lubrication performance employing biophysical and in vitro
friction measurements. Next, in the developed ex vivo tongue-enamel friction system (Chapter 2),
stimulated whole saliva from patients with Sjögren’s syndrome was tested to transfer this strategy to a pre-clinical situation. Out of a library of genetically engineered cationic polypeptides, the variant SUP K108cys that contains 108 positive charges and two cysteine residues at each terminus was identified as the best SUP to restore oral lubrication. Employing this SUP, the Relief period for SCFs from healthy and patient saliva was significantly extended. For patient saliva, the Relief period was increased from 3.8 min to 21 min with SUP K108cys treatment. For healthy saliva, the Relief period even increased from 8 min to 40 min. Investigation of the tribochemical mechanism revealed that the observed lubrication enhancement was due to electrostatic stabilization of SCFs and mucin recruitment. Mucin recruitment is accompanied by strong water fixation and reduced water evaporation.
In Chapter 6, the studies performed in the previous chapters were placed in a broader perspective. The
characteristics of the developed tongue-enamel friction system were discussed as well as that the results regarding the lubrication properties of saliva and saliva substitutes, and strategies to improve those properties were further elucidated. Furthermore, future perspectives for the use of the tongue-enamel friction system were discussed. One of these applications might be the use of the developed ex vivo system
as a platform to test saliva substitutes prior to clinical tests. This way, one might have a better idea of the lubricating potential of a new saliva substitute before it is tested in the clinic.
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