Eyeworld

EW CORNEA 76 December 2017 "The advantage of using the mucins as a coating," Dr. Lieleg said, "is that this coating procedure is simple (the contact lens only needs to be immersed in a mucin solution for awhile) and can be conducted before the contact lens is placed onto the cornea." Winkeljann et al. wrote that contact lens storage solutions could be enriched with the gastric mucin "where they will spontaneously form a protective coating layer on the contact lens material by passive adsorption." The study authors also said the mucin could be incorpo- rated into the actual material of hydrogel contact lenses. Dr. Lieleg said neither the PBS nor the solution containing hyal- uronic acid spontaneously coated the contact lenses during a soak- ing period. Thus, he explained, hyaluronic acid would have to ated the coefficients of friction—the friction between two objects when pushed together, in this case a por- cine cornea and contact lens—with phosphate buffered saline (PBS), PBS with purified gastric mucin, and arti- ficial tears with hyaluronic acid. All three resulted in equally low friction coefficients, according to the study authors. From the standpoint of surface damage to the corneas, however, the researchers used a specialized variety of light microscopy to compare surface topology of porcine cornea samples before and after sliding experiments with the contact lenses. They found that when PBS alone was used, there were "strong alterations in the surface structure of the cornea samples," Dr. Lieleg said. When mucins were added, however, either as a coating or as an eye drop, there was no damage. MUC5AC is a secretory mucin protein produced by goblet cells in the conjunctiva as part of the precorneal tear film. 2 Patients with dry eye often have reduced gob- let cell density, which leads to a reduction in this lubricating mucin. 3 MUC5AC is also a major component of stomach mucus, according to the research by Dr. Lieleg and other in- vestigators of the Winkeljann et al. study. The researchers noted in their paper that stomach mucus occurs in higher quantities than secretory mucus in tears, making it a more viable source for commercial purpos- es. In other areas of medicine, it has already been researched for anti-viral properties and in oral sprays. 4,5 The researchers studied "the friction behavior of corneal tissue probed with contact lenses" before and after the use of purified gastric mucins. The investigators first evalu- Three-dimensional topographical image of a local tissue damage (stripe in the middle) on a cornea sample after a friction experiment with an uncoated contact lens. Source: B. Winkeljann/TUM be applied as drops several times per day to prevent tissue damage. Hyaluronic acid is also not a natural component of the tear film. Dr. Lieleg said they have not tested if the coating is stable for longer periods of time, such as two weeks, which is how long many soft, disposable contact lenses can be used. Immersing contact lenses in a storage solution infused with the mucin, however, could renew the coating on the lenses each evening, he explained. Future experiments will test the long-term stability of the coating and will test coated contact lenses in situ in an animal model. "In the long run, we are trying to develop synthetic, mucin-mi- metic molecules that can take over the role of either native mucins or purified mucins. But this will require more research until we fully understand how the molecular architecture of mucins is responsible for their outstanding lubricating properties," Dr. Lieleg said. Winkeljann et al. also noted the other applications for purified gas- tric mucins as lubricants in medical applications. "[S]cenarios where medical devices mechanically challenge soft, sensitive tissue, such as the inser- tion of a catheter into the urethra or intubation tube into the trachea before surgery, or irritations in the oral cavity caused by braces, might benefit from the use of mucin addi- tives or coatings," they wrote. EW References 1. Winkeljann B, et al. Mucin coatings prevent tissue damage at the cornea-contact lens interface. Adv Mater Interfaces. July 2017. Epub ahead of press. 2. Zhao H, et al. Quantification of MUC5AC protein in human tears. Cornea. 2001;20: 873–7. 3. Pflugfelder SC, et al. Correlation of goblet cell density and mucosal epithelial membrane mucin expression with rose bengal staining in patients with ocular irritation. Ophthalmology. 1997;104:223–35. 4. Lieleg O, et al. Mucin biopolymers as broad- spectrum antiviral agents. Biomacromole- cules. 2012;13:1724–32. 5. Sweeney MP, et al. Clinical trial of a mucin-containing oral spray for treatment of xerostomia in hospice patients. Palliat Med. 1997;11:225–32. Editors' note: Dr. Lieleg has no finan- cial interests related to his comments. Contact information Lieleg: oliver.lieleg@tum.de Pig continued from page 74

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