The molecular structure of hyaluronic acid
Dr Nicholas Young
BSc(Hons) BOptom PhD(Med) PGCertOcTher
Dry Eye Centre, Heathmont VIC
What makes a good artificial tear (AT)? It’s hard to answer this question as most of us who treat dry eye are aware that no single AT is effective for all patients. Many patients appear refractory to all ATs. In some cases, ATs worsen dry eye symptoms.
Comparative studies of ATs are somewhat futile in this context too, as it is difficult to predict how a patient will respond to a given product with variations in their disease severity over time. Successive meta-analyses are also inconclusive on this point,1,2 as is DEWS II (2017 international dry eye workshop) which acknowledged that no two dry eye patients are alike and that no single treatment plan can be effective for all patients.3
Historically, artificial tears (ATs) act as a first-line topical treatment in dry eye. ATs mainly comprise three classes of pharmaceutical: natural cellulose derivatives, (such as carboxymethylcellulose), synthetic polymers (such as PVA and HP-guar)and hyaluronic acid. In their own way, ATs can have a role to play in reducing ocular surface inflammation.
Carboxymethylcellulose (CMC) and hyaluronic acid (HA) are two of the most commonly prescribed and used artificial tears.4 CMC’s anionic cellulose polymer with substituted carboxyl group makes it ‘sticky’ to the ocular surface, enabling greater bio-adhesion and increased tear retention time. In comparison, HA is a glycosaminoglycan with repeating alternating N-acetylglucosamine and glucuronate sequences in linear chains. This structure is viscoelastic and binds water molecules,5 helping to reduce surface dehydration and shear forces during blinking,6,7 effects that indirectly help lessen surface inflammation.
What is hyaluronic acid?
Sodium hyaluronate is a naturally-occurring molecule of the human body, but sodium hyaluronate and hyaluronic acid are different chemicals; the former is a water-soluble salt form of the latter. Although about half the HA in our bodies is found in skin, it was first discovered by Karl Meyer and John Palmer in human vitreous.8 Synthetic HA was patented in 1942 by Endre Balazs for commercial use in baking products, and was originally intended for use as a substitute for egg white.9
The name ‘Healon’ (a highly purified form of HA) was trademarked in 1970 and first used medically in ophthalmic surgery. Today HA has become widely used in ophthalmic surgery, dry eye care and cosmetic medicine to improve skin texture and hydration.10
HA is one of several viscosity enhancers used in ATs. Its ability to retain a significant volume of water11 and its visco-elasticity helps it to maintain corneal surface wettability, reduce tear osmolarity and reduce shear forces associated with blinking and air flow. HA has been shown to improve corneal density of all five layers,12 and suppress specific inflammatory mediators, while up-regulating other protective mediators.13 It is also associated with protection of goblet cell density and a reduction of dry eye associated squamous metaplasia, two characteristics of dry eye.14 Collectively, these properties appear to have stabilising effects on the ocular surface, preventing disease and enhancing epithelial repair processes.15,16
BENEFITS OF HYALURONIC ACID
Excessive tear evaporation and reduced aqueous volume can lead to hyperosmolar tears. These changes cause stress on the ocular surface with resultant corneal and conjunctival cell death, tissue inflammation and a destructive cycle of events.17
Tear osmolarity’s predictive value for dry eye disease was conceptualised from indecision regarding diagnostic criteria for Sjögren’s syndrome, but its experimental origins date from almost 50 years ago. In animal studies, hypo-osmolar electrolyte solutions appear best suited to reducing the effects of dry eye,18 while hypotonic HA is effective in human trials2,19 and may be more effective in lowering tear film osmolarity than carboxymethylcellulose and HP-guar.20
pH balance and phosphate free
The pH of normal tears is 7.4 (tolerance: 6.6–7.8). AT complexity revolves around buffer maintenance of this criterion while ensuring good wettability, lubricity and retention time on the ocular surface. While some ATs use phosphate as a buffer, the use of citrate has been found to be desirable.
Although phosphate buffers in eye-drops are effective, innate corneal calcium can react with phosphate to form calcium phosphate crystals. These have been shown to cause corneal calcification: an accumulation of the insoluble crystals, particularly in severe dry eye and already compromised corneas. In one study, 26 of 59 eye-drops tested had phosphate levels above physiological levels, with very high concentrations being found in three products.21 In some reported cases, affected patients have required corneal transplants.22 Fortunately, the incidence of this adverse response is low; however, given a choice, phosphate-free is more prudent.
Many eye-drops contain the preservative benzalkonium chloride (BAK), a cationic surfactant which disrupts the lipid layer on contact with the eye. It also penetrates the epithelial cell microvilli and goblet cells with consequential cell death. Without clearance of the BAK, cell death allows release of the BAK to affect neighbouring cells;23,24 a single drop of 0.01% BAK is detectable in the epithelial cell layer for up to seven days.25 In addition to cell toxicity, the use of preserved ATs is also associated with allergies in some patients.26 Newer ATs contain neutralising or less-invasive preservatives; however, some of these are also associated with adverse ocular surface effects.27
There are many unpreserved ATs on the market, but only two main delivery systems. Individual-use vials are most common. These are typically used once, prior to disposal.
The alternative is a multi-dose system such as the one used by AFT pharmaceuticals for Hylo-Forte. The ‘COMOD’ (COntinuous MOno Dose multidose) system comprises a pump in a bottle; the solution is contained within the bottle in a sterile flexible bag. By pressing the pump, a drop is expelled from the bag onto the eye. When the pump is released, air pressure is restored in the bottle via a ventilation duct, but not into the bag. The contents of the bag therefore remain sterile.
The active ingredient in Hylo-Forte may also prove useful in patients who use BAK-containing medications for other purposes such as glaucoma treatment, as HA may reduce the toxic effect of benzalkonium chloride.28
Hylo-Forte is one of a number of dry eye products that contribute some significant features and benefits to the choice of AT for our dry eye patients. Apart from the active ingredient 0.2% sodium hyaluronate, the other Hylo-Forte components are citric acid, sodium citrate dehydrate and sorbitol. While we cannot entirely predict the outcome of a treatment, informed choice can help to prevent prescribing errors.
This article was written with the financial support of AFT Pharmaceuticals.
1. Song J, Lee K, Park H, et al. Efficacy of carboxymethylcellulose and hyaluronate in dry eye disease: a systematic review and meta-analysis. Korean Journal of Family Medicine 2017; 38: 1: 2.
2. Lester M, Orsoni G, Gamba G, et al. Improvement of the ocular surface using hypotonic 0.4% hyaluronic acid drops in keratoconjunctivitis sicca. Eye 2000; 14: 6: 892–898.
3. Jones L, Downie L, Korb D, et al. TFOS DEWS II Management and Therapy Report. The Ocular Surface 2017; 15: 3: 575–628.
4. Lee J, Ahn H, Kim E, Kim T. Efficacy of sodium hyaluronate and carboxymethylcellulose in treating mild to moderate dry eye disease. Cornea 2011; 30: 2: 175–179.
5. Balazs EA, Laurent TC, Howe AF, Varga L. Irradiation of mucopolysaccharides with ultraviolet light and electrons. Radiation Research 1959; 11: 2: 149–164.
6. Meyer K. Chemical structure of hyaluronic acid. Fed Proc 1958; 17: 4: 1075–1077.
7. Nakamura M, Hikida M, Nakano T, et al. Characterization of water retentive properties of hyaluronan. Cornea 1993; 12: 5: 433–436.
8. Simoni RD, Hill RL, Vaughan M, Hascall V. The discovery of hyaluronan by Karl Meyer. J Biolog Chem 2002; 277 (39): e27.
9. Balazs EA, Denlinger JL. Clinical uses of hyaluronan. Biology of Hyaluronan 1989; 143: 265–275.
10. Robert L. Hyaluronan, a truly ‘youthful’ polysaccharide. Its medical applications. Pathologie Biologie 2015; 63: 1: 32–34.
11. Balazs EA. The physical properties of synovial fluid and the special role of hyaluronic acid. Disorders of the Knee 1974; 2: 63–75.
12. Wegener AR, Meyer LM, Schönfeld CL. Effect of viscous agents on corneal density in dry eye disease. J Ocular Pharmacol Therap 2015; 31: 8: 504–508.
13. Brignole F, Pisella PJ, Dupas B, et al. Efficacy and safety of 0.18% sodium hyaluronate in patients with moderate dry eye syndrome and superficial keratitis. Graefe’s Arch Clin Exp Ophthalmol 2005; 243: 6: 531–538.
14. Moon JW, Lee HJ, Shin KC, et al. Short term effects of topical cyclosporine and viscoelastic on the ocular surfaces in patients with dry eye. Korean J Ophthalmol 2007; 21: 4: 189–194.
15. Aragona P, Papa V, Micali A, et al. Long term treatment with sodium hyaluronate-containing artificial tears reduces ocular surface damage in patients with dry eye. Brit J Ophthalmol 2002; 86: 2: 181-184.
16. Zhong J, Deng Y, Tian B, et al. Hyaluronate acid-dependent protection and enhanced corneal wound healing against oxidative damage in corneal epithelial cells. J Ophthalmol 2016; 2016: 1–10.
17. Baudouin C, Aragona P, Messmer EM, et al. Role of hyperosmolarity in the pathogenesis and management of dry eye disease: proceedings of the OCEAN group meeting. Ocular Surface 2013; 11: 4: 246–258.
18. Gilbard J, Rossi S, Heyda K. Ophthalmic solutions, the ocular surface, and a unique therapeutic artificial tear formulation. Amer J Ophthalmol 1989; 107: 4: 348–355.
19. Troiano P, Monaco G. Effect of hypotonic 0.4% hyaluronic acid drops in dry eye patients: a cross-over study. Cornea 2008; 27: 10: 1126-1130.
20. Benelli U, Nardi M, Posarelli C, Albert TG. Tear osmolarity measurement using the TearLab Osmolarity System in the assessment of dry eye treatment effectiveness. Contact Lens Ant Eye 2010; 33: 2: 61–67.
21. Bernauer W, Thiel MA, Langenauer UM, Rentsch KM. Phosphate concentration in ATs. Graefe’s Arch Clin Exper Ophthalmol 2006; 244: 8: 1010–1014.
22. Bernauer W, Thiel MA, Kurrer M, et al. Corneal calcification following intensified treatment with sodium hyaluronate ATs. British J Ophthalmol 2006; 90: 3: 285–288.
23. Tønjum AM. Effects of benzalkonium chloride upon the corneal epithelium studied with scanning electron microscopy. Acta Ophthalmol 1975; 53: 3: 358–366.
24. De Saint Jean M, Brignole F, Bringuier AF, et al. Effects of benzalkonium chloride on growth and survival of Chang conjunctival cells. Invest Ophthalmol Vis Sci 1999; 40: 3: 619–630.
25. Champeau EJ, Edelhauser HF. Effect of ophthalmic preservatives on the ocular surface: conjunctival and corneal uptake and distribution of benzalkonium chloride and chlorhexidine digluconate. The preocular tear film in health, disease and contact lens wear. Lubbock, TX: Dry eye Institute, Inc, 1986. p 292–302.
26. Baoudouin C, Labbe A, Liang H, et al. Preservatives in eyedrops: the good, the bad and the ugly. Prog Retinal Eye Res 2010; 29: 4: 321–334.
27. Schrage N, Frentz M, Spoeler F. The Ex Vivo Eye Irritation Test (EVEIT) in evaluation of artificial tears: Purite-preserved versus unpreserved eye drops. Graefe’s Arch Clin Exp Ophthalmol 2012; 250: 9: 1333–1340.
28. Yu F, Liu X, Zhong Y, et al. Sodium hyaluronate decreases ocular surface toxicity induced by benzalkonium chloride-preserved latanoprost: an in vivo study. Invest Ophthalmol Vis Sci 2013; 54: 5: 3385.