MPS POLYMER

The Current Research on Hyaluronan

The addition of hyaluronan to a multipurpose solution may help improve contact lens comfort.

By Marjorie J. Rah, OD, PhD

Dr. Rah, is the manager, Global Medical Affairs, Vision Care, Bausch + Lomb.

Contact lens multipurpose solution (MPS) systems often contain wetting agents, typically surfactants or conditioning polymers, which can reduce surface tension of a lens, helping moisture to spread more easily across its surface (Bennett and Weissman, 2007; Rosen, 1978). The longer the wetting agents remain associated with a lens, the longer they may help to keep it moist (Burke et al, 2011).

A Closer Look at Hyaluronan

One such conditioning polymer is hyaluronan (HA), a high-molecular-weight glycosaminoglycan biopolymer naturally found throughout the body in the connective tissue of the skin, the umbilical cord, in the synovial fluid in joints, and in the eye (Lapcik et al, 1998; Stuart and Linn, 1985; Fraser et al, 1997; Laurent et al, 1996). In the eye, HA can be found in normal human tears (Frescura et al, 1994), lacrimal tissue (Yoshida et al, 1996), and in the corneal epithelium (Lerner et al, 1998).

A high capacity for hydrogen bonding allows HA to attract and to retain moisture within its coiled polymer chains. HA draws in and holds onto up to 1,000 times its weight in water (Presti and Scott, 1994). Under high shear force, such as the blink of an eye, HA polymers align, decrease viscosity, and spread evenly across the ocular surface (Szczotka-Flynn, 2006).

HA can help improve the contact lens wearing experience in many ways. It forms a free-flowing network on the lens surface, attracting water to envelop lenses in a moisture-rich solution. It also helps to stabilize the tear film and reduce friction (Lerner et al, 1998).

In the past two years, new MPS lens care systems have entered the market. Each of these care solutions has a unique formulation designed to improve comfort while maintaining disinfection efficacy. Biotrue MPS (Bausch + Lomb) contains a wetting agent combination system consisting of the surfactants poloxamine 1107 and sulfobetaine and the conditioning polymer HA (Burke et al, 2011). This article highlights current research on some of the outcomes of incorporating these wetting agents into an MPS.

Continuous Release of HA

To evaluate how wetting agents interact with both traditional hydrogel and silicone hydrogel (SiHy) contact lens materials and how the interactions may affect wetting agent release profiles, Scheuer, Doty et al (2011) developed an in vitro model in which lenses were soaked in various contact lens disinfecting solutions. After soaking for eight hours in either an MPS or a saline control solution, saline was dripped over the lenses at a rate that approximates tear secretion in the human eye (Reddy, 1996), and the rinse solutions were collected at two-hour intervals over a period of 20 hours. The surface tensions of the rinse solutions collected were then analyzed versus the surface tension of the control.

Figure 1 shows the results. Only those lenses soaked in Biotrue were shown to have surfactants present after six hours. The lenses soaked in Biotrue exhibited the presence of surfactants continuously for up to 20 hours.

Figure 1: Hours of continuous wetting agent release for all silicone hydrogel lenses tested.

Burke et al (2011) further examined the retention and release of HA from both hydrogel and SiHy lenses by soaking lenses overnight in either Biotrue MPS prepared with HA tagged with fluorescein or a saline control. After soaking, saline was dripped over the lenses at a rate that approximates tear film secretion in the human eye (Reddy, 1996), the rinse solution was collected every two hours over a 20-hour period, and the fluorescein intensity was evaluated. The results also showed continuous release of HA for up to 20 hours for all lens types (Figure 2).

Figure 2: Percentage of hyaluronan remaining on all silicone hydrogel and hydrogel lenses tested after soaking in Biotrue MPS.

Hyaluronan in Human Tears

To evaluate the HA levels in tears, Scheuer, Su et al (2011) described the HA concentration in the tear film after wearing contact lenses that have been soaked in Biotrue MPS. Tears were collected on Schirmer strips at baseline and after two hours of wearing either senofilcon A contact lenses soaked in Biotrue for 14 hours or control lenses rinsed with saline. The results showed an increase in HA concentration in tears collected from eyes that had worn lenses soaked in Biotrue compared to baseline (p=0.009) and compared to those that had worn saline rinsed control contact lenses (p=0.033) (Figure 3). The researchers concluded that after wearing senofilcon A contact lenses soaked in Biotrue MPS, the HA concentration in the tear film may be increased, helping to make wearing contact lenses more comfortable. This is consistent with the previous HA release data reported by Burke et al.

Figure 3: Concentration of HA in the tears at baseline and after wearing lenses soaked in either saline or multipurpose solution containing HA (Biotrue).

Hyaluronan on the Lens Surface

To visualize the presence of HA found in Biotrue on the surface of SiHy lenses, Wygladacz et al (2012) examined lenses (senofilcon A and balafilcon A) that had been soaked for four hours in Biotrue MPS. After rinsing with safranin solution for three minutes and HPLC grade water for three minutes, the lenses were imaged with a confocal laser scanning microscope equipped with a Differential Interference Contrast (DIC) attachment. Control lenses incubated with safranin solution but not exposed to Biotrue were also examined. The confocal and DIC images showed that the high-molecular-weight HA in Biotrue adsorbed consistently across the surface of the SiHy lenses (Figure 4).

Figure 4: DIC image of HA networks adsorbed to a SiHy contact lens surface soaked in Biotrue MPS containing HA (bottom panel), and control lens (top panel).

Conclusion

Surfactants or conditioning polymers such as HA help moisture to spread more easily across the lens by reducing surface tension. Not only has HA been shown to be retained and released by contact lenses soaked in Biotrue MPS (Burke et al, 2011; Scheuer, Doty et al, 2011), but the concentration of HA in the tear film after wearing lenses that have been soaked in Biotrue MPS also increases (Scheuer, Su et al, 2011). This is noteworthy because the longer a wetting agent is associated with the lens, the longer the potential for improved wettability and longer lasting comfort. What's more, the HA can be visualized on the lens via confocal microscopy after a four-hour soak.

By incorporating hyaluronan into the formulation, Biotrue hydrates contact lenses in a way that the eyes hydrate themselves, which can help to improve comfort and make lens wear easier on the eyes. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #198.