Multi-purpose contact lens solutions differ in protein removal

October 15, 2008

Multi-purpose contact lens solutions appear to differ in their protein removal efficiency, a trait that may influence the comfort of the lenses. Further investigation and comparison of the properties of multi-purpose solutions are needed to clarify the hypothesis, which was suggested by findings of a recent small study.

Berkeley, CA-A small, preliminary study has shown that multi-purpose contact lens (CL) solutions differ in their protein removal efficiency. The finding suggests the need for further investigation into the effect of protein deposits on the comfort of contact lenses, according to Meng C. Lin, OD, PhD, FAAO, associate clinical professor and director of the Clinical Research Center, School of Optometry, University of California, Berkeley.

In a previous clinical study, Dr. Lin and colleagues found that subjects who wore high-protein uptake lenses (Acuvue 2, Johnson & Johnson Vision Care), preferred one multi-purpose disinfecting solution (Opti-Free Express [OFX], Alcon Laboratories), saying that it provided better cleaning and comfort. An opposite trend was observed for subjects who wore low-protein uptake lenses (Focus Night & Day, CIBA Vision), however; these subjects preferred another lens solution (Opti-Free [OF], Alcon Laboratories) to OFX.

"It is possible that lens care solutions that have surface-active agents can facilitate removal of the protein a little bit better, hence enhancing wearing comfort for some lenses," Dr. Lin said. The different comfort preferences of the lens users, however, also suggest that some multi-purpose solutions may clean lenses too well by stripping off too much of the natural tear film components on the lens surface.

To test the hypothesis that multi-purpose solutions vary in protein removal efficiency and to further understand how differences in protein removal efficiency affect wearing comfort, Dr. Lin and colleagues conducted three experiments to examine the efficiency of several commercial multi-purpose solutions.

First, they used a very well-characterized silica surface in a controlled environment with single-protein adsorption and removal by multi-purpose solution. For the second experiment, also in vitro, investigators soaked low-protein uptake lenses in model protein mixtures and compared total protein concentration with and without multi-purpose solution exposure. The final experiment was an analysis of total protein with ex vivo lenses cleaned with either OFX or OF solutions. Subjective comfort ratings also were collected.

"In a very controlled environment, the degree of lysozyme removal is very different among different multi-purpose solutions. OFX can remove about 80% of the adsorbed lysozyme as compared with only about 15% removal with OF. Most of the multi-purpose solutions remove less than 20% of lysozyme," she said.

In the second experiment, in which lenses were exposed to the model tear protein, a direct protein quantification agent was used to analyze protein concentration. The results also showed that OFX was more efficient than OF in removing protein.

A clinical setting

After confirming that some solutions have much higher efficiency of protein removal than others, investigators conducted a third experiment to determine whether the trend would hold up in a clinical setting and whether differences in protein removal efficiency could impact comfort while wearing low-protein uptake silicone hydrogel lenses. They conducted a contralateral, double-blind study on nine asymptomatic soft CL wearers. The subjects wore low-protein uptake lenses for 2 weeks and were instructed to use OFX in one eye and OF in the other. The lenses were collected and the total protein was quantified with bicinchoninic acid assay. Subjective wearing comfort also was assessed.

The investigators found that six of the nine subjects had higher total protein concentrations in the eye exposed to the OF solution. The difference in protein concentrations between the two multi-purpose solutions was about 7%, however, which was not statistically significant. Interestingly, 56% of participants obtained better comfort with OF than with OFX; 11% had no preference, while the remaining 33% preferred OFX over OF.

Based on those preliminary results, the team calculated the sample size needed to detect a 20% difference in total protein concentrations and determined that at least 30 subjects would be required to reach statistical power. Furthermore, to establish a relationship between the amount of protein deposits on a lens and wearing comfort, a much larger sample size would be required, Dr. Lin said.

"These results lead to a cascade of questions regarding the role of protein deposits in wearing comfort for low-protein uptake silicone hydrogel lenses," she concluded. "Given the potential significance of variable protein removal efficiencies among multi-purpose CL solutions, further investigation may identify appropriate combinations of lens materials and solutions for enhancing wearing comfort."