(448d) Delivery of a Phospholipid Agent From a Silicone Hydrogel Contact Lens

Silicone hydrogel contact lenses have been shown to quickly absorb dimyristoylphosphatidylcholine (DMPC) from an n-propanol solution, and then release it slowly into water or an artificial tear solution [1,2]. This paper presents an analysis of the transport phenomena associated with the uptake and release. To load a lens, it was carefully removed from the blister pack, lightly blotted on a lint-free tissue (KimWipe, Kimberly-Clark, Neenah, WI), and immersed in 5 mL of loading solution (0.15 % DMPC) for 30, 60 or 120 seconds.  Then the lens was placed in a beaker of distilled deionized water (DD H₂O) at 35°C for 5 seconds, transferred to another beaker of DD H₂O for at least 5 seconds, rinsed with DD H₂O, and finally stored in a capped vial of DD H₂O for 2 to 20 minutes while the other lenses were prepared.  To elute the DMPC from the lens, it was placed in water or artificial tear solution at 35°C and samples were taken at 0, 2, 4, 10, 24, 48 and 72 hrs. 

Analysis of the data showed that amount of DMPC loaded into the lens increased with loading time and loading concentration.  The DMPC eluted continuously from the lens during the 72 hours, but the elution rate decreased steadily with time.  To determine if the elution might be diffusion controlled, the amount eluted was plotted versus the square root of time, and such a plot was linear for the first 10 hours, but deviated to slower elution at longer times.  The first 10 hours of data was fit to a 1-dimensional model in Cartesian coordinates of diffusion from a semi-infinite solid with constant surface concentration.  The model fit well and a diffusivity of 1.34 x 10^-12 cm²/s was estimated.  This value is consistent with the reported diffusivities of other materials in silicone hydrogel contact lenses [3].  Given this diffusivity and a nominal lens thickness of 0.01 cm, the characteristic time constant of this system is t = L²/D_AB = 5200 hrs, where L is the half-thickness of the lens.  Because this value is so much larger than 10 hrs (where deviation from the model occurs), the diffusion of DMPC from the lens must involve a more complex mechanism than the simple diffusion model.  Subsequently we have applied more complicated models that include changing bulk concentrations during the elution, and the multiphase characteristic of silicone hydrogel contact lenses.

While these models may be of academic and perhaps esoteric interest to academicians and researchers who might design such lenses form DMPC delivery, the clinical significance of this research work is that DMPC can be easily loaded into a contact lens and then delivered slowly and consistently over the course of 10 hours.  Since DMPC is a comfort product used in some types of eye drops designed to relieve “dry eye” irritation, the ability to deliver DMPC from a contact lens is of great interest to contact lens users who could obtain relief while wearing a contact lens.

References:

1. Pitt, W.G., Jack, D.R., Zhao, Y., Nelson, J.L., and Pruitt, J.D. “Loading and Release of a Phospholipid from Contact Lenses”, Optometry and Vision Science, 88(4), 502-506 (2011).

2. Pitt, W.G., Jack, D.R., Zhao, Y., Nelson, J.L., and Pruitt, J.D. “Transport of Phospholipid in Silicone Hydrogel Contact Lenses”, J. Biomat. Sci. Polym. Ed., in press (2011).

3. Kim, J., Conway, A. Chauhan, A., “Extended delivery of ophthalmic drugs by silicone hydrogel contact lenses” Biomaterials, 29, 2259-2269 (2008).