Contact lens measures drug concentration in the anterior eye
Although the idea had been proposed by previous researchers, scientists at the University of Strathclyde and Glasgow University (both in Glasgow, Scotland) are the first to actually fabricate and test a fiberoptic-coupled corneal contact lens that provides direct, minimally invasive, real-time measurements of drug concentration in the anterior portion of a human eye.
Although the idea had been proposed by previous researchers, scientists at the University of Strathclyde and Glasgow University (both in Glasgow, Scotland) are the first to actually fabricate and test a fiberoptic-coupled corneal contact lens that provides direct, minimally invasive, real-time measurements of drug concentration in the anterior portion of a human eye.1
To develop any ophthalmic medication, it is necessary to measure the ocular concentration of the drug within the eye. Previously, the only working method was to use animal eyes at various time intervals after the topical application of the drug-a wasteful, laborious, and inaccurate process.
The goal of the scientists is to turn the anterior chamber of the eye into a spectrophotometer cuvette. A specially designed contact lens is attached to optical fibers that guide light from a laser or a xenon arc lamp into the eye of the human test subject and carries it back to a spectrograph for analysis (see figure, top). The contact lens, measuring 25 × 8 × 6 mm and machined from fused silica, is mirrored on two opposite faces to send the beam through a thin tear layer, through the cornea, and across the anterior chamber of the eye, following a path approximately 8.6 mm in length for the human subjects tested.
Because the eye transmits light between 300 and 1400 nm, and because many pharmaceutical drugs of interest for the eye have broad, easily resolvable peaks in the UV and visible (VIS) regions, the UV/VIS spectral region was chosen for analysis. The xenon arc lamp was used for absorption spectroscopy, while an argon-ion laser at 488 nm was used for fluorescence measurements. For all measurements, a reference spectral scan was obtained by filling the concave lens surface of an upturned sensor head with deionized and distilled water. This reference scan was then subtracted from signal scans taken in recorded intervals after application of fluorescein or brimonidine, two ocular solutions used in the experiment.
“The real-time measurement capability is able to easily detect the presence and monitor the time course of a topically applied drug, such as brimonidine, in the human eye,” explains Joe Miller, now at the Lions Eye Institute in Western Australia. The scientists also observed that the human-eye uptake of topical fluorescein was poor as confirmed by previous research.
“Our work is yet another example of the rapid strides being made in the field of biomedical photonics, and its application in drug discovery,” adds Deepak Uttamchandani, a professor at the University of Strathclyde.
1. J. Miller et al., British J. Ophthalmology 89,1147 (September 2005).