OCULAR CONTACT TIME OF A CARBOMER GEL (GELTEARS) IN HUMANS

Background/aims-Carbomers are widely used in products for the treatmen t of dry eye; however, the polymer gel thins on addition of probes (fo r example, fluorescein salt) confounding the comparison of products by objective clinical tests such as spectrophotofluorimetry or scintigra phy. A novel method of radiolabelling carbomer gels, with minimum chan ge to their rheology, has permitted the noninvasive evaluation of prec orneal residence of the gel in volunteers using gamma scintigraphy. Th e technique was used to evaluate the precorneal clearance of the liqui d phase and of a suspended particulate in GelTears. Methods-Low sodium technetium-99m labelled diethylenetriaminepentacetic acid (Tc-99m-DTP A) was used to label carbomer 940 gel, either adsorbed onto sterile ch arcoal to model an entrapped drug, or added directly to the gel to a f inal activity of 1 MBq per 25 mu l dose. The clearance of the labelled gels was then compared with Tc-99m-DTPA labelled saline in 12 volunte ers. Results-The addition of the low sodium radiopharmaceutical produc ed insignificant rheological changes in the gel compared with conventi onal Tc-99m-DTPA labelling. The residence times on the eye of the gel formulations were significantly greater than that of the saline contro l. At 8 minutes postdosing, the label (mean (SD)) on the ocular surfac e were: saline, 7% (7%); Tc-99m-DTPA gel, 42% (27%); and Tc-99m-carbon gel, 42% (20%) of administered dose. There was no difference observed in the precorneal distribution between Tc-99m-DTPA solution and parti culate markers. Conclusions-These data demonstrate that carbomer based gels significantly extend contact of solutes or suspended solids with the corneal surface. The method of labelling does not significantly c hange the initial viscosity and is superior to previous methods which have used sodium salts (for example, sodium fluorescein) and therefore underestimate contact time.