Cytotoxicity of pivalylphenylephrine and pivalic acid to corneal endothelium

Esterase activities in adult rabbit eyes

The rational design of prodrugs to improve the therapeutic efficacy of existing drugs would be expedited if the nature of the in vivo enzymatic conditions that regenerate the drugs from their prodrugs is known. Using albino and pigmented rabbits as models, this research seeks to delineate the esterase activities in their corneas, irises, ciliary bodies, and aqueous humor, which are intimately involved in the disposition of drugs from topical dosing. This was achieved by monitoring the hydrolysis kinetics of alpha-naphthyl acetate, both in the presence and absence of esterase inhibitors, upon incubation with aqueous humor and homogenates of cornea, iris, and ciliary body. It was found that in both breeds of rabbits, esterase activity was the highest in the iris-ciliary body followed by the cornea and then the aqueous humor, and that multiple esterases probably existed in the aqueous humor, and the ocular tissues studied. However, the esterase activity in the cornea and iris-ciliary body of the pigmented rabbit was greater when compared with the albino rabbit. Based on these results, drugs and prodrugs containing ester linkages can undergo varying extents of esterase-mediated hydrolysis while permeating the cornea and upon entering the aqueous humor, iris, and ciliary body. Moreover, in view of the differences in esterase activity that exist between the albino and pigmented rabbits, it would be necessary to employ both breeds of rabbits in evaluating the rate and extent to which ocular ester prodrugs would be converted to their parent compounds.

Effect of phenylephrine on aqueous humor flow

The effect of topical 2.5% phenylephrine hydrochloride, an alpha adrenergic agonist, on the rate of aqueous humor flow was studied in the eyes of 22 normal human subjects. The drug and a placebo were administered by random assignment from coded containers with the fellow eye serving as the control. Intraocular pressure, anterior chamber volume, corneal thickness and the rate of clearance of topically applied fluorescein from the anterior chamber were measured. Four of the 22 subjects underwent a study of the effect of phenylephrine on the permeability to fluorescein of the blood-aqueous barrier. No statistically significant effect of the drug was found on the rate of aqueous humor flow, intraocular pressure, anterior chamber volume, corneal thickness or permeability of the blood-aqueous barrier.

Endothelial physiology and intraocular lens implantation

The endothelium is the cellular monolayer which lines the posterior surface of the cornea. This layer is important in clinical ophthalmology because it is vital to maintenance of the transparency of the cornea and vision through its pump and barrier functions which limit the ingress of fluid into the cornea from the aqueous. When the function of the corneal endothelium becomes compromised, the corneal stroma swells as it hydrates. Subsequently, epithelial bullae form with painful recurring epithelial erosions, and finally corneal scarring and blindness result. The relatively vulnerable position of the corneal endothelium renders it susceptible to iatrogenic injury during intraocular procedures, especially IOL implantation: the poor regenerative (mitotic) capacity of the human corneal endothelium limits its ability to recover normal function once it is injured.