Characterization and tissue localization of ocular carboxylesterases in multiple species

Abstract Introduction Ocular ADME (absorption, distribution, metabolism & elimination) is gaining interest due to rapidly growing incidence of eye diseases and the need of new therapeutic applications. The information regarding drug‐metabolising enzymes (DME) in different ocular tissues is scattered. In addition, animal models may not be predictive of the human situation due to species differences. Aim of the study We sought to obtain quantitative and functional data of ocular carboxylesterases in multiple species to help comparisons between preclinical animal models and humans. Methods Esterase activity assays were optimized using established human‐specific substrates. Quantitative proteomics was done using specific marker peptides to quantify the selected esterases in ocular tissues of different species (porcine, rabbit and human). Finally, the functional and quantification data was compared between tissues and species. Main findings The rabbit ocular tissues had about 3‐fold higher hydrolysis rates for the generic esterase substrate, p‐nitrophenyl acetate, than the pig tissues. Carboxylesterase 1‐mediated activity (D‐luciferin methyl ester as substrate) in pig ocular tissues were low as compared to rabbit. Moreover, carboxylesterase 2‐mediated activities (fluorescein diacetate as substrate) varied over a 5‐ to 20‐fold range among seven ocular tissues (conjunctiva, cornea, aqueous humor, vitreous, retina, RPE and choroid) in the pig and rabbit, respectively. On average, corneal and choroidal tissues showed high expression and functional activity of carboxylesterase isoforms while retinal pigment epithelium and aqueous humor showed the least activity. More detailed enzymatic and proteomic data will be presented at the poster. Conclusion This study provides new insight in ocular esterase expression and esterase‐mediated drug metabolism in multiple species.