Noncomeal absorption

Ahmed, I., and T.F. Patton. 1987. Disposition of timolol and inulin in the rabbit eye following corneal versus noncomeal absorption. Int J Pharm 38 9. 

In a study by Schoenwald et al., the conjunctival/scleral pathway yielded higher iris-ciliary body concentrations for all compounds evaluated with the exception of lipophilic rhodamine B (33). Romanelli et al. demonstrated the absorption of topical ocular bendazac into the retina-choroid via the conjunctival-scleral pathway (34). Noncomeal absorption was influenced more by physicochemical properties of the drag than characteristics of the formulation, whereas conversely transcomeal absorption could be influenced more by characteristics of the formulation. 

The photophysical properties (extinction coefficient, shifts in absorption and emission spectra, quantum yield, and lifetime) of a variety of probes are modified by pH changes, complexation by metal ions, or redox reactions. The resulting changes in photophysical parameters can be used to determine concentration of H+ and metal cations with suitably designed fluorophores. Most of these resulting sensors involve an equilibrium between the analyte, A, and the free probe (unprotonated or noncom-plexed by metal ion), Pf. If the stoichiometry of this reaction is 1 1, the reaction may be represented by... 

As in humans ethical constraints limit sampling sites to peripheral blood vessels, absorption cannot directly be estimated. Hence, the absorption rate (ka, fe0i) as calculated from peripheral plasma concentration data by compartmental or noncom-partmental models reflects rather an oral bioavailability rate, unless any gut wall and liver first-pass elimination is ruled out [112]. 

Routes of absorption that lead to the removal of drag from the precorneal area, and do not result in direct ocular uptake, are referred to as nonproductive. These noncomeal pathways, which are in parallel with comeal absorption and include conjunctival uptake and drainage via the nasolacrimal duct, lead to systemic absorption by way of conjunctival blood vessels in the former case and removal through the nasal mucosa and gastrointestinal tract in the latter. As discussed, drag can penetrate the conjunctiva, and, via the sclera, enter the eye however, blood vessels within the conjunctiva can also lead to systemic absorption. 

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