Timolol Binding to Bovine Ocular Melanin In Vitro

Quantification of Drugs in Distinctly Separated Ocular Substructures of Albino and Pigmented Rats

“…Melanin binding of drugs occurs reversibly via different types of electrostatic and hydrophobic interactions between the drug and the melanin polymer depending on the molecular properties of the drug [26][27][28]. It has been observed that some solvents do not extract all of the bound drug from melanin [15,[29][30][31], which is explained by the different effects of solvents on the drug-melanin interactions. We evaluated solvent extraction efficiency from melanin in vitro, as direct measurement from in vivo samples would require the use of radiolabeled compounds.…”

Establishment of an In Vitro–In Vivo Correlation for Melanin Binding and the Extension of the Ocular Half-Life of Small-Molecule Drugs

“…Even though release kinetics is an important property that seems relevant to understand the ocular half-life, the reports in the literature addressing this are quite limited. 19,21,22 In addition, the measurement of binding isotherms of drug candidates is considered difficult due to solubility limitations and therefore we exploited the combination of kinetic studies and mechanistic modeling to assess a Ratio between the first measured concentrations in ocular homogenates of pigmented (7 h) and albino rats (2 h). kinetic and equilibrium binding parameters.…”

“…In this work, we first targeted the development of a kinetic binding assay, which, in combination with a mechanistic modeling approach, was able to provide rates of compound association and dissociation ( k on and k off ) as well as B max and K D binding parameters. Even though release kinetics is an important property that seems relevant to understand the ocular half-life, the reports in the literature addressing this are quite limited. ,, In addition, the measurement of binding isotherms of drug candidates is considered difficult due to solubility limitations and therefore we exploited the combination of kinetic studies and mechanistic modeling to assess kinetic and equilibrium binding parameters. We attempted to establish a surface plasmon resonance (SPR) method, which would allow derivation of the drug binding parameters in an automated manner; however, the obtained results were unsatisfactory due to unspecific binding issues with the gold sensor (data not shown).…”

“…16 Even though the release kinetics from the pigmented tissue can play an important role in defining the concentration−time profile in vivo, unlike with the binding extent at equilibrium, there have been very limited attempts to address this release experimentally in vitro. 21,22 In addition, the resulting ocular PK profile is believed to be a consequence of other concurrent processes involved such as systemic clearance, plasma protein binding, permeability, pH partitioning, and binding to cell constituents. 23−25 Drug metabolism and transport can also alter the drug's ocular disposition profile.…”

“…This rate is believed to be mainly governed by the drug’s dissociation rate from melanin and permeation across melanosomal and plasma membranes . Even though the release kinetics from the pigmented tissue can play an important role in defining the concentration–time profile in vivo, unlike with the binding extent at equilibrium, there have been very limited attempts to address this release experimentally in vitro. , In addition, the resulting ocular PK profile is believed to be a consequence of other concurrent processes involved such as systemic clearance, plasma protein binding, permeability, pH partitioning, and binding to cell constituents. − Drug metabolism and transport can also alter the drug’s ocular disposition profile. While there is evidence for metabolic activity of phase 1 and phase 2 enzyme systems in the cornea, ciliary body, and RPE, the state of knowledge on the relevance of identified drug transporters in the overall drug ocular tissue distribution and bioavailability is still limited. − Hence, understanding the interplay of melanin binding and these different processes at a mechanistic level could help in the rational design of new drug candidates with desired physicochemical and ADME properties to target the back of the eye …”