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

A large variety of drugs bind effectively to melanin, and this binding influences their ocular pharmacokinetic and distribution profiles. We aimed to establish a correlation between in vitro melanin binding and in vivo ocular pharmacokinetics (PK). The extent of melanin binding in vitro was determined for a set of model drugs; binding kinetics and binding isotherms were generated and fitted to a mechanistic model to derive the drug-melanin binding parameters (Bmax, KD, kon, and koff). In addition, in vitro ADME properties such as cellular permeability, P-glycoprotein-mediated efflux, plasma protein binding, and octanol partition coefficients were determined. Moreover, cellular uptake was measured in the nonpigmented ARPE-19 cells and in lightly pigmented human epidermal melanocytes. Finally, in vivo ocular PK studies were performed in albino and pigmented rats using intravenous injections. Substantial drug enrichment accompanied by a very long residence time was observed in pigmented ocular tissues, which could be linked to the melanin binding determined in vitro and to the intracellular drug uptake into the pigmented cells. The resulting ocular PK profile is shown to be a consequence of the interplay of melanin binding with concurrent processes such as systemic clearance, plasma protein binding, cellular permeation, P-glycoprotein efflux, pH partitioning, and tissue binding. Understanding this interplay at a mechanistic level could help in the rational design and development of new small-molecule drug candidates with the desired PK/pharmacodynamic profile to target the back of the eye.