Abstract
Purpose :
Sustained delivery of small molecule drugs to the back of the eye remains a challenging barrier for the access of novel ophthalmic treatments. A large variety of drugs bind effectively to melanin and this binding is related to changes in their local pharmacokinetic and pharmacodynamic profiles. We aimed to establish an in vitro-in vivo correlation for melanin binding to better understand its qualitative and quantitative aspects and extrapolate in vitro readouts into in vivo ocular PK profiles for model drugs.
Methods :
The extent of melanin binding was determined for a set of model compounds by incubation with melanin from Sepia officinalis at equilibrium. Binding kinetics data was generated and fitted to a mechanistic model to derive the drug binding parameters (kon, koff, Bmax and KD). In vitro ADME profiling was performed to determine membrane permeability, active transport, plasma stability, lipophilicity and ionization states. Cellular uptake and unbound partition coefficients were measured in non-pigmented and pigmented human retinal pigment epithelium cell lines. In vivo ocular intravenous PK studies were performed in albino and pigmented rats and total ocular drug concentrations were quantified.
Results :
Binding kinetics data demonstrated that compounds with similar extent of melanin binding show distinct release profiles in dissociation experiments in vitro. Increased cellular uptake was observed for strong melanin binders in pigmented cell lines, resulting in decreased intracellular unbound fractions. Distribution studies performed in albino and pigmented rats showed a substantial enrichment accompanied by a very long residence time in pigmented ocular tissues, which could be linked to the extent and kinetics of melanin binding determined in vitro as well as other compound’s properties.
Conclusions :
This work suggests that melanin binding measured in an in vitro system can be related to increased enrichment and retention of a molecule in the ocular pigmented tissues. The resulting PK profile is the consequence of the interplay of different processes such as systemic clearance, plasma protein binding, membrane permeability, pH partitioning and lipophilicity. Understanding this interplay at a mechanistic level can help in the rational development of new drug candidates with the desired PK/PD profile targeting the back of the eye.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.