Ocular melanin binding of drugs: in vitro binding studies

Certain drugs accumulate into pigmented tissues due to their binding to melanin, a macromolecule inside pigmented cells. Melanin can affect the drug’s pharmacokinetics by acting as a drug reservoir. Binding of drugs to the ocular melanin might enable ocular drug targeting, prolonged duration of action and retinal delivery after non‐invasive drug administration (eye drops, tablet). Interestingly, chloroquine levels are maintained in the pigmented retinal and choroidal cells for months after discontinuation of dosing. The aim of the experimental part was to study melanin binding of a set of compounds in vitro with isolated melanin from the retinal pigment epithelium (RPE) and choroid of porcine eyes. The in vitro binding with melanin was studied at three different pH (i.e. 5, 6, 7.4) since melanosomal pH is unknown but expected to be acidic, so these range of pH will help us to understand the effect of pH on binding affinity and capacity. The compounds chosen for the study; papaverine, levofloxacin, terazosin, pazopanib, are small molecules with diverse physicochemical properties (octanol/water partitioning coefficient (log P), pKa, acid/base status). The binding parameters (B max and K d ) estimated from Sips isotherm where the actual distribution of binding energies is most likely continuous and unimodal. Binding parameters were calculated based on experimental data and terazosin is the most potent melanin binder by having K value of 13.30 (K = B max /K d n ). In addition, pH has quite large effect on binding for example binding affinity and capacity of levofloxacin at pH 5 is almost as three times as pH 7.4. This study demonstrates the importance of melanosomal pH on melanin binding of drugs and a useful step toward building a bottom‐up pharmacokinetic model for ocular drug delivery.