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Drug Distribution to Retinal Pigment Epithelium: Studies on Melanin Binding, Cellular Kinetics, and Single Photon Emission Computed Tomography/Computed Tomography Imaging
journal contribution
posted on 2016-01-07, 00:00 authored by Anna-Kaisa Rimpelä, Mechthild Schmitt, Satu Latonen, Marja Hagström, Maxim Antopolsky, José A. Manzanares, Heidi Kidron, Arto UrttiMelanin
binding is known to affect the distribution and elimination
of ocular drugs. The purpose of this study was to evaluate if the
extent of drug uptake to primary retinal pigment epithelial (RPE)
cells could be estimated based on in vitro binding
studies with isolated melanin and evaluate the suitability of single
photon emission computed tomography/computed tomography (SPECT/CT)
in studying pigment binding in vivo with pigmented
and albino rats. Binding of five compounds, basic molecules timolol,
chloroquine, and nadolol and acidic molecules methotrexate and 5(6)-carboxy-2′,7′-dichlorofluorescein
(CDCF), was studied using isolated melanin from porcine choroid-RPE
at pH 5.0 and 7.4. The uptake to primary porcine RPE cells was studied
with timolol, chloroquine, methotrexate, and CDCF. The cell study
setting was modeled using parameters from the in vitro binding study. In vivo kinetics of 3-[I-123]-iodochloroquine
was studied by the SPECT/CT method in albino and pigmented rats. All
basic compounds bound to melanin at both pH values, whereas the acidic
compounds bound more at pH 5.0 than at pH 7.4. The basic compounds
(chloroquine, timolol) showed significant cellular uptake, unlike
the acidic compounds (methotrexate, CDCF). On the basis of the modeling,
melanin binding was a major factor governing the overall drug distribution
to the RPE cells. Likewise, melanin binding explained distribution
of 3-[I-123]-iodochloroquine in the pigmented RPE, whereas drug accumulation
was not seen in the albino rat. This study demonstrates the suitability
of noninvasive SPECT/CT imaging in monitoring ocular melanin binding in vivo. These studies are a useful step toward understanding
the pharmacokinetic impact of melanin binding.