Binding of betaxolol, metoprolol and oligonucleotides to synthetic and bovine ocular melanin, and prediction of drug binding to melanin in human choroid-retinal pigment epithelium.
OBJECTIVE
To characterize the binding of betaxolol, metoprolol and oligonucleotides to synthetic and bovine ocular melanin, and to predict the binding to melanin in human choroid-retinal pigment epithelium (RPE).


METHODS
The shape, size and specific surface area of synthetic melanin and isolated melanin granules from bovine choroid-retinal pigment epithelium (RPE) were characterized by SEM, laser diffractometry and BET. The binding of betaxolol, metoprolol, fluorescein isothiocyanate (FITC)-labeled phosphodiesther oligonucleotides and 6-carboxyfluorescein (6-CF) to melanin was determined. The binding of beta-blockers to melanin in human choroid-RPE was estimated based on binding parameters and the melanin content in human choroid-RPE.


RESULTS
Bovine melanin granules were round or oval with a mean diameter of ca. 1 mum. Synthetic granules were slightly smaller and irregular and had a two times higher specific surface area than bovine melanin. Synthetic melanin bound more betaxolol and metoprolol than bovine melanin and both melanin types showed a high affinity and a low affinity binding sites. The human choroid-RPE was predicted to contain 3-19 times more melanin bound drug than unbound drug at typical therapeutic concentrations (1-1,000 ng/ml). FITC-labeled oligonucleotides and 6-CF did not bind to melanin.


CONCLUSIONS
The binding of lipophilic drugs to biological melanin differs from that of synthetic melanin. Lipophilic beta-blockers are expected to bind significantly to melanin in human choroid-RPE: only a small fraction of the drug being in active free form. In contrast, phosphodiesther oligonucleotides do not seem to bind to melanin.
Pitkänen
Leena
L
Department of Pharmaceutics, University of Kuopio, P.O. Box 1627, Kuopio, 70211, Finland. Leena.Pitkanen@uku.fi
Ranta
Veli-Pekka
VP
Moilanen
Hanna
H
Urtti
Arto
A
D000319:Adrenergic beta-Antagonists / Q000378:metabolism*; D000818:Animals; D015784:Betaxolol / Q000378:metabolism*; D002417:Cattle; D002829:Choroid / Q000378:metabolism*; D006801:Humans; D008543:Melanins / Q000378:metabolism*; D008790:Metoprolol / Q000378:metabolism*; D009841:Oligonucleotides / Q000378:metabolism*; D010857:Pigment Epithelium of Eye / Q000378:metabolism*
D000319:Adrenergic beta-Antagonists; D008543:Melanins; D009841:Oligonucleotides; D008790:Metoprolol; D015784:Betaxolol