Timolol binding to bovine ocular melanin in vitro

Area of Science:

• Pharmacology
• Biochemistry
• Ocular Science

Background:

• Melanin in the iris plays a role in drug absorption and distribution.
• Understanding drug-melanin interactions is crucial for ocular drug development.

Purpose of the Study:

• To characterize the binding kinetics and affinity of 3H-timolol to bovine iris melanin.
• To investigate the influence of various factors on timolol-melanin binding.

Main Methods:

• Ligand binding assay using 3H-timolol and bovine iris melanin.
• Kinetic studies for association and dissociation rates.
• Saturation experiments to determine binding capacity and affinity.
• Evaluation of pH, ethanol, isoxuprine, and propranolol enantiomers' effects.

Main Results:

• Timolol binding to melanin was saturable, with a binding maximum of approximately 1 microM timolol/g melanin.
• Binding increased over 5 hours, reaching 24% of added radioactivity.
• Association rate constants were determined using a two-fit model.
• Dissociation was rapid and uniphasic.
• Isoxuprine significantly diminished timolol binding, whereas pH, ethanol, and propranolol enantiomers did not.

Conclusions:

• Bovine iris melanin exhibits saturable binding for timolol.
• Isoxuprine interferes with timolol binding to melanin.
• These findings contribute to understanding timolol's pharmacokinetic behavior in the eye.