Binding Affinity of Bunazosin, Dorzolamide, and Timolol to Synthetic Melanin

Purpose

The purpose of this study was to compare the binding affinity of bunazosin and dorzolamide to synthetic melanin relative to that of timolol.

Methods

Synthetic melanin was prepared from dopa by the action of tyrosinase. Timolol, dorzolamide, and bunazosin were incubated separately at a concentration of 10⁻⁴ M in 2 ml of 0.066 M phosphate buffer containing 5 mg of synthetic melanin. After centrifugation, the absorbance of each free drug in the supernatant was measured at its optimum wavelength. The percentage of each drug bound to melanin was calculated directly from the change in absorbance relative to the initial value.

Results

The increase in the binding rates of all three drugs seemed to reach a plateau after 30 min. After incubating for 60 min, the binding rate of timolol was 22.2% ± 4.9%, bunazosin 36.3% ± 2.5%, and dorzolamide 8.5% ± 1.9%. There were statistically significant differences between the binding rates of each drug.

Conclusions

Under our study conditions, the order of binding affinity of these ocular hypotensive agents to synthetic melanin seems to be as follows: bunazosin ≫ timolol ≫ dorzolamide. Jpn J Ophthalmol 2004;48:34–36 © Japanese Ophthalmological Society 2004