Penetration of timolol into the rabbit eye. Measurements after ocular instillation and intravenous injection.

The distribution of radioactivity in 15 ocular and extraocular tissues or fluids of the rabbit after ocular instillation and intravenous injection of 100 microL of 14C-labeled timolol solution (1.5%) was studied by liquid scintillation counting (LSC). The drug was distributed rapidly among the tissues examined after both routes of administration. Levels of radioactivity were considerably greater in ocular tissues after instillation as compared with intravenous injection, whereas in extraocular tissues the levels were similar after both routes of administration. Using the same experimental protocol, aqueous humor and serum levels of unchanged timolol were measured by gas liquid chromatography. Comparisons with LSC studies indicated that, following ocular instillation, only unchanged timolol was present in aqueous humor, whereas both timolol and metabolites were present in serum. After intravenous administration, timolol was rapidly metabolized and metabolites appeared in the serum and aqueous humor.