The pharmacokinetics of ocularly applied timolol in rabbits

[3H]timolol derived radioactivity and beta-antagonistic activity in the aqueous humour, iris, ciliary body and blood in rabbits were analyzed by liquid scintillation counting (LSC) and radioreceptor assay (RRA) after ocular timolol. After ocular application of tracer labelled timolol, the radioactivity decreased slowly in the iris (Kel = 0.05 h-1) and ciliary body (Kel = 0.02 h-1). In the aqueous humour the elimination rate was faster (Kel = 0.57 h-1). The beta-antagonistic activities of the aqueous humour and plasma decreased rapidly when compared to LSC values. The ethanol extractable pool of the beta-antagonistic activity from the iris and ciliary body was only a few percent of the corresponding LSC estimates. The kinetics of beta-antagonistic activity extractable from iris and ciliary body was identical with that in the aqueous humour. Timolol concentrations in the untreated rabbit eye were sufficiently high to lower the intraocular pressure.