Ocular absorption of propranolol in rabbits

Microdialysis evaluation of the ocular pharmacokinetics of propranolol in the conscious rabbit

PURPOSE

This study was conducted to assess the effects of anesthesia and aqueous humor protein concentrations on ocular disposition of propranolol.

METHODS

Rabbits were anesthetized and a microdialysis probe was inserted into the anterior chamber of one eye; the contralateral eye served as a control. At timed intervals after probe placement, a 100-microl sample of aqueous humor was aspirated from each eye to determine protein concentration. In vitro protein binding parameters were used to simulate the impact of protein concentration on propranolol disposition. To assess the influence of anesthesia, probes were implanted in the anterior chamber of each eye. After >5-day stabilization, conscious and anesthetized rabbits (n = 3/group) received a 200-microg topical dose of [3H]DL-propranolol in each eye; propranolol was assayed in probe effluent.

RESULTS

Changes in aqueous humor protein concentrations were observed following probe insertion. Simulations demonstrated that the unbound propranolol AUC (approximately 2.4-fold) in aqueous humor should be reduced due to protein influx. Intraocular propranolol exposure in anesthetized rabbits was approximately 8-fold higher than in conscious rabbits, and approximately 1.9-fold higher than in rabbits without a post-surgical recovery period.

CONCLUSIONS

Anesthesia and time-dependent aqueous humor protein concentrations may alter ocular pharmacokinetics, and must be taken into account in the design of microdialysis experiments.

Evaluation of microdialysis sampling of aqueous humor for in vivo models of ocular absorption and disposition

The dynamics of beta-adrenergic-associated reductions in aqueous humor production for treatment of elevated intraocular pressure are not well understood. In particular, the relationship between ocular pharmacokinetics and pharmacodynamics has yet to be established. This study was undertaken to develop a procedure for examining the ocular absorption and disposition of topically administered ophthalmic beta-adrenergic antagonists in individual animals. Dogs were anesthetized with isoflurane and a microdialysis probe was implanted in the anterior chamber of one eye and perfused with 0.9% saline at a rate of 2 microliters min-1. 3H-propranolol was administered by intracameral injection or topically. Each dog received intracameral and topical propranolol, in alternate eyes on separate days, in a randomized cross-over fashion. Microdialysis probe effluent was collected every 5 min for > or = 2.5 h; concentrations of propranolol were determined by liquid scintillation spectroscopy and were corrected for probe recovery of the substrate as determined by in vivo retrodialysis (approximately 46%) to estimate aqueous humor concentrations. In separate experiments in rabbits, microdialysis probes were implanted in each eye. 3H-propranolol was administered topically to one eye; the contralateral eye received intracameral 3H-propranolol. Model-independent pharmacokinetic parameters for each treatment phase were calculated. The mean +/- S.D. times to peak concentration of propranolol in aqueous humor were 86.6 +/- 47.6 min in the dog and 54.1 +/- 20.4 min in the rabbit. The terminal rate constant was 0.0189 +/- 0.00429 min-1 in the dog vs. 0.00983 +/- 0.00546 min-1 in the rabbit. Intraocular tissue availability of propranolol differed markedly between the dog (n = 3) and rabbit (n = 3) (approximately 0.056 in the dog vs. approximately 0.55 in the rabbit). These results demonstrate the utility of microdialysis sampling for examination of ocular pharmacokinetics.