Ocular and systemic bioavailability of ophthalmic flurbiprofen

Flurbiprofen, a nonsteroidal antiinflammatory agent which is not ocularly metabolized, was employed as a probe compound to investigate the drug kinetic relationship between systemic and ocular humoral circulation. The ocular and systemic bioavailabilities of topically applied flurbiprofen were also quantitated. Anesthetized albino female rabbits received flurbiprofen doses intracamerally, topically, and intravenously at 2 to 4 week intervals. Aqueous humor and plasma were used as the sampling compartments. Plasma clearance values of flurbiprofen were 6.77 and 7.87 ml/min, after 6-mg and 208-micrograms intravenous doses, respectively. These values were not significantly different and indicated no dose-dependent disposition kinetics over a 30-fold dose range. Both ocular and systemic flurbiprofen dispositions followed a biexponential pattern with a rapid distribution phase. The systemic and ocular distribution half-lives of flurbiprofen were 12 min and 15 min, respectively. The plasma elimination half-life was 74 min and the aqueous humor elimination half-life was 93 min. The latter approximated the turnover rate of aqueous humor and suggested that aqueous humor drainage was the major process of flurbiprofen elimination from the globe. About 99% of flurbiprofen is bound to plasma protein. At distribution equilibrium, the plasma and aqueous humor concentrations of flurbiprofen differed by a hundredfold, suggesting that only free drug entered the aqueous humor after the administration of a systemic dose. In the ophthalmic studies, right eyes were instilled with 50 microliters of 0.3% flurbiprofen in saline (dose = 150 micrograms), and left eyes were instilled with 50 microliters of 0.15% flurbiprofen in saline (dose = 75 micrograms). When the area of the aqueous humor concentration-versus-time curve values was normalized by the administration dose, the 75-micrograms dose was 30% more available to ocular tissues than was the 150-micrograms dose. This demonstrated a disproportionate relationship between the administered dose and the fraction absorbed. The intracameral dose was considered to be completely bioavailable for intraocular effects. The ocular bioavailability of the ophthalmic dose was defined by using intracameral administration as a standard measurement. The ocular bioavailabilities of the 75-micrograms and 150-micrograms topical flurbiprofen doses were 10% and 7%, respectively. Systemic bioavailability after topical administration of 225 micrograms of flurbiprofen was 74%.