Vehicle effects on ocular drug bioavailability III: Shear-facilitated pilocarpine release from ointments

Pilocarpine release from water-in-oil emulsion ointments was studied in vitro and in vivo, using albino rabbits. Pilocarpine release from the vehicle to the ocular fluids was dependent on shear, i.e.,blinking, and the dosing system emulsifying efficiency. A mechanical shearing component was vital for correlating corneal drug penetration and the in vitro pilocarpine release pattern. Simple diffusion studies with the vehicles did not predict drug in vivo release, but the ointment systems were all superior to an aqueous pilocarpine solution. Incorporation of a mechanical shearing component to mimic blinking gave good correlation of in vitro and in vivo results. Also, increasing the vehicle emulsifying efficiency by surfactant addition decreased shear-facilitated drug release and in vivo performance. Finally, increasing the internal aqueous phase volume fraction decreased in vivo performance and was linked to the influence of effective drug concentration in the vehicle.