Vehicle effects on ocular drug bioavailability II: Evaluation of pilocarpine

The influence of vehicle composition on ocular penetration of pilocarpine was studied in the albino rabbit. Increasing the pH of a vehicle promoted increased corneal penetration for pilocarpine, in accordance with the pH-partition hypothesis, but a similar series of experiments with a nonionizable drug, glycerin, gave similar results. The extent of pH-induced lacrimation by the vehicle and its effect on precorneal drug concentration also was determined. Increased pilocarpine absorption at neutral to slightly alkaline pH was due primarily to its peculiar solubility characteristics coupled with less irritation and lacrimation rather than a direct pH effect on the molecule. Incorporation of pilocarpine into a petrolatum-based ointment vehicle resulted in increased aqueous humor pilocarpine levels above those provided by an equivalent dose of aqueous solution. The mechanism of this increase was determined to be a higher effective concentration of pilocarpine in the ointment vehicle coupled with an increase in contact time of the dose. The ointment system also exerted an unusual form of vehicle control in that it promoted the corneal penetration of pilocarpine while impeding uniform mixing of the dose with tears and thereby imposed a restriction on the amount of pilocarpine available to the ocular tissues.