Aqueous chamber drug distribution volume measurement in rabbits

A method was developed for aqueous chamber drug distribution volume measurement in the albino rabbit, and the apparent volume of distribution was determined for inulin, pilocarpine alkaloid, and 1-hexanoic acid. The method consists of injecting a suitable concentration of drug, in an appropriate volume of fluid, into the anterior chamber of the eye and monitoring the decline in drug concentration as a function of time by periodic sampling of the aqueous humor. Graphical analysis of the resulting data yields both the apparent volume of distribution and the turnover rate constant of the aqueous humor. The technique does traumatize the eye, causing formation of plasmoid aqueous, which does not interfere with the apparent drug distribution volume measurement or the determination of aqueous humor turnover. Inulin was used to determine the physiological aqueous volume, 287 mul, in good agreement with literature values. The turnover rate constant was 0.016 min-1, also in good agreement with literature values. The apparent volume of distribution for pilocarpine alkaloid was 575 mul in albino eyes and 760 mul in pigmented irides; for 1-hexanoic acid in albino eyes, it was 760 mul. For pilocarpine alkaloid, literature citations on the fraction of dose absorbed have been based on an assumed apparent volume of distribution of 250-300 mul. Therefore, a factor of two error has been introduced when using albino eyes and a factor of almost three has been introduced when using pigmented eyes. The implication of the apparent volume of distribution for pilocarpine in its ocular disposition is discussed, as is the unexpected observation that pilocarpine alkaloid apparently inhibits formation of plasmoid aqueous and follows one-compartment kinetics during these studies. Is is shown that the one-compartment kinetics for pilocarpine are due to its biological activity in the aqueous chamber.