Polymer effects on ocular bioavailability: the influence of different liquid vehicles on the mydriatic response of tropicamide in humans and in rabbits

Abstract Five viscous polymeric vehicles containing 0.2% tropicamide were tested for mydriatic activity in humans and in rabbits. The polymers were carboxymethylcellulose (CMC), low molecular weight hydroxypropylcellulose (HPCL). medium molecular weight hydroxypropylcellulose (HPCM). poly(vinylpvrrolidone) (PVP) and poly(vinyl alcohol) (PVA), Their concentrations were adjusted in order to give iso-viscous (70 ± 2 cps) Newtonian solutions (HPCL, PVP, PVA) or pseudoplastic solutions with an apparent viscosity of 70 cps at a rate of shear of 700 s −1 (CMC. HPCM). All vehicles increased the ocular bioavailability of the drug in both species, when compared with a non-viscous solution. However, in humans PVP and PVA were significantly more active with respect to the other polymers. The different activity of the polymeric vehicles in humans could neither be correlated with drug binding, nor with surface or interfacial tension, nor with the rheological behaviour of the solutions (Newtonian vs pseudoplastic). On the basis of experimental evidence, the hypothesis is advanced that some polymers may increase the ocular bioavailability of tropicamide not by viscous effects aione, but also by influencing the spreading characteristics and the thickness of the medication layer over the precorneal area. The reasons for the interspecies differences in activity, presumably residing in different precorneal dynamics of the applied solution, are also discussed.