Kinetics and Thermodynamics of Drug Permeation Through Silicone Elastomers iv. Effect of Polymer Backbone Structure

Abstract

The effect of silicone polymer backbone structure on the permeability of progesterone and testosterone through silicone membranes was investigated in the capsule-type drug delivery system at 25, 37 and 50°C, respectively. Three types of polymer were studied: (I) (Me₂SiCH₂)_x, (II) (Me₂Sl-CH₄-SiMe₂O)_x, and (III) [Me₂Si(CH₂)_n SiMe₂O] _x, where n=2, 6 and 8.

Permeabilities of progesterone in these polymers were found to decrease in the following order: Polymer (III) < Polymer (I) < Polymer (II). The same order was also observed for the permeabilities of testosterone, but their values were one order of magnitude lower than those of progesterone. Incorporation of the phenylene group in the polymer backbone in Polymer (II) caused a significant decrease in the diffusion coefficient but an increase in the solubility coefficient. The net effect is a 50% decrease in the permeability with respect to (Me₂ SiO)_x. This effect was found to be less pronounced with alkylene group in Polymer (III).

The activation energies of permeation for progesterone through these polymers were found to be in the range of 12-21 kcal/mole.