Preparation and characterisation of polyphosphazene-based controlled release systems for naproxen.

A new polyphosphazene polymer suitable for preparation of naproxen controlled release systems has been prepared by non complete substitution of the chlorine at the phosphorus atoms of polydichlorophosphazene with phenylalanine ethyl ester and imidazole. Polymeric disks obtained by solvent evaporation were found to maintain their integrity for months either "in vivo" and "in vitro" after incubation in buffer, although the intrinsic viscosity undergoes a rapid decrease. The release rate of naproxen from the films was found to depend on thickness of the matrixes and amount of entrapped drug. "In vitro" release spikes, corresponding to initial in plasma burst after subcutaneous implantation of the films in rats, have been observed with high concentrations of drug in the matrixes. On the other end decreasing the drug concentrations in the polymer matrixes, the naproxen release approximates a zero order kinetic and a controlled release is obtained for weeks. Scanning electron microscopy showed that the disks giving spikes in drug release are characterised by the presence of naproxen crystals on the matrix surface. "In vivo" studies are demonstrating that the initial naproxen concentration spike is useful in the treatment of acute models of inflammation, while a more constant and lasting release is successful in chronic inflammation models. The degradation profile of this new polyphosphazene as well as preliminary pharmacological data are also reported.