Nitric oxide release from a biodegradable cysteine-based polyphosphazene

Nitric oxide (NO) is a unique bioactive molecule that performs multiple physiological functions and has been found to exhibit antithrombotic, antimicrobial, and wound-healing effects as an exogenous therapeutic agent. NO release from polymeric materials intended for use in biomedical applications has been established to reduce their thrombogenicity and decrease the likelihood of infection and inflammation that frequently produce medical complications. As a result, numerous NO-releasing polymers have been developed in an effort to utilize the beneficial properties of NO to improve the performance of implantable materials. The majority of synthetic NO-releasing biodegradable polymers that have been reported to date are polyesters, and there is significant interest in the development of new NO-releasing materials with improved or distinctive physicochemical characteristics. Polyphosphazenes are polymers with inorganic phosphorus-nitrogen backbones, and hydrolytically-sensitive derivatives with organic substituents have been prepared that degrade under physiological conditions. For this reason, biodegradable poly(organophosphazenes) are interesting candidate materials for applications such as tissue engineering, where the addition of NO release capability may be therapeutically useful. Herein, we report the first development and characterization of an NO-releasing poly(organophosphazene) from poly(ethyl S-methylthiocysteinyl-co-ethyl cysteinyl phosphazene) (POP-EtCys-SH). The thiolated polymer was synthesized from the reaction of poly(dichlorophosphazene) with ethyl S-methylthiocysteinate, followed by partial cleavage of the disulfide linkages to form free thiol groups. The conversion of thiol to the NO-releasing S-nitrosothiol functional group with tert-butyl nitrite resulted in a polymer (POP-EtCys-NO) with an average NO content of 0.55 ± 0.04 mmol g^-1 that was found to release a total of 0.35 ± 0.02 mmol NO g^-1 over 24 h under physiological conditions (37 °C, pH 7.4 phosphate buffered saline). Extracts obtained from both the thiolated and S-nitrosated polymers were not found to significantly impair the viability of human dermal fibroblasts or induce morphological changes, indicating that this cysteine-based polyphosphazene may possess potential utility as an NO-releasing biomaterial.