Polyhydroxyalkanoates (PHAs) are natural biopolymers produced by various microorganisms as a reserve of carbon and energy. PHA synthesis generally occurs during fermentation under nutrient limiting conditions with excess carbon. There are two main types of PHAs, short chain length PHAs (scl-PHAs) and medium chain length PHAs (mcl-PHAs). The mechanical and thermal properties of PHAs depend mainly on the number of carbons in the monomer unit and its molecular weight. PHAs are promising materials for biomedical applications because they are biodegradable, non-toxic and biocompatible. The large range of PHAs, along with their varying physical properties and high biocompatibility, make them highly attractive biomaterials for use in drug delivery. They can be used to produce tablets, micro-and nanoparticles as well as drug eluting scaffolds. A large range of different PHAs have been explored and the results obtained suggest that PHAs are excellent candidates for controlled and targeted drug delivery systems.

This study deals with the process of optimization and synthesis of Poly(3-hydroxybutyrate) microspheres with encapsulated Cl-amidine. Cl-amidine is an inhibitor of peptidylarginine deiminases (PADs), a group of calcium-dependent enzymes, which play critical roles in a number of pathologies, including autoimmune and neurodegenerative diseases, as well as cancer. While Cl-amidine application has been assessed in a number of in vitro and in vivo models; methods of controlled release delivery remain to be investigated. P(3HB) microspheres have proven to be an effective delivery system for several compounds applied in antimicrobial, wound healing, cancer, and cardiovascular and regenerative disease models. In the current study, P(3HB) microspheres with encapsulated Cl-amidine were produced in a size ranging from ~4–5 µm and characterized for surface morphology, porosity, hydrophobicity and protein adsorption, in comparison with empty P(3HB) microspheres. Cl-amidine encapsulation in P(3HB) microspheres was optimized, and these were found to be less hydrophobic, compared with the empty microspheres, and subsequently adsorbed a lower amount of protein on their surface. The release kinetics of Cl-amidine from the microspheres were assessed in vitro and expressed as a function of encapsulation efficiency. There was a burst release of ~50% Cl-amidine in the first 24 h and a zero order release from that point up to 16 days, at which time point ~93% of the drug had been released. As Cl-amidine has been associated with anti-cancer effects, the Cl-amidine encapsulated microspheres were assessed for the inhibition of vascular endothelial growth factor (VEGF) expression in the mammalian breast cancer cell line SK-BR-3, including in the presence of the anti-proliferative drug rapamycin. The cytotoxicity of the combinatorial effect of rapamycin with Cl-amidine encapsulated P(3HB) microspheres was found to be 3.5% more effective within a 24 h period. The cells treated with Cl-amidine encapsulated microspheres alone, were found to have 36.5% reduction in VEGF expression when compared with untreated SK-BR-3 cells. This indicates that controlled release of Cl-amidine from P(3HB) microspheres may be effective in anti-cancer treatment, including in synergy with chemotherapeutic agents. Using controlled drug-delivery of Cl-amidine encapsulated in Poly(3-hydroxybutyrate) microspheres may be a promising novel strategy for application in PAD-associated pathologies.

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