Bone regeneration by tuning the drug release from the calcium phosphate scaffolds

Abstract

Bone is among the most transplanted tissues for regeneration of bone defects, caused by trauma, age-related bone loss or bone infections. Due to their similar characteristics to the mineral phase of bone, calcium phosphates (CaPs) have raised a lot of interest. Some properties of CaPs, like biodegradability, biocompatibility, bioactivity and osteoconduction represent a great potential for this application. Among them, calcium phosphate cements (CPCs) have additional advantages like injectability and in situ hardening ability. Moreover, the possibility to tune the porosity of CaPs in general and of CPCs in particular, makes them suitable vehicles for local delivery of drugs. Loading CaPs with drugs allows conferring additional functionalities to the synthetic bone grafts, which is of great interest. The main aim of this thesis is to explore CaP bioceramics as vehicles for local delivery of drugs, covering both low temperature biomimetic ceramics, like calcium deficient hydroxyapatite (CDHA), and high temperature sintered ceramics, like beta tricalcium phosphate (s-TCP), in the form of microporous and macroporous substrates. The physic-chemical nature of these bioceramics, their porosity and textural properties plays an essential role in their drug delivery properties. In order to be able to tailor the drug release kinetics of the bioceramics beyond their intrinsic properties, plasma polymerization has been investigated. Plasma is a particular state of a gas, electrically neutral, which can be employed in different applications. Although plasma polymerization has been widely studied for biomedical applications, its combination with bioceramics is rather unexplored. To select a suitable drug for bone regeneration, an extensive literature review was done on statins, and more particularly on simvastatin as a potential osteogenic and angiogenic promoter. This drug was evaluated within macroporous scaffolds of either CDHA or s-TCP as drug delivery vehicles. The drug-loaded materials were plasma-coated with polycaprolactone: poly ethylene glycol (PCL:PEG) co-polymers. The coating covered the micro and nanopores of the CaPs surface and produced complex geometries presenting a nano and micro rough morphology which led to low wettability despite the hydrophilicity of the copolymer. Plasma coating with PCL-co-PEG on scaffolds loaded with simvastatin acid allowed delaying and modulating the drug release from the bone scaffolds depending on the thickness of the layer deposited, which, in turn depended on the initial specific surface area of the CaP. To further investigate the fundamentals of plasma polymerization on bioceramics, PEG-like polymer coatings of different thickness were deposited on microporous s-TCP loaded with antibiotics. The rough s-TCP surface was associated to strong hydrophobic surface properties, which nevertheless retained their suitable biological behavior with regard to human osteoblast cells. The microbiological activity of the antibiotics was preserved, and the coatings reduced the total amount of drug released as a function of the increasing plasma treatment time. In another approach, a statin that had never before been employed in combination with CaPs, Pitavastatin (PITA), was investigated as potentially osteogenic and angiogenic promoter through in vitro studies which revealed dose-dependent enhancement of mineralization and vascularization. The incorporation of PITA to the liquid phase of an injectable CDHA foam allowed obtaining injectable local drug delivery scaffolds, without altering their macroporosity or textural properties. The drug release kinetics was affected by the evolving microstructure of the setting of the macroporous cement. Overall the results obtained proved that PITA seems to be a suitable novel candidate to enhance the osteogenic potential of synthetic bone grafts and identified the required doses to obtain the desired biological effects.