Abstract
Introduction: The biological activity of autologous grafts is due to a number of proteins (growth factors) that control bone cell differentiation, proliferation and expression. Several of these have been isolated including; bone morphogenetic proteins 2 and 7. These are commercially available and regularly used with the intention of accelerating fracture healing, repairing critical sized defects and combating bone mineral loss. Whilst it is commonly recognised that multiple growth factors are present at differing times in the healing cascade, the usual delivery, both in the clinic and the laboratory, is of one growth factor delivered over a very short and early time period. Commonly growth factors are delivered in solution or from a collagen sponge and are quickly metabolised in the proteolytic wound healing environment. The physiological need for BMPs is later than the acute delivery at the time of surgery. The aim of this study is to develop a granular protein delivery system that enables controlled release of multiple proteins at a variety of time points.
Methods: A series of homogenous polymer granules 8mm3 were prepared by photo-polymerising 12uL of mixtures of methacrylated adipic acid anhydride (MAAA) and methyl methacrylate (MMA) or MAAA and butyl methacrylate (BMA) with molar ratios ranging from 100- 55 % (MSAA). Into each granule 5ug of a model drug, carmoisine was loaded and 1%w/w of 2,2-dimethoxy-2-phenyl-acetophenone (DMPA) photoinitiator was added per granule. The granules were exposed to UV light at 390nm for 14 minutes. Multilayered granules were prepared photo-polymerising 4uL layers of different monomer compositions in a similar method to the single layered method above. The composition of the multilayered granules was chosen to optimise the release profile. Carmoisine release profiles were determined by UV-visible spectroscopy.
Results: Homogenous granules composed of 100% MAAA released 90% of their payload by 24hrs, those composed of 90:10 MAAA:MMA released by 48hrs those composed of 70:30 MAAA:MMA released by 80hrs those composed of 60:40 MAAA:MMA released by 170hrs those composed of 70:30 MAAA: BMA released by 288hrs and those composed of 60:40 MAAA:BMA released by 456hrs. The multilayered granule had a sustained release of the model drug over the test period of 19 days.
Discussion: The limitation of most drug delivery systems, such as microspheres or collagen, is poor control over the release profile. The drug is ether released instantly or well after it is required. This multilayered composite drug delivery system enables the controlled release of different bioactive compounds at different time points between 0 and 19 days. By altering the drug loading in each layer we were able to sustain the release of one compound over this time period. This technology enables us to switch compounds at a given time points for example delivery of angiogenic factors for one week, proliferative factors for the second week and differentiation factors for the third week. This technology enables the pre-programmed release of multiple growth factors at times in the healing cascade when they meet the physiological need. A controlled release of growth factors at the appropriate time should improve bone healing rates.
Correspondence should be addressed to Dr Owen Williamson, Editorial Secretary, Spine Society of Australia, 25 Erin Street, Richmond, Victoria 3121, Australia.
