Drug Delivery Systems
A study of HAP implants for the treatment of bone infections was recently performed in collaboration with Professor A. K. Dash from the Department of Pharmaceutical and Administrative Sciences at Creighton University. HAP micro-spheres containing ciprofloxacin were prepared by precipitation followed by a spray drying process. The drug load in the micro-spheres ranged from 0.25-2% (w/w). SEM, powder X-ray diffraction, and Differential Scanning Calorimetry showed that the surface morphology of the micro-spheres was rough and that the drug was present at a molecularly dispersed state in the HAP. Implants (5x8 mm) were made by pressing the powder into small cylinders. Th e mechanism of in vitro drug release from the implant was found to be controlled by diffusion in the matrix. For this set of experiments, complete release of the drug occurred within 10 hours (see Fig. 1). The release rate can be increased by increasing t he concentration of the drug and decreased by introducing diffusion barriers in the implants. Further information on this study can be found in "Preparation and characterization of novel drug delivery implants for the treatment of bone infections", by H. H. Pham, P. Luo, F. Génin, and A. K. Dash, Proceedings of the AAPS Meeting (New Orleans, November 1999).
Figure 1: In vitro release of ciprofloxacin from HAP implants containing; () 2% (w/w), () 1% (w/w), and () 0.5% (w/w) of theoretical drug load.
Other examples of drug delivery systems:
Other examples of proteins and HAP:
Calcium phosphate apatite (CPA) is known to be one of the most important implantable materials due to its biocompatibility. Natural bone is approximately 70% CPA including hydroxyapatite (HAP) by weight and 50% by volume. Organic substances (such as co llagen) and water are the other constituents of bones. The CPA mineral is nanocrystalline with grains less than 50 nm. These nanocrystals are connected to each other to form the connective hard tissue, (i.e. the bone skeleton).
Bone is a complex structure with macro- and micro-pores. The pores are mostly interconnected to allow body fluid to carry nutrients and provide a medium where interfacial reactions between hard tissue and soft tissue can occur. Two types of bone struct ures have been described: cancellous and cortical structures. Cancellous bone differs from cortical bone in being open-spaced and trabecular. Since osteons average between 180 and 250 Ám in diameter and intercommunicate through Volksmann canals, the size of the interconnected pores have similar dimensions.
Implant materials must provide properties that are similar to that of bone. Currently, only natural materials have been selected as bone substitute for bone grafting surgery. These include auto- or allograft, bovine, and coral blocks. Although these ma terials can closely replicate the structure of human bone, they present a number of disadvantages. They are typically very brittle and experience significant loss of strength during sterilization. They also can cause inflammation problems and carry the po tential for disease transmission.
To address these issues, our research is focussing in developing high strength synthetic biomaterials such as CPA and HAP ceramics. The ceramics are either sintered porous, or dense calcium-based mixtures (including HAP and TCP). Our research team rece ntly completed an extensive characterization of the sintering behavior of our narrow size distribution powder. We also have completed a study of the phase transformations that can occur in our sintered micro-porous HAP. Our research team is now designing high strength three-dimensional cellular biomaterials.
Ping Luo, Nan Liu, and Michael Thelen, "Protein binding potential of biocompatible ceramic hydroxyapatite micro-spheres", 24th Annual Meeting of the Society for Biomaterials, Vol. 21, 278 (1998).
Hai H. Pham, Ping Luo, François Génin, and Alekha K. Dash, " Preparation and characterization of hydroxyapatite microspheres containing ciprofloxacin", AAPS Annual Meeting, New Orleans, November 14-18 1999.
P. Luo, F. Y. Génin, and T. G. Nieh, Synthetic calcium phsophate-based skeletons, Proceedings of the Fifth World Biomaterials Congress, Toronto, 627 (1996).
Ping Luo, "Methods of Treating Nuclear Hydroxyapatite Materials", US Patent 5994609, Nov. 1999
Ping Luo, "Method of Synthesizing Hydroxyapatite Powders and Bulk Materials", US patent 5858318, Jan. 1999
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