A novel skeletal drug delivery system using self-setting bioactive glass bone cement. III: the in vitro drug release from bone cement containing indomethacin and its physicochemical properties

In vitro release kinetics and physical, chemical and mechanical characterization of a POVIAC®/CaCO3/HAP-200 composite

Coralline calcium-hydroxyapatite and calcium carbonate from Porites Porites coral were added to a polymeric matrix based on polyvinyl acetate (POVIAC(®)), to obtain a novel bone substitute composite as well as a system for the controlled drug (cephalexin) release. Composite samples with different compositions were characterized by physical-chemical and mechanical methods. Furthermore, the in vitro release profile of cephalexin and the kinetic behavior of its release from these composites were analyzed by appropriate mathematical models. It was shown that there is no chemical interaction between the inorganic filler and the polymer matrix, each conserving the original properties of the raw materials. The compressive mechanical strength and Young modulus of the composite with 17.5% of POVIAC(®), has better mechanical properties than those of cancellous bone. The variation of POVIAC(®) content can affect the cephalexin release kinetic in the composite. The cephalexin release mechanism from the composites can be considered as the result of the joint contribution of a prevailing Fickian diffusion and of polymer chain relaxation. It was also demonstrated that cephalexin is occluded inside the composites and not on their surface.

Cephalexin-loaded injectable macroporous calcium phosphate bone cement

Different types of calcium phosphate cements (CPCs) have been studied as potential matrices for incorporating different types of antibiotics. All of these matrices were morphologically microporous whereas macroporosity is essential for rapid cement resorption and bone replacement. In this study, liberation of cephalexin monohydrate (CMH) from a macroporous CPC was investigated over 0.5-300 h in simulated body fluid and some mathematical models were fitted to the release profiles. Macroporosity was introduced into the cement matrix by using sodium dodecyl sulfate molecules as air-entraining agents and the effect of both surfactant and CMH on basic properties of the CPC was studied. Incorporation of CMH into the CPC composition increased the setting time, decreased the crystallinity of the formed apatite phase, and improved the injectability of the paste. The use of both CMH and sodium dodecyl sulfate did not affect the rate of conversion of the reactants into apatite phase while soaking the cements in simulated body fluid. Results showed that the liberation rate of the drug from porous CPC was higher than that of the nonporous CPC but same release patterns were experienced in both types of cements, that is, like to nonporous CPC, a time-dependent controlled release of the incorporated drug was obtained from macroporous CPC. The Weibull model was the best fitting-equation for release profiles of all cements. The liberated CMH was as active as fresh cephalexin. It is concluded that this macroporous CPC can be successfully used as drug carrier with controlled release profile for the treatment of bone infections.

Calcium-level responsive controlled drug delivery from implant dosage forms to treat osteoporosis in an animal model

The effects of plasma calcium levels on estradiol release from a self-setting apatite bone cement containing 0.5% estradiol and on the bone mineral density (BMD) of ovariectomized rats were investigated. Apatite cement consisting of an equimolar mixture of tetracalcium phosphate, dicalcium phosphate dihydrate and 0.5% beta-estradiol was prepared. The in vitro release profiles from the cements in simulated body fluid containing 0, 5 and 10 mg/100 ml calcium indicated that estradiol release rate decreased with increasing calcium concentration in the dissolution medium. After subcutaneous implantation of the cement, in vivo estradiol release in diseased rats (ovariectomized rats on a low calcium diet) was significantly higher than that in normal rats. The diseased rats maintained a low calcium level during drug release. The bone mass of the recovery model rat was greater after the experiment than before. The results suggested that the severity of osteoporosis in this animals can be reduced by the implantation of this estradiol-loaded apatite cement.

The kinetics of pentoxifylline release from drug-loaded hydroxyapatite implants

Hydroxyapatite (HAP) was synthesized by the aqueous precipitation method from CaO and H3 PO4 as the reagents. The HAP powders, either subjected or not subjected to preliminary calcination, were mixed with a pore-creating medium and isostatically shaped at a pressure of 350 MPa to form cylindrical samples. A natural product such as flour served as a pore-creating medium. Sintering was performed in the air, at 1200 or 1250 degrees C. The employed procedure allowed for achieving microporous materials of pore sizes ranging from 0.1 to 15 microm and with open porosity values of 23-44%. It was demonstrated that the porosity of the obtained materials depended mainly on the amount of the added pore-creating medium and the temperature of sintering. The implants, shaped as hollow cylinders, were filled with 50 mg of pentoxifylline (PTX) as a model drug. Internal wells for drug placement were drilled in the samples using a high precision drill. The drug release study was performed in pH = 7.35 phosphate buffer, at 37 degrees C. The results showed that the amount and time of PTX release, as well as the lag time were mainly controlled by the open porosity of the carriers.

Effect of geometrical cement size on in vitro and in vivo indomethacin release from self-setting apatite cement

The relationship between in vitro and in vivo indomethacin (IMC) release from a self-setting bioactive apatite cement and cement size were investigated. Differently sized apatite cements (total weight, 500 mg); either 64 of the small size (2 mm diameter x 2 mm thickness), sixteen of the medium size (4 mm x 2 mm) or one of the large size (15 mm x 2 mm) were obtained from cement bulk powder containing tetracalcium phosphate, dicalcium phosphate dihydrate and hydroxyapatite. In vitro IMC release from the 1, 2 and 5% drug-loaded apatite cement systems in simulated body fluid (SBF) (pH 7.25) at 37 degrees C increased with increasing concentrations of IMC and with decreasing geometrical size of the cement. The plots of in vitro IMC release per unit area against the square root of time increased with increasing IMC concentrations, but not with decreasing geometrical size of the cement. After subcutaneous (s.c.) implantation of differently sized 1% IMC-loaded cements in male Wistar rats, the plasma IMC concentration and the area under the curve increased with decreasing cement diameter. The in vivo IMC release profiles of the cement were deconvoluted from the plasma IMC profiles after s.c. administration of IMC solution. The plots of in vivo IMC release per unit area against the square root of time suggested that the initial release from all 1% drug-loaded cements was very rapid, slowed after one day, but continued for over two weeks. The relationship between the in vitro release in SBF and the in vivo release in rats of IMC-loaded cements was linear.

Antibiotic delivery system using bioactive bone cement consisting of Bis-GMA/TEGDMA resin and bioactive glass ceramics

A novel drug delivery system containing cephalexin (CEX) as a model drug using a new bioactive bone cement consisting of 15% bisphenol-alpha-glycidyl methacrylate (Bis-GMA), 15% triethylene-glycol dimethacrylate (TEGDMA) resin and 70% apatite- and wollastonite-containing glass-ceramic (A-W GC) powder was investigated. A-W GC powder containing CEX powder hardened within 5 min after mixing with Bis-GMA/TEGDMA resin, and furthermore its compressive strength was expected to be higher than that of polymethylmethacrylate cement. In vitro CEX release from bioactive bone cement pellets in a simulated body fluid at pH 7.25 and 37 degrees C continued for more than 2 weeks. The drug release rate increased with increasing amount of CEX in the mixture. All of the drug release profiles followed the Higuchi equation at the initial stage, but not at later stages. As hydroxyapatite was precipitated out on the cement surface, the drug release rate decreased. These results suggest that the CEX release rate from bioactive bone cement could be controlled by varying the amount of drug in the cement system.

Oestradiol release from self-setting apatitic bone cement responsive to plasma-calcium level in ovariectomized rats, and its physicochemical mechanism

The effect of plasma calcium levels on the release of oestradiol from a self-setting apatite bone cement containing 0.5% oestradiol was investigated in ovariectomized rats. The profiles of in-vitro release from the cements in simulated body fluid containing 0, 5 or 10 mg calcium per 100 mL indicated that the rate of release of oestradiol decreased with increasing calcium concentration in the dissolution media. After subcutaneous implantation of oestradiol-loaded cement in healthy and vitamin D-deficient rats, oestradiol release in diseased rats with low plasma calcium levels was significantly higher than that in healthy rats. These results suggest that in-vitro release of oestradiol from apatite bone cement was dependent on the calcium concentration in the buffer and that the in-vivo release of oestradiol from apatite bone cement was dependent on plasma calcium levels.