Apatite as carrier for growth hormone: in vitro characterization of loading and release

Previous studies concerning bone drug delivery systems have provided little data about the amount of drug loaded, one of the essential factors for determining the dose/effect relationship. To investigate this factor, an adsorption method involving a therapeutic agent was tested in vitro on an apatitic calcium phosphate (AP). One milligram of human growth hormone (hGH) was deposited onto 0.1, 0.15, and 0.2 g of AP powder over a period of 24 h at 37 degrees C. The amount of hGH loaded was determined by subtracting the dose recovered from that applied on AP. The results show that 1 g of AP absorbed 9.48 mg of hGH. From 0.1 and 0.15 g of hGH-loaded AP, hGH was released in vitro for 2 and 3 weeks, respectively, with a 50% time release (T1/2) at 30 h and 72 h, respectively, for the two quantities. The amount of drug loaded and the determined release kinetics were compatible with the action pattern of hGH, indicating that hGH-loaded calcium phosphate supports are suitable for bone-growth promotion.