Research on drug release performance of hydroxyapatite-gelatin composites produced in SBF medium

Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is a bioseramic material found in bone and teeth composition which has apatite-like structure. In spite of its biocompatibility, bioactivity and nontoxicity, hydroxyapatite has been used for sentetic bone material production, bone and teeth implants and drug delivery applications. In recent studies, it has been concluded that hydroxyapatite nanoparticles has inhibition effect on different kind of tumors. Therefore, hydroxyapatite particles or composites including hydroxyapatite have been preferred in controlled drug delivery studies. However, highly fragile and hard structure of hydroxyapatite limited its usage in clinical applications. The mechanical disadvantages of hydroxyapatite can be overcomed by using a polymer in order to produce a hydroxyapatite-polymer composite. In recent years, studies aiming to use ceramic/polymer materials as drug carrier have gained importance. By adding biodegradable polymers with different degradation rate and mechanism to ceramic materails, variety of release profiles can be obtained. In drug release systems polylactic/polyglicolic acid, proteins (collagen, gelatin, fibrin, etc.), carbohydrates (chitin, chitosan, starch, alginate, etc.) and polymers such as polycarbonate, polypropylene fumarate (PPF), polyethylene (PEG), nylon, polypropylene can be used. While crosslink agents including genipin, calcium chloride, glutaraldehyde etc. are used to crosslink polymers, different solvents can be used in order to provide desired pH and to degrade different kind of polymers. All these materials have an impact on the biocompatibility of the drug release system. One of the polymers used in drug release systems is gelatin which can be used alone or constituent of a composite. Gelatin is obtained by partial hydrolysis of collagen and widely used in food and medicine. Not only being biocompatible, biodegradable and nontoxic, but also economic made gelatin a preferable polymer in drug release studies. In addition, gelatin has an efficient swelling-releasing characteristic, does not produce antigen and does not have a negative effect on cell viability. However, gelatin has some disadvantages such as being soft, very soluble at 37°C and having a low mechanical strength. Several methods are used for drug loading such as soaking, incorporation, encapsulation, impregnation, vacuum impregnation, adsorption, absorption, spray drying, freeze drying etc. Spray drying is one of the most efficient methods for drug loading. During spray drying process, both drying and partical formation take place. Characteristics of obtained products can be controlled by varying inlet temperature, air flow rate etc. and can be kept continuous. With different spray dryer designs, small or big particulated powder, stack or granules cane be obtained. Also, It is possible to produce drug loaded microspheres with uniform size distribution with spray dryer. Given the side effects and high costs of systematical antibiotic treatments, controlled drug delivery studies performed with drug loaded composite materails have gained importance. In drug delivery studies with bioactive materials, it is observed that permeability and porosity of the material effect the release profile. As a result, having a highly similar structure with bone inorganic phase has caused hydroxyapatite to be preferred in several drug loading studies. In drug release systems, drug release profile generally shows a biphasic release including initial burst followed by a slow release with small changes. Studies show that bacterial growth is observed around the implants when the drug concentration is under MIC (Minimum Inhibition Concentration) value. Initial burst releases from drug release systems can cause side effects. Therefore, in an ideal drug release sytem drug concentration should be kept above the MIC value and zero order release profile should be maintained in order to minimize side effects. 5-Fluorouracil (5-FU) is a primidin analogue which is widely used in cancer treatment. By preventing the formation of thymine nucleotid which is a constituent of DNA and blocking the phosphate enzyme, 5-FU inhibits DNA and RNA synthesis leading to restraining and slowing down cancer cell growth. 5-FU is widely used for the treatment of colon, rectal, breast, ovary, pancreas, stomach, brain and skin cancer. During treatment with 5-FU, drug is generally injected to body by injection and infusion methods. Because of the fast absorption and toxic characteristic of 5-FU, digestive, neural, dermatological and cardiovascular side effects can occur. In order to maximize therapeutic feature and minimize side effects, by controlled release of 5-FU, encapsulation studies with natural and synthetic polymers are performed. Chitosan, gelatin, poly(lactide-co-glycolide) (PLGA), poly(d,l-lactide) (PLA), poly(methylidene malonate 2.1.2) and pectin are some of the polymers used in 5-FU encapsulation studies. In addition, studies including 5-FU loaded HAp/PLGA and gelatin/chitosan composites are performed. Drug release mechanism depends on many factors such as drug carrier's pore size, shape, connectivity, drug/carrier interaction, drug/polymer interaction, polymer degradation rate. For that reason, it is important to determine release mechanism before applying. Mathematical models used for determining release kinetics enable evaluating significant physical parameters including drug diffusion coefficient. Also, these models contributes optimization of release process. In this study, hydroxyapatite-gelatin (HAp-GEL) composites are produced in the presence of SBF (Simulated Body Fluid) and spray dryer is used for obtaining drug loaded composites. Composites are produced by wet precipitation method implementing glutaraldehyde (GA) as a cross-linking agent. By running experiments with different amounts of gelatin and glutaraldehyde, their effects on encapsulation efficiency and drug release profiles are studied. Drug encapsulation experiments are performed with 5-FU as a selected drug. Drug release profiles are evaluated according to four different kinetic models including Zero Order, First Order, Higuchi and Korsmeyer-Peppas. The obtained composites are characterized by Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM), Thermogravimetric Analysis (TGA) and X-Ray Diffraction (XRD). With XRD and TGA analysis, production of HAp-GEL composites with different HAp/GEL weight ratio is confirmed. XRD analysis show caharacteristic HAp peaks.In FTIR analysis, characteristic HAp, gelatin and 5-FU peaks are observed. With SEM analysis, production of drug loaded HAp-GEL microspheres are confirmed. Comparing to absorption method, spray drying shows a much greater drug loading efficiency. According to drug release profiles, it is observed that with increasing jelatin content initial burst increases. Total average release rate of drug from composites decreases with increasing jelatin content. As initial burst release can cause side effects and decrease drug's efficiency, it is concluded that HAp-GEL composites with lower gelatin content are more suitable for controlled drug delivery. When crosslinking agent is increased, an increase in initial burst release is also observed. Considering the toxic characteristic of glutaraldehyde, it is concluded that composites cross-linked with lower amount of glutaraldehyde are more appropriate for drug release applications. Among the four different kinetic models, HAp-GEL composites fitted First Order release model the best.