Globular protein gels as carriers of active substances STANISŁAW MLEKO*, MARTA TOMCZYÑSKA-MLEKO, ZDZISŁAW TARGOÑSKI *Corresponding author University of Life Sciences in Lublin Department of Milk and Hydrocolloids Technology Stanisław Mleko Akademicka 13 Lublin, 20-950, Poland ABSTRACT: Article discusses globular protein gels as potential devices for controlled delivery of biologically active substances. By simply adjusting of several gelation variables, it is possible to obtain protein gels with different matrix structure and swelling characteristics. The advantages of using food protein based gels is their biodegrability and natural way of preparation without any chemical cross-linking agents. Advances in technology of globular protein gels as carriers for controlled release of drugs and nutraceutics are presented in the paper. INTRODUCTION In recent years, polymeric microspheres, polymer micelles, and gelled materials have been shown to be effective as drug Globular proteins are very important ingredient of foods. In a and other active ingredients carriers (4). Hydrogels, water- globular protein, the amino acid chain folds in a manner that containing polymer matrices with pores of colloidal length enhances protein’s solubility in water by placing polar groups of scale, are very interesting substrates for the sustained release atoms at its surface. This main characteristic distinguishes them of incorporated drugs and nutraceuticals. The chemical from fibrous proteins, which are practically insoluble. A fibrous composition and microstructure of polymers can be readily protein - gelatin is often used to form gels and was utilized for engineered to suit drugs with different hydrophobicity, obtaining drug release carriers (1). Food globular proteins molecular weight, pI, etc. Through chemical modification, the include: egg albumin, whey proteins, vegetable globulins, polymer can carry a specific delivery information. The surface hemoglobin, myoglobin, serum globulins in blood and many properties and compositions of polymer matrices can be Food technologies enzymes. easily optimized to achieve the desired biocompatibility and A drug has a therapeutic effect, when it reaches a desired drug release kinetics (2). It seems, that gels will have a growing target in the body before degradation of the active ingredient role in biomedical applications. Proper gel matrix design and and when the concentration of the drug is in an optimal level accurate mathematical modelling are keys to engineering for necessary period of time. Recombinant protein technology the drug release rates. To obtain a gel based drug carrier, gels created numerous protein and peptide preparates for different are soaked in drug solution or the gelation process is carried in vol 21 n 3 disease treatment. The effective delivery of these biomolecules the presence of drug. Both methods have some limitations. - is hindered because of their large molecular weights and very High concentration of drug may interfere with gel matrix subtle structures. Such preparates administrated by intravenous formation. Soaking the gel in a drug has limitations because or subcutaneous injection are prone to proteolytic many drugs have low solubilities in water and some can be degradation, what decreases the activity of the drug and excluded from gel networks due to size-exclusion May/June 2010 leads to multiple injections or increased doses to maintain phenomenon (5). An alternative would be to load the drug - effective drug concentration (2). Multiple daily injections are during the polymerization and cross-linking of the polymer very inconvenient for patients and can cause a risk of skin matrix, or to synthesize a drug-polymer conjugate. However, inflammation and infection. A single injection causes fast the polymerization conditions often lead to denaturation of increase in drug concentration in blood plasma, what may the fragile protein drugs. Additionally, when a polymer is industry hi-tech induce local toxicity and negative immune responses. formed by chemical cross-linking of the monomers in the Time-release drug carriers deliver treatment continuously, presence of a drug, byproducts from the gel’s formation can OOD F without under or over dosage of the drug. The purpose to use interfere with the drug. Although gels made from globular gro the controlled delivery carriers is to: maintain the optimal proteins may evoke in some human organisms immune/ A concentration of drug in the blood plasma or in the cell, inflammatory responses, they do offer many advantageous decrease release rate, lower systemic drug toxicity, enhance properties. They are characterized by inherent drug targeting specificity, improve treatment absorption rates, biocompatibility, biodegradability, and biologically provide protection for pharmaceuticals against biochemical recognizable moieties that support cellular activities (2). degradation, enhance drug activity for drugs with short half-life, Protein gels are of particular interest in perspective of active decrease dosing frequency, decrease waste of active ingredients controlled release. They are very sensitive to ingredient of the drug. environmental conditions. Agents as temperature, pH, ionic composition and strength and other chemical compounds may trigger different functional responses of gel systems (6). GELS AS ACTIVE INGREDIENT DELIVERY SYSTEMS Food protein gels can be used in pharmacy for drug delivery and in the food industry as carriers of active ingredients, Many foods need a thick liquid or gelled consistency in order to aromas or nutraceutics. Additionally high nutritional value of give consumers the desired sensation during eating (3). Gels food proteins will have benevolent influence on the diet. are three-dimensional networks that entrap large quantities of Globular protein gels controlled release carriers offer a water while remaining insoluble in aqueous solutions due to sustained release mechanism in which the drug release rates different interactions. Gels exhibit many unique can be controlled by changing gel composition, physicochemical properties that make them good materials for microstructure, average molecular weight of the aggregates different biomedical applications including drug delivery. and degree of aeration. Biodegradable gels are favoured 26 over non-degradable chemically cross-linked polymers, as no temperature, pH or ionic strength. Some correlations between additional surgeries to recover the implanted gels are needed. the swelling ratio and pH were reported (11). Park, Song et al. Gels can be prepared from different food proteins. The (12) investigated thermally denatured egg albumin gels as controlled drug release properties of whey protein potential drug delivery system. The pH of albumin solution concentrate (WPC) gels were studied using caffeine as the affected pH sensitive swelling characteristics of thermally model drug (7). Whey proteins were very good choice, as they denatured albumin gel thereby heat induced protein gel are globally applied as a source of dietary protein, not just for matrices having different pH sensitivities were prepared. Until normal healthy subjects, but also for special applications like pI, the equilibrium swelling ratio of heat induced egg albumin infant formulas, hospital nutrition, slimming and muscle building decreased as the pH value of swelling medium rose. However, (8, 9). Caffeine release from WPC gels was found to be pH the swelling ratio increased as the pH of swelling medium dependent (7). When the pH of the release medium was increased. At the isoelectric point, the swelling ratio was lower (1.8), the release was slower than when the release minimal. The egg albumin sample denatured at pH 11.13 occurred in a medium with a higher pH (7.5). The release showed the highest sensitivity. The sensitivity decreased as kinetics for both cases was diffusion controlled. Equilibrium denaturation pH decreased. The mechanism of pH sensitive swelling ratio (SR) was at the minimum when pH of the swelling swelling is expected to be governed by ionization of carboxyl medium was close to the isoelectric point (pI) of the whey groups in the experimental pH region. High equilibrium swelling protein. For pH far from the pI (from 6.0 to 10.0), the SR ratio was attributed to the electrostatic repulsive force increased. In particular, when the pH was higher than the pI, originating from the negative charge of ionized carboxyl the swelling was highly pH sensitive. The higher the WPC groups. So, different pH sensitivity means a different amount of concentration used in preparing the hydrogel, the lower was ionizable and/or ionized carboxyl groups (12). the SR. Consistent with the swelling behaviour of the gels, More further research is needed to understand correlations Food technologies release was slower when the pH of the medium was lower (pH between in vitro and in vivo release. Even model in vitro 1.8) than when it was higher (pH 7.5). The SR and caffeine research with artificial gastrointestinal tract has its limitations, as release rate decreased significantly when the gels were doesn’t involve food intake. Food may alter the rate of drug surface-coated with alginate (7). diffusion as a result of increased mechanical stress and by interacting with active ingredients and gel matrix. The acting of delivery systems should always be validated using well- DRUG RELEASE MECHANISM designed animal and/or human feeding studies, since it is difficult to mimic the complex physiological processes that Drug