Sodium Alginate, a Natural Polysaccharide has the Stability, Solubility, Viscosity and Safety Needed in a Pharmaceutical Excipient
Sodium alginate consists mainly of the sodium salt of alginic acid, which is a mixture of polyuronic acids composed of 1, 4'-linked �-D-mannuronic acid and �-L-guluronic acid residues. The British chemist E. C. Stanford made the first scientific studies on the extraction of alginates from brown seaweed in 1881. He found that the extracted substance, which he named algin, possessed several interesting properties. These included the ability to thicken solutions, to make gels and to form films. From these, he proposed several industrial applications. However, large-scale industrial production of alginate was not introduced until 50 years later. Commercial production started in 1927, which has now expanded to about 30,000 tons per year worldwide; 30 percent of this tonnage is devoted to the food industry, the rest being used in industrial, pharmaceutical, and dental applications.
The algal plants or seaweeds are classified into four principal groups: The green algae or Chlorophyceae, the blue-green algae or Cyanophyceae, the brown algae or Phaeophyceae, and the red algae or Rodophyceae. Most of the large brown seaweeds or Phaeophyceae are potential sources of alginate. Alginate is the most abundant marine biopolymer and, after cellulose, the most abundant biopolymer in theworld. The main commercial sources are species of Ascophyllum, Durvillaea, Ecklonia, Laminaria, Lessonia, Macrocystis, Sargassum and Turbinaria. Of these the most important are Laminaria, Macrocystis porifera and Ascophyllum nodosum. Bacterial alginates have also been isolated form Azotobacter vinelandii and several Pseudomonas species.
Chemistry of Sodium Alginate
Sodium alginate (C6H7O8Na)n consists mainly of the sodium salt of alginic acid, which is a mixture of polyuronic acids composed of 1, 4-linked �-D-mannuronic acid and �-L-guluronic acid residues. It's non-proprietary names include BP (Sodium alginate), (PhEur: Natrii alginas) and USP-NF (Sodium alginate). It is also known as E400, Kelacid, L-gulo-D-mannoglycuronan, Polymanuronic acid, Protacid and Satialgine H8
The composition and sequential structure of alginate can be determined by high-resolution 1H- and 13C-nuclear magnetic resonance spectroscopy (NMR) and is used to determine the monad frequencies, as well as diads and triads. Alginates are block co-polymers of L-guluronic acid (G) and D-mannuronic acid (M) residues connected by 1:4 glycosidic linkages. Figure: 1 shows a typical structure for the alginic acid block co-polymer. The relative proportions of D-mannuronic and L-guluronic acids are species-dependent and can be influenced by growth conditions. The proportions of various blocks depend on the source of origin of seaweed, the season of harvest, and part of the algae from which alginate is extracted, e.g., blade or stipe, as described in Table: 1 (Smidsrod et al; 1996). The concentration of G and M acids (G: M ratio) contributes to varied structural and biocompatibility characteristics.
The polysaccharides derived from seaweeds - alginates, agars, carrageenans and furcelleran - can be induced to form gels under certain conditions. Solutions of alginate will react with many di- and trivalent cations to form gels; the gels will form at room temperature, or any temperature up to 100oC, and they do not melt when heated. Alginate beads can be prepared by extruding a solution of sodium alginate containing the desired protein, as droplets, into a divalent crosslinking solution such as Ca2+, Sr2+, or Ba2+. Monovalent cations and Mg2+ ions do not induce gelation, while Ba2+ and Sr2+ ions produce stronger alginate gels than Ca2+. Other divalent cations such as Pb2+, Cu2+, Cd2+, Co2+, Ni2+, Zn2+ and Mn2+ will also crosslink alginate gels but their use is limited due to their toxicity.
The gelation and crosslinking of the polymers are mainly achieved by the exchange of sodium ions from the guluronic acids with the divalent cations, and the stacking of these guluronic groups to form the characteristic egg-box structure shown in Figure: 2. In the presence of divalent calcium ions, the calcium is ionically substituted at the carboxylic site. A second alginate strand can also connect at the calcium ion, forming a link in which the Calcium ion attaches two alginate strands together. Calcium helps to hold the molecules together, their polymeric nature and their aggregation bind the calcium more firmly; this has been termed "cooperative binding." The analogy is that the strength and selectivity of cooperative binding is determined by the comfort with, which "eggs" of the particular size may pack in the 'box', and with which the layers of the box pack with each other around the eggs.
The viscosity behavior of alginate solutions is pseudoplastic, the solution flows more readily the more it is stirred or pumped. This effect is reversible except at very high rates of shear. Various grades of sodium alginate are available, yielding aqueous solutions of varying viscosity within a range of 20 to 400 centipoise (0.02 to 0.4 Pa s) in 1 percent solution at 20oC. In the presence of the supporting electrolyte, rheological behavior of polyelectrolyte solution is known to depend on the ionic structure of the aqueous solvent, e.g., increasing the concentration of a strong electrolyte such as NaCl in the alginate solution up to 100 mM was shown to reduce the solution viscosity due to the change in polymer conformation. The viscosity of various types of alginate at different concentration is given in Table 2.
Commercial alginates, like polysaccha-rides in general, are polydisperse with respect to molecular weight (Mw). Therefore, the given Mw of an alginate always represent an average of all of the molecules in the population. The most common ways to express the Mw are as the number average (Mn) and the weight average (Mw). The two averages are defined by the following equations:
Where Ni = number of molecules having a specific molecular weight Mi
Wi = weight of molecules having a specific molecular weight Mi
In a polydispers molecular population the relation Mw > Mn is always valid. The coefficient Mw / Mn is referred to as the polydispersity index, and will typically be in the range 1.5-2.5 for commercial alginates. The most common methods used for determination of molecular weights are calculations based on intrinsic viscosity and light scattering measurements
Alginic acid is slightly soluble in water and insoluble in most organic solvents. It is soluble in alkaline solutions, resulting in viscous solutions. The powders of alginates are wetted with water, the hydration of particles results in each having a tacky surface. Then the particles will rapidly stick together resulting in clumps, which are very slow to completely hydrate and dissolve. It is more difficult to dissolve alginate in water if the water contains compounds, which compete with the alginate for the water necessary for its hydration. The presence of sugars, starches or proteins in the water will reduce the rate of hydration and longer mixing times will be necessary. Salts of monovalent cations (such as NaCl) have a similar effect at levels above about 0.5 percent. A 1 percent solution in distilled water has a pH of approximately 7.2.
Sodium alginate is hygroscopic; the moisture content at equilibrium depends on the relative humidity. Dry alginate is quite stable, when stored in a well-closed container at a temperature of 25oC or less. Alginate solutions are stable between pH 5-9. Degree of polymerization (DP) and molecular weight relate directly to the viscosity of alginate solutions; loss of viscosity on storage is a measure of the extent of depolymerisation of the alginate. Alginates with a high DP are less stable than those with a low DP. Alginates have been reported to undergo proton-catalyzed hydrolysis, which is dependent on time, pH, and temperature. Propylene glycol alginate solutions are stable at room temperature from pH 3-4; below pH 2 and above pH 6 they will lose viscosity quickly even at room temperature.
Immunogenicity and Biocompatibility
Alginate is a natural and biodegradable biopolymer. Chemical composition and the mitogenic contaminants found in alginates are the two main contributors to alginate immunogenicity. There are numerous reports addressing the fibrotic reaction of implanted alginates. It is known that alginates may contain small amounts of pyrogens, polyphenols, proteins and complex carbohydrates. The presence of polyphenols might possibly be harmful to the immobilized cells, and the presence of pyrogens, proteins and complex carbohydrates may induce immunological reactions by the host.
Safety and Toxicity
Alginate obtained a "generally recognized as safe" (GRAS) status as a food and pharmaceutical ingredient by the U.S. Food and Drug Administration (FDA) in the early 1970's. It is generally regarded as a nontoxic and nonirritant material. Calcium alginate gels are found to be nontoxic to cells and hence are suitable for drug delivery.
King et al; (1983) has listed 39 countries which permitted alginate salts as at January 1982; three of them had not approved the propylene glycol ester. The joint Expert Committee of Food Additives of the Food and Agriculture Organization of UN/World Health Organization has also issued specifications for alginates and recommended an Acceptable Daily Intake, for alginic acid salts of 50 mg per kg body weight per day, for propylene glycol alginate of 25 mg/kg/day. Numerous studies have tested that the high level of safety of sodium alginate in the foods.
Pharmaceutical Applications of Alginate
Sodium alginate was introduced into the US Pharmacopoeia as early as 1938. Alginic acid was entered in the British Pharmaceutical Codex in 1963. Alginic acid is insoluble in water but swells when placed in water. So, traditionally, sodium alginate has been used as a tablet-binding agent, while alginic acid is used as a tablet disintegrant in compressed tablets designed for immediate drug release. The effect of sodium alginate on tablet properties is, however, dependent on the amount incorporated in the formulation and in some cases the alginate salt can promote disintegration. It can be added during the granulating process, rather than as a powder after granulation, so the processing is easier. The mechanical strength of the final tablet is greater, compared to using starch.
Alginates are also involved in the production of correcting suspensions, gels, and concentrated emulsions on the basis of fats and oils. Sodium alginate is used in some liquid medicines to increase viscosity and improve the suspension of solids. Propylene glycol alginate can improve the stability of emulsions. Controlled release (CR) drug delivery systems represent one of the very important areas of health care. Bod-meier et al; (1993) prepared sustained-release polymer particles containing drugs with various solubility characteristics (ibuprofen, theophylline, guanifesin, and pseudoephedrine HCl) based on alginate with colloidal polymer dispersions. Actual drug contents close to 50 percent and encapsulation efficiencies between 80 and 98 percent were achieved for all drugs. The stirring time before separation of the particles from the gelation medium had to be minimized with the water-soluble drugs to maximize drug loading. However, it was not critical with water-insoluble drugs. Drug release was found to depend on the solubility of the drug used.
One researcher reported that alginate gel particles show a pH-sensitive swelling property, i.e., the particles remain unchanged in distilled water or acidic medium (pH 1.5 KCl-HCl) but swell rapidly in pH 7.0 phosphate buffer to a size greater than the original size. This property of alginate can be useful for drugs, which are acid sensitive because they can be shielded from attack of gastric juices and can be released at desirable rates in the intestine because of reswelling of xero-gels in the intestine.
Another researcher prepared CR sodium alginate beads containing diclofenac sodium (sparingly water-soluble) by precipitation of sodium alginate in alcohol followed by cross-linking with glutaraldehyde in acidic medium. The percentage entrapment efficiency was found to be between 30 and 71 percent, depending on preparation conditions. The beads prepared at higher temperatures and longer times of exposure to the cross-linking agent have shown lower entrapment efficiency with extended release. �Girish N. Patel, of the Department of Pharmaceutical Technology in the S. K. Patel College of Pharmaceutical Education and Research of Ganpat University (Gujarat, India)