Cellulose derivatives are extensively used for thickening of pharmaceutical solutions and disperse systems such as emulsions and suspensions. Furthermore, these polymers can increase viscosity of non-aqueous pharmaceutical solution likes organic-based coating solutions. Viscosity enhancing of drug solutions poses many advantages such as improving consuming controllability and increasing residence time of drugs in topical and mucosal solutions which lead to improve bioavailability of topical, nasal or ocular preparations. It has been revealed that viscosity enhancement, in some cases, can increase absorption of some poorly- absorb drugs like insulin from oral dosage forms. Cellulose ethers in concentrations lower than minimum gel-forming amounts are used as thickening agents or viscosity builder. These polymers play an important role in stabilizing of pharmaceutical disperse systems especially in suspensions and coarse emulsions. By increasing the viscosity of suspension, based on the stock’s equation, the sedimentation rate of dispersant decreased and thus, the uniformity of dispersion after shaking of product will improve. In the case of emulsions, these polymers can increase the shelf life and their resistance against mechanical and thermal shocks. Among cellulose derivatives, cellulose ethers especially their higher molecular weight grades are more suitable for using as viscosity enhancer and stabilizer for liquid pharmaceutical disperse systems such as suspensions and emulsions. There is a direct proportionality between viscosity of cellulose ether solutions and molecular weights of them.
Bioadhesives and mucoadhesives are drug containing polymeric films with ability of adhering to biological membranes after combining with moisture or mucus compounds. Bioadhesives were developed in mid 1980s as a new idea in drug delivery and nowadays they have been accepted as a promising strategies to prolong the residence time and to improve specific localization of drug delivery systems on various biological membranes.
In compared with tablets, these dosage forms have higher patient compliance due to their small size and thickness. Other advantage of these drug delivery systems is their potential to prolong residence time at the site of drug absorption and thus they can reduce the dosing frequency in controlled release drug formulations. These dosage forms can also intensify the contact of their drug contents with underlying mucosal barrier and improve the epithelial transport of drugs across mucus membranes especially in the case if poorly absorbed drugs. Some special polymers can be used in these formulations with epithelial permeability modulation ability by loosening the tight intercellular junctions. Some of these polymers also can act as proteolytic enzymes inhibitor in orally used adhesive formulations of sensitive drugs.
Bioadhesives considered as novel drug delivery systems. These dosage forms are formulated to use on the skin and mucus membranes of gastrointestinal, ear, nose, eye, rectum and vagina. The main excipients of these formulations are adhesive and film-former polymer(s). Adhesive polymers are synthetic, semi synthetic or natural macromolecules with capability of attaching to skin or mucosal surfaces. Very different types of polymers have been used as bioadhesive polymers. Synthetic polymers such as acrylic derivatives, carbopols and polycarbophil, natural polymers such as carageenan, pectin, acacia and alginates and semi-synthetic polymers like chitosan and cellulose derivatives are used in bioadhesive formulations. Cellulose
derivatives especially cellulose ethers are widely used in bioadhesives. There are used in various types of these formulations such as buccal, ocular, vaginal, nasal and transdermal formulations alone or with combination of other polymers. More recently used cellulose ethers in bioadhesives include nonionic cellulose ethers such as ethyl cellulose (EC), hydroxy ethyl cellulose, hydoxypropyl cellulose (HPC), methyl cellulose (MC), carboxy methyl cellulose (CMC) or hydroxylpropylmethyl cellulose (HPMC) and anionic ether derivatives like sodium carboxymethyl cellulose (NaCMC). Ability of polymer to take up water from mucus and pH of target place are important factors determining the adhesive power of polymers. Some bioadhesive polymers such as polyacrylates show very different adhesion ability in various pH values thus the selection of adhesive polymer should be made based on the type of bioadhesive preparation. One amvantage of cellulose ethers such as NaCMC and HPC is lesser dependency of adhesion time and adhesion force of them to pH of medium in compared with polyacrylate and thiolated bioadhesive polymers. In some studies, other groups of adhesive polymers or polysaccharides are used with cellulose ethers to improve their adhesion characteristics such as adhesion time and adhesion force. Concurrent use of polyvinyl pyrrolidone (VP), hydroxypropyl beta cyclodextrin, polycarbophil, carbopol(s), pectin, dextran and mannitol with HPMC, HEC or NaCMC have been reported in the literatures.
Solid dosage forms such as tablets, pellets, pills, beads, spherules, granules and
microcapsules are often coated for different reasons such as protection of sensitive drugs from humidity, oxygen and all of inappropriate environmental conditions, protection against acidic or enzymatic degradation of drugs, odor or taste masking or making site or time specific release characteristics in pharmaceuticals to prepare various modified release drug delivery systems such as sustained release, delayed release, extended release, immediate release, pulsatile release or step-by-step release dosage forms. Both ether and ester derivatives of cellulose are widely used as coating of solid pharmaceuticals. Cellulose ethers are generally hydrophil and convert to hydrogel after exposing to water. Although, some of the cellulose ethers e.g. ethyl cellulose are insoluble in water but majority of them such as methyl, hydroxypropyl and hydroxylpropylmethy1 cellulose are water soluble. Both of soluble and insoluble cellulose ethers can absorb water and form a gel. After exposing of these coated dosage forms with water, the coating polymers form to hyrogel and gradually dissolve in water until disappear but the insoluble cellulose ether coatings remain as a viscose gel around tablets and drug release is performed by diffusion of drug molecules within this layer. These two types of dosage forms called dissolution-controlled and diffusion-controlled drug delivery systems, respectively. Despite cellulose ethers, the cellulose esters are generally water insoluble or water soluble in a distinct pH range. These polymers like cellulose acetate (CA), cellulose acetate phthalate (CAP) and cellulose acetate butyrate (CAB) do not form gel in presence of water and they are widely used for preparing of pH sensitive and semi-permeable micro-porous membranes. These membranes are employed for wide variety of controlled release coating of pharmaceuticals especially in enteric or osmotic drug delivery devices. These polymers are benefited to make different cellulosic membrane filters applied in pharmaceutical industries.
Gels are semisolid systems consisting of dispersions of very small particles or large
molecules in an aqueous liquid vehicle rendered jellylike by the addition of a gelling agent. In recent decades, synthetic and semi-synthetic macromolecules are mostly used as gelling agents in pharmaceutical dosage forms. Some of these agents include: carbomers, cellulose derivatives and natural gums. Cellulose derivatives such as HPMC and CMC are the most popular gelling agents used in drug formulations. These polymers are less sensitive for microbial contamination than natural gelling agents such as tragacanth, acacia, sodium algininate, agar, pectin and gelatin. Cellulose derivatives generally dissolve better in hot water (except MC grades) and their mechanisms of jellification is thermal. For preparing gel, powder of these polymers with suitable amount initially dispersed in cold water by using mechanical mixture and then, the dispersion is heated to about 60-80°C and gradually cooled to normal room temperature to form a gel (except MC grades). The resulted gels from these polymers are single-phase gels. Adding of electrolytes in the low concentrations
increase the viscosity of these gels by salting out mechanism and higher concentrations (above 3-4%) can precipitate the polymer and breakup the gel system.
Maximum stability and transparency of the gels prepared by these polymers is about
neutral range (pH=7-9) and acidic pHs can precipitate them from gel system. Minimum gel- forming concentrations of cellulose derivatives are different based on the type and the molecular weights of them but the medium range is about 4-6%w/v. The type of cellulose derivative in pharmaceutical gels can significantly affect drug release from gel formulations. These gels also can be used as the base of novel drug delivery systems such as liposomal formulations.
Cellulose and related polymers are commonly used in solid dosage forms like tablets and capsules as filler. Various forms of cellulose have been used in pharmaceutical preparations as multifunctional ingredients thus; they are concerned as precious excipients for formulation of solid dosage forms. Cellulose and its derivatives have many advantages in using as filler in solid pharmaceuticals such as their compatibility with the most of other excipients, pharmacologically inert nature and indigestibility by human gastrointestinal enzymes. These polymers do not cause any irritancy potential on stomach and esophagus protective mucosa. Various forms of pure cellulose and cellulose ether derivatives can be used as filler in these formulations.
Binders are the essential components of solid drug formulations made by wet granulation process. In wet granulation process, drug substance is combined with other excipients and processed with the use of a solvent (aqueous or organic) with subsequent drying and milling to produce granules. Cellulose and some derivatives have excellent binding effects in wet granulation process. A number of MCC grades such as PH-101 are widely used as binder in wet granulation. Other cellulose derivatives such as MC, HPMC and HPC have good binding properties in wet granulation. Low substituted cellulose ethers such as low substituted HPC (L-HPC) also used as binder in wet granulation process. Even though, low substituted cellulose ethers have lower water solubility compared with normal grades, however they have very good binding efficacy. Cross-linked cellulose (CLO) and cross-linked cellulose derivatives such as cross-linked NaCMC can be used as excellent binders in pharmaceuticals as well.
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