Tuesday, July 14, 2009

Oral Dosage Forms and Delivery Systems

Oral dosage forms comprise liquids (solutions, suspensions, and emulsions), semi-solids (pastes), and solids (tablets, capsules, powders, granules, premixes, and medicated blocks).
A solution is a mixture of 2 or more components that form a single phase that is homogeneous down to the molecular level. Solutions offer several advantages over other dosage forms. Compared with solid dosage forms, solutions are absorbed faster and generally cause less irritation of the GI mucosa. Moreover, phase separation on storage is not a concern with solutions, as it may be for suspensions and emulsions. The disadvantages of solutions include susceptibility to microbial contamination and the hydrolysis in aqueous solution of susceptible active ingredients. In addition, the taste of some drugs is more unpleasant when in solution. A range of additives is used in the formulation of oral solutions, including buffers, flavors, antioxidants, and preservatives. Oral solutions provide a convenient means of drug administration to neonates and young animals.
A suspension is a coarse dispersion of insoluble drug particles, generally with a diameter exceeding 1 µm, in a liquid (usually aqueous) medium. Suspensions are useful for administering insoluble or poorly soluble drugs or in situations when the presence of a finely divided form of the material in the GI tract is required. An example of the latter is the treatment of “frothy bloat” with dimethyl polysiloxanes, which relies on a dispersion of finely divided silica in the forestomach of ruminants. The taste of most drugs is less noticeable in suspension than in solution, due to the drug being less soluble in suspension. Particle size is an important determinant of the dissolution rate and bioavailability of drugs in suspension. In addition to the excipients described above for solutions, suspensions include surfactants and thickening agents. Surfactants wet the solid particles, thereby ensuring the particles disperse readily throughout the liquid. Thickening agents reduce the rate at which particles settle to the bottom of the container. Some settling is acceptable, provided the sediment can be readily dispersed when the container is shaken. Because hard masses of sediment do not satisfy this criterion, caking of suspensions is not acceptable.
An emulsion is a system consisting of 2 immiscible liquid phases, one of which is dispersed throughout the other in the form of fine droplets; droplet diameter generally ranges from 0.1-100 µm. The 2 phases of an emulsion are known as the dispersed phase and the continuous phase. Emulsions are inherently unstable and are stabilized through the use of an emulsifying agent, which prevents coalescence of the dispersed droplets. Creaming, as occurs with milk, also occurs with pharmaceutical emulsions. However, it is not a serious problem because a uniform dispersion returns upon shaking. Creaming is, nonetheless, undesirable because it is associated with an increased likelihood of the droplets coalescing and the emulsion breaking. Other additives include buffers, antioxidants, and preservatives. Emulsions for oral administration are usually oil (the active ingredient) in water, and facilitate the administration of oily substances such as castor oil or liquid paraffin in a more palatable form.
A paste is a 2-component semi-solid in which drug is dispersed as a powder in an aqueous or fatty base. The particle size of the active ingredient in pastes can be as large as 100 µm. The vehicle containing the drug may be water; a polyhydroxy liquid such as glycerin, propylene glycol, or polyethylene glycol; a vegetable oil; or a mineral oil. Other formulation excipients include thickening agents, cosolvents, adsorbents, humectants, and preservatives. The thickening agent may be a naturally occurring material such as acacia or tragacanth, or a synthetic or chemically modified derivative such as xanthum gum or hydroxypropylmethyl cellulose. The degree of cohesiveness, plasticity, and syringeability of pastes is attributed to the thickening agent. It may be necessary to include a cosolvent to increase the solubility of the drug. Syneresis of pastes is a form of instability in which the solid and liquid components of the formulation separate over time; it is prevented by including an adsorbent such as microcrystalline cellulose. A humectant (eg, glycerin or propylene glycol) is used to prevent the paste that collects at the nozzle of the dispenser from forming a hard crust. Microbial growth in the formulation is inhibited using a preservative. It is critical that pastes have a pleasant taste or are tasteless. Pastes are a popular dosage form for treating cats and horses, and can be easily and safely administered by owners.
A tablet consists of one or more active ingredients and numerous excipients and may be a conventional tablet that is swallowed whole, a chewable tablet, or a modified-release tablet (more commonly referred to as a modified-release bolus due to its large unit size). Conventional and chewable tablets are used to administer drugs to dogs and cats, whereas modified-release boluses are administered to cattle, sheep, and goats. The physical and chemical stability of tablets is generally better than that of liquid dosage forms. The main disadvantages of tablets are the bioavailability of poorly water-soluble drugs or poorly absorbed drugs, and the local irritation of the GI mucosa that some drugs may cause.
A capsule is an oral dosage form usually made from gelatin and filled with an active ingredient and excipients. Two common capsule types are available: hard gelatin capsules for solid-fill formulations, and soft gelatin capsules for liquid-fill or semi-solid-fill formulations. Soft gelatin capsules are suitable for formulating poorly water-soluble drugs because they afford good drug release and absorption by the GI tract. Gelatin capsules are frequently more expensive than tablets but have some advantages. For example, particle size is rarely altered during capsule manufacture, and capsules mask the taste and odor of the active ingredient and protect photolabile ingredients.
A powder is a formulation in which a drug powder is mixed with other powdered excipients to produce a final product for oral administration. Powders have better chemical stability than liquids and dissolve faster than tablets or capsules because disintegration is not an issue. This translates into faster absorption for those drugs characterized by dissolution rate-limited absorption. Unpleasant tastes can be more pronounced with powders than with other dosage forms and can be a particular concern with in-feed powders, in which it contributes to variable ingestion of the dose. Moreover, sick animals often eat less and are therefore not amenable to treatment with in-feed powder formulations. Drug powders are principally used prophylactically in feed, or formulated as a soluble powder for addition to drinking water or milk replacer. Powders have also been formulated with emulsifying agents to facilitate their administration as liquid drenches.
A granule is a dosage form consisting of powder particles that have been aggregated to form a larger mass, usually 2-4 mm in diameter. Granulation overcomes segregation of the different particle sizes during storage and/or dose administration, the latter being a potential source of inaccurate dosing. Granules and powders generally behave similarly; however, granules must deaggregate prior to dissolution and absorption.
A premix is a solid dosage form in which an active ingredient, such as a coccidiostat, production enhancer, or nutritional supplement, is formulated with excipients. Premix products are mixed homogeneously with feed at rates (when expressed on an active ingredient basis) that range from a few milligrams to ~200 g/ton of feed. They are administered to poultry, pigs, and ruminants. The density, particle size, and geometry of the premix particles should match as closely as possible those of the feed in which the premix will be incorporated to facilitate uniform mixing. Issues such as instability, electrostatic charge, and hygroscopicity must also be addressed. The excipients present in premix formulations include carriers, liquid binders, diluents, anti-caking agents, and anti-dust agents. Carriers, such as wheat middlings, soybean mill run, and rice hulls, bind active ingredients to their surfaces and are important in attaining uniform mixing of the active ingredient. A liquid binding agent, such as a vegetable oil, should be included in the formulation whenever a carrier is used. Diluents increase the bulk of premix formulations, but unlike carriers, do not bind the active ingredients. Examples of diluents include ground limestone, dicalcium phosphate, dextrose, and kaolin. Caking in a premix formulation may be caused by hygroscopic ingredients and is addressed by adding small amounts of anti-caking agents such as calcium silicate, silicon dioxide, and hydrophobic starch. The dust associated with powdered premix formulations can have serious implications for both operator safety and economic losses, and is reduced by including a vegetable oil or light mineral oil in the formulation. An alternate approach to overcoming dust is to granulate the premix formulation.
A medicated block is a compressed feed material that contains an active ingredient, such as a drug, anthelmintic, surfactant (for bloat prevention), or a nutritional supplement, and is commonly packaged in a cardboard box. Ruminants typically have free access to the medicated block over several days, and variable consumption may be problematic. This concern is addressed by ensuring the active ingredient is nontoxic, stable, palatable, and preferably of low solubility. In addition, excipients in the formulation modulate consumption by altering the palatability and/or the hardness of the medicated block. For example, molasses increases palatability and sodium chloride decreases it. Additionally, the incorporation of a binder such as lignin sulfonate in blocks manufactured by compression or magnesium oxide in blocks manufactured by chemical reaction, increases hardness. The hygroscopic nature of molasses in a formulation may also impact the hardness of medicated blocks and is addressed by using appropriate packaging.

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