Tablets & Capsules

TC0918

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44 September 2018 Tablets & Capsules eye on John Lawrence Bulk Solids Innovation Center, Kansas State University excipients This column covers some excipient basics and discusses how moisture content affects excipient properties and behavior. The moisture content of excipi- ents plays an important role in the physical and chemical properties of pharmaceutical products. The char- acteristics of the excipients for a par- ticular dosage of a formulation depend on their moisture sorption behavior, flowability, and stability during manufacturing and storage. Defined as any substance other than the API that has been appropriately evaluated for safety, excipients are included in a drug delivery system for the following reasons: • To aid processing of the system during its manufacture; • To protect and support stability, bioavailability, and patient acceptability; • To assist in product identification; • To enhance any other attribute that promotes the overall safety and efficacy of the API during storage or use [1]. Manufacturers classify excipients according to their function, such as sweetener, preservative, binder, lubricant, flow enhancer, film former, filler, disintegrant, or diluent. According to a study by Dave in 2008, the most common excipients are lactose; microcrystalline cellulose (MCC), a diluent; sodium starch gly- colate; croscarmellose sodium, a dis- integrant; colloidal silicon-dioxide, a glidant; hydroxypropyl methylcellu- lose (HPMC); polyvidone (PVP); lactose hydrous; starch, a binder; magnesium stearate, a lubricant; and titanium dioxide, an opacifier [2]. The presence of water in an excip- ient is very critical in terms of stabil- ity, flow, dissolution, compaction, and storage. The hygroscopicity of an excipient is its ability to interact with moisture from the surrounding atmosphere, and the mechanisms of moisture sorption are different for various excipients. Therefore, formu- lators must understand the mois- ture-sorption characteristics of potential excipients before working on any API formulation. Pharmaceutical scientists refer to the equilibrium relative humidity (ERH) of an excipient (the vapor pressure inside and outside the mate- rial) as the water activity (a w ). The a w is a measure of free moisture in the material and influences a combina- tion of water-solute and water-sur- face interactions and capillary forces. Generally, the a w increases as the temperature increases. Depending on the temperature, relative humidity (RH), and compo- Figure 1 Influence of vapor pressure on adsorption and desorption Lower vapor pressure inside particles a. Adsorption: Moisture moves to material from environment to maintain equilibrium Higher vapor pressure in surrounding environment Higher vapor pressure inside particles b. Desorption: Moisture moves from material to environment to maintain equilibrium Lower vapor pressure in surrounding environment

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