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eye on will likely be used in other strategies to formulate solid dosage forms that include poorly soluble compounds. Polymers once used only as tablet coatings have emerged as significant bioavailability enhancers. The quest for better has also spurred innovation among other types of excipients, including silicon dioxide. The advantageous properties of silica Silicon dioxide, also known as sil- ica, is a well-known and extensively used excipient. Its inert nature and high purity make it an ideal material for pharmaceutical formulations. Over the years, silica manufacturers have perfected processes that make it highly functional and of consistently high quality. Now researchers and commercial groups are leveraging this material as a substrate for use in for- mulating solid dispersions. It's another tool to deal with bioavailability and the challenge of BCS II compounds. For decades, silica functioned pri- marily as a glidant and general pro- cessing aid. As a glidant, it reduces the inter-particulate friction of bulk powders, allowing the particles to flow more efficiently. This function is important because it improves the content uniformity of tablets and capsules and the speed at which they can be produced. When used as a general processing aid, silica is useful for carrying liquids and reducing electrostatic charges. It also serves as an anti-tacking agent in pharmaceuti- cal coatings, among other functions. Whatever its function, silica use lev- els are as small as 0.5 to 2 percent. Additionally, silica can control mois- ture during processing and in the final dosage form, which is useful for protecting moisture-sensitive APIs. This protection can extend the shelf- life of a drug product and/or sustain its therapeutic effect. Many antibi- otics, for example, are protected by silica used as a desiccant. Ultimately, it's the properties of the particles that make silica unique and such a versatile material. By tai- loring porosity, activating the sur- face, and essentially "engineering" various properties of the silica parti- cle, excipient manufacturers have made silica extremely interesting to formulators as a platform for drug delivery. Silica can also be synthe- sized by various routes and, depend- ing on the type of synthesis, its pores are formed differently. The method of synthesis also dictates the concentration of surface hydroxyl groups on the silica. Taken together, these properties deliver the specific shape, size, pore uniformity, and, ultimately, silica's potential as an effective drug delivery platform. Tablets & Capsules May 2017 41 William McCarthy W.R. Grace excipients This edition of the column summarizes the benefits of using silica to enhance the bioavailability of poorly soluble APIs. The pharmaceutical industry has been addressing the challenge of poorly soluble active pharmaceutical ingredients (APIs) and API candi- dates for more than two decades. This can be attributed to shifting methods of drug product design and to the industry's focus on more spe- cific drug candidates that has led to more complex molecules [1]. One study found that more than 75 per- cent of new chemical entities are poorly soluble [2]. Fortunately, there are new tech- nologies and novel approaches to help formulators enhance bioavail- ability. Examples include alternative synthesis, chemical complexing, micronization, emulsion systems, and solid dispersions [1]. Even so, solu- bility challenges aren't going away, and formulation scientists continue to seek more and better methods to address them. Using compendial excipients to improve bioavailability offers an expeditious route because they require no additional regulatory approval. Polymeric excipients, for instance, are now used in spray dry- ing and hot-melt extrusion (HME) to create solid dispersions that improve bioavailability. In fact, spray drying and HME have transformed how formulators use polymer excipients. In addition to their critical role in formulating amorphous solid dispersions, they Silica allows amorphous forms of APIs to be readily delivered into the gastrointestinal tract.