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Tablets & Capsules January 2015 39 grittiness. The USP-NF describes it as containing not less than 15 percent MgO and not less than 67 percent SiO 2 . It is chemi cally stable but should be protected from moisture. Magnesium silicate is non-toxic and non-irritating. The dust particles may be irritating to the eyes. Gloves, eye protection and a respirator should be used when handling large amounts. It is GRAS listed and in cluded in the FDA's IID for use in oral solid dosage forms. Magnesium silicate has been known to decrease the bioavailability of certain APIs by chelation or binding when taken together. Long- term use as an antacid has resulted in bladder and renal stones. Its ability to adsorb moisture makes it useful as an anti-caking agent in food and pharmaceutical products. It also serves as a glidant. Magnesium trisilicate. This com - pound of magnesium oxide and sili - con dioxide (CAS# 14987-04-3) has the chemical formula Mg 2 Si 3 O 8 xH 2 O. This clay-like material occurs natu rally as a mineral, e.g., meers - chaum. Like magnesium silicate, the synthetic form is pre pared by re - acting soluble sodium silicate (water - glass) with a magne sium salt. It is a tasteless, odor less, fine white powder free of grit tiness. The USP-NF des - cribes it as containing not less than 20 per cent MgO and not less than 45 percent SiO 2 . It is chemically stable but highly hygroscopic and should be protected from moisture. Magnesium trisilicate is non-toxic and non-irritating. Gloves, eye pro - tec tion, and a respirator should be used when handling large amounts. It is GRAS-listed and is included in the FDA's IID for use in oral solid dosage forms. Magnesium trisilicate has been known to decrease the bioavailability of certain APIs by chelation or bind - ing when taken together. Long-term use as an antacid has resulted in bladder and renal stones. Magnesium trisilicate is used mainly as an antacid to treat peptic ulcers. It increases the pH of gastric juice via a neutralization reaction. Therapeutically, about 2 grams may be taken daily. It also serves as a gli - dant in oral pharmaceutical formu - lations and food products. Magnesium aluminum silicate. This complex (CAS# 1327-43-1) has the general chemical formula MgAl 2 SiO 6 . It comprises a three-lattice layer of octahedral alumina and two tetra - hedral silica sheets. "Smectite" is the mineralogical term for a group of trilayer clays, in cluding the com - mercially significant varieties mont - morillonite, hectorite, and saponite. Traditionally, smectites have been known as bentonite, a geological term, and bentonite ore contains a sub stantial amount of smectite, usu - ally montmorillonite. When smectite clays come in contact with water, their platelets expand due to hy - dration of the cations associated with the mineral. This expansion permits water to infiltrate the crystal struc - ture, increasing overall volume ten- to twentyfold. It is a tasteless, odor - less, off-white, soft, and slippery powder processed to be free of grit and to remove the ore's non-swelling components. It is chemically stable but should be protected from mois - ture. The USP-NF describes four types, which differ in viscosity and in the ratio of aluminum-to-magnesium they contain, expressed as a percent - age range (Table 2). Magnesium aluminum silicate is non-toxic and non-irritating. Gloves, eye protection, and a respirator should be used when handling large a mounts. It is included in the FDA's IID for use in oral solid dosage forms and suspensions and in rectal, va gi - nal, and topical preparations. Magne - sium aluminum silicate has been known to decrease the bio avail ability of certain actives by chelation or binding when taken together. Smectite clays have a long history of use as excipients in tablets, oint - ments, and creams. In liquids, they are used primarily to increase visco sity and to stabilize suspensions and emul - sions. In ointments and supposi tories, they are used to control drug release. In solid dosage forms they are tradi - tionally used as binders, adsorbents, and disintegrants. Mag ne sium alumi - num silicate is also used in solid dosage forms as a component of drug delivery systems. This ap plication is of in creasing interest be cause this group of natural excipients provides a unique combination of physico - chemical prop erties. The drug-clay interactions in clude cation exchange, anion ex change, hydrogen bonding, high-surface-area adsorption, and intercalation. There are numerous grades and suppliers of magnesium aluminum silicate, including the Veegum line from RT Vanderbilt Minerals, Norwalk, CT, and another line from Brenntag Specialites, South Plainfield, NJ. A totally synthetic grade that meets compendial specifications is Neusilin, from Fuji Health Science, Burlington, NJ. This material has unique properties and applications. Unlike smectite clays, which have a layered crystalline structure, Neusilin is amorphous. As a result, its surface area can reach as much as 300 m 2 /g, a property that increases its functionality. It serves as an anti-caking agent, adsorbent, binder, disintegrant, and glidant, it has also been used to keep drugs amorphous. Neusilin comes in various grades that differ in bulk density, water content, particle size, and pH. While the mined material has a basic pH, Neusilin is available in basic and neutral grades. T&C J o h n A . M c C a r t y i s p r e s i d e n t o f McCarty Pharma Consultants, PO B o x 6 6 1 5 6 8 , M i a m i S p r i n g s , F L 33266. Tel. 305 887 9220, fax 305 888 6 0 4 0 . E - m a i l : j o h n . m c c a r t y @ comcast.net. He is also a member of Tablets & Capsules' Technical Advisory Board. Table 2 USP-NF magnesium aluminum silicate types Type Viscosity Al-to-Mg ratio (mPa•s) (%) IA 225-600 0.5-to-1.2 IB 150-450 0.5-to-1.2 IC 800-2,200 0.5-to-1.2 IIA 100-300 1.4-to-2.8