Tablets

TABLETS

Additional processes are required for tablet production beyond those described previously. Because these processes are not ubiquitous in pharmaceutical manufacturing, they are dealt with only briefly here. Many of them are required for all solid dosage forms. Each process must be conducted while balancing the effects of the respective excesses.

Compressed solids, tablets, or caplets are prepared by placing the blend of component additives in a cylinder or die, above a movable piston or punch. An upper punch is brought into the top of the piston, and pressure applied to the distal ends of the punches forces the powder into a compact (Fig. 14.7). Product quality depends on the cohesive forces acting on the powder under compres-sion. These cohesive forces are influenced by the selection of additives in the dosage formulation. One method of evaluating tablet manufacture considers the effect of applied pressure on porosity of the compressed powder (Carstensen, 1993). Data may be plotted as the negative natural logarithm of porosity against applied pressure in the form of a Heckel plot (Heckel, 1961). The slope is pro-portional to the yield value (φ, elastic limit) l/3φ.

The tooling of a tablet press varies according to the tablet design. Con-sideration must be given to the distribution of forces across the faces of the tablet punches as they are brought together to compress the tablet in the die. As more unusually shaped tablets are produced and more elaborate embossing tools are required, the forces are not distributed evenly across the punches, and care must be taken if they are to have a reasonable, useful life span. Mathematically, finite element analysis can be used to characterize these forces and to calculate the requirement for preserving the tools· for extended periods.

FIGURE 14.7 Tablet manufacture depicting the three steps of filling, compression, and ejection.

Coating is achieved by placing a batch of tablets in a coating pan and spraying or coating from solution with the required polymer. The Accela-Cota (Fig. 14.8) is one of the more common coating systems.

Tablets have been prepared with different characteristics and for different purposes. The most common tablets are uncoated, coated, chewable, or effer-vescent. Some specialized dosage forms have been developed for sublingual and buccal delivery. A typical uncoated conventional tablet might have the com-position shown in Table 14.1. Examples of such systems include generic aspirin and Valium. These tablets are designed for rapid dissolution.

Tablets may be coated for a variety of reasons, including better appear-ance, taste masking, ease of swallowing, protection from light, protection from gastrointestinal irritation, facilitating tablet printing, and control release. The formulation of a coated tablet is similar to that of an uncoated tablet. Usually, it is coated from a solution of polymer, for example, methylcellulose, enteric polymer. Bayer aspirin or erythromycin products are examples of coated tablets.

Chewable tablets are usually flavored and contain additives that contrib-ute to a smooth texture, including glycerin and sugars such as mannitol and sorbitol. An example is Tylenol chewable tablets.

FIGURE 14.8 Accela-Cota. Source: Courtesy of Thomas Engineering, Hoffmann Estates, Illi-nois, U.S.

TABLE 14.1 Tablet Composition

Effervescent tablets are formulated so that an acid-base reaction occurs when they are combined with water. This is achieved by using weak acids (e.g., citric, malic, tartaric, or fumeric acids) or bases (e.g., sodium or potassium carbonates) in the product. The best known of these products is Alka-Seltzer.

Sublingual tablets are designed to disintegrate and dissolve instantly. Hence, they must have structural integrity sufficient for storage, transport, and administration but capable of dissolution on the oral mucosa under the tongue. Nitroglycerin tablets designed for treating angina are prepared in a composi-tionally simple formulation of lactose massed with 60% ethanol. This route of administration is intended to avoid first-pass liver metabolism. Testosterone tablets have been prepared for buccal delivery by slow dissolution. The tablet does not contain a disintegrant and is intended to have an extended residence time in the buccal cavity at the rear of the mouth. Since release is not immediate, drug dosage may be significantly reduced by this route.