Tablet diluents capacity

Minchom, C.M. Armstrong, N.A. A proposed technique for expressing the capacity of directly compressible tablet diluents. J. Pharm. Pharmacol. 1989, 39, 69. 

By definition, direct compression diluents are intended to be mixed with other ingredients. Therefore, not only should the pressure-tablet strength profile of the diluent be determined, but also should those of mixtures of the diluent with an active ingredient. The capacity of a direct compression diluent is the proportion of another ingredient that can be mixed with it while still obtaining tablets of acceptable quality. The definition of acceptable will depend on the purpose for which the tablets are required. 

The magnitude of the effect that a given active ingredient will have on tablet properties will clearly depend on the tabletting properties of that substance. If it is also capable of direct compression, then the effect will not be great. If, however, it is a substance that is difficult to compress into tablets, then it will cause a marked deterioration in tablet quality when mixed with the diluent. Therefore, for a reliable test of capacity, the direct compression diluent should be mixed with a standard substance and tabletted under standardized conditions. The pressure-strength profiles of the mixtures can then be constructed. Paracetamol and ascorbic acid have been used as standards. 

The technique of Malkowska and Khan, " used as described before to determine the capacity of a direct compression diluent, was originally developed as a method of expressing the ability of a formulation to be reworked. Referring to Fig. 2, the upper curve represents the strength of tablets prepared without reworking and the lower curve is the strength of reworked tablets. The reworking index is calculated from the ratio of the areas under the curves as described previously. 

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