Estimation by Calculation

One method to determine the density factor of a drug in a base other than cocoa butter requires the use of the ratio of a blank suppository of the non-cocoa butter base to a blank suppository of the cocoa butter base. This informationis generally obtained by calibrating the mold first with one base and then the other base.

As an example of the method, a mold was calibrated with the PEG base and the average blank suppository weighed 2.24 grams. The same mold was calibrated with cocoa butter and those blank suppositories weighed 1.87 grams on average. Therefore, the ratio of the two weights was:

If 200 mg of aspirin is to be incorporated into each PEG suppository, it is necessary to determine how much PEG base will be displaced by the aspirin. That displacement amount can be calculated as follows:

• density factor of aspirin in cocoa butter = 1.3 (from reference sources)
• density of PEG base relative to cocoa butter = 1.20 (the ratio obtained from the calibrations)
• 0.2 g of aspirin will displace of PEG base

For each PEG suppository to be formulated, 0.2 g of aspirin and 2.06 g (2.24 g - 0.18 g = 2.06 g) of the PEG base will be needed.