Methods of Preparation
Suppositories can be extemporaneously prepared by one of three methods.
1. Hand Rolling is the oldest and simplest method of suppository preparation
and may be used when only a few suppositories are to be prepared in a cocoa
butter base. It has the advantage of avoiding the necessity of heating the cocoa
butter. A plastic-like mass is prepared by triturating grated cocoa butter and
active ingredients in a mortar. The mass is formed into a ball in the palm of
the hands, then rolled into a uniform cylinder with a large spatula or small
flat board on a pill tile. The cylinder is then cut into the appropriate number
of pieces which are rolled on one end to produce a conical shape.
Effective hand rolling requires considerable practice and skill. The suppository
"pipe" or cylinder tends to crack or hollow in the center, especially
when the mass is insufficiently kneaded and softened.
2. Compression Molding is a method of preparing suppositories from a
mixed mass of grated suppository base and medicaments which is forced into
a special compression mold. The method requires that the capacity of the molds
first be determined by compressing a small amount of the base into the dies
and weighing the finished suppositories. When active ingredients are added,
it is necessary to omit a portion of the suppository base, based on the density
factors of the active ingredients.
3. Fusion Molding involves first melting the suppository base, and
then dispersing or dissolving the drug in the melted base. The mixture is removed
from the heat and poured into a suppository mold. When the mixture has congealed,
the suppositories are removed from the mold. The fusion method can be used with
all types of suppositories and must be used with most of them.
Suppositories are generally made from solid ingredients and drugs which are
measured by weight. When they are mixed, melted, and poured into suppository
mold cavities, they occupy a volume the volume of the mold cavity. Since
the components are measured by weight but compounded by volume, density calculations
and mold calibrations are required to provide accurate doses.
When a drug is placed in a suppository base, it will displace an amount of
base as a function of its density. If the drug has the same density as the base,
it will displace an equivalent weight of the base. If the density of the drug
is greater than that of the base, it will displace a proportionally smaller
weight of the base. Density factors for common drugs in cocoa butter are available
in standard reference texts. The density factor is used to determine how much
of a base will be displaced by a drug. The relationship is:
For example, aspirin has a density factor in cocoa butter of 1.3 (see Remington's).
If a suppository is to contain 0.3 g of aspirin, it will replace 0.3 g ÷
1.3 or 0.23 g of cocoa butter. If the blank suppository (suppository without
the drug) weighed 2 g, then 2 g - 0.23 g or 1.77 g of cocoa butter will be needed
for each suppository, and the suppository will weigh 1.77 g + 0.3 g = 2.07 g.
So if a pharmacist was making 12 aspirin suppositories using cocoa butter as
the base, he would weigh 1.77 g × 12 or 21.24 g of cocoa butter and 0.3
g × 12 or 3.6 g of aspirin.
Some example density factors of drugs in cocoa butter are shown in the table
below (see Remington's):