So many materials are used as pharmaceutical necessities and as vehicles in topical systems that they defy thorough analysis. The pharmacist should nevertheless make some effort to learn of the more common constituents and their principal functions. The compositions of formulations as presented on product labels are the main source of such information. The compositions presented in Table 6 offer a glimpse into the compositional natures of semisolids.
Because of the large number of materials that are used in topical preparations and the diverseness of their physical properties, the formulation of topical dosage forms tends to be something of an art perfected through experience. Only by making myriad recipes does one eventually gain insight about the materials and their use in the design of new formulations. Such insight allows the experienced formulator to manipulate the properties of existing formulations to gain a desired characteristic. Often one finds good recipes to use as starting points for formulations in the trade literature. Two factors have to be kept in mind when borrowing the compositions of such trade formulations. First, the trade recipes (recipes supplied with advertising material touting specific components) are often inadequately tested in terms of their long-term stability. Second, the dominant features used in judging the merits of trade-promoted formulas tend to be their initial appearances and overall elegance. Little to no attention can be paid to the drug delivery attributes of the prototypical systems when they are first prepared in the suppliers laboratories because the drug delivery attributes are so compound specific. Thus, it is left up to the pharmacist (industrial research pharmacist) to make adjustments in the formulas, which are consistent
Table 6 Prototype Formulations
I. Ointment (white ointment, USP)
White petrolatum 95% (w/v)
White wax 5%
Melt the white wax and add the petrolatum; continue heating until a liquid melt is formed. Congeal with stirring. Heating should be gentle to avoid charring (steam is preferred), and air incorporation by too vigorous stirring is to be avoided.
II. Absorption ointment (hydrophilic petrolatum, USP)
White petrolatum 86% (w/w) Stearyl alcohol 3% White wax 8% Cholesterol 3%
Melt the stearyl alcohol, white wax, and cholesterol (steam bath). Add the petrolatum and continue heating until a liquid melt is formed. Cool with stirring until congealed.
HI. Water-washable ointment (hydrophilic ointment, USP)
White petrolatum 25% (w/w)
Stearyl alcohol 25%
Propylene glycol 12%
Sodium lauryl sulfate 1%
Purified water 37%
Melt the stearyl alcohol and white petrolatum (steam bath) and warm to about 75 °C. Heat the water to 75°C and add the sodium lauryl sulfate, propylene glycol, methylparaben, and propylparaben. Add the aqueous phase and stir until congealed.
IV. Water-soluble ointment (polyethylene glycol ointment, USP 14)
Polyethylene glycol 4000 (Carbowax 4000) 50%
Polyethylene glycol 400 50%
Melt the PG 4000 and add the liquid PG 400. Cool with stirring until congealed.
V. Cream base, w/o (rose water ointment, NF 14) Oleaginous phase
White wax 12.0%
Almond oil 55.58%
Sodium borate 0.5%
Stronger rose water, NF 2.5%
Purified water, USP 16.5%
Melt the spermaceti and white wax on a steam bath. Add the almond oil and continue heating to 70°C. Dissolve the sodium borate in the purified water and stronger rose water, warmed to 75°C. Gradually add the aqueous phase to the oil phase with stirring. Cool to 45°C with stirring and incorporate the aromatic (rose oil).
Note: This is a typical cold cream formulation. The cooling effect comes from the slow evaporation of water from the applied films. The aromatic is added at as low a temperature as possible to prevent its loss by volatilization during manufacture.
Table 6 Prototype Formulations (Continued)
VI. Cream base, o/w (general prototype)
Stearyl alcohol 15%
Sorbitan monooleate 1.25% Aqueous phase
Sorbitol solution, 70% USP 7.5%
Polysorbate 80 3.75%
Propylparaben 0.015% Purified water, q.s. ad 100%
Heat the oil phase and water phase to 70°C. Add the oil phase slowly to the aqueous phase with stirring to form a crude emulsion. Cool to about 55°C and homogenize. Cool with agitation until congealed.
VII. Cream base, o/w (vanishing cream)
Stearic acid 13%
Stearyl alcohol 1%
Cetyl alcohol 1% Aqueous phase
Potassium hydroxide 0.9%
Purified water, q.s. ad 100%
Heat the oil phase and water phase to about 65°C. Add the oil phase slowly to the aqueous phase with stirring to form a crude emulsion. Cool to about 50°C and homogenize. Cool with agitation until congealed
Note: In this classic preparation, the stearic acid reacts with the alkaline borate to form the emulsifying stearate soap.
VIII. Paste (zinc oxide paste, USP)
Zinc oxide 25%
White petrolatum, q.s. ad 100%
Titrate the calamine with the zinc oxide and starch and incorporate uniformly in the petrolatum by levigation in a mortar or on a glass slab with a spatula. Mineral oil should not be used as a levigating agent, since it would soften the product. A portion of the petrolatum can be melted and used as a levigating agent if so desired.
IX. Gel (lubricating jelly)
Carbopol 934 0.24%
Propylene glycol 16.7%
Sodium hydroxide, q.s. ad pH 7
Purified water, q.s. ad 100%
Disperse the Methocel in 40 mL of hot (80-90°C), water. Chill overnight in a refrigerator to effect solution. Disperse the Carbopol 934 in 20 mL of water. Adjust the pH of the dispersion to 7.0 by adding sufficient 1% sodium hydroxide solution (about 12 mL is required per 100 mL) and bring the volume to 40 mL with purified water. Dissolve the methylparaben in the propylene glycol. Mix the Methocel, Carbopol 934, and propylene glycol fractions using caution to avoid the incorporation of air.
with good delivery of specific drugs. Each drug requires unique adjustments in accord with its singular physicochemical properties.
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