Q: Can you please supply the stability of salicylic acid ointment 20%, with references?
A: Salicylic acid (SA) is available as a white powder or colorless crystals with a sweet, acrid taste. It is odorless, but irritating to nostrils. It has a melting point of 158ºC to 161ºC. Reported solubilities are 1 in 550, water; 1 in 4, ethanol; and 1 in 809, olive or almond oil. It also is soluble in white petrolatum (~1 in 80). SA solubility in water is increased by the presence of borax, ammonium citrate, ammonium acetate, sodium citrate, potassium citrate, and sodium phosphate. SA is incompatible with iodine, iron salts (use a hard rubber spatula), and oxidizing substances.
SA can increase viscosity of solutions containing quaternary ammonium compounds, probably due to increased micelle size. It has been reported that SA was most readily released from emulsion bases, especially oil-in-water vehicles, and that incorporation of SA in ointment base using cold spatulation yielded higher diffusion rates than when fusion technique was used.
SA is bactericidal, fungicidal, and keratolytic. Combined with sulfur, benzoic acid, coal tar, and/or resorcinol, it has been used in treating eczema, psoriasis, seborrhea, and other skin disorders. It has been used as an exfoliant in treatments for skin aging (wrinkles). It is not chemically a beta-hydroxy acid (BHA), although sometimes it is referred to as such. Investigations into effects of ultraviolet light on skin treated with SA and BHA are under way. Applied in concentrations of 10% to 12% in collodion vehicle or in concentrations of 20% to 50% in ointment or "plaster," it will cause sloughing of warts or corns. The FDA has limited concentrations of SA in OTC products to 12% to 40% in ointment, 5% to 17% in collodion-like vehicle, and 15% in karaya gum, glycol plaster vehicle (21CFR358.110).
SA/calcipotriene 0.005% ointment preparation stored at 5ºC and 25ºC reportedly resulted in incompatibility (calcipotriene concentration dropped precipitously). USP29/NF24 <795> provides "beyond-use date" (BUD) guidelines, applicable when no stability studies for a particular preparation can be found. These guidelines state: "For nonaqueous liquids and solid formulations, where a commercially manufactured drug product is the source of active ingredient, the BUD is not later than 25% of the time remaining until the product's expiration date or 6 months, whichever is earlier."
Mr. Erickson is director of professional affairs at Gallipot Inc.
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