Foams

Foams

Stable foams are semisolid preparations that entrap air upon application to form a lightweight, flexible matrix with a large surface area of the liquid. Foams are sometimes used for topical application to areas that are otherwise difficult to reach, such as hairy scalp, or on sensitive skin, such as in acne.

For example, Luxiq® aerosol foam is a topical anti-inflammatory cor-ticosteroid formulation that contains 0.12% w/w betamethasone valerate in a thermolabile hydroethanolic foam vehicle. This foam vehicle consists of ethanol (~60%), cetyl alcohol, stearic acid, polysorbate 80, potassium citrate, propylene glycol, purified water, and cetyl alcohol. It is pressur-ized with a hydrocarbon (propane/butane) propellant. The foam melts upon contact with warm skin and is intended for application to the scalp. Similarly, clobetasol 0.05% foam is an anti-inflammatory corticosteroid formulation intended for application to the scalp. Clindamycin phosphate foam 1% is a topical antibiotic preparation for use in acne.

Foams typically contain a hydrocarbon propellant in the packaging container to pressurize the drug solution. The drug is dissolved in a low boiling point vehicle, such as the one containing a high proportion of etha-nol, which also has a surfactant and a base to dissolve the drug. The vehicle may also contain preservatives and buffering agents. Evaporation of etha-nol upon aerosolization leads to expansion of liquid droplets and formation of foam by entrapment of air.