Drugs in Topical Formulations

To date, topical drugs on the market are mainly chosen from well-established drugs prepared as oral or injectable formulations. For example, steroids with anti-inflammatory effects are used for the treatment of inflammatory skin diseases, such as psoriasis or atopic dermatitis. In addition, antimicrobial drugs are used for skin diseases, such as acne vulgaris and superficial bacterial infections, and antifungal drugs are used for superficial fungal infections. It is necessary that the full dose of the drug formulation penetrates immediately, at the local site of action on the skin, to produce the desired pharmacologic effect. Therefore, a drug having superior skin permeability is desirable for use as a topical drug. From past studies, it is known that the skin permeability of a drug is related to its physical properties, such as molecular weight, lipophilicity (the oil-water partition coefficient), and melting point. It is possible to select a topical drug with superior skin permeability using those physical properties as selection criteria. The focus of this paper is the characteristics of molecular weight, oil-water partition coefficient, and melting point of drugs with superior skin permeability. In addition, the physical properties of the major topical drugs (steroids, nonsteroidal anti-inflammatory drugs, antimicrobials, antifungals, immunosuppressive drugs, and topical anesthetics) presently on the market are discussed.