Mechanism of action and other potential roles of an immune response modifier.

Imiquimod (1-[2-methylpropyl]-1H-imidazo[4,5-c]quinolin-4-amine) is a widely used topical immune response modifier. This drug was initially developed as an antiviral agent and was found to have potent effects on the immune system. Because imiquimod causes the activation of antigen-presenting cells (APCs), it acts as an immunologic adjuvant. By triggering cytokine production, imiquimod enhances the ability of APC to present viral or tumor antigens to reactive T lymphocytes, and amplifies type 1 helper T cell (T(H) 1)-mediated immune responses (interferon [IFN] gamma production as well as other related cytokines). The cellular receptors for imiquimod and its analogues are toll-like receptors (TLR) 7 and 8. These 2 receptors are part of a larger family of TLRs that are critical components of innate immunity, which has evolved to detect dangerous bacterial, viral, fungal, and parasitic infections. Topical imiquimod is a U.S. Food and Drug Administration (FDA)-approved treatment for external genital warts, actinic keratoses (AKs), and superficial basal cell carcinomas (sBCCs). Because there are a number of cell types that express either TLR7 or receptors for cytokines induced by imiquimod, this agent has broad-reaching direct and indirect effects in the skin as well as the related skin immune system. Thus, imiquimod has been demonstrated to be useful in the treatment of a number of conditions beyond the FDA-approved indications (mostly neoplastic and infectious but also fibrotic and some degenerative conditions).