The Antitumoral Mode of Action of Imiquimod and Other Imidazoquinolines

Imiquimod, the lead compound of the imidazoquinoline family of nucleoside analogues, has shown good efficacy against a variety of tumors of different origin. The mode of action of imiquimod and related compounds, which we have begun to understand in some detail in recent years, is complex and interesting inasmuch as it appears to comprise several presumably mutually enhancing components. Predominant amongst its actions is the induction of pro-inflammatory cytokines through agonistic activity towards Toll-like receptor (TLR)-7 and TLR-8, and consecutively, activation of the central transcription factor NF-kappaB. This activity stimulates the production of pro-inflammatory cytokines, chemokines and other mediators resulting in activation of antigen-presenting cells and the mounting of a profound Th1-weighted antitumoral cellular immune response. In addition, there are a number of secondary effects on the molecular and cellular level that can be explained through the activation of NF-kappaB. The pro-inflammatory activity of imiquimod appears to be augmented by suppression of a negative regulatory feedback mechanism which normally limits inflammatory responses. This is achieved independent of TLR-7 and TLR-8 through interference with adenosine receptor signaling pathways, particularly the A(2A) subtype, and receptor-independent reduction of adenylyl cyclase activity. Finally, at higher, albeit therapeutically relevant concentrations, imiquimod exerts a pro-apoptotic activity against tumor cells. Induction of apoptosis by imiquimod appears to be dependent on Bcl-2 proteins and involves caspase activation. The combination of multiple, presumably synergistic anti-tumoral functions by a single compound represents an interesting principle of pathogenesis-oriented, anti-neoplastic therapy.