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.