The small antitumoral immune response modifier imiquimod interacts with adenosine receptor signaling in a TLR7- and TLR8-independent fashion.

Imiquimod, a small-molecule immune response modifier of the imidazoquinoline family, has shown profound antitumoral and antiviral efficacy both in vitro and in clinical applications in vivo. It has been demonstrated that this activity is mediated through the Toll-like receptor (TLR)7- and TLR8-signaling cascade resulting in the secretion of proinflammatory cytokines and, consecutively, induction of a tumor-directed cellular immune response. In addition, imiquimod exerts a direct proapoptotic activity in tumor cells. We demonstrate here that imiquimod induces activation of the transcription factor NF-kappaB and the downstream production of proinflammatory cytokines in the absence of TLR7 and TLR8. In Chinese hamster ovary cells stably transfected with the human adenosine receptor subtypes, we then show in radioligand-binding competition experiments that imiquimod binds to adenosine receptors at concentrations relevant in clinical settings, with highest affinities to the A(1) and A(2A) subtypes. The effect on the receptor-mediated activation of adenylyl cyclase was also studied, and these experiments revealed that imiquimod acts as an adenosine receptor antagonist. In addition, imiquimod had an inhibitory effect on adenylyl cyclase activity downstream from the receptor. Finally, using transformed human keratinocytes, we provide experimental evidence that imiquimod and A(2A) adenosine receptor-specific compounds similarly induce proinflammatory cytokines in the absence of immune cells. Thus, imiquimod appears to suppress an important feedback mechanism of inflammation by antagonism of adenosine receptor-dependent increase of cAMP and a concomitant receptor-independent inhibition of cAMP production. These novel mechanisms presumably act synergistic with the positive induction of proinflammatory cytokines and can, at least in part, explain the profound inflammation observed in some patients in vivo.