1. ¹Rudolf Virchow Center, DFG Research Center for Experimental Biomedicine and Department of Dermatology, Bayerische Julius Maximilians University, Würzburg, Germany
2. ²Institute of Pharmacology and Toxicology, Bayerische Julius Maximilians University, Würzburg, Germany

Correspondence: Professor Michael P. Schön, Rudolf Virchow Center, DFG Research Center for Experimental Biomedicine and Department of Dermatology, University of Würzburg, Versbacher Str. 9, Würzburg 97078, Germany. E-mail: michael.schoen@virchow.uni-wuerzburg.de

³These authors contributed equally to this work.

Received 16 October 2005; Revised 6 February 2006; Accepted 9 February 2006; Published online 30 March 2006.

Abstract

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-B 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 A₁ 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.