Novel curcumin analog (cis-trans curcumin) as ligand to adenosine receptors A2A and A2B: potential for therapeutics.

All four of the adenosine receptor (AR) subtypes mediate pain and have been targeted by pharmacologists to generate new therapeutics for chronic pain. The vanilloid phytochemicals, which include curcumin, capsaicin, and gingerol, have been shown to alleviate pain. However, there is little to no literature on the interaction of vanilloid phytochemicals with ARs. In this study, photochemical methods were used to generate a novel isomer of curcumin (cis-trans curcumin or CTCUR), and the interactions of both curcumin and CTCUR with the two Gs-linked AR subtypes were studied. Competitive binding assays, docking analysis, and confocal fluorescence microscopy were performed to measure binding affinity; cell survival assays were used to measure toxicity; and cAMP assays were performed to measure receptor activation. Competitive binding results indicated that CTCUR binds to both AR A2A and AR A2B with Ki values of 5 µM and 7 µM, respectively, which is consistent with our docking results. Fluorescence microscopy data also shows binding for A2B and A2A. Cell survival results show that CTCUR and CUR are nontoxic at the tested concentrations in these cell lines. Overall, our results suggest that vanilloid phytochemicals may be slightly modified to increase interaction with Gs-ARs, and thereby can be further explored to provide a novel class of non-opioid antinociceptives.

Curcumin Stereoisomer, Cis-Trans Curcumin, as a Novel Ligand to A1 and A3 Adenosine Receptors.

Adenosine receptors (ARs) are being explored to generate non-opioid pain therapeutics. Vanilloid compounds, curcumin, capsaicin, and vanillin possess antinociceptive properties through their interactions with the transient receptor potential channel family. However, their binding with adenosine receptors has not been well studied. The hypothesis in this study was that a vanilloid compound, cis-trans curcumin (CTCUR), binds to each of the two Gi-linked AR subtypes (A1AR and A3AR). CTCUR was synthesized from curcumin (CUR) using the cavitand-mediated photoisomerization technique. The cell lines transfected with the specific receptor (A1AR or A3AR) were treated with CTCUR or CUR and the binding was analyzed using competitive assays, confocal microscopy, and docking. The binding assays and molecular docking indicated that CTCUR had Ki values of 306 nM (A1AR) and 400 nM (A3AR). These values suggest that CTCUR is selective for Gi-linked ARs (A1AR or A3AR) over Gs-linked ARs (A2AAR or A2BAR), based on our previous published research. In addition, the docking showed that CTCUR binds to the toggle switch domain of ARs. Curcumin (CUR) did not exhibit binding at any of these receptors. In summary, CTCUR and other modifications of CUR can be developed as novel therapeutic ligands for the Gi-linked ARs (A1AR and A3AR) involved with pain and cancer.