Impairment of TNF-α production and action by imidazo[1,2-α] quinoxalines, a derivative family which displays potential anti-inflammatory properties

In a previous study, we analysed the synthesis and properties of a series of imidazo[1,2-a]quinoxalines designed in our laboratory as possible imiquimod analogues. We found that these imidazo[2-a]quinoxalines were in fact potent inhibitors of phosphodiesterase 4 enzymes (PDE4). PDE4 inhibition normally results in an increase in intracellular cAMP which, in PBMC, induces the suppression of TNF-alpha mRNA transcription and thus cytokine synthesis. Such an effect is antagonistic to that of imiquimod. Furthermore, some TNF-alpha-induced activity, such as cell apoptosis which is dependent on the intracellular cAMP levels might also be affected. Therefore, by counteracting the properties of TNF-alpha and/or its production, the imidazo[1,2-alquinoxalines could be considered as potential anti-inflammatory drugs. The present study was performed to confirm or refute this hypothesis. For this, we characterized the effects of imidazo[1,2-a]quinoxalines both on TNF-alpha activity and synthesis in regard to their ability to act as inhibitors of PDE4 (IPDE4). We found that the imidazo[1,2-a]quinoxalines dose-dependently prevented the TNF-alpha-triggered death of L929 cells, with the 8-series (-NHCH3 in R4) being the most potent. Moreover, when the effect of the 8-series on TNF-alpha production was investigated using 7962 T cells, it was observed that these compounds impaired the TCR:CD3-triggered TNF-alpha production. Structure-activity analysis revealed that these properties of the drugs did not coincide with their IPDE4 properties. This prompted further exploration into other signalling mechanisms possibly involved in TNF-alpha action and production, notably the p38 MAPK and the PI3K pathway. We demonstrate here that the imidazo[1,2-alquinoxalines targeted these pathways in a different way: they activated the p38 MAPK pathway whilst inhibiting the PI3K pathway. Such effects on cell signalling could account for the imidazo[1,2-a]quinoxalines effects on 1) action and 2) production of TNF-alpha, which define these drugs as potential anti-inflammatory agents.