Pharmacological inhibitors of JNK and ERK kinases SP600125 and U0126 are not appropriate tools for studies of drug metabolism because they activate aryl hydrocarbon receptor.

Mitogen-activated protein kinases (MAPKs) are important regulators of aryl hydrocarbon receptor (AhR). An immense progress in MAPKs' biochemistry was attained with the discovery of their specific pharmacological inhibitors. Unfortunately, the inhibitors of JNK and ERK MAPKs, i.e. SP600125 and U0126, respectively, affect AhR-CYP1A signaling pathway because they are partial agonists of AhR and induce CYP1A genes. This implies that SP600125 and U0126 are inappropriate tools for studies of the role of MAPKs in AhR regulation. The results from studies using SP600125 or U126, past or future, should be interpreted with prudence regarding their stimulatory effects on AhR-CYP1A pathway.