FIGURE 4.

Off-target effects of common ERK5 inhibitors. The earliest ERK5 inhibitor, XMD8-92, has been used extensively to probe supposed roles of ERK5 kinase activity. However, XMD8-92 also inhibits binding of BRD4 to its target acetylated protein targets to modulate transcriptional processes. Indeed, XMD8-92 is equipotent for inhibition of ERK5 kinase activity and blockade of binding of BRD4 to ε-N-acetylated lysine-containing sequences. This “off-target” activity against a critical regulator of gene expression is shared by the closely related XMD17-109 and XMD-17-26 and likely accounts for the anti-inflammatory and anti-proliferative effects of these agents. For this reason they should no longer be employed in cellular or in vivo studies of ERK5 function and studies that have relied upon them should be revisited and viewed with caution. Second generation ERK5i that lack BRD4 activity have now been described and include the dual-ERK5/LRRK2 inhibitors, JWG-045 (XMD10-78), and JWG-071. ADTL-EI1712 inhibits ERK1, ERK2, and ERK5 and may find utility in the treatment of melanoma where resistance to BRAFi, MEKi or ERKi is sometimes driven activation of ERK5; however, it is not known if ADTL-EI1712 has BRD4 activity. AX15836, compound 46, and BAY-885 lack BRD4 binding activity and are currently the most potent and selective inhibitors of ERK5 kinase activity and the preferred options for selective inhibition of ERK5 kinase activity in cells and in vivo. Finally, ERK5 activation can also be prevented by the potent and selective MEK5 inhibitors BIX02188 and BIX02189.