MKC9989 is a Hydroxy aryl aldehydes (HAA) inhibitor and also inhibits IRE1α with an IC50 of 0.23 to 44 μM.

At 10 μM concentration, MKC9989 completely inhibits both basal and thapsigargin induced splicing of XBP1 mRNA. These effects are observed even in cells pre-treated with thapsigargin, indicating that MKC9989 can fully reverse the onset of XPB1 splicing after the UPR is initiated. In parallel analysis, MKC9989, significantly stabilizes the RIDD target CD59 mRNA when co-administered with thapsigargin relative to thapsigargin treatment alone and modestly increases levels of CD59 mRNA in non-stressed cells, the latter likely reflects the inhibition of baseline RIDD activity. In contrast to effects on XBP1 splicing, MKC9989 moderately stabilizes CD59 levels when administered 2 hour post treatment with thapsigargin. Finally, the potency of MKC9989 against the splicing of XBP1 mRNA (EC50=0.33 μM) is comparable to its potency against RNA cleavage in vitro[1].

[1]. Sanches M, et al. Structure and mechanism of action of the hydroxy-aryl-aldehyde class of IRE1 endoribonuclease inhibitors. Nat Commun. 2014 Aug 28;5:4202.