Hypoxia-inducible factors (HIFs) are regulated by the hydroxylation of prolyl residues in oxygen-dependent degradation domains, which leads to degradation by the proteasome. HIF prolyl hydroxylation is catalyzed by prolyl hydroxylase domain enzymes (PHD1, 2, and 3), members of the Fe(II) and 2-oxoglutarate (2OG) oxygenase family. They require dioxygen as a co-substrate, thus acting as the hypoxia-sensing component of the HIF system. IOX4 is a potent inhibitor of PHD2 (IC50 = 1.6 nM). It displays >1,000-fold selectivity for PHD2 over other 2OG-dependent dioxygenases, including JMJD isoforms, FBXL11, JARID1C, BBOX1, FIH, and FTO. IOX4 is active in vivo, inhibiting prolyl hydroxylation and increasing HIF-1α levels in cells (IC50 values range from 5.6 to 11.7 μM) and inducing HIF-1α and HIF-2α expression in mice. The induction of HIF expression in mice occurs in the brain as well as in the liver, kidney, and heart, indicating that IOX4 penetrates the blood-brain barrier.
