In vitro activity: The ratio of total concentration of extracellular ML133 versus intracellular ML133 is 1:0.13 at pH 6.5, 1:9.09 at pH 8.5. ML133 is clean against 3A4 and 2C9 (IC50>30 μM), displays moderate inhibition of 1A2 (IC50 = 3.3 μM) but proves to be a potent inhibitor of 2D6 (IC50 = 0.13 μM) in CYP450 assay. ML133 is highly protein bound (>99%) in both human and rat and also displays high intrinsic clearance in both species. ML133 (10 μM) is not displaced by K+ influx from the wild-type Kir2.1 in HEK 293 cells. M1 and/or M2 transmembrane domains contain the critical molecular determinant for ML133 inhibition of Kir2.1, especially D172 and I176.

Kinase Assay: ML133 hydrochloride is a selective inhibitor of the Kir2 family of inward rectifier (IRK, KCNJ) potassium channels. ML133 inhibits Kir2.1 with IC50 of 1.8 μM at pH 7.4 and 290 nM at pH8.5. It exhibits little selectivity against other members of Kir2.x family channels, but has no effect on Kir1.1 (IC50> 300 μM), and displays weak activity for Kir4.1 (76 μM) and Kir7.1 (33 μM), making ML133 the most selective small molecule inhibitor of the Kir family reported to date. It also showed modest selectivity versus hERG and a larger panel of GPCRs, ion channels and transporters.

Cell Assay: ML133, which inhibits Kir2.1 with IC50 of 1.8 μM at pH 7.4 and 290 nM at pH 8.5, but exhibits little selectivity against other members of Kir2.x family channels. ML133 has no effect on Kir1.1 (IC50 > 300 μM), and displays weak activity for Kir4.1 (76 μM) and Kir7.1 (33 μM), making ML133 the most selective small molecule inhibitor of the Kir family reported to date.