Encequidar, also known as HM-30181, is an oral P-glycoprotein (P-gp) inhibitor developed to enhance the oral bioavailability of P-gp substrate drugs. Encequidar showed the highest potency (IC(50)=0.63nM) among several MDR1 inhibitors, including cycloporin A, XR9576, and GF120918, and effectively blocked transepithelial transport of paclitaxel in MDCK monolayers (IC(50)=35.4nM). Encequidar is currently under clinical trials.