An inhibitor of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter P-glycoprotein (P-gp), with adjuvant activity. Upon oral administration, P-gp inhibitor HM30181AK selectively binds to and inhibits the multidrug resistance (MDR) efflux pump P-gp, which prevents the efflux of various chemotherapeutic agents from intestinal epithelial cells to the gastrointestinal tract. This leads to an increase in both oral bioavailability and therapeutic efficacy. P-gp prevents the intestinal uptake and intracellular accumulation of various cytotoxic agents. HM30181AK is not systemically absorbed.