Overcoming multidrug resistance using small molecule dynamic inhibitors by hijacking nascent and inducing turnover of mature ABCG2 for degradation in lysosomes.

ABCG2 has been associated with multidrug resistance (MDR) and protection of cancer stem cells. ABCG2 knockout had no apparent adverse effect on mice. Thus, ABCG2 is an interesting and perhaps an ideal target for drug discovery to overcome MDR and eliminate cancer stem cells. Although many ABCG2 inhibitors have been identified, few have moved into clinical testing and none has been approved. Thus, there is an unmet need for novel ABCG2 inhibitors. Targeted protein degradation (TPD) using proteolysis-targeting chimeras (PROTAC) and molecular-glues have been gaining traction with many in clinical trials, representing a new way targeting cytosolic proteins. However, TPD agents for membrane proteins are scarce. Recently, ABCG2 inhibitors with dynamic properties have been identified that they not only inhibit ABCG2 activity but also induce ABCG2 degradation. These dynamic inhibitors are unique and may represent a new class of TPD agents for membrane proteins and next generation inhibitors for development. Here, we investigated the mechanism of action of the dynamic inhibitor PZ-39 and its analogue PZ-39C8 and showed that they selectively bound to the extracellular loop between TM5-TM6 of ABCG2. This binding induces clathrin-dependent endocytosis of mature ABCG2 and hijacks nascent ABCG2, targeting them to lysosome via autophagy for degradation. PZ-39 also effectively induced ABCG2 loss and sensitized doxorubicin resistance in xenograft tumors. Thus, further investigation of dynamic ABCG2 inhibitors may lead to the next generation of therapeutics to overcome MDR in cancer chemotherapy and contribute to future design of TPD agents targeting membrane proteins.