ABCG2 and squamous cell carcinoma: Our previous study uncovered that PI3K inhibitor BAY-1082439 was able to reverse P-gp-mediated drug resistance in oral epidermoid carcinoma cell line and non-small cell lung cancer cell line, and the mechanism of action of BAY-1082439 is to inhibit PI3K which can downregulate the expression of P-gp and BCRP [37].