PGP and cancer: Although the possibility of other mechanisms contributing to the resensitization of multidrug-resistant cancer cells remains, and that unexpected adverse drug interactions may occur in combination therapies (Stewart et al., 2004; Shukla et al., 2008; Libby and Hromas, 2010; Robey et al., 2018), we report here an additional action of branebrutinib that could be utilized in combination therapy with conventional anticancer drugs to treat multidrug-resistant cancers associated with the overexpression of P-gp, which should be investigated further.