Overexpression of ABCB1 and ABCG2 contributes to reduced efficacy of the PI3K/mTOR inhibitor samotolisib (LY3023414) in cancer cell lines.

LY3023414 (samotolisib) is a promising new dual inhibitor of phosphoinositide 3-kinase (PI3K) and mammalian target of rapamycin (mTOR). Currently, multiple clinical trials are underway to evaluate the efficacy of LY3023414 in patients with various types of cancer. However, the potential mechanisms underlying acquired resistance to LY3023414 in human cancer cells still remain elusive. In this study, we investigated whether the overexpression of ATP-binding cassette (ABC) drug transporters such as ABCB1 and ABCG2, one of the most common mechanisms for developing multidrug resistance, may potentially reduce the efficacy of LY3023414 in human cancer cells. We demonstrated that the intracellular accumulation of LY3023414 in cancer cells was significantly reduced by the drug efflux function of ABCB1 and ABCG2. Consequently, the cytotoxicity and efficacy of LY3023414 for inhibiting the activation of the PI3K pathway and induction of G0/G1 cell-cycle arrest were substantially reduced in cancer cells overexpressing ABCB1 or ABCG2, which could be restored using tariquidar or Ko143, respectively. Furthermore, stimulatory effect of LY3023414 on the ATPase activity of ABCB1 and ABCG2, as well as in silico molecular docking analysis of LY3023414 binding to the substrate-binding pockets of these transporters provided additional insight into the manner in which LY3023414 interacts with both transporters. In conclusion, we report that LY3023414 is a substrate for ABCB1 and ABCG2 transporters implicating their role in the development of resistance to LY3023414, which can have substantial clinical implications and should be further investigated.

MY-5445, a phosphodiesterase type 5 inhibitor, resensitizes ABCG2-overexpressing multidrug-resistant cancer cells to cytotoxic anticancer drugs.

The overexpression of one or multiple ATP-binding cassette (ABC) transporters such as ABCB1, ABCC1 or ABCG2 in cancer cells often leads to the development of multidrug resistance phenotype and consequent treatment failure. Therefore, these transporters constitute an important target to improve the therapeutic outcome in cancer patients. In this study, we employed a drug repurposing approach to identify MY-5445, a known phosphodiesterase type 5 inhibitor, as a selective modulator of ABCG2. We discovered that by inhibiting the drug transport function of ABCG2, MY-5445 potentiates drug-induced apoptosis in ABCG2-overexpressing multidrug-resistant cancer cells and resensitizes these cells to chemotherapeutic drugs. Our data of MY-5445 stimulating the ATPase activity of ABCG2 and molecular docking analysis of its binding to the substrate-binding pocket of ABCG2 provide additional insight into the manner in which MY-5445 interacts with ABCG2. Furthermore, we found that ABCG2 does not confer resistance to MY-5445 in human cancer cells. Overall, our study revealed an additional action of MY-5445 to resensitize ABCG2-overexpressing multidrug-resistant cancer cells to conventional anticancer drugs, and this should be evaluated in future drug combination trials.