Tariquidar Is an Inhibitor and Not a Substrate of Human and Mouse P-glycoprotein

Abstract

Since its development, tariquidar (TQR; XR9576;4-dihydro-1H-isoquinolin-2-yl)ethyl]phenyl]carbamoyl]-4,5-dimethoxyphenyl]quinoline-3-carboxamide) has been widely regarded as one of the more potent inhibitors of P-glycoprotein (P-gp), an efflux transporter of the ATP-binding cassette (ABC) transporter family. A third-generation inhibitor, TQR exhibits high affinity for P-gp, although it is also a substrate of another ABC transporter, breast cancer resistance protein (BCRP). Recently, several studies have questioned the mechanism by which TQR interfaces with P-gp, suggesting that TQR is a substrate for P-gp instead of a noncompetitive inhibitor. We investigated TQR and its interaction with human and mouse P-gp to determine if TQR is a substrate of P-gp in vitro. To address these questions, we used multiple in vitro transporter assays, including cytotoxicity, flow cytometry, accumulation, ATPase, and transwell assays. A newly generated BCRP cell line was used as a positive control that demonstrates TQR-mediated transport. Based on our results, we conclude that TQR is a potent inhibitor of both human and mouse P-gp and shows no signs of being a substrate at the concentrations tested. These in vitro data further support our position that the in vivo uptake of [ 11 C]TQR into the brain can be explained by its highaffinity binding to P-gp and by it being a substrate of BCRP, followed by amplification of the brain signal by ionic trapping in acidic lysosomes.

Role of P-glycoprotein inhibitors in ceramide-based therapeutics for treatment of cancer

“…Cyclosporin A, a substrate for P-gp, blocks the pumping of drugs in a competitive manner [55] and inhibits drug-activated and basal ATPase activity of P-gp [56]. Tariquidar, a potent inhibitor of P-gp [57, 58], but also a substrate and inhibitor for breast cancer resistance protein (BCRP/ABCG2) [59], shows a noncompetitive interaction with P-gp substrates and inhibits the ATPase activity of P-gp [60]; it could thus be considered to have an allosteric effect on substrate recognition or ATP hydrolysis. There is also a report showing that tariquidar inhibits P-gp drug efflux by blocking transition to the open state during the catalytic cycle [61].…”

Imaging transporters: Transforming diagnostic and therapeutic development

“…A recent comprehensive analysis of the interactions of the third generation inhibitor, tariquidar, with P‐gp demonstrated that tariquidar does not behave as a P‐gp substrate. BCRP was suggested as the major transporter that mediates tariquidar's removal from the brain . The current absence of PET markers that bind with high affinity to P‐gp or BCRP complicates transporter expression studies in humans.…”

How to overcome ATP-binding cassette drug efflux transporter-mediated drug resistance?

“…Several studies report that miRNAs can regulate MDR by modulating the expression of P-gp. For example, it has been shown that miR-451 and miR-27a upregulation was P-gp dependent in cell lines representing ovarian cancer, leukemia and hepatocellular carcinoma [112,114,115] . In contrast, deregulation rather than upregulation of certain miRNAs, such as miR27a and miR 331-5p, can also cause drug resistance reversal and decreased P-gp expression, as seen in doxorubicin resistant leukemia cell lines K562 and HL60 [131][132][133] .…”

“…Since its discovery, tariquidar was described as non-competitive and non-transported inhibitor of P-gp [110] , while Bankstahl et al [111] have shown that tariquidar is concentration-dependently transported by P-gp. Recent transepithelial drug transport assays using radioactive tariquidar in human and mouse cell lines indicate that tariquidar functions as a high-affinity P-gp substrate rather than a non-competitive inhibitor [112] .…”

“…For instance, the dose of the anticancer agents used in combination with the second-generation P-gp inhibitor PSC 833 had to be reduced to prevent toxicities of the anticancer agent because PSC 833 caused a decreased drug clearance via inhibition of its metabolism by cytochrome P450 [112] . Regarding patient selection, initial clinical trials enrolled patients irrespective of their P-gp status, whereas rationally patient selection should be based on adequate transporter(s) expression and/or function in tumor specimen and whether the targeted transporter is the dominant mechanism of drug resistance [91] .…”