By Jessica Lu
A major problem in the treatment of cancer is that cancer cells can acquire resistance to anticancer drugs. This resistance could be specific to the drug used, but an especially difficult problem is the development of multidrug resistance (MDR). MDR occurs when cancer cells develop resistance to various structurally and functionally different anticancer drugs (Choi, 2005). Most patients who die of cancer have metastatic cancers with MDR (Robey et al., 2018). The ATP-binding cassette (ABC) transporters are a family of transporter proteins which play a key role in the development of MDR. ABC transporters are transmembrane proteins which shuttle a multitude of chemically diverse substrates across cell membranes. Some have very broad substrate specificity. In cancer cells, the overexpression of ABC transporters with broad specificity allows a range of anticancer drugs to be pumped out, leading to MDR. In total, 19 ABC transporters have been shown to efflux anticancer drugs (Robey et al., 2018).
The first ABC transporter discovered to play a role in cancer drug resistance was multidrug resistance protein 1 (MDR1), also known as P-glycoprotein (P-gp) (Robey et al., 2018). MDR1 is a 170 kDa membrane protein composed of 12 hydrophobic transmembrane domains, and 2 nucleotide binding domains which bind and hydrolyse ATP. Similarly to all ABC transporters, the binding and hydrolysis of ATP causes conformational changes in both the transmembrane domains and the nucleotide binding domains, allowing substrates to be shuttled out (Choi, 2005). The first structure of human MDR1 was solved in 2018, with the protein in an ATP-bound, outward facing conformation (Kim & Chen, 2018).
After the discovery of MDR1, inhibitors for it were developed and tested in clinical trials in an attempt to overcome MDR (Robey et al., 2018). At least three generations of MDR1 inhibitors have been tested. The first and second generation included inhibitors such as verapamil, cyclosporin and valsdopar. These all failed in clinical trials due to lack of potency or off-target effects. More recently, a third generation of inhibitors have been designed, including drugs such as elacridar, zousuquidar and tariquidar. These inhibitors have shown some positive results, however patient response rates have been variable. This is possibly due to heterogenous MDR1 expression or the coexpression of other efflux drug transporters. Another problem is that there is little knowledge about whether MDR1 inhibitors even reach the cells of interest (Sprachman et al., 2014). To combat these failures, Sprachman et al. (2014) suggest that it is important to develop techniques for real-time imaging of MDR1 expression and inhibition in single cells in vivo. They developed fluorescent versions of the third-generation MDR1 inhibitor, encequidar (HM30181). One of these (derivative 4) was found to be a useful tool for in vivo real-time cellular imaging agent, with potential to aid future research.
Aside from MDR1, there is compelling evidence for the role of two other ABC transporters in MDR: breast cancer resistance protein (BCRP) and multidrug resistance protein 1 (MRP1). However, apart from these three, other ABC transporters are also able to efflux anticancer drugs in vitro (Fletcher et al., 2016). Animal models have shown that several of these affect the pharmacokinetics of anticancer drugs in animal models. For example, the loss of multidrug resistance-associated protein 7 (MRP7) sensitises mice to the cancer drug paclitaxel. However, it is still unclear whether these other ABC transporters contribute directly to MDR in tumours (Fletcher et al., 2016). This is an area with huge potential for future research, especially because, outside of MDR, ABC transporters may play other roles in cancer proliferation, lack of differentiation in cancer stem cells and metastasis (Fletcher et al., 2010).
The role of ABC transporters in MDR in cancer is still being investigated, particularly outside of MDR1, BCRP and MRP1. Even if they do not contribute directly to MDR, they may still play other
important roles in the development of cancer. Although efforts to combat MDR caused by ABC transporters have thus far been unsuccessful, it is still an area which demands further investigation. Earlier clinical trials investigating the efficacy of ABC transporter inhibitors were conducted without selecting patients with tumours that had high levels of ABC transporter expression. Considering that gene expression profiling in acute myeloid leukaemia showed that only 13% of samples were positive for MDR1 or BCRP expression, this overly optimistic trial design likely contributed to the lack of potency observed (Robey et al., 2018). In addition, more is now known about ABC transporters, including the structure of human MDR1. With better trial design, knowledge and imaging techniques, work developing inhibitors for ABC transporters could still be fruitful for tackling MDR.
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