Introduction

Cancer includes a group of diseases that are characterized by abnormal and out of control spreadable cellular growth (Mbaveng et al. 2017). Causative agents of cancers are either external such as tobacco consumption and infections; or internal such as immune conditions, genetic mutations, and hormonal imbalance. The incidence of cancer is not limited to developing countries but also to already developed ones and the burden of cancer affects both. According to the World Health Organization (WHO), malignant neoplasms are ranked the second leading cause of deaths worldwide after cardiovascular diseases. In 2012 alone, a global record of 14.1 million newly diagnosed cancer cases with 8.2 million deaths due to cancer were reported (Torre et al. 2015). Moreover, these estimates are expected to increase by 2030 to about 150% which constitute a ringing alarm. These statistical estimates are based on GLOBOCAN 2012 presented by the International Agency for Research on Cancer (IARC) (Torre et al. 2015; Society A.C 2016).

Although the general term cancer covers many different diseases, most types of cancers share a common feature of not acting to available chemotherapies through development of multidrug resistance (MDR). MDR is a phenomenon by which cancer cells develop broad resistance to a wide variety of structurally and functionally unrelated compounds which may arise from several mechanisms of which the best described is the overexpression of drug efflux proteins such as P-glycoprotein. This ultimately leads to cancer relapse and death in 90% of patients. Some cancers such as gastrointestinal and renal cancers are largely unresponsive to chemotherapy, i.e., they have a high degree of intrinsic MDR, whereas leukemias, lymphomas, ovarian, and breast cancers often respond to initial treatment, but then acquire MDR during the course of the disease. MDR to anticancer drugs is therefore a serious health problem that dramatically affects the efficacy of cancer treatments.

In this article, we review the possible mechanisms of multidrug resistance with focus on efflux transporters-related MDR. We also emphasize how natural products constitute a promising value as chemosensitizers through inhibition of different efflux proteins.

Mechanisms of drug resistance in cancer

The cancer treatments available to patients include chemotherapy, radiotherapy, surgery, immunotherapy, or a combination of them (Gottesman et al. 2002; Saeed et al. 2016; Saluja et al. 2016; Nie et al. 2016). Although many cancer types are curable with chemotherapeutic cytotoxic agents, sometimes chemoresistance against cancer therapeutic agents develops. Chemoresistance against drugs can be either “intrinsic” which describe the pre-existing constitutive overexpression of cancer cell detoxification system before the start of chemotherapeutic regimen, or “acquired” where it develops after the start of the chemotherapy over time or after a secondary chemotherapy with tumor relapse (Gottesman 2002; Quintieri et al. 2007). The mechanisms through which cancer chemotherapy fails include pharmacological, physiological, and/or cellular mechanisms (Sikic 2015). First, the pharmacological mechanisms of chemotherapy failure may include insufficient drug dosing, or suboptimal dosing regimens of the chemotherapeutic regimens (Sikic 2015; Marangolo et al. 2006; Carlson and Sikic 1983).

Second, the physiological mechanisms of chemotherapy failure, however, include lack of optimal distribution of the chemotherapeutic agents to what is called “sanctuary sites” due to the presence of the blood-brain barrier (at the central nervous system) and blood-testicular barrier (at testes) (Fromm 2004).

Another physiological mechanism for the chemotherapy failure is the poor distribution of the chemotherapeutic agent to cancer tissue due to the poor vasculature in angiogenesis process (Kyle et al. 2007). Therefore, the use of anti-angiogenic agents (e.g., sunitinib) helped patients to revert vasculature back to normal and improved the distribution of chemotherapeutic drug to their target cancer tissues (Matsumoto et al. 2011).

Third, the cellular mechanisms involved in the chemotherapy resistance and eventually failure are schematically outlined in Fig. 1.

Fig. 1
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Cellular drug resistance mechanisms, adapted from (Sikic 2015) under permission from Elsevier Inc

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Multi-drug resistance: a specific type of resistance

A specific form of cellular drug resistance in cancer is termed multi-drug resistance (MDR).This is a phenomenon by which cancer cells become cross-resistant to a wide variety of structurally and pharmacologically unrelated cancer cytotoxic drugs such as vinblastine, paclitaxel, and doxorubicin (Callies et al. 2016; Wu et al. 2014; Kuete and Efferth 2015; Eichhorn and Efferth 2012). MDR renders the tumor cells non-responsive to treatment and failure of chemotherapy in 90% of metastatic cancers (Bernardes de Andrade Carli et al. 2013; Turk et al. 2009; Longley et al. 2006).

The main mechanism describing MDR in cancer is the overexpression of ATP binding cassette (ABC) transporter proteins that effectively efflux diverse chemotherapeutic agents outside the cancer cells, decreasing the intracellular drug concentration, rendering chemotherapy ineffective (Fig. 2) (Saraswathy and Gong 2013; Yan et al. 2014; Krishna and Mayer 2000; Gillet et al. 2007; Kadioglu et al. 2016).

Fig. 2
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Schematic representation of MDR in cancer cells with ABC transporter-mediated drug efflux. Adapted from (Avendaño and Menéndez 2015) under permission from Elsevier Inc

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ABC transporters in normal physiology and cancer

There are 49 ABC transporter genes in the human genome (Huang 2007; Gottesman and Ambudkar 2001; Glavinas et al. 2004). In normal physiology, these transporters actively transport endogenous and exogenous substrates through biological membranes into body tissues, such as small intestine, colon, kidney, pancreas, blood-brain barrier, and blood-testes barrier by ATP hydrolysis (Fromm 2004; Abdallah et al. 2015). In addition to the detoxification of xenobiotics, efflux transporters have a role in mediating the transport of some substrates across the cellular membranes such as cholesterol, amino acids, sugars, lipids, peptides, hydrophobic drugs, and antibiotics (Gottesman and Ambudkar 2001; Dean and Annilo 2005; Ifergan et al. 2004; Shi et al. 2007a; Shi et al. 2007b). However, in cancer cells, some of these transporters are responsible for chemotherapy failure.

The identified human drug transporter protein superfamily is divided into seven sub-families: namely ABCA, ABCB, ABCC, ABCD, ABCE, ABCF, and ABCG (Kathawala et al. n.d.) with diverse physiological functions and roles in multidrug resistance (Table 1).

Table 1 Families of human ABC transporters and their functions. Data were adapted from Vasiliou et al. (2009)
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Major ABC transporters involved in MDR of cancer

The assembly of different ABC efflux transporters across cell membrane is similar. It is composed of transmembrane domains (TMDs) each contains a number of membrane-spanning α-helices (5–10 helices) and nucleotide-binding domains (NBDs). The TMD is the site where the substrate binds to the transporter, whereas NBD exerts ATPase activity that hydrolyses ATP molecules to provide the energy required for the substrate (drug) efflux process against concentration gradients to extracellular space (Avendaño and Menéndez 2015; Gottesman and Ling 2006; Yu et al. 2016). ABC transporters appear as full transporters or half transporters that dimerize to form functional full transporter units.

Three efflux transporters have been investigated in much more detail concerning their role for MDR in cancer cells: ABCB1 (also termed P-glycoprotein, P-gp, or MDR1), ABCC1 (also termed MDR-associated protein 1 or MRP1), and ABCG2 (also termed breast cancer resistance protein BCRP or mitoxantrone resistance protein MXR) (Fig. 3).

Fig. 3
figure 3

Schematic presentation showing the structure of major ABC transporters involved in MDR. Adapted from (Avendaño and Menéndez 2015) under permission from Elsevier Inc

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ABCB1 (P-gp, MDR1)

ABCB1 was the first efflux protein to be identified in MDR Chinese hamster ovary cells (CHO) by Juliano and Ling in 1976 (Juliano and Ling 1976). It is a 170 kDa glycoprotein that is expressed in liver, placenta, kidney, intestine- and blood-brain barriers, where it has detoxification and transport physiological functions. ABCB1 is the most extensively studied efflux transporter and accounts for the efflux of about half the number of anticancer drugs used in clinic (Avendaño and Menéndez 2015). In cancer cells, the overexpression of ABCB1 confers MDR phenotype to cells against diverse traditional chemotherapeutic drugs of unrelated chemical structures and variable mechanisms of actions such as paclitaxel, doxorobicin, and vinblastine and many others (Loo and Clarke 2005). In addition, the ABCB1 transporter also mediates the efflux of the marine antileukemia drug imatinib (Avendaño and Menéndez 2015).

The human ABCB1 protein contains 1280 amino acid residues forming 2 similar halves. Each half contains one TMD with six α-helices (TMD1 and TMD2) and a hydrophilic NBD (NBD1 and NBD2) (Fig. 3). The binding of ABCB1 drug substrates to the TMDs causes a subsequent hydrolysis of ATP molecule that in turn leads to a conformational change in the shape of the transporter expelling the drug out of the cells (Hyde et al. 1990; Karthikeyan and Hoti 2015). This prohibits the intracellular accumulation of drugs from reaching their target, and eventually making chemotherapy ineffective. Natural chemosensitizers that proved to modulate the function of ABCB1 are listed in Tables 2 and 3.

ABCC1 (MRP1)

ABCC1 is a 190 kDa ABC transporter, which is expressed in liver, bowel, and excretory organs. It is also expressed in sanctuary sites such as the blood-brain barrier. Although the similarity between amino acid sequence of ABCB1 and ABCC1 is as low as 15%, the resistance conferred through both proteins is significantly overlapping (Leschziner et al. 2006). As displayed in Fig. 3, the structure of ABCC1 is composed of three TMDs (TMD0, TMD1, and TMD2) and two cytoplasmic NBDs. Several chemotherapeutic agents such as doxorobicin, topotecan, and vincristine are substrates of ABCC1 in cancer cells (Kathawala et al. n.d.). However, ABCC1 did not show efflux activity toward taxanes (i.e., paclitaxel as known ABCB1 substrate) (Morrow et al. 2006). Many modulators of ABCB1 such as verapamil and cyclosporine A inhibit the function of ABCC1 as well (Zhou et al. 2008). Natural chemosensitizers that modulate the function of ABCC1 are listed in Tables 1 and 2.

ABCG2 (BCRP, MXR)

ABCG2 is a 72 kDa ABC half transporter and contains only one TMD and one NBD (Fig. 3) and only functions upon dimerization or by tetramer formation (Karthikeyan and Hoti 2015). This transporter was first identified and characterized in a MDR breast cancer cell line (MCF7) (Doyle et al. 1998). It is expressed normally in cells membranes of small intestine, placenta, brain, prostate, and ovaries. ABCG2 is also expressed in many types of cancer cells. Amphipathic molecules are substrates for ABCG2 transporter. This transporter also shares with other transporters the property of transporting structurally unrelated drugs. It can effectively efflux mitoxantrone and camptothecin as well as fluorescent dyes. Natural chemosensitizers that modulate the function of ABCG2 are listed in Tables 1 and 2.

Generations of chemosensitizers

Extensive research work has been performed to inhibit ABC transporter function and expression to re-sensitize cancer cells to chemotherapy. Therefore, inhibitors (chemosensitizers) block the transporter to increase drug accumulation in MDR cancer cells, which results in a better cytotoxic effect by the corresponding chemotherapeutic drug (Wu et al. 2011). Three distinct generations of chemosensitizers have been classified according to the relative affinity, toxicity, and specificity (Palmeira et al. 2012).

First-generation chemosensitizers

Early attempts to screen for ABC transporter inhibitors employed already available drugs that are used in the clinic such as the calcium channel blockers verapamil (Tsuruo et al. 1981), immunosuppressive drugs such as cyclosporine A (Shiraga et al. 2001), and the antimalarial drug quinine (Karthikeyan and Hoti 2015; Krishna and Mayer 2001). However, the original pharmacological activity of these first-generation drugs (chemosensitizers) caused non-desirable toxicity to non-cancerous cells, were non-specific, and had low affinity to the ABC transporter so that they required high doses to function in vivo. Examples of first-generation chemosensitizers are displayed in Fig. 4.

Fig. 4
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Examples of first-generation chemosensitizers

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Second-generation chemosensitizers

The limitations recorded with first-generation chemosensitizers led to subsequent attempts to chemically modify P-gp inhibitors and the second generation of chemosensitizers emerged. Examples are chemically modified analogues of first-generation chemosensitizers such as dexverapamil (verapamil’s R-enantiomer) and PSC833 (valspodar, modified from cyclosporine A). Although second-generation chemosensitizers showed potent chemosensitization in MDR cancer cells in vitro, they displayed toxicity in animal models (Abdallah et al. 2015; Nawrath and Raschack 1987; Pirker et al. 1990). Furthermore, they caused drug-drug interaction in clinical trials, since they showed cytochrome P450 inhibitory activities (Klinkhammer et al. 2009). Examples of second-generation chemosensitizers are displayed in Fig. 5.

Fig. 5
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Examples of second-generation chemosensitizers

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Third-generation chemosensitizers

The advances in quantitative structure-activity relationship (QSAR) and combinatorial chemistry led to the emergence of the third-generation chemosensitizers with potent affinity to P-gp, less toxicity, and strong activity such as R1010933 (laniquidar), LY335979 (zosuquidar), GF120918 (elacridar), VX-710 (biricodar), and XR9576 (tariquidar) (Fig. 6). However, data from clinical trials revealed dual interactions with different types of ABC transporters (less selectivity to inhibit a given transporter) (Avendaño and Menéndez 2015; Toppmeyer et al. 2002; Yanagisawa et al. 1999).

Fig. 6
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Examples of third-generation chemosensitizers

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Mechanism of chemosensitization of MDR cells

Avendano and co-workers (2015) summarized six possible mechanisms of actions of ABCB1/P-gp chemosensitizers (Fig. 7):

  1. 1.

    The chemosensitizer (e.g., verapamil) can be recognized as transporter substrate and lock the transporter in a cycle of transport and ATP hydrolysis, which in turn increases intracellular drug concentration.

  2. 2.

    Competitive inhibition by some chemosensitizers such as zosuquidar with longer and higher affinity to the drug binding site at the TMD of the transporter. Such compounds compete with the actual anticancer drug on the binding site of P-gp and block its transport.

  3. 3.

    Non-competitive inhibition of transporter by some chemosensitizers such as Cis-flupenthixol that bind important amino acid residues on P-gp sites other than the drug binding site (allosteric inhibition) and possibly interference with the conformation responsible for drug efflux.

  4. 4.

    Some surfactants, anesthetics, and fluidizers non-specifically perturb membrane lipids and thereby increase the rates of drug uptake (Ferte 2000; Eytan 2005).

  5. 5.

    Some chemosensitizers interfere with the ATP-binding domain of the transporter. An example of this mechanism is the trapping of ADP by vanadate at the ATP binding site (Urbatsch et al. 1995).

  6. 6.

    Some chemosensitizers can interfere with the intracellular ABCB1-mediated drug sequestration in vesicular membrane (e.g., lysosomal sequestration (Yamagishi et al. 2013)) making the drug more available to its cellular targets.

Fig. 7
figure 7

Possible mechanisms of ABCB1/p-gp chemosensitizers. Adapted from (Avendaño and Menéndez 2015) under permission from Elsevier Inc

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Natural products: the fourth-generation of MDR chemosensitizers

The high biodiversity, good oral bioavailability, and relatively low intrinsic toxicity of natural products enabled the discovery of new chemical scaffolds for drug development. Due to the limitations encountered by three generations of chemosensitizers, natural products are attractive partners for the combination with chemotherapy to enhance their cancer cytotoxic effects and reverse MDR. Edible phytochemicals such as curcumin, quercetin, and kaempferol block ABCB1 function and reverse MDR in human cancer cell lines (Limtrakul et al. 2005). Furthermore, some naturally derived compounds such as trabectedin, cytarabine, and halaven are clinically useful based on their strong chemosensitizing properties (Huang 2007; Shi et al. 2007a; Abraham et al. 2010; Lopez and Martinez-Luis 2014).

Herein, natural compounds such as phytochemicals, marine, or fungal compounds were presented as chemosensitizers of MDR cancer cells (Tables 2 and 3). These natural product chemosensitizers belong to diverse chemical classes, such as flavonoids, coumarines, terpenoids, etc. Listed natural products target the three major transporters ABCB1, ABCC1, and ABCG2.

Table 2 Examples of natural chemosensitizers of ABC transporters isolated from plants
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Table 3 Examples of chemosensitizers of ABC transporters isolated from natural sources (marine organisms, insects, and fungi)
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Conclusion

A major hurdle of successful cancer chemotherapy is MDR caused by ABC transporters. Extensive research has been carried out to identify chemosensitizers with high selectivity, high affinity, and low toxicity. Three generations of chemosensitizers that reverse MDR have emerged without satisfactory clinical success due to limitation of their toxicity, low affinity, and non-selectivity. Natural products may represent attractive alternatives to synthetic compounds for the development as chemosensitizers in combination with chemotherapeutic agents to enhance their efficacy in cancer cells.