Multi-drug resistance in cancer chemotherapy refers to the ability of cancer cells to survive from treatment of a wide range of drugs (Meszaros et al., 2009).
In addition to the MDR induced by drugs in early exposure, the MDR cancer cells may subsequently develop cross-resistance to several unexposed and structurally unrelated chemotherapeutic agents (Biedler et al., 1970).
How to tackle the MDR cells in chemotherapy is a pressing issue in cancer treatments. Verapamil was the first known Pgp inhibitor to increase the intracellular concentration of anti-cancer agents in MDR cells by binding to Pgp and inhibiting the Pgp-mediated efflux (Twentyman, 1992). It was believed that anti-cancer drug resistance could be reversed by drug efflux inhibition. Researchers developed and tested a range of Pgp inhibitors to improve the pharmacological effects of chemotherapy in cancer patients (Tsuruo et al., 1981; Stewart et al., 2000; Toppmeyer et al., 2002).
Mechanisms of MDR include decreased uptake of drugs, alterations in cellular pathways and increased active efflux of drugs (Gottesman, 2002; La Porta, 2007; Watson, 1991).
Overexpression of ATP-binding cassette (ABC) transporters is one of the most common mechanisms. Overexpression of the three major ABC transporters, i.e. P-glycoprotein (Pgp), multi-drug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP/ABCG2), is frequently observed in cancer cell lines selected with chemotherapeutic drugs (Szakacs et al., 2006) and critical to clinical drug resistance (Leonard, 2003).
Fractions from 17 clinically used anti-tumor traditional Chinese medicinal herbs were tested for their potential to restore the sensitivity of MCF-7/ADR and A549/Taxol cells to a known anti-neoplastic agent. Five herbs, Curcuma wenyujin, Chrysanthemum indicum, Salvia chinensis, Ligusticum chuanxiong Hort. and Cassia tora L., could sensitize these resistant cancer cells at a non-toxic concentration (10 µg mL–1), and markedly increased doxorubicin accumulation in MCF-7/ADR cells, which necessitates further investigations into the active ingredients of these herbs and their underlying mechanisms (Yang et al., 2011).
Natural sources are a fertile ground to find novel drugs with activity against MDR cancer cells. In some countries, especially China, traditional herbal medicines are often used together with mainstream chemotherapeutic agents. The clinically used traditional Chinese herbs for the treatment of tumor can be classified into four categories based on the theory of Traditional Chinese Medicine (TCM): drugs (CH group) for 'Clearing away Heat and Toxins', drugs (PB group) for 'Promoting Blood Flow to Remove Stasis', drugs for 'Invigoration' and toxic drugs. Drugs for 'Invigoration' have indirect anti-neoplastic action by enhancing an organism's immunity and have been used clinically to minimise radiotherapy- and chemotherapy-induced toxicity (Fu & Chen, 2008; Chai, To, Lin, 2010).
Some of the recent findings on the circumvention of ABC transporters-mediated MDR by various ingredients and extracts of CM and their formulae, based on whether the MDR reversal involved Pgp alteration, are reviewed below.
Saponins
Ginsenosides are the major active components from Panax ginseng (Renshen). Ginsenosides are mainly triterpenoid dammarane derivatives. Several ginsenosides, namely Rg1, Rg3, Re, Rc and Rd inhibited drug efflux (Kim et al., 2003). A combination of purified saponins containing Rb1, Rb2, Rc, Rd, Re and Rg1 reversed MDR whereas individual ginsenosides did not produce any effect (Park et al., 2006). Ginsenosides reversed MDR of several chemotherapeutic drugs such as homoharringtonine, cytarabine, doxorubicin and etoposide in K562/VCR and in a dose-dependent manner in K562/DOX (Gao et al., 2004).
Pgp expression decreased but bcl-2 expression remained the same (Wang, 2003). Rb1 reversed MDR of harringtonolide and vincristine in K562/HHT and HL60/VCR cell lines respectively (Shi et al. , 2005).
Panax notoginseng (Sanqi) total saponins reversed MDR of doxorubicin in MCF-7/DOX and K562/VCR cell lines. The mechanism may be related to the decrease of Pgp expression (Si & Tien, 2005; Liu, Liu, & Fang, 2008).
Rg3, one of the active ginsenosides from Panax ginseng, restored the sensitivity of resistant KBV20 cell line to various anti-cancer drugs, including vincristine, doxorubicin, etoposide and colchicine in a time-and dose-dependent manner. This ginsenoside competitively inhibited the binding of substrate drugs to Pgp and its binding affinity to Pgp was remarkably higher than that of verapamil. In contrast to the dose-dependent effects in vitro, Rg3 increased animal life span in an in vivo MDR model in a dose-independent manner (Kim et al., 2003).
Flavonoids
Quercetin is one of the most widely distributed flavonoids in natural products including Chinese medicinal herbs such as Sophora japonica (Huai). Quercetin inhibited the binding of heat shock factor at the MDR1 promoter, thereby decreasing MDR1 transcription and reducing Pgp expression (Kim et al., 1998). Quercetin also inhibited the overexpression of Pgp mediated by arsenite (Kioka et al., 1992). In HL-60/DOX and K562/DOX cell lines, quercetin enhanced the anti-cancer sensitivity to daunorubicin and decreased Pgp expression (Cai et al., 2004; Cai et al., 2005). MDR reversal effect of quercetin was probably mediated by its action on mitochondrial membrane potential and the induction of apoptosis. Furthermore, quercetin derivatives rather than quercetin itself reversed MDR (Kothan et al., 2004). Quercetin increased the sensitivity of Pgp-overexpressing KBV1 cell line towards vinblastine and paclitaxel in a dose-dependent manner. Among many active flavonoids, quercetin was less potent than kaempferol but more effective than genistein and daidzein in reversing MDR. Genistein and daidzein had no effect on Pgp expression (Limtrakul, Khantamat, & Pintha, 2005).
Although quercetin may be a potential MDR reversing agent, lethal drug-drug interaction between quercetin and digoxin has been reported. Quercetin (40 mg/kg) elevated the peak blood concentration of digoxin and caused sudden death of tested animals (Wang et al., 2004).
Paeonol is a weak calcium channel blocker isolated from the root of Paeonia suffruticosa (Mudan). In K562/DOX cell line, paeonol showed positive MDR reversal effect towards doxorubicin, daunorubicin, vincristine and vinblastine without modulating Pgp expression [100]. In parental K562 cells, paeonol induced apoptosis in a time-and dose-dependent manner (Sun et al., 2004).
Curcumin, the major component in Curcuma longa (Jianghuang), inhibited the transport activity of all three major ABC transporters, i.e. Pgp, MRP1 and ABCG2 (Ganta & Amiji, 2009). Curcumin reversed MDR of doxorubicin or daunorubicin in K562/DOX cell line and decreased Pgp expression in a time-dependent manner (Chang et al., 2006). Curcumin enhanced the sensitivity to vincristine by the inhibition of Pgp in SGC7901/VCR cell line (Tang et al., 2005). Moreover, curcumin was useful in reversing MDR associated with a decrease in bcl-2 and survivin expression but an increase in caspase-3 expression in COC1/DDP cell line (Ying et al., 2007). The cytotoxicity of vincristine and paclitaxel were also partially restored by curcumin in resistant KBV20C cell line (Um et al., 2008). Curcumin derivatives reversed MDR by inhibiting Pgp efflux (Um et al., 2008).
A chlorine substituent at the meta-or para-position on benzamide improved MDR reversal (Um et al., 2008). Bisdemethoxycurcumin modified from curcumin resulted in greater inhibition of Pgp expression (Limtrakul, Anuchapreeda, & Buddhasukh, 2004). Tetrahydrocurcumin, the major metabolite of curcumin, inhibited all three major ABC transporters (Limtrakul et al., 2007). Curcumin induced atypical and caspase-independent cell death in MDR cells (Piwocka, Bielak-Mijewska, & Sikora, 2002). In leukaemic cells collected from 78 childhood leukaemia patients, curcumin reduced Pgp expression (Anuchapreeda et al., 2006). A specialized nanoemulsion of curcumin is better than conventional solution form drugs in enhancing the efficiency of drug delivery into the cells, down-regulating Pgp expression, inhibiting the NFκB pathway and promoting apoptotic response (Choi et al., 2008).
Other Compounds
Schizandrins, the active constituents of Schisandra chinensis (Wuweizi), were investigated for their MDR reversal effects. Schizandrin A was the most potent in reversing MDR by enhancing apoptosis and down-regulating Pgp and total protein kinase C expression. The crude extract of Schisandra chinensis reversed the resistance against vincristine in vivo (Huang et al., 2008). Deoxyschizandrin and γ-schizandrin, among the nine dibenzo[a,c]cyclooctadiene lignans examined, enhanced intracellular drug concentration and induced cell-cycle arrest at the G2/M phase when combined with sub-toxic dosages of doxorubicin (Slaninová et al., 2009). Gomisin A, on the other hand, altered Pgp-substrate interaction by binding to Pgp simultaneously with substrates (Wan et al., 2006).
Formulae – injections (See Injectables)
'Shengmai Injection', consisting of Panax ginseng and Ophiopogon japonicus (Maidong), down-regulated Pgp expression in peripheral blood lymphocyte membrane. When used together with oxaliplatin, 5-fluorouracil or folinic acid, the injection prolonged the survival rate of colon cancer patients (Cao et al., 2005). The injection also enhanced the efficacy of tamoxifen and nifedipine in combination therapy (Lin et al., 2002).
'KLT Injection' consisting of the extract of Coix lacryma-jobi (Yiyi) enhanced the anti-cancer activities of paclitaxel and docetaxel and reversed MDR in a dose-dependent manner (Dong, Zheng, & Lu, 2002).
Formulae – powders
'Shenghe Powder', consisting of Panax ginseng, Scorophularia ningpoensis (Xuanshen) and Atractylodes macrocephala (Baizhu), increased the intracellular concentration of vincristine in resistant SGC-7901/VCR cell line, possibly due to the induction of apoptosis and down-regulation of Pgp and bcl-2 expression (Wang et al., 2007).
'Modified Sanwubai Powder', consisting of herbs such as Croton tiglium (Badou), Platycodon grandiflorum (Jiegeng) and Fritillaria thunbergii, induced apoptosis in SGC-7901 cell line and down-regulated the gene expressions of p53, bcl-2, rasP21CD44 and Pgp (Xu et al., 2005).
Formulae – others
Three herbal extracts used to treat diseases other than cancer, namely Ams-11, Fw-13 and Tul-17, greatly enhanced the efficacy of vincristine both in vitro and in vivo and reversed MDR in a dose-dependent manner. Tul-17 inhibited Pgp expression (Qu et al., 2006).
Oil emulsion from Brucea javanica (Yadanzi) reversed MDR when used together with other chemotherapeutic drugs such as vincristine, doxorubicin, cisplatin, mitomycin C, 5-fluorouracil or etoposide, probably due to down-regulation of Pgp expression or inhibition of TOPO II or both (Yu, Wu, Zhang, 2001).
'Sangeng Mixture Decoction', consisting of Reynoutria japonica (Huzhang), Actinidia arguta (Mihouligen) and Geum aleppicum (Shuiyangmeigen), reversed MDR of doxorubicin via down-regulation of Pgp expression (Feng et al., 2003).
FFTLG, a formula containing Actinidia arguta, reversed MDR in K562/DOX cell line by increasing the intracellular doxorubicin concentration (Guo, Xie, Feng, 2002).
R1, consisting of Ligusticum chuanxiong, Curcuma longa and Millettia dielsiana (Jixueteng), enhanced the anti-cancer activities of doxorubicin in MCF-7/DOX via down-regulation of Pgp expression (Chen et al., 2003; Lin, 2007).
Formulae
'Ganli Injection', consisting of matrine and tetramethylpyazine hydrochloride, reversed MDR by increasing the sensitivity of 5-fluorouracil and the intracellular concentration of doxorubicin in BEL-7402/5-FU cell line (Gu et al., 2007).
'Bushen Huayu Jiedu Formula', consisting of Cinnamomum cassia (Rougui), Psoralea corylifolia (Buguzhi) and Rheum palmatum, was tested in A549/DDP cell line and S180 tumor-bearing mice. In vitro, the formula significantly increased the intracellular concentration of cisplatin at high doses and inhibited the activity of calcium channel and LRP-56 expression at both high and low doses. In vivo, the formula improved the serum concentration, reduced the inflow and the release of Ca2+ and inhibited the LRP gene expression (Cao et al., 2004; Cao et al., 2008).
Four CM formulae, namely Glycyrrhiza glabra (GLYC), Hedyotis diffusa (OLEN), a formula consisting of 15 herbs including Cistanche deserticola (Roucongrong), Rabdosia rubescens (Donglingcao) and Zanthoxylum nitidum (Liangmianzhen) (SPES), and a formula consisting of eight herbs including Serenoa repens (Juyezhong), Scutellaria baicalensis (Huangqin), Panax ginseng and Glycyrrhiza glabra (PC-SPES) were cytotoxic to cancer cell lines in a dose-dependent manner. SPES, PC-SPES, OLEN decreased the bcl-2 gene expression and were pro-apoptotic, while GLYC was pro-necrotic without altering the over-expression of bcl-2 in MDR cells. Furthermore, OLEN, SPES and PC-SPES exhibited similar pharmacological effects to etoposide and vincristine (Sadava et al., 2002).
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