In vitro evaluation of the efficacy of selected medicinal plant extracts against multidrug resistant cancer cells

Abstract

Cancer is ranked as one of the leading causes of death globally, and a significant number of these diagnosed malignancies are found in lung tissue. Small cell lung cancer (SCLC) is a high grade neuroendocrine cancer, which is responsible for high mortality rates worldwide. The failure of therapeutic regimes in cancer management can be ascribed to increased cancer metastasis and the occurrence of multidrug resistance (MDR). MDR in SCLC is often the result of hyperexpression of several adenosine triphosphates (ATP)-binding cassette (ABC) efflux transporters in these tumours, which can decrease the intracellular accumulation of chemotherapeutic drugs, resulting in sub-therapeutic levels. P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP) are some of the most widely studied efflux transporters involved in cancer MDR. There is an urgent need to identify novel treatment approaches to combat MDR in cancer, and various traditionally used medicinal plants are believed to cure, prevent or manage cancer. In this study, Aloe vera gel material and precipitated polysaccharides, Sutherlandia frutescens and Xysmalobium undulatum were investigated as an ethno-medicinal approach to combat MDR in cancer. This was done through evaluation of their potential in vitro anticancer efficacy against selected chemosensitive and chemoresistant SCLC cell models. These SCLC cell models included a chemosensitive line (H69V), a multidrug resistant line with hyperexpressed MRP1 efflux transporters (H69AR), as well as the multidrug resistant NCI-H69/LX4 line with hyperexpressed P-gp transporters. A porcine kidney non-tumorigenic cell line (LLC-PK1) was also included to evaluate the cytotoxic effects of the selected plant materials. The 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay was used as a preliminary screening method to determine the relative reduction in cell viability (IC) of each selected SCLC cell line, after a 96 h exposure to the various plant materials. Subsequently, the effect of the selected plant materials on intracellular ATP and extracellular adenylate kinase (AK) levels of the different SCLC and LLC-PK1 cells were measured to establish their anticancer efficacy and cytotoxic potential more accurately. All the selected plant materials investigated in this study resulted in a statistically significant reduction in cell viability for all of the SCLC cell lines (p<0.05), although a measure of resistance was observed in the chemoresistant cell lines. The anticancer phytochemicals in the crude extracts are therefore most probably substrates for MRP1 and P-gp related efflux. The aqueous S. frutescens extract was found to possibly induce necrosis in the MRP1 hyperexpressive SCLC cell line at 1.406 mg/ml, while the MTT data suggested that S. frutescens extract had the highest cancer selectivity ratio in P-glycoprotein (P-gp) hyperexpressive SCLC management of all the materials investigated. The cytotoxic effect of the X. undulatum extract on non-cancerous cells (LLC-PK1 cell line) contradicts its use in SCLC management. The selected medicinal plant materials significantly altered both the intracellular ATP and extracellular AK levels of the chemosensitive and chemoresistant SCLC cell lines, indicating anticancer effects against SCLC cells. However, indications of cytotoxicity to some extent were also detected in a non-cancerous LLC-PK1 cell line for all of the plant materials. The affinity of phytochemicals in the plant materials for the efflux transporters may be put to use though combination with standard anticancer drugs with an affinity for these efflux transporters. The phytochemicals may enhance intracellular drug accumulation by saturating the transporter binding sites, or competing with the standard drugs for binding