Rhizoma alimatis and its bioactive component alisol B 23-acetate reverse multidrug resistance phenotype in P-glycoprotein overexpression cancer cells

Abstract

Failure of cancer chemotherapy may be due to intrinsic or acquired resistance of tumor cells to anticancer drugs. When acquired resistance occurs it often takes the form of multidrug resistance (MDR), i.e. the resistance would be against a spectrum of drugs with different structures and modes of action. A number of cellular and molecular alterations may contribute to the development of the MDR phenotype and one that has been well studied is the over-expression of a 170kDa P-glycoprotein (P-gp) which serves as a xenobiotics exporter. Using ATP as the energy source, P-gp is known to facilitate the efflux of a broad range of cytotoxic drugs including anthracyclines, Vinca alkaloids, epipodophyllotoxins and tanxanes. Agents that fully or partly block P-gp activity thus may prevent the undesirable loss of intracellular drugs and may have beneficial, effects during chemotherapy. A number of P-gp inhibitors or modulators have been described. Although many of these agents have been found to overcome drug resistance in in vitro models, their clinical uses have been limited due mainly to a low host tolerance to experimental MDR inhibitors which precluded the attainment of active intracellular levels. In China and East Asia, medical herbs have been used to treat various diseases for hundreds of years and have been proven to be safe. Medical herbs have also been used to help cancer patients and are more often administered with western chemotherapeutic agents. Such attempts were reported to yield beneficial result, but the scientific basis for these claims and mechanisms involved are not totally clear. The aim of this study is to identify novel reversal agents for P-glycoprotein mediated MDR from herbal medicines and to delineate the cellular ad molecular mechanism involved. Thirty-four medicinal herbs were screened for the MDR reversal activity and Rhizoma Alimatis was identified for significant effect on reversing P-glycoprotein mediated multidrug resistance. The chloroform extract of this plant, RAE restored the cytotoxicity of doxorubicin as well as some other functional and structural unrelated chemotherapeutic agents in P-glycoprotein overexpression resistant cell lines. The extract also exhibited a synergistic relationship with these chemotherapeutic agents in MDR cancer cell killing and inhibited P-glycoprotein transport function as demonstrated by changes in doxorubicin accumulation and Rh-123 retention. RAE was further fractionated to identify the active fractions (a chemical or a group of chemicals). By bio-assay guided isolation, a pure compound, alisol B 23-acetate was purified and identified. Its efficacy and mechanism were further studied and it has been demonstrated that: (a) alisol B 23-acetate binds directly to P-glycoprotein and block the function of P-glycoprotein; (b) the binding site differs from that of verapamil; (c) alisol B 23-acetate can completely reverse the resistance to vinblastine at a concentration as low as 5μM in vitro as determined by the cell growth assay and the colony-forming assay, and at such concentration this compound does not exhibit any significant toxicity to both sensitive and resistant cells; (d) in this study, alisol B 23-acetate has no effect on P-glycoprotein expression level or intracellular ATP level, corroborating that the reversal function of this compound is mostly likely attributable to the inhibition of P-glycoprotein function; (e) in animal, alisol B 23-acetate enhances the effect of doxorubicin and can prolonged the life-spans of P388DOX tumor-bearing mice, when compared with mice receiving these chemotherapeutic agents alone. To conclude, Rhizoma Alimatis and its bio-active component alisol B 23-acetate can serve as a modulator of P-gp mediated MDR. Moreover, alsiol B 23-aceatate was demonstrated to be able to bind to P-gp as a competitor to its substrates, such as chemotherapeutic agents. Its efficacy was also confirmed by in vivo studies.

Date of Award	15 Feb 2005
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wang Fun FONG (Supervisor)