Pharmacokinetic-pharmacodynamic correlations in the development of ginger extract as an anticancer agent

Abstract

Anticancer efficacy of ginger phenolics (GPs) has been demonstrated in various in vitro assays and xenograft mouse models. However, only sub-therapeutic plasma concentrations of GPs were detected in human and mouse pharmacokinetic (PK) studies. Intriguingly, a significant portion of GPs occurred as phase II metabolites (mainly glucuronide conjugates) in plasma. To evaluate the disposition of GPs and understand the real players responsible for efficacy, we performed a PK and tissue distribution study in mice. Plasma exposure of GPs was similar on day 1 and 7, suggesting no induction or inhibition of clearance pathways. Both free and conjugated GPs accumulated in all tissues including tumors. While non-cytotoxicity of 6-ginerol glucuronide precluded the role of conjugated GPs in cell death, the free forms were cytotoxic against prostate cancer cells. The efficacy of ginger was best explained by the reconversion of conjugated GPs to free forms by β-glucuronidase, which is over-expressed in the tumor tissue. This previously unrecognized two-step process suggests an instantaneous conversion of ingested free GPs into conjugated forms, followed by their subsequent absorption into systemic circulation and reconversion into free forms. This proposed model uncovers the mechanistic underpinnings of ginger's anticancer activity despite sub-therapeutic levels of free GPs in the plasma.

Figure 1

Comparison of plasma kinetics on day 1 vs. day 7 of Ginger phenolics following daily dose administration of GE at 250 mg/kg. The plasma concentrations of (A) 6G, (B) 8G, (C) 10G and (D) 6S were quantitated in samples collected at various time points (0, 0.08, 0.16, 0.25, 0.5, 1, 2, 3, 4, 6, 8 and 12 h) on day 1 vs day 7 following oral administration of GE. Values and error bars shown in the figure represent mean and standard deviation (SD), respectively.

Figure 2

Tissue distribution of GPs following daily dose administration of GE for 7 days. Concentrations of GPs in untreated and β-glucuronidase treated samples obtained from the intestine, liver, spleen, heart, brain, lung and kidney were compared at different time points (n = 3). Three-dimensional representation of graphs in Microsoft Excel does not permit SD inclusion and hence concentration-time data with SD is presented in Supplementary Tables 2–8.

Figure 3

Distribution of GPs in the stomach following repeated dose administration of GE. Concentrations of GPs in untreated and β-gd treated samples obtained from stomach were compared at different time points (n = 3). Three-dimensional representation of graphs in Microsoft Excel does not permit SD inclusion and hence, the SD values are provided in Supplementary Table 1.

Figure 4

Tumor concentration of GPs following repeated dose administration of GE. Concentrations of GPs in untreated and β-gd treated tumor samples were compared at different time points (n = 3). Three-dimensional representation of graphs in Microsoft Excel does not permit SD inclusion and hence, the SD values are provided in Supplementary Table 9.

Figure 5

Proposed mechanism of action of GPs. After oral administration of GE, the free GPs form glucuronide conjugates in stomach, intestine, and liver. These conjugates enter the systemic circulation and get accumulated in various tissues. Once in the tissues, the conjugates are deconjugated to release the free active forms via the action of the β-glucuronidase enzyme that is expressed in high amounts in the tumor cells compared to normal cells, thus showing pharmacological effect.