The effects of sinomenine on intestinal absorption of paeoniflorin by the everted rat gut sac model

Abstract

Paeoniflorin and sinomenine, derived from the root of Paeonia lactiflora Pall. (family Ranunculaceae) and the stem of Sinomenium acutum Rehder & Wilson (family Menispermaceae), respectively, have been, and are currently, widely used for treatment of rheumatic and arthritic diseases in China and Japan. Our previous studies demonstrated that sinomenine could significantly improve the bioavailability of paeoniflorin in rats, but the underlying mechanisms remain unknown. The present study aims to investigate the intestinal kinetic absorptive characteristics of paeoniflorin as well as the absorptive behavior influenced by co-administration of sinomenine using an in vitro everted rat gut sac model. The results showed a good linear correlation between the paeoniflorin absorption in sac contents and the incubation time from 0 to 90 min. However, the concentration dependence showed that a non-linear correlation exists between the paeoniflorin absorption and its concentrations from 10 to 160 μM, and the absorption was saturated at about 80 μM of the drug. Sinomenine at 16 and 136 μM concentrations could significantly enhance the absorption of paeoniflorin (20 μM) by 1.5- and 2.5-fold, respectively. Moreover, two well-known P-glycoprotein inhibitors, verapamil and quinidine, could significantly elevate the absorption of paeoniflorin by 2.1- and 1.5-fold, respectively. Furthermore, sinomenine in a pattern, which influenced paeoniflorin's absorption, manifested as similar to that of P-glycoprotein inhibitors. In conclusion, sinomenine significantly enhance the intestinal absorption of paeoniflorin, subsequently improve the bioavailability of paeoniflorin. The mechanism underlying the improvement of paeoniflorin's bioavailability was proposed that sinomenine could decrease the efflux transport of paeoniflorin by P-glycoprotein.

Introduction

Paeoniflorin is a characteristic monoterpene glucoside (Fig. 1) derived from the root of a Chinese medicinal plant, Paeonia lactiflora Pall. (family Ranunculaceae). Chinese and Japanese doctors have been using this plant in combination with other herbs to treat inflammatory and arthritic diseases for hundreds of years. The therapeutic effects of this plant as well as its active component, paeoniflorin, have been confirmed by pharmacological results, including potent analgesic, anti-inflammatory, sedative and anti-coagulative activities (Ishida et al., 1987, Liang et al., 1990, Ohta et al., 1994, Watanabe, 1997). However, the pharmacokinetic studies demonstrated that paeoniflorin has a poor absorption and thus a very low bioavailability (3–4%) when administrated orally. This was deduced to be due to the limited transportation of paeoniflorin across gastrointestinal mucosa into the blood (Takeda et al., 1995, Takeda et al., 1997).

Recently, we found that sinomenine (7,8-didehydro-4-hydroxy-3,7-dimethoxy-17-methylmorphinan-6-one; Fig. 1), an alkaloid derived from the Chinese herb Sinomenium acutum Rehder & Wilson (family Menispermaceae), could markedly improve the oral bioavailability of paeoniflorin in rats. In jugular-catheterized male Sprague–Dawley rats, the co-administration of sinomenine (90 mg/kg) markedly elevated the bioavailability of paeoniflorin to more than 12 times in comparison with animals treated with paeoniflorin (150 mg/kg) alone (Liu et al., 2005). Pharmacologically, sinomenine has been demonstrated to significantly inhibit inflammatory reactions caused by various phlogistic agents (Irino, 1958, Cheng et al., 1964). Our previous studies showed that sinomenine significantly ameliorated arthritic pathologies in adjuvant arthritic rats and inhibit cell proliferation of lymphocytes and synovial fibroblasts (Liu et al., 1994, Liu et al., 1996). Clinically, in treating arthritis, Chinese and Japanese doctors usually prescribe both the root of Paeonia lactiflora and the stem of Sinomenium acutum together (Wong and Wu, 1936). This clinical practice likely suggested synergistic effects between the two herbs in which the herb–herb interaction in intestinal drug absorption would be greatly implicated. To study intestinal absorption kinetics of the oral drugs, several in vivo and in vitro models have been developed (Anderberg et al., 1992, Park and Miltra, 1992, Hovgaard and Brondsted, 1995, Lindmark et al., 1995, Stewart et al., 1995, Barthe et al., 1998). The everted rat gut sac technique was first described by Wilson and Wisemen (Rubinstein, 1990). Researchers further developed an improved everted rat gut sac system (Barthe et al., 1998) by using tissue medium, TC199. The reproducibility of this in vitro model suggests that the everted rat gut sac is a useful screening tool for studying transport of P-glycoprotein (P-gp) substrates and potential P-gp modifiers (Carreno-Gomez and Duncan, 2000).

Using the in vitro everted rat gut sac model, Takeda et al. investigated the intestinal absorptive and metabolic behavior of paeoniflorin, showing that paeoniflorin was not metabolized in the gut wall and only has a very low permeability from serosal side to mucosal side of the intestine. This low permeability of paeoniflorin may cause its poor oral bioavailability (Takeda et al., 1995). In our previous studies, it was found that oral administration of paeoniflorin in rats had very low plasma concentration of the drug, while co-administration of sinomenine could markedly elevate the plasma concentration and significantly improve the oral bioavailability of paeoniflorin in rats (Liu et al., 2005). The underlying mechanisms remain unknown. Thus, the present study was to investigate the intestinal absorptive behavior of paeoniflorin using the in vitro everted rat gut sac model in the presence of sinomenine, and some P-gp inhibitors, in order to reveal the possible underlying mechanisms of sinomenine improving the bioavailability of paeoniflorin in vivo.