A colon delivery system has been used to improve the bioavailability of glycyrrhizin, a glycoside of glycyrrhetic acid. The bioavailability of glycyrrhizin is low when administered in conventional oral galenic dosage forms because glycyrrhizin is enzymatically hydrolysed both in the stomach and in the intestine. It was reasoned that if large amounts of glycyrrhizin were directly delivered to the colon, enzymatic activity should be reduced due to saturation so that intact glycyrrhizin could be absorbed into the systemic circulation. Based on this assumption, pressure-controlled colon delivery capsules (PCDCs) were used as a colon delivery system. Eight types of glycyrrhizin solution were prepared and were introduced into PCDCs. After oral administration of the test PCDCs to beagle dogs, blood samples were obtained over 24 h and plasma glycyrrhizin concentrations were measured by an HPLC method. With PCDCs containing aqueous glycyrrhizin and propylene glycol solutions, plasma glycyrrhizin levels were extremely low and the bioavailabilities of glycyrrhizin were 0.6% and 0.4%, respectively. When Labrasol was added to both types of glycyrrhizin solution, the bioavailability was improved to 4.6% for aqueous solution and 3.8% for propylene glycol solution. When a surfactant, Polysorbate 80, was added in combination with Labrasol, synergistic effects were not obtained. Furthermore, dose-dependent effects of Polysorbate 80 were not obtained. Labrasol, which is a component of self-emulsifying drug delivery systems (SEDDS), has been shown to strongly improve the bioavailability of glycyrrhizin from the colon.
The pharmacokinetics of glycyrrhizin (GZ) was compared in albumin-deficient rats (NAR) and normal rats (SDR) after intravenous administration. The study sought to clarify the relationship between GZ concentration and its elimination rate in serum, liver and bile when the serum protein binding of GZ decreased. Serum protein binding in SDR and NAR, respectively, was 99.7% and 68.2% for a GZ concentration of 2.5 microg/ml. At steady-state conditions after i.v. infusion of GZ (0.5-2.0 mg/h), the relationship between the GZ concentration in serum and liver was linear in the SDR but nonlinear in the NAR. For both NAR and SDR, the GZ liver level and the elimination rate was nonlinear, indicating that the elimination of GZ from liver into bile was the rate-limiting step regardless of serum protein binding, and that the liver GZ level was extremely high when serum protein binding was decreased. It is concluded that a typical dose of GZ in chronic hepatitis patients whose serum albumin level is low will not cause a decrease of therapeutic effect compared with patients with a normal serum albumin level.
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