pH-Sensitive Glycyrrhizin Based Vesicles for Nifedipine Delivery.

Glycyrrhizic acid, or glycyrrhizin (GA), a major active component of licorice root, has been widely used in traditional Chinese and Japanese medicine since ancient times. However, only in the last decades has a novel and unusual property of the GA been discovered to form water-soluble, supramolecular complexes with a variety of lipophilic drugs. These complexes show significant advantages over other known delivery systems, in particular, due to strong pH sensitivity, the properties of GA self-associates. In the present study, a supramolecular complex formation of the hypotensive and antiarrhythmic drug nifedipine with GA has been studied at different pH values, corresponding to the different degrees of GA dissociation, including a fully dissociated state of GA. Both NMR experiments and molecular dynamics simulations demonstrate the existence of the nifedipine complex with GA at all dissociation states of GA. However, optical absorption experiments show the decrease of complex stability and solubility at pH > 6 when the GA molecule is fully deprotonated. It means the higher release rate of the drug in a neutral and basic environment compared with acid media. These results could form the basis of follow-up studies of GA self-associates as pH-controlled drug delivery systems.

Effect of Glycyrrhizic Acid and Arabinogalactan on the Membrane Potential of Rat Thymocytes Studied by Potential-Sensitive Fluorescent Probe.

The effect of the natural saponin glycyrrhizic acid (GA) and polysaccharide arabinogalactan (AG) on the transmembrane potential of rat thymocytes was investigated using the potential-sensitive fluorescent probe 4-(p-dimethylaminostyryl)-1-methylpyridinium (DSM). Incubation of cells with GA in micellar form resulted in a decrease of the amplitude of observed fluorescence kinetics that points out to a decrease of the transmembrane potential. The proposed mechanism is an increase of membrane ion permeability (passive ion transport) of the plasma cell membrane due to GA incorporation. The incorporation of GA molecules into the cell membrane is extremely sensitive to the degree of GA dissociation. The neutral form of glycyrrhizic acid enters the lipid bilayer in contrast to the deprotonated anionic form. The incubation of rat thymocytes with anionic form of GA, namely with its disodium salt, has no effect on the fluorescence kinetics. The possible reasons of this phenomenon are discussed in the light of the nuclear magnetic resonance (NMR) and molecular dynamics (MD) data. The treatment of thymocytes with AG affects only the initial rate of the probe incorporation. The proposed mechanism is that AG covers the surface of the cell membrane and forms a barrier for the probe. Additionally, our experiments demonstrated that both polysaccharide AG and GA in the neutral form (but not Na2GA) effectively capture the cationic probe in an aqueous solution and then deliver it to the cell membrane.

Glycyrrhizin-Assisted Transport of Praziquantel Anthelmintic Drug through the Lipid Membrane: An Experiment and MD Simulation.

Praziquantel (PZQ) is one of the most widespread anthelmintic drugs. However, the frequent insufficient application of PZQ after oral administration is associated with its low solubility, penetration rate, and bioavailability. In the present study, the permeation of PZQ through a 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC) membrane was investigated to probe glycyrrhizin-assisted transport. Glycyrrhizin (or glycyrrhizic acid, GA), a natural saponin, shows the ability to enhance the therapeutic activity of various drugs when it is used as a drug delivery system. However, the molecular mechanism of this effect is still under debate. In the present study, the transport rate was measured experimentally by a parallel artificial membrane permeation assay (PAMPA) and molecular dynamics (MD) simulation with DOPC lipid bilayers. The formation of the noncovalent supramolecular complex of PZQ with disodium salt of GA (Na2GA) in an aqueous solution was proved by the NMR relaxation technique. PAMPA experiments show a strong increase in the amount of the penetrating praziquantel molecules in comparison with a saturated aqueous solution of pure drug used as a control. MD simulation of PZQ penetration through the bilayer demonstrates an increase in permeability into the membrane in the presence of a glycyrrhizin molecule. A decrease in the free energy barrier in the middle of the lipid bilayer was obtained, associated with the hydrogen bond between PZQ and GA. Also, GA reduces the local bilayer surface resistance to penetration of PZQ by rearranging the surface lipid headgroups. This study clarifies the mechanism of increasing the drug's bioavailability in the presence of glycyrrhizin.

Intermolecular Voids in Lipid Bilayers in the Presence of Glycyrrhizic Acid.

It is known that glycyrrhizic acid (GA) promotes the enhancement of the activity of several medicines. This is attributed to the fact that GA increases the membrane permeability of small drug molecules. There is an opinion that GA facilitates the formation of additional large voids in the membrane, which enhance the passive diffusion of molecules across the membrane. In this work, we investigate how GA influences the intermolecular voids using the molecular dynamics simulation. We calculate the interstitial spheres (empty spheres inscribed between molecules) in model DPPC and DOPC bilayers, both pure and with the addition of cholesterol. It was observed that the addition of GA does not lead to the formation of new large interstitial spheres; i.e., new large voids do not appear. The distribution of empty volume inside the bilayers is also studied. We calculated the profiles of the empty volume fraction both from the middle plane of the bilayer and from its outer surface (from the lipid-water interface). This analysis has shown that the addition of GA does not cause the increase of the empty volume in the bilayer; moreover, there is a slight decrease in the bilayers with cholesterol. Thus, we have not found a confirmation of the simplest hypothesis that individual GA molecules induce pores in the membrane.