P-glycoprotein (Pgp) is a 170 kDa phosphorylated glycoprotein encoded by human MDR1 gene. It is responsible for the systemic disposition of numerous structurally and pharmacologically unrelated lipophilic and amphipathic drugs, carcinogens, toxins, and other xenobiotics in many organs, such as the intestine, liver, kidney, and brain. Like cytochrome P450s (CYP3A4), Pgp is vulnerable to inhibition, activation, or induction by herbal constituents. This was demonstrated by using an ATPase assay, purified Pgp protein or intact Pgp-expressing cells, and proper probe substrates and inhibitors. Curcumin, ginsenosides, piperine, some catechins from green tea, and silymarin from milk thistle were found to be inhibitors of Pgp, while some catechins from green tea increased Pgp-mediated drug transport by heterotropic allosteric mechanism, and St. John's wort induced the intestinal expression of Pgp in vitro and in vivo. Some components (e.g., bergamottin and quercetin) from grapefruit juice were reported to modulate Pgp activity. Many of these herbal constituents, in particular flavonoids, were reported to modulate Pgp by directly interacting with the vicinal ATP-binding site, the steroid-binding site, or the substrate-binding site. Some herbal constituents (e.g., hyperforin and kava) were shown to activate pregnane X receptor, an orphan nuclear receptor acting as a key regulator of MDR1 and many other genes. The inhibition of Pgp by herbal constituents may provide a novel approach for reversing multidrug resistance in tumor cells, whereas the stimulation of Pgp expression or activity has implication for chemoprotective enhancement by herbal medicines. Certain natural flavonols (e.g., kaempferol, quercetin, and galangin) are potent stimulators of the Pgp-mediated efflux of 7,12-dimethylbenz(a)-anthracene (a carcinogen). The modulation of Pgp activity and expression by these herb constituents may result in altered absorption and bioavailability of drugs that are Pgp substrates. This is exemplified by increased oral bioavailability of phenytoin and rifampin by piperine and decreased bioavailability of indinavir, tacrolimus, cyclosporine, digoxin, and fexofenadine by coadministered St. John's wort. However, many of these drugs are also substrates of CYP3A4. Thus, the modulation of intestinal Pgp and CYP3A4 represents an important mechanism for many clinically important herb-drug interactions. Further studies are needed to explore the relative role of Pgp and CYP3A4 modulation by herbs and the mechanism for the interplay of these two important proteins in herb-drug interactions.
"In contrast to inhibitors that bind the N-terminus, novobiocin derivatives that target the C-terminal domain slow the growth of cancer cells and promote degradation of HSP90 clients without inducing the heat shock response, which is an important and somewhat unexpected distinction . Other non-ATP-competitive HSP90 inhibitors are known, but most are non-selective, which limits their in vivo use; examples include epigallocatechin gallate  (inhibits several nonchaperone targets     ), cisplatin  (damages DNA ), and silybin  (inhibits P-glycoprotein  and cytochrome P450 ). A particularly interesting class of small molecules is capable of modulating co-chaperone access to the EEVD motif in the HSP90 C-terminus. "
"The amount of berberine capable of crossing enterocytes seems to be reduced by about 90% by P-gp, and this suggests that either the use of a potential P-gp inhibitor21 or a chemical modification of berberine that would allow it to overcome P-gp antagonism22 might enhance its poor oral bioavailability, thus increasing its clinical effectiveness. Among the potential P-gp inhibitors, silymarin (derived from Silybum marianum), a herbal drug traditionally used as a liver protectant, could be considered a good candidate, owing to its very poor oral bioavailability and very high safety profile.23 We therefore decided to test the clinical role played by silymarin when added to berberine in the treatment of glycemic and lipid alterations in patients with T2DM. "
[Show abstract][Hide abstract] ABSTRACT: Berberine is an isoquinoline alkaloid widely used to improve the glucidic and lipidic profiles of patients with hypercholesterolemia, metabolic syndrome, and type 2 diabetes. The limitation of berberine seems to be its poor oral bioavailability, which is affected by the presence, in enterocytes, of P-glycoprotein - an active adenosine triphosphate (ATP)-consuming efflux protein that extrudes berberine into the intestinal lumen, thus limiting its absorption. According to some authors, silymarin, derived from Silybum marianum, could be considered a P-glycoprotein antagonist.
The study aimed to evaluate the role played by a possible P-glycoprotein antagonist (silymarin), when added to a product containing Berberis aristata extract, in terms of benefits to patients with type 2 diabetes.
The study enrolled 69 patients with type 2 diabetes in suboptimal glycemic control who were treated with diet, hypoglycemic drugs, and in cases of concomitant alterations of the lipid profile, hypolipidemic agents. The patients received an add-on therapy consisting of either a standardized extract of Berberis aristata (titrated in 85% berberine) corresponding to 1,000 mg/day of berberine, or Berberol®, a fixed combination containing the same standardized extract of Berberis aristata plus a standardized extract of Silybum marianum (titrated as >60% in silymarin), for a total intake of 1,000 mg/day of berberine and 210 mg/day of silymarin.
Both treatments similarly improved fasting glucose, total cholesterol, low-density lipoprotein (LDL) cholesterol, triglyceride, and liver enzyme levels, whereas glycosylated hemoglobin (HbA1c) values were reduced to a greater extent by the fixed combination.
The association of berberine and silymarin demonstrated to be more effective than berberine alone in reducing HbA1c, when administered at the same dose and in the form of standardized extracts in type 2 diabetic patients.
Clinical Pharmacology: Advances and Applications 11/2013; 5:167-174. DOI:10.2147/CPAA.S54308
"The mechanisms by which β-CD and HP-β-CD inhibit Pgp activity may be different from that of other Pgp inhibitors, such as cyclosporine A, quinidine, and verapamil.43 Pgp recognizes many compounds as substrates, and tends to have a high affinity with hydrophobic and positively charged compounds at a certain physiological pH.44 β-CD and HP-β-CD do not appear to be substrates of Pgp because they are both hydrophilic and are electrically neutral cyclic oligosaccharides with a relatively high molecular weight.45 Furthermore, β-CD and HP-β-CD should not compete with Pgp substrates because of their lack of cell permeability.27 "
[Show abstract][Hide abstract] ABSTRACT: Icariin is a bioactive herbal ingredient isolated from Herba epimedii, which has been widely used for the treatment of osteoporosis and male sexual dysfunction in traditional Chinese medicine. The major objective of this work is to investigate the different enhancing effects of β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) on the intestinal absorption of icariin, and to identify the molecular mechanisms of this action. Host-guest-type interactions of icariin with cyclodextrins nanocavities were unambiguously demonstrated by the phase-solubility diagram, ultraviolet spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, X-ray powder diffractometry, and two dimensional proton nuclear magnetic resonance rotating-frame Overhauser effect spectroscopy. These results were further supported using molecular modeling studies. The rat single-pass intestinal perfusion model showed that the absorption of icariin was affected by P-glycoprotein (Pgp). The icariin/HP-β-CD inclusion complex provided greater enhancement in the intestinal absorption than the icariin/β-CD inclusion complex. Therefore, the enhancing effect was involved in a solubilizing effect and/or Pgp inhibitory effect. Finally, fluorescence anisotropy measurements and Pgp adenosine triphosphatase (ATPase) assay demonstrated that β-CD exhibited no effect on Pgp, while HP-β-CD showed inhibition by restraining the Pgp ATPase activity rather than changing the fluidity of cell membrane.
International Journal of Nanomedicine 08/2012; 7:4239-49. DOI:10.2147/IJN.S33014 · 4.38 Impact Factor
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.