Transport characteristics of isorhamnetin across intestinal Caco-2 cell monolayers and the effects of transporters on it

The in vitro intestinal cell model: different co-cultured cells create different applications

As a vitro absorption model, the Caco-2 cells originate from a human colon adenocarcinomas and can differentiate into a cell layer with enterocyte-like features. The Caco-2 cell model is popularly applied to explore drug transport mechanisms, to evaluate the permeability of drug and to predict the absorption of drugs or bioactive substances in the gut. However, there are limitations to the application of Caco-2 cell model due to lack of a mucus layer, the long culture period and the inability to accurately simulate the intestinal environment. The most frequent way to expand the Caco-2 cell model and address its limitations is by co-culturing it with other cells or substances. This article reviews the culture methods and applications of 3D and 2D co-culture cell models established around Caco-2 cells. It also concludes with a summary of model strengths and weaknesses.

A water-soluble preparation for intravenous administration of isorhamnetin and its pharmacokinetics in rats

Flavonoids are considered as health-protecting food constituents. The testing of their biological effects is however hampered by their low oral absorption and complex metabolism. In order to investigate the direct effect(s) of unmetabolized flavonoid, a preparation in a biologically friendly solvent for intravenous administration is needed. Isorhamnetin, a natural flavonoid and a human metabolite of the most frequently tested flavonoid quercetin, has very low water solubility (<3.5 μg/mL). The aim of this study was to improve its solubility to enable intravenous administration and to test its pharmacokinetics in an animal model. By using polyvinylpyrrolidone (PVP10) and benzalkonium chloride, we were able to improve the solubility approximately 600 times to 2.1 mg/mL. This solution was then administered intravenously at a dose of 0.5 mg/kg of isorhamnetin to rats and its pharmacokinetics was analyzed. The pharmacokinetics of isorhamnetin corresponded to two compartmental model with a rapid initial distribution phase (t1/2α: 5.7 ± 4.3 min) and a slower elimination phase (t1/2β: 61 ± 47.5 min). Two sulfate metabolites were also identified. PVP10 and benzalkonium did not modify the properties of isorhamnetin (iron chelation and reduction, and cell penetration) substantially. In conclusion, the novel preparation reported in this study is suitable for future testing of isorhamnetin effects under in vivo conditions.

Solid self-microemulsifying drug delivery system (S-SMEDDS) prepared by spray drying to improve the oral bioavailability of cinnamaldehyde (CA)

The aim of this study was to prepare a solid self-microemulsifying drug delivery system (S-SMEDDS) of cinnamaldehyde (CA) by spray drying technique to improve the oral bioavailability of CA. The preparation of CA S-SMEDDS with maltodextrin as the solid carrier, a core-wall material mass ratio of 1:1, a solid content of 20% (w/v), an inlet air temperature of 150 °C, an injection speed of 5.2 mL/min, and an atomization pressure of 0.1 MPa was determined by using the encapsulation rate as the index of investigation. Differential scanning calorimetry (DSC) revealed the possibility of CA being encapsulated in S-SMEDDS in an amorphous form. The in-vitro release showed that the total amount of CA released by S-SMEDDS was approximately 1.3 times higher than that of the CA suspension. Pharmacokinetic results showed that the relative oral bioavailability of CA S-SMEDDS was also increased to 1.6-fold compared to CA suspension. Additionally, we explored the mechanism of CA uptake and transport of lipid-soluble drugs CA by S-SMEDDS in a Caco-2/HT29 cell co-culture system for the first time. The results showed that CA S-SMEDDS uptake on the co-culture model was mainly an energy-dependent endocytosis mechanism, including lattice protein-mediated endocytosis and vesicle-mediated endocytosis. Transport experiments showed that CA S-SMEDDS significantly increased the permeability of CA in this model. These findings suggested that CA S-SMEDDS is an effective oral solid dosage form for increasing the oral bioavailability of lipid-soluble drug CA.

Citric Acid Enhances the Activities of Astilbin on Psoriasis via Down-Regulation of P-Glycoprotein

Astilbin (AS) has been confirmed to be an attractive candidate drug for psoriasis; however, the low oral absorption limits its further development and utilization. Herein, a simple method was discovered to solve this problem, which was combined with citric acid (CA). The efficiency was estimated by imiquimod (IMQ)-induced psoriasis-like mice, and the absorption was predicted by the Ussing chamber model, HEK293-P-gp cells were used to validate the target. Compared with the AS group, the combination with CA significantly reduced the PASI score and down-regulated the protein expression of IL-6 and IL-22, which showed that the combination of CA enhanced the anti-psoriasis effect of AS. Moreover, AS concentration in psoriasis-like mice plasma was significantly increased (3.90-fold) in the CA combined group, and the mRNA and protein levels of P-gp in the small intestine of the combined group were decreased by 77.95 and 30.00%, respectively. In addition, when combined with CA, AS absorption significantly increased while the efflux ratio decreased in vitro. Furthermore, CA significantly elevated the uptake of AS by 153.37% and decreased the protein expression of P-gp by 31.70% in HEK293-P-gp cells. These results indicated that CA enhanced the therapeutic efficacy of AS by improving its absorption via down-regulation of P-gp.

Effects of Isorhamnetin on Diabetes and Its Associated Complications: A Review of In Vitro and In Vivo Studies and a Post Hoc Transcriptome Analysis of Involved Molecular Pathways

Diabetes mellitus, especially type 2 (T2DM), is a major public health problem globally. DM is characterized by high levels of glycemia and insulinemia due to impaired insulin secretion and insulin sensitivity of the cells, known as insulin resistance. T2DM causes multiple and severe complications such as nephropathy, neuropathy, and retinopathy causing cell oxidative damages in different internal tissues, particularly the pancreas, heart, adipose tissue, liver, and kidneys. Plant extracts and their bioactive phytochemicals are gaining interest as new therapeutic and preventive alternatives for T2DM and its associated complications. In this regard, isorhamnetin, a plant flavonoid, has long been studied for its potential anti-diabetic effects. This review describes its impact on reducing diabetes-related disorders by decreasing glucose levels, ameliorating the oxidative status, alleviating inflammation, and modulating lipid metabolism and adipocyte differentiation by regulating involved signaling pathways reported in the in vitro and in vivo studies. Additionally, we include a post hoc whole-genome transcriptome analysis of biological activities of isorhamnetin using a stem cell-based tool.

Mechanism of the Intestinal Absorption of Six Flavonoids from Zizyphi Spinosi Semen Across Caco-2 Cell Monolayer Model

BACKGROUND

Flavonoid compounds are one kind of active ingredients isolated from a traditional Chinese herb Zizyphi spinosae semen (ZSS). Studies have shown that ZSS flavonoids have significant antioxidant effects.

METHODS

In this study, the Caco-2 cell monolayer model was constructed to investigate the intestinal absorption characteristics and mechanism of Isovitexin (IV), Swertisin (ST), Isovitexin-2''-O-β-D-glucopyranoside (IVG), Spinosin (S), 6'''-p-coumaroylspinosin (6-CS) and 6'''-feruloylspinosin (6-FS).

RESULTS

The results of the bidirectional transport assay showed that the six flavonoids have good intestinal absorption in a near-neutral and 37°C environment, and the absorbability in descending order was 6-FS>6- CS>IVG>S>IV>ST. The results of carrier inhibition experiments and transport kinetics indicated that the absorption mechanism of six flavonoids was energy-dependent monocarboxylate transporter (MCT)-mediated active transport. In particular, the para-cellular pathway also participated in the transport of IV, ST, IVG and S. Furthermore, the efflux process of six flavonoids was mediated by P-glycoprotein (P-gp) and multidrug resistance protein (MRP), which may result in a decrease of bioavailability.

CONCLUSION

Our findings provide significant information for revealing the relationship between the intestinal absorption mechanism of flavonoids and its structure as well as laying a basis for the research of flavonoid preparations.

Transit and Metabolic Pathways of Quercetin in Tubular Cells: Involvement of Its Antioxidant Properties in the Kidney

Quercetin is a flavonoid with antioxidant, antiviral, antimicrobial, and anti-inflammatory properties. Therefore, it has been postulated as a molecule with great therapeutic potential. The renoprotective capacity of quercetin against various toxins that produce oxidative stress, in both in vivo and in vitro models, has been shown. However, it is not clear whether quercetin itself or any of its metabolites are responsible for the protective effects on the kidney. Although the pharmacokinetics of quercetin have been widely studied and the complexity of its transit throughout the body is well known, the metabolic processes that occur in the kidney are less known. Because of that, the objective of this review was to delve into the molecular and cellular events triggered by quercetin and/or its metabolites in the tubular cells, which could explain some of the protective properties of this flavonoid against oxidative stress produced by toxin administration. Thus, the following are analyzed: (1) the transit of quercetin to the kidney; (2) the uptake mechanisms of quercetin and its metabolites from plasma to the tubular cells; (3) the metabolic processes triggered in those cells, which affect the accumulation of metabolites in the intracellular space; and (4) the efflux mechanisms of these compounds and their subsequent elimination through urine. Finally, it is discussed whether those processes that are mediated in the tubular cells and that give rise to different metabolites are related to the antioxidant and renoprotective properties observed after the administration of quercetin.

In vitro evaluation of intestinal absorption of tiliroside from Edgeworthia gardneri (Wall.) Meisn

Although Edgeworthia gardneri (Wall.) Meisn and its main component tiliroside (TIL) show good bioactivity, its intestinal absorption data supporting its low bioavailability have not been reported.The evaluation results of three absorption models in vitro and in vivo indicated that the results of the Ussing chamber model were basically consistent with the results of in vivo experiments. It was thus applied to investigate the characteristics of TIL across various intestinal regions and the interaction between TIL and adenosine triphosphate (ATP)-binding cassette family proteins (ABC) including, P-glycoprotein (P-gp), multidrug resistance-associated protein 2 (MRP₂), and breast cancer resistance protein (BCRP).The data of the bi-directional transport showed that the ileum had the higher apparent permeability coefficient (P_app) of TIL than duodenum and jejunum, suggesting the best absorption of TIL in the ileum.In the presence of the MRP₂ inhibitor, the absorption of TIL from water extracts of E. gardneri (Wall.) Meisn (WAE) was improved, indicating that MRP₂ other than P-gp and BCRP affected the absorption of TIL and might be responsible for its low bioavailability. This study laid the foundation for enhancing the bioavailability of TIL and highlighted the influences of efflux transporters on bioavailability.

Inhibitory Effects of Quercetin and Its Main Methyl, Sulfate, and Glucuronic Acid Conjugates on Cytochrome P450 Enzymes, and on OATP, BCRP and MRP2 Transporters

Quercetin is a flavonoid, its glycosides and aglycone are found in significant amounts in several plants and dietary supplements. Because of the high presystemic biotransformation of quercetin, mainly its conjugates appear in circulation. As has been reported in previous studies, quercetin can interact with several proteins of pharmacokinetic importance. However, the interactions of its metabolites with biotransformation enzymes and drug transporters have barely been examined. In this study, the inhibitory effects of quercetin and its most relevant methyl, sulfate, and glucuronide metabolites were tested on cytochrome P450 (CYP) (2C19, 3A4, and 2D6) enzymes as well as on organic anion-transporting polypeptides (OATPs) (OATP1A2, OATP1B1, OATP1B3, and OATP2B1) and ATP (adenosine triphosphate) Binding Cassette (ABC) (BCRP and MRP2) transporters. Quercetin and its metabolites (quercetin-3'-sulfate, quercetin-3-glucuronide, isorhamnetin, and isorhamnetin-3-glucuronide) showed weak inhibitory effects on CYP2C19 and 3A4, while they did not affect CYP2D6 activity. Some of the flavonoids caused weak inhibition of OATP1A2 and MRP2. However, most of the compounds tested proved to be strong inhibitors of OATP1B1, OATP1B3, OATP2B1, and BCRP. Our data demonstrate that not only quercetin but some of its conjugates, can also interact with CYP enzymes and drug transporters. Therefore, high intake of quercetin may interfere with the pharmacokinetics of drugs.

Characterization of the intestinal absorption of morroniside from Cornus officinalis Sieb. et Zucc via a Caco-2 cell monolayer model

Morroniside is a biologically active polyphenol found in Cornus officinalis Sieb. et Zucc (CO) that exhibits a broad spectrum of pharmacological activities, such as protecting nerves, and preventing diabetic liver damage and renal damage. However, little data are available regarding the mechanism of its intestinal absorption. Here, an in vitro human intestinal epithelial cell model of cultured Caco-2 cells was applied to study the absorption and transport of morroniside. The effects of donor concentration, pH and inhibitors were investigated. The bidirectional permeability of morroniside from the apical (AP) to the basolateral (BL) side and in the reverse direction was studied. When administered at three tested concentrations (5, 25 and 100 μM), the apparent permeability coefficient (Papp) values in the AP-to-BL direction ranged from 1.59 × 10-6 to 2.66 × 10-6 cm/s. In the reverse direction, BL-to-AP, the value was ranged from 2.67 × 10-6 to 4.10 × 10-6 cm/s. The data indicated that morroniside transport was pH-dependent. The permeability of morroniside was affected by treatment with various inhibitors, such as multidrug resistance protein inhibitors MK571 and indomethacin, as well as the breast cancer resistance protein inhibitor apigenin. The mechanisms of the intestinal absorption of morroniside may involve multiple transport pathways, such as the passive diffusion and efflux protein-mediated active transport especially involving multidrug resistance protein 2 and breast cancer resistance protein. After the addition of CO, the Papp values in the AP-to-BL direction increased significantly, therefore, it can be assumed that some ingredients in the CO promote morroniside absorption in the small intestine.

Synergistic Mechanisms of Constituents in Herbal Extracts during Intestinal Absorption: Focus on Natural Occurring Nanoparticles

The systematic separation strategy has long and widely been applied in the research and development of herbal medicines. However, the pharmacological effects of many bioactive constituents are much weaker than those of the corresponding herbal extracts. Thus, there is a consensus that purer herbal extracts are sometimes less effective. Pharmacological loss of purified constituents is closely associated with their significantly reduced intestinal absorption after oral administration. In this review, pharmacokinetic synergies among constituents in herbal extracts during intestinal absorption were systematically summarized to broaden the general understanding of the pharmaceutical nature of herbal medicines. Briefly, some coexisting constituents including plant-produced primary and secondary metabolites, promote the intestinal absorption of active constituents by improving solubility, inhibiting first-pass elimination mediated by drug-metabolizing enzymes or drug transporters, increasing the membrane permeability of enterocytes, and reversibly opening the paracellular tight junction between enterocytes. Moreover, some coexisting constituents change the forms of bioactive constituents via mechanisms including the formation of natural nanoparticles. This review will focus on explaining this new synergistic mechanism. Thus, herbal extracts can be considered mixtures of bioactive compounds and pharmacokinetic synergists. This review may provide ideas and strategies for further research and development of herbal medicines.

Exploring the compatibility mechanism of ShengDiHuang Decoction based on the in situ single-pass intestinal perfusion model

Affecting the absorption of active ingredients in the intestine serves as one of the important compatibility mechanisms of traditional Chinese medicine. The aim of this study was to investigate the compatibility mechanism of ShengDiHuang Decoction (SDHD) by using the single-pass intestinal perfusion in situ model. The major effective ingredients, catalpol, aucubin, acteoside, rehmannioside D, rehmannioside A, rhein, aloe emodin, emodin, chrysophanol, and physcion, were determined by HPLC. By analysing the effects of different concentrations, different pH, intestinal segments, protein inhibitors, and tight junction regulators on SDHD absorption, it was found that catalpol, aucubin, rehmannioside D, rehmannioside A, acteoside, rhein, and chrysophanol may undergo active transport, while aloe-emodin and emodin may undergo passive transport. Catalpol, aucubin, and rehmannioside D may be substrates of BCRP and MRP2, while rehmannioside A and rhein may be substrates of BCRP, and acteoside and chrysophanol may be substrates of P-gp, BCRP and MRP2. By comparing the P_app values of the major effective ingredients between single herb and herb-pairs, the compatibility of rehmannia and rheum could significantly promote the absorption of components in rehmannia. It is verified that rheum has a synergistic effect on the absorption of rehmannia in SDHD.

Transepithelial transport mechanisms of 7,8-dihydroxyflavone, a small molecular TrkB receptor agonist, in human intestinal Caco-2 cells

7,8-Dihydroxyflavone (7,8-DHF), as a high-affinity TrkB receptor agonist, has been extensively explored in many human disorders involving brain-derived neurotrophic factor (BDNF) such as Alzheimer's disease, Parkinson's disease, depression, and obesity. However, to date, the transepithelial transport mechanisms of 7,8-DHF in the intestines remain unclear. The aim of our work was to quantify and to characterize in vitro transport of naturally occurring 7,8-DHF distinguished by its physicochemical and pharmacological properties. We discussed the transport mechanisms of 7,8-DHF using the Caco-2 cell model to determine the bi-directional permeability with different environmental factors (time, concentration, pH, metabolic inhibitors etc.). The influx and efflux characteristics of 7,8-DHF were also clarified. 7,8-DHF was poorly transported across Caco-2 cell monolayers by mainly passive diffusion via a transcellular pathway and not a paracellular pathway. The transport of 7,8-DHF was time and concentration-dependent in both the apical (AP) to basolateral (BL) side and the reverse direction. Interestingly, decreasing the pH from 7.4 to 6.0 markedly enhanced 7,8-DHF transport. It is noteworthy that 7,8-DHF transport was strongly inhibited by metabolic inhibitors and was highly dependent on temperature. The efflux ratio (ER) values at different concentrations were all above 1.5, indicating the existence of the efflux transporter. We found that breast cancer resistance protein (BCRP) was not involved in 7,8-DHF secretion and that the transport mechanism of 7,8-DHF was passive transport with an active efflux mediated by P-glycoprotein (P-gp) and multidrug resistance associated proteins (MRPs), particularly MRP 2. Moreover, the use of various influx transporter inhibitors in Caco-2 cells showed that organic cation transporters (OCTs) and organic anion-transporting polypeptides (OATPs) participated in 7,8-DHF transport. Taken together, the elucidated transport characteristics of 7,8-DHF provide useful information for designing novel and efficient delivery systems and avoiding food-food or food-drug interactions.

Intestinal mucosal metabolites-guided detection of trace-level ginkgo biloba extract metabolome

The characterization of metabolome for poorly absorptive natural medicines is challenging. Previous identification strategy often relies on nontargeted scanning biological samples from animals administered with natural medicines in a data-dependent acquisition (DDA) mode by LC-MS/MS. Substances that displayed significant increases following drug administration are thus assigned as potential metabolites. The accurate m/z of precursors and the corresponding MS/MS fragment ions are used to match with herbal ingredients and to infer possible metabolic reactions. Nevertheless, the low concentration of these metabolites within complex biological matrices has often hampered the detection. Herein we developed a strategy termed intestinal mucosal metabolome-guided detection (IMMD) to tackle this challenge using ginkgo biloba (GBE) as an example. The rationale is that poorly absorptive natural products are usually concentrated and extensively metabolized by enterocytes before they enter the blood stream and distribute to other organs. Therefore, we firstly identified the metabolites from intestinal mucosa of GBE-treated rats, and then used the identified intestinal mucosal GBE metabolome as targeted repository for MRM analysis. The presences of these metabolites were subsequently examined in rat plasma, liver and brain. The resultant GBE metabolome showed significantly improved coverage with 39, 45 and 6 metabolites identified in plasma, liver and brain compared to 22, 16 and 0 metabolites from the corresponding regions via the DDA-based strategy. In addition, we integrated the previously reported nontargeted diagnostic ion network analysis to facilitate the characterization of GBE components, and a chemicalome-metabolome matching approach (CMMA) to assist the identity assignment of GBE metabolome with IMMD. Combinatorially, we establish a multi-faceted platform to streamline the workflow of metabolome characterization for herbal medicines of low bioavailability. The metabolome information is expected to shed light on the elucidation of metabolic pathways for natural products, and the underlying mechanisms of their biological efficacies.

Effects of Gut Microbiota on the Bioavailability of Bioactive Compounds from Ginkgo Leaf Extracts

Ginkgo leaf extract (GLE) is a popular herbal medicine and dietary supplement for the treatment of various diseases, including cardiovascular disease. GLE contains a variety of secondary plant metabolites, such as flavonoids and terpenoids, as active components. Some of these phytochemicals have been known to be metabolized by gut microbial enzymes. The aim of this study was to investigate the effects of the gut microbiota on the pharmacokinetics of the main constituents of GLE using antibacterial-treated mice. The bilobalide, ginkgolide A, ginkgolide B, ginkgolide C, isorhamnetin, kaempferol, and quercetin pharmacokinetic profiles of orally administered GLE (600 mg/kg), with or without ciprofloxacin pretreatment (150 mg/kg/day for 3 days), were determined. In the antibacterial-treated mice, the maximum plasma concentration (C_max) and area under the curve (AUC) of isorhamnetin were significantly (p < 0.05) increased when compared with the control group. The C_max and AUC of kaempferol and quercetin (other flavonol glycosides) were slightly higher than those of the control group, but the difference was not statistically significant, while both parameters for terpenoids of GLE showed no significant difference between the antibacterial-treated and control groups. These results showed that antibacterial consumption may increase the bioavailability of isorhamnetin by suppressing gut microbial metabolic activities.

Intestinal Absorption of Isoalantolactone and Alantolactone, Two Sesquiterpene Lactones from Radix Inulae, Using Caco-2 Cells

BACKGROUND

Isoalantolactone and alantolactone are the main sesquiterpene lactones in Radix Inulae (dried root of Inula helenium L. or I. racemosa Hook. F.), which is a frequently utilized herbal medicine. They also occur in several plants and have various pharmacologic effects. However, they have been found to have poor oral bioavailability in rats.

OBJECTIVES

To understand the intestinal absorptive characteristics of isoalantolactone and alantolactone as well specific influx and efflux transporters in their absorption.

METHODS

Bidirectional permeabilities of isoalantolactone and alantolactone were investigated across Caco-2 cell monolayers. Transport assays were performed using different concentrations of two lactones and specific inhibitors of ATP-binding cassette transporters and influx transporters.

RESULTS

The absorption permeability of isoalantolactone and alantolactone was high at the tested concentrations (5, 20 and 80 μmol/l), and the major permeation mechanism of both lactones was found to be passive diffusion with active efflux mediated by multidrug resistance-associated proteins (MRPs) and breast cancer resistance protein (BCRP).

CONCLUSION

Our results demonstrated that the absorption permeability of isoalantolactone and alantolactone was good in the Caco-2 cell model. The isoalantolactone and alantolactone absorption elucidated in this study provides useful information for further pharmacokinetics studies. Since low intestinal absorption can now be ruled out as a cause, further studies are needed to explain the low oral bioavailability of the two sesquiterpene lactones.

Oyster-Derived Zinc-Binding Peptide Modified by Plastein Reaction via Zinc Chelation Promotes the Intestinal Absorption of Zinc

Zinc-binding peptides from oyster (Crassostrea gigas) have potential effects on zinc supplementation. The aim of this study was to prepare efficient zinc-binding peptides from oyster-modified hydrolysates by adding exogenous glutamate according to the plastein reaction and to further explore the zinc absorption mechanism of the peptide-zinc complex (MZ). The optimum conditions for the plastein reaction were as follows: pH 5.0, 40 °C, substrate concentration of 40%, pepsin dosage of 500 U/g, reaction time of 3 h and l-[1-¹³C]glutamate concentration of 10 mg/mL. The results of ¹³C isotope labelling suggested that the addition of l-[1-¹³C]glutamate contributed to the increase in the zinc-binding capacity of the peptide. The hydrophobic interaction was the main mechanism of action of the plastein reaction. Ultraviolet spectra and scanning electronic microscopy (SEM) revealed that the zinc-binding peptide could bind with zinc and form MZ. Furthermore, MZ could significantly enhance zinc bioavailability in the presence of phytic acid, compared to the commonly used ZnSO₄. Additionally, MZ significantly promoted the intestinal absorption of zinc mainly through two pathways, the zinc ion channel and the small peptide transport pathway. Our work attempted to increase the understanding of the zinc absorption mechanism of MZ and to support the potential application of MZ as a supplementary medicine.

Quercetin and kaempferol increase the intestinal absorption of isorhamnetin coexisting in Elaeagnus rhamnoides (L.) A. Nelson (Elaeagnaceae) extracts via regulating multidrug resistance-associated protein 2

BACKGROUND

Isorhamnetin (IS) is a flavonoid component with many biological activities such as antioxidant, anti-inflammatory, and anticancer, which is also the main active component in total flavones of Elaeagnus rhamnoides (L.) A. Nelson (Elaeagnaceae) (TFH); however, the interaction between IS and other components in TFH is unclear.

PURPOSE

The aim of the present study was to investigate the enhancement of quercetin (QU) or kaempferol (KA) on the intestinal absorption of IS coexisting in TFH, and then preliminarily illuminate the related mechanisms.

METHODS

Firstly, the intestinal absorption of IS in the presence or absence of QU or KA was conducted by in vivo pharmacokinetics model, in situ single-pass intestinal perfusion model (SPIP), and MDCK II-MRP2 monolayer cell model to confirm the enhancement of QU or KA on IS absorption. Secondly, the effects of multidrug resistance-associated protein 2 (MRP2) inhibitors on the IS intestinal absorption were investigated to ascertain the mediation of MRP2 on IS absorption. Finally, the effects of QU or KA on MRP2 activity, protein expression, and mRNA level were performed by SPIP, everted-gut sacs, western blotting, and real-time polymerase chain reaction experiments to elucidate the related mechanisms.

RESULTS

QU or KA increased IS intestinal absorption according to the increased AUC_0-96h, C_max, and P_eff of IS after co-administrated with QU or KA to rats; the oral absorption of IS was mediated by MRP2 based on the facts that the average plasma concentration, AUC_0-96h, and P_eff of IS were increased when co-administrated with PR or MK571 (MRP2 inhibitors) as well as the P_ratio(BL/AP) of IS was decreased by MK571 in MDCK II-MRP2 cell monolayer; the activity, protein expression, and mRNA level of MRP2 were inhibited or down-regulated by QU or KA because of the increased P_eff of MRP2 substrate calcein (CA) and the down-regulated relative protein and mRNA intensity after co-treated with QU or KA.

CONCLUSION

QU and KA increased the intestinal absorption of IS in TFH by regulating the activity and expression of MRP2, which provides useful information for the investigation of the transporter-mediated interaction of flavonoid components in herbal extracts.

The intestinal absorption properties of flavonoids in Hippophaë rhamnoides extracts by an in situ single-pass intestinal perfusion model

The purpose of this study was to investigate the absorption properties of isorhamnetin (IS), quercetin (QU), and kaempferol (KA) in total flavones of Hippophaë rhamnoides L. (TFH) by an in situ single-pass intestinal perfusion model. The results indicated that IS, QU, and KA in TFH were absorbed site-dependently, and both enterohepatic circulation and intestinal flora could participate in their absorption processes. The absorption mechanisms of IS, QU, and KA in TFH were involved in both passive diffusion and active transport, and the mediation of efflux transporter multidrug resistance-associated proteins (MRPs) should not be neglected.

Elucidation of the Intestinal Absorption Mechanism of Loganin in the Human Intestinal Caco-2 Cell Model

Loganin, iridoid glycosides, is the main bioactive ingredients in the plant Strychnos nux-vomica L. and demonstrates various pharmacological effects, though poor oral bioavailability in rats. In this study, the intestinal absorption mechanism of loganin was investigated using the human intestinal Caco-2 cell monolayer model in both the apical-to-basolateral (A-B) and the basolateral-to-apical (B-A) direction; additionally, transport characteristics were systematically investigated at different concentrations, pHs, temperatures, and potential transporters. The absorption permeability (PappAB) of loganin, which ranged from 12.17 to 14.78 × 10-6cm/s, was high at four tested concentrations (5, 20, 40, and 80μM), while the major permeation mechanism of loganin was found to be passive diffusion with active efflux mediated by multidrug resistance-associated protein (MRP) and breast cancer resistance protein (BCRP). In addition, it was found that loganin was not the substrate of efflux transporter P-glycoprotein (P-gp) since the selective inhibitor (verapamil) of the efflux transporter exhibited little effects on the transport of loganin in the human intestinal Caco-2 cells. Meanwhile, transport from the apical to the basolateral side increased 2.09-fold after addition of a MRP inhibitor and 2.32-fold after addition of a BCRP inhibitor. In summary, our results clearly demonstrate, for the first time, a good permeability of loganin in the human intestinal Caco-2 cell model and elucidate, in detail, the intestinal absorption mechanism and the effects of transporters on iridoid glycosides compounds.

Phenolic compounds, antioxidant activity, antiproliferative activity and bioaccessibility of Sea buckthorn (Hippophaë rhamnoides L.) berries as affected by in vitro digestion

Phenolics, antioxidant and antiproliferative properties of Sea buckthorn berries were evaluated using a simulated in vitro digestion and compared with a chemical extraction method. Digested samples were subjected to antiproliferation evaluation against human liver, breast and colon cancer cells. Furthermore, the bioaccessibility of digested berries was evaluated using a Caco-2 cell culture model. Results revealed that after enzymatic digestion the phenolic compounds were quite different from the chemical extracts, more flavonoid aglycones were released, whereas less total phenolics, phenolic acids and flavonoid glycosides were detected. Although the extracellular antioxidant activity of the digesta was lower than that of extracts, the cellular antioxidant activity (CAA) and antiproliferative effects of berries were significantly enhanced by digestion. This was attributed to their higher flavonoid aglycone content and could be verified by testing individual active compounds, suggesting that the cellular uptake of samples might be improved, which was also certified by the Caco-2 cell uptake model. The digested samples showed an almost 5-fold cellular accumulative amount of isorhamnetin than pure isorhamnetin, which was attributed to the significant down regulation of the mRNA expression level of efflux transporters MRP2 and P-gp. This finding indicated that the digestion enhanced the bioaccessibility of bioactive compounds of berries.

Study of Structure and Permeability Relationship of Flavonoids in Caco-2 Cells

Flavonoids exhibit a broad range of biological activities. However, poor absorption of some flavonoids is a major limitation for use of flavonoids as nutraceuticals. To investigate the structure requirements for flavonoids intestinal absorption, transepithelial transport and cellular accumulation (CA) of 30 flavonoids were determined using the Caco-2 cell monolayer. The bilateral permeation of five types of flavonoids followed the order: flavanones ≥ isoflavones > flavones ≥ chalcones > flavonols. The concentration of flavonoids accumulated in cells did not correlate with cell penetration since the correlation coefficient between the apparent permeability coefficient (P_app) and their corresponding CA was poor (R² < 0.3). Most flavonoids exhibited a ratio of 0.8–1.5 for P_{app A to B}/P_{app B to A}, suggesting passive diffusion pathways. However, luteolin, morin and taxifolin may involve the efflux mechanisms. The quantitative structure-permeability relationship (QSPR) study demonstrated that the intestinal absorption of flavonoids can be related to atomic charges on carbon 3′ (Q_C3′), molecule surface area (SlogP_V3), balance between the center of mass and position of hydrophobic region (vsurf_ID1) and solvation energy of flavonoids (E_sol). These results provide useful information for initially screening of flavonoids with high intestinal absorption.

Bioaccessibility, Intestinal Permeability and Plasma Stability of Isorhamnetin Glycosides from Opuntia ficus-indica (L.)

Isorhamnetin glycosides are representative compounds of Opuntia ficus-indica that possess different biological activities. There is slight information about the changes in bioaccessibility induced by the glycosylation pattern of flavonoids, particularly for isorhamnetin. In this study, the bioaccessibility and permeability of isorhamnetin glycosides extracted from O. ficus-indica were contrasted with an isorhamnetin standard. Also, the plasma stability of these isorhamnetin glycosides after intravenous administration in rats was evaluated. Recoveries of isorhamnetin after oral and gastric digestion were lower than that observed for its glycosides. After intestinal digestion, isorhamnetin glycosides recoveries were reduced to less than 81.0%. The apparent permeability coefficient from apical (AP) to basolateral (BL) direction (Papp(AP-BL)) of isorhamnetin was 2.6 to 4.6-fold higher than those obtained for its glycosides. Isorhamnetin diglycosides showed higher Papp(AP-BL) values than triglycosides. Sugar substituents affected the Papp(AP-BL) of the triglycosides. Isorhamnetin glycosides were better retained in the circulatory system than the aglycone. After intravenous dose of the isorhamnetin standard, the elimination half-life was 0.64 h but increased to 1.08 h when the O. ficus-indica extract was administered. These results suggest that isorhamnetin glycosides naturally found in O. ficus-indica could be a controlled delivery system to maintain a constant plasmatic concentration of this important flavonoid to exert its biological effects in vivo.

Exposure Characteristics of the Analogous β-Carboline Alkaloids Harmaline and Harmine Based on the Efflux Transporter of Multidrug Resistance Protein 2

Harmaline and harmine occur naturally in plants and are distributed endogenously in human and animal tissues. The two β-carboline alkaloids possess potential for treating Alzheimer's disease, Parkinson's disease, depression and other central nervous system diseases. However, studies have showed that the two compounds have similar structures but with quite different bioavailability. The aim of this study was to elucidate the exposure difference and characterize the in vitro transport, metabolism, and pharmacokinetic properties of harmaline and harmine. The results showed that the harmaline and harmine transport across the Caco-2 and MDCK cell monolayers was varied as the time, concentration, pH and temperature changed. The absorption of harmaline and harmine was significantly decreased when ES (OATPs inhibitor), TEA (OCTs/OCTNs substrate), NaN₃ (adenosine triphosphate inhibitor), or sodium vanadate (ATPase Na⁺/K⁺-dependent inhibitor) was added. However, when given MK571 and probenecid (the typical MRP2 inhibitor), the P_appAB of harmine was increased (1.62- and 1.27-folds), and the efflux ratio was decreased from 1.59 to 0.98 and from 1.59 to 1.19, respectively. In addition, the uptake ratio of harmine at 1 μM was >2.65 in the membrane vesicles expressing human MRP2. Furthermore, harmine could slightly up-regulate the expression of MRP2, which implying harmine might be the substrate of MRP2. Particularly, the CL_int-value for harmine was ~1.49-folds greater than that of harmaline in human liver microsomes. It was worth noting that the F-value of harmine was increased 1.96-folds after harmine co-administration with probenecid. To summarize, comprehensive analysis indicated that harmaline and harmine were absorbed by transcellular passive diffusion and a pH- and Na⁺-dependent mechanism might be mediated by OATPs and OCTs/OCTNs. MRP2 but MDR1 or BCRP might be involved in the transport of harmine. Furthermore, harmine was more unstable and easily metabolized than harmaline. All these findings suggested that harmine not only appears be an MRP2 substrate, but also possesses weak metabolic stability, and eventually leads to a low oral bioavailability. Taken together, the elucidated absorption, transport, metabolism as well as pharmacokinetic characteristics of harmaline and harmine provide useful information for designing delivery systems, pharmacological applications and avoiding drug-drug interactions.

The transepithelial transport mechanism of polybrominated diphenyl ethers in human intestine determined using a Caco-2 cell monolayer

Oral ingestion plays an important role in human exposure to polybrominated diphenyl ethers (PBDEs). The uptake of PBDEs primarily occurs in the small intestine. The aim of the present study is to investigate the transepithelial transport characteristics and mechanisms of PBDEs in the small intestine using a Caco-2 cell monolayer model. The apparent permeability coefficients of PBDEs indicated that tri- to hepta-BDEs were poorly absorbed compounds. A linear increase in transepithelial transport was observed with various concentrations of PBDEs, which suggested that passive diffusion dominated their transport at the concentration range tested. In addition, the pseudo-first-order kinetics equation can be applied to the transepithelial transport of PBDEs. The rate-determining step in transepithelial transport of PBDEs was trans-cell transport including the trans-pore process. The significantly lower transepithelial transport rates at low temperature for bidirectional transepithelial transport suggested that an energy-dependent transport mechanism was involved. The efflux transporters (P-glycoprotein, multidrug resistance-associated protein, and breast cancer resistance protein) and influx transporters (organic cation transporters) participated in the transepithelial transport of PBDEs. In addition, the transepithelial transport of PBDEs was pH sensitive; however, more information is required to understand the influence of pH.

Evaluation of PepT1 transport of food-derived antihypertensive peptides, Ile-Pro-Pro and Leu-Lys-Pro using in vitro, ex vivo and in vivo transport models

Ile-Pro-Pro (IPP) and Leu-Lys-Pro (LKP) are food-derived antihypertensive peptides which inhibit angiotensin-converting enzyme (ACE) and may have potential to attenuate hypertension. There is debate over their mechanism of uptake across small intestinal epithelia, but paracellular and PepT1 carrier-mediated uptake are thought to be important routes. The aim of this study was to determine their routes of intestinal permeability using in vitro, ex vivo and in vivo intestinal models. The presence of an apical side pH of 6.5 (mimicking the intestinal acidic microclimate) and of Gly-Sar (a high affinity competitive inhibitor and substrate for PepT1) were tested on the transepithelial apical to basolateral (A to B) transport of [³H]-IPP and [³H]-LKP across filter-grown Caco-2 monolayers in vitro and rat jejunal mucosae ex vivo. A buffer pH of 6.5 on the apical side enabled Gly-Sar to reduce the apparent permeability (P_app) of [³H]-IPP and [³H]-LKP, but this inhibition was not evident at an apical buffer pH of 7.4. Gly-Sar reduced the P_app across isolated jejunal mucosae and the area under the curve (AUC) in intra-jejunal instillations when the apical/luminal buffer pH was either 7.4 or 6.5. However, the jejunal surface acidic pH was maintained in rat jejunal tissue even when the apical side buffer pH was 7.4 due to the presence of the microclimate which is not present in monolayers. PepT1 expression was confirmed by immunofluorescence on monolayers and brush border of rat jejunal tissue. This data suggest that IPP and LKP are highly permeable and cross small intestinal epithelia in part by the PepT1 transporter, with an additional contribution from the paracellular route.

Self-renewing Monolayer of Primary Colonic or Rectal Epithelial Cells

BACKGROUND & AIMS

Three-dimensional organoid culture has fundamentally changed the in vitro study of intestinal biology enabling novel assays; however, its use is limited because of an inaccessible luminal compartment and challenges to data gathering in a three-dimensional hydrogel matrix. Long-lived, self-renewing 2-dimensional (2-D) tissue cultured from primary colon cells has not been accomplished.

METHODS

The surface matrix and chemical factors that sustain 2-D mouse colonic and human rectal epithelial cell monolayers with cell repertoires comparable to that in vivo were identified.

RESULTS

The monolayers formed organoids or colonoids when placed in standard Matrigel culture. As with the colonoids, the monolayers exhibited compartmentalization of proliferative and differentiated cells, with proliferative cells located near the peripheral edges of growing monolayers and differentiated cells predominated in the central regions. Screening of 77 dietary compounds and metabolites revealed altered proliferation or differentiation of the murine colonic epithelium. When exposed to a subset of the compound library, murine organoids exhibited similar responses to that of the monolayer but with differences that were likely attributable to the inaccessible organoid lumen. The response of the human primary epithelium to a compound subset was distinct from that of both the murine primary epithelium and human tumor cells.

CONCLUSIONS

This study demonstrates that a self-renewing 2-D murine and human monolayer derived from primary cells can serve as a physiologically relevant assay system for study of stem cell renewal and differentiation and for compound screening. The platform holds transformative potential for personalized and precision medicine and can be applied to emerging areas of disease modeling and microbiome studies.

Esterification of Quercetin Increases Its Transport Across Human Caco-2 Cells

Plant polyphenols showed useful biochemical characteristics in vitro; however, the assessments of their clinical applications in vivo are restricted by their limited bioavailability due to their strong resistance to 1st-pass effects during absorption. In order to improve the bioavailability of quercetin (QU), the ester derivative of QU (3,3',4',5,7-pentahydroxy flavones, TAQU) was synthesized, followed by examining the oil-water partition coefficient as well as the transport mechanisms of QU and its ester derivative (TAQU) using human Caco-2 cells. The transport characteristics of QU and TAQU transport under different conditions (different concentrations, time, pH, temperature, tight junctions, and potential transporters) were systematically investigated. Results showed that QU had a lower permeability coefficient (2.82 × 10(-6) cm/s) for apical-to-basolateral (AP-BL) transport over 5 to 50 μM, whereas the transport rate for AP to BL flux of TAQU (5.23 × 10(-6) cm/s) was significantly greater than that of QU. Paracellular pathways were not involved during the transport of both QU and TAQU. QU was poorly absorbed by active transport, whereas TAQU was mostly absorbed by passive diffusion. Efflux transporters, P-glycoproteins, multidrug resistance proteins were proven to participate in the transport process of QU, but not in that of TAQU. These results suggested that improving the lipophicity of QU by esterification could increase the transport of QU across Caco-2 cells.

One-step self-assembled nanomicelles for improving the oral bioavailability of nimodipine

Our study aimed to develop a self-assembled nanomicelle for oral administration of nimodipine (NIM) with poor water solubility. Using Solutol(®) HS15, the NIM-loaded self-assembled nanomicelles displayed a near-spherical morphology with a narrow size distribution of 12.57 ± 0.21 nm (polydispersity index =0.071 ± 0.011). Compared with Nimotop(®) (NIM tablets), the intestinal absorption of NIM from NIM nanomicelle in rats was improved by 3.13- and 2.25-fold in duodenum and jejunum at 1 hour after oral administration. The cellular transport of NIM nanomicelle in Caco-2 cell monolayers was significantly enhanced compared to that of Nimotop(®). Regarding the transport pathways, clathrin, lipid raft/caveolae, and macropinocytosis mediated the cell uptake of NIM nanomicelles, while P-glycoprotein and endoplasmic reticulum/Golgi complex (ER/Golgi) pathways were involved in exocytosis. Pharmacokinetic studies in our research laboratory have showed that the area under the plasma concentration-time curve (AUC0-∞) of NIM nanomicelles was 3.72-fold that of Nimotop(®) via oral administration in rats. Moreover, the NIM concentration in the brain from NIM nanomicelles was dramatically improved. Therefore, Solutol(®) HS15-based self-assembled nanomicelles represent a promising delivery system to enhance the oral bioavailability of NIM.