Mechanisms of Intestinal Pharmacokinetic Natural Product-drug Interactions

Abstract

The growing co-consumption of botanical natural products with conventional medications has intensified the need to understand potentialeffects on drug safety and efficacy.This review delves into the intricacies of intestinal pharmacokinetic interactions between botanical natural products and drugs, such as alterations in drug solubility, permeability, transporter activity, and enzyme-mediated metabolism. It emphasizes the importance of understanding how drug solubility, dissolution, and osmolality interplay with botanical constituents in the gastrointestinal tract, potentially altering drug absorption and systemic exposure. Unlike reviews that focus primarily on enzyme and transporter mechanisms, this article highlights the lesser known but equally important mechanisms of interaction. Applying the Biopharmaceutics Drug Disposition Classification System (BDDCS) can serve as a framework for predicting and understanding these interactions.Through a comprehensive examination of specific botanical natural products such as byakkokaninjinto, green tea catechins, goldenseal, spinach extract, and quercetin, we illustrate the diversity of these interactions and their dependence on the physicochemical properties of the drug and the botanical constituents involved. This understanding is vital for healthcare professionals to effectively anticipate and manage potential natural product-drug interactions, ensuring optimal patient therapeutic outcomes. By exploring these emerging mechanisms, we aim to broaden the scope of natural product-drug interaction research and encourage comprehensive studies to better elucidate complex mechanisms.

1. Introduction

The National Institutes of Health Office of Dietary Supplements defines botanicalsas plants or plant parts valued for their therapeutic or medicinal properties(National Institutes of Health, 2020).Popular examples include ashwagandha, elderberry, green tea, goldenseal, and St. John’s wort(Smith, Resetar & Morton, 2022). Botanicals are typicallyconsumed as beverages, food, or asa part of anherbal supplement made from extracts or processed plant parts.Historically, botanicals have played a prominent role in drug discovery, and approximately 60% ofFDA-approved medicines from 2001 to 2010 were derived from plants (Kinghorn et al., 2011; Atanasov et al., 2021). An immense amount of information about botanicals and herbal supplements can be found on the internet from peer-reviewed and non-peer reviewed sources. The sale and use of botanicalsor herbal supplementshas gained considerable popularity among consumers due to their perceived health benefits and therapeutic potential(Dickinson & Mackay, 2014; Smith, Resetar & Morton, 2022; Friedman et al., 2023). However, botanicals and herbal supplements pose a risk of natural product-drug interactions occurring because theycontain constituents that can alter drug pharmacokinetics (PK) and/or pharmacodynamics(Asher, Corbett & Hawke, 2017).To prevent these unwanted drug interactions, it is essential to identify the mediators and mechanisms of the drug interactions. This review summarizes the critical mechanisms forPK natural product-drug interactionsin the intestine, emphasizing lesser known and emerging mechanisms involving drug solubility and osmolality, providing a different perspective on PK interactions. Wedo, however, still provide a brief overview of recent studies involving the well-characterized transporter- and enzyme-mediated mechanisms.For some of the studies discussed, only pre-clinical data are available, limiting immediate clinical translation due to challenges associated with extrapolating between in vitro to in vivo data and between rodents and humans.

2. Intestine: a major extra-hepatic site of drug interactions

The oral route is the most common and convenient mode of drug administration (Brown and Wobst 2021).In order to achieve some of the desired clinical outcomes of drug therapy (i.e., maximum efficacy and safety), the active drug must be delivered to its specific site of action to exert its therapeutic effect.The fraction of the administered dose that reaches systemic circulation, known as drug oral bioavailability (F) can impact the concentration of the drug at the site of action. After oral administration and before absorption, which is thetransfer of the drug molecules from the site of administration to systemic circulation,the drugformulationis broken down and undergoes dissolution. Dissolution is the processwhere the active pharmaceutical ingredient is released from the drug formulation. It occurs in the gastric fluids of the stomach andcan continue in the small intestinefor enteric-coated tablets and certain extended-release formulations.Absorption of the dissolved drug molecules primarily occurs in the small intestine due to its unique villous structure and the presence of transporters in enterocytes. Additional factors such as gastrointestinal (GI) conditions, the microbiome composition,or a drug’s physicochemical properties can affect drug dissolution and absorption.The most salient factors are discussed in more detail below and are shown in Figure 1.

Figure 1.

Pathway and factors associated with drug exposure after oral administration.(Abuhelwa et al., 2017). F_a, fraction of dose absorbed from the intestinal lumen; F_g, fraction of dose escaping intestinal metabolism; MW, molecular weight. Created with BioRender.com.

Solubility:

Drug solubility refers to the extent to whicha drug dissolves in a specific liquid such as water or physiological fluids. Poor aqueous drug solubility is a major issue in drug discovery and development because the solubility of a drug in GI fluids is crucial for dissolution. Highly water-soluble drugs tend to dissolve rapidly, whereaslipid-soluble drugs, which account for 40% of available drugs and most new drug candidates, have inherent challenges in dissolution (Fine-Shamir & Dahan, 2024). Drug formulation approaches such as the use of nanocrystals, and/or the inclusion of surfactants, can improve solubility, increase dissolution, and systemic drug exposure (Fischer et al., 2012; Kalvakuntla et al., 2016; Awan et al., 2022).

Permeability:

Permeability can be defined as a drug’s ability to cross the cell membrane, and like solubility, it is a fundamental factor in drug absorption. The lipophilicity of a drug refers to its ability to dissolve in lipids and is important for membrane permeability. Hence, lipophilic drugs with higher logP (logarithm of the partition coefficient) values tend to have greater membrane permeability. Low molecular weight lipid-soluble compounds are absorbed by passive diffusion, whereby the drug molecule permeates the cell membrane from high concentration in the GI tract to low concentration in the portal blood. Conversely, larger polar molecules cross the cell membrane withthe help of a carrier protein either by facilitated or active transport.

Food and GI fluid:

The chemistry and compositionof the intestinal fluid,such as pH, osmolality, and bile salts,differ in the fasted versus fed state. Theeffectof food ona drug’s dissolution and absorption depends on the drug’s physicochemical propertiesand formulation(Singh, 1999). Food, especially food with high-fat content, can increase the rate and extent of absorption of poorly water-soluble drugs (Meng et al., 2001; Sunesen et al., 2005). Conversely, food does not typically affect systemic exposure ofmost water-soluble drugs(Du Souich et al., 1990).The enhanced solubility of lipophilic drugs in the fed state can be attributed to several factors,includingdelayed gastric emptying, change in pH, increased blood flow, and secretion of bile salts that formmixed micelles. These micelles are colloidal structures formed by bile acids, phospholipids, and glycerides that act as surfactants in the intestinal lumen. In the fed state, these constituents are present at higher concentrations to facilitate the absorption of dietary lipids.

The physical and chemical properties of GI fluid, such asintestinal fluid volume, osmolality, and pH, play an essential part in the dissolution and absorption of drugs. Changes in the intestinal fluid volume can have differing effects on drug absorption dependingon the drug’s physicochemical properties and other factors, such as formulation and transit time. Generally, an increased intestinal fluid volume provides more medium for a poorly water-soluble drug to dissolve, potentially improving its dissolution(Nader et al., 2016). Osmolality, defined as the number of solutes per kilogram of solution,can affect drug absorption by influencing intestinal fluid volume viawater movement across cell membranes. Higher osmolality in the gut can draw water into the GI tractthrough osmosis, assisting in the dissolution of drugs(increased dissolution liquid) and improving their absorption(Mudie, Amidon & Amidon, 2010).The pH of the GI fluid varies along different segments of the GI tract and can significantly impact drug solubility and absorption due to its influence on drug ionization (Dahlgren et al., 2021).The extent of ionization depends on the drug’s pKa. For example,compounds with a low pKa are unionized in the low pH of the gastric fluidbut are better absorbed in small intestinebecause of the larger surface area and better permeability(Manallack et al., 2013).

Transporters:

Transporters are membrane proteins that facilitate the movement of drugs across biological membranes. In the intestine, they are expressed in the epithelial cells(enterocytes), and their localization, either apical or basolateral (Figure 2), determines their role in the absorption and secretion of drugs and how they contribute to the first-pass effect. They can be broadly grouped as uptake and efflux transporters because of their ability to move endogenous and exogenous substrates into and out of the cell. Changes in transporter expression and activity can alterdrug bioavailability and potentially influence therapeutic outcomes (Shitara et al., 2004; Misaka et al., 2014; Lynch et al., 2021).

Figure 2.

Localization of uptake and efflux transporters in human enterocytes. Open and closed circles represent uptake and efflux transporters, respectively. (?) indicates membrane localization of transporter is equivocaland not well established. (Said et al., 2004; Estudante et al., 2012; Drozdzik et al., 2014; Wenzel, Drozdzik & Oswald, 2021)ASBT1, apical sodium bile transporter 1; BCRP, breast cancer resistance protein; CHT, choline transporter; ENT1/ENT2, equilibrative nucleoside transporter 1/2; MCT1, monocarboxylate transporter 1; MRP1/MRP2/MRP3, multidrug resistance-associated protein 1/2/3; OATP2B1, organic anion transporting polypeptide 2B1; OCT1/OCT3, organic cation transporter 1/3; OSTα/OSTβ, organic solute transporter; PEPT1, peptide transporter 1; P-gp, P-glycoprotein; PMAT, plasma monoamine transporter; SERT, serotonin transporter; SGLT, sodium/glucose cotransporter; THTR2, thiamine transporter 2. Created with BioRender.com.

Enzymes:

Enterocytesexpress drug-metabolizing enzymes that contribute to the first-pass effect. The fraction of the orally administered drug that escapes metabolism in the gut (F_g) is important for its overall bioavailability. The major drug-metabolizing enzymes expressed in the intestine include cytochrome P450s (CYPs), UDP-glucuronosyltransferases (UGTs), sulfotransferases, carboxylesterases, and alcohol dehydrogenases (Figure 3)(Basit et al., 2022; Murata et al., 2023).Drugs, environmental pollutants,or somedisease conditions can cause induction or inhibition of these enzymes and subsequentlyaffect systemic drug exposure.

Figure 3.

Major drug metabolizing enzymes expressed in human enterocytes.

(Basit et al., 2020, 2022; Murata et al., 2023)

ADH1C, alcohol dehydrogenase 1C; ALDH1A1, aldehyde dehydrogenase 1A1; CES1/CES2, carboxylesterase 1/2; CYP1A1/CYP2C9/CYP2C18/CYP3A4/CYP3A5, cytochrome P450 1A1/2C9/2C18/3A4/3A5; MAOA, monoamine oxidase A; SULT1A1/SULT1A3/SULT1B1/SULT1E1/SULT2A1,sulfotransferase 1A1/1A3/1B1/1E1/2B1; UGT1A1/UGT1A3/UGT1A10/UGT2B7/UGT2B17,UDP-glucuronosyltransferase 1A1/1A3/1A10/2B7/2B17. Created with BioRender.com.

Like conventional medicines, the oral route is also the mainmode of administrationforbotanicals, and their disposition is affectedby the same factors described above. When drugs and botanicals are consumed together, the highest concentrations of both may occur in the intestinal lumen and/or enterocytes, increasing the potential for direct or indirect interactions betweenthe drug and the botanical constituents.

3. Mechanisms ofintestinal PK botanical natural product-drug interactions

In 2005, Wu and Benet proposed the use of the biopharmaceutics drug disposition classification system (BDDCS) as a tool in predicting and understanding drug interactions(Wu & Benet, 2005). The BDDCSis a modified version of the biopharmaceutics classification system (Amidon et al., 1995),which the FDA applies as a guide for bioequivalence studies (Mehta et al., 2017). Within the BDDCS,drugs are grouped into four classes based on their aqueous solubility and extent of metabolism/rate of permeability (Figure 4).Although not every drug can be easily classified using the BDDCS system, it provides a general understanding of how the physicochemical properties of the drug guide its disposition in various organs. Revisions and additions to this classification system are ongoing, with recent examples including the change of repaglinide from BDDCS class 2 to class 1 and omeprazole from class 1 to class 2 (Bocci, Oprea & Benet, 2022). The BDDCScan provide insight into the mechanisms for botanical natural product-drug interactions, and this section discusses multiple major mechanisms mediatingtheseintestinal interactions.

Figure 4.

Predicted transporter and food effects on drugs based on BDDCS (Custodio, Wu & Benet, 2008; Bocci, Oprea & Benet, 2022).

3.1. Intestinal fluid volume and osmolality

Potential drug interactions occur when altered intestinal fluid volume and osmolalitychange a drug’s dissolution rate and/or alters absorption(Nader et al., 2016; de Waal et al., 2020). A recentin vivo study reporteda beverage drug interaction between apple juice and atenolol in rats(Funai et al., 2022). The study investigated the effect of a larger ingested fluid volume and a hyperosmotic liquid (apple juice, 749 mOsm/kg)on intestinal absorption of atenolol, which is a BDDCS class 3 drug with high solubility and low permeability. Orally administered apple juicemoderatelyreduced both area under the plasma concentration-time curve (AUC) and maximum plasma concentration (C_max) of atenolol by 8.8–25.6% and 12.9–41.5%, respectively. This decrease was observed with larger ingested volumes of 1 mL and 2 mL compared to 0.5 mL of apple juice(Funai et al., 2022). In addition, decreased atenolol exposure was observed after administration of a hyperosmotic solution that was adjusted to match the osmolality of the apple juice, indicating the effect was primarily mediated through an osmotic mechanism. Theseobservationscontradict the expected effect of increased intestinal volumes and osmolality on a drug’sdissolution and absorption, but the researchers speculated that for water-soluble drugs like atenolol,increased lumen volume may change the drug’s concentration gradient, thus reducing overall uptake. More mechanistic studies areneeded to verify this hypothesis. Clinical data also suggest a potential osmotic effect of fruit juices on atenolol and fexofenadine PK. Jeon et. al. reported that 600 and 1200 ml of apple juice decreased atenolol AUC by ~60% and ~80%, respectively, in healthy volunteers (Jeon et al., 2013), and Dresser et. al. reported 300 ml of apple juice decreased fexofenadine AUC by ~75% in healthy volunteers (Dresser et al., 2002). The authors of both studies indicated a potential role of osmotic changes in the PK interactions. For BDDCS class 2 drugs that are highly permeable and have poor solubility, it would be expected that larger fluid volume and osmolality could increase drug dissolution and absorption (Tanaka et al., 2017).Additional research is needed to determine whether apple juice or other fluids that can cause a high osmolaritywill affect the PK of poorly water-soluble drugs.

3.2. Drug binding

The binding or complexation of drug moleculesto botanical constituents is an emerging mechanism of PK interactions (Table 1). It results in the formation of an insoluble complex that cannot be absorbed. This interaction typically occurs with poorly water-soluble drugs, such as BDDCS class 2 or class 4 compounds, that may be more prone to hydrophobic chemical interactions and/or rely on GI fluids for improved solubility and absorption (Dahan and Miller 2012).Important experimental considerations in investigating this mechanism include the solutions used forin vitro experiments and the drug formulation. Ideally, physiologically relevant solutions should be used, such as simulated intestinal fluids that closely represent either fasted or fed states in the intestinal environment(Riethorst et al., 2016). Additionally, there may be differences in drug solubility in powder form versus in a pharmaceutical formulation with excipients.

Table 1:

Studies indicating drug binding as a PK botanical natural product-drug interaction mechanism.

Botanical natural product	Drug	BDDCS class	Experimental system	Observed effect	Mechanism	Reference
Green tea extract, EGCG, and EGC	Raloxifene and Nadolol	Raloxifene: Class 2 Nadolol: Class 3	In vitro: Fasted and fed state simulated intestinal fluids In vivo: Mice	In vitro: Decreased concentrations of raloxifene with green tea extract (30%) and EGCG(78%) in FaSSIF. Nadolol’s concentration was unaffected. In vivo: Decreased Cmax of raloxifene and raloxifene-6-glucuronide by 44% and 58%, respectively, with green tea extract.	Combined effect of direct binding of flavan-3-gallate catechins to raloxifene and binding of catechins to taurocholic acid	(Oyanna et al., 2024)
Green tea catechins (EGCG, EGC, epicatechin gallate, epicatechin)	Aripiprazole	Class 2	In vitro: Lactate buffer solution	Decreased amount of aripiprazole (68%) in the mixed solution with EGCG	Formation of an insoluble complex between aripiprazole and flavan-3-gallate catechins	(Ikeda et al., 2022)
Thylakoid-rich spinach extract	Ciprofloxacin	Class 4	In vitro: Ultra-pure water In vivo: Rats	In vitro: Decrease in ciprofloxacin’s concentration by 80% In vivo:Decreased AUC and Cmax of ciprofloxacin by 25% and 40%, respectively	Chelation with calcium in the extract	(Saito et al., 2019)
Byakkokaninjinto	Tetracycline	Class 3	Humans	Decreased AUC and Cmax of tetracycline by 30% and 28%, respectively	Chelation of tetracycline with cationic metals in Byakkokaninjinto	(Ohnishi et al., 2009)

The drug-binding discussed above differs from chelation, which is the formation of a complex between a drug and a metal cation (Flora & Pachauri, 2010), such as that observed with concurrent administration of metallic cation-containing preparations (antacids) and fluoroquinolones such as ciprofloxacin and ofloxacin (Kara et al., 1991; Stojković et al., 2014).A few natural product-drug interaction studies have reported chelation as aninteraction mechanism (Hitoshi et al., 2012; Saito et al., 2019). Unlike other botanical natural product-drug interactions precipitated by bioactive constituents like polyphenols, glycosides, andalkaloids,these chelation-based interactions are caused by metallic cations present in the natural product.Thus, these interactions may not occur with extracts or individual botanical constituents that do not contain the cations.

Byakkokaninjinto

Byakkokaninjinto is a combination herbal medicine preparation that originated in China and is now popular in Japan. The formulation of byakkokaninjinto includes a blend of herbs, but the percentages/amounts vary across manufacturers. The main ingredients include Gypsum fibrosum, Anemarrhena rhizome, Glycyrrhiza radix, Ginseng radix, and Oryzae radix. It is believed to have beneficial effects on inflammation and has been investigated in dermatitis and stomatitis (Shimizu, 2013; Sunagawa et al., 2018). In a study involving20 male participants, the effect of byakkokaninjinto on the PK of ciprofloxacin and tetracycline was investigated (Ohnishi et al., 2009). Tetracycline showed a more pronounced decrease in AUC and urine recovery (30% and 32.6%, respectively) when compared to ciprofloxacin (15% and 9.7%, respectively). The mechanism mediating the interaction was suggested as the formation of insoluble chelates between calcium in byakkokaninjinto and the drugs. A follow-up study in rats demonstrated that staggered administration ofbyakkokaninjinto two hours before tetracycline had no effect on tetracycline’s PK (Hitoshi et al., 2012), suggesting that the interaction is dependent on both being simultaneously present in the intestine.

Green tea

Green tea, made from Camellia sinensis leaves, is a widely consumed beverage, and its main bioactive constituents (catechins) have been extensively investigated in both PK and pharmacodynamic studies.Some of thesePK interaction studies have resulted ina decreased systemic exposure of the drug after co-administration with green tea and its catechins, such as sunitinib whose C_max and AUC_0-∞decreasedby 47.7% and 51.5%, respectively, in rats(Ge et al., 2011; Clarke et al., 2023). Further investigation revealed drugbinding or formation of complexes as the mechanisms of the drug interactions(Ikeda et al., 2012a; Ge et al., 2022). In particular, the flavan-3-gallate catechins [e.g., (−)-epigallocatechin gallate, EGCG], rather than the flavan-3-ol catechins [e.g., (−)-epigallocatechin, EGC],are reported to be responsible for these interactions (Ikeda et al., 1992, 2022; Ogawa et al., 2016; Oyanna et al., 2024). The mechanism reported for some of these drugs, such as aripiprazole and raloxifene (BDDCS class 2), and risperidone (BDDCS class 1)involves direct binding between the flavan-3-gallate catechins and the piperazine or piperidine ring of the drug(Ikeda et al., 2012b, 2022; Oyanna et al., 2024).However,this interaction may not occur for all piperazine derivatives.For example,an in vitro study investigated the interaction between the piperazine derivative, hydroxyzine dihydrochlorideand green tea polyphenols and found no complex was formed(Ohata et al., 2017). Theseauthors noted that the lack of interaction may be due to steric hindrancecaused by the side chains overhanging the nitrogen atoms on the piperazine ring of hydroxyzine dihydrochloride.

This decrease in compoundsolubility by green tea catechins has also been observed with endogenous hydrophobic compounds like cholesterol (logP = 8.72)(Fornasier et al., 2020). Like other poorly water-soluble compounds in intestinal fluids, bile micelles enhance the solubilization and absorption of cholesterol (Carey, Small & Bliss, 1983). Green tea has been shown to inhibit themicellar solubility of cholesterol in these micelles by binding to the bile acids and disrupting the stability of the micelle (Ogawa et al., 2016). The molecular mechanism for this interaction was reported to be hydrogen bonding between the carbonyl group and galloyl moiety of the flavan-3-gallate catechins and taurocholic acid (Sakakibara et al., 2019).

Spinach extract

Spinach (Spinacia oleracea L.) is a green leafy vegetable that is commonly consumed as food. Although its origin is debated to befrom Iran, China,orNepal,productionstatistics indicate China is the world’s largest producer (Roberts & Moreau, 2016).Like other botanicals, spinach containsmany constituents,such as amino acids, minerals, thylakoids, glycolipids, and polyphenols(Lomnitski et al., 2003; Chun et al., 2005). An extensive review has highlighted the potentialhealth benefits of spinach, particularly its antioxidant, anti-inflammatory, and anticancer properties(Roberts & Moreau, 2016). Thylakoids, which are membrane-bound chloroplast compartments where photosynthesis occurs, are enriched in spinach extracts(Mechela, Schwenkert &Soll, 2019).Theseextracts, available as dietary supplements, are advertised to aid in mitigatingobesity(Köhnke et al. 2009; Rebello et al. 2015).

Moreover, studies have indicated that thylakoid-rich spinach extracts may alter thePKof medicines. Saito et al.observed decreasedin vitroconcentrations of ciprofloxacin, levofloxacin, and tetracycline after incubation with thylakoid-rich spinachextracts.The mechanism responsible for the interaction was reported to be a chelation between the metal cations (calcium) in the extract and the antibiotics(Saito et al., 2019). Further investigations with the extract and several poorly water-soluble drugs in vivorevealed decreased systemic concentrationsof only ciprofloxacin’sAUC and C_max by 60% and 75%, respectively(Saito et al., 2019).Additional studies in animals and humans should be performed to confirm these results due to limitations in the study design that may have affected the data (i.e., prolonged fasting conditions,>10% of blood volume collected without fluid replacement).

Another effectobserved with spinach extracts is thylakoids’ binding of bile acids. Spinach extract prepared from plant membranes,were shown in vitro to bind different bile acids, especially hydrophobic bile acids, in a concentration-dependent manner.When the spinach plant membrane extract was administered orally in corn oil, rats experienced the binding effect via increased fecal bile acid excretion(Matsuda et al., 2018). The authors did not explore the specific mechanism for this interaction; however, they hypothesize thatit could be attributed to the light-harvesting complex II proteinsresponsible for capturing and transferring light energy during photosynthesisin thylakoids(Hunter, 1995; Albertsson et al., 2007; Matsuda et al., 2018).The authors acknowledge some loss of these proteins during extraction, emphasizing the importance of understanding the potential effects of whole plant versus extract on PK.

3.3. Gut Microbes

Gut microbiomes are complex colonies of microorganisms, including six main phyla: Firmicutes, Bacteriodetes, Actinobacteria, Proteobacteria, Fusobacteria, and Verrucomicrobia(Hou et al., 2022). These microbiomes play crucial roles in various functions, such as digesting and fermenting dietary fibers, synthesizing essential vitamins, and potentially enhancing immunity (Gomaa, 2020; Pant et al., 2023). The gut microbiota can directly influence the PK of conventional medicines by altering the metabolism of drugs such as amiodarone, digoxin, and nabumetone (Haiser et al., 2013; Yoo et al., 2016; Jourova et al., 2019). Additionally, studies have reported that gut microbiome-derived hydrolases can hydrolyze glycosidic bonds in natural products like ginsenosides and kale (Shin, Seo & Oh, 2014; Shimojo et al., 2018).

However, there is not much known about how gut microbiomes mediate drug interactions when natural products and conventional medicines are concurrently administered. For example, a recent clinical study reported that green tea decreased the geometric mean AUC_0–96h ratio of raloxifene between treatment and vehicle groups to 0.60 (Clarke et al., 2023). One of the hypotheses was green tea interrupted the gut microbiome’s deconjugation of raloxifene glucuronides, but a preliminary follow up study revealed no correlation between the gut microbiome and the AUC change (Clarke et al., 2023). The field is rapidly evolving, and ongoing research continues to shed light on emerging mechanisms of natural product-drug interactions, which could have significant implications for personalized medicine and dietary recommendations.

3.4. Transporters

Constituents of botanical natural products may be inhibitors or inducers of intestinal drug transporters (Figure 2), potentially affecting the intracellular concentrations of co-administered drugs. Transporter-mediated intestinal PKnatural product-drug interactions have been reviewed extensively(Wu et al., 2016; Stieger, Mahdi & Jager, 2017; Murtaza et al., 2017).This section focuses on recentstudies that were not addressed in the previous reviews.

Astragalusmembranaceus

Astragalus membranaceus (Huangqi)is a plant native to Asiathat has been consumed and used traditionally for various conditions such as gastritis, upper respiratory infections, and chronic kidney disease (Wang et al., 2023b).The isoflavones in the root (Astragali radix) have beenreported as the bioactive part of the herb and investigated as an antidiabetic, anti-inflammatory, andanti-tumor agent(Choi et al., 2007; Gong et al., 2018). The effect of Astragali radix on the PK of dapagliflozin, an antidiabetic drug, wastested in vivo in both healthy and type 2 diabetes mellitus-induced rats(Du et al., 2023). The Astragali radix extract (ARE) was administered orally daily for six days. On the seventh day, dapagliflozin was administered orally with a final dose of ARE. AREincreaseddapagliflozin C_maxby 31% and decreased its T_maxandmean residence time (MRT)by 56% and 25%, respectively,in healthy rats. The effects of ARE on dapagliflozin PK in diabetic rats were somewhat different. A similar decrease in T_max(62.5%) was observed, but, in contrast to the healthy rats, ARE decreased dapagliflozin C_max (31.9%). The authors proposedthe inhibition of the efflux transporter, p-glycoprotein (P-gp), as the potential mechanism for the ARE interaction, which is consistent with an earlier T_maxobserved in the ARE-treated healthy groups. Dapagliflozin has been reported as a P-gp substrate (Obermeier et al., 2010). The authors also suggest that the altered PK effects caused by ARE might benefit the antidiabeticeffect of dapagliflozin because AUC_0-inf was unchanged, and dapagliflozin’s reduced C_max could offset the elevated concentrationscaused by renal impairment in diabetes.However, follow-up clinical studies are required to test this potential PK and pharmacodynamic interaction.

Black tea

Black tea is made from the leaves of the Camellia sinensis plant. It differs from green tea because the fresh leaves undergo fermentation, affecting the smell, taste, and composition(Muthumani & Kumar, 2007). Black tea extract contains several constituents similar tothose in other types of tea(Aaqil et al., 2023).The main polyphenolic constituents are theaflavins, and these have been investigated for their beneficial effects on hyperlipidemia, inflammation, and hyperglycemia (Chen et al., 2005; He, 2017; Wang et al., 2023a). The effects of black tea extract and theaflavin derivatives on the PK of the lipid-lowering drug rosuvastatin were recently investigated (Kondo et al., 2019). Rosuvastatin is a BDDCS class 3drug that is a substrate of several transporters, including organic anion transporting polypeptide (OATP) 2B1 transporter, which is expressed in the intestine and liver(Kitamura et al., 2008).The black tea extract and theaflavin derivatives inhibited the uptake of rosuvastatin in OATP2B1 transfected human embryonic kidney (HEK) 293 cells (Kondo et al., 2019). Administration of black tea extract with rosuvastatin in rats significantly decreased rosuvastatin’s C_maxand AUC_0–8hby 48% and 37%,respectively, suggesting that black tea reduces rosuvastatin plasma concentrations by inhibiting intestinal OATP2B1-mediated transportin vivo(Kondo et al., 2019). A major limitation of this study is that blood samples were collected up to 8 hours after dosing of rosuvastatin, whichonly captured approximately one half-life for rosuvastatin in rats(Sun, Li & Zhou, 2022). Hence, although these data suggest an interaction between black tea and rosuvastatin, more thorough data are needed to understand the mechanism and risk.

Goldenseal

Goldenseal (Hydrastis canadensis L.) is a perennial herb native toeastern North America. It has historical significance in Native American medicine for its use in skin conditions and digestive disorders(“Goldenseal | NCCIH”). More recently, ithas also beenresearched for its possible beneficial effects on hyperlipidemia, inflammation, and hyperglycemia(Chen et al., 2013; Pang et al., 2015).Some of the major constituents in goldenseal that have been investigatedfor their beneficial effects include alkaloids such as berberine, (−)-β-hydrastine, and canadine (Mandal et al., 2020). Goldenseal has been implicated as a precipitant of drug interactions due to its ability to modulate the activity and expression of transportersand drug-metabolizing enzymes(Gurley et al., 2008; Nguyen et al., 2021). The study by Nguyen et al. provides a comprehensive examination of the effect of goldenseal on the PKof probe transporter substrates furosemide(organic anion transporter [OAT] 1 and OAT3), metformin (organic cation transporter [OCT] 1, OCT2,multidrug and toxinextrusion[MATE] 1, and MATE2), and rosuvastatin (OATP1B1, OATP1B3, and breast cancer resistance protein [BCRP]) (Nguyen et al., 2021). Goldenseal extract (standardized to berberine)inhibited the efflux transporter BCRP and the uptake transporters OATP1B1 and OATP1B3in vitro. Clinical evaluationin healthy participants found that goldenseal had no effect on the PK of rosuvastatin and furosemide and unexpectedly decreased metformin AUC_∞ by 23%. This suggests that goldenseal extract alters intestinal permeability, transport, and/or other processes involved in metformin absorption. Our subsequent study aimed to elucidate the mechanism underlying the goldenseal-metformin interaction, hypothesizing that goldenseal inhibits intestinal uptake transporters involved in metformin absorption(Oyanna et al., 2023). Metformin is a BDDCS class 3 drugwidely prescribed for type 2 diabetes. Using in vitro assays, the research confirmed that goldenseal extract, and to a lesser extent its alkaloid (berberine), inhibited OCT3, plasma membrane monoamine transporter (PMAT), and thiamine transporter 2(THTR2)activity. These transporters all contribute to metformin uptake from the intestinal lumen into enterocytes. PK studies in mice demonstrated goldenseal extract significantly decreased the C_maxof orally administered metforminby 31%, whereas an equimolar concentration of the major goldenseal constituents, including berberine, had no effect(Oyanna et al., 2023).These data suggest that other constituents in goldenseal contribute to the observed PK interaction.

Jabara juice

Jabara (Citrus jabara) is a rare variety of citrus fruit indigenous to Japan (Omori, Nakahara & Umano, 2011; Shimizu et al., 2016). Jabara fruit contains many compounds present in citrus fruits such as 5-hydroxymethylfurfural and narirutin (Rouseff, Youtsey & Martin, 1987; Uchida et al., 2020). Extracts and individual constituents of the plant have been investigated for their therapeutic potential in asthma, allergic reactions, and airway inflammation (Funaguchi et al., 2007; Iwashita et al., 2017; Uchida et al., 2020). A recent study investigated the effect of Jabara juice on the intestinal absorption of fexofenadine, a BDDCS class 3 drug and histamine receptor antagonist (Han et al., 2023). Jabara juice reduced fexofenadine basolateral-to-apical flux, but not apical-to-basolateral flux, in Caco-2 cells, suggesting inhibition of P-gp. Narirutin did not affect fexofenadine permeability, indicating that it was not the constituent driving the change in permeability. Oral co-administration of Jabara juice with fexofenadine in mice increased fexofenadine AUC by 1.8-fold and C_max by 2.4-fold, further supporting the potential role of P-gp inhibition in the PK interaction.

3.5. Enzymes

Drug metabolizing enzymes are central to the metabolism of botanical natural products and drugs. Enteric enzymes are integral to the first-pass effect and oral bioavailability(Figure 3). The impact of botanical natural products or herbal supplements on enzyme expression and function has been extensively reviewed (Mukherjee et al., 2011; Zuo et al., 2022). Differentiating between intestinal and hepatic enzyme-mediated interactions involves understanding the specific enzymes involved, the route of administration, and the effect on PK parameters. This section focuses on recent botanical natural product-drug interaction studiesinvolving drug-metabolizing enzymes.

Ginseng

Ginseng is a popular herb originating from East Asia. It contains diverse active components, including ginsenosides, polysaccharides, peptides, and fatty acids(Attele, Wu & Yuan, 1999). These compounds have been investigated for potential anti-inflammatory and anticancer effects and to treat cardiovascular diseases (Yu et al., 2023; Wang et al., 2024; Zhou et al., 2024). Due to the popularity of ginseng and the likelihood of co-administration with conventional medicines, its potential to cause a PK natural product-drug interactionhas been investigated. A recent study showed that red ginseng extract increased the systemic exposure of warfarin in rats(Jeon et al., 2024). This study is potentially important because warfarin is a widely used anticoagulant with a narrow therapeutic index, and such drug interactions can influence its safety and efficacy. Bothsingle and repeated red ginseng extract administrationhad similar effects on oral warfarin PK. Single administration increased C_max and AUC_0-∞ of warfarin by 29% and 95.1%, whereas repeated administration increased C_max and AUC_0-∞of warfarin by 49.3% and 132%(Jeon et al., 2024). These increases in warfarin exposure corresponded to decreased formation of 7-hydroxywarfarin, which is the main metabolite of the more potent S-warfarin enantiomer (Jones et al., 2010). However, Cyp2c11 expression decreased in the intestine only after repeated red ginseng extract, indicating that changes in Cyp2c11 expression alone cannot explain the changes in warfarin PK. Further investigation found that ginsenoside Rb1, a major component of red ginseng extract, alsoinhibited the metabolic activity of Cyp2c11, suggesting that the interactioninvolved bothinhibition of metabolism in the intestineand decreased expression of Cyp2c11 for the group who had repeated red ginseng administration(Jeon et al., 2024).These results in rats need to be tested in human subjects before clinical recommendations are warranted.

Polygonum capitatum

Polygonum capitanum is a perennial plant, commonly known as pinkhead smartweed,that is native to East Asia (Huang et al., 2015). Extracts from the plant have been investigated as potential antibacterial and anti-inflammatory agents(Liao et al., 2011; Zhang et al., 2015). Chen et al. investigated its effect on the PK of the antibacterial agent levofloxacin, a BDDCS class 3 drug, in rats (Chen et al., 2021). P. capitatum decreased the C_max and AUC_0-tof levofloxacin by 84.2% and 93%, respectively, after oral co-administration but did not have any effect after intravenous injection of levofloxacin. These observations suggest that the PK interaction occurred in the intestine. The authors speculatedthe induction of drug-metabolizing enzymes or transporters as the mechanism (Chen et al., 2021). Indeed, previous data indicateP. capitanum induced CYP2C9 and CYP3A4 orthologsafter repeated administration over several days (Zheng et al., 2014). A challenge to this interpretation is that Chen et al. only administered a single dose of P. capitatum, which may not be sufficient to induce enzyme expression and cause a PK interaction.

Quercetin

Quercetin is a flavonoid present in many fruits, vegetables, and grains (Kellogg et al., 2017; Ayvazyan et al., 2023). It has been studied for its potential anti-microbial, anti-inflammatory, and anticancer effects(Da Silva et al., 2012; Ren et al., 2024). Quercetin’s effect on the PK of amiodarone, an antiarrhythmic drug, was investigated in rats (Ahmad et al., 2023). Quercetin pretreatment followed by amiodarone or co-administration of quercetin with amiodarone increased amiodarone’s systemic exposure and decreased its T_max and t_1/2by 35.7%and 16.8%, respectively. The quercetin effect was more pronounced in the pretreated group compared to the co-administered group(Ahmad et al., 2023). The authors suggest the mechanism underlying this interaction was inhibition of intestinal CYP enzymes(Cyp3a1, Cyp3a2)(Shayeganpour, El-Kadi & Brocks, 2006)and potentially the efflux transporters P-gp and multidrug resistance-associated protein 2 (MRP2). Unfortunately, the authors did not measure the levels of amiodarone metabolites, such as N-desethylamiodarone, so the contribution ofmetabolism and transporter to this interaction is not known.

Another study reported that quercetin increased the C_max and AUC of orally administered ticagrelor in ratsby 1.5 and 1.88-fold, respectively (Zhang et al., 2021). The authors took an uncommon approach, untargeted metabolomics, to investigate the mechanism for the increased systemic exposure of ticagrelor by quercetin. They reported quercetin changed metabolic pathways associated with ATP generation and suggestedthat these changes could inhibit CYP3a and/or P-gp efflux activity. These claims are speculative and require additional experimentation to be substantiated.

4. Clinical Implications and Recommendations

The data reviewed herein exemplify important considerations and challenges associated with investigating intestinal natural product-drug PK interactions. These include the complexity of the GI environment and how it interacts with the physiochemical properties of natural products and drugs, as well as the potential for transporters and metabolizing enzymes to be involved in the drug PK. As this field progresses, greater attention and care are required in several specific areas to improve the robustness and translatability of the data.

1. Drug binding testsshould be considered during drug development for widely consumed botanical natural products that bind specific drug classes (e.g., green tea catechins). These experiments must be completed in physiologically relevant solutions, such as FaSSIF and FeSSIF,to appraise effects on drug solubility and oral drug bioavailability.

2. In vitro experiments need to be further validated with in vivo experiments. There are notable examples of in vitro predictions not accurately reflecting in vivo effects. For instance, interactions between green tea and ticagrelor, as well as raloxifene, exemplify cases where observations in vitro differed from those in vivo(Wang et al., 2020; Clarke et al., 2023). In both studies, in vitro data showed that inhibition of intestinal UGT and CYP3Aenzymes could possiblylead to increased exposure of the parent drugs, raloxifene and ticagrelor, respectively, but the in vivo data demonstrated decreased exposure of both parent drugs and metabolitesin humans and rats, respectively. Thus, continued development of in vitro models will improve the predictability of clinical risks.Additionally, physiologically based pharmacokinetic (PBPK) modeling could be used to predict natural product-drug interactions within the context of various physiological scenarios in humans(Cox et al., 2021).

3. It is essential that future investigations involvingnatural product extracts, dietary supplements, or beverages explicitly specify the composition of the product and/or the specific constituents used. For example, the specific types of catechins should be described rather than using general terms such as “polyphenols” or “tea catechins”. Using generic terms may overlook the variations in bioactivity associated with specific compounds. Additionally, it is imperative to identify whether a single compound or a combination was utilized to provide a comprehensive understanding of the experimental design. Ultimately, this will promote robust and reliable scientific inquiry.

5. Conclusion

This review has provided an overview of the major mechanisms underlying PK interactions between botanical natural products and drugs in the intestine. These interactions depend not only on the drugs’ physicochemical properties but also on the specific constituents of the botanicals, their concentration in the GI tract, and the dynamic conditions of the intestinal environment. With a focus beyond traditional enzyme and transporter-mediated pathways, thisreviewcovers the impact of drug solubility, osmolality, and drug binding on the PK of co-administered drugs. The highlighted examples illustrate the diverse mechanisms through which botanical natural products can influence drug PK in the intestine. In many cases, further research is still needed to clarify the full extent of botanical natural product-drug interactions and their potential implications for patient care and drug therapy optimization. By advancing our understanding of intestinal PK interactions between botanical natural products and drugs, healthcare professionals can better navigate the complexities of multi-therapy treatments.

Table 2:

List of transporter-mediated mechanisms of botanical natural product-drug interactions

Botanical natural product	Drug	BDDCS class	Experimental system	PK effect	Mechanism	Reference
Black ginger and 5,7-dimethyoxyflavone	3H-Digoxin and 3H-estrone	Digoxin: Class 3	MDCK and Caco-2 cells	Decreased efflux ratioa of 3H-digoxin and 3H-estrone	Inhibition of BCRP and P-gp activity	(Boonnop et al., 2023)
Goldenseal extract	In vitro: 1-methyl-4-phenylpyridinium iodide (MPP+) and thiamine In vivo: Metformin	Metformin: Class 3	In vitro: HEK 293 cells In vivo: Mice	In vitro: Decreased uptake of MPP+ and thiamine In vivo: Decreased Cmax by 31% and unchanged t1/2 of metformin in mice	Inhibition of OCT3, PMAT, and THTR2 activity	(Oyanna et al., 2023)
Garlic and ginkgo biloba extracts	Sofosbuvir	Class 1	Rats	Garlic: decreased Cmax by 18.8% and increased clearance of sofosbuvir by 63%. Ginkgo: increased Cmax of sofosbuvir by 17.6%	Garlic: intestinal induction of P-gp. Ginkgo: inhibition of P-gp activity	(Wasef et al., 2022)
Schisantherin A	Lenvatinib	Unclassified	Rats	Increased Cmax (54.8%), AUC0-∞ (54.3%), and unchanged t1/2 of lenvatinib	Decreased intestinal mRNA and protein expression of P-gp	(Cui et al., 2022)
Hibiscus sabdariffa L.	Captopril	Class 3	Rats	Decreased AUC0-∞ and Cmax of captopril by 83.2% and 78.8%, respectively	Unknown. Potentially reduced expression of peptide transporter 1 by quercetin in the extract or formation of a mixed disulfide captopril complex	(Nurfaradilla, Saputri & Harahap, 2020)
Tanjin (Salvia miltiorrhiza)	Rosuvastatin	Class 3	Rats and healthy human participants	Decreased Cmax and AUC0–12h of rosuvastatin in rats and humans by 26.9% and 19.4%, respectively	Increased intestinal mRNA expression of BCRP in rats	(Yang et al., 2019)

^aEfflux ratio: ratio of secretory permeability to absorptive permeability

Table 3:

List of enzyme-mediated mechanisms of botanical natural product-drug interactions

Botanical natural product	Drug	BDDCS class	Experimental system	PK effect	Mechanism	Reference
Red ginseng extract	Warfarin	Class 2	In vitro: Rat intestinal and liver microsomes In vivo: Rats, orally	In vitro: Potent inhibition of 7-hydroxywarfarin formation in intestinal microsomes In vivo: Increased Cmax and AUC of warfarin with single by 29% and 95.1%, respectively. While repeated administration increased Cmax and AUC of warfarin by 49.3% and 132%, respectively	Inhibition of intestinal Cyp2c11	(Jeon et al., 2024)
Sinapic acid	Dasatinib	Class 2	Rats	Increased Cmax and AUC of dasatinib by 62% and 28.9%, respectively	Decreased Cyp3a2, Bcrp, and P-gp expression in intestine and liver	(Shahid et al., 2023)
Chinese herb Styrax	Midazolam and felodipine	Class 1	Rats, orally	Increased AUC0-∞, Cmax, and t1/2 of midazolam by 2.04, 2.28, and 2.11-fold, respectively. Increased felodipine AUC0-∞ and Cmax by 1.41 and 2.16-fold, respectively	Inhibition of Cyp3a	(Zhang et al., 2022)
Tea polyphenols	Ticagrelor	Unclassified	In vitro: Human and liver microsomes In vivo: Rats	In vitro: Decreased metabolism of ticagrelor In vivo: Decreased AUC0-∞ and Cmax of oral ticagrelor by 53% and 64%, respectively. No change in ticagrelor’s t1/2 while oral clearance increased by 2.47-fold	In vitro: Inhibition of Cyp3a In vivo: Transporter effects	(Wang et al., 2020)

Abbreviations:

ARE

astragali radix extract

AUC

area under the plasma concentration versus time curve

BCRP

breast cancer resistance protein

BDDCS

biopharmaceutics drug disposition classification system

C_max

maximum plasma concentration

CYP

cytochrome P450

EGC

(−)-epigallocatechin

EGCG

(−)-epigallocatechin gallate

FaSSIF

fasted state simulated intestinal fluid

FeSSIF

fed state simulated intestinal fluid

GI

gastrointestinal

OATP

organic anion transporting polypeptide

OCT

organic cation transporter

P-gp

P-glycoprotein

PK

pharmacokinetics

PMAT

plasma monoamine transporter

THTR2

thiamine transporter 2

T_max

time to C_max

UGTs

UDP-glucuronosyltransferases