P-glycoprotein interactions of nefazodone and trazodone in cell culture

Influence of Genetic Polymorphisms on the Pharmacokinetics of Trazodone Hydrochloride: A Scoping Review and Future Perspective

BACKGROUND

Trazodone hydrochloride is an antidepressant used in clinical practice. As a substrate of cytochrome P450 enzymes that is vulnerable to P-glycoprotein transport, several factors can alter its plasma concentration, and hence, dose adjustment may be required. The aim of this scoping review was to identify genetic polymorphisms that influence the pharmacokinetics of trazodone hydrochloride.

METHODS

A literature search was performed using PubMed, PubMed Central, BVS/BIREME, EBSCOhost, Web of Science, Embase, Cochrane Library, and Medline databases for studies published until August 2021. The search strategy was based on the following keywords: Trazodone OR "m-chlorophenyl piperazine" AND "Pharmacogenetics" OR "Genetics" OR "Cytochrome P-450 Enzyme System" OR "Polymorphism, Single Nucleotide" OR "Polymorphism, Genetic."

RESULTS

The search retrieved 684 candidate articles; 307 duplicates were eliminated. In total, 377 articles were eligible for the first screen. However, only 4 met the eligibility criteria, and 12 polymorphisms in 5 different genes (CYP2D6, CYP1A2, CYP3A4, CYP3A5, and ABCB1). Notably, only C3435T ABCB1 influenced the pharmacokinetics of trazodone hydrochloride. Individuals with the T/T genotype had lower area under the curve, half-life, and maximum concentration values with a higher clearance rate.

CONCLUSIONS

Polymorphisms in CYP450 do not seem to directly influence the pharmacokinetics of trazodone hydrochloride or its metabolites. By contrast, genetic polymorphisms in ABCB1 seem to have an important effect on the pharmacokinetics of trazodone hydrochloride by enhancing drug metabolism and elimination.

Major Bleeding Risk in Patients With Non-valvular Atrial Fibrillation Concurrently Taking Direct Oral Anticoagulants and Antidepressants

Direct oral anticoagulants (DOACs) are commonly prescribed with antidepressants that may increase bleeding risk. Here we assessed the association between DOACs with and without concurrent antidepressants and major bleeding risk in patients with atrial fibrillation (AF) by a retrospective cohort study included patients with AF who received prescriptions of DOACs in Taiwan’s National Health Insurance database between 2012 and 2017. Adjusted rate ratio (ARR) of major bleeding was calculated by comparing incidence rate adjusted with Poisson regression and inverse probability of treatment weighting using the propensity score between patient-times with and without antidepressants. Among 98863 patients with AF, concurrent use of bupropion with DOACs increased the risks of all major bleeding (ARR: 1.49, 95% CI: 1.02–2.16) and gastrointestinal hemorrhage (ARR: 1.57, 95% CI: 1.04–2.33). An increased risk of intracerebral hemorrhage (ICH) was associated with the combinations of DOACs with selective serotonin reuptake inhibitors (SSRIs, ARR: 1.38, 95% CI: 1.08–1.76), particularly in paroxetine (ARR: 2.11, 95% CI: 1.17–3.81), and tetracyclic antidepressants (TeCAs, ARR: 1.34, 95% CI: 1.01–1.78). In subgroup analyses stratified by individual NOACs, SSRIs increased the risk of ICH in the dabigatran-treated patients (ARR: 1.55, 95% CI: 1.04–2.33). The combinations of apixaban and serotonin-norepinephrine reuptake inhibitors (SNRIs) were associated with a higher risk of all major bleeding (ARR: 1.63, 95% CI: 1.04–2.55). These results clearly indicate the drug–drug interactions between DOACs and antidepressants, which should be carefully considered when prescribing DOACs in adult patients. Careful monitoring for bleeding should be performed while concurrently prescribing DOACs with bupropion, SSRI, SNRI, and TeCA. Concomitant use of DOACs and TCAs may be a relatively safe strategy for patients with AF.

Acute regulation of apical ABC transporters in the gut. Potential influence on drug bioavailability

The extensive intestinal surface offers an advantage regarding nutrient, ion and water absorptive capacity but also brings along a high exposition to xenobiotics, including drugs of therapeutic use and food contaminants. After absorption of these compounds by the enterocytes, apical ABC transporters play a key role in secreting them back to the intestinal lumen, hence acting as a transcellular barrier. Rapid and reversible modulation of their activity is a subject of increasing interest for pharmacologists. On the one hand, a decrease in transporter activity may result in increased absorption of therapeutic agents given orally. On the other hand, an increase in transporter activity would decrease their absorption and therapeutic efficacy. Although of less relevance, apical ABC transporters also contribute to disposition of drugs systemically administered. This review article summarizes the present knowledge on the mechanisms aimed to rapidly regulate the activity of the main apical ABC transporters of the gut: multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 2 (MRP2) and breast cancer resistance protein (BCRP). Regulation of these mechanisms by drugs, drug delivery systems, drug excipients and nutritional components are particularly considered. This information could provide the basis for controlled regulation of bioavailability of therapeutic agents and at the same time would help to prevent potential drug-drug interactions.

Therapeutic drugs modulate ATP-Binding cassette transporter-mediated transport of amyloid beta(1-42) in brain microvascular endothelial cells

Deposition of amyloid-β peptide (Aβ_(1-42)) is a hallmark of Alzheimer's disease. Clearance of Aβ_(1-42), across the blood-brain barrier (BBB), is mediated by ATP-binding Cassette (ABC) efflux transporters. Many therapeutic drugs inhibit ABC transporters, but little is known of the effect of therapeutic drugs on Aβ_(1-42) transport across BBB endothelial cells. The effects of selected, widely prescribed, therapeutic drugs on ABCB1, ABCC5 and ABCG2 activities were determined by measuring intracellular levels of calcein, GS-MF, and Hoechst 33342 respectively in primary porcine brain endothelial cells (PBECs). The ability of ABCB1, ABCC5 and ABCG2 to transport Aβ_(1-42) was determined using fluorescent Aβ_(1-42). The ability of the ABCB1, ABCC5 and ABCG2 inhibitor telmisartan to modify transcellular Aβ_(1-42) transport was investigated using PBEC monolayers housed in Transwell® inserts. Treatment of PBECs with ABC transporter inhibitory drugs (indomethacin, olanzapine, chlorpromazine, telmisartan, pantoprazole, quinidine, sulfasalazine and nefazodone) increased Aβ_(1-42) intracellular accumulation. Inhibition of ABCB1, ABCC5 and ABCG2 by telmisartan increased Aβ_(1-42) transport in the apical to basal direction and reduced its transport in basal to apical direction in PBEC monolayers. ABCB1, ABCC5 and ABCG2 mediate the efflux transport of Aβ_(1-42) in BBB endothelial cells. Inhibition of ABC transporters by therapeutic drugs, at plasma concentrations, could decrease Aβ_(1-42) clearance from brain, across BBB endothelial cells into blood, and potentially influence levels of the Aβ_(1-42) peptide within the brain.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Effect of ABCB1 C3435T Polymorphism on Pharmacokinetics of Antipsychotics and Antidepressants

P-glycoprotein, encoded by ABCB1, is an ATP-dependent drug efflux pump which exports substances outside the cell. Some studies described connections between C3435T polymorphism T allele and lower P-glycoprotein expression; therefore, homozygous T/T could show higher plasma levels. Our aim was to evaluate the effect of C3435T on pharmacokinetics of 4 antipsychotics (olanzapine, quetiapine, risperidone and aripiprazole) and 4 antidepressants (trazodone, sertraline, agomelatine and citalopram). The study included 473 healthy volunteers receiving a single oral dose of one of these drugs, genotyped by real-time PCR. Multivariate analysis was performed to adjust the effect of sex and genotype of the main cytochrome P450 enzymes. C3435T polymorphism had an effect on olanzapine pharmacokinetics, as T/T individuals showed lower clearance and volume of distribution. T/T individuals showed lower T_1/2 of 9-OH-risperidone, but this difference disappeared after multivariate correction. T/T homozygous individuals showed lower dehydro-aripiprazole and trazodone area under the concentration-time curve, along with lower half-life and higher clearance of trazodone. C/T genotype was associated to higher citalopram maximum concentration. C3435T had no effect on quetiapine, sertraline or agomelatine pharmacokinetics. C3435T can affect the elimination of some drugs in different ways. Regarding risperidone, trazodone and dehydro-aripiprazole, we observed enhanced elimination while it was reduced in olanzapine and citalopram. However, in quetiapine, aripiprazole, sertraline and agomelatine, no changes were detected. These results suggest that P-glycoprotein polymorphisms could affect CNS drugs disposition, but the genetic factor that alters its activity is still unknown. This fact leads to consider the analysis of ABCB1 haplotypes instead of individual variants.

Impaired Rivaroxaban Clearance in Mild Renal Insufficiency With Verapamil Coadministration: Potential Implications for Bleeding Risk and Dose Selection

Pharmacokinetics and antithrombotic effects of the Factor Xa inhibitor rivaroxaban were studied in subjects with mild renal insufficiency concurrently taking the P-glycoprotein and moderate CYP3A inhibitor verapamil, a drug commonly administered to patients with hypertension, ischemic heart disease, or atrial fibrillation. Age-matched controls with normal renal function were studied concurrently. Subjects' overall mean age was 59 years. Mean creatinine clearance values in the 2 groups were 105 and 71 mL/min. After single 20-mg oral doses, rivaroxaban area under the curve (AUC) was increased by a factor of 1.11 (ratio of geometric means [RGM]) in mild renal insufficiency compared to controls. Verapamil coadministration independently increased AUC to the same extent in both the mild renal insufficiency and control groups (RGM, 1.39 and 1.43). Concurrent mild renal insufficiency and verapamil produced additive inhibition compared to controls without verapamil (RGM, 1.58). Prothrombin time (PT) prolongation and Factor Xa inhibition tracked plasma rivaroxaban, and were enhanced by verapamil. Concentration-response relationships for PT (linear) and Factor Xa inhibition (hyperbolic) were unaffected by renal function or verapamil. The absolute and relative increases in rivaroxaban AUC caused by verapamil in mild renal insufficiency subjects are potentially associated with an increased bleeding risk. Modification of recommended dosage may be required in this combination of circumstances to reduce risk to patients.

Pharmacogenetics of trazodone in healthy volunteers: association with pharmacokinetics, pharmacodynamics and safety

Aim: The aim was to evaluate the effect of polymorphisms in metabolizing enzymes and transporters on the pharmacokinetics, pharmacodynamics and adverse effects of trazodone in healthy volunteers. Materials & methods: 36 healthy volunteers receiving a single 100-mg oral dose of trazodone were genotyped for 11 variants in CYP3A4, CYP3A5, CYP2D6 and ABCB1 by real-time PCR. Plasma concentrations were measured using liquid chromatography-tandem mass spectrometry method. Results & conclusion: Sex affected the pharmacokinetics of trazodone with higher clearance in women. Polymorphisms in ABCB1, but not in CYP3A or CYP2D6, influenced trazodone pharmacokinetics. Trazodone decreased blood pressure and prolonged the corrected QT interval interval. CYP2D6 and ABCB1 polymorphisms were associated with the incidence of dizziness and prolonged corrected QT interval, respectively. Subjects with adverse drug reactions had lower concentrations of trazodone suggesting its metabolite (m-chlorophenylpiperazine) could be responsible for these effects.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Three- and four-class classification models for P-glycoprotein inhibitors using counter-propagation neural networks

P-glycoprotein (P-gp) is an ATP binding cassette (ABC) transporter that helps to protect several certain human organs from xenobiotic exposure. This efflux pump is also responsible for multi-drug resistance (MDR), an issue of the chemotherapy approach in the fight against cancer. Therefore, the discovery of P-gp inhibitors is considered one of the most popular strategies to reverse MDR in tumour cells and to improve therapeutic efficacy of commonly used cytotoxic drugs. Until now, several generations of P-gp inhibitors have been developed but they have largely failed in preclinical and clinical studies due to lack of selectivity, poor solubility and severe pharmacokinetic interactions. In this study, three models (SION, SIO, SIN) to classify specific 'true' P-gp inhibitors as well as three other models (CPBN, CPB1, CPN) to distinguish between P-gp inhibitors, CYP 3A inhibitors and co-inhibitors of these proteins with rather high accuracy values for the test set and the external set were generated based on counter-propagation neural networks (CPG-NN). Such three and four-class classification models helped provide more information about the bioactivities of compounds not only on one target (P-gp), but also on a combination of multiple targets (P-gp, CYP 3A).

Sucralose, a synthetic organochlorine sweetener: overview of biological issues

Sucralose is a synthetic organochlorine sweetener (OC) that is a common ingredient in the world's food supply. Sucralose interacts with chemosensors in the alimentary tract that play a role in sweet taste sensation and hormone secretion. In rats, sucralose ingestion was shown to increase the expression of the efflux transporter P-glycoprotein (P-gp) and two cytochrome P-450 (CYP) isozymes in the intestine. P-gp and CYP are key components of the presystemic detoxification system involved in first-pass drug metabolism. The effect of sucralose on first-pass drug metabolism in humans, however, has not yet been determined. In rats, sucralose alters the microbial composition in the gastrointestinal tract (GIT), with relatively greater reduction in beneficial bacteria. Although early studies asserted that sucralose passes through the GIT unchanged, subsequent analysis suggested that some of the ingested sweetener is metabolized in the GIT, as indicated by multiple peaks found in thin-layer radiochromatographic profiles of methanolic fecal extracts after oral sucralose administration. The identity and safety profile of these putative sucralose metabolites are not known at this time. Sucralose and one of its hydrolysis products were found to be mutagenic at elevated concentrations in several testing methods. Cooking with sucralose at high temperatures was reported to generate chloropropanols, a potentially toxic class of compounds. Both human and rodent studies demonstrated that sucralose may alter glucose, insulin, and glucagon-like peptide 1 (GLP-1) levels. Taken together, these findings indicate that sucralose is not a biologically inert compound.

ABCB1 gene variants influence tolerance to selective serotonin reuptake inhibitors in a large sample of Dutch cases with major depressive disorder

P-glycoprotein (P-gp), an ATP-driven efflux pump in the blood-brain barrier, has a major impact on the delivery of antidepressant drugs in the brain. Genetic variants in the gene ABCB1 encoding for P-gp have inconsistently been associated with adverse effects. In order to resolve these inconsistencies, we conducted a study in a large cohort of patients with major depressive disorder with the aim to unravel the association of ABCB1 variants with adverse effects of antidepressants and in particular with selective serotonin reuptake inhibitors (SSRIs), which display affinity as substrate for P-gp. The Netherlands Study of Depression and Anxiety (NESDA) study was used as a clinical sample. For 424 patients data were available on drug use, side effects. We selected six ABCB1 gene variants (1236T>C, 2677G>T/A, 3435T>C, rs2032583, rs2235040 and rs2235015) and analyzed them for association with adverse drug effects using multinomial regression analysis for both single variants and haplotypes. We found a significant association between the number of SSRI-related adverse drug effects and rs2032583 (P=0.001), rs2235040 (P=0.002) and a haplotype (P=0.002). Moreover, serotonergic effects (sleeplessness, gastrointestinal complaints and sexual effects) were significantly predicted by these variants and haplotype (P=0.002/0.003). We conclude that adverse drug effects with SSRI treatment, in particular serotonergic effects, are predicted by two common polymorphisms of the ABCB1 gene.

Interactions between antidepressants and P-glycoprotein at the blood-brain barrier: clinical significance of in vitro and in vivo findings

The drug efflux pump P-glycoprotein (P-gp) plays an important role in the function of the blood-brain barrier by selectively extruding certain endogenous and exogenous molecules, thus limiting the ability of its substrates to reach the brain. Emerging evidence suggests that P-gp may restrict the uptake of several antidepressants into the brain, thus contributing to the poor success rate of current antidepressant therapies. Despite some inconsistency in the literature, clinical investigations of potential associations between functional single nucleotide polymorphisms in ABCB1, the gene which encodes P-gp, and antidepressant response have highlighted a potential link between P-gp function and treatment-resistant depression (TRD). Therefore, co-administration of P-gp inhibitors with antidepressants to patients who are refractory to antidepressant therapy may represent a novel therapeutic approach in the management of TRD. Furthermore, certain antidepressants inhibit P-gp in vitro, and it has been hypothesized that inhibition of P-gp by such antidepressant drugs may play a role in their therapeutic action. The present review summarizes the available in vitro, in vivo and clinical data pertaining to interactions between antidepressant drugs and P-gp, and discusses the potential relevance of these interactions in the treatment of depression.

The role of multi-drug resistance p-glycoprotein in glucocorticoid function: studies in animals and relevance in humans

Entry of glucocorticoid hormones into cells is tightly regulated by membrane transporters. One of these transporters, the multi-drug resistance p-glycoprotein, has been extensively described to confer treatment resistance to tumour cells as well as to regulate the intracellular levels of glucocorticoid hormones. Moreover, multi-drug resistance p-glycoprotein is also present on the endothelial cells of the blood-brain-barrier, and in neurones, where it limits the access of glucocorticoids to the brain. Finally, this transporter also has the ability to limit the entry of some antidepressants to the brain, with potential consequences for the clinical therapeutic effects of these drugs. This review will focus on the studies that have used multi-drug resistance p-glycoprotein knockout animals in such context, and will discuss the potential clinical relevance of these transporters for psychiatric disorders. In particular, we will discuss the reciprocal interactions between this transporter and antidepressants, both as its inhibitors and as its substrates. We believe that the interaction between antidepressants and multi-drug resistance p-glycoprotein is one of the most potentially exciting developments in psychopharmacological research.

Sertraline and its metabolite desmethylsertraline, but not bupropion or its three major metabolites, have high affinity for P-glycoprotein

The ATP-binding cassette (ABC) transporter protein subfamily B1 line (ABCB1) transporter P-glycoprotein (P-gp) plays an important role in the blood-brain barrier limiting a broad spectrum of substrates from entering the central nervous system. In the present study, the transport activity of P-gp for sertraline, desmethylsertraline, bupropion, and the major metabolites of bupropion, threo-amino alcohol (TB), erythro-amino alcohol (EB), and hydroxy metabolite (HB) was studied using an ATPase assay in expressed human P-gp membranes by measuring concentrations of inorganic P(i) in expressed human P-gp membranes. Verapamil was included as a positive control. The Michaelis-Menten equation was used for characterizing the kinetic data. Sertraline and desmethylsertraline showed high affinity for P-gp. The V(max)/K(m) values of sertraline (1.6 min(-1) x 10(-3)) and desmethylsertraline (1.4 min(-1) x 10(-3)) were comparable with that of verapamil (1.7 min(-1) x 10(-3)). Bupropion and its three metabolites showed very weak affinity for P-gp, with V(max)/K(m) values lower than 0.01 min(-1) x 10(-3). The results of the present study indicate that sertraline and desmethylsertraline have high affinity for P-gp, whereas bupropion and its three major metabolites TB, EB, and HB have very weak affinity for P-gp. These findings may help to explain observed drug-drug interactions among antidepressants.

The emerging importance of transporter proteins in the psychopharmacological treatment of the pregnant patient

P-glycoprotein, breast cancer resistance protein, and multidrug resistance proteins have physiological functions in placental tissue. Several antidepressants, antipsychotics, and anti-epileptic drugs have been found to be substrates of P-glycoprotein and other transporters. The extent that drugs pass through the placental barrier is likely influenced by drug transporters. The rational choice of psychoactive drugs to treat mental illness in women of child-bearing age should incorporate knowledge of both drug disposition as well as expected pharmacologic effects. This review summarizes the current data on drug transporters in the placental passage of medications, with a focus on medications used in clinical psychopharmacology.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

An overview of psychotropic drug-drug interactions

The psychotropic drug-drug interactions most likely to be relevant to psychiatrists' practices are examined. The metabolism and the enzymatic and P-glycoprotein inhibition/induction profiles of all antidepressants, antipsychotics, and mood stabilizers are described; all clinically meaningful drug-drug interactions between agents in these psychotropic classes, as well as with frequently encountered nonpsychotropic agents, are detailed; and information on the pharmacokinetic/pharmacodynamic results, mechanisms, and clinical consequences of these interactions is presented. Although the range of drug-drug interactions involving psychotropic agents is large, it is a finite and manageable subset of the much larger domain of all possible drug-drug interactions. Sophisticated computer programs will ultimately provide the best means of avoiding drug-drug interactions. Until these programs are developed, the best defense against drug-drug interactions is awareness and focused attention to this issue.

Rapid induction of P-glycoprotein expression by high permeability compounds in colonic cells in vitro: a possible source of transporter mediated drug interactions?

P-glycoprotein (PGP) substrates with high membrane permeability, such as propranolol and verapamil, are considered to be essentially "transparent" to PGP since the transporter does not significantly limit their absorption or elimination. However, the question of whether such compounds can modulate PGP expression in epithelial cells following short-term exposure, with potential consequences for drug interactions, has not been addressed. LS180 colonic epithelial cells were exposed to propranolol or verapamil at concentrations (50-300 microM) consistent with those likely to be present in the gut lumen during oral dosing. Both compounds stimulated four to six-fold increases in MDR1 mRNA and PGP protein expression measured by quantitative real-time PCR and immunoblotting, respectively. These changes were accompanied by an induction in transporter activity measured by rhodamine 123 efflux. In contrast, metoprolol, a compound with similar permeability but no affinity for PGP had no effect on PGP expression. The induction of PGP by propranolol and verapamil was rapid with significant increases occurring within 3h with maximal stimulation after 6h exposure. Rifampicin, shown to cause clinical drug interactions via a PXR-mediated increase in PGP expression, exhibited a very similar time-course and extent of induction. In conclusion, verapamil and propranolol, whose trans-epithelial permeability are unaffected by PGP, appear to be effective inducers of PGP expression in gut epithelial cells in vitro. While the in vivo significance of these observations is unknown, this questions whether high permeability, "PGP-transparent" compounds, currently favoured in drug selection strategies, should be evaluated in terms of their potential for transporter-mediated drug interactions.

Classification analysis of P-glycoprotein substrate specificity

Prediction of P-glycoprotein substrate specificity (S(PGP)) can be viewed as a constituent part of a compound's "pharmaceutical profiling" in drug design. This task is difficult to achieve due to several factors that raised many contradictory opinions: (i) the disparity between the S(PGP) values obtained in different assays, (ii) the confusion between Pgp substrates and inhibitors, (iii) the confusion between lipophilicity and amphiphilicity of Pgp substrates, and (iv) the dilemma of describing class-specific relationships when Pgp has no binding sites of high ligand specificity. In this work, we compiled S(PGP) data for 1000 compounds. All data were represented in a binary format, assigning S(PGP) = 1 for substrates and S(PGP) = 0 for non-substrates. Each value was ranked according to the reliability of experimental assay. Two data sets were considered. Set 1 included 220 compounds with S(PGP) from polarized transport across MDR1 transfected cell monolayers. Set 2 included the entire list of 1000 compounds, with S(PGP) values of generally lower reliability. Both sets were analysed using a stepwise classification structure-activity relationship (C-SAR) method, leading to derivation of simple rules for crude estimation of S(PGP) values. The obtained rules are based on the following factors: (i) compound's size expressed through molar weight or volume, (ii) H-accepting given by the Abraham's beta (that can be crudely approximated by the sum of O and N atoms), and (iii) ionization given by the acid and base pKa values. Very roughly, S(PGP) can be estimated by the "rule of fours". Compounds with (N + O) > or = 8, MW > 400 and acid pKa > 4 are likely to be Pgp substrates, whereas compounds with (N + O) < or = 4, MW < 400 and base pKa < 8 are likely to be non-substrates. The obtained results support the view that Pgp functioning can be compared to a complex "mini-pharmacokinetic" system with fuzzy specificity. This system can be described by a probabilistic version of Abraham's solvation equation, suggesting a certain similarity between Pgp transport and chromatographic retention. The chromatographic model does not work in the case of "marginal" compounds with properties close to the "global" physicochemical cut-offs. In the latter case various class-specific rules must be considered. These can be associated with the "amphiphilicity" and "biological similarity" of compounds. The definition of class-specific effects entails construction of the knowledge base that can be very useful in ADME profiling of new drugs.

The effect of age on P-glycoprotein expression and function in the Fischer-344 rat

We investigated the effect of age on P-glycoprotein (P-gp) expression and function in rat liver, intestine, kidney, and endothelial cells of the blood-brain barrier (BBB) and lymphocytes. Flow cytometric analysis was used to examine P-gp expression in lymphocytes from male Fischer-344 rats from three age groups (young at 3-4 months, intermediate at 13-14 months, and old at 25-26 months). In addition, P-gp function in lymphocytes was assessed by measuring the ability of the P-gp inhibitor verapamil to limit the efflux of the fluorescent P-gp substrate rhodamine 123. P-gp expression was evaluated in the remaining four tissues by Western blot analysis. The effect of age on P-gp expression was tissue-specific. Although lymphocytic and hepatic P-gp expression increased with age, renal P-gp content was lower in the old kidneys. No statistical difference was observed in P-gp expression in intestinal microsomes or in BBB cell lysates among the three age groups. P-gp function was also increased by 6- to 8-fold in lymphocytes from the old rats. When P-gp expression was compared with CYP3A expression in these rats (reported elsewhere in this journal), we found that P-gp expression increased with age, whereas CYP3A expression and activity declined in the old livers. The converse pattern was observed in the kidney. Thus, age-related changes in P-gp expression and function are likely to be tissue-specific, and these changes may be inversely related to differences in CYP3A expression.

Rapid assessment of P-glycoprotein inhibition and induction in vitro

PURPOSE

Using rhodamine123 (RH123) cell exclusion. 17 clinically used compounds were screened for their inhibitory effect on P-glycoprotein (P-gp), which was compared with the drugs' inhibitory activity against CYP3A4. The same assay was used to study induction of P-gp activity.

METHODS

P-gp inhibition was assessed using RH123 accumulation into LS180V cells as well as Rh123 transport across Caco-2 mono-layers. Inhibition of CYP3A4 was determined in human liver microsomes using triazolam-4-hydroxylation. Induction of P-gp expression and activity was measured using western blot analysis and RH123 accumulation into LS180V cells, respectively.

RESULTS

The observed inhibition of RH123 cell exclusion ranged from little or no effect (digoxin, indinavir, fexofenadine) up to a nearly 10-fold increase in RH 123 accumulation (ivermectin, terfenadine). No correlation between P-gp and CYP3A4 inhibition was observed. The rank order in P-gp inhibitory potency for terfenadine, verapamil, ritonavir. and indomethacin was identical in both LS180V and Caco-2 models. Ritonavir and St. John's wort extract showed a concentration-dependent P-gp induction, with good correlation between western blot analysis and RH123 accumulation.

CONCLUSIONS

The RH123 accumulation assay in LS180V cells can be used as a valuable screening tool to study both inhibition and induction of P-gp activity and expression. This assay has the potential to predict P-gp-mediated alterations in intestinal absorption of drugs.

Short-term exposure to low-dose ritonavir impairs clearance and enhances adverse effects of trazodone

Antiretroviral agents may participate in drug interactions that influence the efficacy and toxicity of other antiretrovirals, as well as pharmacologic treatments of coincident or complicating diseases. The viral protease inhibitor, ritonavir, may cause drug interactions by inhibiting the activity of cytochrome P450-3A (CYP3A) isoforms. In a single-dose, blinded, four-way crossover study, 10 healthy volunteer subjects received 50 mg of trazodone hydrochloride or matching placebo concurrent with low-dose ritonavir (four doses of 200 mg each) or with placebo. Compared to the control condition, ritonavir significantly reduced apparent oral clearance of trazodone (155 +/- 23 vs. 75 +/- 12 ml/min, p < 0.001), prolonged elimination half-life (6.7 +/- 0.7 vs. 14.9 +/- 3.9 h, p < 0.05), and increased peak plasma concentrations (842 +/- 64 vs. 1125 +/- 111 ng/ml, p < 0.05) (mean +/- SE). Coadministration of trazodone with ritonavir increased sedation, fatigue, and performance impairment compared to trazodone plus placebo; differences reached significance only for the digitsymbol substitution test. Three subjects experienced nausea, dizziness, or hypotension when trazodone was given with ritonavir; 1 of these subjects also experienced syncope. Thus short-term low-dose administration of ritonavir impairs oral clearance of trazodone and increases the occurrence of adverse reactions. The findings are consistent with impairment of CYP3A-mediated trazodone metabolism by ritonavir.

Effect of zolpidem on human cytochrome P450 activity, and on transport mediated by P-glycoprotein

The influence of high concentrations of zolpidem (100 microM, corresponding to approximately 200 times maximum therapeutic concentrations) on the activity of six human Cytochrome P450 (CYP) enzymes was evaluated in a model system using human liver microsomes. Zolpidem produced negligible or weak inhibition of human CYP1A2, 2B6, 2C9, 2C19, 2D6, and 3A. Transport of rhodamine 123, presumed to be mediated mainly by the energy-dependent efflux transport protein P-glycoprotein, was studied in a cell culture system using a human intestinal cell line. High concentrations of zolpidem (100 microM), exceeding the usual therapeutic range by more than 100-fold, produced only modest impairment of rhodamine 123 transport. The findings indicate that zolpidem is very unlikely to cause clinical drug interactions attributable to impairment of CYP activity or P-gp mediated transport.

Fexofenadine transport in Caco-2 cells: inhibition with verapamil and ritonavir

This study investigated fexofenadine (FXD) transport and the inhibition of FXD transport in Caco-2 cell monolayer transwells, using rhodamine 123 (RH123) transport as a positive control. FXD transport from the basolateral (B) to apical (A) compartment was fivefold higher than A to B transport. FXD transport was linear with respect to time (up to 270 min) and concentration (up to 300 microm). Similar results were seen with the positive control RH123. Ritonavir (100 PM) and verapamil (100 microm) reduced transport of FXD and RH123 by more than 80%, whereas transport was not inhibited by 100 m indomethacin or 2 mM probenecid. This suggests predominantly P-glycoprotein (P-gp)-mediated transport as opposed to transport by multidrug resistance protein. In concentration-response experiments, FXD transport was inhibited by verapamil and ritonavir with IC50 values of 6.5 microm and 5.4 microm, respectively. Results from this in vitro study demonstrate differential transport of FXD across Caco-2 cell monolayers and inhibition of FXD transport by established P-gp inhibitors. Thefindings support the use of FXD as an index or probe compound to reflect P-gp activity in vivo.

Saint John's wort: an in vitro analysis of P-glycoprotein induction due to extended exposure

1. Chronic use of Saint John's wort (SJW) has been shown to lower the bioavailability for a variety of co-administered drugs including indinavir, cyclosporin, and digoxin. Decreases in intestinal absorption through induction of the multidrug resistance transporter, P-glycoprotein (P-gp), may explain decreased bioavailability. 2. The present study characterized the response of P-gp to chronic and acute exposure of SJW and hypericin (HYP, a presumed active moiety within SJW) in an in vitro system. Experiments were performed with 3 to 300 microg ml(-1) of methanol-extracted SJW and 0.03 to 3 microM HYP, representing low to high estimates of intestinal concentrations. 3. In induction experiments, LS-180 intestinal carcinoma cells were exposed for 3 days to SJW, HYP, vehicle or a positive control (ritonavir). P-gp was quantified using Western blot analysis. P-gp expression was strongly induced by SJW (400% increase at 300 microg ml(-1)) and by HYP (700% at 3 microM) in a dose-dependent fashion. Cells chronically treated with SJW had decreased accumulation of rhodamine 123, a P-gp substrate, that was reversed with acute verapamil, a P-gp inhibitor. Fluorescence microscopy of intact cells validated these findings. In Caco-2 cell monolayers, SJW and HYP caused moderate inhibition of P-gp-attributed transport at the maximum concentrations tested. 4. SJW and HYP significantly induced P-gp expression at low, clinically relevant concentrations. Similar effects occurring in vivo may explain the decreased bioavailability of P-gp substrate drugs when co-administered with SJW.