Drug-drug interactions affected by the transporter protein, P-glycoprotein (ABCB1, MDR1) I. Preclinical aspects

When certain coadministered drugs interfere with the transport of one another at the P-glycoprotein (Pgp) level, we observe the phenomenon of "drug-drug interactions". We describe here the physical and biochemical ways that drugs react with Pgp, the induction of Pgp by xenobiotics and the polymorphisms of Pgp related to drug-drug interactions. We also describe methods that can be used to detect potential drug-drug interactions. Among the widely used methods are flow cytometry, cell culture with polarized expression of Pgp, liposome and inside-out membrane preparations, ex vivo studies, in silico calculations, cDNA microarrays, in vivo mouse models, positron emission tomography (PET), and nuclear magnetic resonance (NMR) methods with live animals.     

Multidrug transporters as drug targets

Transport molecules can significantly affect the pharmacodynamics and pharmacokinetics of drugs. An important transport molecule, the 170 kDa P-glycoprotein (Pgp), is constitutively expressed at several organ sites in the human body. Pgp is expressed at the blood-brain barrier, in the kidneys, liver, intestines and in certain T cells. Other transporters such as the multidrug resistance protein 1 (MRP1) and MRP2 also contribute to drug distribution in the human body, although to a lesser extent than Pgp. These three transporters, and especially Pgp, are often targets of drugs. Pgp can be an intentional or unintentional target. It is directly targeted when one wants to block its function by a modifier drug so that another drug, also a substrate of Pgp, can penetrate the cell membrane, which would otherwise be impermeable. Unintentional targeting occurs when several drugs are administered to a patient and as a consequence, the physiological function of Pgp is blocked at different organ sites. Like Pgp, MRP1 also has the capacity to mediate transport of many drugs and other compounds. MRP1 has a protective role in preventing accumulation of toxic compounds and drugs in epithelial tissue covering the choroid plexus/cerebrospinal fluid compartment, oral epithelium, sertoli cells, intesticular tubules and urinary collecting duct cells. MRP2 primarily transports weakly basic drugs and bilirubin from the liver to bile. Most compounds that efficiently block Pgp have only low affinity for MRP1 and MRP2. There are only a few effective and specific MRP inhibitors available. Drug targeting of these transporters may play a role in cancer chemotherapy and in the pharmacokinetics of substrate drugs.     

P-glycoprotein as a drug target in the treatment of multidrug resistant cancer

Multidrug resistance (MDR) is a major obstacle to successful cancer chemotherapy. One important mechanism of MDR involves the multidrug transporter, P-glycoprotein (Pgp), which confers upon cancer cells the ability to resist lethal doses of certain cytotoxic drugs by pumping the drugs out of the cells and thus reducing their cytotoxicity. Pgp belongs to the ATP-binding cassette (ABC) family of transporter molecules which require hydrolysis of ATP to run the transport mechanism. The substrates of Pgp may be endogenous (steroid hormones, cytokines) or exogenous (cytostatic drugs). A number of studies have demonstrated a negative correlation between Pgp expression levels and chemosensitivity or survival in a range of human malignancies. In principle, Pgp mediated drug resistance can be circumvented by treatment regimens that either exclude Pgp substrate drugs or include Pgp inhibitory agents. Experimental studies have demonstrated that certain structural modifications of anthracyclines confer the ability to escape Pgp transport. The therapeutic benefit of Pgp inhibitors as chemosensitizers is currently being explored in phase III clinical trials, and the first promising results have already been reported. Another therapeutic option for Pgp inhibitors has recently evolved as several Pgp inhibitors, many of which are generally low-toxic substances, by themselves constrain proliferation and cause cell death by apoptosis in certain MDR cancer cell lines. The dual effect of Pgp inhibitors, targeting MDR cancer cells selectively, may translate into improved efficacy of cancer chemotherapy and perhaps new and less toxic drug treatment strategies in human MDR cancer.     

Antipsychotic polypharmacy

The administration of more than one drug for a single medical condition is considered to be polypharmacy. There are many possible reasons for polypharmacy: (1) psychosis is a chronic disease that cannot be cured; (2) expectations to improve patients' quality of life beyond what drugs can actually do is high; (3) the lack of side effects and interactions can cause physicians to be more daring in terms of potential complications; (4) information from the Internet may cause patients and their families to demand medications; (5) the diluted mental health system allows legal guardians and other mental health professionals to force physicians to provide multiple drugs; (6) many new drugs are available; and (7) physicians are forced to shorten hospitalization days. The 1997 American Psychiatric Association Practice Research Network found that 17% of 146 patients with schizophrenia were treated concurrently with more than one antipsychotic medication. Polypharmacy may increase the risk of adverse effects, drug interactions, noncompliance, and medication errors. It is not wise to use polypharmacy only to prevent side effects and drug and interactions. Our attempts to reduce polypharmacy may fail, as academicians also propagate polypharmacy, and all of the algorithms indicate polypharmacy as an option, putting physicians in a legal and ethical bind. Techniques such as experimental ward, peer review, computer information feedback, and comparing different techniques may temporarily reduce polypharmacy but long-term outcome is not affected. Scientific data on the efficacy of polypharmacy is needed in order to sort out good and bad polypharmacy.     

Drug-induced phospholipidosis caused by combinations of common drugs in vitro

Drug-induced phospholipidosis (DIPLD), characterized by the accumulation of phospholipids within lysosomes, is suspected to impair lysosomal function and considered an adverse side effect of the administered medication. The increasing use of polypharmacy and the resultant elevated risks of adverse drug reactions raise the need to explore the effects of drug combinations with respect to their influence on side effects, such as DIPLD. In this study, we utilized an in vitro assay to investigate DIPLD that was caused by 24 commonly used drugs applied alone and in binary combinations with each other. Moreover, we attempted to predict the extent of DIPLD resulting from the combinations using a simple additive approach based on the increase in phospholipid levels caused by the single drugs. The results suggest that DIPLD, which was caused by combinations of drugs, occurs in an additive manner, depending on total drug concentration. Furthermore, we show that the extent of DIPLD can be predicted from the DIPLD caused by the single drugs. Thus, the simultaneous use of multiple drugs with PLD-inducing properties increases the event risk, as well as the severity of drug-induced phospholipidosis. The findings underline the importance of considering the DIPLD-inducing properties of drugs, especially in the context of polypharmacy.     

Effects of isoprenoid analogues of SDB-ethylenediamine on multidrug resistant tumor cells alone and in combination with chemotherapeutic drugs

Multidrug resistance (MDR) mediated by P-glycoprotein (Pgp) remains the major obstacle for successful treatment of cancer. Inhibition of Pgp transport is important for higher efficacy of anticancer drugs. Lipophilic cationogenic amines with at least one tertiary N atom, such as verapamil, are classical PgP-blocking agents. In a search for novel accessible compounds potent against MDR tumor cells, we synthesized a series of arylalkylamines that contain isoprenoid side chains of different length. Two out of seven new analogues of the known N,N'-bis(3,4-dimethoxybenzyl)-N-solanesylethylenediamine (SDB-ethylenediamine), namely, compounds with C10 and C15 side chains, at low micromolar concentrations were preferentially toxic for several mammalian tumor cell lines that acquired MDR during prolonged drug selection. Moreover, at noncytotoxic concentrations, these compounds potently sensitized MDR cells to Pgp substrates vinblastine and adriamycin. We conclude that these analogues of SDB-ethylenediamine may have dual therapeutic advantage because (i) they are preferentially toxic for MDR cells when administered alone and (ii) they potentiate the cytotoxicity of Pgp-transported anticancer drugs.     

Future perspectives for the development of P-glycoprotein modulators.

Resistance to chemotherapeutic agents constitutes one of the major obstacles to the successful treatment of cancer. While several mechanisms underlying drug resistance have been elucidated, the most widely studied mechanism involves the efflux of antineoplastic drugs from cancer cells by P-glycoprotein, the 170 kD glycoprotein product of the MDR-I gene. The observation that several compounds are able to inhibit P-glycoprotein in vitro created optimism that the problem of multidrug resistance in cancer could be quickly resolved by moving these compounds into the clinic. However, despite a large number of clinical trials with several different putative Pgp modulators, the value of Pgp modulation in clinical oncologic practice remains unresolved. While these initial trials have not answered the question of whether Pgp is an important mechanism of resistance in human cancers, or whether modulation of Pgp is likely to positively impact on the treatment of cancer, they have provided insights regarding the problems inherent in conducting trials of this nature. These clinical insights, along with knowledge gained from continued basic research on drug resistance mediated by Pgp and related transporters, will form a strong foundation for future research into the role of Pgp and Pgp modulation in the treatment of cancer. The ubiquitous nature of transporters and the high prevalence of transporter substrates among antineoplastic drugs, compel the development of modulators that can be used to prevent or reverse drug resistance.     

Clinical aspects of the MDR1 (ABCB1) gene polymorphism

Transporter proteins, in particular P-glycoprotein (Pgp), are important determinants in absorption, tissue targeting, and elimination of drugs. In addition to physiological and environmental factors, its expression and function are modified by genetic polymorphisms of the MDR1 gene. So far, several MDR1 SNPs have been identified, and mutations at positions 2677 and 3435 were associated with alteration of Pgp expression and/or function. In contrast to drug-metabolizing enzymes (eg, CYP2D6), for which loss of function mutations or gene amplification manifests as distinct phenotypes in the population, the impact of MDR1 polymorphisms on pharmacokinetics and pharmacodynamics of Pgp substrates is moderate. Clinical studies on the effects of the C3435T polymorphism and drug treatment with cardiac glycosides, the immunosuppressants cyclosporine and tacrolimus, HIV protease inhibitors, and tricyclic antidepressants are discussed.     

Biochemical mechanism of modulation of human P-glycoprotein (ABCB1) by curcumin I, II, and III purified from Turmeric powder

P-glycoprotein (Pgp, ABCB1) is an ATP-dependent drug efflux pump linked to development of multidrug resistance (MDR) in cancer cells. Previously [Biochem Pharmacol 2002;64:573-82], we reported that a curcumin mixture could modulate both function and expression of Pgp. This study focuses on the effect of three major curcuminoids--curcumin I, II and III purified from a curcumin mixture--on modulation of Pgp function in a multidrug resistant human cervical carcinoma cell line (KB-V1). The similar IC(50) values for cytotoxicity of curcuminoids of KB-V1, and KB-3-1 (parental drug sensitive cell line) suggest that these curcuminoids may not be substrates for Pgp. Treating the cells with non-toxic doses of curcuminoids increased their sensitivity to vinblastine only in the Pgp expressing drug resistant cell line, KB-V1, and curcumin I retained the drug in KB-V1 cells more effectively than curcumin II and III, respectively. Effects of each curcuminoid on rhodamine123, calcein-AM, and bodipy-FL-vinblastine accumulation confirmed these findings. Curcumin I, II and III increased the accumulation of fluorescent substrates in a dose-dependent manner, and at 15 microM, curcumin I was the most effective. The inhibitory effect in a concentration-dependent manner of curcuminoids on verapamil-stimulated ATPase activity and photoaffinity labeling of Pgp with the [(125)I]-iodoarylazidoprazosin offered additional support; curcumin I was the most potent modulator. Taken together, these results indicate that curcumin I is the most effective MDR modulator among curcuminoids, and may be used in combination with conventional chemotherapeutic drugs to reverse MDR in cancer cells.     

Evaluation of the role of P-glycoprotein in the uptake of paroxetine, clozapine, phenytoin and carbamazapine by bovine retinal endothelial cells

Expression of the drug transport proteins, including P-glycoprotein (Pgp), in the brain vascular endothelium represents a challenge for the effective delivery of drugs for the treatment of several central nervous system (CNS) disorders including depression, schizophrenia and epilepsy. It has been hypothesized that Pgp plays a major role in drug efflux at the blood-brain barrier, and may be an underlying factor in the variable responses of patients to CNS drugs. However, the role of Pgp in the transport of many CNS drugs has not been directly demonstrated. To explore the role of Pgp in drug transport across an endothelial cell barrier derived from the central nervous system, the expression and activity of Pgp in bovine retinal endothelial cells (BRECs) and the effects of representative CNS drugs on Pgp activity were examined. Significant Pgp expression in BRECs was demonstrated by western analyses, and expression was increased by treatment of the cells with hydrocortisone. Intracellular accumulation of the well-characterized Pgp-substrate Taxol was markedly increased by the non-selective transporter inhibitor verapamil and the Pgp-selective antagonist PGP-4008, demonstrating that Pgp is active in these endothelial cells. In contrast, neither verapamil nor PGP-4008 affected the intracellular accumulation of [3H]paroxetine, [14C]phenytoin, [3H]clozapine or [14C]carbamazapine, indicating that these drugs are not substrates for Pgp. Paroxetine, clozapine and phenytoin were shown to be Pgp inhibitors, while carbamazapine did not inhibit Pgp at any concentration tested. These results indicate that Pgp is not likely to modulate patient responses to these drugs.     

Astemizole: an old anti-histamine as a new promising anti-cancer drug

Mortality-to-incidence ratio in cancer patients is extremely high, positioning cancer as a major cause of death worldwide. Despite hundreds of clinical trials for anti-cancer drugs that are currently in progress, most clinical trials for novel drug treatments fail to pass Phase I. However, previously developed drugs with novel anti-tumor properties offer a viable and cost-effective alternative to fight cancer. Histamine favors the proliferation of normal and malignant cells. Several anti-histamine drugs, including astemizole, can inhibit tumor cell proliferation. Astemizole has gained enormous interest since it also targets important proteins involved in cancer progression, namely, ether à-go-go 1 (Eag1) and Eag-related gene (Erg) potassium channels. Furthermore, Eag1 is thought to be an important marker and a therapeutic target for several different cancers. Astemizole inhibits Eag1 and Erg channel activity, and in cells expressing the Eag1 channel it decreases tumor cell proliferation in vitro and in vivo. It should be noted that some cardiovascular side effects have been reported for astemizole in a few rare cases. Nevertheless, astemizole stands as a very promising anti-cancer tool because it displays several anti-proliferative mechanisms, may serve as the basis to synthesize new anti-cancer agents, and has been previously administered clinically. In this review we will summarize the main findings relating to histamine and anti-histamines in cancer cell proliferation focusing on astemizole targets (Eag1 and Erg channels), and its anti-cancer effects in vitro and in vivo. We will also describe the side effects of astemizole and discuss proposals to overcome such effects in cancer patients. Finally, we will remark on the relevance of developing novel astemizole-related compounds.     

Cyclosporins: structure-activity relationships for the inhibition of the human MDR1 P-glycoprotein ABC transporter

Cyclic undecapeptide cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)], the immunosuppressive and antifungal antibiotic cyclosporin A (CsA), was reported to interfere with the MDR1 P-glycoprotein (Pgp), a transmembranous adenosine 5'-triphosphate binding cassette (ABC) transporter with phospholipid flippase or "hydrophobic vacuum cleaner" properties that mediate multidrug resistance (MDR) of cancer cells. By use of photoaffinity-labeled cyclosporins and membranes from Pgp-expressing cells, it was recently shown that in vitro, Pgp molecules could bind a large cyclosporin domain involving residues 4-9 as well as the side chain of residue 1. Tumor cell MDR can also be reversed by a product more distantly related to cyclosporin with the structure [Thr(2), Leu(5), D-Hiv(8), Leu(10)]-CsA (SDZ 214-103). In a standardized assay that measures Pgp function in vivo (on intact live cells) by the Pgp-mediated efflux of the calcein-AM Pgp substrate and uses human lymphoblastoid MDR-CEM (VBL(100)) cells as highly resistant Pgp-expressing cells, SDZ 214-103 was found to be one of the most active Pgp inhibitors among naturally occurring cyclosporins, with an IC(50) of 1.6 microM in an assay where CsA gives an IC(50) of 3.4 microM. Using the in vivo assay, 60, mostly natural, cyclosporin analogues were analyzed to establish structure-activity relationships (SAR). Our SAR are compatible with the in vitro-defined Pgp binding domain model and further disclose that in vivo Pgp inhibition is favored by larger hydrophobic side chains on cyclosporin residues 1, 4, 6, and 8 and a smaller one on residue 7, although with no effect on the residue 5 side chain; moreover, larger hydrophobic side chains on other residues 2, 3, 10, and 11 (outside the in vitro-defined Pgp binding domain) also favor the eventual inhibition of Pgp function. The N-desmethylation of any of the seven N-methylated amides, as naturally occurring in numerous cyclosporins, regularly leads to a decreased Pgp inhibitory activity (Pgp-InhA), up to its abrogation if it occurs at residues 4 and 9. Nevertheless, despite unfavorable use of [Thr(2)] and [Leu(10)] residues, all [D-Hiv(8)] analogues whose lead is SDZ 214-103 show a large Pgp-InhA. The SAR for Pgp inhibition by cyclosporins are thus very complex. Because CsA and SDZ 214-103 show largely different conformations when free in solution, but remarkably similar ones when bound to the cytosolic cyclophilins, SAR for Pgp inhibition must similarly include requirements for occurrence of suitable conformers for insertion in the cell membrane, sufficient conformational plasticity for gaining access to Pgp binding sites, and an adequate conformer structure there to achieve such binding with a high enough affinity and possibly escape from sequestration on cyclophilins.     

Studies on pyrrolopyrimidines as selective inhibitors of multidrug-resistance-associated protein in multidrug resistance

Multidrug resistance mediated by P-glycoprotein (Pgp) or multidrug-resistance-associated protein (MRP) remains a major obstacle for successful treatment of cancer. Inhibition of Pgp and MRP transport is important for high efficacy of anticancer drugs. While several Pgp inhibitors have entered clinical trials, the development of specific MRP1 inhibitors is still in its infancy. In our screening program, we have identified a pyrrolopyrimidine (4) as a novel and selective MRP1 inhibitor. Subsequent SAR work on the 4-position of the template revealed the phenethylpiperazine side chain as a potent replacement of the benzylthio group of the lead molecule. Introduction of groups at the 2-position seems to have no detrimental effect on activity. Modifications to the nitrile group at the 7-position resulted in the identification of analogues with groups, such as amides, with superior pharmacokinetic profiles. In vivo efficacy has been demonstrated by xenograft studies on selected compounds.     

P-glycoprotein--a clinical target in drug-refractory epilepsy?

ATP-binding cassette transporters such as P-glycoprotein (Pgp), multidrug resistance-associated protein, and breast cancer resistance protein are known to transport a wide range of substrates and are highly expressed in the capillary endothelial cells that form part of the blood-brain barrier. It is noteworthy that P-glycoprotein has been shown to be up-regulated in animal models of refractory epilepsy, and adding a Pgp inhibitor to treatment regimens has been shown to reverse the drug-resistant phenotype. Limited data have suggested a role for Pgp in epilepsy in humans as well. However, few epilepsy drugs have been shown to be transported by Pgp, leading to controversy over whether Pgp actually plays a role in drug-resistant epilepsy. In this issue of Molecular Pharmacology, Bauer et al. (p. 1444) demonstrate that glutamate can cause localized up-regulation of Pgp via cyclooxygenase-2 (COX-2) and that this phenomenon can be prevented with COX-2 inhibitors. Localized rather than global up-regulation of Pgp may explain some of the difficulty investigators have had in proving a role for Pgp in epilepsy. The results add new support for future clinical trials targeting Pgp expression in drug-refractory epilepsy.     

The medicinal chemistry of multidrug resistance (MDR) reversing drugs

Multidrug resistance (MDR) is a kind of resistance of cancer cells to multiple classes of chemotherapic drugs that can be structurally and mechanistically unrelated. Classical MDR regards altered membrane transport that results in lower cell concentrations of cytotoxic drug and is related to the over expression of a variety of proteins that act as ATP-dependent extrusion pumps. P-glycoprotein (Pgp) and multidrug resistance protein (MRP1) are the most important and widely studied members of the family that belongs to the ABC superfamily of transporters. It is apparent that, besides their role in cancer cell resistance, these proteins have multiple physiological functions as well, since they are expressed also in many important non-tumoural tissues and are largely present in prokaryotic organisms. A number of drugs have been identified which are able to reverse the effects of Pgp, MRPI and sister proteins, on multidrug resistance. The first MDR modulators discovered and studied in clinical trials were endowed with definite pharmacological actions so that the doses required to overcome MDR were associated with unacceptably high side effects. As a consequence, much attention has been focused on developing more potent and selective modulators with proper potency, selectivity and pharmacokinetics that can be used at lower doses. Several novel MDR reversing agents (also known as chemosensitisers) are currently undergoing clinical evaluation for the treatment of resistant tumours. This review is concerned with the medicinal chemistry of MDR reversers, with particular attention to the drugs that are presently in development.     

The functions and structure of ABC transporters: implications for the design of new inhibitors of Pgp and MRP1 to control multidrug resistance (MDR)

Multidrug resistance (MDR) is a kind of acquired resistance of microorganisms and cancer cells to chemotherapic drugs that are characterized by different chemical structure and different mechanism of action. Classic MDR is the consequence of the over-expression of a variety of proteins that extrude the chemotherapic from the cell, lowering its concentration below the effective one. The ABC (ATP Binding Cassette) is a ubiquitous and important family of such transporter proteins. Members of this super family are present in mammals as well as in prokaryotic organisms and use ATP as the energy source to activate the extrusion process. P-glycoprotein (Pgp) and Multidrug Resistance Proteins (MRP1 and sister proteins) are the most important and widely studied members of ABC super family. Our knowledge about the structures and functions of transporter proteins has definitely improved in recent years, following the resolution of the structure of bacterial pumps which opened the way to the building of homology models for the more complex Pgp and MRP. It can be anticipated that these results will have a strong impact on the design of more potent and safer MDR reverters. A huge number of small molecules, many of natural origin, are able to reverse multidrug resistance by inhibiting the functions of Pgp, MRP1 and sister proteins and their action has been considered a possible way to reverse MDR. However, while a few compounds have reached clinical trials, none of them has, so far, been cleared for therapeutic use. Two main reasons are at the base of this difficulty: i) MDR is a complex phenomenon that may arise from several different biochemical mechanisms, with the consequence that inhibition of transporter proteins may be insufficient to reverse it; ii) the physiological role of Pgp and sister proteins requires more potent modulators with proper selectivity and pharmacokinetic in order to avoid unwanted side effects. This paper first reviews the most recent discoveries on the structures and functions of the ABC super family, in particular Pgp and MRP. Then, the medicinal chemistry of MDR reverters, in light of these findings, is discussed and the molecules that are presently in development are reviewed.     

Impact of small molecules immunosuppressants on p-glycoprotein activity and T-cell function

Purpose. P-glycoprotein (Pgp) is a member of the ABC-transporter family that transports substances across cellular membranes acting as an efflux pump extruding drugs out of the cells. Pgp plays a key role on the pharmacokinetics of several drugs. Herein, we have studied the effects of immunosuppressants on Pgp function, assessing rhodamine-123 (Rho123) uptake and efflux in different T-cell subsets. Methods. Different immunosuppressants such as Cyclosporine (CsA), Rapamycin (Rapa) and Tacrolimus (Tac) were used to assess the in vitro effect on Pgp function of main T-cell subsets among healthy volunteers. We measured Rho123 uptake, efflux and kinetic of extrusion in CD4+ and CD8+ subsets by flow cytometry. Antigen-specific memory T-cell responses were assessed by measuring T-cell proliferation and cytokine secretion using an allogeneic mixed lymphocyte reaction. Results. Rho123 uptake in groups treated with CsA and CsA+Rapa was significantly decreased compared to non-treated group and the other immunosupressants in both T cells subsets. Pgp activity was also reduced in CsA and CsA+Rapa compared to the other immunosupressants but it was only significant in the CsA group for CD8+ subset. Kinetic extrusion of Rho123 by Pgp in all groups was faster in CD8+ T cells. All immunosuppressants and the specific Pgp inhibitor PSC833 diminished antigen-primed T-cell proliferation, especially CD8+ T-cell subset. Conclusions. Our data indicate that small molecules immunosuppressants, especially CsA, inhibit Pgp activity and T-cell function being the CD8+ T cells more susceptible to this effect. These findings support the importance of Pgp when designing combined immunosuppressive regimens. This article is open to POST-PUBLICATION REVIEW. Registered readers (see "For Readers") may comment by clicking on ABSTRACT on the issue's contents page.     

The barrier function of CYP3A4 and P-glycoprotein in the small bowel

CYP3A4 present in small bowel enterocytes can catalyze substantial metabolism of some orally administered drugs and, thus, exerts a first-pass effect. Recent data indicate that the P-glycoprotein (the MDR 1 gene product) in the enterocyte brush border also limits the bioavailability of many of the same drugs that interact with CYP3A. It has been proposed that P-glycoprotein and CYP3A4 may be functionally linked because (a) the two proteins are co-localized within the digestive tract and within enterocytes, (b) they share many of the same substrates and (c) they are co-inducible in response to at least some xenobiotics. There are several potential mechanisms whereby the functions of P-glycoprotein and CYP3A4 could be complimentary. First, Pgp may limit absorption in the proximal small bowel, shifting it to more distal, less catalytically efficient segments that contain lower amounts of CYP3A4. Second, Pgp may function to prolong the duration of absorption. This might increase the duration of exposure of drug to and, hence, the extent of metabolism by enterocyte CYP3A4. Finally, Pgp may preferentially remove from the enterocyte primary drug metabolites that are themselves substrates for CYP3A4. This would limit product inhibition and facilitate primary metabolism catalyzed by CYP3A4. Characterization of the roles of CYP3A4 and Pgp in limiting oral drug availability may be aided by recent success in the development of human intestinal cell lines that stably express both CYP3A4 and Pgp.     

Schisandrol A from Schisandra chinensis reverses P-glycoprotein-mediated multidrug resistance by affecting Pgp-substrate complexes

Recent studies have shown that dibenzocyclooctadiene lignans may reverse P-glycoprotein-mediated multidrug resistance (Pgp-MDR) in cancer cells; however, the mechanism of action remains unknown. Through screening of herbs, we found that schisandrol A (SCH) isolated from Fructus Schisandrae (the dried fruit of Schisandra chinensis (Turcz.) Baill.) sensitized Pgp-MDR HepG2-DR cells by interfering with the function of Pgp-substrate complexes. In Pgp-MDR cells, SCH enhanced the cytotoxicity of cancer drugs that are Pgp substrates and restored vinblastine-induced G2/M arrest without lowering Pgp expression. SCH increased cellular retention of Pgp substrates such as rhodamine 123. In Pgp-overexpressing membrane preparations, SCH stimulated basal Pgp-ATPase thus showing some substrate-like function. However, SCH was not a competitive inhibitor for verapamil or progesterone and decreased their Km. In the presence of substrates, SCH decreased the reactivity between Pgp and the monoclonal antibody UIC-2 which is normally increased with active substrate-Pgp complexes. The labeling of active Pgp transport sites by [125I]-iodoarylazidoprazosin was partially blocked by SCH. SCH did not affect the activity of the mutant Pgp F983A suggesting that SCH acted differently than the thioxanthene type of Pgp allosteric inhibitors. Our results suggest that SCH acts by affecting the normal formation and functioning of the Pgp-substrate complexes.