A human lymphocyte based ex vivo assay to study the effect of drugs on P-glycoprotein (P-gp) function

Impact of P-glycoprotein on intracellular drug concentration in peripheral blood mononuclear cells and K562 cells

P-glycoprotein (P-gp) expression in lymphocytes is variable and 2-fold higher in rheumatoid arthritis (RA) patients with treatment resistance than in healthy subjects. To date the information on P-gp-mediated drug interaction in lymphocyte is limited. We analyzed the importance on P-gp in lymphocytes using peripheral blood mononuclear cells (PBMCs) together with K562, K562/Adr, and K562/Vin cells, which have various P-gp levels, as cell models, and dexamethasone, nintedanib and apafant as weak to good P-gp substrates. P-gp levels in K562, K562/Adr, and K562/Vin cells were 0.3-, 20-, and 106-fold of healthy PBMCs, respectively. While cell accumulation of apafant and nintedanib decreased in all cells with increasing P-gp levels, dexamethasone accumulation in K562/Adr was comparable to that in healthy PBMCs and K562 cells. Cell accumulations of substrates in cells with low P-gp expression were not significantly changed by the P-gp inhibitors at therapeutic concentrations. However, accumulation increased to 1.4-fold at highest in K562/Adr cells with higher P-gp expression than in PBMCs of the RA patients. These results suggest P-gp controls the cellular concentration of P-gp substrates in PBMCs or K562 cells but cellular concentration of a weak P-gp substrate would not be apparently affected even in cells with a sufficient P-gp expression.

The Effects of Jabara Juice on the Intestinal Permeation of Fexofenadine

Jabara juice and its component narirutin inhibit the activity of organic anion-transporting polypeptides (OATPs) 1A2 and OATP2B1, which are considered to play significant roles in the intestinal absorption of fexofenadine. In this study, we investigated the effects of jabara juice on the intestinal absorption of fexofenadine in mice and the inhibitory effects of jabara juice and narirutin on the permeation of fexofenadine using Caco-2 cell monolayers and LLC-GA5-COL300 cell monolayers. In the in vivo study, the area under the plasma concentration-time curve (AUC) of fexofenadine in mice was increased 1.8-fold by jabara juice. In the permeation study, 5% jabara juice significantly decreased the efflux ratio (ER) of fexofenadine for Caco-2 monolayers. Furthermore, the ERs of fexofenadine and digoxin, which is a typical substrate of P-glycoprotein (P-gp), for LLC-GA5-COL300 cell monolayers were decreased in a concentration-dependent manner by jabara juice extract, suggesting that jabara juice may increase the intestinal absorption of fexofenadine by inhibiting P-gp, rather than by narirutin inhibiting OATPs. The present study showed that jabara juice increases the intestinal absorption of fexofenadine both in vivo and in vitro. The intestinal absorption of fexofenadine may be altered by the co-administration of jabara juice in the clinical setting.

The History of the ABC Proteins in Human Trypanosomiasis Pathogens

Human trypanosomiasis affects nearly eight million people worldwide, causing great economic and social impact, mainly in endemic areas. T. cruzi and T. brucei are protozoan parasites that present efficient mechanisms of immune system evasion, leading to disease chronification. Currently, there is no vaccine, and chemotherapy is effective only in the absence of severe clinical manifestations. Nevertheless, resistant phenotypes to chemotherapy have been described in protozoan parasites, associated with cross-resistance to other chemically unrelated drugs. Multidrug resistance is multifactorial, involving: (i) drug entry, (ii) activation, (iii) metabolism and (iv) efflux pathways. In this context, ABC transporters, initially discovered in resistant tumor cells, have drawn attention in protozoan parasites, owing to their ability to decrease drug accumulation, thus mitigating their toxic effects. The discovery of these transporters in the Trypanosomatidae family started in the 1990s; however, few members were described and functionally characterized. This review contains a brief history of the main ABC transporters involved in resistance that propelled their investigation in Trypanosoma species, the main efflux modulators, as well as ABC genes described in T. cruzi and T. brucei according to the nomenclature HUGO. We hope to convey the importance that ABC transporters play in parasite physiology and chemotherapy resistance.

Targeting Glycoproteins as a therapeutic strategy for diabetes mellitus and its complications

OBJECTIVES

Glycoproteins are organic compounds formed from proteins and carbohydrates, which are found in many parts of the living systems including the cell membranes. Furthermore, impaired metabolism of glycoprotein components plays the main role in the pathogenesis of diabetes mellitus. The aim of this study is to investigate the influence of glycoprotein levels in the treatment of diabetes mellitus.

METHODS

All relevant papers in the English language were compiled by searching electronic databases, including Scopus, PubMed and Cochrane library. The keywords of glycoprotein, diabetes mellitus, glycan, glycosylation, and inhibitor were searched until January 2019.

RESULTS

Glycoproteins are pivotal elements in the regulation of cell proliferation, growth, maturation and signaling pathways. Moreover, they are involved in drug binding, drug transportation, efflux of chemicals and stability of therapeutic proteins. These functions, structure, composition, linkages, biosynthesis, significance and biological effects are discussed as related to their use as a therapeutic strategy for the treatment of diabetes mellitus and its complications.

CONCLUSIONS

The findings revealed several chemical and natural compounds have significant beneficial effects on glycoprotein metabolism. The comprehension of glycoprotein structure and functions are very essential and inevitable to enhance the knowledge of glycoengineering for glycoprotein-based therapeutics as may be required for the treatment of diabetes mellitus and its associated complications. Graphical abstract.

P-glycoprotein overexpression in bone marrow-derived multipotent stromal cells decreases the risk of steroid-induced osteonecrosis in the femoral head

P‐glycoprotein (P‐gp) plays a role in steroid‐induced osteonecrosis of the femoral head (ONFH), but the underlying mechanism remains unknown. We hypothesized that P‐gp overexpression can prevent ONFH by regulating bone marrow–derived multipotent stromal cell (BMSC) adipogenesis and osteogenesis. BMSCs from Sprague–Dawley rats were transfected with green fluorescent protein (GFP) or the multidrug resistance gene 1 (MDR1) encoding GFP and P‐gp. Dexamethasone was used to induce BMSC differentiation. Adipogenesis was determined by measuring peroxisome proliferator‐activated receptor (PPAR‐γ) expression and the triglyceride level. Osteogenesis was determined by measuring runt‐related transcription factor 2 (Runx2) expression and alkaline phosphatase activity. For in vivo experiments, rats were injected with saline, BMSCs expressing GFP (GFP‐BMSCs) or BMSCs expressing GFP‐P‐gp (MDR1‐GFP‐BMSCs). After dexamethasone induction, adipogenesis was determined by measuring PPAR‐γ expression and fatty marrow, whereas osteogenesis was detected by measuring Runx2 expression, trabecular parameters and the mineral apposition rate, followed by evaluation of the incidence of ONFH. Overexpression of P‐gp in BMSCs resulted in markedly decreased expression of adipogenic markers and increased expression of osteogenic markers. Compared with rats injected with saline, rats injected with GFP‐BMSCs showed reduced ONFH, and the injected GFP‐positive BMSCs attached to trabecular surfaces and exhibited an osteoblast‐like morphology. Compared with the rats injected with BMSCs expressing GFP alone, rats injected with BMSCs overexpressing GFP and P‐gp showed lower adipocytic variables, higher osteogenic variables and lower incidence of ONFH. Overexpression of P‐gp inhibited BMSC adipogenesis and promoted osteogenesis, which reduced the incidence of steroid‐induced ONFH.

New fluphenazine analogues as inhibitors of P-glycoprotein in human lymphocyte cultures

Aim of the study

To evaluate the inhibitory effect of 17 new analogues of FPh on the Pgp transport function, by estimation of the rhodamine 123 (Rod-123) accumulation inside cultured lymphocytes.

Material and methods

Lymphocyte were cultured in the presence of a lectin (PHA; 2%, v/v), incubated with benzo[α]pyrene (B[α]P; 7.5 µM, 48 h) to induce genotoxic damage and to increase Pgp expression in the cells. Lymphocytes cultured without the tested compounds were considered as controls.

Results

It was established that 10 analogues of FPh, among 17 tested, significantly increased Rod-123 accumulation in lymphocytes at the concentration of 10 µM. As compared to the control cultures the Pgp transport function was the most strongly inhibited by 1a, 1b, 1d, 3f, 3h and 3i analogues (approximately by 25%).

Conclusions

FPh analogues 1a, 1b, 1d, 3f, 3h and 3i should be further studied as promising candidates for adjuvant cancer chemotherapeutics.

Tacrolimus, a calcineurin inhibitor, overcomes treatment unresponsiveness mediated by P-glycoprotein on lymphocytes in refractory rheumatoid arthritis

OBJECTIVE

Tacrolimus, a calcineurin inhibitor, is used for treatment of rheumatoid arthritis (RA). It also inhibits functions of P-glycoprotein, which is involved in drug resistance. We examined the mechanisms of early response to 2-week tacrolimus treatment in patients with RA.

METHODS

One hundred thirteen patients with refractory RA despite at least 3 antirheumatic agents, including methotrexate, were treated with tacrolimus (1.5-3 mg/day) and the response was assessed at 2 weeks. Expression of the multidrug resistance (MDR-1) gene and P-glycoprotein was assessed in peripheral blood mononuclear cells (PBMC) collected from 113 patients and 40 healthy subjects. The drug exclusion function by the P-glycoprotein was measured by the residual amount of intracellular tritium-labeled dexamethasone cell/medium ratio (C/M ratio).

RESULTS

The disease activity of enrolled patients was 5.8 +/- 1.2 (mean +/- SD) by DAS28 erythrocyte sedimentation rate. A good response to tacrolimus was noted at 2 weeks in 22 of 113 patients. At baseline, PBMC of patients with RA showed upregulated expression of MDR-1 gene and P-glycoprotein and low C/M ratio. The response to tacrolimus correlated with P-glycoprotein expression and C/M ratio. A significant improvement in C/M ratio was noted after 2 weeks of treatment. The C/M ratio correlated significantly with P-glycoprotein expression on CD4+ lymphocytes.

CONCLUSION

Early efficacy of tacrolimus treatment depended on its inhibitory effect on the drug exclusion function of P-glycoprotein, leading to restoration of intracellular therapeutic levels of corticosteroids and clinical improvement. Evaluation of P-glycoprotein expression on lymphocytes is potentially useful for predicting the response to RA treatment.

Energy allocations to xenobiotic transport and biotransformation reactions in rainbow trout (Oncorhynchus mykiss) during energy intake restriction

Limited energy intake may result in the down-regulation of cellular defense mechanisms, or if maintained, result in trade-offs with other physiological systems. To examine this, juvenile rainbow trout (Oncorhynchus mykiss) were fed full-rations (1.17% body weight [BW]/day), half-rations (0.59% [BW]/day), or fasted for 9 weeks followed by refeeding at full-rations. BW and liver somatic index (LSI), P-glycoprotein (P-gp), ethoxyresorufin-o-deethylase (EROD), and glutathione-S-transferase (GST) activities were measured to determine if they are maintained under limited resources. P-gp and EROD activities were maintained at baseline values in ration-restricted (P-gp: 119+/-29 pg R123/min/mg cells, EROD: 0.58+/-0.17 nmol/min/mg protein) and fasted fish (P-gp: 120+/-14 pg R123/min/mg cells, EROD: 0.47+/-0.14 nmol/min/mg protein), suggesting they may be prioritized systems during fasting. GST activity was attenuated within 6 weeks of fasting (34% decrease from control), but recoverable to baseline values after refeeding. Changes in BW and LSI of calorie-restricted (BW: 16% decrease from control; LSI: 33% decrease from baseline value) and fasted trout (BW: 38% decrease from control; LSI: 44% decrease from baseline value) suggest that resources were mobilized from body stores partly to support these systems. Condition indices and defense activities in groups also varied over time, suggesting that environmental temperature may modulate these parameters.

Modulation of human cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp) in Caco-2 cell monolayers by selected commercial-source milk thistle and goldenseal products

In this study, we used an in vitro Caco-2 cell monolayer model to evaluate aqueous extracts of commercial-source goldenseal (Hydrastis canadensis) and milk thistle (Silybum marianum) capsule formulations, their marker phytochemicals (berberine and silibinin, respectively), as well as dillapiol, vinblastine, and the HIV protease inhibitor saquinavir for their ability to modulate CYP3A4 and ABCB1 expression after short-term exposure (48 h). Both upregulation and downregulation of CYP3A4 expression was observed with extracts of varying concentrations of the two natural health products (NHPs). CYP3A4 was highly responsive in our system, showing a strong dose-dependent modulation by the CYP3A4 inhibitor dillapiol (upregulation) and the milk thistle flavonolignan silibinin (downregulation). ABCB1 was largely unresponsive in this cellular model and appears to be of little value as a biomarker under our experimental conditions. Therefore, the modulation of CYP3A4 gene expression can serve as an important marker for the in vitro assessment of NHP-drug interactions.

In vitro evaluation of human cytochrome P450 and P-glycoprotein-mediated metabolism of some phytochemicals in extracts and formulations of African potato

African potato (Hypoxis hemerocallidea, AP) is a traditional herbal medicine widely used as an immune booster and also for the treatment of various ailments such as urinary diseases, prostrate hypertrophy and cancer. Amongst the chemical components contained in AP, the norlignan glycoside, hypoxoside (HYP) is purported to be the most important phytochemical in terms of AP's medicinal value. Additional constituents in AP include the sterols, beta-sitosterol (BSS), stigmasterol (STG), and the stanol, stigmastanol (STN). The potential of extracts of AP, AP formulations as well as HYP, its aglycone rooperol (ROP) and the sterols to inhibit in vitro metabolism of drug marker substrates by human cytochrome P450 (CYP) enzymes such as CYP3A4, 3A5 and CYP19 were investigated. Samples were also assessed for their effect on drug transport proteins such as P-glycoprotein (P-gp). The effects on CYP-mediated metabolism were studied by fluorometric microtitre plate assay. The potential interaction with P-gp was investigated by measuring the efflux of the fluorescent dye rhodamine 123 (Rh 123) in the CaCo-2 (colon carcinoma) cell line. Various extracts of AP, AP formulations, only STG and the norlignans, in particular the aglycone ROP, exhibited inhibitory effects on CYP3A4-, 3A5- and 19-mediated metabolism. The extracts and the formulations that contained a significant amount of HYP showed high induction of P-gp compared to the positive control, ritonavir. Whilst extrapolation of the current in vitro findings to clinical effects may well be considered speculative, these in vitro data should be heeded as a signal of possible in vivo interactions. Appropriate measures are therefore necessary to explore the possibility of such in vitro-in vivo correlations.

Expression of multidrug resistance P-glycoprotein on lymphocytes from nephrotic children treated with cyclosporine A and ACE-inhibitor

The aim of this work was to determine the expression of P-glycoprotein (P-gp) on peripheral lymphocytes (CD3) in children with steroid-dependent nephrotic syndrome (SDNS) during cyclosporine A (CyA) and ACE-inhibitor (ACE-I) treatment. The study group (I) consisted of 20 children with SDNS aged 5-18 years, with a subsequent proteinuria relapse at the time of prednisone dose reduction. All nephrotic syndrome (NS) children were examined three times: A--at proteinuria relapse, before CyA treatment; B--after 3 months; C--after 12 months of CyA administration. The control group (II) consisted of 20 healthy children. CD3/P-gp was measured using a flow cytometry assay. The serum CyA level was assessed by means of the immunofluorescence method. The expression of CD3/P-gp in NS relapse, prior to CyA+ACE-I administration, was much higher (median 9.15%, range 1.50-13.50%) when compared to healthy controls (median 1.20%, range 0.30-5.70%). The absolute number of CD3/P-gp in this examination was almost five times higher when compared to healthy controls (p<0.01). After 3 months of CyA+ACE-I therapy, the expression of CD3/P-gp decreased dramatically and was similar to the controls. Similar results were obtained after 12 months of treatment. A strong negative correlation was found between CD3/P-gp and serum CyA concentration in both examinations (r=-0.624, p<0.01; r=-0.464, p<0.01). We conclude that the results of our studies indicate that CyA+ACE-I in SDNS inhibits the expression of P-gp. CyA is an alternative therapy that may lead to the optimization of glucocorticoid (GC) doses, thus, reducing the risk that is associated with the treatment.

Functional studies on the activity of efflux transporters in an ex vivo model with chicken splenocytes and evaluation of selected fluoroquinolones in this model

The efflux proteins P-glycoprotein (P-gp), BCRP and members of the MRP-family (MRPs) are increasingly recognized as determinants of the absorption, tissue distribution and excretion of numerous drugs. A widely applied in vitro screening method, to assess the effect of these efflux transporters in transmembrane transport of drugs is based on the use of peripheral blood mononuclear cells (PBMC), in which the efflux of fluorescent dye Rhodamine 123 (Rh-123) can be easily measured. In avian species, the isolation of PBMCs is compromised by the presence of thrombocytes having approximately the same size. As an alternative, we validated the use of isolated splenocytes to assess Rhodamine 123 transport in the presence and absence of specific inhibitors for P-gp, MRPs and BCRP. Rh-123 efflux was concentration-dependent with the percentage of efflux that decreased with increasing concentrations. P-gp inhibitors, PSC833 and GF120918, significantly inhibit Rh-123 efflux, whereas inhibitors for MRPs and BCRP, MK571 and Ko-143, respectively, have a limited inhibitory effect. However, the effect of GF120918 was more pronounced as compared to PSC833, suggesting an additional role for BCRP next to P-gp in Rh-123 efflux. Moreover, fluoroquinolones were selected to test the applicability of the described model. None of these fluoroquinolones significantly inhibit P-gp function at concentrations up to 50 microM, with exception of danofloxacin and danofloxacin mesylate that were found to reduce Rh-123 efflux by approximately 15%.

Induction of P-glycoprotein in lymphocytes by carbamazepine and rifampicin: the role of nuclear hormone response elements

AIMS

Carbamazepine (CBZ) is an inducer of cytochrome P450 enzymes, which have been implicated in many drug interactions. However, for immunosuppressant and anti-HIV drugs, whose main site of action is the lymphocyte, induction of P-glycoprotein (Pgp) may also be important. In this study, we have investigated whether CBZ acts as an inducer of Pgp in lymphocytes.

METHODS

Pgp expression was assessed by flow cytometry and real-time reverse transcriptase-polymerase chain reaction using lymphocytes from four healthy subjects after incubation with therapeutic concentrations of CBZ, using rifampicin as a positive control. Binding to DR-4 elements in the MDR1 promoter was assessed by electrophoretic mobility shift assay (EMSA) and a luciferase-reporter construct.

RESULTS

CBZ increased MDR1 mRNA expression at 6 h by 3.7-fold [95% confidence interval (CI) 0, 7.6) when compared with controls. CBZ increased lymphocyte Pgp expression at 72 h by 7.6-fold (95% CI 2.1, 13.2) over control values. EMSA revealed a 2.1-fold (95% CI 1.5, 2.7) increased binding to the DR-4 element of CBZ when compared with control values. Activation of the DR-4 element was confirmed using reporter constructs. Rifampicin also had similar effects in all experiments.

CONCLUSIONS

Carbamazepine induces Pgp in a manner comparable to rifampicin, by increasing binding to the DR4 element. This has implications for interactions involving drugs whose site of action is the lymphocyte.

MDR1 G1199A polymorphism alters permeability of HIV protease inhibitors across P-glycoprotein-expressing epithelial cells

OBJECTIVE

To evaluate the impact of the human multidrug resistance gene (MDR1) G1199A polymorphism (amino acid change Ser400Asn) on P-glycoprotein (P-gp)-dependent transepithelial permeability and uptake kinetics of HIV protease inhibitors (PI), by using recombinant epithelial cells expressing wild-type MDR1 (MDR1wt) or the G1199A variant (MDR1(1199A)).

METHODS

Using a recombinant expression system developed previously, the transepithelial permeability and uptake kinetic parameters of five PI, amprenavir, indinavir, lopinavir, ritonavir, and saquinavir were estimated across polarized epithelial cells.

RESULTS

For all PI, the transepithelial permeability ratio (basolateral-to-apical transport divided by apical-to-basolateral transport) was significantly greater in MDR1(1199A) cells than MDR1wt cells: amprenavir (1.7-fold), indinavir (1.8-fold), lopinavir (1.5-fold), ritonavir (2.8-fold), and saquinavir (2.1-fold). However, the impact of G1199A on P-gp activity appeared to primarily influence drug permeability in the apical-to-basolateral direction. Kinetic analysis of ritonavir and saquinavir uptake by MDR1wt- and MDR1(1199A)-expressing cells showed that Vmax was similar, while uptake Km was significantly higher in cells expressing the G1199A variant suggesting that alterations in P-gp-dependent efflux mediated by G1199A were due to changes in transporter affinity.

CONCLUSIONS

Alterations in transepithelial permeability of HIV PI due to the G1199A polymorphism may impact oral bioavailability of PI and penetration into cells and tissues of the lymphoid and central nervous systems.

Discrimination of live and early apoptotic mononuclear cells by the fluorescent SYTO 16 vital dye

Accurate detection of apoptotic cells is important for the determination of cell viability. The aim of this study was to compare the sensitivity of the cell permeant SYTO 16 fluorescent dye for detecting early apoptotic mononuclear cells (MNCs) in normal donor blood with other apoptosis assays [i.e. Annexin-V, light scatter/7-amino-actinomycin-D (7-AAD) and chloromethyl-X-rosamine (CMXRos)] and to identify critical parameters for optimal SYTO 16 staining. Apoptosis was induced in normal human leukocytes from adult peripheral blood or cord blood, or the Jurkat T-lymphocytic cell line and assessed by fluorescence microscopy and flow cytometry. Dual labelling showed that SYTO 16 detected more apoptotic MNCs compared to Annexin-V. SYTO 16 staining intensity was consistent with the light scatter profiles expected of live, apoptotic and necrotic MNCs and was more objective than light scatter/7-AAD. CMXRos staining required considerable care and may not be a robust marker of apoptotic primary MNCs. For SYTO 16 flow cytometric analysis, the optimal conditions for staining 1x10(6) leukocytes were 4 nM SYTO 16 in the presence of 30 muM verapamil for 25-45 min at 37 degrees C in media containing calcium/magnesium supplemented with protein. A P-glycoprotein inhibitor, such as verapamil, and calcium/magnesium are essential for optimal loading of SYTO 16 into live MNCs and discrimination of apoptotic MNCs in normal blood samples. SYTO 16 is a sensitive, simple, inexpensive 'live cell' method for the discrimination of live, apoptotic and necrotic normal blood MNCs and is more sensitive for detecting apoptosis in these cells than Annexin-V or light scatter/7-AAD.

Selamectin is a potent substrate and inhibitor of human and canine P-glycoprotein

The transport of the antiparasitic agents, ivermectin, selamectin and moxidectin was studied in human intestinal epithelial cell monolayers (Caco-2) and canine peripheral blood lymphocytes (PBL). Both models expressed the mdr1-coded 170 kDa ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp). Fluxes of the P-gp substrate rhodamine-123 (Rh-123) across Caco-2 monolayers showed that ivermectin and selamectin acted as potent P-gp inhibitors with IC50 values of 0.1 microm. In contrast, moxidectin was a weaker P-gp inhibitor with an IC50 of 10 microm. The transport of radiolabelled ivermectin, selamectin and moxidectin through Caco-2 monolayers showed that ivermectin, selamectin and moxidectin were P-gp substrates with secretory/absorptive ratios of 7.5, 4.7 and 2.6 respectively. Secretory transport of [3H]-ivermectin and [3H]-selamectin was blocked by the P-gp inhibitor, verapamil. Ivermectin and selamectin inhibited the efflux of Rh-123 from PBL and the concentration of inhibition was similar to that of verapamil. In contrast, moxidectin did not have a significant effect on Rh-123 efflux from PBL. The data suggest that ivermectin and selamectin are potent P-gp substrates, while moxidectin is a weak one.

Fexofenadine does not affect omeprazole pharmacokinetics: both are putative P-glycoprotein substrates

An in vitro study has recently suggested that a proton pump inhibitor, omeprazole, is a modest substrate of P-glycoprotein. Several studies have shown P-glycoprotein is involved in the absorption and excretion of fexofenadine. Therefore, we examined the effect of fexofenadine on the pharmacokinetics of omeprazole. Eight healthy volunteers participated in this study. They received a single oral dose of 40 mg omeprazole before and after 60 mg fexofenadine (10 doses over 6 days). Blood samplings were performed up to 8 hr after each dosing. Plasma concentrations of omeprazole and its two metabolites were quantified with high-performance liquid chromatography. In addition, the effect of fexofenadine on P-glycoprotein function was examined by flow cytometry using rhodamine 123 and CD56-positive lymphocytes. Comparison of the pharmacokinetic parameters of omeprazole before and after fexofenadine revealed that there were no differences in peak concentration, time to peak concentration, area under the time concentration curve up to 8 hr, and elimination half-life. There were also no differences in these pharmacokinetic parameters for the two metabolites of omeprazole. Flow cytometric analysis revealed that fexofenadine did not inhibit the efflux of rhodamine 123. This study indicated that there was probably no drug interaction between omeprazole and fexofenadine, which might be due to less contribution of P-glycoprotein to omeprazole absorption, insufficient inhibitory effect of fexofenadine on P-glycoprotein, or the involvement of other transporters such as organic anion transporting polypeptides.

Cholesterol modulates P-glycoprotein activity in human peripheral blood mononuclear cells

P-glycoprotein (P-gp) is expressed in a wide range of cell types including peripheral blood mononuclear cells (PBMCs) where it may restrict intracellular accumulation of substrates like antineoplastic agents, HIV protease inhibitors, or rhodamine123. P-gp is known to be located in membrane microdomains, whose structure and function are susceptible to cholesterol alterations. This study evaluated the effect of cholesterol alteration in human PBMCs on P-gp activity. Whereas cholesterol depletion had no effect, cholesterol repletion of depleted cells significantly decreased intracellular rhodamine123 concentrations in lymphocytes to 32.2%+/-2.7 (p<0.001) and to 41.9%+/-3.5 (p<0.001) in monocytes. After cholesterol saturation of native cells intracellular rhodamine123 fluorescence decreased to 12.4%+/-1.6 (p<0.001) in lymphocytes and 12.9%+/-3.5 (p<0.001) in monocytes. These data demonstrate that elevated cellular cholesterol levels can markedly increase P-gp activity in human PBMCs.

Whole-blood cultures from renal-transplant patients stimulated ex vivo show that the effects of cyclosporine on lymphocyte proliferation are related to P-glycoprotein expression

BACKGROUND

Cyclosporine (CsA) is a substrate for the MDR-1 gene product P-glycoprotein (P-gp). CsA efficacy may be modulated by lymphocyte P-gp expression levels. In this study, CsA inhibition of lymphocyte proliferation in whole-blood cultures ex vivo has been related to (1) lymphocyte P-gp expression and (2) the C3435T polymorphism in the MDR-1 gene, which has been reported to alter P-gp function.

METHODS

In 30 renal-transplant recipients taking CsA monotherapy, P-gp expression was measured by flow cytometry. Whole-blood samples were stimulated with purified protein derivative (PPD) and phytohemagglutinin (PHA). CsA resistance ex vivo was defined as less than 10% reduction in proliferation with either PPD or PHA at 2 hours compared with 0 hours.

RESULTS

CsA resistance was associated with greater P-gp expression using either PPD (median expression, resistant 1.89 vs. sensitive 0.96, P =0.02) or PHA (1.66 vs. 0.96, respectively, P =0.02). Whole-blood CsA levels in resistant and sensitive patients were similar. The C3435T polymorphism did not affect inhibition of proliferation by CsA (P >0.05 for all between genotype group comparisons).

CONCLUSIONS

Our results indicate that lymphocyte P-gp expression determines the degree of inhibition of proliferation by CsA ex vivo; whether this also affects CsA effectiveness in vivo and therefore graft survival requires further study.

Optimizing chemotherapy by measuring reversal of P-glycoprotein activity in plasma membrane vesicles

The appearance of multidrug resistance (MDR) of cancer cells is a major obstacle to successful chemotherapy. Several proteins have been identified that pump chemotherapeutic drugs out of cells, thus bringing about MDR. One representative pump is the P-glycoprotein, whose function can be inhibited by blockers (also known as reversers, modulators or chemosensitizers). In clinical application, many of these blockers are often not effective because they become bound to the plasma of the patients. The extent of plasma binding of the blocker varies in different persons and we have developed a 96-well kit to assay such inter-person differences. The assay uses membrane vesicles isolated from a human lymphoblastoid cell line (CEM Col1000). Uptake of rhodamine into the vesicles was measured with different concentrations of the blockers verapamil and XR9576 in presence of human plasma. The reverser XR9576 is nearly 30 times more effective than the classical blocker verapamil, the relevant K(m) values ranging from 2.66 to 45 nM for XR 9576 and 0.7 to 5.5 microM for verapamil. An even greater difference between these two drugs, nearly 1,000-fold, could be shown also in intact cells by calcein AM uptake experiments.

Effect of calcineurin inhibitor therapy on P-gp expression and function in lymphocytes of renal transplant patients: a preliminary evaluation

Cyclosporine and tacrolimus are substrates and potent inhibitors of the multidrug transporter, P-glycoprotein, in vitro. The authors have investigated the effect of chronic therapy with these and other immunosuppressive drugs on the expression and function of P-glycoprotein in T lymphocytes. Using a P-gp antibody, the authors studied the level of expression of P-gp in CD4 and CD8 T cells over a period of time in renal transplant patients. For comparison, a group of healthy volunteers and patients who did not receive any calcineurin inhibitors but were maintained on mycophenolate mofetil was included. The P-gp expression on lymphocytes from these two groups remained constant (over several months' time). However, patients who were started on tacrolimus or cyclosporine had an initial decline in expression of P-gp on CD4 T cells. Patients who were initiated on calcineurin therapy on day 1 posttransplant also had a decrease in expression of P-gp on CD4 T lymphocytes. This preliminary analysis suggests that the calcineurin inhibitors might be modulating the expression and function of transporters in lymphocytes, thus changing not only the drug concentration but also the apparent efficacy of these drugs. Further understanding and elucidation of such effects would be important in understanding the relationship between pharmacokinetics and pharmacodynamics of these and other drugs, especially for immunosuppressive and anti-AIDS therapy.