Multidrug Resistance Modulators Research Articles

Novel steroid carbamates reverse multidrug-resistance in cancer therapy and show linkage among efficacy, loci of drug action and P-glycoprotein's cellular localization

P-glycoprotein (Pgp) is a major ABC transporter responsible for multidrug-resistance (MDR) in cancer chemotherapy. Pre-clinical MDR modulation studies identified promising chemosensitizers, but none are in the clinic yet. Two novel progesterone-derived carbamates (11-carbamic acid N,N-dibenzyl progesterone ester and 11-carbamic acid N,N-dibutyl progesterone ester) were examined as potential chemosensitizers in the Pgp-expressing human colon cancer line HCT-15, applying the classical MDR-drugs paclitaxel and doxorubicin. The major findings were: (1) Pgp was expressed in the HCT-15 cells in both the cell and the nuclear membranes, (2) at the low dose range of 1–5μM, each new candidate: (i) increased cytotoxicity of doxorubicin (15-fold) and (separately) of paclitaxel (40-fold), (ii) induced an increase in intracellular accumulation, 60% (4h) for doxorubicin and 300% (18h) for paclitaxel, (iii) reduced drug efflux from the cell, 2-fold and 4-fold for doxorubicin and for paclitaxel, respectively. Based on detailed kinetic analysis, using liposomes to model paclitaxel diffusion through cell membranes, efflux slowdown can be attributed to reduction in the rate constant of drug diffusion through Pgp, and not to Pgp blockage. Chemosensitization was consistently-better for paclitaxel (cytosol-operating) than for doxorubicin (nuclear-operating) implying linkage between P-glycoprotein localization and loci of drug action. Mapping intracellular locations of MDR-pumps may assist therapeutic strategies.

Targeting efflux pumps—In vitro investigations with acridone derivatives and identification of a lead molecule for MDR modulation

To search multi drug resistance modulators, acridones carrying hydroxyl amine substituent at N-10 and COOH/Cl at C-4 were investigated for their interactions with the three components of efflux pump viz. P-gp, ATP, and Mg 2+. Experimental and theoretical results indicated that the compounds with COOH group at C-4 interact with P-gp and Mg 2+ while other set of compounds with Cl at C-4 interact with ATP and Mg 2+. Spot assay and R6G influx/efflux assay of compound 3 using Candida albicans showed decrease in the fungal growth and efflux of R6G, respectively, in presence of compound 3 suggesting the suitability of this compound for MDR modulation.

Enhanced antitumor efficacy by Paclitaxel-loaded Pluronic P123/F127 mixed micelles against non-small cell lung cancer based on passive tumor targeting and modulation of drug resistance

The aim of this work was to demonstrate the advantage of using paclitaxel (PTX)-loaded Pluronic P123/F127 mixed micelles (PF-PTX) against non-small cell lung cancer (NSCLC) compared to Taxol. Modulation of multidrug resistance (MDR) by Pluronic mixed micelles was evaluated in lung resistance protein (LRP)-overexpressing human lung adenocarcinoma A-549 cell line. Influence of PF-PTX on in vitro cytotoxicity was determined by MTT assay, while cellular apoptosis was detected by cell nuclei staining and Annexin V-FITC apoptosis detection kit. Cell cycle arrest was also confirmed by flow cytometry. Additionally, in vivo fate and antitumor efficacy of PF-PTX were extensively evaluated in comparison with Taxol. It was demonstrated that PF-PTX had superior anti-proliferation activity against A-549 cells compared with Taxol as measured by IC 50. The enhanced anti-cancer efficacy of PF-PTX was associated with PTX-induced apoptosis and cell arrest in the G 2/M phase. Intracellular ATP depletion and decreased mitochondrial potential caused by Pluronic copolymers were found to be related to modulation of MDR. PF-PTX also exhibited significant advantages in pharmacokinetics and A-549 xenograft tumor model versus Taxol. The PF-PTX formulation achieved 3.0-fold longer mean residence time in circulation, 2.2-fold larger area under the plasma concentration–time curve than Taxol. At 28 days, tumor volume in PF-PTX group was only 31.8% that of the Taxol. Therefore, PF-PTX significantly enhanced the anti-cancer activity of PTX and might be considered a promising drug delivery system to overcome MDR in lung cancer.

Transporter-Mediated Multidrug Resistance and its Modulation by Chinese Medicines and Other Herbal Products

The main cause of failure in cancer drug therapy is the emergence of cellular resistance to drugs. Cancer cells, after exposure to one drug, can become simultaneously insensitive to mechanistically and chemically unrelated drugs, a phenotype known as multidrug resistance (MDR). Although a number of mechanisms have been proposed to mediate MDR, the classical cellular mechanism involves the overexpression of several members of the ATP-binding cassette (ABC) superfamily of transporters, leading to increased efflux and decreased intracellular drug accumulation. Among these, P-glycoprotein (P-gp, ABCB1), MRP1 (ABCC1) and BCRP (ABCG2) are the main transporters conferring MDR. These transporters are frequently detected in recurrent cancer cells or cancer stem cells. To overcome MDR, various studies have been conducted to investigate potential to discover effective MDR modulators from Chinese medicines (CMs) and other herbal products because many of these have been used for centuries without harmful side effects. This review summarizes: i) The contribution of P-gp, MRP1 and BCRP in cancer drug resistance; ii) known mechanisms of action for MDR modulators; iii) commonly used methods for identification and evaluation of novel modulators of transporter-mediated MDR; and iv) the modulating effects of CMs and other natural products on ABC transporters and MDR. The CM and their active components with potent modulating effects on MDR can be considered as promising lead agents for the design of more effective and less toxic drugs to overcome MDR.

The Structure and Functions of P-Glycoprotein

P-glycoprotein (P-gp) is an ATP-driven transmembrane transporter capable of effluxing a wide variety of structurally diverse and functionally unrelated hydrophobic compounds out of the cell. Multidrug resistance (MDR), often associated with the over-expression of P-gp, has been implicated as a major obstacle to effective chemotherapy for cancer, parasitic diseases, AIDS, and other diseases. Drug efflux mediated by P-gp is also involved in decreasing the oral bioavailability of drugs by limiting intestinal absorption. Our appreciation of the structural and functional aspects of P-gp has definitely improved in recent years, benefiting from the deciphering of the structure of some bacterial transporters that paved the way for construction of homology models for more complex transporters. Here, we will review the recent advances in the studies of the structure and functional characteristics of P-gp with the hopes of facilitating rational drug design in developing novel potent MDR modulators.

Reversal of P-gp mediated multidrug resistance in-vitro and in-vivo by FG020318.

Overexpression of P-glycoprotein (P-gp) by tumours results in multidrug resistance (MDR) to structurally and functionally unrelated chemotherapeutic drugs. Combined therapy with MDR-related cytotoxins and MDR modulators is a promising strategy to overcome clinical MDR. This study was performed to explore the MDR reversal activity of a novel compound 2-[4-(2-pyridin-2-yl-vinyl) phenyl]-4,5-bis-(4-N,N-diethylaminophenyl)-1(H)-imidazole (FG020318) in-vitro and in-vivo. Tetrazolium (MTT) assay was used to evaluate the ability of FG020318 to reverse drug resistance in two P-gp-expressing tumour cell lines, KBv200 and MCF-7/adr. Intracellular doxorubicin accumulation was determined by fluorescence spectrophotometry in MCF-7/adr cell line. The effect of FG020318 on P-gp function was demonstrated by rhodamine 123 (Rh123) accumulation in KBv200 cells. KBv200 cell xenograft models were established to study the in-vivo effect of FG020318 on reversing MDR. FG020318 was not cytotoxic by itself against P-gp expressing KBv200 cells and MCF-7/adr cells and their parental drug-sensitive KB cells and MCF-7 cells. FG020318 could significantly increase the sensitivity of MDR cells to antitumour drugs including doxorubicin and vincristine in MCF-7/adr cells and KBv200 cells, respectively. It was much stronger than the positive control verapamil in reversal of MDR. FG020318 also increased the intracellular accumulation of doxorubicin in a concentration-dependent manner in MCF-7/adr cells, but did not affect the accumulation of doxorubicin in drug-sensitive MCF-7 cells. The Rh123 accumulation in resistant KBv200 cells was also increased by the addition of FG020318, but Rh123 accumulation was not affected by FG020318 in drug-sensitive KB cells. FG020318 potentiated the antitumour activity of vincristine to KBv200 xenografts and was an efficacious modulator in-vivo. Our results suggested that FG020318 was a highly potent, efficacious MDR modulator not only in-vitro but also in-vivo. The reversal of drug resistance by FG020318 was probably related to the increased anticancer drug accumulation and its inhibition of P-gp function of MDR tumour cells.

Multidrug resistance modulator interactions with neutral and anionic liposomes: membrane binding affinity and membrane perturbing activity

A variety of cationic lipophilic compounds (modulators) have been found to reverse the multidrug resistance of cancer cells. In order to determine the membrane perturbing efficacy and the binding affinity of such drugs in neutral and anionic liposomes, the leakage of Sulfan blue induced by five modulators bearing different electric charges was quantified using liposomes with and without phosphatidic acid (xEPA=0 and 0.1), at four lipid concentrations. The binding isotherms were drawn up using the indirect method based on the dependency of the leakage rate on the modulator and the lipid concentrations. Upon inclusion of negatively charged lipids in the liposomes: (i) the binding of cationic drugs was favoured, except in a case where modulator aggregation occurred in the lipid phase; (ii) the drugs with a net electric charge greater than 1.1 displayed a greater enhancement in their potency to produce membrane perturbation; and (iii) the EPA effect on membrane permeation was due mainly to that on membrane perturbation (>or=50%) and, to a lesser extent, to that on the binding affinity (<or=50%). The present study provides evidence that drug-membrane interactions are the result of a complex interplay between the structural and electrical characteristics of the drugs and those of the membranes.

Synthesis of highly functionalized barbituric acids and study of their interactions with p-glycoprotein and Mg 2+ – Potential candidates for multi drug resistance modulation

A number of barbituric acids with appropriate substituent at C-5 position were synthesized and investigated for their interactions with p-gp and Mg 2+. Compounds 5, 6, 8– 10, 12– 14 and 16 increased the basal activity of p-gp by more than 50% at 0.05 μM concentration. Molecular docking indicate a number of H-bond interactions between these molecules and the amino acid residues of ATP binding site of p-gp. These molecules also showed appreciable interactions with Mg 2+, an important component of efflux pump. All the results of these investigations favor the suitability of barbituric acids toward MDR modulation.

Inhibition of tetramethylpyrazine on P-gp, MRP2, MRP3 and MRP5 in multidrug resistant human hepatocellular carcinoma cells

Some membrane transporters in liver, such as P-glycoprotein, multidrug resistance-associated protein 2 (MRP2), MRP3, and MRP5 can lead to a complex multidrug resistance (MDR) to antineoplastic agents. How to inhibit these proteins is still an issue. Tetramethylpyrazine is a bioactive constituent isolated from the root of Ligusticum chuanxiong Hort, a Chinese herb. Recent studies showed that it can enhance the chemosensitivity effects of a drug on human hepatocellular carcinoma cells, acting as a multidrug resistance modulator. In this study, the reversal effect of TMP on MDR was evaluated and its activity mechanism in vitro was explored. The IC50 value shows that TMP reversed the multidrug resistance of BEL-7402/ADM cells 9.23-fold (P<0.01) at the concentration of 600 microM. The mean fluorescence intensity of ADM in BEL-7402/ADM cells with TMP was found to be 163.78+/-39.5% (P<0.01) versus in BEL-7402/ADM cells without TMP by flow cytometry and 126.73+/-28.72% in BEL-7402/ADM cells with TMP versus in BEL-7402/ADM cells without TMP (P<0.01) by high performance liquid chromatography, respectively. It was also found that the mRNA level of multidrug resistant gene MDR1, MRP2, MRP3 and MRP5 and the level of the proteins they encode were decreased after treatment with TMP, indicating that TMP can effectively reverse the MDR in BEL-7402/ADM cells, and its activity mechanism may be correlated with the down-regulation of expression in these transporters.

Complex formation between metabolic enzymes in tumor cells: Unfolding the MDR1-IDE paradigm

To the Editor We greatly appreciated the recent article by Orina and colleagues ([1][1]) reporting on progress in identifying novel modulators of multidrug resistance (MDR). However, we think that, in addition to profiling ATP-binding cassette (ABC) transporter gene expression toward improved

Purine analogs sensitize the multidrug resistant cell line (NCI-H460/R) to doxorubicin and stimulate the cell growth inhibitory effect of verapamil

The resistant cell line NCI-H460/R and its counterpart NCI-H460 were used to investigate the ability of purine analogs to overcome multidrug resistance (MDR) that seriously limit the efficacy of lung cancer regimens with chemotherapeutic agents. Two purine analogs, sulfinosine (SF) and 8-Cl-cAMP, exerted dose-dependent effects on cell growth in both parental and resistant cell lines. They significantly decreased mdr1 expression in NCI-H460/R cells. Low concentrations (1 microM) of SF and 8-Cl-cAMP in combination with doxorubicin (DOX) exerted synergistic growth inhibition in both cell lines. Pretreatment with SF and 8-Cl-cAMP improved the sensitivity to DOX more than verapamil (VER), the standard modulator of MDR. The increased accumulation of DOX observed after the treatment with SF and 8-Cl-cAMP was consistent with the results obtained with VER. VER stimulated the effect of 8-Cl-cAMP on DOX cytotoxicity and mdr1 expression. Combinations of either SF or 8-Cl-cAMP with VER at clinically acceptable concentrations exhibited synergistic effects on cell growth inhibition in the resistant cell line. SF and 8-Cl-cAMP modulated MDR in NCI-H460/R cells, especially when applied before DOX administration. This feature, together with their ability to reverse MDR, renders the purine analogs (in combination with VER) as potential candidates for improving the clinical activity of existing lung cancer therapeutics.

Structure–Activity Relationships of Tariquidar Analogs as Multidrug Resistance Modulators

The review summarizes the most recent achievements in structure-activity relationship (SAR) studies of tariquidar and its analogs. Tariquidar is one of the most promising representatives of the third generation of multidrug resistance (MDR) modulators created so far. This fact determines the strong interest of different research groups in the development of tariquidar-like structures as selective inhibitors of MDR transporters in resistant human cancer cells. After the discovery of tariquidar, a number of analogs have been synthesized and pharmacologically tested, thus supplying good data for comprehensive analyses of their structure-activity relationships. In the review, the structural and pharmacological data of newly synthesized tariquidar-like compounds are first presented. Next, the main achievements in the SAR studies are described focusing on two main transport proteins: P-glycoprotein and breast cancer resistance protein. The reported results are discussed from the point of view of their significance and importance for future directions in the rational design of effective MDR modulators.

Synthesis and antimultidrug resistance evaluation of icariin and its derivatives

A series of icariin derivatives were synthesized. Their multidrug resistance (MDR) reversal activities were evaluated by MTT assay and the results indicated that the derivatives were the potent modulators of MDR. It was showed that the derivatives significantly increased the intracellular accumulation of ADR in MCF-7/ADR cells compared with drug sensitive MCF-7 cells. The results of bi-directional assay and reverse transcription polymerase chain reaction (RT-PCR) assay showed that the derivatives had high inhibitory activity against P-gp efflux function and significantly down-regulated on the expression of P-gp.

Dynamic Assessment of Mitoxantrone Resistance and Modulation of Multidrug Resistance by Valspodar (PSC833) in Multidrug Resistance Human Cancer Cells

P-glycoprotein (Pgp), a member of the ATP-binding cassette transporter family, is one of the major causes for multidrug resistance (MDR). We report using confocal microscopy to study the roles of Pgp in mediating the efflux of the anticancer agent mitoxantrone and the reversal of MDR by the specific Pgp inhibitor valspodar (PSC833). The net uptake and efflux of mitoxantrone and the effect of PSC833 were quantified and compared in Pgp-expressing human cancer MDA-MB-435 (MDR) cells and in parental wild-type cells. The MDR cells, transduced with the human Pgp-encoding gene MDR1 construct, were approximately 8-fold more resistant to mitoxantrone than the wild-type cells. Mitoxantrone accumulation in the MDR cells was 3-fold lower than that in the wild-type cells. The net uptake of mitoxantrone in the nuclei and cytoplasm of MDR cells was only 58 and 67% of that in the same intracellular compartment of the wild-type cells. Pretreatment with PSC833 increased the accumulation of mitoxantrone in the MDR cells to 85% of that in the wild-type cells. In living animals, the accumulation of mitoxantrone in MDA-MB-435mdr xenograft tumors was 61% of that in the wild-type tumors. Administration of PSC833 to animals before mitoxantrone treatment increased the accumulation of mitoxantrone in the MDR tumors to 94% of that in the wild-type tumors. These studies have added direct in vitro and in vivo visual information on how Pgp processes anticancer compounds and how Pgp inhibitors modulate MDR in resistant cancer cells.

Lathyrane diterpenes from Euphorbia lathyris as modulators of multidrug resistance and their crystal structures

Two lathyrane diterpenes ( 1–2) together with previous ones ( 3–6) were isolated from Euphorbia lathyris. The structures of 1 and 2 were established on the basis of extensive spectroscopic methods, including two-dimensional NMR techniques. Stereo configuration and conformation of 1 and 2 were analyzed by X-ray diffraction for the first time. The configuration of the unique isolathyrol diterpene 2 from the seeds of E. Lathyris, was rectified from S∗ to R∗ at C-5. All compounds were examined as modulators of multidrug resistance on MCF-7/ADM cell lines in vitro. Their structure–activity relationship was discussed from aspect of different skeletons. A class of potential P-gp inhibitors with five different types (jokinol, isolathyrol, epoxylathyrol, 7-hydroxylathyrol, lathyrol) was discovered.

The impact of dose escalation and resistance modulation in older patients with acute myeloid leukaemia and high risk myelodysplastic syndrome: the results of the LRF AML14 trial

The acute myeloid leukaemia (AML)14 trial addressed four therapeutic questions in patients predominantly aged over 60 years with AML and High Risk Myelodysplastic Syndrome: (i) Daunorubicin 50 mg/m(2) vs. 35 mg/m(2); (ii) Cytarabine 200 mg/m(2) vs. 400 mg/m(2) in two courses of DA induction; (iii) for part of the trial, patients allocated Daunorubicin 35 mg/m(2) were also randomized to receive, or not, the multidrug resistance modulator PSC-833 in a 1:1:1 randomization; and (iv) a total of three versus four courses of treatment. A total of 1273 patients were recruited. The response rate was 62% (complete remission 54%, complete remission without platelet/neutrophil recovery 8%); 5-year survival was 12%. No benefits were observed in either dose escalation randomization, or from a fourth course of treatment. There was a trend for inferior response in the PSC-833 arm due to deaths in induction. Multivariable analysis identified cytogenetics, presenting white blood count, age and secondary disease as the main predictors of outcome. Although patients with high Pgp expression and function had worse response and survival, this was not an independent prognostic factor, and was not modified by PSC-833. In conclusion, these four interventions have not improved outcomes in older patients. New agents need to be explored and novel trial designs are required to maximise prospects of achieving timely progress.

P-glycoprotein Versus MRP1 on Transport Kinetics of Cationic Lipophilic Substrates: A Comparative Study Using [99mTc]Sestamibi and [99mTc]Tetrofosmin

The multidrug resistance (MDR) phenotype in cancer is closely related with the overexpression of P-glycoprotein (Pgp) and multidrug resistance protein-1 (MRP1). Although conferring resistance to a similar spectrum of drugs, these proteins present distinct transport mechanisms and have their own substrates. In this work, we compared the functional properties of Pgp and MRP1 in the transport kinetics of two cationic lipophilic tracers, [(99m)Tc]sestamibi and [(99m)Tc]tetrofosmin, in cellular models of resistance. Cellular transport kinetics of both tracers was evaluated in Small-cell lung cancer cell line H69 and in its drug-resistant sublines, H69LX4 and H69AR, overexpressing Pgp and MRP1, respectively. Studies were performed in the absence and in the presence of MDR modulators. Kinetic parameters extracted from time-activity curves were analyzed through receiver-operating characteristics curve analysis. The uptake and the efflux rate of both radiotracers were significantly higher (p < 0.05) in sensitive cells. However, MRP1 was more effective than Pgp in removing tracers from the intracellular medium. The addition of verapamil and PSC833 significantly reduced the efflux rate and restored the accumulation of both tracers in H69LX4 cells. Only verapamil was effective in the inhibition of MRP1; however, the effects were more pronounced with [(99m)Tc]sestamibi, when compared to [(99m)Tc]tetrofosmin. Outward transport of radiotracers by MRP1 was dependent on the intracellular glutathione levels. We concluded that both tracers can detect Pgp- and MRP1-mediated drug resistance, based on transport kinetics; however, MRP1 is more effective than Pgp on outward transport of radiotracers. We postulate that this finding can be useful to distinguish between the two resistance mechanisms.

Jatrophane diterpenes from Euphorbia spp. as modulators of multidrug resistance in cancer therapy

A phytochemical investigation of the aerial parts of Euphorbia spp., considered one of the most common elements of Mediterranean landascape, led to the isolation of a large number of bioactive plant metabolites, belonging to the diterpenes family. Above all, over seventy jatrophane, modified jatrophane, segetane, pepluane, and paraliane diterpenoids, fifty of them reported for the first time, were extracted, purified and characterized from Euphorbia dendroides, Euphorbia characias, Euphorbia peplus, Euphorbia paralias and Euphorbia helioscopia. These compounds showed interesting pharmacological activities. In particular, jatrophanes, modified jatrophanes and lathyranes exhibited a potent inhibitory activity against P-glycoprotein (Pgp), a membrane protein that confers cellular ability to resist lethal doses of certain cytotoxic drugs by pumping them to the outside, thus resulting in a reduced cytotoxicity. Among the others, our chemical survey led to isolation of the most powerful inhibitors of daunomycin-efflux activity isolated to date for this class of inhibitors, named euphodendroidin D and pepluanin A. Their efficiency was found to be at least two-fold higher than the conventional modulator cyclosporin A, taken as a reference. In addition, the isolation of a high number of natural structurally-related analogues allowed us to perform Structure Activity Relationship (SAR) studies, without any chemical modification, which gave information on the key pharmacophoric elements of these new class of promising drugs. A further set of diterpene analogues, very recently isolated from sun spurge, E. helioscopia, individually investigated for their Pgp- and BCRP-inhibiting properties, appeared to be specific inhibitors of Pgp since they showed no significant activity against BCRP, thus resembling to the third-generation class of specific MDR inhibitors.

Perspectives of inhibition of multidrug resistance during cancer chemotherapy, in vitro and in vivo experiments

The development of pharmacological agents able to counteract the mechanisms of multidrug resistance in oncology has remained a major goal for the past ten years. Our purpose was to find multidrug resistance reversal agents less toxic than verapamil among various synthetic compounds: cinnamylidene ketones; 1,4-dihydropyridines; phenothiazines; heat shock 90 inhibitor peptides; betti base derivative of tylosin and among some naturally occurring plant derived jatrophane and lathyrane-type diterpenes. The first part of this thesis presents the inhibition of multidrug resistance through inhibition of the P-glycoprotein efflux pump in various cell lines. In general, the newly identified multidrug resistance modifiers were able to enhance the antiproliferative activity of selected anticancer drugs in a synergistic or additive way in in vitro experiments. The in vitro activity of betti base derivative of tylosin was confirmed by further in vivo efficacy studies in DBA/2 mice. As an alternative way of antitumor effect, apoptosis inductions of resistance modifiers were studied. The substituted dihydropyridine 13 was the most promising apoptosis inducer on mouse lymphoma cells. Human cytomegalovirus was used in a modified in vitro model for characterizing lathyrane compounds with antipromotion effect on human lung cancer cells. All the six macrocyclic lathyrane-type diterpenoids reduced the promotion in vitro , except latilagascene D, decreased IE-antigen expression of cytomegalovirus to prevent progression of tumor malignancy.

Synthesis and biological evaluation of a small molecule library of 3rd generation multidrug resistance modulators

The development of new modulators possessing high efficacy, low toxicity and high selectivity is a pivotal approach to overcoming P-glycoprotein (P-gp) mediated multidrug resistance (MDR) in tumour cells. In this study 39 compounds are presented which have been synthesized and pharmacologically investigated in our laboratory. Similarly to the potent 3rd generation MDR modulator tariquidar (XR9576) the compounds contain a tetrahydroisoquinoline–ethyl-phenylamine substructure that, in contrast to XR9576, is connected to a smaller hydrophobic part, thus leading to molecules of lower molecular weight. The connection between the tetrahydroisoquinoline–ethyl-phenylamine substructure and the hydrophobic part was achieved through four different types of linkers: amide, urea, amide-ether and amide-styryl. A number of structural modifications in the hydrophobic part were created. The calcein AM assay served as test system to determine the P-gp transport inhibitory potencies of the compounds. For the amide linker derivatives a structure–activity relationship analysis was performed outlining which structural modifications contributed to the inhibitory potency. The compounds containing a bicyclic hydrophobic part with a particular substituent in a specific orientation were identified as the most potent amide derivatives. Among the urea derivatives the compounds with highest inhibitory potency possessed an ortho-nitro substituent. The conformational analysis revealed that this position enables the formation of a hydrogen bond to the urea linker thus stabilizing the conformation. Regarding the amide-styryl derivatives the elongation of the amide linker seemed to be most decisive for the observed increase in activity. The most promising candidate in the whole library possess an amide-ether linker and an ortho-nitro substituent in the hydrophobic part. This compound inhibites P-gp slightly less than tariquidar and can serve as a lead structure for new potent P-gp modulators.