Circumvention of multidrug-resistance in P388 cells is associated with a rise in the cellular content of phosphatidylcholine

Phosphatidylcholine-Derived Lipid Mediators: The Crosstalk Between Cancer Cells and Immune Cells

To become resistant, cancer cells need to activate and maintain molecular defense mechanisms that depend on an energy trade-off between resistance and essential functions. Metabolic reprogramming has been shown to fuel cell growth and contribute to cancer drug resistance. Recently, changes in lipid metabolism have emerged as an important driver of resistance to anticancer agents. In this review, we highlight the role of choline metabolism with a focus on the phosphatidylcholine cycle in the regulation of resistance to therapy. We analyze the contribution of phosphatidylcholine and its metabolites to intracellular processes of cancer cells, both as the major cell membrane constituents and source of energy. We further extended our discussion about the role of phosphatidylcholine-derived lipid mediators in cellular communication between cancer and immune cells within the tumor microenvironment, as well as their pivotal role in the immune regulation of therapeutic failure. Changes in phosphatidylcholine metabolism are part of an adaptive program activated in response to stress conditions that contribute to cancer therapy resistance and open therapeutic opportunities for treating drug-resistant cancers.

Sphingolipid abnormalities in cancer multidrug resistance: Chicken or egg?

The cancer multidrug resistance (MDR) phenotype encompasses a myriad of molecular, genetic and cellular alterations resulting from progressive oncogenic transformation and selection. Drug efflux transporters, in particular the MDR P-glycoprotein ABCB1, play an important role in MDR but cannot confer the complete phenotype alone indicating parallel alterations are prerequisite. Sphingolipids are essential constituents of lipid raft domains and directly participate in functionalization of transmembrane proteins, including providing an optimal lipid microenvironment for multidrug transporters, and are also perturbed in cancer. Here we postulate that increased sphingomyelin content, developing early in some cancers, recruits and functionalizes plasma membrane ABCB1 conferring a state of partial MDR, which is completed by glycosphingolipid disturbance and the appearance of intracellular vesicular ABCB1. In this review, the independent and interdependent roles of sphingolipid alterations and ABCB1 upregulation during the transformation process and resultant conferment of partial and complete MDR phenotypes are discussed.

Biophysics of cell membrane lipids in cancer drug resistance: Implications for drug transport and drug delivery with nanoparticles

In this review, we focus on the biophysics of cell membrane lipids, particularly when cancers develop acquired drug resistance, and how biophysical changes in resistant cell membrane influence drug transport and nanoparticle-mediated drug delivery. Recent advances in membrane lipid research show the varied roles of lipids in regulating membrane P-glycoprotein function, membrane trafficking, apoptotic pathways, drug transport, and endocytic functions, particularly endocytosis, the primary mechanism of cellular uptake of nanoparticle-based drug delivery systems. Since acquired drug resistance alters lipid biosynthesis, understanding the role of lipids in cell membrane biophysics and its effect on drug transport is critical for developing effective therapeutic and drug delivery approaches to overcome drug resistance. Here we discuss novel strategies for (a) modulating the biophysical properties of membrane lipids of resistant cells to facilitate drug transport and regain endocytic function and (b) developing effective nanoparticles based on their biophysical interactions with membrane lipids to enhance drug delivery and overcome drug resistance.

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.

Lipid rafts: dream or reality for cholesterol transporters?

As a key constituent of the cell membranes, cholesterol is an endogenous component of mammalian cells of primary importance, and is thus subjected to highly regulated homeostasis at the cellular level as well as at the level of the whole body. This regulation requires adapted mechanisms favoring the handling of cholesterol in aqueous compartments, as well as its transfer into or out of membranes, involving membrane proteins. A membrane exhibits functional properties largely depending on its lipid composition and on its structural organization, which very often involves cholesterol-rich microdomains. Then there is the appealing possibility that cholesterol may regulate its own transmembrane transport at a purely functional level, independently of any transcriptional regulation based on cholesterol-sensitive nuclear factors controling the expression level of lipid transport proteins. Indeed, the main cholesterol "transporters" presently believed to mediate for instance the intestinal absorption of cholesterol, that are SR-BI, NPC1L1, ABCA1, ABCG1, ABCG5/G8 and even P-glycoprotein, all present privileged functional relationships with membrane cholesterol-containing microdomains. In particular, they all more or less clearly induce membrane disorganization, supposed to facilitate cholesterol exchanges with the close aqueous medium. The actual lipid substrates handled by these transporters are not yet unambiguously determined, but they likely concern the components of membrane microdomains. Conversely, raft alterations may provide specific modulations of the transporter activities, as well as they can induce indirect effects via local perturbations of the membrane. Finally, these cholesterol transporters undergo regulated intracellular trafficking, with presumably some relationships to rafts which remain to be clarified.

The effect of fatty acids and analogues upon intracellular levels of doxorubicin in cells displaying P-glycoprotein mediated multidrug resistance

Multidrug resistance mediated by overexpression of P-glycoprotein (P-gp) is a major obstacle in the chemotherapeutic management of cancer. The objectives of the current work were to examine if fatty acids affect the intracellular transport and dynamics of doxorubicin in drug-resistant cancer cell lines, and to assess if such effects were mediated through modulation of P-gp efflux pump activity. Among the range of fatty acids tested in this study, eicosapentaenoic acid diester (EPADI) increased doxorubicin accumulation [A] to 137% and retention [R] to 212% in doxorubicin-resistant MCF-7/ADR breast carcinoma cells, and [A] to 147% and [R] to 163% in vinblastine-resistant KBVI nasopharyngeal carcinoma cells. Consistent with EPADI-induced increases in intracellular doxorubicin concentrations, EPADI (10 microg/ml) sensitized MCF-7/ADR cells to the cytotoxic effects of doxorubicin (1 microg/ml) as assessed by MTT assay (viability < 50% of control), while EPADI itself displayed no cytotoxicity. The combination of EPADI (10 microg/ml) with verapamil (1 microM) resulted in a considerable increase in the [A] and [R] of the model P-gp substrate rhodamine-123 within drug-resistant cells compared to when either agent were used alone. KBV1 cells treated with combination of EPADI (10 microg/ml) and verapamil (1 microM) achieved 160% and 1120% greater [A] and [R] of rhodamine-123, respectively, compared to untreated cells. The P-gp modulatory effects of EPADI either alone, or as part of a combination with more potent inhibitors, should be further investigated.

Analysis of the tangled relationships between P-glycoprotein-mediated multidrug resistance and the lipid phase of the cell membrane

P-glycoprotein (Pgp), the so-called multidrug transporter, is a plasma membrane glycoprotein often involved in the resistance of cancer cells towards multiple anticancer agents in the multidrug-resistant (MDR) phenotype. It has long been recognized that the lipid phase of the plasma membrane plays an important role with respect to multidrug resistance and Pgp because: the compounds involved in the MDR phenotype are hydrophobic and diffuse passively through the membrane; Pgp domains involved in drug binding are located within the putative transmembrane segments; Pgp activity is highly sensitive to its lipid environment; and Pgp may be involved in lipid trafficking and metabolism. Unraveling the different roles played by the membrane lipid phase in MDR is relevant, not only to the evaluation of the precise role of Pgp, but also to the understanding of the mechanism of action and function of Pgp. With this aim, I review the data from different fields (cancer research, medicinal chemistry, membrane biophysics, pharmaceutical research) concerning drug-membrane, as well as Pgp-membrane, interactions. It is emphasized that the lipid phase of the membrane cannot be overlooked while investigating the MDR phenotype. Taking into account these aspects should be useful in the search of ways to obviate MDR and could also be relevant to the study of other multidrug transporters.

Tamoxifen activates cellular phospholipase C and D and elicits protein kinase C translocation

The antiestrogen tamoxifen is widely used for endocrine therapy of breast cancer; however, the mechanisms of estrogen receptor-independent interactions of tamoxifen remain ill defined. Here we examine the effect of tamoxifen on the initial steps of cell signal transduction. To this end, phospholipid metabolism and protein kinase C (PKC) translocation were assessed in CCD986SK human mammary fibroblasts treated with tamoxifen. The addition of tamoxifen resulted in dose-dependent and time-dependent increases in the cellular second messengers phosphatidate (PA) and diacylglycerol (DG). On addition of ethanol to the medium, tamoxifen induced the formation of phosphatidylethanol, demonstrating that tamoxifen activates phospholipase D (PLD). Cellular DG also increased in the presence of ethanol, showing that tamoxifen also activates phospholipase C (PLC). In cells prelabeled with choline and ethanolamine, tamoxifen caused increases in choline, phosphorylcholine, ethanolamine and phosphorylethanolamine. Structure-activity relationship studies for activation of PLD revealed that tamoxifen was the most effective, whereas 4-hydroxy tamoxifen was nearly devoid of activity. Phorbol diesters also activated PLD, but estrogen had no influence. Pretreatment of cells with phorbol dibutyrate (PKC down-regulation protocol) blocked phorbol diester- and tamoxifen-induced PLD activity. Exposure of cells to the PKC inhibitor GF 109203X diminished tamoxifen-induced PLD activity. Addition of tamoxifen to cultures elicited selective membrane association of PKC epsilon. We conclude that tamoxifen exerts considerable extra-nuclear influence at the transmembrane signaling level. These events may contribute to effects beyond the scope of estrogen receptor-dependent actions.

Synergistic effects of tamoxifen and cepharanthine for circumventing the multidrug resistance

We examined the synergistic effects of tamoxifen (TAM) and cepharanthine (CEP) for doxorubicin (DOX) sensitivity using MTT assay. The augmentation of DOX sensitivity by TAM and CEP was significantly correlated with the P-glycoprotein expression. The cytotoxic effect of DOX with TAM and CEP was significantly higher than that of DOX alone, or DOX with TAM, and this synergistic effect was dominant in cell lines with high expression of P-glycoprotein. It was also examined that the intracellular concentration of DOX was increased in combined exposure of TAM and CEP, compared with the exposure of TAM, because TAM and CEP promoted the influx and inhibited the efflux of DOX. Thus, TAM and CEP might be able to circumvent DOX-resistance for treatment in cancer patients.

The effects of cyclosporin A, tamoxifen, and medroxyprogesterone acetate on the enhancement of adriamycin cytotoxicity in primary cultures of human breast epithelial cells

Adriamycin (Adr), the single most active agent used in the treatment of breast cancer, may become ineffective as treatment progresses due to the development of multidrug resistant (MDR) tumors. A major mechanism associated with MDR is increased P-glycoprotein (Pgp) expression. This study examined the abilities of the anti-estrogen tamoxifen (TAM) and the progestin medroxyprogesterone acetate (MPA) as well as cyclosporin A (CsA), a known resistance modifier, to enhance the cytotoxic effects of Adr on human breast epithelial cells (HBEC) in primary culture. Pgp and estrogen receptor (ER) expression were determined in each of the cultures by immunocytochemical assays using the monoclonal antibodies C219 and H222 Sp gamma, respectively. The Adr-sensitive, Pgp-, ER+ MCF-7 cell line and the Adr-resistant, Pgp+, ER- MCF7-AdrR cell line were used as controls. Primary cultures were categorized as HBEC from tissues with or without previous chemotherapy. Pgp was detected in 1 of the 15 cell cultures from tissues without previous chemotherapy and in 5 of the 6 cell cultures from tissues previously exposed to chemotherapy. Incubation with either CsA or MPA plus Adr enhanced Adr toxicity in Pgp+ but not Pgp- cell cultures, whereas TAM had no effect on the sensitivity of any of the cultures. Of the 21 primary cultures of HBEC, 3 were ER+. There was no correlation between the enhancement of Adr cytotoxicity and ER status. The data suggest that MPA as well as CsA may be useful as modifying agents in overcoming Pgp-associated multidrug resistance.

Inhibition of lysosomal acid sphingomyelinase by agents which reverse multidrug resistance

An increasing body of evidence appears to implicate the lipid bilayer of multidrug resistant (MDR) cells with P-glycoprotein activity. Several cationic amphiphilic drugs (CADs) have been extensively described as modulators of MDR. These same agents are also known to (1) inhibit lysosomal acid sphingomyelinase (ASmase), a phospholipid degrading enzyme, and/or (2) induce phospholipidosis in animal tissues or cultured cell lines. In this report, we randomly selected 17 CADs and evaluated their potency in modulating MDR in the murine MDR P388/ADR leukemia cell line. We compared these results with their ability to inhibit ASmase and observed a significant dose-dependent linear relationship (95% central confidence interval), between ASmase inhibition and MDR reversal. This approach permitted us to identify three new modestly potent chemosensitizers: trimipramine, desipramine, and mianserine. Modulation of MDR was not cell line specific, since CADs at 10 microM increased doxorubicin (DOX) and vinblastine (VBL) (but not methotrexate, MTX) cytotoxicity in both P388/ADR and the human MDR cell lines MES-SA/Dx5 and K562/R7, but not in the parental drug-sensitive cells. Although all chemosensitizing CADs at 10 microM significantly increased Rhodamine-123 (Rho-123) accumulation in the human leukemia MDR cell line K562/R7 and most presented significant displacement of the photoaffinity labelling probe iodoarylazidoprazosin, no correlation between these observations and the ability of CADs to sensitize MDR cells to DOX and VBL was found. In conclusion, our study strongly suggests that the chemosensitizing potency of agents such as CADs may be due to a dual mechanism of action: direct antagonism of P-gp activity and indirect modulation of P-gp activity through the disruption of cellular lipid metabolism.

Potentiation of anticancer-drug cytotoxicity by multidrug-resistance chemosensitizers involves alterations in membrane fluidity leading to increased membrane permeability

We are studying the mechanism underlying chemosensitization of anticancer-drug cytotoxicity in wild-type and multidrug-resistant (MDR) mammalian cells. We show here that the chemosensitizers, reserpine and verapamil, display a dramatic potentiation of taxol, anthracycline and Vinca alkaloids cytotoxicity in P-glycoprotein-(P-gp)-deficient hamster and human nasopharyngeal carcinoma cells. We have therefore utilized this phenomenon to probe for the putative P-gp-independent component of cytotoxicity chemosensitization. These chemosensitizers yielded a marked increase in the accumulation of taxol in parental hamster and human carcinoma cells that are devoid of P-gp. These chemosensitizers and non-ionic detergents brought about a pronounced increase in the accumulation of structurally and mechanistically diverse lipophilic chromophores in parental and MDR hamster cells. Furthermore, non-toxic concentrations of these non-ionic detergents yielded a marked potentiation of taxol cytotoxicity in parental cells. These findings were consistent with a chemosensitizer-mediated, P-gp-independent increase in membrane permeability. Thus, several aspects of chemosensitizers' interaction with lipid bilayers and biomembranes were studied. In this respect, like various mild detergents, chemosensitizers induced a dose-dependent leakage of carboxyfluorescein encapsulated in liposomes. Like specialized membrane fluidizers, various chemosensitizers induced a dose-dependent membrane fluidization (and sometimes rigidification) in both liposomes and various wild-type and MDR animal and human cells, as revealed by diphenylhexatriene fluorescence polarization. Furthermore, a favorable correlation was observed between the ability of chemosensitizers to permeabilize lipid bilayers and their capacity to potentiate anticancer-drug cytotoxicity. Thus, we propose that chemosensitizer-mediated changes in the physical properties of biomembranes, including altered fluidity and increased permeability, may be important factors in achieving potentiation of anticancer-drug cytotoxicity in wild-type and MDR mammalian cells. This study offers a basis for the chemosensitizer-mediated potentiation of drug toxicity to healthy tissues, thus emphasizing the importance of a prior evaluation of the potential untoward toxicity when simultaneously using MDR chemosensitizers and cytotoxic agents in the clinic.

Interaction of multidrug-resistant Chinese hamster ovary cells with the peptide ionophore gramicidin D

A major form of multidrug resistance results from the overexpression of P-glycoprotein, a 170 kDa membrane protein. Multidrug resistant (MDR) Chinese hamster ovary (CHO) cells and mdrl transfectants displayed cross-resistance to the channel-forming peptide ionophore gramicidin D, which was reversed by various chemosensitizers, thus directly implicating P-glycoprotein as the mediator of resistance. However, gramicidin D was not able to inhibit [3H]azidopine photolabelling of P-glycoprotein. MDR cells were not resistant to other pore-forming ionophores, but showed a modest level of cross-resistance to the mobile ionophore valinomycin. There was no difference in 125I-gramicidin D uptake by resistant and sensitive cells. Resistant cells showed lower 86Rb+ uptake, relative to the drug-sensitive parent. Addition of GmD increased both the rate and the level of 86Rb+ uptake in sensitive cells, but had no effect on MDR cells. MDR cells also showed much lower rates of gramicidin D-dependent 86Rb+ efflux than sensitive cells, and this was greatly increased by verapamil. These results suggest that P-glycoprotein interferes with the formation of ion-conducting gramicidin D channels. In contrast, valinomycin had the same effect on gramicidin D-dependent cation efflux in MDR and sensitive cells. Gramicidin D is thus unique among the ionophores is being a substrate for P-glycoprotein, which appears to greatly reduce the formation of active dimeric channels in the plasma membrane of MDR cells.

Interaction of multidrug-resistant Chinese hamster ovary cells with amphiphiles

The interaction of membrane-active amphiphiles with a series of MDR Chinese hamster ovary (CHO) cell lines was investigated. Cross-resistance to cationic amphiphiles was observed, which was effectively sensitised by verapamil. MDR cells showed collateral sensitivity to polyoxyethylene amphiphiles (Triton X-100/Nonidet P-40), which reached a maximum at 9-10 ethylene oxide units. Resistant lines were also highly collaterally sensitive (17-fold) to dibutylphthalate. mdrl transfectants showed cross-resistance to cationic amphiphiles, but no collateral sensitivity to nonionic species. Triton X-100/Nonidet P-40 inhibited 3H-azidopine photoaffinity labelling at low concentrations, perhaps reflecting a specific interaction with P-glycoprotein. Further investigation of the molecular basis of collateral sensitivity revealed that association of 3H-Triton X-100 with MDR cells reached steady state levels rapidly, and occurred by a non-mediated mechanism. The equilibrium level of X-100 uptake was inversely related to drug resistance. Collateral sensitivity is thus not a result of decreased Triton X-100 association with the cell. The fluorescent probe merocyanine 540 was used to examine the MDR plasma membrane microenvironment for physicochemical changes. Increasing levels of drug resistance correlated with a progressive shift in the mean cell fluorescence to lower levels, which suggests that the packing density in the outer leaflet of MDR cells is increased relative to that of the drug-sensitive parent.

Effect of cell density on cytotoxicity of ether lipid analogues in variants of B16 murine melanoma

Ether lipids are defined here as analogues of naturally occurring lysophosphatidylcholines with cytotoxic activity against neoplastic cells. The activity of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET18OMe) and 3-hexadecylmercapto-2-methoxymethyl-propyl-1-phosphocholine (Ilmofosine) (BM 41.440) was tested in variants of B16 murine melanoma, grown in adhesion cultures (B16F1 with low metastatic potential; B16F10 and B16BL6 with high metastatic potential). Cytotoxicity was evaluated by counting the cells that survived after 24 h of drug exposure. Cholesterol, sphingomyelin, total phospholipid and phosphatidylcholine levels were determined. After 24 h of drug exposure, cultures of the B16BL6 variant contained a larger number of cells, especially when high drug concentrations (100-250 microM) were used, than cultures of the B16F1 and B16F10 variants. The sensitivity to ET18OMe of the three variants was evaluated at different cell densities (at each density the dose was equalized per number of cells/well; 0.1 mumol/10(6) cells/well). In B16F1 and B16F10 cultures the dose-response curve was not affected by the number of cells/well, while in B16BL6 no more than 20% of the cells were killed at all cell densities measured. A linear relationship was noted between cell density and cholesterol/phospholipid and sphingomyelin/phosphatidylcholine ratios in the resistant variant B16BL6, confirming that lipid composition modulates the cytotoxic activity of ether lipids.

Increased accumulation of drugs in a multidrug resistant cell line by alteration of membrane biophysical properties

Growth of CHRC5 multidrug resistant cells in media enriched in a saturated C-17 fatty acid, heptadecanoic acid, resulted in these cells accumulating vinblastine at a rate and to an extent comparable to that of the parental cell line AB1. The fatty acid-enriched growth media had no effect on the ability of AB1 cells to take up vinblastine. The action of amphiphiles on the uptake of rhodamine dyes by CHRC5 cells was compared with the increased dye accumulation affected by verapamil. Membrane rigidifying agents, such as the saturated fatty acid stearic acid, or the cholesterol derivatives, cholesteryl hemisuccinate and cholesteryl phosphorylcholine, as well as a membrane fluidizing unsaturated fatty acid, linoleic acid, could significantly increase dye uptake, although not as well as verapamil. These results taken in conjunction with other reports in the literature, demonstrate that multidrug resistance is sensitive to alterations of membrane properties. They suggest that perturbation of the membrane to either increased or to decreased membrane fluidity can lower the level of resistance.

Probing daunorubicin accumulation defects in non-P-glycoprotein expressing multidrug-resistant cell lines using digitonin

Multidrug resistance (MDR) in tumor cells is frequently associated with reduced cellular cytostatic drug accumulation, caused by the drug efflux protein, P-glycoprotein (Pgp). The action of Pgp in tumor cells can be detected by measuring the increase of daunorubicin accumulation upon blocking Pgp with drugs such as verapamil. A number of MDR cell lines have been described, characterized by decreased drug accumulation without Pgp being present. For such non-Pgp MDR cells no gene probes or functional assays are available to study this phenotype in clinical tumor specimens. We have worked out a method which enables the detection of drug-transport-related decreases in cellular daunorubicin accumulations without the need for the use of specific Pgp blockers. The cells used were SW-1573-, GLC4- and HT1080-sensitive cell lines, which accumulated (corrected for DNA content) 272%, 1,288% and 203% more daunorubicin than the non-Pgp MDR sublines SW-1573/2R120, GLC4/ADR and HT1080/DR4. When the plasma membranes of these MDR lines were permeabilized with 20 microM digitonin an increase to 282%, 1,260% and 239% of 14C-daunorubicin control accumulation was measured (at pH = 7.35). The intracellular pH measured with BCECF was the same in parent and corresponding MDR cells, excluding the role of pH differences in the measured effects. This method provides a tool allowing the detection of cellular mechanisms (including Pgp) which are related to active outward transport of daunorubicin.

Reversal of multidrug resistance by phenothiazines and structurally related compounds

The multidrug-resistance (MDR)-reversal activity of 232 phenothiazines and structurally related compounds was tested in MDR P388 cells. Such activity was found among compounds exhibiting two ring structures (phenyl, cyclopentyl, cyclohexyl, thienyl or 5-norbornen-2-yl but not pyridinyl) linked by a variety of bridge types and possessing a secondary or tertiary amine group. Among 192 such compounds, 31.8% displayed good activity (MDR-reversal ratio, greater than or equal to 10) and 8.3%, outstanding activity (MDR-reversal ratio, greater than or equal to 30). In a subgroup comprising 56 compounds with a carbonyl residue, 4 with sulfuryl residue and 1 with thienyl residue, 42.7% showed good activity and 18%, outstanding activity. The contribution of these residues to the MDR-reversal activity was particularly evident among compounds containing a cyclic tertiary amine. Among 49 such compounds, 51% displayed good activity and 20.4%, outstanding activity, whereas among the 85 compounds lacking such groups, only 31.8% showed good activity and 4.7%, outstanding activity. Enhancement of this activity by the carbonyl group is also obtained when the latter is part of an amide bond of a tertiary amine. As compounds with a carbonyl group located on the rings, on the bridge to the amine group or beyond the amine are efficient MDR reversers, it seems that the exact molecular location of the carbonyl group is not critical for the elicitation of this activity.

Reduced ouabain-sensitive potassium entry as a possible mechanism of multidrug-resistance in P388 cells

Multidrug-resistant P388 cells were found to be resistant also to a variety of ammonium, phosphonium and arsonium compounds. As previously shown for anthracyclines and vinca alkaloids, the resistance to the permanently charged lipophilic cationic compounds could be circumvented by verapamil. Relative to drug-sensitive cells, K+ uptake and plasma membrane Mg-ATPase activity in multidrug-resistant cells are ouabain resistant. The intracellular K+ concentration in drug-resistant cells is maintained at a normal level by increased activity of the furosemide sensitive transport system. It is suggested that the reduced activity of the electrogenic Na(+)-K+ pump in multidrug-resistant, cells could result in a lower transmembrane potential and therefore reduced accumulation of cationic lipophilic compounds.