Multidrug resistance and chemosensitization: therapeutic implications for cancer chemotherapy

Differential effects of verapamil and quinine on the reversal of doxorubicin resistance in a human leukemia cell line

We studied the restoration of doxorubicin accumulation and sensitivity by verapamil and quinine in a variant of the human erythroleukemia cell line K562 selected for resistance to doxorubicin and presenting a multidrug-resistance (MDR) phenotype. Verapamil was able to completely restore doxorubicin accumulation in the resistant cells to the level obtained in sensitive cells, but only partially reversed doxorubicin resistance. Quinine, in contrast, had a relatively weak effect on doxorubicin accumulation but was able to completely restore doxorubicin sensitivity in the resistant cells. In addition, verapamil was able to decrease azidopine binding to P-glycoprotein, whereas quinine was not. Quinine also modified the intracellular tolerance to doxorubicin, which suggests that it is able to modify drug distribution within the cells. Confocal microscopy revealed that verapamil and quinine were able to restore nuclear fluorescence staining of doxorubicin in resistant cells; since this was obtained for quinine without significant increase of doxorubicin accumulation, this observation confirms that quinine acts principally on doxorubicin redistribution within the cells, allowing the drug to reach its nuclear targets. When used in association, verapamil and quinine reversed doxorubicin resistance in a synergistic fashion. We conclude that verapamil and quinine do not share the same targets for reversal of MDR in this cell line; whereas verapamil directly interferes with P-glycoprotein and mainly governs drug accumulation, quinine has essentially intracellular targets involved in drug redistribution from sequestration compartments.

Azelastine and flezelastine as reversing agents of multidrug resistance: pharmacological and molecular studies

The effects of two new phthalazinone derivatives, azelastine (AZ) and flezelastine (FZ), on the reversal of resistance to doxorubicin (dox) were studied using two variants of the rat C6 glioblastoma cell line, selected with dox (C6 0.5) or with vincristine (C6 1V). Both lines presented a multidrug-resistant phenotype which was, in the case of C6 0.5 cells, likely to be accompanied by an additional mechanism leading to intracellular tolerance of the drug. Both AZ and FZ reversed dox resistance in a concentration-dependent manner, and FZ was shown to be at least three times more potent than AZ. FZ was able, at a relatively high concentration (30 microM), to completely restore dox sensitivity in both cell lines. Both drugs were able to virtually restore dox accumulation to the level reached in sensitive cells, and, interestingly, this complete restoration occurred at lower concentrations of modulator than required for complete reversal of resistance. FZ was able to reverse dox intracellular tolerance of C6 0.5 cells and to restore dox accumulation at the IC50 to the level observed in sensitive cells. AZ and FZ both inhibited azidopine binding to membrane preparations of C6 0.5 and C6 1V cells, although FZ was more potent. Both drugs more successfully inhibited azidopine binding to membranes prepared from C6 1V cells (which express the mdr1b gene product) than to membranes from C6 0.5 cells (which express the mdr1a gene product). In view of its potent activity on MDR, further preclinical evaluation of FZ is warranted.

Characterization of the ATPase activity of P-glycoprotein from multidrug-resistant Chinese hamster ovary cells

P-Glycoprotein (Pgp) was isolated from CHRC5 membranes by selective detergent extraction and further purified by lentil lectin affinity chromatography. The purified product displayed a very high basal ATPase activity (1.65 mumol/min per mg protein in the absence of added drugs or lipids) with an apparent Km for ATP of 0.4 mM. There was no evidence of cooperativity, suggesting that the two ATP sites operate independently of each other. Pgp ATPase activity was stimulated by verapamil, trifluoperazine and colchicine, and inhibited by daunomycin and vinblastine. All drugs and chemosensitizers acted as mixed activators or inhibitors, producing changes in both the Vmax of the ATPase and the Km for ATP. ADP competitively inhibited Pgp ATPase, with a Ki of 0.2 mM. The macrolide antibiotics bafilomycin A1, concanamycin A and concanamycin B, inhibited Pgp ATPase at concentrations of 0.1-10 microM, and at an inhibitor:protein stoichiometry of 0.65-1.0 mumol/mg protein, which is at the low end of the range characteristic of P-type ATPases. Pgp ATPase was relatively selective for adenine nucleotides. Several phospholipids stimulated Pgp ATPase activity in a dose-dependent manner, whereas others produced inhibition. Metabolic labelling showed that the endogenous phospholipids associated with purified Pgp consisted largely of phosphatidylethanolamine and phosphatidylserine, with only a small amount of phosphatidylcholine. 32P-Labelling studies indicated that purified Pgp was partially phosphorylated. It can be concluded that Pgp is a constitutively active, adenine nucleotide-specific ATPase whose catalytic activity can be modulated by both drugs and phospholipids.

Mechanisms of resistance to combinations of vincristine, etoposide and doxorubicin in Chinese hamster ovary cells

We have isolated from Chinese hamster ovary cells, 30 sublines resistant to vincristine, doxorubicin or etoposide and 43 sublines evading treatment with a pair of these drugs. Isolated in one step and under low selective pressure, sublines were 3- to 25-fold more resistant to their selecting drug(s) than the parental cells. Possible P-glycoprotein-associated multidrug resistance was investigated through pgp gene copy number and mRNA expression level. DNA topoisomerase II alteration was evaluated from the ability of nuclear extracts to form cleavable complexes. Vincristine (all sublines) and doxorubicin (6/7 sublines) preferentially selected for pgp gene amplification and mRNA overexpression, whereas selection with etoposide resulted in a decrease of cleavable complex formation in 11 out of 13 sublines. A common pgp gene-mediated resistance was found in the 13 doxorubicin plus vincristine-selected sublines, whereas all but one of the 12 etoposide plus vincristine-resistant sublines displayed both pgp mRNA overexpression and decreased ability to form cleavable complexes. Among the 18 doxorubicin plus etoposide selected sublines, five exhibited a decreased ability to form cleavable complexes only, six exhibited pgp mRNA overexpression only and six exhibited both alterations. Overall, drug resistance could not be attributed to either mechanism in three of the 73 sublines. We conclude that under low selective pressure it is possible to find a combination of drugs which require simultaneous selection of more than one resistance mechanism; such cells emerge with very low frequency.

P-glycoprotein of blood brain barrier: cross-reactivity of Mab C219 with a 190 kDa protein in bovine and rat isolated brain capillaries

P-glycoprotein (P-gp), an active efflux pump of antitumor drugs, is strongly expressed in endothelial cells of the blood brain barrier (BBB). Two proteins (155 and 190 kDa) were detected by Western blot analysis of beef and rat capillaries with the monoclonal antibody (MAb) C219. In order to characterize the nature of these proteins, their profile of solubilization by different detergents was established and compared with that of P-gp from the CHRC5 tumoral cell line. The 155 kDa protein (p155) of capillaries and the P-gp of CHRC5 cells were well solubilized by deoxycholate and Elugent, whereas the 190 kDa kDa protein (p190) was only solubilized by sodium dodecylsulfate (SDS). Both proteins have different patterns of extraction by Triton X-114, p155 partitioning as a membrane protein, while p190 was insoluble. Deglycosylation of capillary proteins resulted in a 27-28 kDa decrease in the apparent molecular weight of p155, similar to that observed for the P-gp of CHRC5 cells, but a decrease of only 7-8 for p190. Only p155 was immunoprecipitated by MAb C219. These results suggest that only p155 is the P-gp in BBB and that MAb C219 cross-reacts with a 190 kDa MDR-unrelated glycosylated protein. Consequently, the use of this antibody, which is frequently used to detect P-gp in tumors, could be a pitfall of immunohistochemistry screening for cancer tissues and lead to false positive in the diagnosis of MDR.

Benzimidazoles, potent anti-mitotic drugs: substrates for the P-glycoprotein transporter in multidrug-resistant cells

P-glycoprotein is though to mediate the energy-dependent efflux of many structurally and functionally unrelated lipophilic compounds. Presently, the molecular mechanism underlying the binding and efflux of drugs by P-glycoprotein is not well understood. However, it has been suggested that two planar benzene ring structures and a cationic charge are commonly found in many drugs that interact with P-glycoprotein. The benzimidazoles (BZs) are potent anti-tumour, anti-fungal and anti-parasitic agents, whose mode of action is thought to result from their inhibition of microtubule functions. Although other classes of microtubule inhibitors, such as colchicine and vinblastine, have been studied extensively with respect to their interaction and efflux by P-glycoprotein, the BZ group of drugs has not been characterized. In this study, we have characterized the interaction of BZ with multidrug-resistant cells and found that resistant cells accumulated substantially less BZ compared with drug-sensitive cells. Furthermore, BZ was more toxic to sensitive than to drug-resistant cells, suggesting that BZ is likely to be a substrate for the P-glycoprotein drug efflux pump. In addition, we used a photoactive analogue of BZ ([125I]ASA-BZ) to demonstrate a direct binding between BZ and P-glycoprotein. Results showing that a molar excess of vinblastine, unmodified BZ, verapamil and rhodamine 123, but not colchicine, inhibited the photoaffinity labelling of P-glycoprotein by [125I]ASA-BZ confirmed the binding specificity of BZ to P-glycoprotein. Protease digestion of [125I]ASA-BZ photoaffinity labelled P-glycoprotein yielded two peptides that were similar to those obtained with other P-glycoprotein-associated drugs, e.g. azidopine and iodoaryl azidoprazosin. Taken together, these results demonstrate a direct and specific interaction between P-glycoprotein and BZ in a manner that is probably similar to other previously characterized P-glycoprotein-associated drugs.

Heterogeneity in P-glycoprotein (multidrug resistance) activity among murine peripheral T cells: correlation with surface phenotype and effector function

P-glycoprotein (P-gly) is the transmembrane efflux pump responsible for multidrug resistance in tumor cells. Functional P-gly activity can be conveniently assessed microfluorometrically using the fluorescent dye rhodamine 123 (Rh123), which is an artificial substrate for the P-gly transporter. Here we assess P-gly activity in subsets of mouse peripheral T lymphocytes using the Rh123 efflux assay. Our data indicate that virtually all CD8+ cells extrude Rh123 efficiently, whereas only a subset of CD4+ cells exhibit P-gly activity. Correlation of P-gly activity in CD4+ cells with the expression of a panel of surface markers revealed that cells bearing an "activated/memory" phenotype (CD45RB-, CD44hi, CD62L-, CD25+, CD69+) were exclusively found in the fraction that can extrude Rh123. In contrast "naive" phenotype CD4+ cells (CD45RB+, CD44lo, CD62L+, CD25-, CD69-) could be further subdivided into two major subsets based on P-gly activity. In functional studies of sorted cell populations the Rh123-extruding subset of "naive" CD4+ cells proliferated more strongly and secreted higher levels of interleukin (IL)-2 than its Rh123-retaining counterpart when activated by a variety of polyclonal stimuli. Furthermore, this subset produced detectable levels of interferon (IFN)-gamma upon stimulation but no IL-4 or IL-10. As expected, the Rh123-retaining "naive" subset produced only IL-2 after stimulation, whereas the "memory" subset produced IFN-gamma, IL-4 and IL-10 in addition to low levels of IL-2. Collectively, our data indicate that P-gly activity is a novel parameter that can be used to distinguish a subset of "preactivated" CD4+ cells that would be considered as naive on the basis of their surface phenotype.

P-glycoprotein, multidrug resistance and tumor progression

P-glycoprotein (Pgp) is a plasma membrane protein that was first characterised in multidrug resistant cell lines. The occurrence of Pgp in clinical tumors has been widely studied. Recent investigations have begun to focus on the relationship between Pgp detection in tumors and treatment outcome. In several types of tumors, detection of Pgp correlates with poor response to chemotherapy and shorter survival. P-glycoprotein over-expression often occurs upon relapse from chemotherapy but may also occur at the time of diagnosis. Studies of experimental rat liver carcinogenesis have shown that Pgp expression increases in late stages of carcinogenesis, suggesting that Pgp may be involved in tumor progression. While some of the Pgp isoforms are known to transport hydrophobic chemotherapeutic drugs out of tumor cells, the biologic effects of Pgp overexpression in tumor cells are not fully understood, because the spectrum of substrates for Pgp-mediated transport has not been determined. In the rat liver carcinoma model, strong expression of Pgp is associated with a highly vascular stroma, suggesting that Pgp in tumor cells may affect the connective tissue stroma. The regulation of Pgp appears to be complex, and little is known about how it is up-regulated during carcinogenesis. Further studies of the role of Pgp in malignancy may contribute to our understanding of molecular mechanisms which underlie tumor progression.

Detection of oligomeric and monomeric forms of P-glycoprotein in multidrug resistant cells

P-glycoprotein (P-gp) is thought to function as a drug efflux pump in multidrug resistant (MDR) cells. The functional form of P-gp in its native state is not known. Previous results from radiation target size analysis have suggested that P-gp occurs as dimers in MDR cell plasma membranes [Boscoboinik et al. (1990) Biochim. Biophys. Acta 1027, 225-228]. In this study, we used sucrose gradient velocity sedimentation to determine if P-gp oligomers could be retrieved from detergent extracts of hamster and human MDR cell lines. The proportion of P-gp recovered as higher order oligomers was dependent on the detergents used for solubilization of the cells. When a detergent such as CHAPS was used, 50% or more of the P-gp sedimented as higher order oligomers. In contrast, in the presence of SDS, only monomers were retrieved, but naturally occurring oligomers could be preserved if the cells were treated with a cross-linker prior to detergent solubilization. The oligomers and monomers were both able to bind the photoactive analog of ATP (8-azido[alpha-32P]ATP) or the drug [3H]azidopine in membrane preparations. P-gp is a phosphoprotein, and its phosphorylated state is thought to be important for function. When MDR cells were labeled with [32P]orthophosphate in vivo, we observed that the monomer and dimer were more highly phosphorylated than the larger oligomers, suggesting that these different forms of P-gp may be functionally distinct. The assembly of oligomers appears to occur in an early bisynthetic compartment, and asparagine-linked glycosylation is not required for their formation. Our findings indicate that oligomers of P-gp exist in MDR cells and raise the possibility that the dynamics of oligomer formation and dissociation may be important in the mechanism of action of P-gp.

Reversal of the human and murine multidrug-resistance phenotype with megestrol acetate

MA is an orally active PG derivative with an excellent safety profile that is used primarily for the treatment of carcinomas of the breast and endometrium. We investigated the potential application of MA as an MDR-reversal agent using cell culture and human tumor xenograft models. The reversing activity of MA in vitro was compared with that of PG and VER in two human MDR cell lines, the colon carcinoma HCT-116/VM46 and the breast carcinoma MCF-7/ADR, and in a murine cell line, J774.2. At concentrations as low as 3 microM, MA was capable of partially restoring sensitivity to Act D in the HCT-116/VM46 cells and sensitivity to DOX in the MCF-7/ADR cells. Although less effective than VER, MA was about 2.5 times more potent than PG in reversing MDR at equimolar concentrations. Increased accumulation of DOX in drug-resistant cells that were treated simultaneously with MA was observed by flow cytometry. In vivo, using established human colon and breast carcinoma xenografts implanted s.c. in athymic mice, the combined therapy with MA and DOX resulted in enhanced antitumor activity relative to that of DOX alone in the MDR sublines. These results suggest that MA may be a promising clinical MDR-reversing agent.

Differential effectiveness of a range of novel drug-resistance modulators, relative to verapamil, in influencing vinblastine or teniposide cytotoxicity in human lymphoblastoid CCRF-CEM sublines expressing classic or atypical multidrug resistance

A series of five potential modulators of resistance were tested for their relative ability, as compared with verapamil, to sensitize CEM lymphoblastoid leukemia drug-resistant tumor sublines expressing either the classic or the atypical multidrug-resistance (MDR) phenotype to vinblastine or teniposide. Maximal non-cytotoxic concentrations of each modulator were tested and sensitization induces (SIs) were derived by comparing the drug concentration required to inhibit growth by 50% in their presence or absence. Like verapamil (10 microM) itself, three of the other modulators tested, namely, S9788 (4 microM), flunarizine (20 microM) and quinidine (30 microM), resulted in 2- to 3-fold sensitization of vinblastine against the parental CEM cells, and comparable effects were noted in the CEM/VM-1 cells, which were not cross-resistant to vinblastine. In contrast, cyclosporin A (0.5 microM) and B859-35 (2 microM) did not enhance vinblastine growth inhibition in these lines. However, the greatest sensitization with all the modulators was noted in the classic MDR VBL1000 cells, with SIs ranging from 40- to 350-fold, except for cyclosporin A, which proved ineffective at the concentration tested (SI, 2.6). The greatest extent of differential sensitization of these VBL1000 tumor cells occurred with quinidine or B859-35, which proved significantly more effective than verapamil alone. Combinations of modulators resulted in additive effects, with B859-35 plus cyclosporin A proving superior to B859-35 plus verapamil. In contrast, none of these compounds proved effective as a sensitizer to teniposide. The growth-inhibitory effects of this drug were not modified significantly in either the 92-fold teniposide-resistant VM-1 cells or in the parental cells. Addition of verapamil itself also failed to modulate teniposide growth inhibition in the VBL1000 cells, which express significant cross-resistance to this drug (36-fold). However, SI values of 3- to 5-fold were obtained using quinidine or B859-35. These results serve (a) to emphasise the need to monitor the effects of modulators not only on drug-resistant cells but also on their drug-sensitive counterparts so as to ensure differential sensitization such that normal sensitive tissues are not likely to be adversely influenced and (b) to highlight the observation that the extent of modulation differs depending not only on the antitumor drug used but also on the mechanism of drug resistance expressed. This in vitro model system appears to provide a useful screening system for resistance modulators and certainly could be used in attempts to identify alternative agents that may influence teniposide sensitivity in these drug-resistant sublines.

Evaluation of S9788 as a potential modulator of drug resistance against human tumour sublines expressing differing resistance mechanisms in vitro

Significant activity has been identified using S9788, a triazineaminopiperidine derivative, as a new modulator of multi-drug resistance against a series of drug-resistant human tumour-cell lines in vitro. Maximal non-cytotoxic concentrations (i.e., those resulting in < or = 10% cytotoxicity) of S9788 or verapamil were tested in combination with vinblastine, Adriamycin or vincristine and cytotoxicity was evaluated using a clonogenic assay, or the metabolic dye reduction MTT assay, or by monitoring growth inhibition. Under these conditions, the extent of resistance modulation by verapamil and by S9788 was comparable in the various tumour cell lines tested, although a definite concentration-dependent modulation was noted with both compounds. The highest dose-modification factors were noted in the highly vinblastine-resistant classic multi-drug-resistant subline CEM/VLB100, although resistance reversal was only partial. Resistance modulation by both verapamil and S9788 was noted in 4 drug-selected resistant sublines and 4 "intrinsically" resistant human tumour cell lines, which all exhibited significant P-glycoprotein expression. In contrast, in 2 drug-resistant human tumour sublines (GLC4/ADR and CEM/VM-1) characterized by altered topoisomerase-II activity and proving to be P-glycoprotein-negative, no resistance modulation relative to parental cells was observed. These data are consistent with the proposal that resistance modulation is mediated by interaction between S9788 and P-glycoprotein and support its clinical evaluation in patients with P-glycoprotein-positive tumours.

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.

Tamoxifen resistance in breast cancer

Tamoxifen (TAM) resistance is the underlying cause of treatment failure in many breast cancer patients receiving TAM. The mechanism(s) involved in TAM resistance are poorly understood. A variety of mechanisms have been proposed but only limited evidence exists to substantiate them. Studies have now shown that in many patients TAM resistance is not related to the down regulation or loss of estrogen receptors (ER). Variant ER have been identified, but their significance clinically remains to be proven. Since breast cancer cells secrete several estrogen-regulated growth factors and growth inhibitors that may have autocrine or paracrine activity, altered growth factor production is another possible mechanism for TAM resistance. Tissue-specific transcription activating factors that may alter how the signal induced by TAM binding to the receptor is interpreted by the cell also require further investigation. An increase in antiestrogen binding sites (AEBS), which could effectively partition TAM and reduce its concentration at the ER has also been proposed as a potential mechanism. Pharmacologic mechanisms, such as a shift in metabolism toward the accumulation of estrogenic metabolites, are supported by recent data demonstrating metabolite E and bisphenol in tumors from TAM-resistant patients. Furthermore, a decrease in tumor TAM accumulation and an altered metabolite profile have been reported in TAM-resistant breast tumors grown in nude mice. These and other studies suggest that TAM resistance may be multifactorial in nature, but definitive identification of mechanisms that are operative in clinical TAM resistance requires further study.

Chloroquine: mechanism of drug action and resistance in Plasmodium falciparum

Quinoline-containing drugs such as chloroquine and quinine have had a long and successful history in antimalarial chemotherapy. Although these drugs are known to accumulate by a weak base mechanism in the acidic food vacuoles of intraerythrocytic trophozoites and thereby prevent hemoglobin degradation from occurring in that organelle, the mechanism by which their selective toxicity for lysosomes of malaria trophozoites is achieved has been subject to much discussion and argument. In this review the recent discovery that chloroquine and related quinolines inhibit the novel heme polymerase enzyme that is also present in the trophozoite food vacuole is introduced. The proposal that this inhibition of heme polymerase can explain the specific toxicity of these drugs for the intraerythrocytic malaria parasite is then developed by showing that it is consistent with much of the disparate information currently available. The clinical usefulness of chloroquine, and in some recent cases of quinine as well, has been much reduced by the evolution and spread of chloroquine resistant malaria parasites. The mechanism of resistance involves a reduced accumulation of the drug, although again the mechanism involved is controversial. Possible explanations include an energy-dependent efflux of preaccumulated drug via an unidentified transmembrane protein pump, or an increase in vacuolar pH such that the proton gradient responsible for drug concentration is reduced. New data are also presented which show that heme polymerase isolated from chloroquine resistant trophozoites retains full sensitivity to drug inhibition, consistent with the observation that resistance involves a reduced accumulation of the drug at the (still vulnerable) target site. The significance of this result is discussed in relation to developing new strategies to overcome the problem presented by chloroquine resistant malaria parasites.

Antiproliferative activity of thermosensitive liposome-encapsulated doxorubicin combined with 43 degrees C hyperthermia in sensitive and multidrug-resistant MCF-7 cells

Thermosensitive liposome-encapsulated doxorubicin (TLED) was compared to free doxorubicin, at 37 degrees C or combined with 43 degrees C hyperthermia, in sensitive and multidrug-resistant MCF-7 human tumour cells using clonogenic assays. In the resistant subline, TLED was found to partly circumvent multidrug resistance (MDR). The reversal was comparable to that obtained when verapamil was added to free doxorubicin. When hyperthermic treatment was applied, no difference in thermosensitivity was found between sensitive and resistant cells. The combination of hyperthermia with free doxorubicin did not reverse MDR. Hyperthermia and TLED yielded additive effects in the resistant cells while potentiation was observed in the sensitive cells. These results confirmed the usefulness of the liposome encapsulation of doxorubicin in reversing MDR. The possibility of obtaining additive cytotoxicity using TLED combined with hyperthermia may represent an alternative way of intensification of doxorubicin cytotoxicity concomitant with the circumvention of MDR without using MDR reversing agents, which often generate limiting toxic side-effects.

Non-glucocorticoid steroid analogues (21-aminosteroids) sensitize multidrug resistant cells to vinblastine

Several members of a group of compounds developed to treat stroke and trauma of the central nervous system are shown also to reverse multidrug resistance in human KB-V1 cells. The most potent reversal agents studied are 21-aminosteroid derivatives (lazaroids), tirilazad mesylate (tirilazad, U-74006F) and U-74389F. Tirilazad sensitizes resistant human cells (KB-V1) to killing by vinblastine by 66-fold, but does not change the sensitivity of the nonresistant parental line, KB-3-1, to vinblastine. KB-V1 cell membranes have high levels of P-glycoprotein, a protein that acts as an efflux pump and is thought to be the major cause of multidrug resistance in these cells. Tirilazad inhibits the photoaffinity labeling of P-glycoprotein with [3H]azidopine in KB-V1 cells more effectively than does verapamil, a standard reversal agent. In addition, tirilazad causes the increased accumulation of [3H]vinblastine in multidrug resistant KB-V1 cells. Studies of the resistance reversal potential of related compounds suggest that the complex amine portion of tirilazad is important for its reversal activity, while the steroid portion is less important.

Sensitization of P388 murine leukemia cells to epirubicin cytotoxicity by reserpine

Reserpine, the crystalline active substance isolated from the Rauvolfia plant, produces a characteristic vasodepressor effect in hypertensive patients. Apart from its antihypertensive property, reserpine also possesses transquillising and vasodepressor action, hence it is employed as supportive therapy in the treatment of cardiac disorders. Doxorubicin is a potent anticancer agent, the use of which is limited by its cumulative dose-dependent cardiotoxicity. Epirubicin is a derivative of doxorubicin having more favourable therapeutic index than doxorubicin and possessing less hematologic and cardiac toxicity at comparable doses. The data presented in this paper show the effect of reserpine as a chemosensitizer, when used in combination with epirubicin on P388 murine leukemia cells sensitive (P388/S) and resistant to doxorubicin (P388/DOX) cells. Inhibition of 3H-TdR incorporation into DNA was used as an index of the cytotoxic effects of drug when used alone or in combination. The combination of reserpine (1 microM) and epirubicin (1.7, 8.6 and 17.2 microM) indicated a significant enhancement in the DNA biosynthesis inhibition in P388/S and P388/DOX cell lines. The most prominent feature of the multidrug-resistant cell is the reduced accumulation of the drug intracellularly. P388/DOX cells showed less accumulation of epirubicin in the cell as compared to that of the parental cell line. Further studies demonstrated that reserpine significantly enhanced the intracellular accumulation of epirubicin in both the cell lines. The nature of DNA damage caused by the combination of reserpine and epirubicin was irreversible when studied in P388/DOX cell line. The combination of reserpine (5mg/kg) and epirubicin (1mg/kg) significantly potentiated the antitumor activity of epirubicin in P388/DOX tumor bearing mice. These studies suggest that reserpine can be used as an adjuvant in the cancer chemotherapy to potentiate the antiproliferative activity of anticancer drugs.

Reversal of multidrug resistance by a new lipophilic cationic molecule, S9788. Comparison with 11 other MDR-modulating agents in a model of doxorubicin-resistant rat glioblastoma cells

We have compared the properties of the novel multidrug resistance modulator, S9788, to a panel of 11 well-known modulators in a model of rat glioblastoma cells resistant to doxorubicin and displaying a P-glycoprotein-mediated multidrug-resistance phenotype complemented by a mechanism of intracellular drug tolerance not yet identified (Br J Cancer 1992, 65, 538-544). S9788, like most modulators, was able to completely restore drug accumulation in the resistant line to the level obtained in the sensitive cells. This was obtained with 10 mumol/l of modulator, which is slightly higher than required for cyclosporine A (3 mumol/l) verapamil and nicardipine (6 mumol/l), but lower than for amiodarone, trifluoperazine and dipyridamole (20 mumol/l), tamoxifen and diltiazem (40 mumol/l), quinine, quinidine and nifedipine (> 100 mumol/l). Complete restoration of drug cytotoxicity was, however, obtained only with amiodarone, and a residual resistance factor of 4 could not be overcome by cyclosporine A or S9788, while other modulators gave residual resistance factors of 5-20 (trifluoperazine, tamoxifen, verapamil, quinine, nicardipine, dipyridamole) or even higher (diltiazem, quinidine, nifedipine). When studying doxorubicin accumulation obtained for an exposure to the IC50 of this drug, it appeared that some modulators were able to decrease this "intracellular IC50" independently of their efficiency in resistance reversal (cyclosporine A, S9788, amiodarone, trifluoperazine, quinine, tamoxifen), thus reversing intracellular drug tolerance, whereas other modulators could not reduce this parameter (verapamil, nicardipine, dipyridamole, diltiazem, quinidine). It is suggested that drugs of the first group could be able to segregate doxorubicin in subcellular compartments from which it could not reach its nuclear targets.

High cell density-dependent resistance and P-glycoprotein-mediated multidrug resistance in mitoxantrone-selected Chinese hamster cells

Mitoxantrone (MIT) resistance has been studied in a colony selected from the CHO AA8 parental line in one step under a low degree of selective pressure (9 nM). The cells of the clonal isolate AA8/MIT C1(0) were sensitive to 9 nM MIT at low cell density but able to grow at high density. Parental AA8 cells were not able to grow under the latter condition. Decreased MIT accumulation (-20%) was observed at this step (step 0) in the absence of overexpression of mdr RNA coding for the drug efflux pump P-glycoprotein. Furthermore, AA8/MIT C1(0) did not exhibit cross resistance to vincristine, Adriamycin and etoposide at low cell density. During subsequent controlled growth for 2 months at high cell density in the presence of 9 nM drug, an additional selection occurred leading to a 4-fold MIT-resistant subline AA8/MIT C1(+). This subline was characterized at this step (step I) and after an additional 4 months of culture in the presence of 9 nM MIT (step II). Analysis of mdr gene expression and gene copy number showed an increase in mdr RNA and a pattern of mdr gene amplification which changed between step I and II. AA8/MIT C1(+)II exhibited a classical multidrug resistance phenotype with decreased accumulation of [14C]MIT and cross-resistance to vincristine, Adriamycin and etoposide. The ability to form the cleavable complex in the presence of etoposide in DNA topoisomerase II-containing nuclear extracts was identical in AA8/MIT C1(+)II and AA8 cell lines. These results demonstrate a new sequence of events in MIT resistance: low level of drug resistance at high cell density followed by mdr gene amplification.

SDZ 280-446, a novel semi-synthetic cyclopeptolide: in vitro and in vivo circumvention of the P-glycoprotein-mediated tumour cell multidrug resistance

SDZ 280-446 is a semi-synthetic derivative of a natural cyclic peptolide. Its ability to sensitise in vitro tumour cells whose resistance is due to P-glycoprotein-mediated anticancer-drug efflux was shown using four different pairs of parental drug-sensitive (Par-) and multidrug-resistant (MDR-) cell lines, from three different species (mouse, human, Chinese hamster) representing four different cell lineages (monocytic leukaemia, nasopharyngeal epithelial carcinoma, colon epithelial carcinoma, ovary fibroblastoid carcinoma), and using four different drug classes (colchicine, vincristine, daunomycin/doxorubicin and etoposide). By measuring its capacity to restore normal drug sensitivity of MDR-cells in culture in vitro, it appeared that SDZ 280-446 belongs to the same class of very potent chemosensitisers as the cyclosporin derivative SDZ PSC 833: both are about one order of magnitude more active than cyclosporin A (CsA), which is itself about one order of magnitude more active than other known chemosensitisers (including verapamil, quinidine and amiodarone which have already entered clinical trials in MDR reversal). Low concentrations of SDZ 280-446 could also restore cellular daunomycin retention in MDR-P388 cells to the levels found in the Par-P388 cells. SDZ 280-446 was also effective as a chemosensitiser when given orally in vivo. In a syngeneic mouse model, combined therapy with vinca alkaloids given i.p. and SDZ 280-446 given per os for 5 consecutive days significantly prolonged the survival of MDR-P388 tumour-bearing mice, when compared with mice receiving vinca alkaloids alone. Another protocol, using three cycles of i.p. doxorubicin at 4 day intervals, could also not increase MDR-P388 tumour-bearing mouse survival unless the mice received SDZ 280-446 orally 4 h before each doxorubicin injection. Though only very few combined therapy treatment protocols have been tested so far, clear increases in survival time of MDR-tumour-bearing mice were regularly obtained, leaving hope for major improvement of the therapy using other dosing schedules.

Identification of distinct P-glycoprotein gene sequences in rat

In higher vertebrates, P-glycoprotein is usually encoded by a small family of genes. We have determined that the rat contains three P-glycoprotein genes and have cloned distinct genomic fragments containing the putative 3' untranslated regions of these P-glycoprotein genes. Sequence analysis indicates that the rat P-glycoprotein genes belong to the three P-glycoprotein classes identified in mammals. These cloned sequences will be useful for delineating the expression of P-glycoprotein genes in the rat. We have also isolated a fourth clone which contains only a short, but highly conserved P-glycoprotein domain. This clone appears not be a member of the P-glycoprotein gene family, and its relationship to P-glycoprotein is unknown.

In vivo evidence of complete circumvention of vincristine resistance by a new triazinoaminopiperidine derivative S 9788 in P388/VCR leukemia model

S 9788, a new triazinoaminopiperidine derivative, was found to be a potent reversant of vincristine resistance in the in vivo murine leukemic P388/VCR model. In two treatment regimens (Q4D days 1, 5 and 9 and QD days 1-9), S 9788 enhanced the antitumor activity of vincristine in a dose-dependent manner, resulting in a complete circumvention of drug resistance for well-tolerated doses of S 9788. S 9788 was also effective in enhancing therapeutic effects of vincristine in the treatment of sensitive P388-bearing mice. These results strongly suggest that S 9788 may be a potential candidate for circumvention of multidrug resistance (MDR) in clinical practice.

Comparison of cyclosporin A and SDZ PSC833 as multidrug-resistance modulators in a daunorubicin-resistant Ehrlich ascites tumor

Recent studies by Boesch et al. have demonstrated that a nonimmunosuppressive cyclosporin analog, SDZ PSC 833 (an analog of cyclosporin D), is an active multidrug-resistance modifier that is at least 10 times more potent than cyclosporin A. In vitro accumulation and cytotoxicity experiments using daunorubicin (DNR) and vincristine (VCR) under the influence of SDZ PSC 833 and cyclosporin A were performed in wild-type (EHR2) and the corresponding highly DNR-resistant (about 80-fold) Ehrlich ascites tumor cells (EHR2/DNR+). In accumulation experiments, both SDZ PSC 833 and cyclosporin A were found to reverse the multidrug-resistant (MDR) phenotype, but to the same degree at equimolar concentrations. Thus, in EHR2/DNR+ cells, both cyclosporins at 5 micrograms/ml enhanced DNR and VCR accumulation to sensitive levels, but only a negligible effect on DNR accumulation in the drug-sensitive cells was seen. In the clonogenic assay, the cytotoxicity of the two modulators was equal. The lethal dose for 50% of the cell population (LD50) was approx. 7 micrograms/ml for both compounds, and no toxicity was observed at concentrations below 2 micrograms/ml. At nontoxic doses, both cyclosporins effectively increased the cytotoxicity of DNR and VCR in a concentration-dependent manner. The dose-response curves were nearly identical and did not demonstrate differences in modulator potency. These data permit the conclusion that cyclosporin A and SDZ PSC 833 do raise the intracellular accumulation of DNR and VCR to the same levels and that SDZ PSC 833 does not potentiate cytotoxicity better than cyclosporin A in EHR2/DNR+ cells. However, since the new compound is nonimmunosuppressive and causes less organ toxicity, clinical studies of its MDR modulating effect seem highly relevant.

Modulation of ATP and drug binding by monoclonal antibodies against P-glycoprotein

The role of P-glycoprotein in mediating the drug-resistance phenotype in multidrug resistant cells is now well documented. It is thought to function as an energy-dependent drug-efflux pump of broad specificity. Structurally, P-glycoprotein is an internally duplicated molecule containing two large multi-spanning transmembrane domains and two cytoplasmic ATP binding domains. In this report we demonstrate that monoclonal antibodies C219, C494, and C32 directed against short linear regions of the P-glycoprotein molecule inhibit ATP binding to P-glycoprotein in vitro. We also provide direct evidence that both predicted ATP-binding domains bind ATP and that there is co-operativity between the two sites. In addition, the capacity of P-glycoprotein to bind the calcium channel blocker, azidopine, is inhibited differentially by the antibodies. These observations are the first evidence linking specific perturbations of the P-glycoprotein molecule with ATP and drug binding.

Strategies for the purification of P-glycoprotein from multidrug-resistant Chinese hamster ovary cells

P-glycoprotein, a hydrophobic 170-kDa integral protein overexpressed in the plasma membrane of multidrug-resistant cells, is proposed to function as an ATP-dependent drug efflux pump. Plasma membrane preparations highly enriched in P-glycoprotein were isolated from multidrug-resistant cells by discontinuous sucrose gradient and Ca2+ precipitation methods. Several strategies were used for P-glycoprotein purification, with the goal being to achieve both good yields and purity, while keeping experimental manipulation to a minimum. P-glycoprotein was solubilized from the plasma membrane using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. Immunoaffinity chromatography using C219 monoclonal antibody produced low yields of moderately pure protein. Sequential lectin affinity chromatography on RCA-120 followed by lentil lectin resulted in a P-glycoprotein preparation that showed a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A fraction of P-glycoprotein did not bind to RCA-120, most likely as a result of heterogeneous glycosylation. A combination of chromatography on RCA-120 followed by immunoaffinity chromatography on C219 resulted in low yields of very pure P-glycoprotein.