Modulators of P-glycoprotein-associated multidrug resistance

Classical and Targeted Anticancer Drugs: An Appraisal of Mechanisms of Multidrug Resistance

The mechanisms by which tumor cells resist the action of multiple anticancer drugs, often with widely different chemical structures, have been pursued for more than 30 years. The identification of P-glycoprotein (P-gp), a drug efflux transporter protein with affinity for multiple therapeutic drugs, provided an important potential mechanism and further work, which identified other members of ATP-binding cassette (ABC) family that act as drug transporters. Several observations, including results of clinical trials with pharmacological inhibitors of P-gp, have suggested that mechanisms other than efflux transporters should be considered as contributors to resistance, and in this review mechanisms of anticancer drug resistance are considered more broadly. Cells in human tumors exist is a state of continuous turnover, allowing ongoing selection and "survival of the fittest." Tumor cells die not only as a consequence of drug therapy but also by apoptosis induced by their microenvironment. Cell death can be mediated by host immune mechanisms and by nonimmune cells acting on so-called death receptors. The tumor cell proliferation rate is also important because it controls tumor regeneration. Resistance to therapy might therefore be considered to arise from a reduction of several distinct cell death mechanisms, as well as from an increased ability to regenerate. This review provides a perspective on these mechanisms, together with brief descriptions of some of the methods that can be used to investigate them in a clinical situation.

Use of ribozymes and antisense oligodeoxynucleotides to investigate mechanisms of drug resistance

Chemotherapy can cure a number of human cancers but resistance (either intrinsic or acquired) remains a significant problem in many patients and in many types of solid tumour. Combination chemotherapy (using drugs with different cellular targets/mechanisms) was introduced in order to kill cells which had developed resistance to a specific drug, and to allow delivery of a greater total dose of anti-cancer chemicals by combining drugs with different side-effects (Pratt et al., 1994). Nearly all anti-cancer drugs kill tumour cells by activating an endogenous bio-chemical pathway for cell suicide, known as programmed cell death or apoptosis.

Cyclosporine A enables vincristine-induced apoptosis during reversal of multidrug resistance phenotype in chronic myeloid leukemia cells

Multidrug resistance (MDR) is considered a multifactorial phenotype which prevents a successful clinical cancer treatment. This phenomenon is mainly associated with mechanisms that include drug extrusion by P-glycoprotein (Pgp) overexpression and resistance to apoptosis derived by members of the inhibitor of apoptosis proteins (IAPs), such as XIAP. Studies have proposed the use of compounds that are able to inhibit or modulate Pgp function, with no changes in the physiological expression of this protein. Based on that, the present study aimed to evaluate the reversal of MDR phenotype through modulation of Pgp efflux pump activity in leukemia multidrug-resistant cells, using a low dose of cyclosporine A (CsA). We showed that modulation of Pgp activity by using CsA did not induce cytotoxic effects in leukemia cells, independently of Pgp expression. However, during the modulation condition, we could observe that vincristine-induced apoptosis was significant in resistant cells, which was also coupled with decreasing expression of the inhibitor of apoptosis protein XIAP. In summary, our data suggest that CsA is able to reversing MDR phenotype in vitro, inducing sensibility in multidrug-resistant cells with no alterations in Pgp expression. These findings contribute to our knowledge for the circumvention of MDR in cancer cells and could be helpful for new treatment approaches.

Carrier mediated transport of chlorpheniramine and chlorcyclizine across bovine olfactory mucosa: Implications on nose‐to‐brain transport

Delivery to the CNS via the nasal cavity has been pursued as a means to circumvent the blood-brain barrier (BBB), yet the mechanism of drug transport across this novel route is not well understood. Hydroxyzine and triprolidine have been reported to readily reach the CNS following nasal administration, whereas no measurable amounts of chlorcyclizine or chlorpheniramine, structurally similar antihistamines, were observed in the CSF. The permeation of chlorpheniramine and chlorcyclizine in vitro across the bovine olfactory mucosa was studied to investigate the biological and physicochemical characteristics that contribute to the limited CNS disposition of these compounds following nasal administration. The submucosal to mucosal fluxes (J(s-m)) of chlorcyclizine and chlorpheniramine across the olfactory mucosa were significantly greater than the mucosal to submucosal fluxes (J(m-s)). Moreover, the submucosal-mucosal permeability of both compounds was temperature dependent and saturable. In the presence of metabolic inhibitors (ouabain and 2,4-dinitrophenol) and P-glycoprotein (P-gp)/multidrug resistance protein 1 (MRP1) inhibitors (quinidine and verapamil), the J(m-s) increased and J(s-m) decreased significantly. These results indicate that chlorpheniramine and chlorcyclizine are effluxed from the olfactory mucosa by efflux transporters such as P-gp and MRP1. Transport studies across inert polymeric membranes demonstrated that the permeability of chlorpheniramine and chlorcyclizine decreased at donor concentrations higher than 3 mM suggesting that physicochemical properties such as self-aggregation also play a role in the reduced olfactory mucosal permeability of these compounds at higher concentrations.

Interaction of cytochrome P450 3A inhibitors with P-glycoprotein

Many clinically important drug interactions occur due to inhibition of human liver cytochrome P450 3A (CYP3A) metabolism. The drug efflux pump P-glycoprotein (Pgp) can be an additional locus contributing to these drug interactions because there is overlap in drugs that are substrates for both proteins. We screened a number of CYP3A inhibitors (macrolide antibiotics, azole antifungals, and ergotpeptides) for their ability to interact with Pgp, compared with prototypical Pgp inhibitors. We used cell lines expressing human, mouse, and rat mdr1 genes. Pgp antagonism was defined by interactions of the drugs with four cell lines (LLC-PK1, L-MDR1, L-mdr1a, and L-mdr1b) using a microfluorometric calcein-AM assay and characterized for their inhibitor constant (K(i)) toward calcein-AM. The compounds were further defined for their ability to inhibit MDR1 by their effect on vinblastine accumulation into L-MDR1 cells. Representative compounds from each class of drugs were further tested as Pgp substrates, defined by the ability of human Pgp or mouse mdr1a/Pgp to transport them across a polarized kidney epithelial cell in vitro. These same compounds were administered radiolabeled in vivo to mdr1a (+/+) and (-/-) mice and the distribution of radioactivity compared. The results are summarized as follows: 1) Some drug interactions with Pgp were substrate- and/or assay-dependent. 2) Ergot alkaloids were identified as a class of MDR1/Pgp chemosensitizers. 3) The Ergot alkaloids revealed species differences in the structure-activity relationships for inhibition of Pgp. Simultaneous inhibition of Pgp by many CYP3A inhibitors contributes to human variation in the extent of drug-drug interactions.

Role of P-glycoprotein in restricting propranolol transport in cultured rabbit conjunctival epithelial cell layers

PURPOSE

To determine the role of P-glycoprotein (P-gp) in propranolol transport in cultured rabbit conjunctival epithelial cell layers (RCEC).

METHODS

The localization of P-gp in the cultured RCEC as well as in the excised conjunctiva was determined by immunofluorescence technique. The role of P-gp in transepithelial transport and uptake of propranolol in conjunctival epithelial cells cultured on Transwell filters was evaluated in the presence and absence of P-gp competing substrates, an anti-P-gp monoclonal antibody (4E3 mAb), or a metabolic inhibitor, 2,4-dinitrophenol (2,4-DNP).

RESULTS

Immunofluorescence studies revealed positive staining in the apical membrane of cultured RCEC and in the apical surface of the superficial cell layers in the excised conjunctiva, but not the basolateral membrane of cultured RCEC. Transport of propranolol showed preference in the basolateral-to-apical direction. The net secretory flux was saturable with a Km of 71.5 +/- 24.0 nM and a Jmax of 1.45 +/- 0.17 pmol/cm2/hr. Cyclosporin A, progesterone, rhodamine 123, verapamil, 4E3 mAb and 2,4-DNP all increased apical 50 nM propranolol uptake by 43% to 66%. On the other hand, neither beta-blockers (atenolol, metoprolol, and alprenolol) nor organic cation transporter substrates (tetraethylammonium (TEA) and guanidine), affected apical 50 nM propranolol uptake.

CONCLUSIONS

The energy-dependent efflux pump P-gp appears to be predominantly located on the apical plasma membrane of the conjunctival epithelium. It may play an important role in restricting the conjunctival absorption of some lipophilic drugs.

Multi-drug resistance in chronic lymphocytic leukemia

We evaluated 45 chronic lymphocyte leukemia (CLL) patients for the presence of multi-drug resistance (MDR) by the ex vivo techniques: 1) a functional assay utilizing doxorubicin (dox) retention with modulation; 2) a cytotoxicity assay (MTT) with modulation; 3) and four monoclonal antibodies. Ex vivo tests were correlated with disease stage and prior treatment, and were repeated as patients became resistant to alkylating agents, fludarabine and VAD chemotherapy (infusion of vincristine, dox, and oral dexamethasone). The majority of patients (64.4%) were in early stage and were untreated (62.2%). P-glycoprotein (p-gp 170) was detected most frequently by the monoclonal antibody MRK-16 (48%) and by functional modulation of dox retention by PSC-833 (40.6%) and by functional modulation of the MTT assay with vincristine (0.29) and dox (0.39) with PSC-833 at 1.0 microg/mL. Functional modulation of dox retention with PSC-833 was significantly associated with stage, but not with either the MTT assay or any of the monoclonal antibodies. None of the tests correlated with prior chlorambucil treatment. Correlation of dox retention with the monoclonal antibodies was mild to moderate and became stronger following chlorambucil treatment. Three patients who became resistant to VAD were found to express p-gp 170. We conclude that MDR can frequently be detected in patients with CLL. Furthermore, the expression of p-gp 170 increases with advancing stage, but not prior alkylating agent therapy. The functional expression of p-gp 170 increases with advancing stage and prior treatment and correlates well with monoclonal antibody detection (especially MRK-16). Patients who become resistant to VAD more frequently express p-gp 170 by a variety of techniques. PSC-833 is a more potent modulator of MDR than cyclosporin-A (CsA) ex vivo, and correlates better with stage of disease.

Effect of modulators on the ATPase activity and vanadate nucleotide trapping of human P-glycoprotein

P-Glycoprotein (Pgp) is responsible for the energy-dependent efflux of many natural product oncolytics. Overexpression of Pgp may result in multidrug resistance (MDR). Modulators can block Pgp efflux and sensitize multidrug resistant cells to these oncolytics. To study the interaction of modulators with Pgp, Pgp-ATPase activity was examined, using plasma membranes isolated from the multidrug-resistant cell line CEM/VLB100. A survey of modulators indicated that verapamil, trifluoperazine, and nicardipine stimulated ATPase activity by 1.3- to 1.8-fold, whereas two others, trimethoxybenzoylyohimbine (TMBY) and vindoline, had no effect. Further evaluation showed that TMBY completely blocked the stimulation by verapamil of ATPase activity by competitive inhibition, with a Ki of 2.1 microM. When the effects of these two modulators on the formation of the enzyme-nucleotide complex important in the catalytic cycle were examined, verapamil increased the amount of vanadate-trapped 8-azido-[alpha-32P]ATP bound to Pgp by two-fold, whereas TMBY had no effect. Moreover, TMBY blocked the verapamil stimulation of vanadate-8-azido-[alpha-32P]ATP. Together, these data indicate that verapamil and TMBY bind to Pgp at a common site or overlapping sites, but only verapamil results in enhanced Pgp-ATP hydrolysis and formation of the vanadate-nucleotide-enzyme complex.

Characterization of a novel bisacridone and comparison with PSC 833 as a potent and poorly reversible modulator of P-glycoprotein

Novel compounds, composed of two acridone moieties connected by a propyl or butyl spacer, were synthesized and tested as potential modulators of P-glycoprotein (P-gp)-mediated multidrug resistance. The propyl derivative 1,3-bis(9-oxoacridin-10-yl)-propane (PBA) was extremely potent and, at a concentration of 1 microM, increased steady state accumulation of vinblastine (VLB) approximately 9-fold in the multidrug-resistant cell line KB8-5. In contrast to the readily reversible effects of VRP and cyclosporin A on VLB uptake and similar to the effects of the cyclosporin analog PSC 833, this modulation by PBA was not fully reversed 6-8 hr after transfer of cells to PBA-free medium. Continuous exposure to 3 microM PBA was nontoxic and could completely reverse VLB resistance in KB8-5 cells. Consistent with its effects on VLB transport, the drug resistance-modulating effect of PSC 833 was significantly more persistent than that of VRP. However, the effect of PBA was, like that of VRP, rapidly reversed once the modulator was removed from the extracellular environment. PBA was able to compete with radiolabeled azidopine for binding to P-gp and to stimulate P-gp ATPase activity. However, both the steady state accumulation of PBA and the rate of efflux of PBA were similar in drug-sensitive KB3-1 and drug-resistant KB8-5 cells, suggesting that this compound is not efficiently transported by P-gp. These results indicate that PBA represents a new class of potent and poorly reversible synthetic modulators of P-gp-mediated VLB transport.

Resistance to the chemosensitizer verapamil in a multi-drug-resistant (MDR) human multiple myeloma cell line

Inhibitors of P-glycoprotein (P-gp) or chemosensitizers, such as verapamil, are used to reverse multi-drug resistance (MDR) in cancer patients. Clinical studies in patients with myeloma have shown that some patients with P-gp-positive cancer cells respond to the chemosensitizing effect of verapamil. However, this response is short-lived and tumor cells ultimately become resistant to chemosensitizers. To study mechanisms of resistance to chemosensitizers, a human myeloma cell line, 8226/MDR10V, was selected from a P-gp-positive cell line, 8226/Dox40, in the continuous presence of doxorubicin and verapamil. MDR10V cells are consistently more resistant to MDR drugs than parent cells, Dox40. Chemosensitizers, including verapamil and cyclosporin A, were less effective in reversing resistance in MDR10V compared with Dox40 cells. Verapamil and cyclosporin A were only partially effective in blocking P-gp drug efflux in MDR10V compared to Dox40 cells. Despite higher resistance to cytotoxic agents, MDR10V cells express less P-gp in the plasma membrane than do its parent cells, Dox40. [3H]Azidopine photoaffinity labeling of P-gp and its binding competition with unlabeled verapamil showed similar affinity for P-gp between Dox40 and MDR10V cell lines. Non-P-gp-mediated mechanisms of drug resistance, including over-expression of MRP and alterations in topoisomerase II, were not different for MDR10V cells compared with Dox40 cells.

Verapamil suppresses the emergence of P-glycoprotein-mediated multi-drug resistance

Selection protocols were designed to determine whether non-cytotoxic chemomodifiers can influence the evolution of the drug-resistant phenotype. To this end, the human multiple myeloma cell line RPMI 8226 (8226/S) was selected with either doxorubicin, verapamil or doxorubicin plus verapamil. Using this approach low-level multi-drug-resistant (MDR) cell lines were obtained when 8226/S was selected with doxorubicin only or doxorubicin plus verapamil but not with verapamil only. The MDR phenotypes obtained were mechanistically distinct. In doxorubicin only-selected cells (8226/dox4), drug resistance was mediated by over-expression of the MDR1 gene and its cognate protein P-glycoprotein. In contrast, the drug resistance seen in the doxorubicin plus verapamil-selected cells was mediated through decreases in topoisomerase II protein levels and catalytic activity and not by P-glycoprotein over-expression. Cells selected with verapamil alone did not become resistant to any of the drugs tested. None of the 3 selected cell lines showed any changes in MRP gene expression when compared with 8226/S. Our results indicate that the inclusion of verapamil during drug selection with doxorubicin influences the drug-resistant phenotype by preventing the selection of MDR1/P-glycoprotein-positive cells.

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.

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.

Kinetics of cellular permeability of phenoxazine and its dependence on P-glycoprotein expression

We present here the initial characterization of the mechanism of reversal of cellular resistance to Vinca alkaloids by phenoxazine (PZ). Changes in fluorescence upon cellular accumulation of PZ allowed measurement of the membrane transport kinetics in a sensitive KB-3-1 cell line and two multi-drug resistant (MDR) counterparts. The accumulation of PZ is characterized by two uptake routes, with pseudo-first order rate constants of 0.3 s-1 and 0.07 s-1, while efflux of PZ from cells revealed rate constants of 0.2 s-1. PZ rapidly reaches steady-state concentrations within cells, which may make it more clinically useful than modulators that accumulate more slowly (e.g. verapamil).

Reversal of multidrug resistance in Friend leukemia cells by dexniguldipine-HCl

Dexniguldipine-HCl (DNIG)--a prospective clinical modulator of p170-glycoprotein (pgp170)-mediated multidrug resistance (MDR)--was evaluated in a drug-accumulation assay in MDR murine leukemia cell strain F4-6RADR expressing pgp170. The compound elevated low accumulation of either doxorubicin (DOX), daunorubicin (DNR), or mitoxantrone (MITO) in resistant F4-6RADR cells to the very levels observed in drug-sensitive F4-6 precursor cells. In parallel with the increase in DNR content (F4-6RADR, solvent: 303 +/- 27 pmol/mg protein; DNIG (3.3 mumol/l): 1,067 +/- 174 pmol/mg protein; F4-6P, solvent: 948 +/- 110 pmol/mg protein; n = 8-9, SEM), the amount of DNR tightly bound to the acid precipitate pellet obtained from F4-6RADR (i.e., protein, DNA, RNA) increased 3.9-times to the levels observed in sensitive F4-6 cells. The main pyridine metabolite of DNIG displayed similar activity. Concentration-response analysis revealed that DNIG and R,S-verapamil (VER) induced 100% reversal of the DNR accumulation shortage associated with the MDR phenotype but DNIG was 8 times more potent than VER (50% inhibitory concentration (IC50), 0.73 vs 5.4 mumol/l). In keeping with the accumulation assay, DNIG was about 10 times more potent than VER in sensitizing F4-6RADR cells to the cytostatic and cytotoxic effects of DNR in proliferation assays. In conclusion, DNIG is a potent in vitro modulator, improving (a) the accumulation of anthracycline-like cytostatics, (b) drug access to cellular binding sites, and (c) the cytostatic action of DNR in F4-6RADR leukemia cells of the MDR phenotype.

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.