Clinical reversal of multidrug resistance

Down-regulation of ABCB1 by Collateral Sensitivity Drugs Reverses Multidrug Resistance and Up-regulates Enolase I

The emergence of drug resistance remains an obstacle in the clinical treatment of cancer. Recent developments in the studies of drug resistance have identified compounds such as verapamil and tamoxifen that specifically target ABCB1-expressing multidrug resistant (MDR) cells, through an ATP-dependent ROS-generating mechanism. In this report, we demonstrate that treatment of ABCB1-expressing multidrug resistant cells (CHORC5 or MDA-Doxo400) or individual clones of the latter with sub-lethal concentrations of tamoxifen or verapamil down-regulates ABCB1 protein and mRNA expression in surviving clones. Consequently, tamoxifen- and verapamil-treated cells show increased sensitivity to chemotherapeutic drugs (e.g., colchicine and doxorubicin) and decreased sensitivity to collateral sensitivity drugs (e.g., verapamil and tamoxifen). Importantly, we show for the first time that down-regulation of ABCB1 expression resulting from tamoxifen treatment and CRISPR-knockout of ABCB1 expression up-regulate α-enolase (enolase I) protein levels and activity. These findings demonstrate a possible effect of ABCB1 expression on the metabolic homeostasis of MDR cells. Moreover, given the use of tamoxifen to prevent the recurrence of estrogen receptor-positive breast cancer, the findings of this study may be clinically important in modulating activity of other drugs.

Discovery of New 4-Indolyl Quinazoline Derivatives as Highly Potent and Orally Bioavailable P-Glycoprotein Inhibitors

The major drawbacks of P-glycoprotein (P-gp) inhibitors at the clinical stage make the development of new P-gp inhibitors challenging and desirable. In this study, we reported our structure-activity relationship studies of 4-indolyl quinazoline, which led to the discovery of a highly effective and orally active P-gp inhibitor, YS-370. YS-370 effectively reversed multidrug resistance (MDR) to paclitaxel and colchicine in SW620/AD300 and HEK293T-ABCB1 cells. YS-370 bound directly to P-gp, did not alter expression or subcellular localization of P-gp in SW620/AD300 cells, but increased the intracellular accumulation of paclitaxel. Furthermore, YS-370 stimulated the P-gp ATPase activity and had moderate inhibition against CYP3A4. Significantly, oral administration of YS-370 in combination with paclitaxel achieved much stronger antitumor activity in a xenograft model bearing SW620/Ad300 cells than either drug alone. Taken together, our data demonstrate that YS-370 is a promising P-gp inhibitor capable of overcoming MDR and represents a unique scaffold for the development of new P-gp inhibitors.

Melding Pharmacogenomic Effect of MDR1 and CYP3A5 Gene Polymorphism on Tacrolimus Dosing in Renal Transplant Recipients in Northern India

Introduction

Tacrolimus (TAC) is the mainstay immunosuppressant for renal transplantation. A narrow therapeutic index, multiple drug interactions, and interindividual variability in pharmacokinetics make it obligatory to monitor therapeutic drug levels. The Multidrug resistance gene 1 (MDR1) and CYP3A5 gene polymorphism may blend to achieve the optimal level. The optimal dose as per body weight is difficult to single out in the early posttransplantation period. In this study, we aimed to analyze the melding effect of both gene polymorphisms and to elicit the dose depending on the combination of genetic single nucleotide polymorphisms (SNPs) in northern Indian transplant recipients, for whom data are limited.

Methods

The daily TAC dose, weight-adjusted doses (mg/kg per day), TAC trough blood concentration (average of at least 3 levels), dose normalized with a corresponding dose using TAC concentration/weight-adjusted dose ratio (ng/ml per mg/kg per day) of 248 patients were recorded. All recipients were genotyped for the SNPs of CYP3A5 at intron 3 A6986G (the *3 or *1 allele), MDR1 at exons 12 (C1236T), 21 (G2677A/T), and 26 (C3435T). We analyzed the blending effect of mutant SNPs of the MDR gene and CYP3A5 for optimized TAC levels.

Results

Among CYP3A5 genotypic variants, the dose-adjusted TAC level was significantly lower, and the TAC dose required to achieve the target level was significantly higher, in CYP3A5*1*1 (expressor) than that of CYP3A5*1*3 and CYP3A5*3*3. Of the MDR1 gene SNPs, only the G2677T/A homozygous mutant was significantly associated with TAC level, and it was strongly correlated with P-gp expression.The daily TAC dose requirement was highest with a combination of CYP3A5*1*1 and homozygous mutant TT+AA genotype of G2677T/A, and was lowest with CYP3A5*3*3 and wild-type GG of the G2677T/A genotype.

Conclusion

Both CYP gene and MDR1 gene polymorphism affect TAC dose requirements, and there is a need to look for both in an individual to achieve the target trough concentration.

Non-Hodgkin's B-cell lymphoma: advances in molecular strategies targeting drug resistance

Non-Hodgkin's lymphoma (NHL) is a heterogeneous class of cancers displaying a diverse range of biological phenotypes, clinical behaviours and prognoses. Standard treatments for B-cell NHL are anthracycline-based combinatorial chemotherapy regimens composed of cyclophosphamide, doxorubicin, vincristine and prednisolone. Even though complete response rates of 40-50% with chemotherapy can be attained, a substantial proportion of patients relapse, resulting in 3-year overall survival rates of about 30%. Relapsed lymphomas are refractory to subsequent treatments with the initial chemotherapy regimen and can exhibit cross-resistance to a wide variety of anticancer drugs. The emergence of acquired chemoresistance thus poses a challenge in the clinic preventing the successful treatment and cure of disseminated B-cell lymphomas. Gene-expression analyses have increased our understanding of the molecular basis of chemotherapy resistance and identified rational targets for drug interventions to prevent and treat relapsed/refractory diffuse large B-cell lymphoma. Acquisition of drug resistance in lymphoma is in part driven by the inherent genetic heterogeneity and instability of the tumour cells. Due to the genetic heterogeneity of B-cell NHL, many different pathways leading to drug resistance have been identified. Successful treatment of chemoresistant NHL will thus require the rational design of combinatorial drugs targeting multiple pathways specific to different subtypes of B-cell NHL as well as the development of personalized approaches to address patient-to-patient genetic heterogeneity. This review highlights the new insights into the molecular basis of chemorefractory B-cell NHL that are facilitating the rational design of novel strategies to overcome drug resistance.

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.

HG-829 is a potent noncompetitive inhibitor of the ATP-binding cassette multidrug resistance transporter ABCB1

Transmembrane drug export mediated by the ATP-binding cassette (ABC) transporter P-glycoprotein contributes to clinical resistance to antineoplastics. In this study, we identified the substituted quinoline HG-829 as a novel, noncompetitive, and potent P-glycoprotein inhibitor that overcomes in vitro and in vivo drug resistance. We found that nontoxic concentrations of HG-829 restored sensitivity to P-glycoprotein oncolytic substrates. In ABCB1-overexpressing cell lines, HG-829 significantly enhanced cytotoxicity to daunorubicin, paclitaxel, vinblastine, vincristine, and etoposide. Coadministration of HG-829 fully restored in vivo antitumor activity of daunorubicin in mice without added toxicity. Functional assays showed that HG-829 is not a Pgp substrate or competitive inhibitor of Pgp-mediated drug efflux but rather acts as a noncompetitive modulator of P-glycoprotein transport function. Taken together, our findings indicate that HG-829 is a potent, long-acting, and noncompetitive modulator of P-glycoprotein export function that may offer therapeutic promise for multidrug-resistant malignancies.

Dihydropyridines and multidrug resistance: previous attempts, present state, and future trends

Multidrug resistance is defined as the resistance of a tumor cell to the cytotoxic action of divergent drugs used in chemotherapy. Dihydropyridines are a class of calcium channel antagonists that were discovered to have a multidrug resistance reversing effect and prompted investigations resulting in the synthesis of hundreds of new derivatives. Most of the investigators tried to achieve two goals: a decrease in Ca²(+) channel-blocking activity and an increase in the multidrug resistance reversing effect. Most of the synthesized compounds failed in the later stages of studies especially in clinical trials because of pharmacokinetic or pharmacodynamic limitations. Therefore, it will be necessary to include new methods, such as combinatorial synthesis, and, more importantly, to apply computational methods based on global structure-activity relationship models that consider all problems. Moreover, some compounds should be synthesized that are effective on several multidrug resistance targets.

Sorafenib and sunitinib: novel targeted therapies for renal cell cancer

Renal cell cancer (RCC) is a relatively uncommon malignancy, with 51,190 cases expected to be diagnosed in 2007. Localized disease is curable by surgery; however, locally advanced or metastatic disease is not curable in most cases and, until recently, had a limited response to drug treatment. Historically, biologic response modifiers or immunomodulating agents were tested in clinical trials based on observations that some cases of RCC can spontaneously regress. High-dose aldesleukin is approved by the United States Food and Drug Administration as a treatment for advanced RCC; however, the drug is associated with a high frequency of severe adverse effects. Responses have been observed with low-dose aldesleukin and interferon alfa, but with little effect on overall survival. Sorafenib and sunitinib are novel therapies that target growth factor receptors known to be activated by the hypoxia-inducible factor and the Ras-Raf/MEK/ERK pathways. These pathways are important in the pathophysiology of RCC. Sorafenib and sunitinib have shown antitumor activity as first- and second-line therapy in patients with cytokine-refractory metastatic RCC who have clear-cell histology. Although complete responses are not common, both drugs promote disease stabilization and increase progression-free survival. This information suggests that disease stabilization may be an important determinant for response in RCC and possibly other cancers. Sorafenib and sunitinib are generally well tolerated and are considered first- and second-line treatment options for patients with advanced clear cell RCC. In addition, sorafenib and sunitinib have shown promising results in initial clinical trials evaluating antitumor activity in patients who are refractory to other antiangiogenic therapy. The most common toxicities with both sorafenib and sunitinib are hand-foot syndrome, rash, fatigue, hypertension, and diarrhea. Research is directed toward defining the optimal use of these new agents.

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

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

Development of ruthenium antitumor drugs that overcome multidrug resistance mechanisms

Organometallic ruthenium(II) complexes of the general formula [Ru(eta6-p-cymene)Cl2(L)] and [Ru(eta6-p-cymene)Cl(L)2][BPh4] with modified phenoxazine- and anthracene-based multidrug resistance (MDR) modulator ligands (L) have been synthesized, spectroscopically characterized, and evaluated in vitro for their cytotoxic and MDR reverting properties in comparison with the free ligands. For an anthracene-based ligand, coordination to a ruthenium(II) arene fragment led to significant improvement of cytotoxicity as well as Pgp inhibition activity. A similar, but weaker effect was also observed when using a benzimidazole-phenoxazine derivative as Pgp inhibitor. The most active compound in terms of both Pgp inhibition and cytotoxicity is [Ru(eta6-p-cymene)Cl2(L)], where L is an anthracene-based ligand. Studies show that it induces cell death via inhibition of DNA synthesis. Moreover, because the complex is fluorescent, its uptake in cells was studied, and relative to the free anthracene-based ligand, uptake of the complex is accelerated and accumulation of the complex in the cell nucleus is observed.

The prognostic significance of P-glycoprotein, multidrug resistance-related protein 1 and lung resistance protein in pediatric acute lymphoblastic leukemia: a retrospective study of 295 newly diagnosed patients by the Children's Oncology Group

Multidrug resistance (MDR) is a phenomenon by which cells become resistant to an array of structurally unrelated chemotherapeutic agents. The prognostic value that P-glycoprotein (Pgp), multidrug resistance-related protein 1 (MRP1), and lung resistance protein (LRP) have in the setting of pediatric acute lymphoblastic leukemia (ALL) is controversial. In a retrospective study, we analyzed samples obtained from 295 similarly treated pediatric ALL patients to assess whether the overexpression and/or function of these proteins at diagnosis affects outcome. Most patients (70%, 207/295) did not overexpress an MDR protein. A small number of patients expressed functional Pgp (1%, 3/295) and some overexpressed functional MRP1 (10%, 19/295), with a statistically significant number of the latter being of T-lineage as opposed to pre-B (P < 0.001). A small number of patients (2%, 6/295) also overexpressed both Pgp and MRP1. Additional patients expressed increased levels of LRP. Elevated levels of these proteins at diagnosis did not correlate with risk factors and did not predict an adverse prognosis. Life-table estimates and Kaplan-Meier plots did not show any significant differences between patients who overexpressed an MDR protein compared with those who did not, nor was any difference noted when the different MDR + groups were compared with one another. These data strongly support the conclusion that the overexpression of these functional drug efflux pumps at diagnosis does not contribute to treatment failure in pediatric ALL.

Pumping of drugs by P-glycoprotein: a two-step process?

The apparent inhibition constant, Kapp, for the blockade of P-glycoprotein (P-gp) by four drugs, verapamil, cyclosporin A, XR9576 (tariquidar), and vinblastine, was measured by studying their ability to inhibit daunorubicin and calcein-AM efflux from four strains of Ehrlich cells with different levels of drug resistance and P-gp content. For daunorubicin as a transport substrate, Kapp was independent of [P-gp] for verapamil but increased strictly linearly with [P-gp] for vinblastine, cyclosporin A, and XR9576. A theoretical analysis of the kinetics of drug pumping and its reversal shows that Kapp for inhibition should increase linearly with the amount of pumps present in the membrane for a reverser that inhibits pumping from the cytoplasmic face. In contrast, if the reverser acts by blocking transport from the outer face, i.e., preemptively, Kapp should be independent of the number of pumps present. The experimental data suggest that verapamil blocks pumping at the extracellular face of the membrane, whereas the other three blockers act on pumping from the cytoplasmic phase. The maximum degree of inhibition was the same for all four blockers; thus, they do not act in parallel but rather, in serial, i.e., a drug that is pumped from the cytoplasmic phase has to pass the preemptive route upon leaving the cell. Our results are consistent with the Sauna-Ambudkar two-step model for pumping by P-gp. We suggest that the vinblastine/cyclosporin A/XR9576-binding site accepts daunorubicin at the cytoplasmic face and transfers it to the verapamil-binding site, from where daunorubicin is emptied at the extracellular surface.

Multidrug resistance-1 (MDR-1) in autoimmune disorders III: increased P-glycoprotein activity in lymphocytes from immune thrombocytopenic purpura patients

OBJECTIVE

P-glycoprotein (P-gp) expression has been widely observed in normal and neoplastic cells. The physiologic role of P-gp involves hormone and metabolite secretion, bacterial product detoxification, and transport of several drugs to the extracellular space. Multidrug resistance-1 is characterized by drug extrusion through P-gp, reducing the intracellular levels of drugs and diminishing their pharmacological effects. Treatment of immune thrombocytopenic purpura (ITP) includes agents that are substrates of P-gp; hence, the objective of this study was to analyze the functional activity of P-gp in lymphocytes from patients with ITP.

PATIENTS AND METHODS

30 ITP patients (9 refractory, 5 dependent, 14 responders to treatment, and 2 with stable disease) and 25 healthy controls were studied. Peripheral blood mononuclear cells were isolated by gradient centrifugation and incubated with daunorubicin (a fluorescent drug extruded by P-gp). Functional activity of P-gp was analyzed by flow cytometry. Results were expressed as the percentage of lymphocytes able to extrude daunorubicin.

RESULTS

ITP patients showed an increased number of lymphocytes with P-gp activity (mean=12.3%+/-16%) when compared to controls (mean=0.87%+/-0.72%) (p<0.05). P-gp function was higher in the refractory group (median=9.4%) than in the treatment-dependent (median=5.4%), responder (median=6.4%), and stable disease (median=5.2%) groups, although no statistical differences were found among them.

CONCLUSION

Enhanced P-gp activity in ITP may be related to an unfavorable clinical outcome and poor response to treatment. Furthermore, P-gp function might affect therapeutic requirements for disease control.

The medicinal chemistry of multidrug resistance (MDR) reversing drugs

Multidrug resistance (MDR) is a kind of resistance of cancer cells to multiple classes of chemotherapic drugs that can be structurally and mechanistically unrelated. Classical MDR regards altered membrane transport that results in lower cell concentrations of cytotoxic drug and is related to the over expression of a variety of proteins that act as ATP-dependent extrusion pumps. P-glycoprotein (Pgp) and multidrug resistance protein (MRP1) are the most important and widely studied members of the family that belongs to the ABC superfamily of transporters. It is apparent that, besides their role in cancer cell resistance, these proteins have multiple physiological functions as well, since they are expressed also in many important non-tumoural tissues and are largely present in prokaryotic organisms. A number of drugs have been identified which are able to reverse the effects of Pgp, MRPI and sister proteins, on multidrug resistance. The first MDR modulators discovered and studied in clinical trials were endowed with definite pharmacological actions so that the doses required to overcome MDR were associated with unacceptably high side effects. As a consequence, much attention has been focused on developing more potent and selective modulators with proper potency, selectivity and pharmacokinetics that can be used at lower doses. Several novel MDR reversing agents (also known as chemosensitisers) are currently undergoing clinical evaluation for the treatment of resistant tumours. This review is concerned with the medicinal chemistry of MDR reversers, with particular attention to the drugs that are presently in development.

Interaction of the P-glycoprotein multidrug transporter (MDR1) with high affinity peptide chemosensitizers in isolated membranes, reconstituted systems, and intact cells

P-glycoprotein-mediated multidrug resistance can be reversed by the action of a group of compounds known as chemosensitizers. The interactions with P-glycoprotein of two novel hydrophobic peptide chemosensitizers (reversins 121 and 205) have been studied in model systems in vitro, and in a variety of MDR1-expressing intact tumor cells. The reversins bound to purified P-glycoprotein with high affinity (77-154 nM), as assessed by a quenching assay using fluorescently labeled purified protein. The peptides modulated P-glycoprotein ATPase activity in Sf9 insect cell membranes expressing human MDR1, plasma membrane vesicles from multidrug-resistant cells, and reconstituted proteoliposomes. Both peptides induced a large stimulation of ATPase activity; however, higher concentrations, especially of reversin 205, led to inhibition. This pattern was different from that of simple linear peptides, and resembled that of chemosensitizers such as verapamil. In both membrane vesicles and reconstituted proteoliposomes, 1-2 microM reversins were more effective than cyclosporin A at blocking colchicine transport. Reversin 121 and reversin 205 restored the uptake of [3H]daunorubicin and rhodamine 123 in MDR1-expressing cells to the level observed in the drug-sensitive parent cell lines, and also effectively inhibited the extrusion of calcein acetoxymethyl ester from intact cells. In cytotoxicity assays, reversin 121 and reversin 205 eliminated the resistance of MDR1-expressing tumor cells against MDR1-substrate anticancer drugs, and they had no toxic effects in MDR1-negative control cells. We suggest that peptides of the reversin type interact with the MDR1 protein with high affinity and specificity, and thus they may be good candidates for the development of MDR1-modulating agents to sensitize drug resistance in cancer.

Mechanism of action of P-glycoprotein in relation to passive membrane permeation

This review presents a survey of studies of the movement of chemotherapeutic drugs into cells, their extrusion from multidrug-resistant (MDR) cells overexpressing P-glycoprotein (Pgp), and the mode of sensitization of MDR cells to anticancer drugs by Pgp modulators. The consistent features of the kinetics from studies of the operation of Pgp in cells were combined in a computer model that enables the simulation of experimental scenarios. MDR-type drugs are hydrophobic and positively charged and as such bind readily to negatively charged phospholipid head groups of the membrane. Transmembrane movement of MDR-type drugs, such as doxorubicin, occurs by a flip-flop mechanism with a lifetime of about 1 min rather than by diffusion down a gradient present in the lipid core. A long residence time of a drug in the membrane leaflet increases the probability that P-glycoprotein will remove it from the cell. In a manner similar to ion-transporting ATPases, such as Na+,K(+)-ATPase, Pgp transports close to one drug molecule per ATP molecule hydrolyzed. Computer simulation of cellular pharmacokinetics, based on partial reactions measured in vitro, show that the efficiency of Pgp, in conferring MDR on cells, depends on the pumping capacity of Pgp and its affinity toward the specific drug, the transmembrane movement rate of the drug, the affinity of the drug toward its pharmacological cellular target, and the affinity of the drug toward intracellular trapping sites. Pgp activities present in MDR cells allow for the efficient removal of drugs, whether directly from the cytoplasm or from the inner leaflet of the plasma membrane. A prerequisite for a successful modulator, capable of overcoming cellular Pgp, is the rapid passive transbilayer movement, allowing it to reenter the cell immediately and thus successfully occupy the Pgp active site(s).

Membrane fluidization by ether, other anesthetics, and certain agents abolishes P-glycoprotein ATPase activity and modulates efflux from multidrug-resistant cells

The anesthetics benzyl alcohol and the nonaromatic chloroform and diethyl ether, abolish P-glycoprotein (Pgp) ATPase activity in a mode that does not fit classical competitive, noncompetitive, or uncompetitive inhibition. At concentrations similar to those required for inhibition of ATPase activity, these anesthetics fluidize membranes leading to twofold acceleration of doxorubicin flip-flop across lipid membranes and prevent photoaffinity labeling of Pgp with [125I]-iodoarylazidoprazosin. Similar concentrations of ether proved nontoxic and modulated efflux from Pgp-overexpressing cells. A similar twofold acceleration of doxorubicin flip-flop rate across membranes was observed with neutral mild detergents, including Tween 20, Nonidet P-40 and Triton X-100, and certain Pgp modulators, such as verapamil and progesterone. Concentrations of these agents, similar to those required for membrane fluidization, inhibited Pgp ATPase activity in a mode similar to that observed with the anesthetics. The mode of inhibition, i.e. lack of evidence for classical enzyme inhibition and the correlation of Pgp ATPase inhibition with membrane fluidization over a wide range of concentrations and structures of drugs favors the direct inhibition of Pgp ATPase activity by membrane fluidization. The unusual sensitivity of Pgp to membrane fluidization, as opposed to acceleration of ATPase activity of ion transporters, could fit the proposed function of Pgp as a 'flippase', which is in close contact with the membrane core.