Multidrug resistance in cancer chemotherapy

Relevance of ABC Transporters in Drug Development

ATP-binding cassette (ABC) transporters play a critical role in protecting vital organs such as the brain and placenta against xenobiotics, as well as in modulating the pharmacological and toxicological profile of several drug candidates by restricting their penetration through cellular and tissue barriers. This review paper provides a description of the structure and function of ABC transporters as well as the role of P-glycoprotein, multidrug resistance-associated protein 2 and breast cancer resistance protein in the disposition of drugs. Furthermore, a review of the in vitro and in vivo techniques for evaluating the interaction between drugs and ABC transporters are provided.

The role of endolysosomal trafficking in anticancer drug resistance

Multidrug resistance (MDR) remains a major obstacle towards curative treatment of cancer. Despite considerable progress in delineating the basis of intrinsic and acquired MDR, the underlying molecular mechanisms remain to be elucidated. Emerging evidences suggest that dysregulation in endolysosomal compartments is involved in mediating MDR through multiple mechanisms, such as alterations in endosomes, lysosomes and autophagosomes, that traffic and biodegrade the molecular cargo through macropinocytosis, autophagy and endocytosis. For example, altered lysosomal pH, in combination with transcription factor EB (TFEB)-mediated lysosomal biogenesis, increases the sequestration of hydrophobic anti-cancer drugs that are weak bases, thereby producing an insufficient and off-target accumulation of anti-cancer drugs in MDR cancer cells. Thus, the use of well-tolerated, alkalinizing compounds that selectively block Vacuolar H⁺-ATPase (V-ATPase) may be an important strategy to overcome MDR in cancer cells and increase chemotherapeutic efficacy. Other mechanisms of endolysosomal-mediated drug resistance include increases in the expression of lysosomal proteases and cathepsins that are involved in mediating carcinogenesis and chemoresistance. Therefore, blocking the trafficking and maturation of lysosomal proteases or direct inhibition of cathepsin activity in the cytosol may represent novel therapeutic modalities to overcome MDR. Furthermore, endolysosomal compartments involved in catabolic pathways, such as macropinocytosis and autophagy, are also shown to be involved in the development of MDR. Here, we review the role of endolysosomal trafficking in MDR development and discuss how targeting endolysosomal pathways could emerge as a new therapeutic strategy to overcome chemoresistance in cancer.

Gold nanoparticle-conjugated nanomedicine: design, construction, and structure-efficacy relationship studies

In comparison with conventional therapies, nanomedicine shows prominent clinical performance, with better therapeutic efficacy and less off-target toxicity. As an important component of nanomedicine, gold nanoparticle (GNP)-based nanodrugs have attracted considerable interest because of their excellent performance given by the unique structure. Although no pharmaceutical formulations of GNP-associated nanodrugs have been officially marketed yet, a substantial amount of research on this aspect is being carried out, producing numerous GNP-based drug delivery systems with potential clinical applications. In this review, we present an overview of our progress on GNP-based nanodrugs combined with other achievements in biomedical applications, including drug-conjugated GNPs prepared for disease treatments and specific tumour targeting, structure-efficacy relationship (SER) studies on GNP-conjugated nanodrugs, and therapeutic hybrid nanosystems composed of GNPs. In addition, we also put forward some proposals to guide future work in developing GNP-based nanomedicine. We hope that this review will offer some useful experience for our peers and GNP-based nanodrugs will be utilized in the clinic with further persistent efforts.

Synthesis and Anticancer Activity of Hybrid Molecules Based on Lithocholic and (5Z,9Z)-Tetradeca-5,9-dienedioic Acids Linked via Mono(di,tri,tetra)ethylene Glycol and α,ω-Diaminoalkane Units

For the first time, hybrid molecules were synthesized on the basis of lithocholic and (5Z,9Z)-1,14-tetradeca-5,9-dienedicarboxylic acids, obtained in two stages using the homo-cyclomagnesiation reaction of 2-(hepta-5,6-diene-1-yloxy)tetrahydro-2H-pyran at the key stage. The resulting hybrid molecules containing 5Z,9Z-dienoic acids are of interest as novel synthetic biologically active precursors to create modern drugs for the treatment of human oncological diseases. The synthesized hybrid molecules were found to exhibit extremely high in vitro inhibitory activity against human topoisomerase I, which is 2-4 times higher than that of camptothecin, a known topoisomerase I inhibitor. Using flow cytometry and fluorescence microscopy, it was first shown that these new molecules are efficient apoptosis inducers in HeLa, U937, Jurkat, K562, and Hek293 cell cultures. In addition, the results of investigations into the effect of the synthesized acids on mitochondria and studies of possible DNA damage in Jurkat tumor cells are also presented.

Transcriptome profiling reveals Silibinin dose-dependent response network in non-small lung cancer cells

Silibinin (SIL), a natural flavonolignan from the milk thistle (Silybum marianum), is known to exhibit remarkable hepatoprotective, antineoplastic and EMT inhibiting effects in different cancer cells by targeting multiple molecular targets and pathways. However, the predominant majority of previous studies investigated effects of this phytocompound in a one particular cell line. Here, we carry out a systematic analysis of dose-dependent viability response to SIL in five non-small cell lung cancer (NSCLC) lines that gradually differ with respect to their intrinsic EMT stage. By correlating gene expression profiles of NSCLC cell lines with the pattern of their SIL IC50 response, a group of cell cycle, survival and stress responsive genes, including some prominent targets of STAT3 (BIRC5, FOXM1, BRCA1), was identified. The relevancy of these computationally selected genes to SIL viability response of NSCLC cells was confirmed by the transient knockdown test. In contrast to other EMT-inhibiting compounds, no correlation between the SIL IC50 and the intrinsic EMT stage of NSCLC cells was observed. Our experimental results show that SIL viability response of differently constituted NSCLC cells is linked to a subnetwork of tightly interconnected genes whose transcriptomic pattern can be used as a benchmark for assessment of individual SIL sensitivity instead of the conventional EMT signature. Insights gained in this study pave the way for optimization of customized adjuvant therapy of malignancies using Silibinin.

Immunoadjuvants for cancer immunotherapy: A review of recent developments

Cancer immunotherapy evolved as a new treatment modality to eradicate tumor cells and has gained in popularity after its successful clinical transition. By activating antigen-presenting cells (APCs), and thus, inducing innate or adaptive immune responses, immunoadjuvants have become promising tools for cancer immunotherapy. Different types of immunoadjuvants such as toll-like receptor (TLR) agonists, exosomes, and metallic and plant-derived immunoadjuvants have been studied for their immunological effects. However, the clinical use of immunoadjuvants is limited by short response rates and various side-effects. The rapid progress made in the development of nanoparticle systems as immunoadjuvant carrier vehicles has provided potential carriers for cancer immunotherapy. In this review article, we describe different types of immunoadjuvants, their limitations, modes of action, and the reasons for their clinical adoption. In addition, we review recent progress made in the nanoparticle-based immunoadjuvant field and on the combined use of nanoparticle-based immunoadjuvants and chemotherapy, phototherapy, radiation therapy, and immune checkpoint inhibitor-based therapy. STATEMENT OF SIGNIFICANCE: Cancer immunotherapy emerged as a new hope for treating malignant tumors. Different types of immunoadjuvants serve as an important tool for cancer immunotherapy by activating an innate or adaptive immune response. Limitation of free immunoadjuvant has paved the path for the development of nanoparticle-based immunoadjuvant therapy with the hope of prolonging the therapeutic efficacy. This review highlights the recent advancement made in nanoparticle-based immunoadjuvant therapy in modulating the adaptive and innate immune system. The application of the combinatorial approach of chemotherapy, phototherapy, radiation therapy adds synergy in nanoparticle-based immunoadjuvant therapy. It will broaden the reader's understanding on the recent progress made in immunotherapy with the aid of immunoadjuvant-based nanosystem.

DHPAC, a novel synthetic microtubule destabilizing agent, possess high anti-tumor activity in vincristine-resistant oral epidermoid carcinoma in vitro and in vivo

Multidrug resistance (MDR) is one of major obstacles to effective chemotherapeutic treatment of cancer. This study showed that DHPAC, 2-(6-ethoxy-3-(3-ethoxyphenylamino) -1-methyl-1,4-dihydroindeno[1,2-c]pyrazol-7-yloxy) acetamide, a novel compound that binds to the same site on microtubules as colchicine, has high anti-tumour activity in vincristine-resistant oral epidermoid carcinoma (KB/V) cells. It found that the presence of DHPAC strongly inhibited KB/V cell growth in vivo and in mice xenograft. The inhibitory effect of DHPAC is much stronger than that by colchicine in these KB/V cells (IC₅₀: 64.4nM and 458.0nM respectively). Treatment of the cells with DHPAC induced cell apoptosis by reducing mitochondrial membrane potential and altered the expression of several apoptosis-related proteins such as Bcl-2, Bax, Caspase-9, Cytochrome c and PARP. DHPAC treatment also caused cell rest in G2/M phase by regulating of the expression of a number of cell cycle-related proteins (e.g. Cyclin B1, Cdc2, Cdc25b, Cdc25c, RSK2). Furthermore, DHPAC presence inhibits PTEN phosphorylation and PTEN/Akt/NF-κB signalling. Thus, DHPAC has potent anti-cancer activity in MDR tumuors and may be a potential therapeutic agent for the treatment of vincristine-resistant human oral epidermoid carcinoma.

Design, synthesis, and structure-activity relationships of pyrimido[4,5-b]indole-4-amines as microtubule depolymerizing agents that are effective against multidrug resistant cells

To identify the structural features of 9H-pyrimido[4,5-b]indoles as microtubule depolymerizers, pyrimido[4,5-b]indoles 2-8 with varied substituents at the 2-, 4- and 5-positions were designed and synthesized. Nucleophilic displacement of 2,5-substituted-4-chloro-pyrimido[4,5-b]indoles with appropriate arylamines was the final step employed in the synthesis of target compounds 2-8. Compounds 2 and 6 had two-digit nanomolar potency (IC₅₀) against MDA-MB-435, SK-OV-3 and HeLa cancer cells in vitro. Compounds 2 and 6 also depolymerized microtubules comparable to the lead compound 1. Compounds 2, 3, 6 and 8 were effective in cells expressing P-glycoprotein or the βIII isotype of tubulin, mechanisms that are associated with clinical drug resistance to microtubule targeting drugs. Proton NMR and molecular modeling studies were employed to identify the structural basis for the microtubule depolymerizing activity of pyrimido[4,5-b]indoles.

Applying biodegradable particles to enhance cancer vaccine efficacy

One of the primary goals of our group and our collaborators here at the University of Iowa is to develop therapeutic cancer vaccines using biodegradable and biocompatible polymer-based vectors. A major advantage of using discretely packaged immunogenic cargo over non-encapsulated vaccines is that they promote enhanced cellular immunity, a key requirement in achieving antitumor activity. We discuss the importance of co-encapsulation of tumor antigen and adjuvant, with specific focus on the synthetic oligonucleotide adjuvant, cytosine-phosphate-guanine oligodeoxynucleotides. We also discuss our research using a variety of polymers including poly(α-hydroxy acids) and polyanhydrides, with the aim of determining the effect that parameters, such as size and polymer type, can have on prophylactic and therapeutic tumor vaccine formulation efficacy. Aside from their role as vaccine vectors per se, we also address the research currently underway in our group that utilizes more novel applications of biodegradable polymer-based particles in facilitating other types of immune-based therapies.

SKI-II reverses the chemoresistance of SGC7901/DDP gastric cancer cells

Cisplatin is frequently used in treating gastric cancers; however, acquired resistance to the drug often reduces the efficacy of therapy. The present study analyzed the efficacy of the combination of 4-[4-(4-chloro-phenyl)-thiazol-2-ylamino]-phenol (SKI-II) and cisplatin [cis-diamminedichloroplatinum (II); DDP] on the gastric cancer SGC7901/DDP cell line. The results revealed that SKI-II and DDP had a clear synergistic effect. Glutathione (GSH) and glutathione S-transferase (GST) levels decreased significantly subsequent to the cells being treated with the combination of DDP and SKI-II compared with the cells that were treated with DDP or SKI-II alone. Phosphorylated extracellular-signal-regulated kinase (p-ERK) and phosphorylated c-Jun N-terminal kinase (p-JNK) expression levels also decreased following treatment with SKI-II. The results suggested that SKI-II is able to reverse the drug resistance in human gastric carcinoma cells and enhance the antitumor effect of DDP through the ras/mitogen-activated protein kinase (MAPK) proliferation pathway.

Phase 0 and phase III transport in various organs: combined concept of phases in xenobiotic transport and metabolism

The historical phasing concept of drug metabolism and elimination was introduced to comprise the two phases of metabolism: phase I metabolism for oxidations, reductions and hydrolyses, and phase II metabolism for synthesis. With this concept, biological membrane barriers obstructing the accessibility of metabolism sites in the cells for drugs were not considered. The concept of two phases was extended to a concept of four phases when drug transporters were detected that guided drugs and drug metabolites in and out of the cells. In particular, water soluble or charged drugs are virtually not able to overcome the phospholipid membrane barrier. Drug transporters belong to two main clusters of transporter families: the solute carrier (SLC) families and the ATP binding cassette (ABC) carriers. The ABC transporters comprise seven families with about 20 carriers involved in drug transport. All of them operate as pumps at the expense of ATP splitting. Embedded in the former phase concept, the term "phase III" was introduced by Ishikawa in 1992 for drug export by ABC efflux pumps. SLC comprise 52 families, from which many carriers are drug uptake transporters. Later on, this uptake process was referred to as the "phase 0 transport" of drugs. Transporters for xenobiotics in man and animal are most expressed in liver, but they are also present in extra-hepatic tissues such as in the kidney, the adrenal gland and lung. This review deals with the function of drug carriers in various organs and their impact on drug metabolism and elimination.

Norcantharidin, derivative of cantharidin, for cancer stem cells

Cancer stem cells (CSCs) existing in human cancers have been demonstrated to be a major cause of cancer treatment resistance, invasion, metastasis, and relapse. Self-renewal pathways, Wnt/β-catenin, Sonic hedgehog (Shh), and the Notch signaling pathway play critical roles in developing CSCs and lead to angiogenesis, migration, invasion, and metastasis. Multidrug resistance (MDR) is an unfavorable factor causing the failure of treatments against cancer cells. The most important and thoroughly studied mechanism involved in MDR is the active efflux of chemotherapeutic agents through membrane drug transporters. There is growing evidence that Norcantharidin (NCTD), a water-soluble synthetic small molecule derivative of naturally occurring cantharidin from the medicinal insect blister beetle (Mylabris phalerata Pallas), is capable of chemoprevention and tumor inhibition. We summarize investigations into the modulation of self-renewal pathways and MDR in CSCs by NCTD. This review may aid in further investigation of using NCTD to develop more effective strategies for cancer treatment to reduce resistance and recurrence.

Hormone response in ovarian cancer: time to reconsider as a clinical target?

Ovarian cancer is the sixth most common cancer worldwide among women in developed countries and the most lethal of all gynecologic malignancies. There is a critical need for the introduction of targeted therapies to improve outcome. Epidemiological evidence suggests a critical role for steroid hormones in ovarian tumorigenesis. There is also increasing evidence from in vitro studies that estrogen, progestin, and androgen regulate proliferation and invasion of epithelial ovarian cancer cells. Limited clinical trials have shown modest response rates; however, they have consistently identified a small subset of patients that respond very well to endocrine therapy with few side effects. We propose that it is timely to perform additional well-designed trials that should include biomarkers of response.

Hormone response in ovarian cancer: time to reconsider as a clinical target?

Ovarian cancer is the sixth most common cancer worldwide among women in developed countries and the most lethal of all gynecologic malignancies. There is a critical need for the introduction of targeted therapies to improve outcome. Epidemiological evidence suggests a critical role for steroid hormones in ovarian tumorigenesis. There is also increasing evidence from in vitro studies that estrogen, progestin, and androgen regulate proliferation and invasion of epithelial ovarian cancer cells. Limited clinical trials have shown modest response rates; however, they have consistently identified a small subset of patients that respond very well to endocrine therapy with few side effects. We propose that it is timely to perform additional well-designed trials that should include biomarkers of response.

Small-molecule synthetic compound norcantharidin reverses multi-drug resistance by regulating Sonic hedgehog signaling in human breast cancer cells

Multi-drug resistance (MDR), an unfavorable factor compromising treatment efficacy of anticancer drugs, involves upregulated ATP binding cassette (ABC) transporters and activated Sonic hedgehog (Shh) signaling. By preparing human breast cancer MCF-7 cells resistant to doxorubicin (DOX), we examined the effect and mechanism of norcantharidin (NCTD), a small-molecule synthetic compound, on reversing multidrug resistance. The DOX-prepared MCF-7R cells also possessed resistance to vinorelbine, characteristic of MDR. At suboptimal concentration, NCTD significantly inhibited the viability of DOX-sensitive (MCF-7S) and DOX-resistant (MCF-7R) cells and reversed the resistance to DOX and vinorelbine. NCTD increased the intracellular accumulation of DOX in MCF-7R cells and suppressed the upregulated the mdr-1 mRNA, P-gp and BCRP protein expression, but not the MRP-1. The role of P-gp was strengthened by partial reversal of the DOX and vinorelbine resistance by cyclosporine A. NCTD treatment suppressed the upregulation of Shh expression and nuclear translocation of Gli-1, a hallmark of Shh signaling activation in the resistant clone. Furthermore, the Shh ligand upregulated the expression of P-gp and attenuated the growth inhibitory effect of NCTD. The knockdown of mdr-1 mRNA had not altered the expression of Shh and Smoothened in both MCF-7S and MCF-7R cells. This indicates that the role of Shh signaling in MDR might be upstream to mdr-1/P-gp, and similar effect was shown in breast cancer MDA-MB-231 and BT-474 cells. This study demonstrated that NCTD may overcome multidrug resistance through inhibiting Shh signaling and expression of its downstream mdr-1/P-gp expression in human breast cancer cells.

Starvation, detoxification, and multidrug resistance in cancer therapy

The selection of chemotherapy drugs is based on the cytotoxicity to specific tumor cell types and the relatively low toxicity to normal cells and tissues. However, the toxicity to normal cells poses a major clinical challenge, particularly when malignant cells have acquired resistance to chemotherapy. This drug resistance of cancer cells results from multiple factors including individual variation, genetic heterogeneity within a tumor, and cellular evolution. Much progress in the understanding of tumor cell resistance has been made in the past 35 years, owing to milestone discoveries such as the identification and characterization of ABC transporters. Nonetheless, the complexity of the genetic and epigenetic rewiring of cancer cells makes drug resistance an equally complex phenomenon that is difficult to overcome. In this review, we discuss how the remarkable changes in the levels of glucose, IGF-I, IGFBP-1 and in other proteins caused by fasting have the potential to improve the efficacy of chemotherapy against tumors by protecting normal cells and tissues and possibly by diminishing multidrug resistance in malignant cells.

Can we develop biomarkers that predict response of cancer patients to immunotherapy?

PRIMARY OBJECTIVE

The primary objective is to delineate the potential utility of cancer biomarkers that correlate and predict response to immunotherapy in cancer patients who are refractory to conventional therapeutics. Unlike significant development of biomarkers that predict response to chemotherapy, very few biomarkers have been developed to predict the response to immunotherapy.

MAIN OUTCOMES AND RESULTS

This article describes briefly the importance of characterizing and validating biomarkers for immunotherapy. A few examples have been provided, such as the transcription factor NF-kappaB, the transcription repressor Yin-Yang 1 (YY1), the pro-apoptotic gene product (Smac/DIABLO) and the circulating Fas and Fas ligand. These biomarkers have been determined to be of prognostic significance in different cancers.

CONCLUSIONS

Immunotherapy is considered as an alternative therapy in the treatment of cancer patients who are refractory to chemotherapy/radiation/hormonal therapies. Cross-resistance to apoptosis develops between cancer cells that are resistant to conventional therapeutics and immunotherapy. Therefore, it is important to develop biomarkers that will determine patient response to immunotherapy.

Frequency of C3435T single nucleotide MDR1 genetic polymorphism in an Asian population: phenotypic-genotypic correlates

AIMS

To investigate the frequency of the single nucleotide polymorphism C3435T in exon 26 of the MDR1 gene in Asians and to determine the functional significance of this SNP with the clinical pharmacokinetics of oral cyclosporin (Neoral) in 10 stable heart transplant patients.

METHODS

The MDR1 C3435T polymorphism was investigated in 290 healthy Asian subjects (98 Chinese, 99 Malays and 93 Indians). We also compared the MDR1 polymorphism between the Asian population studied here and the published data on Africans and Caucasians. The clinical relevance of this SNP on oral bioavailability of a known P-gp substrate, cyclosporin, was assessed in 10 stable Chinese heart transplant patients.

RESULTS

The homozygous TT genotype was observed in 32%, 28% and 43% of Chinese, Malays and Indians. The homozygous CC genotype was found in 25% of Chinese and Malays compared with 18% of Indians. The Indians had a lower frequency of the C allele [0.38 (0.31-0.45)] compared with the Chinese [0.46 (0.39-0.53)] and Malays [0.48 (0.42-0.55)]. Chi-squared test showed that the distribution of allele frequencies between the Malays and Indians differed significantly (P = 0.04). In this Asian population, the overall distribution of genotypes (CC, CT and TT) and allele frequencies were significantly different from those in Africans (P < 0.001). The results were also significant when the Chinese, Malays and Indians were compared separately with the African group (P < 0.001). Compared with the Caucasian data, the overall distribution of genotype and allele frequencies in the Asian population were also significantly different (P < or = 0.05). However, when each Asian ethnic group was compared separately with the Caucasians, only the Indians were found to be significantly different (P < or = 0.004). Genotypic-phenotypic correlations of this SNP were assessed in 10 stable Chinese heart transplant patients. The median AUC(0,4 h) was 11% lower in patients with CC genotype compared with subjects with TT genotype. However, the interpatient variability in AUC(0,4 h) was high in patients, especially in those with CC genotype.

CONCLUSIONS

The distribution of the SNP C3435T in exon 26 in the Chinese and Malay population was found to be similar to the Caucasians whereas the Indians were different. The Asian population also differed significantly from the African and Caucasian population in the distribution of the C3435T SNP. The low frequency of the T allele in the Indian population implies lower expression of P-gp and may have important therapeutic and prognostic implications for use of P-gp dependent drugs in individuals of Indian origin.

A new challenge for successful immunotherapy by tumors that are resistant to apoptosis: two complementary signals to overcome cross-resistance

Tumor resistance to conventional therapies is a major problem in cancer treatment. While tumors initially respond to radiation or chemotherapies, subsequent treatments with these conventional modalities are ineffective against relapsed tumors. The problem of tumor resistance to chemotherapy and radiation has led to the development of immunotherapy and gene-based therapies. These alternative therapeutic approaches are intensely explored because they are supposed to be more tumor specific and better tolerated than the conventional therapies. Recent advances in apoptosis have revealed that resistance to apoptosis is one of the major mechanisms of tumor resistance to conventional therapies. Resistance to apoptosis is a naturally acquired characteristic during oncogenesis and is selected for after successive rounds of conventional therapies. Resistance to apoptosis involves dysregulation and/or mutation of apoptotic signaling molecules that render tumor cells unresponsive to apoptotic stimuli. Since both immunotherapy and chemotherapy kill tumors by apoptosis and the killings are signaled through a central core apoptotic program, dysregulation of this central program and development of resistance to apoptosis in chemoresistant cells could render them cross-resistant to immunotherapy. Therefore, in order to establish an effective antitumor response and to complement immunotherapy and gene-based therapies, cross-resistance due to resistance to apoptosis must be overcome. In this review, based on prior findings and recent evidence, we put forth a model, verified experimentally, in which chemoresistant tumor cells can be sensitized to immune-mediated killing by subtoxic concentrations of chemotherapeutic drugs/factors. The model involves two complementary signals. The first signal is a sensitizing signal that regulates pro/antiapoptotic targets, thus facilitating the apoptotic signal. The second apoptotic signal initiates a partial activation of the apoptotic signaling pathway, and activation is completed by complementation with signal one. Thus, effective killing of immunoresistant cells is achieved by both signals. The two-signal approach provides a new strategy to overcome cancer cross-resistance to immunotherapy and opens new avenues for the development of more effective and selective immunosensitizing agents.

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.

Adenoviral delivery of A-FOS, an AP-1 dominant negative, selectively inhibits drug resistance in two human cancer cell lines

Activator protein-1 (AP-1) transcription factor has been linked to chemotherapeutic resistance. To assess the clinical efficacy of AP-1 inhibition toward reversing drug resistance, we have developed an adenovirus expressing a dominant negative that inhibits AP-1 DNA binding, namedAdA-FOS. We examined the consequence of AdA-FOS infection on two paired human cancer cell lines, each pair consisting of a parental cell and the drug- resistant derivative. The first pair of cells is the parental human ovarian cancer cell line A2780 and the cisplatin-resistant A2780/CP70 cell line. The second pair of cells is the parental epidermal carcinoma cell line KB8 and the multidrug-resistant (mdr) KB85 cell line. Because of an association of up-regulated AP-1 activity with their drug resistance, these cell lines were considered good targets of AdA-FOS therapy. Following infection of the drug-sensitive and drug-resistant cells, we observed a significant decrease in cell viability of KB85 and A2780/CP70 cells at drug doses normally not lethal to the cell. The parental cell lines, A2780 and KB8 cells, were not similarly affected by AdA-FOS. This decrease in viability was specific to AdA-FOS as an adenovirus control (Advector) did not reverse drug resistance. Although the efficiency of AdA- FOS in therapy would need to be further analyzed with other cisplatin-resistant and mdr cell lines, these results suggest that AP-1 is a therapeutic molecular target and that inhibition of AP-1 DNA binding may be of clinical value in treating chemotherapeutic resistance.

The gut as a barrier to drug absorption: combined role of cytochrome P450 3A and P-glycoprotein

Intestinal phase I metabolism and active extrusion of absorbed drug have recently been recognised as major determinants of oral bioavailability. Cytochrome P450 (CYP) 3A, the major phase I drug metabolising enzyme in humans, and the multidrug efflux pump, P-glycoprotein, are present at high levels in the villus tip of enterocytes in the gastrointestinal tract, the primary site of absorption for orally administered drugs. The importance of CYP3A and P-glycoprotein in limiting oral drug delivery is suggested to us by their joint presence in small intestinal enterocytes, by the significant overlap in their substrate specificities, and by the poor oral bioavailability of joint substrates for these 2 proteins. These proteins are induced or inhibited by many of the same compounds. A growing number of preclinical and clinical studies have demonstrated that the oral bioavailability of many CYP3A and/or P-glycoprotein substrate drugs can be increased by concomitant administration of CYP3A inhibitors and/or P-glycoprotein inhibitors. We believe that further understanding the physiology and biochemistry of the interactive nature of intestinal CYP3A and P-glycoprotein will be important in defining, controlling, and improving oral bioavailability of CYP3A/P-glycoprotein substrates.

Phase I and pharmacokinetic study of the novel MDR1 and MRP1 inhibitor biricodar administered alone and in combination with doxorubicin

PURPOSE

To evaluate the safety, tolerability, and pharmacokinetics of biricodar (VX-710), an inhibitor of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP1), alone and with doxorubicin in patients with advanced malignancies. The effect of VX-710 on the tissue distribution of (99m)Tc-sestamibi, a P-gp and MRP1 substrate, was also evaluated.

PATIENTS AND METHODS

Patients with solid malignancies refractory to standard therapy first received a 96-hour infusion of VX-710 alone at 20 to 160 mg/m(2)/h. After a 3-day washout, a second infusion of VX-710 was begun, on the second day of which doxorubicin 45 mg/m(2) was administered. Cycles were repeated every 21 to 28 days. (99m)Tc-sestamibi scans were performed before and during administration of VX-710 alone.

RESULTS

Of the 28 patients who enrolled, 25 patients were eligible for analysis. No dose-limiting toxicity (DLT) was observed in the nine assessable patients who received 120 mg/m(2)/h or less. Among seven patients receiving VX-710 160 mg/m(2)/h, two DLTs were seen: reversible CNS toxicity and febrile neutropenia. All other adverse events were mild to moderate and reversible. Plasma concentrations of VX-710 in patients who received at 120 and 160 mg/m(2)/h were two- to fourfold higher than concentrations required to fully reverse drug resistance in vitro. VX-710 exhibited linear pharmacokinetics with a harmonic mean half-life of 1.1 hours. VX-710 enhanced hepatic uptake and retention of (99m)Tc-sestamibi in all patients.

CONCLUSION

A 96-hour infusion of VX-710 at 120 mg/m(2)/h plus doxorubicin 45 mg/m(2) has acceptable toxicity in patients with refractory malignancies. The safety and pharmacokinetics of VX-710 plus doxorubicin warrant efficacy trials in malignancies expressing P-gp and/or MRP1.

Total synthesis of ningalin B utilizing a heterocyclic azadiene Diels-Alder reaction and discovery of a new class of potent multidrug resistant (MDR) reversal agents

A concise, efficient approach to the total synthesis of ningalin B (1) based on a heterocyclic azadiene Diels-Alder strategy (1,2,4,5-tetrazine-->1,2,-diazine-->pyrrole) ideally suited for construction of the densely functionalized pyrrole core found in the natural product is detailed. Examination of the natural product and a number of synthetic intermediates revealed that while lacking inherent cytotoxic activity, many reverse the multidrug-resistant (MDR) phenotype, resensitizing a human colon cancer cell line (HCT116/VM46) to vinblastine and doxorubicin at lower doses than the prototypical agent verapamil.

Modulation of daunorubicin toxicity by liposomal encapsulation and use of specific inhibitors in vitro

Anthracyclines serve as a valuable tool in chemotherapy, but their usefulness is often limited by the occurrence of resistance mechanisms in tumor cells. Resistance of tumor cells is a multifactorial event, where several mechanisms act concurrently, including drug efflux and enzymatic drug inactivation. Liposomal encapsulation of anthracyclines has been discussed as a successful regimen to overcome drug resistance. Our investigations were carried out on a daunorubicin (DRC) sensitive breast cancer cell line and two DRC resistant sublines generated thereof. In all three cell lines, the extent of DRC detoxification via carbonyl reduction to daunorubicinol (DRCOL) was determined. In addition, rutin, the most effective inhibitor of carbonyl reducing enzymes, was tested to affect DRCOL formation. DRC IC(50) values were determined in relation to several combinations of DRC administration, (a) liposomal encapsulated DRC, (b) addition of verapamil (inhibitor of drug efflux), (c) addition of rutin (inhibitor of DRC carbonyl reduction). We could show that DRC sensitive and resistant breast cancer cell lines are able to catalyze DRC detoxification via carbonyl reduction to DRCOL. Rutin was shown to inhibit this reaction, but could not serve as an enhancer of DRC toxicity in MTT tests. Verapamil was effective only in resistant cells due to the overexpression of P-glycoprotein 170. Liposomal encapsulation of DRC did not show the expected increase in DRC toxicity in the present tumor cell model.

Use of chemical chaperones in the yeast Saccharomyces cerevisiae to enhance heterologous membrane protein expression: high-yield expression and purification of human P-glycoprotein

Utilizing human P-glycoprotein (P-gp), we investigated methods to enhance the heterologous expression of ATP-binding cassette transporters in Saccharomyces cerevisiae. Human multidrug resistance gene MDR1 cDNA was placed in a high-copy 2 mu yeast expression plasmid under the control of the inducible GAL1 promoter or the strong constitutive PMA1 promoter from which P-gp was expressed in functional form. Yeast cells expressing P-gp were valinomycin resistant. Basal ATPase activity of P-gp in yeast membranes was 0. 4-0.7 micromol/mg/min indicating excellent functionality. P-glycoprotein expressed in the protease-deficient strain BJ5457 was found in the plasma membrane and was not N-glycosylated. By use of the PMA1 promoter, P-gp could be expressed at 3% of total membrane protein. The expression level could be further enhanced to 8% when cells were grown in the presence of 10% glycerol as a chemical chaperone. Similarly, glycerol enhanced protein levels of P-gp expressed under control of the GAL1 promoter. Glycerol was demonstrated to enhance posttranslational stability of P-gp. Polyhistidine-tagged P-gp was purified by metal affinity chromatography and reconstituted into proteoliposomes in milligram quantities and its ATPase activity was characterized. Turnover numbers as high as 12 s(-1) were observed. The kinetic parameters K(MgATP)(M), V(max), and drug activation were dependent on the lipid composition of proteoliposomes and pH of the assay and were similar to P-gp purified from mammalian sources. In conclusion, we developed a system for cost-effective, high-yield, heterologous expression of functional P-gp useful in producing large quantities of normal and mutant P-gp forms for structural and mechanistic studies.

Synthesis of C-ring aromatic taxoids and evaluation of their multi-drug resistance reversing activity

The C-aromatic taxoids were synthesized to develop effective inhibitors against drug efflux mediated by p-glycoproteins. Among those tested using multi-drug resistant tumor cells (2780AD), the benzoate 11 exhibited significant activity as potent as verapamil, a well-established MDR reversing agent.

Synthesis and evaluation of artificial taxoids with antitumor and multi-drug resistance reversing activities

Artificial taxoids were synthesized and subjected to evaluation of their ability of multi-drug resistance reversing and antitumor activities. While the taxoid 4 could not increase cellular accumulation of vincristine in multi-drug resistant tumor cells, the C4-hydroxy analog 15 showed significant effect. However, these compounds showed weak activities on growth inhibition of cancer cells.

Reversal of P-glycoprotein-mediated multidrug resistance in vitro by AV200, a new ardeemin derivative

The activity of AV200, a synthetic ardeemin derivative, in reversing the multidrug resistance phenotype has been investigated. At non-toxic doses, AV200 was able to completely restore vincristine and paclitaxel toxicities and partially restore that of doxorubicin in multidrug-resistant cells. The potency of AV200 as a modulator of the resistance to doxorubicin, vincristine and paclitaxel resulted to be seven-, 59 and 12-fold, respectively, higher than that of verapamil. In vitro measurements of rhodamine 123 accumulation in human resistant cells suggest that AV200 reverses multidrug resistance by directly inhibiting the P-glycoprotein-mediated drug efflux. This work underscores the possibility of utilizing ardeemin derivatives as a source of non-toxic modulators of the multidrug resistance phenotype.

Molecular interactions of cyclosporin A with P-glycoprotein. Photolabeling with cyclosporin derivatives

The interaction between P-glycoprotein (140-180 kDa) from the multidrug-resistant Chinese hamster ovary cell line CHRC5 and cyclosporin A was characterized using three different photoactivable cyclosporin A analogs. Two monoclonal antibodies, which are able to discriminate between two major domains of cyclosporin A (the cyclophilin and calcineurin binding domains), were used to detect the photolabeled proteins. A protein of 155 kDa corresponding to P-glycoprotein was much more strongly photolabeled in membranes of CHRC5 cells than in membranes of their drug-sensitive parent cell line AuxB1. The antitumor drug vinblastine and the reversal agents verapamil and cyclosporin A inhibited the photolabeling, and the nonimmunosuppressive derivative PSC-833 caused a stronger inhibition than cyclosporin A. P-glycoprotein photolabeled with cyclosporin A analogs was only detected with the monoclonal antibody that recognizes cyclosporin A and its metabolites, indicating that the calcineurin binding domain recognized specifically by the other antibody is not exposed. These results suggest that the portion of cyclosporin A that binds to calcineurin plays a role in the interaction of cyclosporin A with P-glycoprotein.

Comparison of action of the anti-neoplastic drug lonidamine on drug-sensitive and drug-resistant human breast cancer cells: 31P and 13C nuclear magnetic resonance studies

Lonidamine (LND) is a relatively new anti-cancer drug, and several clinical trials have indicated that it may be effective in combinations with other therapeutic modalities. LND is classified within the metabolic inhibitor agents. Multidrug resistance (MDR) phenomenon is often associated with increased energy requirements, and enhanced glycolysis rate. These studies were performed to delineate the mechanism of action of LND on MDR human breast cancer cells, and to investigate whether LND as a single agent, or in combination with another anti-metabolism drug, 2-deoxyglucose (2-DG), may be useful against MDR tumors. The effects of LND on intact perfused drug-sensitive (WT) and 33-fold resistant to Adriamycin (Adr) MCF-7 cells, embedded in alginate micro capsules, were continuously monitored by 31P and 13C nuclear magnetic resonance (NMR) spectroscopy. 31P NMR studies showed that LND induced intracellular acidification and depletion of NTP in both WT and Adr cells. However, pH and NTP levels decreased less in the Adr cells than in the WT cells (p < 0.05 for both parameters). 13C NMR demonstrated that LND inhibited lactate transport, and lactate signals were elevated in both cell lines. However, the intracellular lactate levels increased to a greater extent in the WT than in the Adr cells (p < 0.05). There were major differences in the effects of LND on metabolism between sensitive and resistant cells. While LND enhanced glucose uptake in the WT cells, and its administration was followed by continuous increase of lactate signal, both processes were not affected by LND in the Adr cells. 2-DG is a glucose analogue that inhibits both cellular uptake and utilization of glucose, leading to cell starvation. Combined treatment with LND and 2-DG yielded at best additive, but not synergistic, cellular toxicity, and the metabolic effects of LND were attenuated by 2-DG. These results showed that the principal mechanism of action of LND is inhibition of lactate transport leading to intracellular lactate accumulation and acidification in both WT and Adr cells. The Adr cells were only 2-fold resistant to LND (compared to the WT cells), and since cellular uptake of alkaloid chemotherapy is improved in acidic environment, LND may have a role in the treatment protocols of MDR tumors, especially when given as the initial means for induction of intracellular acidification.

Reversal of P-glycoprotein-associated multidrug resistance by ivermectin

P-Glycoprotein (P-gp) causes a multidrug resistance (MDR) phenotype in tumour cells. In some cancers, the expression of P-gp has been correlated with low clinical response to chemotherapy and survival of patients. Previous studies have shown that certain lipophilic drugs bind to P-gp and reverse the MDR phenotype of tumour cells. In this study, we extend that list of compounds and present evidence for the capacity of a potent and clinically safe anthelmintic, ivermectin (IVM), as an MDR-reversing drug. Using a highly drug-resistant human cell line, we compared IVM with other MDR-reversing agents and showed that IVM is 4- and 9-fold more potent than cyclosporin A and verapamil, respectively. The capacity of IVM to inhibit iodoaryl-azidoprazosin photolabeling of P-gp is consistent with direct binding to P-gp. Studies showed that [3H]IVM binding to membranes from resistant cells is specific and saturable with KD and Bmax values of 10.6 nM and 19.8 pmol/mg, respectively. However, while cyclosporin A or vinblastine inhibited [3H]IVM binding to membranes from drug-resistant but not drug-sensitive cells, neither verapamil nor colchicine had any effect. Furthermore, both IVM and cyclosporin A and, to a lesser extent, verapamil also inhibited [3H]vinblastine binding to membranes from drug-resistant cells. Drug transport studies showed that [3H]IVM is a substrate for the P-gp drug efflux pump. However, it was transported less efficiently by P-gp than [3H]vinblastine. Moreover, only cyclosporin A was effective in potentiating the accumulation of [3H]IVM in drug-resistant cells. Taken together, the high efficiency of MDR reversal by IVM combined with its low toxicity are consistent with the properties of an ideal MDR-reversing agent.

Vinorelbine: an active, non cross-resistant drug in advanced breast cancer. Results from a phase II study

PURPOSE

To evaluate efficacy and toxicity of vinorelbine and to investigate its cross-resistance with other current drug treatments for metastatic breast cancer.

PATIENTS AND METHODS

From July 1992 to December 1993, 57 histologically proven breast cancer patients entered this Phase II study. Patients were stratified according to their status of previous treatment, namely, no prior chemotherapy or relapse more than 12 months since the end of adjuvant chemotherapy (Group A) and other patients (Group B).

RESULTS

Fifty three patients were evaluable for response, 27 in Group A and 26 in Group B. All patients were evaluable for toxicity. Vinorelbine was initially administered at the dose of 30 mg/sqm weekly by i.v. infusion in 100 ml of normal saline over 20 minutes. A frequency analysis of drug administration in the first 20 cases revealed two main treatment periodicities, corresponding to one week and to three weeks. Thereafter the drug was administered at 30 mg/sqm on day 1 and 8, every 3 weeks. With the new drug schedule, the mean dose intensity increased from 19.7 to 21.1 mg/sqm per week. Overall, an objective response rate of 47% (95% C.I. 33%-61%) was documented. Four patients achieved complete response (7%, CI: 2%-18%) and 21 partial response (40%, CI: 26%-54%). Fifty nine percent of patients in Group A and 35% in Group B showed objective tumor response. The analysis of response rate in previously treated patients failed to show evidence of cross-resistance with vinorelbine. Main side effects, i.e. neutropenia, local pain, and gastrointestinal and flu-like symptoms, were moderate and short lasting.

CONCLUSION

Vinorelbine has clinically significant activity in metastatic breast cancer, and no cross-resistance with prior anthracyclines and CMF treatments. The drug schedule of 30 mg/sqm iv bolus on day 1 and 8 every 3 weeks was found effective and tolerable.

Polyaromatic alkaloids from marine invertebrates as cytotoxic compounds and inhibitors of multidrug resistance caused by P-glycoprotein

The effects of several members of the family of lamellarins, polyaromatic alkaloids isolated from tunicates belonging to the genus Didemnum, on the growth of several tumour cell lines and on P-glycoprotein (P-gp)-mediated multidrug resistance (MDR), were investigated. Cytotoxicity experiments of lamellarins were performed on a panel of tumour cell lines, including two multidrug-resistant cell lines. Some lamellarins showed good anti-tumour activity, with similar levels of cytotoxicity against both the resistant and their corresponding parental cell lines. Two lamellarins displayed a high potency against lung carcinoma cells. Studies of the resistance modifier activity of the different lamellarins at non-toxic concentrations were also carried out in cells exhibiting MDR, and lamellarin I was selected for the highest chemosensitising activity. At non-toxic doses, verapamil and lamellarin I effectively increased the cytotoxicity of doxorubicin, vinblastine and daunorubicin in a concentration-dependent manner in multidrug-resistant cells, but the potency of lamellarin I as a MDR modulator was 9- to 16-fold higher than that of verapamil. In vitro measurements of rhodamine 123 accumulation in the multidrug-resistant Lo Vo/Dx cells suggest that lamellarin I reverses MDR by directly inhibiting the P-gp-mediated drug efflux. This work underscores the possibility of using these marine-derived compounds as a potential new source of anti-tumoral drugs active on resistant cells as well as of non-toxic modulators of the MDR phenotype.

Chemosensitization and drug accumulation assays as complementary methods for the screening of multidrug resistance reversal agents

Two methods based on the reversion of adriamycin-resistance o the increase of Rhodamine 123 accumulation in a multidrug resistant (MDR) cell line have been simplified and adapted for the screening of MDR reversal agents. Both methods are carried out in microtiter plates, are highly sensitive and can be easily automated. In both assays verapamil and the cyclosporine derivative PSC 833 could be detected at concentrations lower than 1 and 0.05 microM, respectively. Depending on the MDR cell line used, drugs exhibiting the collateral sensitivity phenomenon can be selected in the cytotoxicity assay, while interferences due to sample toxicity are easily avoided in the dye accumulation assay.

Anthracycline resistance in breast cancer: clinical applications of current knowledge

Anthracyclines are highly effective antineoplastic agents for the treatment of breast cancer. Nevertheless, essentially all breast cancer patients have tumours which are intrinsically resistant or which develop resistance during the course of therapy. Clinical trials provide indirect information on the nature of anthracycline resistance and work in the basic sciences has demonstrated molecular mechanisms which play a role. Initial clinical attempts to exploit and translate these mechanisms to predict, and interfere with, anthracycline resistance have met with mixed success, and have not yet led to accepted clinical applications.