Future perspectives for the development of P-glycoprotein modulators

Tariquidar-Related Chalcones and Ketones as ABCG2 Modulators

ABC transporters, including ABCG2, play a vital role in defending the human body against the vast range of xenobiotics. Even though this is beneficial for human health, these protein transporters have been implicated in the emerging resistance of cancer cells to a variety of structurally and functionally diverse anticancer drugs. In order to investigate their role in resistance, potent and selective ABCG2 modulators have been described in the literature. A leading class of modulators are the tariquidar analogues; however, their susceptibility to hydrolysis limits their applicable use. To overcome this, we synthesized a novel series of chalcone- and ketone-based compounds inspired by reported tariquidar analogues. Compounds were characterized and evaluated for their ABCG2 modulatory activity and ABC transporter selectivity. When compared to transporters ABCB1 and ABCC1, the chalcone-based compounds exhibited selectivity for ABCG2, while the ketone-based compounds showed only a slight preference for ABCG2. From the former series, chalcone 16d (UR-DP48) displayed similar activity to the reference fumitremorgin C, both producing comparable maximal effects. The compound exhibited marked antiproliferative activity, while cytotoxicity was less pronounced for the most active compound 17f from the ketone series. Chalcone-containing tariquidar analogues are promising modulators to aid in functional investigations of ABCG2 transporters.

The Onus of Sphingolipid Enzymes in Cancer Drug Resistance

Chemotherapy resistance, inherent or acquired, represents a serious barrier to the successful treatment of cancer. Although drug efflux, conducted by plasma membrane-resident proteins, detoxification enzymes, cell death inhibition, and DNA damage repair are ensemble players in this unwanted biology, a full understanding of the many in concert molecular mechanisms driving drug resistance is lacking. Recent discoveries in sphingolipid (SL) metabolism have provided significant insight into the role of these lipids in cancer growth; however, considerably less is known with respect to SLs and the drug-resistant phenotype. One exception here is enhanced ceramide glycosylation, a hallmark of multidrug resistance that is believed responsible, in part, for diminishing ceramides tumor-suppressor potential. This chapter will review various aspects of SL biology that relate to chemotherapy resistance and extend this topic to acknowledge the role of chemotherapy selection pressure in promoting dysregulated SL metabolism, a characteristic in cancer and an exploitable target for therapy.

Chrysosplenetin inhibits artemisinin efflux in P-gp-over-expressing Caco-2 cells and reverses P-gp/MDR1 mRNA up-regulated expression induced by artemisinin in mouse small intestine

CONTEXT

CYP3A4 and P-gp together form a highly efficient barrier for orally absorbed drugs and always share the same substrates. Our previous work revealed that chrysosplenetin (CHR) significantly augmented the rat plasma level and anti-malarial efficacy of artemisinin (ART), partially due to the uncompetitive inhibition effect of CHR on rat CYP3A. But the impact of CHR on P-gp is still unknown.

OBJECTIVE

The present study investigates whether CHR interferes with P-gp-mediated efflux of ART and elucidates the underlying mechanism.

MATERIALS AND METHODS

P-gp-over-expressing Caco-2 cells were treated with ART (10 μM) or ART-CHR (1:2, 10:20 μM) in ART bidirectional transport experiment. ART concentration was determined by UHPLC-MS/MS method. Healthy male ICR mice were randomly divided into nine groups (n = 6) including negative control (0.5% CMC-Na solution, 13 mL/kg), ART alone (40 mg/kg), verapamil (positive control, 40 mg/kg), ART-verapamil (1:1, 40:40 mg/kg), CHR alone (80 mg/kg), ART-CHR (1:0.1, 40:4 mg/kg), ART-CHR (1:1, 40:40 mg/kg), ART-CHR (1:2, 40:80 mg/kg) and ART-CHR (1:4, 40:160 mg/kg). The drugs were administrated intragastrically for seven consecutive days. MDR1 and P-gp expression levels in mice small intestine were examined by performing RT-PCR and western blot analysis. ABC coupling ATPase activity was also determined.

RESULTS

After combined with CHR (1:2), Papp (AP-BL) and Papp (BL-AP) of ART changed to 4.29 × 10 - 8 (increased 1.79-fold) and 2.85 × 10 - 8 cm/s (decreased 1.24-fold) from 2.40 × 10 - 8 and 3.54 × 10 - 8 cm/s, respectively. Efflux ratio (PBA/PAB) declined 2.21-fold (p < 0.01) versus to ART alone. ART significantly up-regulated both MDR1 mRNA and P-gp levels compared with vehicle, while CHR in combination ratio of 0:1, 0.1:1, 1:1, 2:1 and 4:1 with ART, reversed them to normal levels as well as negative control (p < 0.05). The ATPase activities in ART-CHR 1:4 and CHR alone groups achieved a slight increase (p < 0.05) when compared with ART alone.

DISCUSSION AND CONCLUSION

These results confirm that CHR inhibited P-gp activity and reverse the up-regulated P-gp and MDR1 levels induced by ART. It suggested that CHR potentially can be used as a P-gp reversal agent to obstruct ART multidrug resistance.

Concomitance of P-gp/LRP Expression with EGFR Mutations in Exons 19 and 21 in Non-Small Cell Lung Cancers

PURPOSE

Traditional chemotherapy is the main adjuvant therapy for the treatment of non-small cell lung cancer (NSCLC). However, the emergence of multi-drug resistance (MDR) has greatly restricted the curative effect of chemotherapy. Therefore, it is necessary to find a method to treat MDR NSCLC clinically. It is worth investigating whether NSCLCs that are resistant to traditional chemotherapy can be effectively treated with tyrosine kinase inhibitors targeting epidermal growth factor receptor (EGFR).

MATERIALS AND METHODS

The expression of P-glycoprotein (P-gp) and lung resistance-related protein (LRP) was detected by immunohistochemistry, and mutations in EGFR (exons 19 and 21) and Kirsten rat sarcoma viral oncogene homolog (KRAS) (exon 2) were detected by high-resolution melting analysis (HRMA) of surgical NSCLC specimens from 127 patients who did not undergo traditional chemotherapy or radiotherapy. A Pearson chi-square test was performed to analyze the correlations between the expression of P-gp and LRP and mutations in EGFR and KRAS.

RESULTS

The expression frequencies of P-gp and LRP were significantly higher in adenocarcinomas from non-smoking patients; the expression frequency of LRP was significantly higher in cancer tissue from female patients. The frequency of EGFR mutations was significantly higher in well to moderately differentiated adenocarcinomas from non-smoking female patients. The frequency of EGFR mutations in the cancers that expressed P-gp, LRP, or both P-gp and LRP was significantly higher than that in cancers that did not express P-gp or LRP.

CONCLUSION

NSCLCs expressing P-gp/LRP bear the EGFR mutation in exon 19 or 21 easily.

Targeting the ABCG2-overexpressing multidrug resistant (MDR) cancer cells by PPARγ agonists

BACKGROUND AND PURPOSE

Multidrug resistance (MDR), usually mediated by overexpression of efflux transporters such as P-gp, ABCG2 and/or MRP1, remains a major obstacle hindering successful cancer chemotherapy. There has been great interest in the development of inhibitors towards these transporters to circumvent resistance. However, since the inhibition of transporter is not specific to cancer cells, a decrease in the cytotoxic drug dosing may be needed to prevent excess toxicity, thus undermining the potential benefit brought about by a drug efflux inhibitor. The design of potent MDR modulators specific towards resistant cancer cells and devoid of drug-drug interactions will be needed to effect MDR reversal.

EXPERIMENTAL APPROACH

Recent evidence suggests that the PTEN/PI3K/Akt pathway may be exploited to alter ABCG2 subcellular localization, thereby circumventing MDR. Three PPARγ agonists (telmisartan, pioglitazone and rosiglitazone) that have been used in the clinics were tested for their effect on the PTEN/PI3K/Akt pathway and possible reversal of ABCG2-mediated drug resistance.

KEY RESULTS

The PPARγ agonists were found to be weak ABCG2 inhibitors by drug efflux assay. They were also shown to elevate the reduced PTEN expression in a resistant and ABCG2-overexpressing cell model, which inhibit the PI3K-Akt pathway and lead to the relocalization of ABCG2 from the plasma membrane to the cytoplasma, thus apparently circumventing the ABCG2-mediated MDR.

CONCLUSIONS AND IMPLICATIONS

Since this PPARγ/PTEN/PI3K/Akt pathway regulating ABCG2 is only functional in drug-resistant cancer cells with PTEN loss, the PPARγ agonists identified may represent promising agents targeting resistant cells for MDR reversal.

The connection between the toxicity of anthracyclines and their ability to modulate the P-glycoprotein-mediated transport in A549, HepG2, and MCF-7 cells

Multidrug resistance (MDR) is a major obstacle to the successful chemotherapy of solid tumors. We compared the resistance of the most popular solid tumors, breast adenocarcinoma (MCF-7 cell line) and nonsmall cell lung (A549 cell line) hepatocellular liver carcinoma (HepG2 cells), to aclarubicin (ACL) and doxorubicin (DOX). This research aimed at determining the relation between the toxicity of ACL and DOX, their cell accumulation, and then effect on P-glycoprotein functionality. ACL is more cytotoxic for tumor cells compared to DOX. The intracellular concentration of drugs in cancer cells was dependent on the dose of the drugs and the time of incubation. The P-gp inhibitor Verapamil (V) increased DOX accumulation in all tested cell lines. By contrast, the intracellular level of ACL was not affected by this modifying agent. The assessment of the uptake of 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolocarbocyanine iodide (JC-1) or Rhodamine 123 (R123) allows the evaluation of the different influence of drugs on P-gp activity which is in agreement with the estimation of expression measured by MDR-1 shift assay. These data suggest that ACL is less P-gp dependent than DOX and consequently may be used in a clinical setting to increase treatment efficacy in resistant human tumors.

Signal transduction pathways and transcriptional mechanisms of ABCB1/Pgp-mediated multiple drug resistance in human cancer cells

Multiple drug resistance (MDR), defined as the ability of tumour cells to survive exposure to many chemotherapeutic agents, is a major cause of treatment failure in human cancers. The membrane transporter P-glycoprotein (Pgp, encoded by the ABCB1 [adenosine triphosphate-binding cassette, subfamily B, member 1] gene) is the main mechanism for decreased intracellular drug accumulation in human MDR cancer. ABCB1/Pgp-mediated MDR involves several signal transduction pathways and transcription factors. Activation of these signal transduction pathways influences the prognosis of MDR human cancer. Signalling pathways involved in ABCB1/Pgp-mediated MDR include the mitogen-activated protein kinase (MAPK), c-Jun NH(2)-terminal kinase (JNK), p38, cyclic adenosine monophosphate-dependent protein kinase, phosphatidylino sitol 3-kinase and protein kinase C signalling pathways. This review summarizes the biological characteristics, target points and signalling cascade mediators of these pathways. Drugs targeted against these pathways may provide new therapies for treatment of ABCB1/Pgp-mediated MDR.

Verotoxin-1 treatment or manipulation of its receptor globotriaosylceramide (gb3) for reversal of multidrug resistance to cancer chemotherapy

A major problem with anti-cancer drug treatment is the development of acquired multidrug resistance (MDR) of the tumor cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the globotriaosylceramide membrane receptor (Gb3), a glycolipid associated with multidrug resistance. Gb3 is overexpressed in many human tumors and tumor cell lines with inherent or acquired MDR. Gb3 is co-expressed and interplays with the membrane efflux transporter P-gp encoded by the MDR1 gene. P-gp could act as a lipid flippase and stimulate Gb3 induction when tumor cells are exposed to cancer chemotherapy. Recent work has shown that apoptosis and inherent or acquired multidrug resistance in Gb3-expressing tumors could be affected by VT-1 holotoxin, a sub-toxic concentration of the holotoxin concomitant with chemotherapy or its Gb3-binding B-subunit coupled to cytotoxic or immunomodulatory drug, as well as chemical manipulation of Gb3 expression. The interplay between Gb3 and P-gp thus gives a possible physiological approach to augment the chemotherapeutic effect in multidrug resistant tumors.

Targeted chemotherapy in drug-resistant tumors, noninvasive imaging of P-glycoprotein-mediated functional transport in cancer, and emerging role of Pgp in neurodegenerative diseases

Multidrug resistance (MDR) mediated by overexpression of P-glycoprotein (Pgp) is one of the best characterized transporter-mediated barriers to successful chemotherapy in cancer patients and is also a rapidly emerging target in the progression of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Therefore, strategies capable of delivering chemotherapeutic agents into drug-resistant tumors and targeted radiopharmaceuticals acting as ultrasensitive molecular imaging probes for detecting functional Pgp expression in vivo could be expected to play a vital role in systemic biology as personalized medicine gains momentum in the twenty-first century. While targeted therapy could be expected to deliver optimal doses of chemotherapeutic drugs into the desired targets, the interrogation of Pgp-mediated transport activity in vivo via noninvasive imaging techniques (SPECT and PET) would be beneficial in stratification of patient populations likely to benefit from a given therapeutic treatment, thereby assisting management of drug resistance in cancer and treatment of neurodegenerative diseases. Both strategies could play a vital role in advancement of personalized treatments in cancer and neurodegenerative diseases. Via this tutorial, authors make an attempt in outlining these strategies and discuss their strengths and weaknesses.

Rhodamine inhibitors of P-glycoprotein: an amide/thioamide "switch" for ATPase activity

We have examined 46 tetramethylrosamine/rhodamine derivatives with structural diversity in the heteroatom of the xanthylium core, the amino substituents of the 3- and 6-positions, and the alkyl, aryl, or heteroaryl group at the 9-substituent. These compounds were examined for affinity and ATPase stimulation in isolated MDR3 CL P-gp and human P-gp-His(10), for their ability to promote uptake of calcein AM and vinblastine in multidrug-resistant MDCKII-MDR1 cells, and for transport in monolayers of MDCKII-MDR1 cells. Thioamide 31-S gave K(M) of 0.087 microM in human P-gp. Small changes in structure among this set of compounds affected affinity as well as transport rate (or flux) even though all derivatives examined were substrates for P-gp. With isolated protein, tertiary amide groups dictate high affinity and high stimulation while tertiary thioamide groups give high affinity and inhibition of ATPase activity. In MDCKII-MDR1 cells, the tertiary thioamide-containing derivatives promote uptake of calcein AM and have very slow passive, absorptive, and secretory rates of transport relative to transport rates for tertiary amide-containing derivatives. Thioamide 31-S promoted uptake of calcein AM and inhibited efflux of vinblastine with IC(50)'s of approximately 2 microM in MDCKII-MDR1 cells.

Curcumin as chemosensitizer

This overview presents curcumin as a significant chemosensitizer in cancer chemotherapy. Although the review focuses on curcumin and its analogues on multidrug resistance (MDR) reversal, the relevance of curcumin as a nuclear factor (NF)-KB blocker and sensitizer of many chemoresistant cancer cell lines to chemotherapeutic agents will also be discussed. One of the major mechanisms of MDR is the enhanced ability of tumor cells to actively efflux drugs, leading to a decrease in cellular drug accumulation below toxic levels. Active drug efflux is mediated by several members of the ATP-binding cassette (ABC) superfamily of membrane transporters, which have now been subdivided into seven families designated A through G. Among these ABC families, the classical MDR is attributed to the elevated expression of ABCB1 (Pgp), ABCC1 (MRP1), and ABCG2 (MXR). The clinical importance of Pgp, MRP1, and MXR for MDR and cancer treatment has led to the investigation of the inhibiting properties of several compounds on these transporters. At present, due in part to the disappointing results associated with the many side effects of synthetic modulators that have been used in clinical trials, current research efforts are directed toward the identification of novel compounds, with attention to dietary natural products. The advantage is that they exhibit little or virtually no side effects and do not further increase the patient's medication burden.

Glycosphingolipids and drug resistance

Drug resistance, an all too frequent characteristic of cancer, represents a serious barrier to successful treatment. Although many resistance mechanisms have been described, those that involve membrane-resident proteins belonging to the ABC (ATP binding cassette) transporter superfamily are of particular interest. In addition to cancer, the ABC transporter proteins are active in diseases such as malaria and leishmaniasis. A recent renaissance in lipid metabolism, specifically ceramide and sphingolipids, has fueled research and provided insight into the role of glycosphingolipids in multidrug resistance. This article reviews current knowledge on ceramide, glucosylceramide synthase and cerebrosides, and the relationship of these lipids to cellular response to anticancer agents.

In silico modelling of the interaction of flavonoids with human P-glycoprotein nucleotide-binding domain

A three-dimensional model of human ABCB1 nucleotide-binding domain (NBD) was developed by homology modelling using the high-resolution human TAP1 transporter structure as template. Interactions between NBD and flavonoids were investigated using in silico docking studies. Ring-A of unmodified flavonoid was located within the NBD P-loop with the 5-hydroxyl group involved in hydrogen bonding with Lys1076. Ring-B was stabilised by hydrophobic stacking interactions with Tyr1044. The 3-hydroxyl group and carbonyl oxygen were extensively involved in hydrogen bonding interactions with amino acids within the NBD. Addition of prenyl, benzyl or geranyl moieties to ring-A (position-6) and hydrocarbon substituents (O-n-butyl to O-n-decyl) to ring-B (position-4) resulted in a size-dependent decrease in predicted docking energy which reflected the increased binding affinities reported in vitro.

Design, synthesis and structure-activity relationships of novel taxane-based multidrug resistance reversal agents

A series of novel taxane-based multidrug resistance (MDR) reversal agents (TRAs) has been designed and synthesized. Structure-activity relationship (SAR) study clearly indicates that modification of the C-7 position with hydrophobic arenecarbonylcinnamoyl groups brings about high potency against drug efflux mediated by P-glycoprotein (P-gp). Six TRAs exhibit ability to modulate a wide range of ATP-binding cassette (ABC) transporters, such as P-gp, multidrug resistance-associated protein 1 (MRP1), and breast cancer resistance protein (BCRP), which may serve as novel broad-spectrum modulators of ABC transporters.

Overexpression of glucosylceramide synthase and P-glycoprotein in cancer cells selected for resistance to natural product chemotherapy

Resistance to natural product chemotherapy drugs is a major obstacle to successful cancer treatment. This type of resistance is often acquired in response to drug exposure; however, the mechanisms of this adverse reaction are complex and elusive. Here, we have studied acquired resistance to Adriamycin, Vinca alkaloids, and etoposide in MCF-7 breast cancer cells, KB-3-1 epidermoid carcinoma cells, and other cancer cell lines to determine if there is an association between expression of glucosylceramide synthase, the enzyme catalyzing ceramide glycosylation to glucosylceramide, and the multidrug-resistant (MDR) phenotype. This work shows that glucosylceramide levels increase concomitantly with increased drug resistance in the KB-3-1 vinblastine-resistant sublines KB-V.01, KB-V.1, and KB-V1 (listed in order of increasing MDR). The levels of glucosylceramide synthase mRNA, glucosylceramide synthase protein, and P-glycoprotein (P-gp) also increased in parallel. Increased glucosylceramide levels were also present in Adriamycin-resistant KB-3-1 sublines KB-A.05 and KB-A1. In breast cancer, detailed analysis of MCF-7 wild-type and MCF-7-AdrR cells (Adriamycin-resistant) demonstrated enhanced glucosylceramide synthase message and protein, P-gp message and protein, and high levels of glucosylceramide in resistant cells. Similar results were seen in vincristine-resistant leukemia, etoposide-resistant melanoma, and Adriamycin-resistant colon cancer cell lines. Cell-free glucosylceramide synthase activity was higher in lysates obtained from drug-resistant cells. Lastly, glucosylceramide synthase promoter activity was 15-fold higher in MCF-7-AdrR compared with MCF-7 cells. We conclude that selection pressure for resistance to natural product chemotherapy drugs selects for enhanced ceramide metabolism through glucosylceramide synthase in addition to enhanced P-gp expression. A possible connection between glucosylceramide synthase and P-gp in drug resistance biology is suggested.

Resistance to cytotoxic and anti-angiogenic anticancer agents: similarities and differences

Intrinsic resistance to anticancer drugs, or resistance developed during chemotherapy, remains a major obstacle to successful treatment. This is the case both for resistance to cytotoxic agents, directed at malignant cells, and for resistance to anti-angiogenic agents, directed at non-malignant endothelial cells. In this review, we will discuss mechanisms of resistance which have a bearing on both these conceptually different classes of drugs. The complexity of drug resistance, involving drug transporters, such as P-glycoprotein, as well as resistance related to the tissue structure of solid tumors and its consequences for drug delivery is discussed. Possible mechanisms of resistance to endothelial cell-targeted drugs, including inhibitors of the VEGF receptor and EGF receptor family, are reviewed. The resistance of cancer cells as well as endothelial cells related to anti-apoptotic signaling events initiated by cell integrin-matrix interactions is discussed. Current strategies to overcome resistance mechanisms are summarized; they include high-dose chemotherapy, tumor targeting of cytotoxics to improve tumor uptake, low-dose protracted (metronomic) chemotherapy and combinations of classical agents with anti-angiogenic agents. This review discusses primarily literature published in 2001 and 2002.

Considerations in the design and development of transport inhibitors as adjuncts to drug therapy

With the realization of the importance of drug efflux transporters in disease processes and treatment, development of inhibitors to these transporters has been sought for use as adjuncts to therapy. To date, inhibitors that have been best studied are modulators of P-glycoprotein, a transporter important in the removal of anticancer agents from cells and overexpression of this transporter results in multidrug resistance. There is a delicate balance between efficacy and toxicity. This review summarizes key learning points in the development of P-glycoprotein inhibitors. Topics covered include specificity of the inhibitor for the target transporter, effect on metabolism of coadministered drugs, pharmacokinetic interactions, toxicity and the salient features needed for efficacy. These points will have general application to the development of inhibitors of transporters.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

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.

The role of half-transporters in multidrug resistance

ATP-binding cassette proteins comprise a superfamily of transporter proteins, a subset of which have been implicated in multidrug resistance. Although P-glycoprotein was described over 15 years ago, the recent expansion in the number of transporters identified has prompted renewed interest in the role of drug transporters in clinical drug resistance. These newly identified transporters include additional members of the MRP family, ABC2, and a new half-transporter, MXR/BCRP/ABCP1. This half-transporter confers high levels of resistance to mitoxantrone, anthracyclines, and the camptothecins SN-38 and topotecan. At 72 kDa, MXR localizes to the plasma membrane in cells which highly overexpress the protein either through gene amplification or though gene rearrangement. Future studies will be aimed at identifying an inhibitor, and attempting to translate recognition of this new transporter into a target for anticancer treatment.

Breast cancer response to paclitaxel in vivo

Clinical trials using neoadjuvant (primary, preoperative) chemotherapy demonstrate that breast cancer reduction relates to survival. To date, no pre-treatment pathologic, phenotypic, or genotypic tumor characteristics predict a patient's likely benefit from paclitaxel. This has led to pilot clinical studies that have attempted to identify whether early cellular responses in vivo can be used to predict the effectiveness of chemotherapy. A potential benefit of such predictive studies will be the ability to tailor specific therapeutic approaches to individual patients. Important issues surrounding this field include how to accurately measure and/or categorize the extent of tumor reduction, and how and when to assess breast cancer cellular responses in vivo. Preliminary data indicate that initial apoptotic responses are critical to tumor reduction, and that the timing of tumor samples for assessment of response is important. Although inherent complete resistance of breast cancer to paclitaxel occurs in a minority of patients, mechanisms of acquired or partial resistance require further study. However, the initial apoptotic response to paclitaxel has been shown to transiently reduce both cell density and intratumoral pressure, providing a window of time when there can be improved penetration of paclitaxel into the tumor. Thus a strong initial apoptotic response can set up a compounding benefit from subsequent treatments. Knowledge of breast cancer response to paclitaxel in vivo could lead to therapeutic strategies that enhance the apoptotic response and optimize the dosing schedule, to improve the tumor reduction for most patients.

Cross-talk between signalling pathways and the multidrug resistant protein MDR-1

The multidrug resistant protein MDR-1 has been associated with the resistance to a wide range of anti-cancer drugs. Taxol is a substrate for this transporter system and is used in the treatment of a wide range of human malignancies including lung, breast and ovarian cancer. We have generated a series of ovarian cell lines resistant to this compound, all of which overexpress MDR-1 through gene amplification. We present novel evidence that a constitutive activation of the ERK1/2 MAP kinase pathway was also observed although the level of active JNK and p38 remained unchanged. Inhibition of the ERK1/2 MAP kinase pathway using UO126 or PD098059 re-sensitised the Taxol resistant cells at least 20-fold. Importantly, when Mdr-1 cDNA was stably expressed in the wild-type cell line to generate a highly Taxol-resistant sub-line, 1847/MDR5, ERK1/2 MAP kinases again became activated. This result demonstrated that the increased activity of the signalling pathway in the Taxol-resistant lines was directly attributable to MDR-1 overexpression and was not due to the effects of Taxol itself. Additionally, we demonstrated that inhibition of the P13K pathway with LY294002 sensitised the MDR-1-expressing 1847/TX0.5 cells and 1847/MDR5 cells at least 10-fold but had no effect in the wild-type cells. This finding suggests a possible role for this pathway, also, in the generation of resistance to Taxol.

A Phase I study of infusional vinblastine in combination with the P-glycoprotein antagonist PSC 833 (valspodar)

BACKGROUND

PSC 833 is a second-generation P-glycoprotein (Pgp) antagonist developed to reverse multidrug resistance (MDR). The authors conducted a Phase I study of orally administered PSC 833 in combination with vinblastine administered as a 5-day continuous infusion.

METHODS

Seventy-nine patients with advanced malignant disease were enrolled in the trial and treated with escalating doses of PSC 833. Pharmacokinetic interactions between PSC 833 and vinblastine were anticipated. Accordingly, when dose limiting toxicities were observed, the dose of vinblastine was reduced as PSC 833 was escalated. Three schedules and two formulations of PSC 833 were used in the study.

RESULTS

The maximum tolerated doses of PSC 833 were 12.5 mg/kg orally every 12 hours for 8 days for the liquid formulation in combination with 0.9 mg/m(2) per day vinblastine as a continuous intravenous infusion (CIV) for 5 days; and 4 mg/kg orally every 6 hours for 8 days for the microemulsion formulation in combination with 0.6 mg/m(2) per day vinblastine CIV for 5 days. The principal toxicities for PSC 833 were ataxia and paresthesias and for the combination, constipation, fever. and neutropenia. Increased oral bioavailability and increased peak and trough concentrations were observed with the microemulsion formulation. Significant interpatient variability in pharmacokinetic parameters was observed. Ten patients studied at the MTD for PSC 833 (4 mg/kg orally every 6 hours for 8 days) had inhibition of rhodamine efflux from CD56 positive peripheral lymphocytes as a surrogate for Pgp antagonism. Among 43 evaluable patients with clear cell carcinoma of the kidney, 3 patients had complete responses, and 1 patient had a partial response.

CONCLUSIONS

PSC 833 in combination with vinblastine can be administered safely to patients provided the vinblastine dose is adjusted for pharmacokinetic interactions. The high interpatient variability is a significant confounding factor. Surrogate studies with CD56 positive cells suggest that Pgp inhibition in the clinical setting is achievable. Improved methods for predicting pharmacokinetic interactions should improve future studies.

Phase I study of infusional paclitaxel in combination with the P-glycoprotein antagonist PSC 833

PURPOSE

PSC 833 (valspodar) is a second-generation P-glycoprotein (Pgp) antagonist developed to reverse multidrug resistance. We conducted a phase I study of a 7-day oral administration of PSC 833 in combination with paclitaxel, administered as a 96-hour continuous infusion.

PATIENTS AND METHODS

Fifty patients with advanced cancer were enrolled onto the trial. PSC 833 was administered orally for 7 days, beginning 72 hours before the start of the paclitaxel infusion. Paclitaxel dose reductions were planned because of the pharmacokinetic interactions known to occur with PSC 833.

RESULTS

In combination with PSC 833, maximum-tolerated doses were defined as paclitaxel 13.1 mg/m(2)/d continuous intravenous infusion (CIVI) for 4 days without filgrastim, and paclitaxel 17.5 mg/m(2)/d CIVI for 4 days with filgrastim support. Dose-limiting toxicity for the combination was neutropenia. Statistical analysis of cohorts revealed similar mean steady-state concentrations (C(pss)) and areas under the concentration-versus-time curve (AUCs) when patients received paclitaxel doses of 13.1 or 17.5 mg/m(2)/d for 4 days with PSC 833, as when they received a paclitaxel dose of 35 mg/m(2)/d for 4 days without PSC 833. However, the effect of PSC 833 on paclitaxel pharmacokinetics varied greatly among individual patients, although a surrogate assay using CD56+ cells suggested inhibition of Pgp was complete or nearly complete at low concentrations of PSC 833. Responses occurred in three of four patients with non-small-cell lung cancer, and clinical benefit occurred in five of 10 patients with ovarian carcinoma.

CONCLUSION

PSC 833 in combination with paclitaxel can be administered safely to patients provided the paclitaxel dose is reduced to compensate for the pharmacokinetic interaction. Surrogate studies with CD56+ cells indicate that the maximum-tolerated dose for PSC 833 gives serum levels much higher than those required to block Pgp. The variability in paclitaxel pharmacokinetics, despite complete inhibition of Pgp in the surrogate assay, suggests that other mechanisms, most likely related to P450, contribute to the pharmacokinetic interaction. Future development of combinations such as this should include strategies to predict pharmacokinetics of the chemotherapeutic agent. This in turn will facilitate dosing to achieve comparable CPss and AUCs.

The relevance of drug sequence in combination chemotherapy

The concept of combining chemotherapeutic agents to increase the cytotoxic efficacy has evolved greatly over the past several years. In the past, the rationale for combination chemotherapy centered on attacking different biochemical targets, overcoming drug resistance in heterogenous tumors, and increasing the dose-density of combination chemotherapy to take advantage of tumor growth kinetics. The overall goal was to improve clinical efficacy with acceptable clinical toxicity. It is now apparent that the sequence of drug administration can significantly enhance the therapeutic effect of chemotherapy. These sequence-dependent effects can be explained by chemotherapy-induced cell cycle perturbations, or by pharmacodynamic interactions between the agents in combination. In this review, we focus on drug combinations with taxanes and camptothecins, which we believe best illustrate the importance of the cell cycle and pharmacologic interactions in the sequential administration of chemotherapy. As our understanding of the cell cycle grows, our ability to appropriately sequence chemotherapy can have a great impact on the treatment of human cancers. Copyright 2000 Harcourt Publishers Ltd.

Does P-glycoprotein play a role in anticancer drug pharmacokinetics?

The multidrug-resistance P-glycoprotein is a drug efflux transport protein abundantly present in various types of human cancer. The protein is encoded by the MDR1 gene and its function is sensitive to modulation by competitive inhibition. Clinical studies have indicated that inhibitors of P-glycoprotein function dramatically decrease the systemic clearance of anticancer agents, necessitating dose reduction. This dose reduction not only complicated the interpretation of toxicity and response data, but also presented a serious obstacle in the development and rational use of P-glycoprotein inhibitors. It is now evident that the pharmacokinetic interference between anticancer drugs and P-glycoprotein inhibitors is due primarily to competition for drug metabolizing enzymes. A wealth of recent experimental data shows that many of the previously tested P-glycoprotein inhibitors, including verapamil, cyclosporin A, and valspodar (SDZ PSC 833), are substrates and/or potent inhibitors of cytochrome P450 3A4 (CYP3A4). Future development and clinical use of potent P-glycoprotein modulators lacking high affinity for CYP3A4 should decrease the impact of these important drug interactions and will eventually result in improved therapeutic specificity and efficacy. Copyright 2000 Harcourt Publishers Ltd.

P-glycoprotein in primary acute myeloid leukemia and treatment outcome of idarubicin/cytosine arabinoside-based induction therapy

The expression of the drug transport protein, P-glycoprotein (Pgp/MDR1) has been found to be of prognostic significance for the achievement of complete remission (CR) or the duration of survival after daunorubicin (DNR)-containing induction therapy in acute myeloid leukemia (AML). This would suggest that the expression of Pgp in AML is high enough to have significant impact on intracellular DNR concentrations and on clinical therapy failure in AML. Recently, DNR has been replaced in many centers by idarubicin (IDA) as the first choice anthracycline in AML treatment. We have, therefore, performed a study in a group of 98 primary AML patients, who all received IDA, but not DNR during induction therapy in order to determine if the response to IDA-containing induction therapy might be related to the biologic characteristic of Pgp expression in AML. The AML samples were studied for Pgp expression by MRK16 antibody staining and for Pgp activity measured as the modulation of rhodamine 123 uptake by 2 microM PSC 833. No correlation of Pgp with complete response rate, event-free survival or overall survival was found. In addition to Pgp, the expression of another protein that has been implicated by some studies in response failure to DNR-containing therapy, the major vault protein (Mvp/LRP), was studied. This marker did not correlate with CR or survival after IDA-containing therapy. The results of this patient study are consistent with model studies showing that the steady-state cellular accumulation of lipophilic anthracyclines such as IDA are little affected by Pgp. Therefore, putative beneficial effects of the inclusion of PSC 833 in IDA-containing therapy might rather be related to alternative mechanisms than to inhibition of Pgp-mediated IDA efflux.

Options for modulation of drug resistance in ovarian cancer

The objective of this paper is to present an update of mechanisms responsible for drug resistance in ovarian cancer and the possible therapeutic options to modulate this resistance using literature review with emphasis on data acquired in studies comprising ovarian tumor samples. The classic concepts on resistance in ovarian cancer, namely platinum and multidrug resistance, are briefly discussed, followed by a description of more recent insights concerning the role of apoptosis in the development of chemoresistance. A wide variety of mechanisms may be responsible for drug resistance in ovarian cancer. However, a growing body of evidence indicates that defects in the intra- and extracellular apoptotic pathways are an important cause of resistance to cytotoxic agents which opens several new treatment strategies.

Reversal of resistance by GF120918 in cell lines expressing the ABC half-transporter, MXR

The emergence of several newly identified members of the ABC transporter family has necessitated the development of antagonists that are able to inhibit more than one transporter. We assessed the ability of the chemosensitizer GF120918 to function as a multispecific antagonist using cytotoxicity assays, rhodamine and calcein efflux assays, and confocal microscopy in cell lines expressing different multidrug resistance transporters. At a concentration of 1 microM in cytotoxicity assays, GF120918 was able to sensitize both S1-B1-20, a subline expressing P-glycoprotein (Pgp), and S1-M1-80, a subline expressing a newly identified mitoxantrone transporter, MXR. GF120918 was ineffective in sensitizing MRP-overexpressing MCF-7 VP-16 cells to etoposide as determined by cytotoxicity studies. In flow cytometry experiments, rhodamine 123 efflux in S1-B1-20 cells was decreased at GF120918 concentrations as low as 25-50 nM, with 250 nM giving complete inhibition of rhodamine efflux. Complete inhibition of rhodamine efflux in mitoxantrone-resistant S1-M1-80 cells required 10 microM. Examination of intracellular mitoxantrone accumulation by confocal microscopy confirmed higher levels of mitoxantrone in S1-B1-20 and S1-M1-80 cells when incubated in the presence of GF120918 than when incubated with mitoxantrone alone. Thus, GF120918 appears to fit the paradigm of a multispecific blocker and is able to block rhodamine and mitoxantrone efflux by the newly identified mitoxantrone transporter. Further studies of this compound should be pursued to determine its feasibility for use in the clinic.

Direct activation of caspases by RGD-peptides may increase drug sensitivity of tumour cells

The realization that cellular homeostasis is dependent on the continuous integration of survival and death signals picked up from the environment and the recent advances in identifying the molecular players involved in these networks may increase our ability to manipulate apoptosis for therapeutic purposes. A recent paper by Buckley et al. in Nature(1)brings this goal one step closer by identifying peptides, containing the motif arginine-glycine-aspartate (RGD), that induce apoptosis by direct activation of caspase-3. We put this finding in the context of what is known about the RGD motif in the light of cancer treatment and suggest the possibility of a synergistic action with anticancer drugs. Copyright 1999 Harcourt Publishers Ltd.

Efflux of Rhodamine From CD56+ Cells as a Surrogate Marker for Reversal of P-Glycoprotein–Mediated Drug Efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononuclear cells allowed us to use an ex vivo assay as a surrogate measure of Pgp antagonism. Efflux of rhodamine from CD56+cells was measured before the start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56+ cells. Time course studies showed that following a single oral dose of PSC 833, decreased rhodamine efflux was found in some patients within 15 minutes of treatment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes. A dose-response relationship was shown between the concentration of PSC 833 in the blood and the inhibition of rhodamine efflux, with an apparent plateau of the inhibition of rhodamine efflux at approximately 1,000 ng/mL. The Ki, defined as the concentration required for half-maximal inhibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly different, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar among all patients. We conclude that measurement of rhodamine efflux from CD56+ cells provides a surrogate assay with the potential for monitoring Pgp antagonism in clinical trials.

Efflux of Rhodamine From CD56+ Cells as a Surrogate Marker for Reversal of P-Glycoprotein–Mediated Drug Efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononuclear cells allowed us to use an ex vivo assay as a surrogate measure of Pgp antagonism. Efflux of rhodamine from CD56+cells was measured before the start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56+ cells. Time course studies showed that following a single oral dose of PSC 833, decreased rhodamine efflux was found in some patients within 15 minutes of treatment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes. A dose-response relationship was shown between the concentration of PSC 833 in the blood and the inhibition of rhodamine efflux, with an apparent plateau of the inhibition of rhodamine efflux at approximately 1,000 ng/mL. The Ki, defined as the concentration required for half-maximal inhibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly different, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar among all patients. We conclude that measurement of rhodamine efflux from CD56+ cells provides a surrogate assay with the potential for monitoring Pgp antagonism in clinical trials.