Multidrug Resistance In Cancer Therapy Research Articles

γH2AX is a biomarker of modulated cytostatic drug resistance.

ONE reason for reduced therapeutic efficiency of cancer chemotherapies is the appearance of multidrug resistance. Plasma membrane glycoprotein (P-gp) is recognized as one of the best characterized drug efflux pumps playing a central role in the absorption and disposition of many drugs, including chemotherapeutic agents. Increased P-gp activity has been suggested and reported to be an indicator of chemotherapeutic failure. Therefore, P-gp activity has been used as a potential target in being able to reverse multidrug resistance. Cytostatic drugs such as etoposide in combination with the immunosuppressive cyclosporine A may inhibit P-gp activity, ultimately resulting in intracellular enrichment of the targeted drug (1). One issue in cancer therapy involves the highly varying drug response among patients. To assess the multidrug resistance status for personalized cancer therapy, an assay that can provide results to monitor the cytostatic effect of antineoplastic agents in combination with different drugs is highly desirable. Many of these drugs, like etoposide, cause DNA double strand breaks (DSBs) that may eventuate in clonogenic death. DSBs are one of the most biologically significant DNA damage lesions that leads to chromosome breakage and/or rearrangement, mutagenesis and loss or gain of genetic information (2). The phosphorylation of H2AX histone proteins which are located in the vicinity of the DSBs is known as one of the earliest responses to DNA DSBs in cells. Induction of DNA DSBs in live mammalian cells triggers the phosphorylation of Ser139 in the SQ motif near the C-terminal of H2AX protein, which results in the phosphorylated form of H2AX, termed cH2AX. cH2AX signals appear as discrete nuclear “foci” which can be visualized and quantified by a number of methods including fluorescence microscopy and flow cytometry, following immunostaining procedures with fluorescence-coupled antibodies. The cH2AX foci counting approach has been used in numerous studies to assess the relationship between cH2AX foci removal and the rate of DSBs repair (3). Each DSB is represented by an individual cH2AX focus with a ratio of 1:1 and after successful repair of DSBs, the level of cH2AX foci decline over time in the presence of normal physiological repair mechanisms. The presence of persistent cH2AX indicates impaired DNA repair. Therefore, cH2AX quantification may prove to be a sensitive biomarker of DNA DSBs in human cells and has been suggested for biomonitoring tumor progression and treatment response (4). DSBs are directly generated by exogenous agents such as ionizing radiation and antitumor drugs (bleomycin, mitoxantrone, etoposide). Because mammalian cells respond to DSBs by activating a multitude of proteins involved in signalling and DNA repair pathways, the very nature of DSBs poses such a threat to cell survival that DNA damage checkpoint proteins may be activated to initiate cellular division arrest. This provides time for DNA repair to proceed before mitosis is completed or in the case of overwhelming damage, apoptosis ensues (5). Therefore, detection of accumulated cH2AX foci may be an efficient method for assessing multidrug resistance in cancer therapy as presented by Reddig et al. in this issue of Cytometry A (page XXX).

Salinomycin enhances doxorubicin-induced cytotoxicity in multidrug resistant MCF-7/MDR human breast cancer cells via decreased efflux of doxorubicin.

Salinomycin is a monocarboxylic polyether antibiotic, which is widely used as an anticoccidial agent. The anticancer property of salinomycin has been recognized and is based on its ability to induce apoptosis in human multidrug resistance (MDR). The present study investigated whether salinomycin reverses MDR towards chemotherapeutic agents in doxorubicin-resistant MCF-7/MDR human breast cancer cells. The results demonstrated that doxorubicin-mediated cytotoxicity was significantly enhanced by salinomycin in the MCF-7/MDR cells, and this occurred in a dose-dependent manner. This finding was consistent with subsequent observations made under a confocal microscope, in which the doxorubicin fluorescence signals of the salinomycin-treated cells were higher compared with the cells treated with doxorubicin alone. In addition, flow cytometric analysis revealed that salinomycin significantly increased the net cellular uptake and decreased the efflux of doxorubicin. The expression levels of MDR-1 and MRP-1 were not altered at either the mRNA or protein levels in the cells treated with salinomycin. These results indicated that salinomycin was mediated by its ability to increase the uptake and decrease the efflux of doxorubicin in MCF-7/MDR cells. Salinomycin reversed the resistance of doxorubicin, suggesting that chemotherapy in combination with salinomycin may benefit MDR cancer therapy.

TPGS-stabilized NaYbF4:Er upconversion nanoparticles for dual-modal fluorescent/CT imaging and anticancer drug delivery to overcome multi-drug resistance

Multi-drug resistance (MDR) is a major cause of failure in cancer chemotherapy. Tocopheryl polyethylene glycol 1000 succinate (TPGS) has been extensively investigated for overcoming MDR in cancer therapy because of its ability to inhibit P-glycoprotein (P-gp). In this work, TPGS was for the first time used as a new surface modifier to functionalize NaYbF4:Er upconversion nanoparticles (UNCPs) and endowed the as-prepared products (TPGS-UCNPs) with excellent water-solubility, P-gp inhibition capability and imaging-guided drug delivery property. After the chemotherapeutic drug (doxorubicin, DOX) loading, the as-formed composites (TPGS-UCNPs-DOX) exhibited potent killing ability for DOX-resistant MCF-7 cells. Flow-cytometric assessment and Western blot assay showed that the TPGS-UCNPs could potently decrease the P-gp expression and facilitate the intracellular drug accumulation, thus achieving MDR reversal. Moreover, considering that UCNPs process efficient upconversion emission and Yb element contained in UCNPs has strong X-ray attenuation ability, the as-obtained composite could also serve as a dual-modal probe for upconversion luminescence (UCL) imaging and X-ray computed tomography (CT) imaging, making them promising for imaging-guided cancer therapy.

Codelivery of Chemotherapeutics via Crosslinked Multilamellar Liposomal Vesicles to Overcome Multidrug Resistance in Tumor

Multidrug resistance (MDR) is a significant challenge to effective cancer chemotherapy treatment. However, the development of a drug delivery system that allows for the sustained release of combined drugs with improved vesicle stability could overcome MDR in cancer cells. To achieve this, we have demonstrated codelivery of doxorubicin (Dox) and paclitaxel (PTX) via a crosslinked multilamellar vesicle (cMLV). This combinatorial delivery system achieves enhanced drug accumulation and retention, in turn resulting in improved cytotoxicity against tumor cells, including drug-resistant cells. Moreover, this delivery approach significantly overcomes MDR by reducing the expression of P-glycoprotein (P-gp) in cancer cells, thus improving antitumor activity in vivo. Thus, by enhancing drug delivery to tumors and lowering the apoptotic threshold of individual drugs, this combinatorial delivery system represents a potentially promising multimodal therapeutic strategy to overcome MDR in cancer therapy.

Doxorubicin-loaded magnetic silk fibroin nanoparticles for targeted therapy of multidrug-resistant cancer.

A strategy to prepare doxorubicin-loaded magnetic silk fibroin nanoparticles is presented. The nanoparticles serve as a nanometer-scale drug-delivery system in the chemotherapy of multidrug-resistant cancer under the guidance of a magnetic field. The magnetic tumor-targeting ability broadens the range of biomedical applications of silk fibroin, and the nanoparticle-assisted preparation strategy is useful for the advancement of other biomacromolecule-based materials.

Paclitaxel-Loaded N-Octyl-O-sulfate Chitosan Micelles for Superior Cancer Therapeutic Efficacy and Overcoming Drug Resistance

The nanoparticle-based drug delivery system holds great attraction to overcome or circumvent multidrug resistance (MDR) in cancer to date. In this work, a synthesized amphiphilic graft copolymer, N-octyl-O-sulfate chitosan (NOSC), and its paclitaxel (PTX)-encapsulated micelles (PTX-M) have been systematically investigated on the MDR reversal effect in vitro and in vivo as well as the mechanism of P-glycoprotein (P-gp) inhibition. NOSC in a wide concentration range even above the critical micelle concentration showed an effective effect on inhibiting P-gp-mediated PTX efflux, which was remarkably different from the surfactants and the Pluronic copolymers. Multiple mechanisms were involved in this effect of NOSC, such as stimulating P-gp ATPase, competitively impeding the binding of PTX with P-gp and reducing the fluidity of the cell membrane. PTX-M presented the highest cellular uptake and the lowest efflux rate of PTX, thereby yielding the optimal cytotoxicity on both the human hepatocellular liver carcinoma (HepG2) cells and the multidrug resistance HepG2 (HepG2-P) cells, which resulted from a combination of the inhibiting P-gp effect of NOSC and the bypassing P-gp action of the intact PTX-M. Additionally, PTX-M had superior blood persistence, tumor accumulation, and therapeutic efficacy after intravenous injection into the tumor-bearing mice. Furthermore, it was demonstrated that most of PTX-M as an intact form was delivered at the tumor site, which ensures the synergetic effect of NOSC micelles on drug delivery and P-gp inhibition. The aforementioned results suggested that NOSC micelles presented promising potential as an anticancer drug carrier for enhanced MDR cancer therapy.

Novel function of N,N-bis(2-chloroethyl)docos-13-enamide for reversal of multidrug resistance in tongue cancer

Multidrug resistance (MDR) is a key element in the failure of chemotherapies, and development of agents to overcome MDR is crucial to improving cancer treatments. The overexpression of glutathione-S-transferases (GSTs) is one of the major mechanisms of MDR. Because some agents used in traditional Chinese medicine have strong antitumor effects coupled with low toxicity; we investigated the ability of N,N-bis(2-chloroethyl)docos-13-enamide (compound J), the synthesized analog of a highly unsaturated fatty acid from Isatis tinctoria L., to reverse the MDR induced by adriamycin (ADM) in TCA8113/ADM cells. We found that compound J significantly increased the cytotoxicity of ADM in TCA8113/ADM cells, with a reversal fold of 2.461. Analysis of the mechanisms through which compound J reversed MDR indicated that compound J significantly decreased the activity of GSTs and enhanced the depletion of GSH in TCA8113/ADM cells, but did not affect the P-glycoprotein (P-gp) efflux. Taken together, our data suggested that compound J was an excellent candidate for reversing MDR in cancer therapy.

Combating P-glycoprotein-Mediated Multidrug Resistance Using Therapeutic Nanoparticles

The development of multidrug resistance (MDR) to chemotherapy is a major obstacle for the successful treatment of cancer. A number of mechanisms have been postulated to account for MDR in cancer. The most common and best-studied mechanism of resistance is mediated through the drug efflux protein P-glycoprotein (P-gp), which is overexpressed in drug-resistant cancer cells and is responsible for the removal of many chemotherapeutic agents. Therapeutic nanoparticles (NPs) have emerged as an innovative and promising option to combat P-gp-mediated MDR and have shown enhanced therapeutic efficacy and reduced toxicity compared to their small molecule counterparts. This review focuses on recent studies using therapeutic NPs to circumvent P-gp-mediated MDR in cancer therapy. The advantages and strategies by which therapeutic NPs were used to overcome P-gp-mediated MDR in cancer are discussed.

Smart pH-Sensitive and Temporal-Controlled Polymeric Micelles for Effective Combination Therapy of Doxorubicin and Disulfiram

The combination of a chemotherapeutic drug with a multidrug resistance (MDR) modulator has emerged as a promising strategy for treating MDR cancer. To ensure two drugs could be simultaneously delivered to tumor region at the optimum ratio, and the MDR modulator could be released earlier and faster than the chemotherapeutic drug to inactivate P-glycoprotein (P-gp) and subsequently inhibit the pumping out of the chemotherapeutic drug, a smart pH-sensitive polymeric micelles system with high drug loading and precise drug ratio was designed and prepared by conjugating doxorubicin (DOX) to poly(styrene-co-maleic anhydride) (SMA) derivative with adipic dihydrazide (ADH) through a acid-cleavable hydrazone bond, and then encapsulating disulfiram (DSF), a P-gp inhibitor as well as an apoptosis inducer, into the micelles formed by the self-assembly of SMA-ADH-DOX (SAD) conjugate. The pH-sensitive polymeric micelles system enabled a temporal release of two drugs: encapsulated DSF was released fast to inhibit the activity of P-gp and restore cell apoptotic signaling pathways, while conjugated DOX was released in a sustained and pH-dependent manner and highly accumulated in drug resistant cells to exert therapeutic effect, due to the inactivation of P-gp by DSF. The smart co-delivery system was very effective in enhancing the cytotoxicity by increasing the intracellular accumulation of DOX and promoting the apoptotic response, and showed the most effective inhibitory effect on the growth of drug-resistant breast cancer xenografts as compared to other combinations of both drugs. In a word, this smart co-delivery system has significant promise for the clinical therapy of MDR cancer.

Abstract 878: Tanshinone-1 kills multidrug resistant tumor cells via reducing phospho-705-Stat3, potentiated by inhibiting secondary activation of signaling networks.

Abstract One of the severest hurdles in cancer treatment is multidrug resistance (MDR), resulting from overexpression of drug transporters and/or deregulation of cellular signaling transduction. Persistent efforts have been made to find new agents and strategies to overcome MDR. MDR involves almost all clinically-used anticancer drugs with natural origins. In this study, however, we found that the natural product tanshinone-1 could circumvent MDR via reducing phospho-705-Stat3, potentiated by inhibiting secondary activation of signaling networks. Tanshinone-1 is a bioactive ingredient in traditional Chinese medicine that exhibits diverse biologic properties, including anticancer properties. Tanshinone-1 displayed equipotent or even more potent cytotoxicity in three MDR tumor cell lines than in their corresponding parental cell lines. Tanshinone-1 induced apoptosis of the MDR cells by activating the intrinsic apoptotic pathway via disturbing mitochondria membrane potential and eliciting the cleavage of caspase-9 and caspase-3. The killing of MDR cells by tanshinone-1 was independent of drug transporters, but correlated tightly with its decreasing phospho-705-Stat3, highly possibly via activating certain phosphatase(s). The expression of the target genes of Stat3 including Mcl-1, cyclin D1, cIAP-2 and c-myc, was downregulated by the treatment with tanshinone-1. Knockdown of stat3 expression with specific small interfering RNA led to reduced sensitivity of the tested MDR cells to tanshinone-1. Treatment with Na3VO4, a pan inhibitor of phosphatases, completely reversed tanshinone-1-induced decrease of phospho-705-Stat3 and significantly lowered its cytotoxicity. Knockdown of phosphatase SHP-2 or PTP1B prevented reduction of phospho-705-Stat3 and induction of cell apoptosis driven by tanshinone-1. Further investigation revealed that tanshinone-1 activated signaling networks involving Akt, p38 and Erk pathways, possibly secondary to its reducing phospho-705-Stat3. Notably, simultaneous suppression of those pathways enhanced the cytotoxic effect of tanshinone-1 against the tested MDR cells. Cotreatment of the PI3K inhibitor PI103, the p38 inhibitor SB203580 and/or the MEK inhibitor AZD6244 with tanshinone-1 potentiated its apoptotic induction. However, PI103 and SB580203 appeared to show more potent synergism than AZD6244 when combined with tanshinone-1. Together, this study presents tanshinone-1 as an MDR-overcoming natural product, and more importantly, from our results, a therapeutic strategy of targeting signaling network(s) may evolve for MDR cancer therapy. Citation Format: Lei Xu, Jian-Ming Feng, Jia-Xin Li, Jin-Mei Zhu, Shan-Shan Song, Lin-Jiang Tong, Yi Chen, Fu-Lin Lian, Dong-Hai Lin, Jian Ding, Ze-Hong Miao. Tanshinone-1 kills multidrug resistant tumor cells via reducing phospho-705-Stat3, potentiated by inhibiting secondary activation of signaling networks. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 878. doi:10.1158/1538-7445.AM2013-878

Mechanism of inhibition of P-glycoprotein mediated efflux by Pluronic P123/F127 block copolymers: Relationship between copolymer concentration and inhibitory activity

The aim of this study was to clarify the relationship between the concentration of Pluronic P123/F127 block copolymers and P-glycoprotein (P-gp) inhibitory potency. Modulation of multidrug resistance (MDR) by Pluronic P123/F127 was evaluated in P-gp over-expressing human breast cancer cell line MCF-7/ADR and its non-P-gp over-expressing counterpart MCF-7 cells. Four different probes (known as P-gp substrates) including rhodamine 123 (R-123), rhodamine 6G (R-6G), doxorubicin (DOX), and paclitaxel (PTX) were applied to investigate the impact of Pluronic P123/F127 copolymers with different concentrations on the intracellular accumulation of these probes. Additionally, the intracellular ATP and mitochondrial transmembrane potential in MCF-7/ADR cells were determined over a wide concentration range of Pluronic P123/F127. Furthermore, the endocytic mechanisms of Pluronic micelles were performed. It was suggested that P-gp substrate hydrophobicity and the concentration of P123/F127 copolymers had little impact on P-gp inhibitory activity of Pluronic P123/F127 itself. Intracellular ATP depletion was the main mechanism of Pluronic P123/F127 for P-gp inhibition. In vitro cytotoxicity study was also conducted in order to compare cytotoxic effect among different PTX formulations. It indicated that the IC50 of PTX-loaded Pluronic P123/F127 mixed micelles was 6.3-fold lower than free PTX and 2.3-fold lower than Taxol, respectively. Therefore, Pluronic P123/F127 polymeric micelles could be considered a promising drug delivery system to overcome MDR in cancer therapy.

Abstract B39: Mechanisms of multidrug resistance in AMLs and selective targeting via small molecules

Abstract Multidrug resistance (MDR) is a major hurdle in the treatment of cancer and there is a pressing need for new therapies. We have recently developed a small molecule, CXL017, with selective cytotoxicity towards MDR cancer cells. In this study, we offer new mechanistic insights about multidrug resistance in acute myeloid leukemias and present evidence for the selective therapeutic potential of CXL017 in the treatment of MDR AMLs. CXL017 significantly and selectively suppresses the growth of tumors derived from the MDR AML cells. In addition, drug resistant AML cells failed to develop stable resistance to CXL017 upon prolonged exposure. Remarkably, instead of acquiring cross-resistance, drug resistant AML cells exposed to CXL017 were re-sensitized to standard therapies (10 to 100-fold). Through gene profiling, protein characterization, and genetic up/down-regulation, we have identified several key signaling processes both at the transcription, such as Topo IIβ, and post-transcription levels, such as Mcl-1, that contribute to the multidrug resistance. Some of these changes also contribute to the selective sensitivity of multidrug resistant AMLs to CXL017. These results overall demonstrate the possibility of developing small-molecule therapies selective against multidrug resistance in cancer therapy upon gaining the mechanistic insights of drug resistance.

Reversion of Multidrug Resistance in a Chemoresistant Human Breast Cancer Cell Line by β-Elemene

Background: Multidrug resistance (MDR) presents a problem in cancer chemotherapy, and developing new agents to overcome MDR is important. This study intends to investigate the reversal effect of β-elemene on MDR in human breast carcinoma MCF-7 and doxorubicin-resistant MCF-7 cells. Methods: MTT cytotoxicity assays, flow cytometry, and Western blot analysis were performed to investigate the antiproliferative effects of the combination of anticancer drugs with β-elemene, to study the reversal of drug resistance, and to examine the inhibitory effects on protein expression. Results: The results showed that β-elemene (30 µmol/l) had a strong potency to increase the cytotoxicity of doxorubicin to MCF-7/DOX cells, with a reversal fold of 6.38. In addition, the mechanisms of β-elemene in reversing P-glycoprotein (P-gp)-mediated MDR demonstrated that β-elemene significantly increases the intracellular accumulations of doxorubicin and Rh123 via inhibition of the P-gp transport function in MCF-7/DOX cells. Flow cytometry and Western blot analyses revealed that β-elemene could inhibit the expression of P-gp, while it had little effect on the expression of MRP1 protein. In addition, β-elemene had little inhibitory effect on the intracellular GSH levels and GST activities in MCF-7/DOX cells. Conclusions: β-Elemene might represent a promising agent for overcoming MDR in cancer therapy.

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.

Multifunctional QD-based co-delivery of siRNA and doxorubicin to HeLa cells for reversal of multidrug resistance and real-time tracking

Co-delivery of siRNA and chemotherapeutic agents has been developed to combat multidrug resistance in cancer therapy. Recently, we developed a series of quantum dots (QDs) functionalized by β-cyclodextrin (β-CD) coupled to amino acids, some of which can be used to facilitate the delivery of siRNA. In this study, two CdSe/ZnSe QDs modified with β-CD coupled to L-Arg or L-His were used to simultaneously deliver doxorubicin (Dox) and siRNA targeting the MDR1 gene to reverse the multidrug resistance of HeLa cells. In this co-delivery system, Dox was firstly encapsulated into the hydrophobic cavities of β-CD, resulting in bypass of P-glycoprotein (P-gp)-mediated drug efflux. After complex formation of the mdr1 siRNA with Dox-loaded QDs via electrostatic interaction, significant down-regulation of mdr1 mRNA levels and P-gp expression was achieved as shown by RT-PCR and Western blotting experiments, respectively. The number of apoptotic HeLa cells after treatment with the complexes substantially exceeded the number of apoptotic cells induced by free Dox only. The intrinsic fluorescence of the QDs provided an approach to track the system by laser confocal microscopy. These multifunctional QDs are promising vehicles for the co-delivery of nucleic acids and chemotherapeutics and for real-time tracking of treatment.

Reversal of Multidrug Resistance by Morning Glory Resin Glycosides in Human Breast Cancer Cells

Reversal of multidrug resistance (MDR) by thirty resin glycosides from the morning glory family (Convolvulaceae) was evaluated in vinblastine-resistant human breast carcinoma cells (MCF-7/Vin). The effects of these amphipathic compounds on the cytotoxicity and P-glycoprotein (P-gp)-mediated MDR were estimated with the sulforhodamine B colorimetric assay. Active noncytotoxic compounds exerted a potentiation effect of vinblastine susceptibility by 1- to over 1906-fold at tested concentrations of 5 and 25 μg/mL. Murucoidin V (1) enhanced vinblastine activity 255-fold when incorporated at 25 μg/mL and also, based on flow cytometry, significantly increased the intracellular accumulation of rhodamine 123 with the use of reserpine as a positive control for a MDR reversal agent. Incubation of MCF-7/Vin cells with 1 caused an increase in uptake and notably lowered the efflux rate of rhodamine 123. Decreased expression of P-glycoprotein by compound 1 was detected by immunofluorescence flow cytometry after incubation with an anti-P-gp monoclonal antibody. These results suggest that resin glycosides represent potential efflux pump inhibitors for overcoming MDR in cancer therapy.

PEO-PPO Block Copolymers for Passive Micellar Targeting and Overcoming Multidrug Resistance in Cancer Therapy

Drug carriers tailored to fit the physicochemical properties of anticancer agents and the therapeutic peculiarities of tumor management are envisioned for improving the effectiveness/toxicity ratio of the current treatments. Polymeric micelles are attracting much attention owing to their unique beneficial features: i) core-shell structure capable to host hydrophobic drugs, raising the apparent solubility in aqueous medium; ii) size adequate for a preferential accumulation (passive targeting) within the tumor, exhibiting enhanced permeability and retention (EPR effect), and iii) unimers that modulate the activity of efflux pumps involved in multidrug resistance (MDR). This review focuses on amphiphilic poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) block copolymers, namely the linear poloxamers (Pluronic® or Lutrol®) and the X-shaped poloxamines (Tetronic®), as components of polymeric micelles able to play these three roles. Specific facets of poloxamers have been highlighted some years ago, but recently their wide range of possibilities is beginning to be fully elucidated and understood. Poloxamines are new excipients in the cancer arena and the comparison of their performance with that of poloxamers may enable to identify aspects of their architecture relevant for the optimization of micellar carriers. Clinical trials in progress indicate that drug-loaded polymeric micelles are beneficial regarding efficiency, safety, and compliance of the treatment and quality of life of the patients. The fact that some copolymers are already approved for internal use and several chemotherapy agents will be off patent soon may help to bring the clinical use of poloxamer- or poloxamine-based micelles into a reality in the coming years.

Reversal effects of two new milbemycin compounds on multidrug resistance in MCF-7/adr cells in vitro

Development of agents to overcome multidrug resistance (MDR) is important in cancer chemotherapy, and the overexpression of P-glycoprotein (P-gp) is one of the major mechanisms of MDR. In this paper, we evaluated the effects of two new milbemycin compounds, milbemycin β 14 and secomilbemycin D, isolated from fermentation broth of S. bingchenggensis on reversing MDR of adriamycin-resistant human breast carcinoma (MCF-7/adr) cells. We observed that the both milbemycins (5 μM) showed strong potency to increase adriamycin cytotoxicity toward MCF-7/adr cells with reversal fold (RF) of 13.5 and 10.59, respectively. In addition, the mechanisms of milbemycins on reversing P-gp-mediated MDR demonstrated that they significantly increased the accumulations of adriamycin and Rh123 via inhibiting P-gp efflux in MCF-7/adr cells. Furthermore, the results also revealed that milbemycin β 14 and secomilbemycin D could regulate down the expression of P-gp, but not affect the expression of MDR1 gene. In conclusion, our observations suggest that the two new milbemycin compounds probably represent the promising agents for reversing MDR in cancer therapy.

Biometric Evaluation of the Interaction Between Anticancer Agents and Multidrug-resistance Modulators

The objective of the study was to evaluate the interaction of multidrug-resistance modulators with anticancer agents using combination index analysis. In cancer chemotherapy, combinations of drugs and adjuvants are routinely used, and it is important to determine if the nature of the combined effect is additive, antagonistic, or synergistic. In recent years efforts have been made to identify compounds that can lower drug resistance in multidrug-resistant tumour cells. For the therapy of multidrug-resistant cancer, synergistic potentiation of the cytotoxic activity of anticancer drugs by compounds that modulate drug resistance is highly desirable. We have evaluated the nature of interaction between clinically used anticancer agents (doxorubicin and etoposide) and investigational modulators of multidrug resistance (dipyridamole and tamoxifen). Multidrug-resistant tumour cells, B16VDXR murine melanoma and MCF-7/ADR human breast carcinoma, were used as in-vitro models to evaluate the cytotoxicity of individual drugs or drug combinations using colony-formation assays. The nature of drug interactions was assessed using combination index analysis as a function of fraction affected (fraction of cells that did not survive drug treatment). Interactions were considered antagonistic when the combination index was greater than 1, additive when equal to 1, and synergistic when less than 1. For doxorubicin and dipyridamole as well as mitoxantrone and tamoxifen interactions, the combination index vs fraction affected plot indicated that both these drug combinations resulted in synergistic interaction for fraction affected values greater that 0.1. The etoposide and dipyridamole combination was synergistic over the entire range of cell kill. The combination index at the 50% level of cell kill (CI50), suggested that in B16VDXR cells the synergistic potentiation of cytotoxicity was markedly stronger for the etoposide and dipyridamole combination (CI50 = 0.25) relative to the synergy observed for the doxorubicin and dipyridamole combination (CI50 = 0.48). The CI50 for the mitoxantrone and tamoxifen combination was about 0.35. This study thus demonstrates the use of combination index analysis in the evaluation of the interaction between anticancer agent and multidrug-resistance modulators. Such biometric evaluations can help to identify drug combinations that result in synergistic potentiation of cytotoxicity against multidrug-resistant cells.

Reversal of P-glycoprotein-mediated multidrug resistance in vitro by milbemycin compounds in adriamycin-resistant human breast carcinoma (MCF-7/adr) cells

The effects of milbemycin A 4 (MB A 4), milbemycin oxime A 4 (MBO A 4) and milbemycin β 1 (MB β 1) on reversing multidrug resistance (MDR) of tumor cells were firstly conducted according to the following research, including MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay, the accumulation of adriamycin, the accumulation and efflux of rhodamine 123 (Rh123), the regulations of MDR1 gene, and expression of P-gp. The three milbemycins (5 μM) showed strong potency to increase adriamycin cytotoxicity toward adriamycin-resistant human breast carcinoma cells MCF-7/adr with reversal fold (RF) of 21.42, 19.06 and 14.89, respectively. In addition, the mechanisms of milbemycins on P-glycoprotein (P-gp)-mediated MDR demonstrated that the milbemycins significantly increased the intracellular accumulations of adriamycin and Rh123 via inhibiting P-gp transport function. Based on the analysis of the P-gp and MDR1 gene expression using flow cytometry and RT-PCR, the results revealed that milbemycin compounds, particularly MB A 4, could regulate down the expression of the P-gp and MDR1 gene. These findings suggest that the milbemycins probably represent promising agents for overcoming MDR in cancer therapy, and especially MB A 4 is better modulator with the lowest toxicity.