Polyethyleneimine grafted with pluronic P85 enhances Ku86 antisense delivery and the ionizing radiation treatment efficacy in vivo.

In an effort to improve the efficacy of antisense delivery, we evaluated polyethyleneimine (PEI, 2 kDa) alone or grafted with nonionic amphiphilic block copolymer Pluronic (P85) as a carrier for Ku86 antisense oligonucleotide (ASO) delivery. Ku86 is an abundant nuclear protein that plays an important role in nonhomologous DNA end joining and has implications in tumorigenesis and acquired drug resistance. Transfection of adherent and suspension cell lines with Ku86 ASOs complexed with P85-g-PEI (2 kDa) conjugates was associated with a specific decrease in Ku86 mRNA levels (EC50<75 nM and EC50<250 nM, respectively, n=3). More importantly, no requirement for reduced serum conditions was necessary during transfection. In contrast, whereas Ku86 ASOs complexed with PEI (2 kDa) alone were effective in decreasing Ku86 mRNA levels in adherent cell lines (EC50<75 nM, n=3), the formulation did not produce any detectable decrease in Ku86 mRNA levels in suspension cell lines. Transfection of adherent cell lines with 500 nM Ku86 ASOs formulated with P85-g-PEI (2 kDa) was associated with a specific decrease (<10% remaining of control) in Ku86 protein expression and a two-fold increased cell death after treatment with ionizing radiation (IR). In athymic nude mice bearing subcutaneous human HT29 colon adenocarcinoma xenografts, Ku86 ASO-P85-g-PEI (2 kDa) administration (15 mg/kg, subcutaneously) with a Q1D x 7 treatment schedule, when combined with a single dose of IR (6 Gy), caused a significant inhibition of HT29 tumor growth compared with mismatch- and naked antisense-pretreated control groups (time from 200 to 1000 mm3, 126.9 versus 84.18 and 87.76 days, P<0.005). A potentiation of the antitumor activity was observed in all mice treated with Ku86 ASO-P85-g-PEI (2 kDa) formulation; however, tumor growth inhibition was reversible upon treatment cessation. No morbidity/mortality or changes in histopathology were observed under this treatment regiment. Our results indicate that P85-g-PEI (2 kDa) conjugates may increase the efficacy of Ku86 ASO delivery in management of resistant malignancies, thus providing a rationale for their evaluation in cancer patients in combination with conventional anticancer therapies.

Phase I dose escalation and pharmacokinetic study of pluronic polymer-bound doxorubicin (SP1049C) in patients with advanced cancer.

SP1049C is a novel anticancer agent containing doxorubicin and two nonionic pluronic block copolymers. In preclinical studies, SP1049C demonstrated increased efficacy compared to doxorubicin. The objectives of this first phase I study were to determine the toxicity profile, dose-limiting toxicity, maximum tolerated dose and pharmacokinetic profile of SP1049C, and to document any antitumour activity. The starting dose was 5 mg m(-2) (doxorubicin content) as an intravenous infusion once every 3 weeks for up to six cycles. A total of 26 patients received 78 courses at seven dose levels. The dose-limiting toxicity was myelosuppression and DLT was reached at 90 mg m(-2). The maximum tolerated dose was 70 mg m(-2) and is recommended for future trials. The pharmacokinetic profile of SP1049C showed a slower clearance than has been reported for conventional doxorubicin. Evidence of antitumour activity was seen in some patients with advanced resistant solid tumours. Phase II trials with this agent are now warranted to further define its antitumour activity and safety profile.

Wheat germ cell-free translation system as a tool for in vitro selection of functional proteins.

We have demonstrated that mRNA, ribosome and resulting protein form complexes (ternary complexes) in wheat germ cell-free translation system and these complexes are stable for at least several hours. The protein folds into a proper conformation capable of specific binding with the inhibitor of its enzymatic activity. The removal of the stop codon from mRNA does not affect translation and mRNA-ribosome-protein complex stability. We have used these results to develop a method of isolation of mouse dihydrofolate reductase (mDHFR) encoding mRNA from native pool of mouse liver mRNA. The native pool of mouse liver mRNA was translated in vitro in a wheat germ cell-free translation system (WG-CFS), and enzyme-specific ternary complexes were affinity selected on a methotrexate-BSA coated 96-well microtiter plate (methotrexate, MTX, is an inhibitor of DHFR enzymatic activity). Bounded ternary complexes were eluted by MTX treatment. mRNA from eluates was amplified by template-switch RT-PCR and products of RT-PCR analyzed by gel electrophoresis. The cDNA was amplified by one-step reverse transcription-PCR and used for transcription, followed by translation and determination of the DHFR enzymatic activity in translation mixtures. This method is suitable for direct cDNA cloning from mRNA or cDNA libraries and for investigation of protein-protein interactions.

Altered organ accumulation of oligonucleotides using polyethyleneimine grafted with poly(ethylene oxide) or pluronic as carriers.

Passive targeting provides a simple strategy based on natural properties of the carriers to deliver DNA molecules to desired compartments. Polyethylenimine (PEI) is a potent non-viral system that has been known to deliver efficiently both plasmids and oligonucleotides (ODNs) in vitro. However, in vivo systemic administration of DNA/PEI complexes has encountered significant difficulties because these complexes are toxic and have low biodistribution in target tissues. This study evaluates PEI grafted with poly(ethylene oxide) (PEO(8K)-g-PEI(2K)) and PEI grafted with non-ionic amphiphilic block copolymer, Pluronic P85 (P85-g-PEI(2K)) as carriers for systemic delivery of ODNs. Following i.v. injection an antisense ODN formulated with PEO(8K)-g-PEI(2K) accumulated mainly in kidneys, while the same ODN formulated with P85-g-PEI(2K) was found almost exclusively in the liver. Furthermore, in the case of the animals injected with the P85-g-PEI(2K)-based complexes most of the ODN was found in hepatocytes, while only a minor portion of ODN was found in the lymphocyte/monocyte populations. The results of this study suggest that formulating ODN with PEO(8K)-g-PEI(2K) and P85-g-PEI(2K) carriers allows targeting of the ODN to the liver or kidneys, respectively. The variation in the tissue distribution of ODN observed with the two carriers is probably due to the different hydrophilic-lipophilic balance of the polyether chains grafted to PEI in these molecules. Therefore, polyether-grafted PEI carriers provide a simple way to enhance ODN accumulation in a desired compartment without the need of a specific targeting moiety.

Inducing neutrophil recruitment in the liver of ICAM-1-deficient mice using polyethyleneimine grafted with Pluronic P123 as an organ-specific carrier for transgenic ICAM-1.

Coordinated expression of cell adhesion molecules and chemokines on the surface of vascular endothelium is responsible for the homing of immune effector cells to targeted sites. One way to attract non-activated immune cells to targeted organs is to use transgenically expressed adhesion molecules responsible for leukocyte recruitment. We have previously shown that polyethyleneimine (PEI) grafted with non-ionic amphiphilic Pluronic P123 block copolymer (P123PEI) modifies biodistribution of plasmid DNA toward the liver. In the present study, a P123PEI-formulated plasmid carrying the gene encoding for the murine ICAM-1 molecule was injected i.v. into transgenic ICAM-1-deficient mice. The RT-PCR analysis of ICAM-1 mRNA expression showed that P123PEI induced a dose-dependent expression of ICAM-1 in the liver. Furthermore, this expression of ICAM-1 induced neutrophil invasion in the liver, while no such invasion was observed in mice injected with formulated control plasmid or naked DNA. These results suggest that P123PEI allows functional transgene expression in the liver following i.v. injection and that ICAM-1 could be used to enhance immune response locally by attracting immune effector cells.

Combinatorial approaches to formulation development.

This review describes the use of combinatorial methods for the development of drug formulations. Combinatorial methods are applied to find solutions to various formulation problems, including drug solubilization, controlled release, oral drug administration, and others. Various methods are described, including the synthesis of carrier libraries, high-throughput screening and computational analysis, which are used during the formulation development process, starting from initial assays through the optimization of formulation composition, to the optimization of the manufacturing process. This review also describes an integrated approach to drug formulation development using libraries of block copolymers as the drug carriers.

Block copolymeric biotransport carriers as versatile vehicles for drug delivery.

This review describes block copolymer-based systems that are used in drug formulation development. The use of amphiphilic block copolymers to modify pharmacological performance of various classes of drugs attracts more and more attention. This is largely attributable to the high tendency of block copolymer-based drug formulations to self-assemble, as well as flexibility of block copolymer chemistry, which allows precise tailoring of the carrier to virtually any chemical entity. Combination of these features allows adjustment of block copolymer-based drug formulations to achieve the most beneficial balance in drug biological interactions with the systems that control its circulation in and removal from the body and its therapeutic activity. The following major aspects are considered: 1) physical properties of formulations and the methods used to adjust these properties towards the highest pharmacological performance of the product; 2) combinatorial methods for optimisation of block copolymer-based formulations; 3) biological response modifying properties of block copolymer-based formulations.

Mechanism of sensitization of MDR cancer cells by Pluronic block copolymers: Selective energy depletion.

This paper, for the first time, demonstrates that exposure of cells to the poly(ethylene oxide)-poly(propylene oxide) block copolymer, Pluronic P85, results in a substantial decrease in ATP levels selectively in MDR cells. Cells expressing high levels of functional P-glycoprotein (MCF-7/ADR, KBv; LLC-MDR1; Caco-2, bovine brain microvessel endothelial cells [BBMECs]) are highly responsive to Pluronic treatment, while cells with low levels of P-glycoprotein expression (MCF-7, KB, LLC-PK1, human umbilical vein endothelial cells [HUVECs] C2C12 myoblasts) are much less responsive to such treatment. Cytotoxicity studies suggest that Pluronic acts as a chemosensitizer and potentiates cytotoxic effects of doxorubicin in MDR cells. The ability of Pluronic to inhibit P-glycoprotein and sensitize MDR cells appears to be a result of ATP depletion. Because many mechanisms of drug resistance are energy dependent, a successful strategy for treating MDR cancer could be based on selective energy depletion in MDR cells. Therefore, the finding of the energy-depleting effects of Pluronic P85, in combination with its sensitization effects is of considerable theoretical and practical significance.

Mechanism of pluronic effect on P-glycoprotein efflux system in blood-brain barrier: contributions of energy depletion and membrane fluidization.

Pluronic block copolymer, P85, inhibits the P-glycoprotein (Pgp) drug efflux system and increases the permeability of a broad spectrum of drugs in the blood-brain barrier (BBB). This study examines the mechanisms by which P85 inhibits Pgp using bovine brain microvessel endothelial cells (BBMEC) as an in vitro model of the BBB. The hypothesis was that simultaneous alterations in intracellular ATP levels and membrane fluidization in BBMEC monolayers by P85 results in inhibition of the drug efflux system. The methods included the use of 1) standard Pgp substrate rhodamine 123 to assay the Pgp efflux system in BBMEC, 2) luciferin/luciferase assay for ATP intracellular levels, and 3) 1,6-diphenyl-1,3,5-hexatriene for membrane microviscosity. Using 3H-labeled P85 and fluorescein-labeled P85 for confocal microscopy, this study suggests that P85 accumulates in the cells and intracellular organelles such as the mitochondria where it can interfere with metabolic processes. Following exposure of BBMEC to P85, the ATP levels were depleted, and microviscosity of the cell membranes was decreased. Furthermore, P85 treatment decreased Pgp ATPase activity in membranes expressing human Pgp. A combination of experiments examining the kinetics, concentration dependence, and directionality of P85 effects on Pgp-mediated efflux in BBMEC monolayers suggests that both energy depletion (decreasing ATP pool available for Pgp) and membrane fluidization (inhibiting Pgp ATPase activity) are critical factors contributing to the activity of the block copolymer in the BBB.

Pluronic P85 enhances the delivery of digoxin to the brain: in vitro and in vivo studies.

Drug delivery across the blood-brain barrier is limited by several mechanisms. One important mechanism is drug efflux, mediated by several transport proteins, including P-glycoprotein. The goal of this work was to examine the effect of a novel drug delivery system, Pluronic block copolymer P85, on P-glycoprotein-mediated efflux from the brain using in vitro and in vivo methods. The hypothesis was that specific Pluronic copolymer systems enhance drug delivery to the central nervous system through the inhibition of P-glycoprotein. The effect of P85 on the cellular accumulation and transport of digoxin, a model P-glycoprotein substrate, was examined in porcine kidney epithelial cells (LLC-PK1) transfected with the human MDR1 gene. The effect of P85 on the directional flux across an in vitro BBB was also characterized. In vivo brain distribution studies were accomplished using wild-type and P-glycoprotein knockout mice. Pluronic increased the cellular accumulation of digoxin 3-fold in LLC-PK1 cells and 5-fold in the LLC-PK1-MDR1-transfected cells. Similar effects were observed for a prototypical P-glycoprotein substrate rhodamine-123. P85 treatment decreased the basolateral-to-apical and increased the apical-to-basolateral digoxin flux across LLC-PK1-MDR1 cell monolayers, and analogous results were observed with the in vitro BBB monolayers. The coadministration of 1% P85 with radiolabeled digoxin in wild-type mice increased the brain penetration of digoxin 3-fold and the digoxin level in the P85-treated wild-type mice was similar to that observed in the P-glycoprotein-deficient animals. These data indicate that Pluronic P85 can enhance the delivery of digoxin to the brain through the inhibition of the P-glycoprotein-mediated efflux mechanism.

Epitope-specific antibody response to HT-1080 fibrosarcoma cells by mimotope immunization.

Mouse monoclonal antibody (mAb) BCD-F9, which recognizes an unknown antigen found on the surface of many tumor cells, was used to screen a phage display library expressing random peptide decamers. The phage that was selected encoded the unique sequence GRRPGGWWMR, representing the peptide capable of binding to the BCD-F9 mAb. The peptide was synthesized and found to specifically inhibit the binding of mAb to HT-1080 fibrosarcoma cells. Alanine mutagenesis of the sequence encoding this peptide indicated that three residues, PXXWW, were critical for its binding to the BCD-F9 mAb. Polyclonal antibodies generated by immunization of rabbits with the synthetic peptide GRRPGGWWMR (anti-mimotope antiserum or AM-F9) bound specifically to HT-1080 cells and inhibited the binding of the BCD-F9 mAb to these cells. Using an experimental animal model in which CD-1 nude mice are inoculated i.v. with HT-1080 cells, develop lung metastasis, and die within 30 days, we have shown that AM-F9 could significantly prolong the life span of these animals. Our results suggest that a peptide mimotope can potentially be used as a novel immunotherapy to induce a beneficial antitumor response.

A combination of poloxamers increases gene expression of plasmid DNA in skeletal muscle.

Intramuscular administration of plasmid DNA is a promising strategy to express therapeutic genes, however, it is limited by a relatively low level of gene expression. We report here that a non-ionic carrier, SP1017, composed of two amphiphilic block copolymers, pluronics L61 and F127, also known as poloxamers, significantly increases intramuscular expression of plasmid DNA. Two reporter genes, luciferase and beta-galactosidase, and one therapeutic gene, erythropoietin, were injected intramuscularly with and without SP1017 into C57Bl/6 and Balb/C mice and Sprague-Dawley rats. SP1017 increased gene expression by about 10-fold and maintained higher gene expression compared with naked DNA. Comparison of SP1017 with polyvinyl pyrrolidone (PVP) showed that SP1017 exhibited a significantly higher efficacy and its optimal dose was 500-fold lower. Experiments with beta-galactosidase using X-gal staining suggested that SP1017 considerably increased plasmid DNA diffusion through the tissue. SP1017 also improved expression of the erythropoietin gene leading to an increase in its systemic level and hematocrits. Previous toxicity studies have suggested that SP1017 has over a 1000-fold safety margin. Poloxamers used in SP1017 are listed in the US Pharmacopeia as inactive excipients and are widely used in a variety of clinical applications. We believe that the described system constitutes a simple and efficient gene transfer method to achieve local or systemic production of therapeutic proteins.

Block and graft copolymers and NanoGel copolymer networks for DNA delivery into cell.

Self-assembling complexes from nucleic acids and synthetic polymers are evaluated for plasmid and oligonucleotide (oligo) delivery. Polycations having linear, branched, dendritic. block- or graft copolymer architectures are used in these studies. All these molecules bind to nucleic acids due to formation of cooperative systems of salt bonds between the cationic groups of the polycation and phosphate groups of the DNA. To improve solubility of the DNA/polycation complexes, cationic block and graft copolymers containing segments from polycations and non-ionic soluble polymers, for example, poly(ethylene oxide) (PEO) were developed. Binding of these copolymers with short DNA chains, such as oligos, results in formation of species containing hydrophobic sites from neutralized DNA polycation complex and hydrophilic sites from PEO. These species spontaneously associate into polyion complex micelles with a hydrophobic core from neutralized polyions and a hydrophilic shell from PEO. Such complexes are very small (10-40 nm) and stable in solution despite complete neutralization of charge. They reveal significant activity with oligos in vitro and in vivo. Binding of cationic copolymers to plasmid DNA forms larger (70-200 nm) complexes. which are practically inactive in cell transfection studies. It is likely that PEO prevents binding of these complexes with the cell membranes ("stealth effect"). However attaching specific ligands to the PEO-corona can produce complexes, which are both stable in solution and bind to target cells. The most efficient complexes were obtained when PEO in the cationic copolymer was replaced with membrane-active PEO-b-poly(propylene oxide)-b-PEO molecules (Pluronic 123). Such complexes exhibited elevated levels of transgene expression in liver following systemic administration in mice. To increase stability of the complexes, NanoGel carriers were developed that represent small hydrogel particles synthesized by cross-linking of PEI with double end activated PEO using an emulsification/solvent evaporation technique. Oligos are immobilized by mixing with NanoGel suspension, which results in the formation of small particles (80 nm). Oligos incorporated in NanoGel are able to reach targets within the cell and suppress gene expression in a sequence-specific fashion. Further. loaded NanoGel particles cross-polarized monolayers of intestinal cells (Caco-2) suggesting potential usefulness of these systems for oral administration of oligos. In conclusion the approaches using polycations for gene delivery for the design of gene transfer complexes that exhibit a very broad range of physicochemical and biological properties, which is essential for design of a new generation of more effective non-viral gene delivery systems.

Evaluation of polyether-polyethyleneimine graft copolymers as gene transfer agents.

Cationic copolymers consisting of polycations linked to non-ionic polymers are evaluated as non-viral gene delivery systems. These copolymers are known to produce soluble complexes with DNA, but only a few studies have characterized the transfection activity of these complexes. This work reports the synthesis and characterization of a series of cationic copolymers obtained by grafting the polyethyleneimine (PEI) with non-ionic polyethers, poly (ethylene oxide) (PEO) or Pluronic 123 (P123). The PEO-PEI conjugates differ in the molecular mass of PEI (2 kDa and 25 kDa) and the degree of modification of PEI with PEO. All of these conjugates form complexes upon mixing with plasmids, which are stable in aqueous dispersion for several days. The sizes of the particles formed in these systems vary from 70 to 200 nm depending on the composition of the complex. However, transfection activity of these systems is much lower than that of PEI (25 kDa) or Superfect as assessed in in vitro transfection experiments utilizing a luciferase reporter expression in Cos-7 cells as a model system. In contrast, conjugate of P123 with PEI (2 kDa) mixed with free P123 (9:1(wt)) forms small and stable complexes with DNA (110 nm) that exhibit high transfection activity in vitro. Furthermore, gene expression is observed in spleen, heart, lungs and liver 24 h after i.v. injection of this complex in mice. Compared to 1,2-bis(oleoyloxy)-(trimethylammonio) propane:cholesterol (DOTAP:Chol) and PEI (25 kDa) transfection systems, the P123-PEI system reveals a more uniform distribution of gene expression between these organs, allowing a significant improvement of gene expression in liver.

Fundamental relationships between the composition of pluronic block copolymers and their hypersensitization effect in MDR cancer cells.

PURPOSE: Previous studies have demonstrated that Pluronic block copolymers hypersensitize multiple drug resistant (MDR) cancer cells, drastically increasing the cytotoxic effects of anthracyclines and other anticancer cytotoxics in these cells. This work evaluates the dose dependent effects of these polymers on (i) doxorubicin (Dox) cytotoxicity and (ii) cellular accumulation of P-glycoprotein probe, rhodamine 123 (R123) in MDR cancer cells. METHODS: Dox cytotoxicity and R123 accumulation studies are performed on monolayers of drug-sensitive (KB, MCF-7, Aux-B1) and MDR (KBv, MCF-7/ADR, CHrC5) cells. RESULTS: Both tests reveal strong effects of Pluronic copolymers observed at concentrations below the critical micelle concentration (CMC) and suggest that these effects are due to the copolymer single chains ("unimers"). Using block copolymers with various lengths of hydrophobic propylene oxide (PO) and hydrophilic ethylene oxide (EO) segments these studies suggest that the potency of Pluronic unimers in MDR cells increases with elevation of the hydrophobicity of their molecule. Optimization of Pluronic composition in R123 accumulation and Dox cytotoxicity studies reveals that Pluronic copolymers with intermediate lengths of PO chains and relatively short EO segments have the highest net efficacy in MDR cells. CONCLUSIONS: The relationship between the structure of Pluronic block copolymers and their biological response modifying effects in MDR cells is useful for determining formulations with maximal efficacy with respect to MDR tumors.

Multidrug-resistance drug-binding peptides generated by using a phage display library.

A phage display library of random decapeptides was used to generate peptide ligands that can bind multidrug-resistance (MDR) drugs mimicking, in this respect, the drug-binding activity of P-glycoprotein. Seven peptide sequences were identified that specifically bound doxorubicin. Five of these sequences expressed the core consensus motif WXXW. The displacement assay showed that the phages expressing these peptides bound MDR type drugs (vinblastine, doxorubicin, verapamil, and genistein) with the same selectivity as P-glycoprotein and did not interact with non-MDR type drugs, such as arabinosylcytosine (Ara-C) and melphalan. One of the selected peptides that showed a highest capacity for the binding (VCDWWGWGIC) was synthesized and displayed competition with the phage for doxorubicin binding. The structure modeling suggested that all the selected sequences contained a hydrophobic envelope in which MDR drugs could be docked with substantial energy minimization. Western blot analysis showed that monospecific antibody obtained against the phage expressing VCDWWGWGIC peptide could specifically recognize P-glycoprotein in the membrane fraction of MDR phenotype MCF-7ADR cells. The MDR drug-binding sequences generated during this work could provide an important tool for design and screening of new chemotherapeutic agents.

Block copolymeric biotransport carriers as versatile vehicles for drug delivery.

This review describes block co-polymer-based systems that are used in drug delivery. The main focus is on amphiphilic block co-polymers, the application of which modifies the pharmacological performance of various classes of drugs and is attracting more and more attention. The two main reasons for this are the high tendency of block co-polymer-based drug formulations to self-assemble and the flexibility of block co-polymer chemistry, which allows precise tailoring of the carrier to virtually any chemical entity. The combination of these and some other features makes it possible to adjust block co-polymer-based drug formulations to achieve the most beneficial balance in their biological interactions (biotransport), with systems that control drug removal from the body and those that are responsible for drug therapeutic activity. The following major aspects are considered: The role of physical properties of formulations in their pharmacological performance. The chemistry and physico-chemistry of block co-polymers and structure-function relationships in these systems. Examples describing the effects of biotransport systems on drug transport and activity in cells and some results on their in vivo applications with various drugs.

Anthracycline antibiotics non-covalently incorporated into the block copolymer micelles: in vivo evaluation of anti-cancer activity.

The chemosensitising effects of poly(ethylene oxide)-poly(propylene oxide)-poly-(ethylene oxide) (PEO-PPO-PEO) block copolymers (Pluronic) in multidrug-resistant cancer cells has been described recently (Alakhov VY, Moskaleva EY, Batrakova EV, Kabanov AV 1996, Biocon. Chem., 7, 209). This paper presents initial studies on in vivo evaluation of Pluronic copolymers in the treatment of cancer. The anti-tumour activity of epirubicin (EPI) and doxorubicin (DOX), solubilised in micelles of Pluronic L61, P85 and F108, was investigated using murine leukaemia P388 and daunorubicin-sensitive Sp2/0 and -resistant Sp2/0(DNR) myeloma cells grown subcutaneously (s.c.). The study revealed that the lifespan of the animals and inhibition of tumour growth were considerably increased in mice treated with drug/copolymer compositions compared with animals treated with the free drugs. The anti-tumour activity of the drug/copolymer compositions depends on the concentration of the copolymer and its hydrophobicity, as determined by the ratio of the lengths of hydrophilic PEO and hydrophobic PPO segments. The data suggest that higher activity is associated with more hydrophobic copolymers. In particular, a significant increase in lifespan (T/C> 150%) and tumour growth inhibition (> 90%) was observed in animals with Sp2/0 tumours with EPI/P85 and DOX/L61 compositions. The effective doses of these compositions caused inhibition of Sp2/0 tumour growth and complete disappearance of tumour in 33-50% of animals. Future studies will focus on the evaluation of the activity of Pluronic-based compositions against human drug-resistant tumours.

Hypersensitizing effect of pluronic L61 on cytotoxic activity, transport, and subcellular distribution of doxorubicin in multiple drug-resistant cells.

The present study demonstrated that poly(oxypropylene) and poly(oxyethylene) block copolymer pluronic L61 (L61)-hypersensitized multidrug-resistant CHRC5 Chinese hamster ovary cells and MCF-7/ADR human breast carcinoma cells to the cytotoxic action of doxorubicin (Dox). CHRC5 and MCF-7/ADR cells manifested 290- and 700-fold increases, respectively, in their sensitivity to Dox/L61 formulation compared with free Dox. Their sensitive counterparts Aux-B1 and MCF-7 displayed only marginal or no increase at all in their response to Dox/L61. The study of the drug transport performed by flow cytometry showed that L61 enhanced the drug uptake and reduced the P-glycoprotein-mediated drug efflux. Visualization of Dox subcellular distribution in CHRC5 cells by fluorescent microscopy revealed that Dox was sequestered in cytoplasmic vesicles, whereas incubation of the cells with Dox/L61 altered the drug compartmentalization by releasing the drug from these vesicles and shifting it to the nucleus. These findings suggested that the hypersensitive response of multidrug-resistant cells to the action of Dox/L61 was caused by an increase in the drug accumulation and changes in its subcellular distribution.