The efflux of anthracyclines in multidrug-resistant cell lines

Enhancing Cellular Uptake and Doxorubicin Delivery of Mesoporous Silica Nanoparticles via Surface Functionalization: Effects of Serum

In this study, we demonstrate how functional groups on the surface of mesoporous silica nanoparticles (MSNPs) can influence the encapsulation and release of the anticancer drug doxorubicin, as well as cancer cell response in the absence or presence of serum proteins. To this end, we synthesized four differently functionalized MSNPs with amine, sulfonate, polyethylene glycol, or polyethylene imine functional surface groups, as well as one type of antibody-conjugated MSNP for specific cellular targeting, and we characterized these MSNPs regarding their physicochemical properties, colloidal stability in physiological media, and uptake and release of doxorubicin in vitro. Then, the MSNPs were investigated for their cytotoxic potential on cancer cells. Cationic MSNPs could not be loaded with doxorubicin and did therefore not show any cytotoxic and antiproliferative potential on osteosarcoma cells, although they were efficiently taken up into the cells in the presence or absence of serum. In contrast, substantial amounts of doxorubicin were loaded into negatively charged and unfunctionalized MSNPs. Especially, sulfonate-functionalized doxorubicin-loaded MSNPs were efficiently taken up into the cells in the presence of serum and showed an accelerated toxic and antiproliferative potential compared to unfunctionalized MSNPs, antibody-conjugated MSNPs, and even free doxorubicin. These findings stress the high importance of the surface charge as well as of the protein corona for designing and applying nanoparticles for targeted drug delivery.

Utilizing the protein corona around silica nanoparticles for dual drug loading and release

A protein corona forms spontaneously around silica nanoparticles (SNPs) in serum-containing media. To test whether this protein corona can be utilized for the loading and release of anticancer drugs we incorporated the hydrophilic doxorubicin, the hydrophobic meloxicam as well as their combination in the corona around SNPs. The application of corona-covered SNPs to osteosarcoma cells revealed that drug-free particles did not affect the cell viability. In contrast, SNPs carrying a protein corona with doxorubicin or meloxicam lowered the cell proliferation in a concentration-dependent manner. In addition, these particles had an even greater antiproliferative potential than the respective concentrations of free drugs. The best antiproliferative effects were observed for SNPs containing both doxorubicin and meloxicam in their corona. Co-localization studies revealed the presence of doxorubicin fluorescence in the nucleus and lysosomes of cells exposed to doxorubicin-containing coated SNPs, suggesting that endocytotic uptake of the SNPs facilitates the cellular accumulation of the drug. Our data demonstrate that the protein corona, which spontaneously forms around nanoparticles, can be efficiently exploited for loading the particles with multiple drugs for therapeutic purposes. As drugs are efficiently released from such particles they may have a great potential for nanomedical applications.

Autophagic flux inhibition and lysosomogenesis ensuing cellular capture and retention of the cationic drug quinacrine in murine models

The proton pump vacuolar (V)-ATPase is the driving force that mediates the concentration of cationic drugs (weak bases) in the late endosome-lysosome continuum; secondary cell reactions include the protracted transformation of enlarged vacuoles into autophagosomes. We used the inherently fluorescent tertiary amine quinacrine in murine models to further assess the accumulation and signaling associated with cation trapping. Primary fibroblasts concentrate quinacrine ∼5,000-fold from their culture medium (K_M 9.8 µM; transport studies). The drug is present in perinuclear granules that are mostly positive for Rab7 and LAMP1 (microscopy). Both drug uptake and retention are extensively inhibited by treatments with the V-ATPase inhibitor bafilomycin A1. The H⁺ ionophore monensin also prevented quinacrine concentration by fibroblasts. However, inhibition of plasma membrane transporters or of the autophagic process with spautin-1 did not alter quinacrine transport parameters. Ancillary experiments did not support that low micromolar concentrations of quinacrine are substrates for organic cation transporters-1 to -3 or P-glycoprotein. The secondary autophagy induced by quinacrine in cells may derive from the accumulation of incompetent autophagolysosomes, as judged from the accumulation of p62/SQSTM1 and LC3 II (immunoblots). Accordingly, protracted lysosomogenesis is evidenced by increased expression of LAMP1 and LAMP2 in quinacrine-treated fibroblasts (48 h, immunoblots), a response that follows the nuclear translocation of the lysosomal genesis transcription factor TFEB and upregulation of LAMP1 and −2 mRNAs (24 h). Quinacrine administration to live mice evidenced variable distribution to various organs and heterogeneous accumulation within the lung (stereo-microscopy, extraction). Dose-dependent in vivo autophagic and lysosomal accumulation was observed in the lung (immunoblots). No evidence has been found for transport or extrusion mechanisms modulating the cellular uptake of micromolar quinacrine at the plasma membrane level. As shown in vitro and in vivo, V-ATPase-mediated cation sequestration is associated, above a certain threshold, to autophagic flux inhibition and feed-back lysosomogenesis.

A fluorescence-based assay for monitoring clinical drug resistance

Background and aims

Multidrug resistance (MDR) limits effectiveness in treating malignancy by modifying internalisation and/or externalisation of drugs through cancer cell membranes. In this study we describe an assay to monitor patients’ responses to chemotherapy.

Methods

The assay is based on the fluorescent properties of doxorubicin alone as well as in combination with methotrexate, vinblastine, doxorubicin and cisplatin (MVAC). The slide-based cell imaging technique was first optimised using a panel of breast and urothelial cancer cell lines and then extended to fine needle breast aspiration biopsy and urine cytology.

Results

The drug fluorescence behaviour observed on smears of clinical specimens is identical to that obtained using fixed cultured cells. The fluorescence of sensitive cells to chemotherapy is mainly localised in the nucleus, whereas resistant cells show a weak fluorescence signal localised in the cytoplasm. The difference in terms of fluorescence intensity is also highlighted through fluorescence spectra.

Conclusions

The results suggest that the assay provides clinically valuable information in predicting responses to doxorubicin and/or MVAC therapy. Originally set up on a confocal microscope, the assay was also effective using a standard epifluorescence microscope; as such it is technically simple, reliable and inexpensive.

The intracellular drug delivery and anti tumor activity of doxorubicin loaded poly(gamma-benzyl L-glutamate)-b-hyaluronan polymersomes

We have investigated the intracellular delivery of doxorubicin (DOX) loaded poly(gamma-benzyl L-glutamate)-block-hyaluronan (PBLG-b-HYA) based polymersomes (PolyDOX) in high (MCF-7) and low (U87) CD44 expressing cancer cell models. DOX was successfully loaded into polymersomes using nanoprecipitation method and in vitro drug release pattern were achieved at pH 5.5 and 7.4 up to 10 days. Block copolymer vesicles without loaded DOX were non cytotoxic in both cells at concentration 150-650 microg/mL. Flow cytometry data suggested successful uptake of PolyDOX in cells and high accumulation was found in MCF-7 than U87 cells. Microscopy imagings revealed that in MCF-7 cells PolyDOX was more in cytoplasm and free DOX in nuclei, whereas in U87 cells free DOX was also found in the cytoplasm. Cytotoxicity of the drug was concentration and exposure time dependent. In addition, PolyDOX significantly enhanced reactive oxygen species (ROS) level in both cells. PolyDOX also suppressed growth of breast tumor on female Sprague-Dawley (SD) rats as compared to phosphate buffer saline pH 7.4 (PBS) control group. In addition reduced level of serum enzymes (LDH and CPK) by PolyDOX formulation indicated less cardiotoxicity of DOX after loading in polymersomes. Results suggest that intracellular delivery of PolyDOX was depended on the CD44 expression level in cells due to presence of hyaluronic acid on the surface of polymersomes, and could be used as a self-targeting drug delivery cargo in over-expressed CD44 glycoprotein cells of breast cancer.

Multidrug resistance in a urothelial cancer cell line after 1-hour mitomycin C exposure

PURPOSE

A factor pertinent to the design of cancer chemotherapy is multidrug resistance. Research in this area conventionally involves in vitro models using resistant cell lines generated by continuous low dose drug exposure for many months, unlike the exposure experienced by residual superficial bladder cancer cells during chemotherapy adjuvant to resection. Recently we noted a measure of multidrug resistance induced by 3 short exposures to mitomycin C during 10 weeks. We currently report detectable functional resistance after a single 1-hour insult.

MATERIALS AND METHODS

RT112 bladder cancer cells (Catalog No. ACC 418, Deutsche Sammlung von Mikroorganismen und Zellkulturen, Braunschweig, Germany) were exposed to a range of mitomycin C concentrations for 1 hour. Cells regrew in 3 of 24 cultures at 15.6, 3.91 and 0.98 mg/ml exposure. These cells were subjected to 3 functional tests of cross resistance to epirubicin, including MTT cytotoxicity assay, quantitative accumulation by flow cytometry and nuclear uptake or exclusion by live cell fluorescence microscopy.

RESULTS

MTT assay and flow cytometry revealed clear indications of resistance. Intracellular distribution, in which nuclear exclusion indicates resistance, was distinctively resistant in 1 subline and another 2 were equivocal.

CONCLUSIONS

Results indicate that some multidrug resistance potential exists even in a cloned cell line that is capable of surviving 1 short drug exposure and expanding after that insult. The exposures used are consistent with those probably experienced by many superficial transitional cell carcinoma cells during an intravesical chemotherapy application. The result gives added weight to considering multidrug resistance induction in dose scheduling or drug combinations for topical chemotherapy.

Interleukin-13 receptor-targeted nanovesicles are a potential therapy for glioblastoma multiforme

The difficulties associated with treatment of malignant brain tumors are well documented. For example, local infiltration of high-grade astrocytomas prevents the complete resection of all malignant cells. It is, therefore, critical to develop delivery systems for chemotherapeutic agents that ablate individual cancer cells without causing diffuse damage to surrounding brain tissue. Here, we describe sterically stable human interleukin-13 (IL-13)-conjugated liposomes, which efficiently bind to the brain cancer cells that overexpress the IL-13 receptor alpha2 protein. The conjugated liposomes bind to glioblastoma multiforme tissue specimens but not to normal cortex. Conjugating the liposomes with human IL-13 allows for specific binding to glioma cells and uptake of the liposomes via endocytosis. Delivering doxorubicin to glioma cells by IL-13-conjugated liposomes results in enhanced cytotoxicity and increased accumulation and retention of drug in the glioma cells compared with delivery of free drug. The therapeutic potential and targeting efficacy of the IL-13-conjugated liposomes carrying doxorubicin was tested in vivo using a s.c. glioma tumor mouse model. Animals receiving i.p. injections of IL-13-conjugated liposomes carrying doxorubicin for 7 weeks had a mean tumor volume of 37 mm3 compared with a mean volume of 192 mm3 in animals injected with nontargeted liposomes. These results strongly suggest that IL-13-conjugated liposomes carrying cytotoxic agents are a feasible approach for creating a nanovesicle drug delivery system for brain tumor therapy.

Aclarubicin-loaded cationic albumin-conjugated pegylated nanoparticle for glioma chemotherapy in rats

Traditional glioma chemotherapy with those second-line drugs such as anthracyclines usually failed because they are inaccessible to blood-brain barrier (BBB) in tumor. In our study, we incorporated aclarubicin (ACL) into cationic albumin-conjugated pegylated nanoparticle (CBSA-NP-ACL) to determine its therapeutic potential of rats with intracranially implanted C6 glioma cells. When labeled with fluorescent probe, 6-coumarin, CBSA-NP was shown to accumulate much more in tumor mass than nanoparticle without conjugated CBSA (NP) 1 hr post intravenous injection, as well as better retention after 24 hr. Tumor drug concentration of CBSA-NP-ACL displayed 2.6- and 3.3-fold higher than that of NP-ACL and ACL solution 1 hr post injection, while 2.7 and 6.6-fold higher after 24 hr, respectively. Moreover, using tumor microdialysis sampling, AUC(0-24 hr) of free drug amount in tumor interstitium delivered by CBSA-NP-ACL was about 2.0- and 2.7-fold higher than that of NP-ACL and ACL solutions, respectively. When the tumor rat model was subjected to 4 cycles of 2 mg/kg of ACL in different formulations, a significant increase of median survival time was found in the group of CBSA-NP-ACL compared with that of saline control animals, animals treated with NP-ACL and ACL solution. By terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeling, CBSA-NP-ACL can extensively make the tumor cell apoptosis. Histochemical evaluation by periodic acid Shiff staining and biochemical analysis depicted that the incorporation of ACL into CBSA-NP reduced its toxicity to liver, kidney and heart. Besides, CBSA-NP-ACL was not shown to open tight junction evaluated by BBB coculture. It was concluded that CBSA-NP-ACL could have a therapeutic potential for treatment of glioma.

The nuclear membrane in multidrug resistance: microinjection of epirubicin into bladder cancer cell lines

OBJECTIVE

To assess whether microinjecting epirubicin into cells showing multidrug resistance (MDR, common to many cancers, including bladder cancer, with resistance to, e.g. anthracyclines and mitomycin C) spares the nucleus, as when these drugs accumulate, distribution in MDR cells characteristically spares the nucleus, suggesting that the nuclear membrane is responsible for excluding cytotoxic drugs from MDR nuclei.

MATERIALS AND METHODS

Nuclear exclusion of drugs is an important feature of resistance in MDR cells, as many MDR-susceptible drugs have cytotoxic actions within the nucleus. Drug accumulation in 'classical' P-glycoprotein-mediated MDR cells is greatly reduced by efflux. Microinjection of epirubicin into the cytoplasm of MDR cells bypasses the P-glycoprotein efflux pump on the plasma membrane. Nuclear sparing would directly implicate the nuclear membrane in this phenomenon. Because of their fluorescence properties, which allow study by confocal microscopy and flow cytometry, anthracyclines have also been used extensively to investigate MDR. Thus sensitive (MGH-U1 and RT112) and MDR (MGH-U1R and MGH-U1-MMC) bladder cancer cell lines were used. Adherent cells from each cell line were individually microinjected with epirubicin (0.5 mg/mL) and a 77 kDa fluorescein isothiocyanate (FITC)-dextran (0.5 mg/mL). The pattern of nuclear epirubicin uptake in injected cells was then evaluated by confocal microscopy. The 77 kDa FITC-dextran allowed easier identification of injected cells and was also excluded from their nuclei.

RESULTS

Sensitive bladder cancer cell lines all showed a nuclear accumulation pattern of epirubicin, consistent with their normal uptake after exposure to epirubicin. The MDR cell lines showed the characteristic nuclear-sparing pattern of epirubicin uptake, similar to the normal uptake pattern after epirubicin exposure. The 77 kDa FITC-dextran showed clearly which cells had been microinjected, and was excluded from the nuclei of all injected cells. Cell viability was confirmed by acridine-orange staining after initial visualization of injected cells.

CONCLUSION

The nuclear membrane is responsible for the nuclear exclusion of epirubicin in MDR cells. Further work is necessary to determine the mechanisms involved.

Partial circumvention of P-glycoprotein-mediated multidrug resistance by doxorubicin-14-O-hemiadipate

Previously, we have reported partial circumvention of P-glycoprotein (Pgp)-associated resistance to doxorubicin (Dox) in MCF7/R human breast carcinoma and P388/R murine leukemia cell lines by doxorubicin-14-O-hemiadipate (H-Dox) [Povarov L.S. et al. (1995) Russian J. Bioorganic Chemistry 21: 797-803]. We felt that these changes were due to alterations in the cellular pharmacokinetics of the analog in multidrug (MDR) resistant cells, as compared to that of Dox. To address this hypothesis, we performed comparative studies of the accumulation, retention and intracellular localization of H-Dox and Dox in Dox-sensitive murine leukemia cell line P388/S and its Dox-selected. Pgp-positive drug resistant P388/R subline. These studies were performed in the presence or absence of cyclosporin A (CsA), a competitive inhibitor of Pgp. Flow cytometric analysis revealed significant differences in Dox and H-Dox accumulation in P388/R cells when compared to P388/S cells. In P388/R versus P388/S cells, there was a 38-fold decrease in Dox accumulation, but only a 5-fold decrease in H-Dox accumulation, indicating over a 7-fold increase in H-Dox buildup in resistant cells. CsA did not affect uptake or retention of either drug by sensitive cells. However, coincubation with CsA resulted in a 54-fold increase in Dox accumulation and only a 5-fold increase in H-Dox uptake in P388/R cells, restoring anthracycline levels in P388/R to 100% of that found in P388/S cells. Once internalized by the resistant cells, H-Dox was retained better than Dox regardless of presence or absence of CsA. Confocal microscopic analysis revealed the presence of H-Dox but no Dox in cellular nuclei of P388/R cells. Thus, increased activity of H-Dox toward P388/R cells was correlated with its enhanced ability to enter and be retained in these cells, and also with redistribution of H-Dox into the nuclei of the resistant cells as compared to Dox. Overall, our findings support our initial hypothesis and provide evidence that H-Dox, a 14-O-hemiadipate of doxorubicin, is affected by Pgp-mediated MDR to a lesser extent than parental Dox due to changes iin the cellular pharmacokinetics of the analog.

Anthracyclines in haematology: preclinical studies, toxicity and delivery systems

The anthracyclines are widely used in the treatment of haematological and non-haematological malignancy and there is now more than 30 years' clinical experience with these agents but despite this, their mechanism of action is incompletely understood. The anthracyclines have been shown to intercalate with DNA and indirectly inhibit the activity of the enzyme topoisomerase II, resulting in DNA strand breaks. More recently, workers have focused on induction of apoptosis and have shown that daunorubicin stimulates production of the apoptotic mediator, ceramide and that the activity of doxorubicin can be blocked by inhibitors of CD95 (fas). One of the major problems with anthracycline therapy is the development of resistance which may be mediated by p-glycoprotein or by other mechanisms. Much recent research has concentrated on methods to modulate the drug-resistant phenotype and these include development of new analogues and use of specific reversal agents. The toxicity profile of the anthracyclines includes bone marrow suppression, severe local reaction following extravasation, radiation recall, alopecia, gastrointestinal and hepatic effects, development of secondary malignancies and significant cardiac toxicity. The risk factors for the development of anthracycline-related cardiac toxicity are well documented and several methods have been exploited in attempts at prevention. Finally, a number of drug delivery systems have been developed in order to improve therapeutic response and reduce toxicity to normal tissues, including the use of liposomal preparations.

Biophysical aspects of P-glycoprotein-mediated multidrug resistance

In the 45 years since Burchenal's observation of chemotherapeutic drug resistance in tumor cells, many investigators have studied the molecular basis of tumor drug resistance and the phenomenon of tumor multidrug resistance (tumor MDR). Examples of MDR in microorganisms have also become topics of intensive study (e.g., Plasmodium falciparum MDR and various types of bacterial MDR) and these emerging fields have, in some cases, borrowed language, techniques, and theories from the tumor MDR field. Serendipitously, the cloning of MDR genes overexpressed in MDR tumor cells has led to elucidation of a large family of membrane proteins [the ATP-binding cassette (ABC) proteins], an important subset of which confer drug resistance in many different cells and microorganisms. In trying to decipher how ABC proteins confer various forms of drug resistance, studies on the structure and function of both murine and human MDR1 protein (also called P-glycoprotein or P-gp) have often led the way. Although various theories of P-gp function have become popular, there is still no precise molecular-level description for how P-gp overexpression lowers intracellular accumulation of chemotherapeutic drugs. In recent years, controversy has developed over whether the protein protects cells by translocating drugs directly (as some type of drug pump) or indirectly (through modulating biophysical parameters of the cell). In this ongoing debate over P-gp function, detailed consideration of biophysical issues is critical but has often been neglected in considering cell biological and pharmacological issues. In particular, P-gp overexpression also changes plasma membrane electrical potential (delta psi zero) and intracellular pH (pHi), and these changes will greatly affect the cellular flux of a large number of compounds to which P-gp overexpression confers resistance. In this chapter, we highlight these biophysical issues and describe how delta psi zero and pHi may in fact be responsible for many MDR-related phenomena that have often been hypothesized to be due to direct drug translocation (e.g., drug pumping) by P-gp.

Human hepatoma cells rich in P-glycoprotein are sensitive to aclarubicin and resistant to three other anthracyclines

Drug resistance is a major obstacle to successful chemotherapy of primary liver cancer, which is associated with high expression of the multidrug resistance (MDR) gene product P-glycoprotein (Pgp), a multidrug efflux transporter. The most effective single agents in treatment of primary liver carcinoma belong to the anthracycline family, yet several anthracyclines are known to be substrates for Pgp. In the present study, we compared four anthracyclines with respect to cell growth inhibition, intracellular accumulation and cellular efflux using the HB8065/R human hepatoma cell line which is rich in Pgp, and the Pgp-poor parental line HB8065/S. The anthracyclines were also administered in conjunction with the Pgp-modifying agents verapamil and SDZ PSC 833 to assess modulation of resistance. The HB8065/R cells were sensitive to aclarubicin (ACL) and highly resistant to epirubicin (EPI), doxorubicin (DOX) and daunorubicin (DNR). SDZ PSC 833 enhanced accumulation, decreased efflux and increased cytotoxicity of EPI, DOX and DNR in the HB8065/R cells, but none of these effects was seen with ACL. In conclusion, ACL is apparently not transported by Pgp and retains its activity in a multidrug-resistant human hepatoma cell line; such properties can be exploited for clinical purposes.

Cellular uptake, cytotoxicity, and transport kinetics of anthracyclines in human sensitive and multidrug-resistant K562 cells

Multidrug resistance in tumor cells is often associated with the presence of an approximately 170 kDa plasma membrane glycoprotein (Pgp) that acts as a drug-efflux pump and decreases intracellular antitumor drug concentration. We measured the uptake of seven anthracyclines (daunorubicin, doxorubicin, 4'-epi-doxorubicin, 4'-deoxy-doxorubicin, iododoxorubicin, 3'-(3-methoxymorpholino)-doxorubicin (FCE23762) and 4-demethoxy-daunorubicin) into K562 cells sensitive and resistant (K562/DNR) to daunorubicin. The K562/DNR subline expresses Pgp at the membrane surface, whereas its sensitive counterpart does not. Laser flow cytometry was used to quantitate intracellular anthracycline content. Uptake of daunorubicin, doxorubicin, 4'-epi-doxorubicin, and 4'-deoxy-doxorubicin was minimal in the K562/DNR subline as compared to their uptake in sensitive cells. On the contrary, iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin accumulate to nearly the same extent into sensitive and resistant K562 cells. Growth inhibition data indicated that the resistance factor for iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin is markedly decreased as compared to the other drugs. Fluorescence measurements were carried out to determine the kinetic parameters associated with the influx and efflux of the drugs into and out of K562 cells. Kinetic data indicated that iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin are not actively rejected from resistant cells, suggesting that they are poor substrates for Pgp-mediated transport. This observation is related to their ability to overcome the multidrug-resistant phenotype of K562/DNR cells in vitro.

Modulation of P-glycoprotein mediated drug accumulation in multidrug resistant CCRF VCR-1000 cells by chemosensitisers

P-glycoprotein (PGP) mediated transport of cytostatic drugs out of resistant cancer cells is a major cause of experimental and probably also of clinical multidrug resistance, which often leads to treatment failure during chemotherapy. The broad substrate specificity of PGP strongly restricts effective chemotherapy and diminishes the patients' prognosis. Inhibition of PGP's pumping function by chemosensitisers is one way to restore cellular responsiveness to otherwise ineffective cytostatics. Clinical trials with several chemosensitisers are under way. To date, it is not clear whether a certain chemosensitiser potentiates the action of different cytostatic drugs, transported by PGP equally well, or whether the chemosensitising potency is dependent on the cytostatic drugs used. Therefore, we compared the effects of five potent chemosensitisers on cellular accumulation using [3H]daunomycin, [3H]vincristine and rhodamine-123 as substrates for PGP. The acridonecarboxamide derivative GF 120918 was the most potent compound and a half-maximal effect was seen at concentrations ranging from 5 nM for rhodamine-123 accumulation to 14 and 19 nM for [3H]vincristine or [3H]daunomycin accumulation, respectively. The new chemosensitiser B9203-016 was slightly less effective than GF 120918 in all three test systems. Dexniguldipine was of intermediate potency with half-maximal effects at concentrations between 62 and 194 nM. The cyclic undecapeptide SDZ PSC 833 showed somewhat lower potency ranging from 151 to 331 nM. Cyclosporin A was less potent than SDZ PSC 833. Furthermore, enhancement of drug accumulation produced by each chemosensitiser was similar, regardless of which PGP substrate was measured, that is, the rank order of potency to increase accumulation was the same in each of the assays used. Our data point to similar, if not identical, mechanisms of drug transport by PGP and inhibition of drug transport by chemosensitisers at least for the substrates rhodamine-123, vincristine and daunomycin.

Verapamil competes with doxorubicin for binding to anionic phospholipids resulting in increased internal concentrations and rates of passive transport of doxorubicin

It is well documented that the Ca2+ channel antagonist verapamil can reverse multidrug resistance in cancer cells by decreasing P-glycoprotein mediated drug efflux. However, less information is available about effects of verapamil on drug-phospholipid interactions and on passive diffusion of drugs across the membrane, which both may play an important role in resensitizing cells to anti-cancer drugs. Therefore we studied the binding of verapamil to model membranes (large unilamellar vesicles) composed of various phospholipids and biological membranes. An increase of the amount of anionic phospholipids resulted in an enhanced binding of verapamil. Competition between verapamil and the anti-cancer drug and P-glycoprotein substrate doxorubicin for binding to anionic phospholipids was observed in model membranes composed of synthetic lipids, or composed of native Escherichia coli phospholipid mixtures, and in cytoplasmic membrane vesicles of this organism. Furthermore, verapamil specifically increased the rate of passive diffusion of doxorubicin across model membranes containing anionic phospholipids. It can be concluded that besides the decrease of P-glycoprotein mediated efflux at least two other effects may account for an increase of the internal (free and DNA-bound) doxorubicin concentration in the presence of verapamil; (i) a decrease of binding to anionic phospholipids in plasma-and intracellular membranes and (ii) an increase of the rate of passive import of doxorubicin across the plasma membrane.