MDR1 gene overexpression confers resistance to imatinib mesylate in leukemia cell line models

Inappropriate expression of the multidrug resistance (MDR1) gene encoding the P-glycoprotein (Pgp) has been frequently implicated in resistance to different chemotherapeutic drugs. We have previously generated chronic myeloid leukemia (CML) cell lines resistant to the tyrosine kinase inhibitor imatinib mesylate (STI571), and one line (LAMA84-r) showed overexpression not only of the Bcr-Abl protein but also of Pgp. In the present study, we investigated this phenomenon in other cell lines overexpressing exclusively Pgp. Thus, cells from the K562/DOX line, described as resistant to doxorubicin due to MDR1 gene overexpression, grew continuously in the presence of 1 microM imatinib, but died in 4 to 5 days if the Pgp pump modulators verapamil or PSC833 were added to the imatinib-treated culture. Analysis of cell proliferation by the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay confirmed the differential sensitivity of K562/DOX to imatinib, which was also reversed by verapamil or PSC833. Flow cytometric analysis of the total phosphotyrosine content by intracytoplasmic staining after a 2-hour incubation with escalating doses of imatinib showed that the inhibitory concentrations of 50% (IC(50)) for inhibition of cellular protein tyrosine phosphorylation were 15, 10, and 5 microM for K562/DOX, K562/DOX plus verapamil, and K562, respectively. Retroviral-mediated transfection of the BCR-ABL(+) AR230 cell line with the MDR1 gene decreased its sensitivity to imatinib, an effect that was also reversed by verapamil. The possible role of MDR overexpression in clinical resistance to imatinib remains to be defined. We therefore confirm that imatinib should be added to the extensive list of drugs that can be affected by the MDR phenomenon.

Resistance to spontaneous apoptosis in acute myeloid leukaemia blasts is associated with p-glycoprotein expression and function, but not with the presence of FLT3 internal tandem duplications

The ability of acute myeloid leukaemia (AML) blasts to survive in culture has been associated with poor patient response to chemotherapy. Other biological factors predicting an adverse outcome include p-glycoprotein (pgp) expression, which is associated with a reduced remission rate, and the presence of fms-like tyrosine kinase 3 gene (FLT3) internal tandem duplications (ITDs), predictive of a high rate of leukaemic relapse. Our previous work has indicated a drug efflux-independent role for pgp in apoptosis resistance. We measured spontaneous in vitro apoptosis in 58 primary AML samples to establish its relationship with functional and phenotypic pgp and with FLT3 ITDs. Cells were incubated for 48 h in a suspension culture, and the remaining viable cells were counted by flow cytometry. Median survival was 38% of baseline values. Resistance to spontaneous apoptosis was strongly associated with pgp (MRK-16 antibody) expression (P = 0.001) and with pgp functional activity (P < 0.001). FLT3 ITDs, found in 20 cases, were inversely associated with functional pgp activity: thus, the median pgp modulation ratio was 2.0 in FLT3 wild-type cases and 1.38 in ITD cases (P = 0.018). Also, the presence of FLT3 ITDs was not associated with in vitro apoptosis resistance. In conclusion, we have found that the presence of FLT3 ITDs is not related to AML blast survival in vitro, and is inversely associated with pgp activity, whereas pgp expression and activity are associated with resistance to spontaneous apoptosis. These results may help to explain the differing adverse effects of pgp (on remission induction) and FLT3 ITDs (on relapse) in AML.

Role of antioxidants in the O-hydroxyethyl-D-(Ser)8-cyclosporine A (SDZ IMM125)-induced apoptosis in rat hepatocytes

The mechanisms underlying the apoptotic activity of the immunosuppressive drug cyclosporine A and its O-hydroxyethyl-D-(Ser)(8)-derivative SDZ IMM125 in rat hepatocytes are not yet fully understood. It was the purpose of the present study to investigate the role of anti- and pro-oxidants and of caspase-3 and intracellular Ca(2+) in SDZ IMM125-induced apoptosis in rat hepatocytes. SDZ IMM125 induced an increase in chromatin condensation and fragmentation, and the activation of caspase-3. Supplementing the cell cultures with the antioxidants, D,L-alpha-tocopherol-polyethylene-glycol-1000-succinate, ascorbic acid, and the reducing agent, dithiothreitol, significantly inhibited the SDZ IMM125-mediated increase in chromatin condensation and fragmentation, and caspase-3 activity. D,L-alpha-tocopherol-polyethylene-glycol-1000-succinate and dithiothreitol caused significant inhibition on SDZ IMM125-mediated cellular Ca(2+) uptake. The glutathione synthetase inhibitor, buthionine sulfoximine, increased SDZ IMM125-mediated caspase-3 action in parallel to chromatin condensation and fragmentation as well as Ca(2+) influx. Supplementation the culture medium with the intracellular Ca(2+) chelator bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid as well as omission of calcium in the medium reduced SDZ IMM125-induced apoptosis whereas the calcium supplementation of the culture medium elevated SDZ IMM125-induced apoptosis. Calcium antagonists inhibited SDZ IMM125-induced caspase-3 activation. Our data indicate that SDZ IMM125-mediated apoptosis in rat hepatocytes can be inhibited by antioxidants, and that the intracellular redox-state can act as a modulator of cytotoxicity and apoptosis. Further, the results suggest that SDZ IMM125-induced uptake of extracellular calcium is also a redox-sensitive process and that the increased intracellular calcium might directly cause apoptosis by increasing the caspase-3 activity as a central event in the cyclosporine-induced apoptotic mechanism.

Clinical development of P glycoprotein modulators in oncology

The last two decades have witnessed dramatic advances into the mechanisms of drug resistance in cancer. The identification of P glycoprotein (Pgp) as a specific mechanism led to the initial hope and expectation that it would be possible to modulate this and increase sensitivity to drug therapy. Clinical trials using first- and second-generation Pgp modulators did establish proof of principle that in some settings, clinical drug resistance could be overcome with the addition of a Pgp modulator-for example, clinical resistance to paclitaxel, a Pgp substrate, in women with ovarian cancer was shown to be overcome in approximately 20% with the addition of PSC 833, a highly effective Pgp modulator. However, evolutionary and adaptive redundancy in resistance mechanisms have tempered clinical results, even with very effective second- and third-generation modulators. The lessons from oncology establish sound methodology for the evaluation of Pgp modulators for safety, tolerability and efficacy in Phase I, II and III clinical trials. This review will focus on some of the early-phase clinical trials with earlier and newer Pgp modulators, either as single agents or in combination with chemotherapy.

Reversal of multidrug resistance: lessons from clinical oncology

Modulation of P glycoprotein (Pgp) in clinical oncology has had limited success. Contributing factors have included the limitation in our understanding of the tumours in which Pgp overexpression is mechanistically important in clinical drug resistance; the failure to prove that concentrations of modulators achieved in patients were sufficient to inhibit Pgp; and the inability to conclusively prove that Pgp modulation was occurring in tumours in patients. New approaches are needed to determine the clinical settings in which Pgp overexpression plays a major role in resistance. (Clinical trials with third generation modulators are ongoing, including trials with the compounds LY335979, R101933 and XR9576. Using the Pgp substrate Tc-99m Sestamibi as an imaging agent, increased uptake has been seen in normal liver and kidney after administration of PSC 833, VX710 and XR9576. These studies confirm that the concentrations of modulator achieved in patients are able to increase uptake of a Pgp substrate. Furthermore, CD56+ cells obtained from patients treated with PSC 833 demonstrate enhanced rhodamine retention in an ex vivo assay after administration of the antagonist. Finally, a subset of patients treated with Pgp antagonists show enhanced Sestamibi retention in imaged tumours. These results suggest that Pgp modulators can increase drug accumulation in Pgp-expressing tumours and normal tissues in patients. Using third generation Pgp antagonists and properly designed clinical trials, it should be possible to determine the contribution of modulators to the reversal of clinical drug resistance.

The cyclosporin PSC 833 increases survival and delays engraftment of human multidrug-resistant leukemia cells in xenotransplanted NOD-SCID mice

Circumvention of chemoresistance in cancer may involve several modulator drugs with high affinity for the multidrug transporter P-glycoprotein (Pgp), which is expressed in a number of multi-resistant malignancies. Pgp acts as a membrane efflux pump with broad substrate specificity including antineoplastic drugs and endogenous substances such as certain cytokines and sphingolipids. Therefore, the consequence of Pgp blockade could be far more complex than intracellular drug retention. In the present study exposure of the Pgp inhibitor, PSC 833 (1200 ng/ml), to Pgp expressing KG1a/200 human leukemia cells provoked cell cycle arrest and apoptosis in vitro. This finding was put to test in vivo using a xenotransplant model of KG1a/200 human cells intravenously inoculated into non-obese diabetic severe combined immunodeficient (NOD-SCID) mice. The animals were randomly allocated to receive treatment with PSC 833 (n = 32) or placebo (n = 24). PSC 833 (30 mg/kg) was subcutaneously injected six or 12 times separated by 48-96 h. The overall mean whole blood concentration of PSC 833 was 1191 +/- 60 ng/ml (s.e.m.) at 20 h after administration. Tumor engraftment was significantly reduced in the treatment group (P = 0.037), which also had prolonged survival compared to control animals (P = 0.0016). This is the first study that demonstrates antileukemic effects of a Pgp inhibitor as single agent therapy in vivo, and the present data raise the possibility of alternative exploitation of modulators in cancer chemotherapy.

PSC 833 modulation of multidrug resistance to paclitaxel in cultured human ovarian carcinoma cells leads to apoptosis

BACKGROUND

Multidrug resistance (MDR) modulator PSC 833 has been shown to modulate multidrug resistance in Pg-p-positive human ovarian carcinoma cells A2780/ADR. Co-treatment of A2780/ADR cells with paclitaxel (PTX) and PSC 833 resulted in the restoration of PTX-sensitivity comparable to that in parental A2780 cells.

RESULTS

The flow cytometry experiments presented here showed PTX-(A2780) and PTX plus PSC 833 (A2780/ADR)-induced cell accumulation in the G2/M-phase of the cell cycle with concomitant appearance of apoptotic cells with sub-G0 (hypodiploid) DNA content. Furthermore, these events were accompanied by the appearance of poly(ADP-ribose) polymerase (PARP) cleavage, up-regulation of Bax, p53 and p21WAF1/CIP1 proteins and internucleosomal DNA fragmentation. Interestingly, we did not detect any significant alterations in Bcl-xL, CD95/Fas and Fas-L protein levels.

CONCLUSION

These results demonstrate the PSC 833 reduced the Pg-p-mediated multidrug resistance in human ovarian carcinoma cells to PTX-induced apoptosis in vitro.

Transport of paclitaxel (Taxol) across the blood-brain barrier in vitro and in vivo

Paclitaxel concentrations in the brain are very low after intravenous injection. Since paclitaxel is excluded from some tumors by p-glycoprotein (p-gp), the same mechanism may prevent entry into the brain. In vitro, paclitaxel transport was examined in capillaries from rat brains by confocal microscopy using BODIPY Fl-paclitaxel. Western blots and immunostaining demonstrated apical expression of p-gp in isolated endothelial cells, vessels, and tissue. Secretion of BODIPY Fl-paclitaxel into capillary lumens was specific and energy-dependent. Steady state luminal fluorescence significantly exceeded cellular fluorescence and was reduced by NaCN, paclitaxel, and SDZ PSC-833 (valspodar), a p-gp blocker. Leukotriene C(4) (LTC(4)), an Mrp2-substrate, had no effect. Luminal accumulation of NBDL-cyclosporin, a p-gp substrate, was inhibited by paclitaxel. In vivo, paclitaxel levels in the brain, liver, kidney, and plasma of nude mice were determined after intravenous injection. Co-administration of valspodar led to increased paclitaxel levels in brains compared to monotherapy. Therapeutic relevance was proven for nude mice with implanted intracerebral human U-118 MG glioblastoma. Whereas paclitaxel did not affect tumor volume, co-administration of paclitaxel (intravenous) and PSC833 (peroral) reduced tumor volume by 90%. Thus, p-gp is an important obstacle preventing paclitaxel entry into the brain, and inhibition of this transporter allows the drug to reach sensitive tumors within the CNS.

The role of the MDR1 gene in the development of multidrug resistance in human hepatoblastoma: clinical course and in vivo model

BACKGROUND

The P-glyprotein (P-gp), which is a membrane channel encoded by the MDR1 gene, represents a possible explanation for multidrug resistance in human hepatoblastoma (HB). P-gp shows up-regulation in tumor cells after chemotherapy; however, to date, its exact role in HB has not been described. The authors investigated the role of the MDR1 gene in the clinical course of patients with HB and in an in vivo model of HB. They also studied the effects of the MDR1 antagonizer PSC 833 on chemotherapy in mice xenotransplanted with HB.

METHODS

Resected tumor specimens, including both primary tumors and recurrent tumors, from a child suffering from HB were investigated histologically. Cell suspensions from the originally removed tumor were incorporated subcutaneously into nude mice. Animals were treated with cisplatin (CDDP) plus PSC 833. MDR1 gene expression levels in the different resected tumors from the patient and in the xenotransplants after treatment were determined with polymerase chain reaction analysis.

RESULTS

MDR1 gene expression was increased in the patient's tumors after every course of chemotherapy from 30% to > 190%. In the xenotransplants, MDR1 gene expression was enhanced significantly after chemotherapy (P(CDDP) = 0.008; P(CDDP+PSC) = 0.002). Tumor volumes (P < 0.001) and serum alpha-fetoprotein levels (P = 0.0002) were significantly lower in the animals that were treated with CDDP + PSC compared with the animals that were treated with CDDP alone.

CONCLUSIONS

The current results suggest that MDR1 gene expression and P-gp are a potential mechanism of drug resistance in HB. The chemosensitizer PSC 833 significantly improved the effects of chemotherapy in animals xenotransplanted with HB. These data encourage further studies concerning the role of chemosensitizers in overcoming multidrug resistance in patients with HB.

Cyclosporins: structure-activity relationships for the inhibition of the human FPR1 formylpeptide receptor

The human formylpeptide receptor (FPR) is a seven-transmembranous G-protein-coupled receptor (7TM-GPCR) for chemotactic peptides of bacterial origins, possibly involved in the recruitment and activation of neutrophils in various inflammatory diseases of mucosal epithelia. Mutational analyses suggest that interactions of formylated peptides with FPR occur on the outer exoplasmic leaflet/domains of the plasma membrane. The immunosuppressive and antifungal antibiotic cyclic undecapeptide cyclosporin A (CsA; cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)]) and some tested analogues such as [Ala(2)]-CsA, [Thr(2)]-CsA, [Val(2)]-CsA, and [Nva(2)]-CsA were able of inhibiting the binding of formylpeptides to the FPR, with [D-MeVal(11)]-CsA (CsH) being much more active than the other analogues. CsH is devoid of immunosuppressive and antifungal activities, and its large potency for human FPR inhibition is of inverse agonism origin. Formylpeptide binding to FPR-expressing cells does not only induce chemotaxis; it also causes a rapid release of granule enzymes in the extracellular medium, allowing the easy monitoring of any inhibition of FPR function "in vivo" (with intact live cells). With such an assay, CsH was confirmed to be the most potent FPR inhibitory cyclosporin, although a far related immunosuppressive cyclosporin analogue, FR901459 ([Thr(2), Leu(5), Leu(10)]-CsA), was found to display a high FPR inhibitory activity (FPR-InhA). To establish structure-activity relationships (SAR) for FPR function inhibition, 59 cyclosporins were now studied by this standardized assay (with differentiated human leukemic cell line HL-60 as FPR-expressing cells and with N-acetyl-beta-D-glucosaminidase release as read-out). These SAR confirmed the low FPR-InhA of classical cyclosporins, where such activity was only seldom found: the most active ones ([Thr(2), Ile(5)]-CsA, [aMeIle(11)]-CsA, and [MeAla(11)]-CsA) remained 3-10-fold less potent than CsH. In contrast, the SAR disclosed that N(10)-desmethylated cyclosporins were particularly prone to display a large FPR-InhA: their most potent one was a [Thr(2), Gly(3), Leu(5), D-Hiv(8), Leu(10)]-CsA, found to be only 2-4-fold less active than [D-MeVal(11)]-CsA (CsH), with which it shows six differences out of 11 residues. Because the free conformations of both CsH and N(10)-desmethylated cyclosporins differ from those of "classical" (N(10)-methylated, [L-MeVal(11)]-using) cyclosporins, these potent FPR inhibitory cyclosporins probably bind to FPR pharmacophores for which classical cyclosporins show little affinity. Moreover, because the conformations of the N(10)-desmethylated cyclosporins widely differ from the CsH one, they probably bind to different pharmacophores on the FPR molecules.

Cyclosporins: structure-activity relationships for the inhibition of the human MDR1 P-glycoprotein ABC transporter

Cyclic undecapeptide cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)], the immunosuppressive and antifungal antibiotic cyclosporin A (CsA), was reported to interfere with the MDR1 P-glycoprotein (Pgp), a transmembranous adenosine 5'-triphosphate binding cassette (ABC) transporter with phospholipid flippase or "hydrophobic vacuum cleaner" properties that mediate multidrug resistance (MDR) of cancer cells. By use of photoaffinity-labeled cyclosporins and membranes from Pgp-expressing cells, it was recently shown that in vitro, Pgp molecules could bind a large cyclosporin domain involving residues 4-9 as well as the side chain of residue 1. Tumor cell MDR can also be reversed by a product more distantly related to cyclosporin with the structure [Thr(2), Leu(5), D-Hiv(8), Leu(10)]-CsA (SDZ 214-103). In a standardized assay that measures Pgp function in vivo (on intact live cells) by the Pgp-mediated efflux of the calcein-AM Pgp substrate and uses human lymphoblastoid MDR-CEM (VBL(100)) cells as highly resistant Pgp-expressing cells, SDZ 214-103 was found to be one of the most active Pgp inhibitors among naturally occurring cyclosporins, with an IC(50) of 1.6 microM in an assay where CsA gives an IC(50) of 3.4 microM. Using the in vivo assay, 60, mostly natural, cyclosporin analogues were analyzed to establish structure-activity relationships (SAR). Our SAR are compatible with the in vitro-defined Pgp binding domain model and further disclose that in vivo Pgp inhibition is favored by larger hydrophobic side chains on cyclosporin residues 1, 4, 6, and 8 and a smaller one on residue 7, although with no effect on the residue 5 side chain; moreover, larger hydrophobic side chains on other residues 2, 3, 10, and 11 (outside the in vitro-defined Pgp binding domain) also favor the eventual inhibition of Pgp function. The N-desmethylation of any of the seven N-methylated amides, as naturally occurring in numerous cyclosporins, regularly leads to a decreased Pgp inhibitory activity (Pgp-InhA), up to its abrogation if it occurs at residues 4 and 9. Nevertheless, despite unfavorable use of [Thr(2)] and [Leu(10)] residues, all [D-Hiv(8)] analogues whose lead is SDZ 214-103 show a large Pgp-InhA. The SAR for Pgp inhibition by cyclosporins are thus very complex. Because CsA and SDZ 214-103 show largely different conformations when free in solution, but remarkably similar ones when bound to the cytosolic cyclophilins, SAR for Pgp inhibition must similarly include requirements for occurrence of suitable conformers for insertion in the cell membrane, sufficient conformational plasticity for gaining access to Pgp binding sites, and an adequate conformer structure there to achieve such binding with a high enough affinity and possibly escape from sequestration on cyclophilins.

Increased CNS uptake and enhanced antinociception of morphine-6-glucuronide in rats after inhibition of P-glycoprotein

Morphine-6-glucuronide (M6G) is a substrate of P-glycoprotein (P-gp), which forms an outward transporter at the blood-brain barrier. Inhibition of P-gp may therefore be expected to cause increased CNS uptake of M6G. We directly assessed the spinal concentrations of M6G and its antinociceptive effects in rats following pharmacological inhibition of P-gp. Spinal cord tissue concentrations of M6G were assessed by microdialysis with probes transversally implanted through the dorsal horns of the spinal cord at level L4. Ten rats received M6G intravenously (0.018 mg/kg loading dose plus 0.00115 mg/kg/min for an 8-h infusion), five of them together with PSC833 to inhibit P-gp (32-h infusion, starting 24 h before the addition of M6G). Antinociceptive effects were explored by means of formalin tests. After having obtained evidence for enhanced CNS uptake and antinociception of M6G in the presence of PSC833, additional behavioural experiments were performed in another 32 rats to assess the dose dependency of the antinociceptive effects of M6G either with or without PSC833 in comparison with both PSC833 alone and placebo. Inhibition of P-gp increased the M6G concentrations in the spinal cord approximately three-fold whereas the plasma concentrations were increased only by a factor of 1.4, which resulted in a more than doubled spinal cord/plasma concentration ratio (from 0.08 +/- 0.03 for M6G alone to 0.17 +/- 0.08 for M6G plus PSC833). Antinociceptive effects of M6G were significantly enhanced by inhibition of P-gp. Inhibition of P-gp alters the transport of M6G across the blood-brain barrier, resulting in enhanced spinal cord uptake and enhanced antinociception.

Calcineurin inhibitors and sirolimus: mechanisms of action and applications in dermatology

Controlled trials and clinical experience indicate that systemic cyclosporin A and tacrolimus are effective treatments for psoriasis, and that cyclosporin A also improves atopic eczema. A variety of other inflammatory and non-inflammatory skin diseases are probably also responsive to these drugs. However, the widespread and longer-term use of cyclosporin A and tacrolimus are limited by side effects. The molecular mechanisms of action of cyclosporin A, tacrolimus and a related drug, sirolimus, have been well defined in T cells and involve inhibition of critical signalling pathways that regulate T cell activation. For example cyclosporin and tacrolimus inhibit calcineurin phosphatase activity and thereby inhibit activation of the transcription factor NFAT. The therapeutic efficacy of topical calcineurin inhibitors in atopic eczema have restimulated interest in the mechanism of action of these drugs in skin disease. Recently the expression pattern of calcineurin and NFAT has been defined in non-immune tissues including the akin. The relevance of this to the mechanism of action of systemic and topical calcineurin inhibitors and sirolimus in skin disorders is discussed.

Elevated uptake of low density lipoprotein by drug resistant human leukemic cell lines

Overexpression of a 170kD membrane glycoprotein, P-glycoprotein (Pgp), which acts as an energy dependent efflux pump for cytotoxic drugs is believed to be one of the factors that is responsible for clinical drug resistance. Recent studies suggest that Pgp is also responsible for the intracellular transport of cholesterol from the plasma membrane to the endoplasmic reticulum. Leukemic cells from patients with acute myelogenous leukemia have an elevated uptake of low density lipoprotein (LDL) when compared with white blood cells from healthy individuals. Since elevated LDL receptor expression and multidrug resistance are both common events in leukemic cells, we investigated LDL receptor expression in sensitive and drug resistant human leukemic cell lines. We found a 2- to 10-fold higher uptake of LDL in five out of five drug resistant K562 cell lines. All three drug resistant HL60 cell lines studied also had higher uptake than the parental cells. The LDL receptor expression in vincristine resistant Pgp positive K562 cells was less sensitive to downregulation by sterols than in parental cells. There was no selective effect of the Pgp inhibitor PSC-833 or other Pgp modulators on LDL receptor activity in Pgp positive cells. Since also resistant Pgp, multidrug resistance protein 1, and breast cancer resistance protein negative cells exhibited an elevated LDL receptor activity, we conclude that overexpression of these proteins is not the mechanism behind the elevated LDL uptake in the drug resistant leukemic cell lines. The findings are of interest for the concept of using lipoproteins as carriers of cytotoxic drugs in cancer treatment.

P-glycoprotein in acute myeloid leukaemia: therapeutic implications of its association with both a multidrug-resistant and an apoptosis-resistant phenotype

P-glycoprotein (Pgp) expression is an independent prognostic factor for response to remission-induction chemotherapy in acute myeloblastic leukaemia, particularly in the elderly. There are several potential agents for modulating Pgp-mediated multi-drug resistance, such as cyclosporin A and PSC833, which are currently being evaluated in clinical trials. An alternative therapeutic strategy is to increase the use of drugs which are unaffected by Pgp. However, in this review, we explain why this may be more difficult than it appears. Evidence from in vitro studies of primary AML blasts supports the commonly held supposition that chemoresistance may be linked to apoptosis-resistance. We have found that Pgp has a drug-independent role in the inhibition of in vitro apoptosis in AML blasts. Modulation of cytokine efflux, signalling lipids and intracellular pH have all been suggested as ways by which Pgp may affect cellular resistance to apoptosis; these are discussed in this review. For a chemosensitising agent to be successful, it may be more important for it to enhance apoptosis than to increase drug uptake.

Reduced effect of gemtuzumab ozogamicin (CMA-676) on P-glycoprotein and/or CD34-positive leukemia cells and its restoration by multidrug resistance modifiers

Gemtuzumab ozogamicin (CMA-676), a calicheamicin-conjugated humanized anti-CD33 mouse monoclonal antibody, has recently been introduced clinically as a promising drug for the treatment of patients with acute myeloid leukemia (AML), more than 90% of which express CD33 antigen. However, our recent study suggested that CMA-676 was excreted by a multi- drug-resistance (MDR) mechanism in P-glycoprotein (P-gp)-expressing leukemia cell lines. We analyzed the in vitro effects of CMA-676 on leukemia cells from 27 AML patients in relation to the amount of P-gp, MDR-associated protein 1 (MRP1), CD33 and CD34, using a multi-laser-equipped flow cytometer. The cytocidal effect of CMA-676, estimated by the amount of hypodiploid portion on cell cycle, was inversely related to the amount of P-gp estimated by MRK16 monoclonal antibody (P = 0.004), and to the P-gp function assessed by intracellular rhodamine-123 accumulation in the presence of PSC833 or MS209 as a MDR modifier (P = 0.0004 and P = 0.002, respectively). In addition, these MDR modifiers reversed CMA-676 resistance in P-gp-expressing CD33(+) leukemia cells (P = 0.001 with PSC833 and P = 0.0007 with MS209). In CD33(+) AML cells from 13 patients, CMA-676 was less effective on CD33(+)CD34(+) than CD33(+)CD34(-) cells (P = 0.002). PSC833 partially restored the effect of CMA-676 in CD33(+)CD34(+) cells. These results suggest that the combined use of CMA-676 and a MDR modifier will be more effective on CD33(+) AML with P-gp-related MDR.

The influence of P-glycoprotein on morphine transport in Caco-2 cells. Comparison with paclitaxel

In vitro monolayer studies using Caco-2 cells were employed here to explore P-glycoprotein mediated transport of morphine. Bi-directional transport studies of 10-75 microM morphine showed efflux to be twofold higher than influx (4 x 10(-6) compared to 2 x 10(-6) cm/s) and cellular accumulation in the efflux direction was eightfold higher. The cyclosporin analogue (PSC-833) equilibrated morphine transport in both directions. Depletion of intracellular glutathione had a greater effect on increasing cellular morphine accumulation than P-glycoprotein inhibitors, suggesting a role for glutathione in morphine transport. P-glycoprotein had a substantially greater effect on paclitaxel accumulation, efflux and bi-directional transport than for morphine. Paclitaxel transport was below detection (<0.1 x 10(-6) cm/s) in the influx direction, yet efflux was very high (18.4 x 10(-6) cm/s) and P-glycoprotein inhibition increased accumulation >100-fold. These results reinforce the substantial role P-glycoprotein has in paclitaxel transport. Conversely, P-glycoprotein regulated morphine transport is weak. Nevertheless, morphine transport rates could be doubled when administered with P-glycoprotein substrates. Therefore, increased analgesia through P-glycoprotein inhibition should be possible.

Selective susceptibility of transformed T lymphocytes to induction of apoptosis by PSC 833, an inhibitor of P-glycoprotein

P-glycoprotein is a cellular efflux pump. The P-glycoprotein inhibitor PSC 833 causes apoptosis of cancer cells and induces a rise in the intracellular levels of ceramide. Our aims were to determine whether a cause and effect relationship exists between these two actions of PSC 833, and to assess whether the PSC 833-induced apoptosis is restricted to transformed cells. Apoptosis was determined by flow cytometry and radioactive quantitation of DNA fragmentation. PSC 833 induced apoptosis in the human T leukemia cell lines: Molt-4 and Jurkat. Analysis of the apoptosis in Molt-4 and Jurkat cells revealed that PSC 833 induced a rise in the cellular ceramide levels (as measured by the DG kinase assay). PSC 833-induced apoptosis was significantly reduced by specific inhibitors of ceramide de novo synthesis (i.e., fumonisin B1 and L-cycloserine). On the other hand, PSC 833 did not induce apoptosis in normal peripheral blood T cells regardless of whether these cells were quiescent, activated, or proliferating. Our results suggest that PSC 833 induces apoptotic death in human transformed T lymphocytes through an increase in ceramide de novo synthesis. In addition, normal lymphocytes are not susceptible to induction of apoptosis by PSC 833. This difference between normal lymphocytes and leukemia cells presents a potential target for chemotherapy.

Monitoring of cellular resistance to cancer chemotherapy

Cellular resistance to a broad spectrum of natural products used as antitumor drugs is believed to be a major cause for the failure of chemotherapy. Flow cytometry has been used for monitoring the expression of drug resistance markers, determining accumulation of fluorescent drugs, and for screening of drugs that enhance chemosensitivity by blocking efflux and enhancing drug retention. This article reviews recent developments in our understanding of the multiple drug resistance phenotype and the use of flow cytometry for the study of drug efflux and its modulation in human tumor cells.

P-glycoprotein inhibition by the multidrug resistance-reversing agent MS-209 enhances bioavailability and antitumor efficacy of orally administered paclitaxel

PURPOSE

Recent studies in humans and mice have demonstrated that intestinal P-glycoprotein plays a causative role in the limited absorption of orally administered paclitaxel. Multidrug resistance (MDR)-reversing agents, such as cyclosporin A and PSC 833, are known to increase the systemic exposure to orally administered paclitaxel by enhancing absorption in the intestinal tract and decreasing elimination via the biliary tract. In this study, we demonstrated that coadministration of the MDR-reversing agent MS-209, which is known to inhibit P-glycoprotein function by direct interaction, improved the bioavailability of orally administered paclitaxel and consequently enhanced its antitumor activity.

METHODS

The pharmacokinetics of paclitaxel were examined by measuring [(3)H]paclitaxel in plasma drawn from rats and mice given the drug with or without MS-209. The influence of MS-209 on the intestinal transport of [(3)H]paclitaxel was studied using a human colorectal cancer cell line, Caco-2. The in vivo efficacy of orally administered paclitaxel in combination with MS-209 was further evaluated in B16 melanoma-bearing mice.

RESULTS

The plasma concentration of [(3)H]paclitaxel following oral administration was significantly increased by coadministration of MS-209 at 100 mg/kg in both rats and mice. In rats, the AUC of [(3)H]paclitaxel following oral administration was strikingly increased (1.9-fold) by coadministration of MS-209, whereas the AUC of [(3)H]paclitaxel following i.v. injection was slightly increased (1.3-fold) by MS-209. The increase in apparent bioavailability of oral paclitaxel due to MS-209 was 1.4-fold. To demonstrate this enhancing action in vitro, we studied the influence of MS-209 on the transport of [(3)H]paclitaxel using Caco-2 cells, which is a well-known model of intestinal efflux. The transport of [(3)H]paclitaxel across the Caco-2 monolayer was markedly inhibited in the presence of MS-209, and the apparent K(i)of MS-209 for the active transport of [(3)H]paclitaxel was 0.4 microM. Moreover, paclitaxel administered orally at 100 mg/kg per day with MS-209 at 100 mg/kg per day showed significant antitumor activity in B16 melanoma-bearing mice, whereas paclitaxel administered orally alone at the same dose showed no antitumor activity. These results suggest that the coadministration of MS-209 improved low systemic exposure to paclitaxel through inhibition of P-glycoprotein, which is involved in drug excretion via the intestinal tract, resulting in a clear antitumor activity of paclitaxel administered orally.

CONCLUSION

The present study suggests that coadministration of MS-209 may be a useful way to improve the bioavailability of drugs not suitable for oral administration due to elimination via the intestinal tract.

Effect of topical immunomodulators on acute allergic inflammation and bronchial hyperresponsiveness in sensitised rats

We examined the effects of different immunomodulators administered topically on asthmatic responses in a rat model of asthma. Sensitised Brown-Norway rats were administered rapamycin, SAR943 (32-deoxorapamycin), IMM125 (a hydroxyethyl derivative of D-serine(8)-cyclosporine), and budesonide by intratracheal instillation 1 h prior to allergen challenge. Allergen exposure induced bronchial hyperresponsiveness, accumulation of inflammatory cells in bronchoalveolar lavage fluid, and also an increase in eosinophils and CD2+, CD4+ and CD8+ T cells in the airways. Interleukin-2, interleukin-4, interleukin-5, interleukin-10, and interferon-gamma mRNA expression was upregulated by allergen exposure. Budesonide abolished airway inflammation, suppressed the mRNA expression for interleukin-2, interleukin-4, and interleukin-5 (P<0.03), and bronchial hyperresponsiveness (P<0.05). IMM125 suppressed airway infiltration of eosinophils, and CD8+ T cells (P<0.02), and prevented the upregulated mRNA expression for interleukin-4, interleukin-5, and interferon-gamma (P<0.02). Rapamycin suppressed CD8+ T cell infiltration in airway submucosa (P<0.03), and mRNA expression for interleukin-2 (p<0.002), while SAR943 suppressed interleukin-2, interleukin-4, and interferon-gamma mRNA (P<0.05). IMM125, rapamycin and SAR943 did not alter airway submucosal CD2+ and CD4+ T cell infiltration, and bronchial hyperresponsiveness. CD8+ T cells, in contrast to CD4+ T cells, are more susceptible to the inhibition by IMM125 and rapamycin, which also caused greater suppression of Th1 compared to Th2 cytokine mRNA expression. In this acute model of allergic inflammation, differential modulation of Th1 and Th2 cytokines may determine the effects of various immunomodulators on airway inflammation and bronchial hyperresponsiveness.

Transport activity and surface expression of the Na+-Ca2+ exchanger NCX1 are inhibited by the immunosuppressive agent cyclosporin A and by the nonimmunosuppressive agent PSC833

Cyclosporin A (CsA) treatment of HEK 293 cells expressing the rat heart RHE-1 (NCX1.1, EMBL accession number ) or the rat brain RBE-2 (NCX1.5, GenBank(TM) accession number ) Na(+)-Ca(2+) exchanger inhibited their transport activity in a concentration-dependent manner. The inhibition was detectable at 2 microm CsA, and exposure of the cells to 20 microm CsA resulted in a decrease of the Na(+)-dependent Ca(2+) uptake to about 20% relative to that of untreated cells. Determination of the surface expression of the exchanger protein revealed a parallel concentration-dependent reduction in the amount of the immunoreactive protein. No reduction was detected in the amount of total immunoreactive exchanger protein in CsA-treated cells relative to untreated ones. Among the different drugs tested, only PSC833, an analog of cyclosporin D, mimicked the effects of CsA. Exposure of the transfected cells to the chemically related cyclosporin D and macrolide drugs (FK506 or rapamycin) had no effect on the transport activity or the surface expression of the Na(+)-Ca(2+) exchanger. Co-expression of the human multidrug transporter P-glycoprotein (of which both drugs are modulators) with the cloned Na(+)-Ca(2+) exchanger revealed that transport activity and surface expression of each transporter in the co-transfected system were similar to those of each transporter alone in both the presence and absence of CsA or PSC833. CsA and PSC833 inhibited the surface expression of the NCX1 protein but did not alter the surface expression of P-glycoprotein. Unlike some P-glycoprotein endoplasmic reticulum-retained mutants (Loo, T. W., and Clarke, D. M. (1997) J. Biol. Chem. 272, 709-712), CsA did not rescue RBE-2/F913-->Stop, an endoplasmic reticulum-retained function-competent mutant of the Na(+)-Ca(2+) exchanger (Kasir, J., Ren, X., Furman, I., and Rahamimoff, H. (1999) J. Biol. Chem. 274, 24873-24880) and did not induce its kinesis to the surface membrane, further demonstrating molecular differences between P-glycoprotein and NCX1 mutants for interaction with CsA.

[Reversal of multidrug-resistance in human leukemia cell line K562/A02 by a cyclosporin D analogue PSC 833]

OBJECTIVE

To explore the efficacy of PSC 833 on multidrug resistance (MDR) reversal and its mechanism.

METHODS

Human erythroleukemic cell line K562 and its doxorubicin-resistant counterpart K562/A02 were used in the study. Cytotoxicity was assessed by MTT assay, P-gp expression by direct immunofluorescence and mdr1 mRNA expression by reverse transcriptase polymerase chain reaction (RT-PCR) with beta-actin as internal control. Intracellular DNR retention was measured with flow cytometry.

RESULTS

K562/A02 cells displayed high levels of mdr1 mRNA and P-glycoprotein and reduced DNR retention compared to their parental K562 cells. 1 micromol/L of PSC 833 had no effect on the levels of mdr1 mRNA and P-gp expression in K562/A02 cells (P > 0.05). PSC 833 conferred a dose-dependent increase on chemosensitivity of K562/A02 to DNR, and its effect was at least 3-fold more potent than that of CsA or Ver. PSC 833 could increase DNR retention in K562/A02 cells. A 100.9% restoration of intracellular DNR retention of the level of K562 cells was gained by PSC 833 at 1.0 micromol/L in K562/A02 cells, whereas only a 86.9% restoration of DNR retention was obtained by CsA at 10 micromol/L in the K562/A02 cells. No effect on DNR sensitivity and retention was found in K562 cells (P > 0.05).

CONCLUSION

PSC 833 is at least 3 approximately 10 fold more potent than CsA or Ver with respect to MDR reversing activity, and it may function by inhibiting the function of P-gp and not reducing the levels of mdr1 mRNA and P-gp directly.

Xenobiotic efflux pumps in isolated fish brain capillaries

To identify specific transporters that drive xenobiotics from the central nervous system to blood, the accumulation of fluorescent drugs was studied in isolated capillaries from killifish and dogfish shark brain using confocal microscopy and quantitative image analysis. In killifish brain capillaries, luminal accumulation of fluorescent derivatives of cyclosporin A and verapamil was concentrative, specific, and energy dependent (inhibition by KCN). Transport was reduced by PSC-833, but not by leukotriene C4, indicating the involvement of P-glycoprotein. The ability of capillaries to transport the cyclosporin A derivative was unchanged over 20 h, demonstrating the long-term viability of the preparation. Luminal accumulation of the fluorescent organic anions sulforhodamine 101 and fluorescein-methotrexate was also concentrative, specific, and energy dependent. Transport of these compounds was reduced by leukotriene C4, but not by PSC-833, indicating the involvement of a multidrug resistance-associated protein (Mrp). Similar results were obtained for isolated capillaries from dogfish shark. Immunostaining localized P-glycoprotein and Mrp2 to the luminal surface of the killifish brain capillary endothelium. These findings validate a new and long-lived comparative model for studying drug transport across the blood-brain barrier and, as in mammals, implicate P-glycoprotein and Mrp2 in transport from the central nervous system to blood in fish.

Assessment of the classical MDR phenotype in epithelial ovarian carcinoma using primary cultures: a feasibility study

This in vitro feasibility study has assessed a number of techniques and their applicability when looking at the role of multidrug resistance (MDR) in solid tumours. Fresh tumour material was obtained from 34 patients, (11 previously treated, 23 untreated) with ovarian adenocarcinoma. Doxorubicin sensitivity was measured using the MTT assay +/- the cyclosporins, Pgp expression was assessed by immunocytochemistry with the MRK-16 MoAb and flow cytometry was used to assess intracellular drug accumulation +/- PSC 833. 85% of samples showed some evidence of modest chemosensitisation by the cyclosporins (median 1.74-fold). We saw a marked variation in the number of Pgp positive cells between patients (1-87%, median 31%). 63% of samples tested showed an enhancement of DNR accumulation in the presence of PSC 833, with a median increase of 7% (sample range 0-29%). The present study highlights some of the technical difficulties encountered when working with fresh tumour material ex vivo. We conclude that screening of patients for their suitability to enter clinical trials incorporating MDR modulating agents is technically demanding, but feasible.

Evidence of P-glycoprotein mediated apical to basolateral transport of flunisolide in human broncho-tracheal epithelial cells (Calu-3)

1. Transepithelial transport of flunisolide was studied in reconstituted cell monolayers of Calu-3, LLC-PK1 and the MDR1-P-glycoprotein transfected LLC-MDR1 cells. 2. Flunisolide transport was polarized in the apical (ap) to basolateral (bl) direction in Calu-3 cells and was demonstrated to be ATP-dependent. In LLC-MDR1 cells, flunisolide was transported in the bl to ap direction and showed no polarization in LLC-PK1 cells. 3. Non-specific inhibition of cellular metabolism at low temperature (4 degrees C) or by 2-deoxy-D-glucose (2-d-glu) and sodium azide (NaN(3)) abolished the polarized transport. Polarized flunisolide transport was also inhibited by the specific Pgp inhibitors verapamil, SDZ PSC 833 and LY335979. 4. Under all experimental conditions and in the presence of all used inhibitors, no decrease in the TransEpithelial Electrical Resistance (TEER) values was detected. From all inhibitors used, only the general metabolism inhibitors 2-deoxy-D-glucose and NaN(3), decreased the survival of Calu-3 cells. 5. Western blotting analysis and confocal laser scanning microscopy demonstrated the presence of MDR1-Pgp at mainly the basolateral side of the plasma membrane in Calu-3 cells and at the apical side in LLC-MDR1 cells. Mass spectroscopy studies demonstrated that flunisolide is transported unmetabolized across Calu-3 cells. 6. In conclusion, these results show that the active ap to bl transport of flunisolide across Calu-3 cells is facilitated by MDR1-Pgp located in the basolateral plasma membrane.

Overcoming MDR by ultrasound-induced hyperthermia and P-glycoprotein modulation

We assessed the effects of combining ultrasound-induced hyperthermia (USHT) with the P-glycoprotein modulator PSC 833 on cellular retention and cytotoxicity of rhodamine 123 (R123) and doxorubicin in the parent and multidrug resistance (MDR) variants of two human cancer lines. USHT significantly increased cellular uptake of R123 and doxorubicin. Without PSC 833, release of R123 and doxorubicin from both USHT-treated and untreated cells was rapid. As expected, PSC 833 (0.5 microM) only slowed their release into the MDR lines. Interestingly, despite the differences in their starting amounts, PSC 833 was effective in prolonging R123 and doxorubicin release from both USHT-treated and untreated MDR cells. PSC 833 did not augment the cytotoxicity of doxorubicin in parent lines but did cause a significant increase in cytotoxicity of doxorubicin in the MDR lines. However, the combined effect of USHT and PSC 833 on cytotoxicity of doxorubicin far exceeded that produced by USHT or PSC 833 alone.

[Study on the reversion of drug resistance in human cervical cancer cell lines]

OBJECTIVE

To determine the resistance reversion of mitomycin (MMC) by 3'-Keto-bmt1-val2-cyclosporin (SDZ PSC 833) in human cervical cancer in vitro and in vivo. METHEDS: A xenografted mitomycin resistant mice model of cervical cancer was devolped. The reversion of mitomycin resistance by SDZ PSC 833 (1 or 3 mg/L) was detected from human cervical cancer cell (Hela) and its resistant subline Hela/MMC in vitro and in vivo. Studies in vitro include drug resistance reversion experiment and the changes of morphology. Studies in vivo including tumor volume and tumor related histopathological changes in the autopsied specimen were evaluated by comparing random sections of each group.

RESULTS

Nontoxic doses of SDZ PSC 833 could result in almost partial reversion of MMC-resistance of Hela/MMC. In vivo studies also showed SDZ PSC 833 augmented the growth inhibitory effect of mitomycin on Hela/MMC xenografted in nude mice.

CONCLUSION

SDZ PSC 833 can overcome mitomycin resistance of Hela/MMC in vitro and in vivo, so SDZ PSC 833 will be a better candidate clinically for reversing multidrug resistance.

Influence of P-glycoprotein modulators on cardiac uptake, metabolism, and effects of idarubicin

PURPOSE

The clinical utility of anthracyclines like idarubicin (IDA) is limited by the occurrence of multidrug resistance and cardiotoxicity. Previous studies have demonstrated that the multidrug transporter P-glycoprotein (P-gp) is present in the heart and have suggested that it exerts a protective function. We sought to determine the influence of P-gp inhibitors verapamil and PSC 833 on myocardial uptake, metabolism, and actions of IDA.

METHODS

In Langendorff-perfused rat hearts, the outflow concentration-time curve and the residual amount in cardiac tissue of IDA and its active metabolite idarubicinol (IDOL) were measured after 0.5 mg dose of IDA in the absence and presence of the P-gp inhibitors verapamil and PSC 833.

RESULTS

During perfusion (80 min), 2% of the IDA dose was converted to IDOL in the heart. Myocardial uptake of IDA was significantly increased by verapamil but not by PSC 833, which increased the recovery of IDA and IDOL. IDA significantly decreased left ventricular developed pressure to approximately 40% and increased coronary vascular resistance to 140% of baseline level, respectively. The vasoconstrictive effect was markedly potentiated by PSC 833.

CONCLUSIONS

The enhancement of myocardial IDA uptake by verapamil could be due to a decrease in P-gp-mediated efflux. PSC 833 inhibits cardiac metabolism (non-IDOL pathways) and increases the acute cardiotoxicity of IDA.

In vitro prevention of the emergence of multidrug resistance in a pediatric rhabdomyosarcoma cell line

We have established preclinical models for the development of drug resistance to vincristine (a major drug used in the treatment of pediatric rhabdomyosarcoma) using cell lines. The RD cell line has a mutant P53 phenotype and does not have detectable P-glycoprotein (P-gp) or multidrug resistance-related protein (MRP) despite expressing low levels of mdr-1 mRNA, which encodes P-gp and mrp1 mRNA. Resistant variants of RD were derived by exposure to increasing concentrations of vincristine. This was repeated on six occasions, resulting in three cell lines which could tolerate 64 x the IC(50) concentration. Six independent agents were tested for their ability to prevent the development of resistance in this model. Despite at least 10 attempts, resistance did not develop in the presence of the multidrug resistance (MDR) modulators PSC833, VX710, and XR9576. This strongly suggests that these agents may delay or even prevent the development of resistance to vincristine. This was also confirmed in a second rhabdomyosarcoma cell line, Rh30. In contrast, the agents indomethacin (MRP1 modulator), CGP41251 (protein kinase C inhibitor), and dexrazoxane (putative MDR prevention agent) did not affect the development of resistance in the RD model. Characterization of the resistant cell lines indicated the presence of increased mdr-1 and P-gp expression, which resulted in resistance to the agents doxorubicin, etoposide, and vincristine but not cisplatin. The resistance could be modulated using PSC833 or VX710, confirming that functional P-gp is present. No apparent differences were seen between the resistant cell lines derived in the absence and presence of the various agents. These experiments strongly suggest that the development of MDR may be preventable using modulators of MDR and merit clinical studies to test this hypothesis.

Intracellular levels of two cyclosporin derivatives valspodar (PSC 833) and cyclosporin a closely associated with multidrug resistance-modulating activity in sublines of human colorectal adenocarcinoma HCT-15

P-Glycoprotein, which mediates multidrug resistance (MDR) in cancer chemotherapy, is a principal target of cyclosporin A and [3'-keto-Bmt(1)]-[Val(2)]-cyclosporin (valspodar; PSC 833). To clarify mechanisms contributing to the different MDR-modulating activities of valspodar and cyclosporin A, we investigated the relation of the intracellular levels of the two cyclosporin derivatives to their modulating effect on MDR in different P-glycoprotein-expressing human colorectal carcinoma HCT-15 cells (parental HCT-15 and adriamycin-resistant sublines). In this study, valspodar was found to be much more potent than cyclosporin A in both sensitizing resistant cells to MDR-related anticancer drugs (e.g., adriamycin, vincristine and paclitaxel (taxol)) and increasing 2-[6-amino-3-imino-3H-xanthen-9-yl]benzoic acid methyl ester (rhodamine 123) retention and [G-(3)H]vincristine sulfate ([(3)H]vincristine) accumulation in these cells. Furthermore, a good correlation was detected between P-glycoprotein levels and the MDR-reversing effect of valspodar. In contrast, the effects of cyclosporin A could not be linked to P-glycoprotein levels in the MDR cells. In addition, the intracellular accumulation of valspodar was found to be 3 - 6 fold higher than that of cyclosporin A in four sublines and verapamil, an inhibitor of P-glycoprotein-mediated transport, enhanced the accumulation of cyclosporin A, but not valspodar. These results suggested that valspodar accumulation is not actively regulated by the P-glycoprotein-mediated efflux system.

Cross-resistance to cytosine arabinoside in a multidrug-resistant human promyelocytic cell line selected for resistance to doxorubicin: implications for combination chemotherapy

The pyrimidine analogue cytosine arabinoside (AraC) is one of the most effective drugs used in the treatment of acute leukaemia. Overexpression of the multidrug resistance (MDR-1) gene and its product, P-glycoprotein (P-gp), is associated with cellular resistance to drugs, such as anthracyclines and vinca alkaloids. This resistance can be reversed by cyclosporine analogues or verapamil (ver). We investigated the in vitro cross-resistance to AraC in a doxorubicin-resistant HL60 cell line, with an elevated expression of the MDR-1 gene. The resistant clone showed an eightfold increased resistance to AraC and a two- to fourfold resistance to the other analogues, as measured by cytotoxicity test. There was no significant increase in the activity of 5'-nucleotidase or in the amount of deoxyribonucleotide pools between cell lines. We could, however, detect a reduction in deoxycytidine kinase (dCK) activity (30%, P = 0.021, using deoxycytidine as substrate) and the level of AraC triphosphates was significantly reduced in the resistant cells (70%, P = 0.009). When the cells were exposed to cyclosporin A (CsA) or the cyclosporine analogue PSC 833 (PSC) in combination with AraC, there was more extensive apoptosis, as measured by formation of oligonucleosomal DNA fragmentation and caspase-3-like activity, than with exposure to AraC alone. We also found an increased retention of AraC in the resistant cells when incubated with AraC in combination with CsA. Ver in combination with AraC, failed to increase apoptosis for the resistant cell line. Our data suggests that the resistance to AraC for the P-gp-expressing cells is a result of a reduction of dCK activity and an increase in efflux, the latter possibly depending on P-gp. A combination of CsA or PSC with AraC may improve the effect of AraC in vivo.

Multidrug-resistance P-glycoprotein (MDR1) secretes platelet-activating factor

The human multidrug-resistance (MDR1) P-glycoprotein (Pgp) is an ATP-binding-cassette transporter (ABCB1) that is ubiquitously expressed. Often its concentration is high in the plasma membrane of cancer cells, where it causes multidrug resistance by pumping lipophilic drugs out of the cell. In addition, MDR1 Pgp can transport analogues of membrane lipids with shortened acyl chains across the plasma membrane. We studied a role for MDR1 Pgp in transport to the cell surface of the signal-transduction molecule platelet-activating factor (PAF). PAF is the natural short-chain phospholipid 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. [(14)C]PAF synthesized intracellularly from exogenous alkylacetylglycerol and [(14)C]choline became accessible to albumin in the extracellular medium of pig kidney epithelial LLC-PK1 cells in the absence of vesicular transport. Its translocation across the apical membrane was greatly stimulated by the expression of MDR1 Pgp, and inhibited by the MDR1 inhibitors PSC833 and cyclosporin A. Basolateral translocation was not stimulated by expression of the basolateral drug transporter MRP1 (ABCC1). It was insensitive to the MRP1 inhibitor indomethacin and to depletion of GSH which is required for MRP1 activity. While efficient transport of PAF across the apical plasma membrane may be physiologically relevant in MDR1-expressing epithelia, PAF secretion in multidrug-resistant tumours may stimulate angiogenesis and thereby tumour growth.

Effects of the novel cyclosporine derivative PSC833 on glucose metabolism in rat primary cultures of neuronal and glial cells

Cyclosporine A (CsA) and the cyclosporine degrees congener PSC833 are known to cause transient CNS symptoms at high dosages in animal and man. Since impaired glucose metabolism plays a fundamental role in many heriditary and drug-induced neurological disorders, it was the purpose of the present study to evaluate whether this mechanism of pathogenesis might apply to PSC833 and CsA, using neural cells from rats. PSC833 and CsA were investigated in primary cultures of rat neuronal and glial cells at the concentration of 0.1, 1, 10, and 20 microM for 24 and 48 hr. Lactate dehydrogenase was determined as a marker of cytotoxicity. Cell proliferation was determined in astrocytes. Cellular glucose metabolism was investigated by 13C-NMR using [1-13C]glucose as a substrate. Glucose and lactate concentrations in the cell culture supernatants were determined spectrophotometrically. PSC833 at 10 microM was not cytotoxic in neuronal or glial cells nor did it inhibit proliferation in astrocytes 24 hr after incubation. Under the same conditions, the determination of [1-13C]glucose and [3-13C]lactate revealed significantly increased glucose consumption and lactate production in both cell types, as well as decreased levels of Krebs cycle intermediates. In the cell culture medium of both cell types after treatment with 10 microM PSC833, the rates of glucose consumption and lactate formation increased in comparison to controls, between 60-83% and 54-78%, respectively. PSC833 (10 microM) and CsA (20 microM) resulted in nearly similar increased glucose consumption and lactate production. The major PSC833 metabolite in rats, M9, which was devoid of CNS effects, did not cause significant changes in glucose metabolism. The present data suggest that PSC833-impaired tricarboxylic acid cycle activity, resulting in decreased Krebs cycle metabolites, can cause energy depletion and acidosis, which might contribute to the transient neurological symptoms of PSC833 and CsA.

Induction of apoptosis in MDR1 expressing cells by daunorubicin with combinations of suboptimal concentrations of P-glycoprotein modulators

The application of most agents with the capacity to reverse multidrug resistance (MDR) via modulation of the multidrug transporter P-glycoprotein (Pgp) was shown to be associated with toxic side-effects. For this reason, we have investigated the effect of combinations of suboptimal concentrations of Pgp blockers on the induction of apoptosis and growth arrest in daunorubicin (D) treated, MDR1 gene transfected cells. We used verapamil, PSC833 and Cremophor EL as Pgp modulators, which affect the function of Pgp by different mechanisms. Treatment of NIH3T3/MDR1 cells with combinations of suboptimal concentrations of Pgp modulators in the presence of D caused apoptosis and G(2) arrest to the same extent as optimal concentrations of singly used blockers. We conclude that combinations of suboptimal concentrations of Pgp modulators may cause effective sensitization of resistant tumor cells, and at the same time, may avoid the frequently observed toxic effects experienced in clinical trials with a single modifier applied at the optimal dose.

The P-glycoprotein antagonist PSC 833 increases the plasma concentrations of 6alpha-hydroxypaclitaxel, a major metabolite of paclitaxel

PURPOSE

Overexpression of P-glycoprotein (Pgp) is one mechanism of drug resistance in cancer chemotherapy. A Phase I trial was conducted using PSC 833, a Pgp antagonist, in combination with paclitaxel in patients with refractory cancer. The objective of this study was to assess the effect of PSC 833 on the metabolism of paclitaxel and characterize the differences in 6alpha-hydroxypaclitaxel pharmacokinetics. In addition, we examined the possibility of enhanced cytotoxicity of paclitaxel by the coexistence of 6alpha-hydroxypaclitaxel.

EXPERIMENTAL DESIGN

Patients received paclitaxel 35 mg/m(2)/day by continuous intravenous infusion (CIVI) x 4 days without PSC 833 in cycle 1 and escalating doses of paclitaxel (13.1, 17.5, or 21.3 mg/m(2)/day CIVI x 4 days) with 5 mg/kg PSC 833 by mouth every 6 h x 7 days in cycle 2. Plasma samples were analyzed for both paclitaxel and its major metabolite with high-performance liquid chromatography methods. Using human liver microsomes, we studied the effect of PSC 833 on the metabolism of paclitaxel. In addition, the in vitro cytotoxicity of 6alpha-hydroxypaclitaxel alone and in combination with paclitaxel was evaluated.

RESULTS

Twenty-one of 22 patients had a metabolite peak (6alpha-hydroxypaclitaxel) observed in the chromatogram of plasma samples from cycle 2 when they received paclitaxel in combination with PSC 833. This metabolite was not detectable in plasma obtained during the first cycle when they received paclitaxel without PSC 833. During cycle 2, the mean concentrations of 6alpha-hydroxypaclitaxel and paclitaxel were 0.10 +/- 0.074 and 0.079 +/- 0.041 microg/ml, respectively. A moderate association was observed between total bilirubin and 6alpha-hydroxypaclitaxel concentrations (P = 0.015, r = 0.52; n = 21). Human liver microsome experiments showed that a PSC 833 concentration as high as 10 microM did not affect the production of 6alpha-hydroxypaclitaxel. Paclitaxel cytotoxicity in HL60 and K562 human leukemia cells was increased in the presence of noncytotoxic concentrations of 6alpha-hydroxypaclitaxel.

CONCLUSIONS

PSC 833 increases the plasma concentration of 6alpha-hydroxypaclitaxel during paclitaxel therapy. Inhibition of cytochrome P-450 3A4 by PSC 833 may explain this in part, although other mechanisms cannot be excluded.

Treatment with inhibitors of caspases, that are substrates of drug transporters, selectively permits chemotherapy-induced apoptosis in multidrug-resistant cells but protects normal cells

Many chemotherapeutic agents induce apoptosis in tumor cells, but killing of normal cells remains a major obstacle. Development of multidrug resistance further limits chemotherapy in cancer. Here, I show that multidrug resistance can be exploited for selective killing of multidrug-resistant cells by a combination of an apoptosis-inducing agent that is not a substrate of either Pgp or MRP (e.g. flavopiridol) with a caspase inhibitor that is a substrate (e.g. Z-DEVD-fmk). In normal cells, treatment with caspase inhibitors prevented PARP cleavage, nuclear fragmentation, and cell death caused by flavopiridol or epothilone B. In contrast, Pgp- and MRP-expressing cells were not rescued by caspase inhibitors. Furthermore, reversal of drug resistance renders Pgp cells sensitive to caspase inhibitors abolishing therapeutic advantage. Thus, caspase inhibitors, that are inactive in multidrug-resistant cells, protect normal but not multidrug-resistant cells against chemotherapy, permitting selective eradication of multidrug-resistant cells. Clinical application of this approach may diminish the toxic side-effects of chemotherapy in patients with multidrug-resistant tumors.

A phase I trial of doxorubicin, paclitaxel, and valspodar (PSC 833), a modulator of multidrug resistance

PURPOSE

P-glycoprotein is an efflux pump for many drugs including doxorubicin and paclitaxel. This study evaluated the coadministration of these drugs with the P-glycoprotein inhibitor valspodar (PSC 833) with the aim of determining: (a) maximum tolerated doses (MTDs) of doxorubicin followed by paclitaxel (DP); (b) the MTD of DP combined with PSC 833 (DPV), without and with filgrastim (G-CSF); and (c) the pharmacokinetic interactions of PSC 833 with doxorubicin and paclitaxel.

EXPERIMENTAL DESIGN

For the first cycle, patients received doxorubicin as a 15-min infusion followed by paclitaxel as a 1-h infusion. For the second cycle, patients received reduced doses of DP with PSC 833 at 5 mg/kg p.o., four times a day for 12 doses.

RESULTS

Thirty-three patients with various refractory malignancies were enrolled and assessable. The MTD of DP without PSC 833 was 35 mg/m(2) doxorubicin and 150 mg/m(2) paclitaxel. The MTD of DPV without G-CSF was 12.5 mg/m(2) doxorubicin and 70 mg/m(2) paclitaxel. The dose-limiting toxicity for both DP and DPV was neutropenia without thrombocytopenia. With G-CSF, the MTD for DPV was 20 mg/m(2) doxorubicin and 90 mg/m(2) paclitaxel. No grade 4 nonhematological toxicities were observed. Five partial and two minor tumor remissions were observed. Paired pharmacokinetics with and without PSC 833 revealed substantial drug interactions with both doxorubicin and paclitaxel.

CONCLUSIONS

PSC 833 can be administered safely with doxorubicin and paclitaxel. The pharmacokinetic profiles of these drugs are significantly affected by PSC 833, requiring approximately 60% dose reductions for equivalent degrees of myelosuppression.

Evolving treatment strategies for inflammatory bowel disease

Crohn's disease and ulcerative colitis are idiopathic inflammatory bowel diseases characterized by dysregulated intestinal immune responses in genetically susceptible hosts. Conventional approaches to the medical therapy of ulcerative colitis and Crohn's disease can now be directed at either induction or maintenance of remission to improve therapeutic efficacy while minimizing complications. Newer approaches have expanded the utility of conventional therapies by improving both safety and efficacy and highlight the importance of specific targets along the immunoinflammatory pathways. The combination of conventional and novel approaches now offers the potential of modifying the natural history of these diseases.

Dexrazoxane significantly impairs the induction of doxorubicin resistance in the human leukaemia line, K562

Dexrazoxane combined with doxorubicin (+ 5-fluorouracil + cyclophosphamide - the FAC regime) leads to a significant decrease in doxorubicin cardiotoxicity and a significant increase in median survival time for patients with advanced breast cancer responsive to FAC. The reason for this increase in survival may be due to interference with the mechanism involved in the emergence of multidrug resistance (MDR). In order to test this hypothesis, we induced resistance to doxorubicin in the K562 cell line by growing cells in increasing concentrations of doxorubicin (10-30 nM) in the presence and absence of dexrazoxane (20 nM). The doxorubicin sensitivity of all resultant sublines was measured using the MTT assay. Flow cytometry was used to assess the MDR1 phenotype, measuring P-glycoprotein expression with MRK 16 antibody and drug accumulation in the presence and absence of PSC 833 for functional P-glycoprotein. Long-term growth in doxorubicin increased the cellular resistance (IC(50)) of K562 cells in a concentration-dependent manner (r(2 )= 0.908). Doxorubicin resistance was not induced in the presence of dexrazoxane (P< 0.0001) for several months. In parallel, the expression of functional P-glycoprotein was delayed after concomitant addition of dexrazoxane to the selecting medium (P< 0.001). Dexrazoxane did not act as a conventional modulator of P-glycoprotein. These results suggest that dexrazoxane may delay the development of MDR1, thus allowing responders to the FAC regime to continue to respond.

Phase I/II study of the P-glycoprotein modulator PSC 833 in patients with acute myeloid leukemia

PURPOSE

To determine the maximum-tolerated dose, pharmacokinetic interaction, and activity of PSC 833 compared with daunorubicin (DNR) and cytarabine in patients with poor-risk acute myeloid leukemia.

PATIENTS AND METHODS

Patients received ara-C 3 g/m(2)/d on 5 consecutive days, followed by an IV loading dose of PSC 833 (1.5 mg/kg) and an 84-hour continuous infusion escalating from 6, 9, or 10 mg/kg/d. Daunorubicin was administered as a 72-hour continuous infusion at 34 or 45 mg/m2/d [corrected]. Responding patients received consolidation chemotherapy with DNR pharmacokinetics performed without PSC-833 on day 1, and with PSC-833 on day 4. Response was correlated with expression of P-glycoprotein and lung resistance protein (LRP), and in vitro sensitization of leukemia progenitors to DNR cytotoxicity by PSC 833.

RESULTS

All 43 patients are assessable for toxicity and response. Grade 3 or greater hyperbilirubinemia (70%) was the only dose-dependent toxicity. Four patients (9%) succumbed to treatment-related complications. Twenty-one patients (49%) achieved a complete remission or restored chronic phase, including 10 of 20 patients treated at the maximum-tolerated dose of 10 mg/kg/d of PSC-833 and 45 mg/m(2) of DNR. The 95% confidence interval for complete response was 33.9% to 63.7%. Administration of PSC 833 did not alter the mean area under the curve for DNR, although clearance decreased approximately two-fold (P =.04). Daunorubicinol clearance decreased 3.3-fold (P =.016). Remission rates were not effected by mdr-1 expression, but LRP overexpression was associated with chemotherapy resistance.

CONCLUSION

Combined treatment with infused PSC 833 and DNR is well tolerated and has activity in patients with poor risk acute myeloid leukemia. Administration of PSC 833 delays elimination of daunorubicinol, but yields variable changes in DNR systemic exposure.

Assessment of active transport of HIV protease inhibitors in various cell lines and the in vitro blood--brain barrier

OBJECTIVE

To investigate the involvement of P-glycoprotein (Pgp) and the multidrug resistance-associated protein (MRP) on the active transport of the HIV protease inhibitors amprenavir, ritonavir and indinavir.

METHODS

The transport behaviour of ritonavir, indinavir and amprenavir in the presence and absence of Pgp modulators and probenecid was investigated in an in vitro blood--brain barrier (BBB) co-culture model and in monolayers of LLC-PK1, LLC-PK1:MDR1, LLC-PK1:MRP1 and Caco-2 cells.

RESULTS

All three HIV protease inhibitors showed polarized transport in the BBB model, LLC-PK1:MDR1 and Caco-2 cell line. The Pgp modulators SDZ-PSC 833, verapamil and LY 335979 inhibited polarized transport, although their potency was dependent on both the cell model and the HIV protease inhibitor used. Ritonavir and indinavir also showed polarized transport in the LLC-PK1 and LLC-PK1:MRP1 cell line, which could be inhibited by probenecid. HIV protease inhibitors were not able to inhibit competitively polarized transport of other HIV protease inhibitors in the LLC-PK1:MDR1 cell line.

CONCLUSIONS

Amprenavir, ritonavir and indinavir are mainly actively transported by Pgp, while MRP also plays a role in the transport of ritonavir and indinavir. This indicates that inhibition of Pgp could be useful therapeutically to increase HIV protease inhibitor concentrations in the brain and in other tissues and cells expressing Pgp. The HIV protease inhibitors were not able to inhibit Pgp-mediated efflux when given simultaneously, suggesting that simultaneous administration of these drugs will not increase the concentration of antiretroviral drugs in the brain.

A reexamination of PSC 833 (Valspodar) as a cytotoxic agent and in combination with anticancer agents

BACKGROUND

The cyclosporins have been thought as being mainly immunosuppressive agents which interfere with the function of the MDR pump and thus play a role in resistance to drug anticancer effects. We reexamined their cytotoxicity in defined cell lines both as single agents and in combination with agents which may be of value in human malignant disease.

METHODS

Cells were grown to confluence following inoculation at 5,000-8,000 cells/well in 96-well dishes, and growth patterns and death were determined by an MTT assay. Median effect analysis was used to look for synergy, additive effects, or antagonism between the cyclosporins and drugs with antitumor effects in humans.

RESULTS

Cyclosporin A and PSC 833 were found to have cytotoxic activity at clinically achievable concentrations in breast, leukemia, and prostate cell lines. Synergistic or additive effects were demonstrated in all three prostate cell lines when PSC 833 was combined with estramustine, etoposide, ketoconazole, suramin, or vinorelbine in the prostate cancer cell lines. Cell line-selective additive effects or synergism were also identified with bicalutamide, carboplatin, cisplatinum, cis-retinoic acid, dexamethasone, 5-fluorouracil, liarozole, and trans-retinoic acid.

CONLCLUSIONS

PSC 833 or cyclosporin alone or in combination with other agents may have an anticancer effect independently of their modulatory action on MDR. Several of the synergistic combinations which are not mediated by the MDR pump need to be tested in vivo for efficacy.

P-glycoprotein-mediated transport of digitoxin, alpha-methyldigoxin and beta-acetyldigoxin

Digoxin is a drug with a narrow therapeutic index, which is substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidneys. Whereas particle size, dissolution rate and lipophilic properties have been identified as determinants for absorption of digitalis glycosides, little is known about P-glycoprotein transport characteristics of digitalis glycosides such as digitoxin, alpha-methyldigoxin, beta-acetyldigoxin and ouabain. Using polarized P-glycoprotein-expressing cell lines we therefore studied whether these compounds are substrates of P-glycoprotein. Polarized transport of digitalis glycosides was assessed in P-glycoprotein-expressing Caco-2 and L-MDR1 cells (LLC-PK1 cells stably transfected with the human MDR1 P-glycoprotein). Inhibition of P-glycoprotein-mediated transport of these compounds in Caco-2 cells was determined using the cyclosporine analogue PSC-833 (valspodar) as inhibitor of P-glycoprotein. No polarized transport was observed for ouabain. However, basal-to-apical transport of digitoxin, alpha-methyldigoxin and beta-acetyldigoxin was greater than apical-to-basal transport in Caco-2 and L-MDR1 cells. In Caco-2 cells net transport rates of these compounds were similar to those of digoxin (digoxin: 16.0+/-4.4%, digitoxin: 15.0+/-3.3%, beta-acetyldigoxin: 16.2+/-1.6%, alpha-methyldigoxin: 13.5+/-4.8%). Furthermore, polarized transport of these compounds could be completely inhibited by 1 microM PSC-833. In summary, these data provide evidence that not only digoxin, but also digitoxin, alpha-methyldigoxin and beta-acetyldigoxin are substrates of P-glycoprotein.

Constitutive and acquired resistance to calcineurin inhibitors in renal transplantation: role of P-glycoprotein-170

The present study examines whether resistance to Cyclosporin A (CyA) and Tacrolimus (FK506) develops in T cells from individual patients and the role of P-glycoprotein 170 (P-gp) in mediating drug resistance. IC50s were established for CyA and FK506 in cell cultures from 46 renal allograft recipients. P-gp expression and functional activity were determined by flow cytometry. Mean ID50 for CyA was 29 microg/li (range 2.5-100) and for FK506 1.2 microg/li (range 0.085-5.5). The sensitivities to the two drugs were correlated (P = 0.0001). There was variation in the ratio of the ID50s depending on the drug used for treatment (P = 0.02). There was no difference in P-gp expression and functional activity in patients with sensitive or resistant cells. The data indicate an association between the sensitivities to CyA and FK506 and evidence of selective resistance to whichever drug was used. P-gp drug transport does not explain this variation.

Comparison of (99m)Tc-sestamibi and doxorubicin to monitor inhibition of P-glycoprotein function

P-glycoprotein (Pgp) overexpression is a well-recognized factor in resistance to chemotherapy. Doxorubicin flow cytometry is used to monitor Pgp function in haematological specimens and biopsies from other cancers, and radionuclide imaging with sestamibi has recently shown promise for non-invasive monitoring. In the present study the two methods were directly compared in single-cell suspensions of three variants of the human breast carcinoma cell line MCF7: sensitive MCF7/WT, doxorubicin-selected MCF7/AdrR, and MDR1-gene-transfected MCF7/BC19 cells with doxorubicin resistance factors of 1, 192, and 14, respectively. Accumulation of sestamibi and mean fluorescence of doxorubicin (5.5 microM) were assessed over 60 min in the presence and absence of Pgp modulators GG918 (0.01 to 0.2 microM) and PSC833 (0.05 to 2.0 microM). Accumulation curves for sestamibi and doxorubicin differed among the cell variants under control conditions, with sestamibi showing a significantly greater difference between WT and resistant cells than doxorubicin. Both GG918 and PSC833 reversed uptake deficits to WT levels for sestamibi in MCF7/BC19 cells and doxorubicin in MCF7/BC19 and MCF7/AdrR cells, but failed to show the same effect for sestamibi in MCF7/AdrR cells (approximately 30% of MCF7/WT level). Thus, both methods clearly distinguished sensitive from resistant MCF7 variants, with the radionuclide method showing greater sensitivity.

Screening of multidrug-resistance sensitive drugs by in situ brain perfusion in P-glycoprotein-deficient mice

PURPOSE

This study was conducted to assess the influence of P-glycoprotein (P-gp) on brain uptake of multidrug resistance sensitive drugs using an in situ brain perfusion technique in P-gp-deficient (mdr1a[-/-]) and wild-type mice.

METHODS

The blood-brain transport of radiolabeled vinblastine, vincristine, doxorubicin, colchicine, and morphine was evaluated in mdr1a(-/-) and wild-type CF-1 mice with the in situ brain perfusion technique. Brain uptake of drugs after intravenous pretreatment with P-gp reversal agents, (PSC 833, GF 120918, or (+/-)-verapamil), or vehicle also was studied in wild-type mice. In all experiments, cerebral vascular volume was determined by co-perfusion of sucrose.

RESULTS

Cerebral vascular volume was preserved during perfusion, indicating maintenance of blood-brain barrier integrity in both types of mice within the concentration range of substrates in the perfusate. The apparent brain transport of colchicine. vinblastine, doxorubicin, and morphine was increased 3.0, 2.7, 1.5, and 1.4-fold, respectively, in mdr1a(-/-) mice compared with the wild-type: the brain uptake of vincristine was not affected by P-gp. Preadministration of PSC 833 or GF 120918 in wild-type mice led to a -3-fold increase in the brain transport of colchicine and vinblastine, but no effect was observed for the other compounds. Intravenous verapamil enhanced colchicine brain transport (1.8-fold), but failed to increase the brain uptake of vinblastine and morphine.

CONCLUSION

The in situ brain perfusion technique appears to be a sensitive and powerful tool for medium throughput screening of the brain uptake of multidrug resistance sensitive drugs. The effect of P-gp is characterized more efficiently with mdr1a(-/-) mice than by using modulators of P-gp in wild-type mice.

In vitro and in vivo reversal of P-glycoprotein-mediated multidrug resistance by a novel potent modulator, XR9576

The overexpression of P-glycoprotein (P-gp) on the surface of tumor cells causes multidrug resistance (MDR). This protein acts as an energy-dependent drug efflux pump reducing the intracellular concentration of structurally unrelated drugs. Modulators of P-gp function can restore the sensitivity of MDR cells to such drugs. XR9576 is a novel anthranilic acid derivative developed as a potent and specific inhibitor of P-gp, and in this study we evaluate the in vitro and in vivo modulatory activity of this compound. The in vitro activity of XR9576 was evaluated using a panel of human (H69/LX4, 2780AD) and murine (EMT6 AR1.0, MC26) MDR cell lines. XR9576 potentiated the cytotoxicity of several drugs including doxorubicin, paclitaxel, etoposide, and vincristine; complete reversal of resistance was achieved in the presence of 25-80 nM XR9576. Direct comparative studies with other modulators indicated that XR9576 was one of the most potent modulators described to date. Accumulation and efflux studies with the P-gp substrates, [3H]daunorubicin and rhodamine 123, demonstrated that XR9576 inhibited P-gp-mediated drug efflux. The inhibition of P-gp function was reversible, but the effects persisted for >22 h after removal of the modulator from the incubation medium. This is in contrast to P-gp substrates such as cyclosporin A and verapamil, which lose their activity within 60 min, suggesting that XR9576 is not transported by P-gp. Also, XR9576 was a potent inhibitor of photoaffinity labeling of P-gp by [3H]azidopine implying a direct interaction with the protein. In mice bearing the intrinsically resistant MC26 colon tumors, coadministration of XR9576 potentiated the antitumor activity of doxorubicin without a significant increase in toxicity; maximum potentiation was observed at 2.5-4.0 mg/kg dosed either i.v. or p.o. In addition, coadministration of XR9576 (6-12 mg/kg p.o.) fully restored the antitumor activity of paclitaxel, etoposide, and vincristine against two highly resistant MDR human tumor xenografts (2780AD, H69/LX4) in nude mice. Importantly all of the efficacious combination schedules appeared to be well tolerated. Furthermore, i.v. coadministration of XR9576 did not alter the plasma pharmacokinetics of paclitaxel. These results demonstrate that XR9576 is an extremely potent, selective, and effective modulator with a long duration of action. It exhibits potent i.v. and p.o. activity without apparently enhancing the plasma pharmacokinetics of paclitaxel or the toxicity of coadministered drugs. Hence, XR9576 holds great promise for the treatment of P-gp-mediated MDR cancers.