The clinical relevance of multidrug resistance

Reversal effect of Tween-20 on multidrug resistance in tumor cells in vitro

Multidrug resistance (MDR) is a major barrier for chemotherapy of many cancers. Non-ionic surfactants have great potential to reverse the MDR by preventing onset or delay progression of the carcinogenic process. However, the role of Tween-20 in the development of MDR remains unknown. The aim of this study was to explore the reversal effect and potential mechanism of Tween-20 on tumor cells in vitro. Alamar Blue assay was used to examine the reversal index of Tween-20 to vincristine (VCR), doxorubicin (DOX) and 5-fluorouracil (5-FU) in KBv200, HepG2/R and Bel-7402/5-FU, respectively. Morphological change was determined by Gimsa and Hoechst 33258 staining. The acumulation of DOX was confirmed by spectrofluorimetric assay. Cell cycle analysis was performed using flow cytometry. The mRNA and protein expression levels of MDR were assessed by semiquantitative RT-PCR and dot blot, respectively. The results showed that Tween-20 at concentrations of 0.0025%, 0.005%, 0.01% had little cytotoxicity. When combined with the cancer drugs, it significantly promoted the sensitivity of MDR cells. Fluorescence staining confirmed that the percentage of apoptotic cell increased when combined with Tween-20. This notion was further supported by the observation that Tween-20 treatment potentiated VIN-induced G2/M arrest of the cell cycle. Furthermore, Tween-20 treatment increased significantly intracellular accumulation of DOX. RT-PCR and dot blot revealed that Tween-20 could downregulate the expression of MDR and P-glycoprotein. Low concentrations of Tween-20 can efficiently reverse the multidrug resistance phenotype by enhancing accumulation of the anticancer drugs. The potential mechanism may be via inhibiting the multidrug-resistant gene expression.

The Multixenobiotic Resistance Mechanism in Aquatic Organisms

Many aquatic organisms thrive and reproduce in polluted waters. This fact indicates that they are well equipped with a defense system(s) against several toxic xenobiotics simultaneously because water pollution is typically caused by a mixture of a number of pollutants. We have found that the biochemical mechanism underlying such "multixenobiotic" resistance in freshwater and marine mussel, in several marine sponges, and in freshwater fish is similar to the mechanism of multidrug resistance (MDR) found in tumor cells that became refractory to treatment with a variety of chemotherapeutic agents. All these organisms possess a verapamil-sensitive potential to bind 2-acetylaminofluorene and vincristine onto membrane vesicles. They all express mRNA for mdr1 gene, and mdr1 protein product, the glycoprotein P170. Finally, in in vivo experiments, the accumulation of xenobiotics is enhanced in all investigated organisms in the presence of verapamil, the inhibitor of the P170 extrusion pump. The knowledge that the presence of one xenobiotic may block the pumping out, and hence accelerating accumulation, of others, may help us to understand and interpret our present and past data on different environmental parameters obtained using indicator organisms.

Expression of heat-shock proteins is associated with major adverse prognostic factors in acute myeloid leukemia

To identify prognostic factors alternative or additional to drug-resistance and apoptosis proteins, we studied the impact of the expression of heat-shock proteins (HSPs) in 98 newly diagnosed acute myeloid leukemia (AML). HSP27 was expressed by 39%, HSP60 by 26%, HSP70 by 58%, HSP90 by 41%, and HSP110 by 30% of cases. HSP expressions were correlated with that of differentiation antigens (CD34, CD14, CD15, CD33) and that of drug-resistance (MRP, MRK) and apoptosis (Bcl-2) proteins. HSP90 and HSP110 were correlated with FAB subtype and karyotypic grouping. Complete remission (CR) was obtained in 68 cases (69%). Median disease-free survival (DFS) of the 68 remitters was 18.1 months with a 3-year DFS rate of 41%. CR rates were higher in patients with lower expression of HSPs. Overall survival (OS) was significantly longer in patients with lower expression of HSPs. Cytogenetics, CD34 positive expression, MRK positive expression, and HSP110 positive expression remained as pejorative prognostic factors for OS in the multivariate analysis. When considering patients with intermediate risk cytogenetics, HSP110 and MRP positive expressions and CD33 negative expression were of poor outcome, while HSP27 and HSP60 positive expressions appeared of pejorative prognostic value in patients with unfavorable karyotypes.

Repetitive doxorubicin treatment of glioblastoma enhances the PGP expression--a special role for endothelial cells

The glioblastoma is the highest dedifferentiated form of astrocytic brain tumors, which is refractory to chemotherapy in most cases. The lack of chemotherapeutic success is correlated with overexpression of the product P-glycoprotein (PGP) coded by the multidrug resistance 1 (MDR1) gene and a subsequent release of drugs from the tumor cells. For the chemotherapeutical treatment of glioblastomas, the endothel cell is of special importance since due to its manifold metabolic and protective tasks within the blood-brain barrier, it already has a relatively high PGP expression under physiological conditions. The aim of the present study was to analyze the uptake of the antimitotic drug Doxorubicin (DOX) and the expression of PGP in human and rat glioblastoma cell lines and in a human endothelial cell line at different time points. In the following in vivo approach DOX enriched glioblastoma cells were transplanted into rats and the developed tumor was investigated histologically. The results showed an increased uptake and an enhanced expression of PGP at certain time points in every cell line. In the tissue a DOX release was mainly observed in perivascular surroundings. It was concluded that DOX enhanced the constitutive PGP expression which led to a subsequent exclusion of DOX in tumor cells but also in the endothelial cells of the tumor vasculature. Since the vascularization is a prerequisite for tumor growth, the inhibition of the PGP expression in tumor endothelial cells might be a clinical approach to make the DOX treatment more effective.

Modulation of P-glycoprotein-mediated multidrug resistance by flavonoid derivatives and analogues

Flavonoid derivatives were synthesized and tested for their ability to modulate P-glycoprotein (Pgp)-mediated multidrug resistance (MDR) in vitro. These compounds belong to various flavonoid subclasses, namely: chromones, azaisoflavones, and aurones. Among the investigated compounds, three showed potent reversing activity. 2-(4-methylpiperazin-1-ylcarbonyl)-5-hydroxychromone (4a), 5,7-dimethoxy-3-phenyl-4-quinolone (5), and 4,6-dimethoxyaurone (6) potentiated daunorubicin cytotoxicity on resistant K562 cells. They were also able to increase the intracellular accumulation of rhodamine-123, a fluorescent molecule which acts as a probe of P-glycoprotein-mediated MDR. This suggests that these compounds act, at least in part, by inhibiting P-glycoprotein activity. The most active compound, 5-hydroxy-2-(4-methylpiperazin-1-ylcarbonyl)chromone (4a) was found to be a powerful reversal agent, more potent than cyclosporin A, used as the reference molecule. No effect was observed on MRP transport nor on cell proliferation. Little apoptosis was induced on K562S cells with 4a compared to K562R, probably due to the extrusion of the compound by Pgp.

PET for in vivo pharmacokinetic and pharmacodynamic measurements

Positron emission tomography (PET) scanning is evolving as a unique tool for drug development in oncology for improving both the efficacy of established treatment and in evaluating novel anticancer agents. As a non-invasive functional imaging modality, PET has an unrivalled sensitivity when monitoring the pharmacokinetics and pharmacodynamics of drugs and biochemicals when radiolabelled with short living positron-emitting radioisotopes. This is of particular relevance in assessing newer molecular-targeted therapy where conventional evaluation criteria (maximum tolerated dose and tumour shrinkage for example) may be inappropriate. PET has already been applied to a wide number of drugs to demonstrate activity in vivo from standard chemotherapy such as 5-fluorouracil (5-FU) [J Clin Oncol 17 (1999) 1580], to novel molecular agents such as those involved in tumour angiogenesis [Br J Cancer 83 (2000) P6] and antivascular therapy [Proc Annu Meet Am Soc Clin Oncol 19 (2000) 179a]. This review will evaluate the achievements of PET in the drug development process, an approach that promises to facilitate the rapid translation of scientific research into current clinical practice.

Recent advances in the discovery of flavonoids and analogs with high-affinity binding to P-glycoprotein responsible for cancer cell multidrug resistance

P-glycoprotein (P-gp) is a plasma membrane glycoprotein that confers multidrug resistance on cells by virtue of its ability to exclude cytotoxic drugs in an ATP-dependent manner. The most commonly considered hypothesis is that P-gp acts as an ATP-driven drug-export pump, the mechanism of which is not understood in detail. Therefore, a tremendous effort is being made to find out modulator molecules to inhibit P-gp. We have been developing flavonoid derivatives as a new class of promising modulators using a new in vitro rational-screening assay based on measurements of the binding-affinity toward the C-terminal nucleotide-binding domain (NBD2) of P-gp. This review is focused on our results obtained with a variety of flavonoids. Structure-activity relationships of flavonoids as potential MDR modulators are reported.

Assessment of caspase activity as a possible prognostic factor in acute myeloid leukaemia

Caspases are one of the key effector molecules in apoptosis. Caspase-3 activity (identified by cleavage of the peptide DEVD) was analysed in bone marrow blasts (minimum 70%) from 45 acute myeloid leukaemia (AML) patients. 12 patients (25%) exhibited high levels of specific DEVDase activity. These blast cells, despite having activated caspase-3, displayed none of the classical caspase-dependent morphological characteristics of apoptosis, such as degradation of DNA fragmentation factor 45, DNA fragmentation, and appeared to be more resistant to drug-induced apoptosis. Our results suggest that in these AML cells, resistance to apoptosis occurred downstream of caspase-3 activation.

Induction of chemoresistance in HL-60 cells concomitantly causes a resistance to apoptosis and the synthesis of P-glycoprotein

The appearance of multidrug-resistant (MDR) proteins or the acquisition of a defective apoptotic programme are major drawbacks in the treatment of cancers since both induce a resistance to classical chemotherapy. However, a link between the two mechanisms has not, as yet, been clearly established. In this study, HL-60 cells cultured in the continual presence of a sub-lethal dose of doxorubicin (dox; HL-60/Dox) were used as a model to study acquired chemoresistance. During the induction of chemoresistance, the appearance of a functional P-glycoprotein (P-gp), in addition to the expression of anti-apoptotic Bcl-2, Bcl-XL and pro-apoptotic Bax proteins was assessed. Parental cells which are sensitive to dox, have no P-gp activity and express Bcl-2 and Bax. After 4 weeks of treatment, a functional P-gp was detected in HL-60/Dox cells. In addition, the synthesis of Bcl-2 appeared to be replaced by Bcl-XL while that of Bax remained unchanged. These cells were also resistant to apoptosis induced by both P-gp and non-P-gp substrates. This inability to induce apoptosis could have resulted from the induction of the expression of the inhibitor of apoptosis protein (XIAP). Our data show that acquired chemoresistance could involve a parallel induction of P-gp and an impairment of the apoptotic pathway.

Reversal of anticancer drug resistance by macrolide antibiotics in vitro and in vivo

1. The combined effects of the macrolide antibiotics erythromycin, josamycin, clarithromycin and YM17K (3,4'-dideoxy mycaminosyl tylonolide hydrochloride) on in vitro intracellular accumulation of vinblastine or cyclosporine (Cs)A and on the in vivo antitumour activity of vinblastine were investigated using mouse leukaemia P388 cells (P388/S) and anticancer drug-resistant (P388/ADR) cells. These effects were compared with those of a calcium antagonist (verapamil) or immunosuppressants (FK506 and CsA). 2. All tested macrolide antibiotics increased the accumulation of both vinblastine and CsA in P388/ADR cells in a dose-dependent manner, but their potency was lower than that of verapamil, CsA or FK506. 3. When vinblastine (200 microg/kg) was administered intraperitoneally with each of the macrolide antibiotics (10 or 100 mg/kg) or with verapamil (25 mg/kg) once a day for 10 days in P388/ADR-bearing mice, combined effects of vinblastine with the macrolide antibiotics (erythromycin, clarithromycin and YM17K) or verapamil were observed. 4. The present study suggests that macrolide antibiotics may overcome anticancer drug resistance by inhibiting the binding of vinblastine or CsA to P-glycoprotein in P388/ADR cells. 5. We believe that these results are encouraging for combination chemotherapy to overcome P-glycoprotein-dependent anticancer drug-resistant tumours in clinical practice.

In vivo monitoring of drugs using radiotracer techniques

There is an increasing realization of the role of non-invasive monitoring of drug pharmacology. In this review, we discuss the role of positron emission tomography in such monitoring of tumour and normal tissue drug pharmacokinetics as well as assessment of tumour response, drug-receptor interactions and mechanisms of drug action and resistance. These studies represent a multidisciplinary research effort involving radiochemists, imaging scientists, clinicians, pharmacologists and mathematical modellers. This review evaluates achievements in the field from assessment of commonly used therapeutic agents such as 5-fluorouracil to target specific molecules such as markers for gene expression. It is envisaged that application of this technology will facilitate rational drug design and rapid translation of new ideas to the bedside.

Inhibition of P-glycoprotein activity and reversal of multidrug resistance in vitro by rosemary extract

The transmembrane transport pump P-glycoprotein (Pgp) causes the efflux of chemotherapeutic agents from cells and is believed to be an important mechanism in multidrug resistance (MDR) in mammary tumours. In the present study we demonstrate that an extract of the common dietary herb rosemary (Rosemarinus officinalis Labiatae), increases the intracellular accumulation of commonly used chemotherapeutic agents, including doxorubicin (DOX) and vinblastine (VIN), in drug-resistant MCF-7 human breast cancer cells which express Pgp. Rosemary extract (RE) inhibits the efflux of DOX and VIN, which are known to be substrates of Pgp, but does not affect accumulation or efflux of DOX in wild type MCF-7 cells, which lack Pgp. Treatment of drug-resistant cells with RE increases their sensitivity to DOX, which is consistent with an increased intracellular accumulation of the drug. RE blocks the binding of the VIN analogue azidopine to Pgp. Thus, it appears that RE directly inhibits Pgp activity by inhibiting the binding of drugs to Pgp.

Multiple cytochrome P-450 subfamilies are co-induced with P-glycoprotein by both phenothiazine and 2-acetylaminofluorene in rats

We studied the effects of two P-glycoprotein (P-gp) inducers, 2-acetylaminofluorene (2-AAF) and phenothiazine (PTZ), administered intraperitoneally, on the activities and content of hepatic cytochrome P-450 (CYP) subfamilies in hepatic microsomes of Sprague-Dawley rats. After 4-day administration of 2-AAF or PTZ, the P-gp content was increased. The total CYP content after PTZ treatment was significantly increased compared with that of controls. The CYP1A, CYP2B and CYP3A2 contents were induced, while the CYP2C6, CYP2C11 and CYP2E1 contents remained unaffected. A marked increase in CYP1A1 was found after administration of each compound. Ethoxyresorufin O-deethylase, pentoxyresorufin O-deethylase, and testosterone 6beta hydroxylation activities showed a significant increase after both 2-AAF and PTZ treatments. In particular, ethoxyresorufin O-deethylase exhibited more than ten times greater activity than that of the controls after the treatments. These results suggest that P-gp inducers affect several CYP subfamilies in addition to CYP3A, which is reported to be up-regulated coordinately with P-gp by a CYP3A inducer.

Presence of classical multidrug resistance and P-glycoprotein expression in human neuroblastoma cells

BACKGROUND

The role of P-glycoprotein (Pgp) associated multidrug resistance for neuroblastoma patients is controversial. Therefore we asked whether at all the typical functional features of the multidrug resistance phenotype could be found in neuroblastoma cells and studied the prognostic relevance of Pgp expression.

PATIENTS AND METHODS

Tumor touch preparations and tumor cell infiltrated bone marrow smears of 62 neuroblastoma patients were investigated. The expression of Pgp was determined by a highly sensitive immunosandwich technique. Drug resistance studies were performed by exposing cells to Pgp-dependent cytostatic drugs in tissue cultures. Intracellular drug accumulation was examined by rhodamine-123 fluorescence microscopy.

RESULTS

Pgp expression was demonstrable for the SK-N-SH cell line, but not detectable in CHP-100 and ten other neuroblastoma cell lines by immunocytochemistry. In tissue cultures, SK-N-SH cells showed a relative resistance to vincristine and adriamycin (45.1 and 12.7-fold resp.) and reduced intracellular accumulation of rhodamine-123 which could be normalized by the Pgp blocker verapamil. Pgp expression was detected by immunocytochemistry in 14 out of 62 tumors (22.6%). No correlation was found to the stage of the disease (P = 0.33), histopathological grading (P = 0.82), N-myc oncoprotein expression (P = 0.76) or N-myc oncogene amplification (P = 0.20). Kaplan-Meier analysis of event free survival for stage 4 tumors revealed a weak trend of inferior survival for patients with Pgp positive tumors (log-rank analysis: P = 0.069).

CONCLUSIONS

Though Pgp expression is detectable and functional in neuroblastoma cells, but its presence does not provide much information to the complex phenomenon of chemotherapy resistance in patients.

Enzymatic synthesis of [4-methoxy-11C]daunorubicin for functional imaging of P-glycoprotein with PET

One of the mechanisms for multidrug resistance (MDR) of tumors is an overexpression of the P-glycoprotein (P-gp). The cytostatic agent daunorubicin was labeled with carbon-11 to probe P-gp with PET. An enzymatic route for the conversion of carminomycin to [4-methoxy-11C]daunorubicin ([4-methoxy-11C]DNR) was investigated, since attempts failed to prepare daunorubicin chemically using [11C]methyl iodide. In the enzymatic synthesis methylation was accomplished by S-adenosyl-L-[methyl-11C]methionine ([11C]SAM), which was synthesized from L-[methyl-11C]methionine. This methylation is catalyzed by carminomycin-4-O-methyltransferase (CMT). The overall radiochemical yield of [4-methoxy-11C]DNR is 1% (EOB), with a total synthesis time of 75 min. In conclusion, [4-methoxy-11C]DNR can be successfully prepared from carminomycin and [11C]SAM using enzymes.

Modulating effect of lonidamine on response to doxorubicin in metastatic breast cancer patients: results from a multicenter prospective randomized trial

Previous results from our preclinical studies have shown that lonidamine (LND) can positively modulate the antiproliferative activity of doxorubicin (DOX) on breast cancer cell lines. To evaluate the effect of LND in a clinical setting, a multicenter randomized trial was carried out on patients with advanced breast cancer. From September 1991 to July 1993, 181 patients were enrolled in the trial and received an initial treatment of DOX at 75 mg/m2 for 3 cycles. The 137 patients who reached complete remission, partial remission, or stable disease were randomized to receive either DOX alone (75 mg/m2 day 1) (arm A) or DOX plus LND (600 mg orally/day) (arm B). The patients enrolled in the two arms were fairly homogeneous in terms of major clinical characteristics. Toxicity was similar in both arms except for myalgia: WHO grade > or=2 was observed in 57% of arm B patients. Overall response rate to DOX + LND was 50% and to DOX alone 38% in evaluable patients, and 48% vs 37% in all registered patients, as determined by an intention-to-treat analysis. The differences did not reach statistical significance. Conversely, in agreement with previous findings, we observed a significant difference in response rate in the subgroup of patients with liver metastases, regardless of the extent of hepatic involvement (DOX + LND 68% vs DOX 33%, p=0.03). This observation makes LND an important tool in association with anthracyclines in the treatment of this subgroup of patients.

Substantial excretion of digoxin via the intestinal mucosa and prevention of long-term digoxin accumulation in the brain by the mdr 1a P-glycoprotein

1. We have used mice with a disrupted mdr 1a P-glycoprotein gene (mdr 1a (-/-) mice) to study the role of P-glycoprotein in the pharmacokinetics of digoxin, a model P-glycoprotein substrate. 2. [3H]-digoxin at a dose of 0.2 mg kg-1 was administered as a single i.v. or oral bolus injection. We focussed on intestinal mucosa and brain endothelial cells, two major pharmacological barriers, as the mdr 1a P-glycoprotein is the only P-glycoprotein normally present in these tissues. 3. Predominant faecal excretion of [3H]-digoxin in wild-type mice shifted towards predominantly urinary excretion in mdr 1a (-/-) mice. 4. After interruption of the biliary excretion into the intestine, we found a substantial excretion of [3H]-digoxin via the gut mucosa in wild-type mice (16% of administered dose over 90 min). This was only 2% in mdr 1a (-/-) mice. Biliary excretion of [3H]-digoxin was not dramatically decreased (24% in wild-type mice versus 16% in mdr 1a (-/-) mice). 5. After a single bolus injection, brain levels of [3H]-digoxin in wild-type mice remained very low, whereas in mdr 1a (-/-) mice these levels continuously increased over a period of 3 days, resulting in a approximately 200 fold higher concentration than in wild-type mice. 6. These data demonstrate the in vivo contribution of intestinal P-glycoprotein to direct elimination of [3H]-digoxin from the systemic circulation and to the pattern of [3H]-digoxin disposition, and they underline the importance of P-glycoprotein for the blood-brain barrier.

Primary breast cancer imaging with technetium-99m sestamibi and its relation with P-glycoprotein overexpression

The aim of this preliminary study was to evaluate retrospectively sestamibi scintigraphy in relation to the presence of the 170-kDa P-glycoprotein (Pgp), which represents an expression of multidrug resistance in patients with primary breast cancer. Fifteen women (age range 37-76 years) were referred for technetium-99m sestamibi scintigraphy because of suspicious breast lesions detected by mammography and ultrasonography, and subsequently assessed by fine-needle aspiration. Scintigraphy was performed 30 min following the injection of 500 MBq 99mTc-sestamibi. Three planar anterior and oblique images were obtained with the patient in the supine position. Excised tumours were assessed for cytosolic CA 15.3, oestrogen (OR) and progesterone (PR) receptors and c-erb B2 neu oncogene. Pathology revealed that only 13 of the 15 patients had malignant tumours. The two benign tumours were sestamibi-negative and Pgp-positive. Sestamibi scintigraphy was positive in 10 of the 13 malignant lesions (including nine of ten infiltrating ductal carcinomas). Two of the three lesions with false-negative scintigraphy were Pgp-negative; in one of these cases histology revealed an invasive lobular carcinoma and in the other, mucinous adenocarcinoma. The third false-negative lesion was a Pgp-positive infiltrating ductal carcinoma which was c-erb B2 neu-negative but CA 15.3-, OR- and PR-positive. This preliminary study confirms that the resistance to chemotherapy which may occur in patients with primary breast cancer can be a cause of negative sestamibi scintigraphy.

Cisplatin, epirubicin, and vindesine with or without lonidamine in the treatment of inoperable nonsmall cell lung carcinoma: a multicenter randomized clinical trial

BACKGROUND

Lonidamine (LND) is an indazol-carboxylic acid derivative that selectively inhibits the energy metabolism of neoplastic cells, and increases the permeability of cell membranes. In vitro studies have demonstrated that LND can potentiate the oncolytic activity of cytotoxic drugs and is able to reverse the acquired multidrug resistance of neoplastic cells. Some clinical trials have suggested a synergism of LND with alkylating agents, cisplatin, and anthracyclines in various solid tumors.

METHODS

From June 1990 to June 1993, 158 previously untreated patients with Stage IIIB and IV nonsmall cell lung cancer (NSCLC) were enrolled into a multicentric randomized trial to evaluate the addition of LND to a cisplatin-epirubicin-vindesine regimen. Eighty patients in the control arm (A) received cisplatin, 60 mg/m2 intravenously (i.v.); epirubicin, 60 mg/m2 i.v.; and vindesine, 3 mg/m2 i.v. (PEV), on Day 1 every 4 weeks, whereas 78 patients in the experimental arm (B) received the same regimen with the addition of LND from 75 mg orally three times on Day 1 to 150 mg orally three times on Day 7+ until tumor progression occurred.

RESULTS

The experimental treatment achieved a significantly higher proportion of major responses in comparison with the control regimen (43% vs. 24%; P=0.02). The addition of LND apparently potentiated the activity of this cytotoxic treatment, particularly in patients with metastatic disease (overall response rate, 39% vs. 17%). The median time to progression (5 vs. 8 months; P=0.0007) and the median survival time (7.6 vs. 11 months; P=0.0013) were also statistically improved in Arm B. The acute toxicity of the 2 treatments was low: only 6% of patients in Arm A and 4% of patients in Arm B had to withdraw from treatment due to Grade 4 World Health Organization toxicity. The main additional side effects related to the administration of LND were epigastralgia, myalgia, asthenia, and orchialgia. However, these symptoms were mild and controlled by the concomitant administration of low doses of steroids.

CONCLUSIONS

The mild acute toxicity of the PEV regimen and the acceptable and nonoverlapping additional side effects of LND render our experimental therapy worthy of consideration for the management of NSCLC patients with poor performance status or low tolerance to more aggressive therapeutic approaches.

Saturable function of P-glycoprotein as a drug-efflux pump in multidrug-resistant tumour cells

P-glycoprotein acts as an active drug-efflux pump in multidrug-resistant tumour cells. We studied the capacity of P-glycoprotein to extrude drugs from the cells. For nanomolar concentrations of vinblastine P388/ADR cells, which overexpress P-glycoprotein in the plasma membrane, accumulated vinblastine, at 37 degrees C for 30 min, to a much lower extent than the sensitive cells (P388/S), while in the micromolar range the cellular concentration was similar for both types of cells. When cells were incubated with a low (10 nM) or high concentration (1 microM) of vinblastine while energy deprived, the vinblastine concentration increased only in the resistant cells incubated with the low concentration of vinblastine, and this increased level was lowered to the level under the normal conditions by addition of glucose. In contrast, the cellular concentrations in other cases were increased to the normal level by glucose. After cells were loaded with the low concentration of vinblastine, the cellular vinblastine was extruded more rapidly from the resistant cells than from the sensitive cells. The courses of vinblastine efflux from the cells loaded with the high concentration of vinblastine were similar in both types of cells. NA-382, a reported P-glycoprotein inhibitor, effectively increased the intracellular vinblastine and inhibited the drug efflux only from multidrug-resistant cells, P388/ADR and AH66 cells, which were incubated with the low concentration of vinblastine. Cellular uptake of NA-382 was also less in P388/ADR cells than in P388/S cells in culture with 10 nM but not 1 microM of the agent, and this low level was reversed to the level in the sensitive cells by 10 microM vinblastine. These results indicate that P-glycoprotein as a drug-efflux pump works effectively under low extracellular concentrations of substrates, but does not under the high concentrations.

Antitumour effects and pharmacokinetics of combination of vinblastine with a staurosporine derivative, NA-382, in P388/ADR-bearing mice

The effects of a staurosporine derivative, N-ethoxycarbonyl-7-oxostaurosporine (NA-382), on the pharmacokinetics of vinblastine were evaluated, compared with those of verapamil, in multidrug-resistant P388/ADR-bearing mice. At first, the in-vitro experiments indicated that NA-382 permeated into the cells better and were more effective in combined cytotoxicity with vinblastine and on accumulation of vinblastine than with verapamil in P388/ADR cells. In combined intraperitoneal injection with vinblastine (200 micrograms kg-1) into P388/ADR-bearing mice, NA-382 in a suspension form (10 mg kg-1) prolonged the life-span of the mice near to that of P388/S-bearing mice treated with vinblastine alone, but verapamil even at the maximum tolerated dosage (30 mg kg-1) barely affected the in-vivo antitumour effect of vinblastine. When simultaneously administered with vinblastine to P388/ADR-bearing mice, NA-382 maintained significantly higher vinblastine levels in the tumour cells for 24 h and gave a larger area under the time-intracellular vinblastine concentration curve (0 to 24 h) than those receiving vinblastine alone, with long retention of the agent in ascitic fluid. Verapamil increased the cellular vinblastine content for only 6 h, accompanying a rapid elimination of the agent from the ascitic fluid. This study indicates that NA-382 is more effective against multidrug-resistance than verapamil, and its suspension is also advantageous for cancer chemotherapy of multidrug-resistant tumours.

Collateral sensitivity of multidrug resistant cells to narcotic analgesics is due to effects on the plasma membrane

It has previously been demonstrated that opiates interact directly with P-glycoprotein in drug resistant Chinese hamster ovary (CHO) cells (Callaghan, R. and Riordan, J.R. (1993) J. Biol. Chem. 268, 16059-16064). In this study we have examined the effects of several opiates on the growth of drug sensitive and resistant CHO and human MCF7 cell lines. The growth of P-glycoprotein expressing cells was inhibited by the opiates pentazocine, pethidine and naloxone to a greater extent than in drug sensitive cells. Since P-glycoprotein is localised at the plasma membrane the effects of opiates on membrane biophysical properties were investigated. The opiates caused a fluidizing effect in membranes from P-glycoprotein expressing cells and decreased the basal level of P-glycoprotein phosphorylation. In addition, they were able to increase the leakage of the membrane impermeant compound 6-carboxyfluorescein entrapped in model membrane vesicles. The ability to alter membrane biophysical properties correlated with the inhibitory effects on growth of drug resistant cells. These results suggest that the collateral sensitivity of P-glycoprotein expressing cell lines to opiates is mediated by the drugs' effects on the plasma membrane.

P-glycoprotein: clinical significance and methods of analysis

Multidrug resistance (MDR) is responsible for a decrease in sensitivity of tumor cells tumor cells to unrelated, naturally occurring anticancer drugs. This resistance is correlated with expression and activity of a membrane protein, P-gp 170, functioning as a drug-extruding pump. It has been well described in in vitro situations; however, the clinical detection and implications are not yet clear. Multiple detection assays have been developed based on the discovery of the MDR gene family and the corresponding protein. Southern, Northern, or Western blot analysis, S1 nuclease protection or PCR-based assays, immunohistochemical detection or functionality tests by flow cytometry have been used extensively. However, by use of these techniques on clinical material, both normal and malignant, contradictory results have emerged. The sensitivity and specificity of a certain technique are always limited by unavoidable parameters, for example, skill of the technician. Moreover, the complexity of the development of resistance against anticancer agents (external determinants), such as the diversity of tumor tissues, the simultaneous presence of other resistance mechanisms, and the low expression level, make MDR detection equivocal and can lead to contradictory results. Previous treatment influencing the MDR profile and inappropriate timing of the test make a possible correlation between MDR expression and chemotherapeutic resistance difficult to establish and can lead to discordant results. In this review, the need for proper criteria is stressed. No single detection technique provides the ideal test to detect MDR. Tandem testing could give more certainty, although small sample size limit this application. Formulation of a standard assay with better definition of a positivity is essential before clinical trials are started.

Validation of a single point flow cytometric assay for determining P-glycoprotein activity in multidrug resistant cell lines

P-glycoprotein, a transmembrane protein which acts as an energy dependent efflux pump, has been implicated as one mechanism of multidrug resistance (MDR) in human tumours. Commonly employed assays measure P-glycoprotein immunohistochemically or mdr1 messenger RNA. In this study we compared a single point flow cytometric assay for determining activity of P-glycoprotein with cellular expression of P-glycoprotein determined by Western blot. Five cell lines, with varying levels of multiple drug resistance, were incubated with daunorubicin (DNR) in the presence (treated) and absence (control) of cyclosporine or verapamil, agents known to inhibit the activity of P-glycoprotein. The treated cell lines, along with non-treated controls were examined for intracellular concentrations of DNR measured by fluorescence intensity using a flow cytometer. The ratio of fluorescence intensity expressed in the treated/control was used as an index of functional activity of P-glycoprotein. Functional activity of the P-glycoprotein as determined by flow cytometry correlates highly with cellular content of P-glycoprotein measured by western blot (correlation coefficients of r = 0.90-0.98 for the various cell line combinations). This method represents a rapid single point flow cytometric assay which may be suitable for screening clinical samples for P-glycoprotein activity.

Multiple drug resistance, the MDR gene, and the law of maximum perversity as it applies to oncology: an hypothesis

The so-called Law of Maximum Perversity, generally stated, says that when more than one outcome is possible, that which is the more adverse is the outcome most likely to occur. In medical oncology, the most obvious expression of this law is the fact that all the neoplasms that are most sensitive to cytotoxic drugs and are most curable by chemotherapy, are rare and in numerical terms are not important as causes of cancer-related deaths. Conversely, the most commonly encountered neoplasms that make up the bulk of oncologic practice and that cause over 90 per cent of cancer-related deaths, are all relatively resistant to cytotoxic agents and are virtually never curable by chemotherapy administered in standard (i.e. non-transplant) doses. It is postulated that the biological properties and the normal tissue distribution of the multidrug resistance (MDR) gene and its product p-170 glycoprotein explain the observed incidences and distribution of tumours that are sensitive or insensitive to cytotoxic agents. The normal role of MDR is to protect cells from environmental carcinogens, and the tissues that are most at risk, and most richly supplied with MDR, will produce drug-resistant neoplasms. Current attempts at MDR reversal may facilitate the treatment of some tumours that are resistant to multiple drugs but may cause severe toxic effects as a consequence of abrogating the largely unknown physiologic functions of P-170.

In vitro effects of pentoxifylline and doxorubicin on cell survival and DNA damage in sensitive and MDR-P388 leukemia cells

The utility of chemosensitizers to improve efficacy of chemotherapy is now gaining importance. This report investigated whether an active hemorheological agent, pentoxifylline (PTX), can circumvent drug resistance in parental (P388/S) and multidrug resistant (P388/DOX) P388 leukemia cells. For detection of doxorubicin (DOX) resistance and reversal of this resistance by PTX, the incorporation of nucleic acid precursor was measured after addition of DOX and PTX, respectively. The effect of PTX on the induction of DNA strand breaks by DOX was also examined. Increased fragmentation of DNA was illustrated in P388/DOX leukemia cells exposed to the combination of DOX and PTX. The most prominent feature of the multidrug-resistant cell is the reduced accumulation of the drug intracellularly. P388/DOX cells showed less accumulation of DOX in the cell as compared to that of the parental cell line. Further studies demonstrated that PTX significantly enhanced the intracellular accumulation of DOX in both the cell lines. These studies warrant the use of PTX as an adjuvant in cancer chemotherapy.

Reversal of the human and murine multidrug-resistance phenotype with megestrol acetate

MA is an orally active PG derivative with an excellent safety profile that is used primarily for the treatment of carcinomas of the breast and endometrium. We investigated the potential application of MA as an MDR-reversal agent using cell culture and human tumor xenograft models. The reversing activity of MA in vitro was compared with that of PG and VER in two human MDR cell lines, the colon carcinoma HCT-116/VM46 and the breast carcinoma MCF-7/ADR, and in a murine cell line, J774.2. At concentrations as low as 3 microM, MA was capable of partially restoring sensitivity to Act D in the HCT-116/VM46 cells and sensitivity to DOX in the MCF-7/ADR cells. Although less effective than VER, MA was about 2.5 times more potent than PG in reversing MDR at equimolar concentrations. Increased accumulation of DOX in drug-resistant cells that were treated simultaneously with MA was observed by flow cytometry. In vivo, using established human colon and breast carcinoma xenografts implanted s.c. in athymic mice, the combined therapy with MA and DOX resulted in enhanced antitumor activity relative to that of DOX alone in the MDR sublines. These results suggest that MA may be a promising clinical MDR-reversing agent.

Multidrug resistance in the laboratory and clinic

Multidrug resistance represents a major obstacle in the successful therapy of neoplastic diseases. Studies have demonstrated that this form of drug resistance occurs in cultured tumor cell lines as well as in human cancers. P-glycoprotein appears to play an important role in such cells by acting as an energy-dependent efflux pump to remove various natural-product drugs from the cell before they have a chance to exert their cytotoxic effects. Using the tools of molecular biology, studies are beginning to reveal the true incidence of multidrug resistance, as mediated by the MDR1 gene, in the clinical setting. It has been demonstrated, at least in the laboratory, that resistance mediated by P-glycoprotein may be modulated by a wide variety of compounds, including verapamil and cyclosporine A. These are compounds which, by themselves, generally have little or no effect on the tumor cells, but when used in conjunction with antineoplastic agents act to decrease, and in some instances eliminate, drug resistance. The mechanism(s) by which these agents act to reverse resistance is not fully understood. Clinical trials to modulate P-glycoprotein activity are now under way to determine whether such strategies will be feasible. The detection of the P-glycoprotein in patient samples is very important in the design of these studies, as it appears that drug-resistant cells lacking P-glycoprotein will be unaffected by agents such as verapamil. Clinical studies are needed in which patients are stratified into chemotherapy protocols based on levels of MDR1 mRNA or P-glycoprotein expression in the primary tumors. Several research areas have been identified that are important to the transfer of the discovery of the MDR1 gene and its protein product from the research laboratory to the clinical environment. There is an immediate need for comprehensive information on the prevalence and levels of expression of the human MDR genes and their protein products in human organs and tissues. Data are needed on P-glycoprotein levels in specific subpopulations (e.g., according to age, sex, race, and diet), and the study of the heterogeneity and variability of expression of P-glycoprotein in normal human tissues should be given high priority. Since early studies have indicated some successes in identifying patients with classic multidrug resistance who might be responsive to chemosensitization, it can be anticipated that clinical research will accelerate in this area. The next wave of clinical studies will provide clinical investigators with opportunities to develop and evaluate P-glycoprotein tests and correlate test results with clinical outcomes.

Expression of xenobiotic metabolizing enzymes in breast cancer

We have studied the expression of different xenobiotic metabolizing enzymes in primary operable breast cancer of no special type. The expression of two forms of cytochrome P450, microsomal epoxide hydrolase, and three classes of glutathione S-transferase was investigated using immunohistochemistry. The tumours were characterized by consistent expression of microsomal epoxide hydrolase and by variable expression of the two forms of cytochrome P450 and the three types of glutathione S-transferase. Cytochrome P450 1A and cytochrome P450 3A were identified in 39 and 22 per cent of tumours, respectively. In each case, immunostaining was present only in areas of invasive carcinoma. Epoxide hydrolase was identified in 89 per cent of tumours and glutathione S-transferases pi, mu, and alpha were identified in 56, 65, and 44 per cent of tumours, respectively. Immunoreactivity for epoxide hydrolase and glutathione S-transferases was identified in both tumours and non-neoplastic breast tissue. The presence of different xenobiotic metabolizing enzymes may have a role in determining the intrinsic drug resistance of breast cancer to a variety of anti-cancer drugs, and the expression of these enzymes can readily be assessed using immunohistochemistry.

Detection of activity of P-glycoprotein in human tumour samples using rhodamine 123

Based on the fluorescent properties of the dye rhodamine 123 (Rh123), which is transported by the membrane efflux pump P-glycoprotein (P-gp), we developed a functional flow cytometric assay for the detection of multidrug-resistant (MDR) cells. Using drug sensitive cell lines (KB-3-1) and MDR mutants (KB-8-5, KB-C1) experimental conditions were established that enabled demonstration of significant differences in Rh123 efflux and accumulation. Subsequently we investigated the applicability of this functional assay for the prediction of MDR in human peripheral blood and bone marrow samples. Using two-colour flow cytometry, the leukaemic blast cells of six patients suffering from acute myeloid leukaemia (AML) were analysed. In three cases the blast cells showed a rapid and marked Rh123 efflux. In the presence of MDR inhibitors these cells retained Rh123. To determine whether the efflux of Rh123 was associated with P-gp expression, the leukaemic cells were stained with the monoclonal antibody MRK-16. In addition extracted RNA was analysed by polymerase chain reaction to evaluate the expression of mdr 1 mRNA. In all three Rh123+ cases mdr 1 mRNA was detectable whereas only one AML case expressed P-gp. In comparing Rh123 with daunorubicin, which also allows the detection of MDR cells, accumulation studies proved Rh123 to be the more sensitive drug for flow cytometric MDR screening. Additionally, two-colour flow cytometry was much easier to perform with Rh123 than with daunorubicin. Our results indicate that flow cytometric measurement of Rh123 accumulation/efflux proves applicable to detect MDR cells in heterogenous clinical samples.

SDZ 280-446, a novel semi-synthetic cyclopeptolide: in vitro and in vivo circumvention of the P-glycoprotein-mediated tumour cell multidrug resistance

SDZ 280-446 is a semi-synthetic derivative of a natural cyclic peptolide. Its ability to sensitise in vitro tumour cells whose resistance is due to P-glycoprotein-mediated anticancer-drug efflux was shown using four different pairs of parental drug-sensitive (Par-) and multidrug-resistant (MDR-) cell lines, from three different species (mouse, human, Chinese hamster) representing four different cell lineages (monocytic leukaemia, nasopharyngeal epithelial carcinoma, colon epithelial carcinoma, ovary fibroblastoid carcinoma), and using four different drug classes (colchicine, vincristine, daunomycin/doxorubicin and etoposide). By measuring its capacity to restore normal drug sensitivity of MDR-cells in culture in vitro, it appeared that SDZ 280-446 belongs to the same class of very potent chemosensitisers as the cyclosporin derivative SDZ PSC 833: both are about one order of magnitude more active than cyclosporin A (CsA), which is itself about one order of magnitude more active than other known chemosensitisers (including verapamil, quinidine and amiodarone which have already entered clinical trials in MDR reversal). Low concentrations of SDZ 280-446 could also restore cellular daunomycin retention in MDR-P388 cells to the levels found in the Par-P388 cells. SDZ 280-446 was also effective as a chemosensitiser when given orally in vivo. In a syngeneic mouse model, combined therapy with vinca alkaloids given i.p. and SDZ 280-446 given per os for 5 consecutive days significantly prolonged the survival of MDR-P388 tumour-bearing mice, when compared with mice receiving vinca alkaloids alone. Another protocol, using three cycles of i.p. doxorubicin at 4 day intervals, could also not increase MDR-P388 tumour-bearing mouse survival unless the mice received SDZ 280-446 orally 4 h before each doxorubicin injection. Though only very few combined therapy treatment protocols have been tested so far, clear increases in survival time of MDR-tumour-bearing mice were regularly obtained, leaving hope for major improvement of the therapy using other dosing schedules.

Altered expression of P-glycoprotein and cellular adhesion molecules on human multi-drug-resistant tumor cells does not affect their susceptibility to NK- and LAK-mediated cytotoxicity

Drug resistance has been associated with resistance to NK- and LAK-cell-mediated cytotoxicity. We evaluated this issue in human cell lines, using multiple myeloma cells (8226) and 2 multi-drug-resistant (MDR) sublines selected using doxorubicin (8226/Dox40) and mitoxantrone (8226/MR40). In parallel, we studied the human breast carcinoma cell line series MCF7, MCF7/D40 and MCF7/Mitox. Unlike the sensitive parental cell lines, all 4 sublines display MDR-patterns of resistance, with the P-glycoprotein pump (P-170) detected only in the doxorubicin-selected sublines. Flow cytometric and immunocytochemical analyses showed expression of cellular adhesion molecules ICAM-I and LFA-3, and MHC-Class-I (MCF7/D40 only), to be decreased in the doxorubicin-selected MDR-sublines, whereas expression of CD56 (Leu 19) was strongly up-regulated in 8226/Dox40. Lysis of P-170-positive MDR tumor cells by NK or LAK cells was, however, unaffected by these alterations, suggesting redundancy in effector:target-cell adhesion pathways. Mitoxantrone-selected tumor cells did not display P-170, nor did they show altered expression of cellular adhesion molecules. Their susceptibility to NK or LAK cytolysis was also unimpaired as compared to the parental cell lines. Clinically, these results imply that immunotherapeutic modalities aiming at increased natural killer functions deserve full consideration even in patients who have become refractory to further cytostatic drug treatment.