Detoxification Mechanisms and Tumor Cell Resistance to Anticancer Drugs

Synthesis and activity of novel glutathione analogues containing an urethane backbone linkage

The new GSH analogues H-Glo(-Ser-Gly-OH)-OH (5), its O-benzyl derivative 4, and H-Glo(-Asp-Gly-OH)-OH (9), characterized by the replacement of central cysteine with either serine or aspartic acid, and containing an urethanic fragment as isosteric substitution of the scissile gamma-glutamylic junction, have been synthesized and characterized. Their ability to inhibit human GST P1-1 (hGST P1-1) in comparison with H-Glu(-Ser-Gly-OH)-OH and H-Glu(-Asp-Gly-OH)-OH, which are potent competitive inhibitors of rat GST 3-3 and 4-4, has been evaluated. In order to further investigate the effect of the isosteric substitution on the binding abilities of the new GSH analogues 4, 5 and 9, the previously reported cysteinyl-containing analogue H-Glo(-Cys-Gly-OH)-OH has been also evaluated as a co-substrate for hGSTP1-1.

Chemical genomics in the global study of protein functions

Small, cell-permeable and target-specific chemical ligands offer great therapeutic value. They can also be used to dissect diverse biological processes, such as cellular metabolism, signal transduction and intracellular protein trafficking. With cutting-edge technologies in synthetic chemistry and ligand screening and identification, chemical ligands have become more readily available for research. Chemical ligands are used increasingly in genomics approaches to understand the global functions of proteins, an emerging frontier called 'chemical genomics'. Chemical genomics should greatly accelerate discovery in biology and medicine in the near future.

Mechanisms of transport across cell membranes of complexes contained in antitumour drugs

Various mechanism of antitumour drug transport across cell membranes has been described. Particular attention has been paid to a passive transport, active transport and multidrug resistance of complexes contained in antitumour drugs. A drug supply to the target site depends on the blood circulation within the tumour, on characteristic drug diffusion in the tissue, and also on binding protein. The physiologic transfer of hydrophilic compounds across the membrane is usually intermediated by means of a specific receptor or a carrier in that membrane, which facilitates the transport of compounds to and from the cell. Some drugs, e.g. doxorubicin and annamycin, can pass across the membrane by intermediacy of liposomes which exhibit a great activity in penetrating into tumour cells. The efficiency of antitumour drugs is limited by the appearance of resistance, i.e. by the lack of sensitivity of the cell to the administered drug. The presence in the membrane of specific proteins belonging to the ABC carriers group is postulated in a resistance theory; they would be responsible for 'pumping out' lipophilic drug molecules from the cell. Participation of high-energy ATP molecule is required by P-glycoprotein (Pgp) and by MRP protein described in this paper for their action. The mechanisms that are responsible for the cell resistance to drugs have been presented by analysing the resistance to antimetabolites, particularly to folate and fluoropyrimidine analogues, to alkylating agents, e.g. cisplatinum, and to heterocyclic compounds being responsible for so-called multidrug resistance.

Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.

Intracellular glutathione content of urothelial cancer in correlation to chemotherapy response

We evaluated the possible correlation between intracellular glutathione (GSH) and drug sensitivity of urothelial cancer. Tissue GSH content of surgical specimens from 20 patients with urothelial cancer was assayed with high performance liquid chromatography (HPLC). GSH levels of cancer tissue (7.887 +/- 6.176 microM/mg protein) were significantly higher than GSH levels of normal mucosa (1.345 +/- 1.252 microM/mg). All patients having measurable lesions were then treated with methotrexate, epirubicin and cisplatin (MEC). These patients were classified into three groups according to clinical response criteria. GSH content in cancer tissue from four patients with complete response was 0.804 +/- 1.183 microM/mg protein. However, the cancer cells from patients with partial response and non-response contained a significantly higher level of GSH (6.295 +/- 2.459 (n = 8) and 12.955 +/- 6.141 microM/mg protein (n = 8), respectively). Intracellular glutathione content may play an important role in intrinsic resistance of urothelial cancer to MEC chemotherapy. It might be potentially used to predict drug sensitivity in urothelial cancer patients before starting chemotherapy.

Purification and characterization of class alpha and Mu glutathione S-transferases from porcine liver

Six cytosolic GSTs from porcine liver were purified by a combination of glutathione affinity chromatography and ion-exchange HPLC. The isoenzymes were characterized by SDS-PAGE, gel filtration, isoelectric focusing, immunoblotting analysis and determination of substrate specificities and inhibition characteristics. The purified GSTs belong to the alpha and mu classes, respectively. No class pi isoenzyme was isolated or detected. The class alpha GST pA1-1* exists as a homodimer (M(r) = 25.3 kDa), whereas GST pA2-3* consists of two subunits with different M(r) values (27.0 and 25.3 kDa). The estimated pI values were 9.5 and 8.8, respectively. Furthermore, four class mu porcine GSTs, pM1-1*, pM1-2*, pM3-?* and pM4-?*, were isolated. The isoenzyme pM1-1* possesses a relative molecular mass of 27.2 kDa and a pI value of 6.2. Additional pM1 isoenzymes hybridize with the subunit pM2* (M(r) = 25.2) to furnish a heterodimer, which shows a pI value of 5.8. The other class mu isoenzymes are heterodimers with pI values of 5.45 and 5.05. Substrate specificities and inhibition characteristics correlate very well with those of the corresponding human isoenzymes. The results are discussed with regard to the usefulness of porcine GSTs as an in vitro testing model.

Phosphono analogues of glutathione as new inhibitors of glutathione S-transferases

Phosphono-analogues of glutathione containing the O = P(OR)2 moiety in place of the cysteinyl residue CH2SH 1a-1d were prepared by solution phase peptide synthesis. Benzyl, benzyloxy-carbonyl, and tert-butyl protecting groups were used to mask the individual amino acid functional groups. The formation of peptide bonds was achieved by the usual peptide synthesis via activation of carboxylic functions with cyclohexylcarbodiimide and subsequent reaction with free amino groups. The thus obtained, fully-protected peptides were each purified by normal phase column chromatography. Deprotection was accomplished by hydrogenolysis and by treatment with HBr/acetic acid yielding the desired phosphonic acid diester 1a-1d. The inhibition of the glutathione conjugation of 1-chloro-2,4-dinitrobenzene by human placental glutathione S-transferase was studied by determining the IC50 values of the new glutathione analogues. The IC50 values were 291 microM, 139 microM, 64 microM, and 21 microM for the dimethyl, diethyl, diisopropyl, and di-n-butyl esters, respectively. The results clearly show that the formal substitution of the glutathione thiol function by phosphonic acid esters leads to a new class of glutathione S-transferase inhibitors. Further investigations directed at the question of whether or not these glutathione analogues are suitable for a modulation in chemotherapy are in progress.

Antioxidant and glutathione-associated enzymes in Wilms' tumour after chemotherapy

The present study demonstrates the activities of antioxidant and glutathione-associated enzymes and the level of glutathione in Wilms' tumour (nephroblastoma) samples after chemotherapy (mainly actinomycin D and vincristine). We observed higher activity of superoxide dismutase in Wilms' tumour compared to adjacent morphologically unchanged kidney. On the other hand, in this tumour lower activities of catalase and the glutathione-associated enzymes glutathione synthetase, gamma-glutamyl transpeptidase, glutathione reductase and total glutathione S-transferases (GST) were found. Using isoelectric focusing we separated different forms of GST in tested tissues and revealed lower activities of the basic enzymes in Wilms' tumour, which may be responsible for the decrease of total GST activity. Moreover, we found the acidic isoenzymes to be the predominant class of GST in nephroblastoma. In Wilms' tumours with unfavourable histology a high activity of these isoenzymes together with a high level of GSH were observed. We suggest that these parameters may participate in the known phenomenon of anticancer drug resistance of tumours with unfavourable histology.

Mechanisms of resistance to topoisomerases poisons

1. Drug resistance remains a major obstacle to cancer treatment. Resistance to chemotherapy can be intrinsic, characterised by the nonresponsiveness of the tumour to the initial treatment. Alternatively, cancers that initially respond to chemotherapy can relapse after various times because of acquired resistance. 2. Resistance to drugs used as single agents is generally accompanied by the development of resistance to other drugs that can be structurally and functionally different. 3. Among the drugs commonly used in cancer treatment there are compounds that have been shown to inhibit DNA topoisomerases (Topos). These critical enzymes regulate the topological conformation of the DNA and participate in essential cellular processes. 4. This paper reviews the Topos' cellular functions, their catalytic activities and the mechanisms of resistance to inhibitors of Topos, with particular attention to the atypical multidrug resistance phenotype.

Modulation of cisplatin sensitivity and accumulation by interferon alpha-2A in human squamous carcinoma cell lines

This study was undertaken to elucidate the mechanism(s) of potentiation of cisplatin (CDDP) cytotoxicity by interferon alpha-2a (IFN alpha-2a) in human squamous carcinoma cell lines SCC-25 and SCC-4. IFN alpha-2a treatment significantly increased the cytotoxicity of CDDP in both cell lines in a dose-dependent manner. In SCC-25 cells, the cytotoxicity of CDDP was increased by about 2- and 4-fold, respectively, by treating the cells with 400 and 800 IU/ml IFN alpha-2a. Sensitivity of SCC-4 cells to CDDP was increased by about 3- and 7-fold, respectively, by 400 and 800 IU/ml IFN alpha-2a treatment. Drug uptake experiments revealed approximately 1.4- to 5-fold higher platinum accumulation in IFN alpha-2a-treated cells as compared to respective controls. Cellular levels of glutathione (GSH) and GSH transferase, which have been suggested to be important determinants of tumor cell sensitivity to CDDP, were not altered by IFN alpha-2a treatment in either of the cell lines. Northern blot analysis showed a moderate increase (about 30-40%) in the level of MT-IIA mRNA by IFN alpha-2a treatment in these cells. Our results suggest that IFN alpha-2a-mediated sensitization of SCC-25 and SCC-4 cell lines to CDDP in vitro may be due to an increase in intracellular platinum accumulation.

Mechanism of cross-resistance to cisplatin in a mitomycin C-resistant human bladder cancer cell line

This study was undertaken to elucidate the mechanism(s) of cross-resistance to cisplatin (CDDP) in a mitomycin C (MMC)-resistant human bladder cancer cell line, J82/MMC. The J82/MMC cell line displayed 2- to 3-fold cross-resistance to CDDP and carboplatin when compared to the parental J82/WT cells. Drug uptake studies revealed that cross-resistance to CDDP in the J82/MMC cell line was independent of reduced platinum accumulation. The J82/MMC cell line exhibited approximately a 1.5-fold resistance to cadmium chloride, an indicator for increased metallothionein (MT) content, when compared to the J82/WT cells. Northern blot analysis showed a 2.7-fold higher level of MT-IIA mRNA in the J82/MMC cell line compared with J82/WT. We have reported previously that, whereas glutathione (GSH) level is comparable in these cells, GSH transferase (GST) activity is significantly higher in the J82/MMC cell line compared with J82/WT. Results of the present study showed that the elevated GST activity in the J82/MMC cell line was due to an over-expression of pi-type GST protein. Although buthionine-S,R-sulfoximine (BSO)-induced GSH depletion significantly enhanced CDDP cytotoxicity in both cell lines, the magnitude of potentiation was markedly higher in J82/MMC cells (about 2.1-fold) relative to J82/WT (about 1.6-fold). Our results suggest that cross-resistance to CDDP in the J82/MMC cell line may be due to alterations in cellular thiols.

Activity of melphalan in combination with the glutathione transferase inhibitor sulfasalazine

Glutathione (GSH) transferases (GST), a family of detoxification enzyme proteins, are suggested to play an important role in tumor cell resistance to melphalan. The GST-activity inhibitor ethacrynic acid has been shown to increase the antitumor activity of melphalan in vitro as well as in vivo. In this study we determined the activity and toxicity of melphalan in combination with another GST-activity inhibitor, sulfasalazine, an agent used to treat ulcerative colitis. We entered 37 previously treated patients with advanced cancer of different histologies on sulfasalazine given at the individually calculated maximum tolerated dose (MTD) and melphalan given at doses beginning at 20 mg/m2. The main toxicity arising from this combination was nausea and vomiting, whereas increased myelosuppression was not observed. A partial response was seen in 2/4 of the ovarian cancer patients only. Plasma sulfasalazine levels varied between 2.5 and 47.1 micrograms/ml. Although reductions in GSH/GST levels were observed in peripheral mononuclear cells of certain patients following sulfasalazine treatment, there was no correlation between the extent of reduction and the plasma sulfasalazine level. A larger patient population must be studied to determine the usefulness of this combination.

Development of extended multidrug resistance in HL60 promyelocytic leukaemia cells

In an attempt to mimic clinical conditions for the treatment of leukaemia, the HL60 promyelocytic cell line was treated for 18 h with low, clinically relevant, levels of the anthracycline epirubicin and the Vinca alkaloid vinblastine. The resulting drug-resistant sublines not only expressed P-glycoprotein and the MDR phenotype but were also cross-resistant to chlorambucil, methotrexate and cisplatinum, and had increased resistance to radiation. Development of resistance was associated with an aberrant differentiation phenotype with decreased expression of myeloid antigens and expression of glycophorin A, an antigen normally associated with erythroid differentiation. The ability of HL60 cells to terminally differentiate in response to all-trans-retinoic acid (vitamin A acid) was lost in the sublines. These results suggest that either a single novel mechanism is responsible for multiple drug resistance or the initial response to drug treatment is the co-induction of multiple mechanisms. These cells and the method by which they were generated therefore provide a clinically relevant model for the study of the initial events in the development of not only multidrug resistance but also the extended multiple drug resistance usually encountered in the treatment of leukaemia.

Glutathione-linked detoxification pathway in normal and malignant human bladder tissue

This study compares the levels of glutathione (GSH) and GSH-dependent detoxification enzymes, which have been implicated in anti-cancer drug resistance, in paired normal and malignant human bladder tissues, a tumor with high incidence of inherent drug resistance. Although the mean GSH transferase (GST) activity did not differ significantly in normal and neoplastic bladder tissues, this enzyme activity was relatively higher in tumor tissues of five out of ten patients as compared with corresponding normal tissues. Similarly, the mean GSH content and GSH reductase activity did not differ significantly between normal and neoplastic bladder tissues. On the other hand, the mean GSH peroxidase activity towards cumene hydroperoxide and catalase activity in bladder tumors was higher by about 1.5 and 1.4 times, respectively (P < 0.05), compared with those of normal tissues. GST isoenzymes corresponding to the three major classes (alpha, mu and pi) were expressed in every normal bladder tissue examined in the present study. Overexpression of GST pi was observed in 60% of the bladder tumors, whereas alpha and mu type GST proteins in tumor tissues were lower at frequencies of 62.5% and 37.5%, respectively, compared with the corresponding normal tissues. These results suggest that (a) elevated levels of GSH peroxidase, catalase and GST pi in human bladder tumors may contribute, at least in part, to the intrinsic drug resistance of this neoplasm and (b) anti-oxidative enzymes GSH peroxidase and/or catalase may represent markers for this neoplasia, although a large number of tissue specimens must be analyzed to validate this hypothesis.

In vitro drug sensitivity testing of tumor cells from patients with non-Hodgkin's lymphoma using the fluorometric microculture cytotoxicity assay

BACKGROUND

Tumor cell drug sensitivity is an important determinant of chemotherapy response. Its measurement in vitro would aid in therapy individualization and new drug development.

MATERIALS AND METHODS

The fluorometric microculture cytotoxicity assay (FMCA), based on production by viable cells of fluorescent fluorescein after 3 days of culture, was used for cytotoxic drug sensitivity testing of 73 samples of tumor cells from patients with non-Hodgkin's lymphoma (NHL).

RESULTS

The technical success rate was 92%, and FMCA data showed good correlation to the Disc assay. NHL samples were considerably more drug sensitive than were samples from in vivo resistant tumors. There was no obvious difference in drug sensitivity for high- vs. low-grade or untreated vs. previously treated low-grade NHL. For 26 patients, clinical outcome was correlated to in vitro response giving a sensitivity and specificity of 93 and 48%, respectively. Cross-resistance between standard drugs was frequent in vitro. Resistance modulators potentiated the effect of vincristine and doxorubicin in 10-29% of the samples, most frequently from previously treated patients.

CONCLUSIONS

The FMCA seems to report clinically relevant drug sensitivity data for NHL, and thus it could serve as a tool for optimization of chemotherapy in the future.

Enhanced glutathione S-transferase activity and glutathione content in human bladder cancer. Followup study: influence of smoking

Glutathione content and glutathione S-transferase activity have been studied in human bladder specimens obtained from controls and from patients with superficial transitional cell carcinoma (tumor samples and peri-tumor normal tissues from the same patient). After combining an earlier study from our laboratory with the additional material presented (9 healthy controls and 25 transitional cell carcinoma patients), it can be observed that glutathione S-transferase activity was significantly greater in tumor than in peri-tumor normal tissue (34 patients, p < 1 x 10(-7)) or in normal mucosa (17 controls, p < 1 x 10(-3)). Glutathione content was significantly greater in tumor than in peri-tumor normal tissue (p < 5 x 10(-3)) or in normal mucosa (p < 2 x 10(-2)), with this increase being evident only in smokers. When comparing normal mucosa and peri-tumor samples no significant differences were found either for glutathione S-transferase activity or for glutathione content. Results demonstrate the relationship between the glutathione S-transferase/glutathione system and development of transitional cell carcinoma, as well as its role in cellular resistance to chemotherapy.

Doxorubicin resistance in human melanoma cells: MDR-1 and glutathione S-transferase pi gene expression

Cellular drug resistance is believed to involve P-glycoprotein-related drug efflux as well as xenobiotic detoxification. In the present study, we analyzed five human melanoma cell lines with 1- to 6-fold doxorubicin resistance for doxorubicin retention and MDR-1 and GST pi gene expression. All the cell lines had high doxorubicin retention, and efflux blockers such as trifluoperazine and verapamil did not have a major effect on drug retention or cytotoxicity. Even though all the cell lines carried the MDR-1 and GST pi genes, gene amplification was not associated with drug resistance. Both laser flow cytometry and immunoperoxidase staining showed high expression of C-219 reactive P-glycoprotein in some of the resistant cells which was not accompanied by either high drug efflux or sensitivity to doxorubicin efflux blockers.

MDR-1 gene expression, anthracycline retention and cytotoxicity in human lung-tumor cells from refractory patients

Lung-tumor cells from pleural effusion of four refractory patients and in cell lines established from them were analyzed for anthracycline retention, cytotoxicity, and MDR-1 gene and P-glycoprotein expression. Murine leukemic P388 and doxorubicin-resistant P388/R84 lines were used as controls. The 50% growth-inhibitory concentration (IC50) for doxorubicin among lung-tumor lines varied from 0.16 to 0.31 microM in soft agar. Heterogeneity in doxorubicin or daunorubicin retention and response to the efflux-blocking action of 25 microM prochlorperazine was noted in pleural effusion of FCCL-1, -4, and -8. Among the cell lines established, an efflux-blocking effect in a subpopulation was noticed only in FCCL-1 and -4. Although the MDR-1 gene was present in all cell lines, including P388, its expression was pronounced only in P388/R84 and FCCL-1. In situ hybridization of antisense RNA probe to tumor cells showed high heterogeneity for MDR-1 message in the human lung-tumor cells as compared with the murine cells. Northern and slot blot hybridization confirmed in situ hybridization in lines with high levels of MDR-1 expression. The synthesis of MDR-1 mRNA and P-glycoprotein in tumor lines was correlated. The results suggest that because of extensive tumor-cell heterogeneity in human tumors, monitoring of MDR expression by in situ hybridization, quantitation of P-glycoprotein content by laser flow cytometry (and/or immunohistochemical methods), and drug efflux (by laser flow cytometry) may be the best ways to monitor multidrug resistance in human tumors.