Dipyridamole increases VP16 growth inhibition, accumulation and retention in parental and multidrug-resistant CHO cells

Dipyridamole increases the cytotoxicity of cisplatin in human larynx cancer cells in vitro

This paper describes the effect of dipyridamole (DIP) on the cytotoxicity of cisplatin in HEp-2 human larynx cancer cells in vitro and the nature of the interaction between cisplatin and dipyridamole. Cytotoxic assays were performed to obtain the IC50 for cisplatin. The cells were treated with 0, 20, 40, 80, 120 or 200 microM cisplatin, with or without a single concentration of DIP and incubated for 60 min at 37 masculine C and 5% CO2 for 3 days and then counted with a hemocytometer. The accumulation of cisplatin in the cells was measured by atomic absorption and fluorescence was used to determine the membrane binding constant of DIP. In the presence of 10, 20 and 30 microM DIP, the IC50 of cisplatin was reduced by 25, 60 and 82% in HEp-2 cells. Combination index analysis revealed that cisplatin and DIP interact synergistically. In larynx cancer cells, the accumulation of cisplatin increased by 13, 27 and 65% as the DIP concentration was increased from 10 to 20 and 30 microM, respectively. The binding constant of DIP to the cell membrane was estimated to be 0.36 +/- 0.12 mg/ml(-1) (N = 2) by fluorescence and cisplatin did not suppress DIP fluorescence. These results suggest that DIP significantly enhances cisplatin cytotoxicity in HEp-2 cells by increasing cisplatin accumulation, probably by altering the cell membrane as suggested by its binding constant. The results obtained reinforce the importance of combination therapy to reduce the doses of chemotherapeutic drugs and therefore the side effects of chemotherapy.

Future potential of thymidylate synthase inhibitors in cancer therapy

Thymidylate synthase (TS) catalyses the de novo synthesis of deoxythymidylate and is a key rate-limiting enzyme of DNA synthesis. The primary site of action of the classic antifolate methotrexate is direct inhibition of dihydrofolate reductase, but it also inhibits TS indirectly by diminishing levels of the TS cosubstrate 5,10-methylenetetrahydrofolate. Polyglutamated metabolites of methotrexate also directly bind and inhibit TS. The prototype fluoropyrimidine fluorouracil is metabolised to an irreversible inhibitor of TS and is the standard chemotherapy for gastrointestinal carcinomas. It is also frequently used in combination with other anticancer drugs against breast cancer and head and neck cancers. The clinical efficacy of fluorouracil is routinely increased by concomitant administration of the biomodulating compound leucovorin (folinic acid). Both the success and limitations of these early drugs led to a search for new, more efficacious TS inhibitors active against a broader range of neoplasms. Raltitrexed (ZD1694, Tomudex) is an antifolate TS inhibitor developed over the last decade that is similarly effective, yet better tolerated, than fluorouracil against colorectal cancer. Additional antifolate and fluoropyrimidine-based TS inhibitors continue to be developed. Many of these experimental drugs have been designed to exploit or thwart selective metabolism in neoplasms, including specific mechanisms of resistance. As the curative potential of relatively non-selective antiproliferative drugs like TS inhibitors is limited against most neoplasms, the future role of TS inhibitors will likely continue to be adjunctive in surgically resectable tumours and palliative in combination with other agents for non-resectable disease. Although TS inhibitors will eventually be supplanted by yet to be discovered agents targeting more tumour-specific cellular signalling pathways, they will probably remain important for the above uses for some time. Future advances in the effective use of TS inhibitors may be forthcoming in the form of improved dosing, fewer untoward effects and increased tumour selectivity with novel fluorouracil prodrug formulations. Furthermore, there is emerging evidence that some novel antifolate TS inhibitors are active against a broader range of neoplams, including lung carcinomas and mesothelioma, compared to classical TS inhibitors. Other possible advances to come include effective biomodulation of antifolate TS inhibitors with nucleoside transport inhibitors and individualised patient therapy based on tumour gene expression and resistance patterns (pharmacogenetics).

MDR1-mediated interaction of digoxin with antiarrhythmic or antianginal drugs

The multidrug transporter, MDR1-mediated interaction of digoxin with antiarrhythmic or antianginal drugs was examined in vitro by using the MDR1-overexpressing LLC-GA5-COL150 cells, which were established by transfection with human MDR1 cDNA into porcine kidney epithelial LLC-PK(1) cells. Amiodarone, its active metabolite monodesethyl-amiodarone (DEA), and quinidine markedly inhibited the basal-to-apical transport (renal secretion) of [(3)H]digoxin and increased the apical-to-basal transport (reabsorption), but cibenzoline and lidocaine showed slight inhibition of the transport, and disopyramide and mexiletin had no such effects. The IC(50) values for amiodarone, DEA and quinidine on [(3)H]digoxin transport in LLC-GA5-COL150 cells were 5.48 microM, 1.27 microM and 9.52 microM, respectively. These were comparable to, or only several times the achievable concentration in clinical use, suggesting that MDR1 could be responsible for the drug interaction between digoxin and amiodarone found in clinical reports and that DEA contributes the elevation of digoxin serum concentration. Similarly, dipyridamole altered the transport, but isosorbide showed only slight modification of the transport. The IC(50) value for dipyridamole was 40.0 microM, also only several times the achievable concentration in clinical use, indicating a risk of interaction.

Non-ionic detergent Tween 80 modulates VP-16 resistance in classical multidrug resistant K562 cells via enhancement of VP-16 influx

The non-ionic detergent Tween 80, which is used as a solvent for lipophilic drugs such as VP-16 and Taxotere, was found to reverse VP-16 resistance of the P-glycoprotein-associated multidrug resistance phenotype via increasing VP-16 influx. In adriamycin-resistant human chronic myelogenous leukemia K562 cells (K562/ADM), which overexpress mdr1 mRNA, the accumulation of VP-16 was only about 10% that in wild-type K562 cells. Tween 80 enhanced VP-16 accumulation in K562/ADM cells but did not influence VP-16 accumulation in parental K562 cells. VP-16 efflux was rapid and similar in both sensitive and resistant cell lines and was not blocked by Tween 80 or verapamil. Under glucose-free conditions, VP-16 accumulation in K562/ADM cells was only half of that in K562 cells. Tween 80 increased VP-16 accumulation in K562/ADM cells in glucose-free medium. In growth inhibition assay, Tween 80 reversed K562/ADM sensitivity to VP-16 without cell damage. Taken together, Tween 80 reverses VP-16 sensitivity in multidrug-resistant K562 cells by increasing influx, which is considered to be the primary mechanism of VP-16 resistance in K562/ADM cells.

Dipyridamole-mediated reversal of multidrug resistance in MRP over-expressing human lung carcinoma cells in vitro

Expression of the multidrug resistance-associated protein (MRP) is widespread in human malignancies, high levels are associated with poor prognosis and may be responsible for intrinsic and radiotherapy-induced chemoresistance. In this study, the nucleoside transport inhibitor, dipyridamole (DP), was investigated as a chemosensitiser of MRP. In growth inhibition assays MRP-over-expressing COR L23/R cells were 20 times more resistant to VP16 and doxorubicin compared with the parental COR L23/R human lung carcinoma cells. DP caused an approximately 8-fold sensitisation of the resistant cells and a 2-fold sensitisation of the parental cells. DP enhanced the accumulation of VP16 1.5 to 2-fold in the parental cells, but had only a modest effect on VP16 accumulation in the resistant cells. VP16 efflux was rapid in both cell lines. DP caused a modest and transient inhibition of the initial efflux in the resistant cells but not the parental cells. Incubation with DP caused a progressive decrease in GSH levels which was more rapid and profound in COR L23/R cells than in COR L23/P cells. Thus, chemosensitisation to VP16 by DP in MRP-overexpressing COR L23/R cells appears to be caused by depletion of cellular GSH rather than a direct effect of DP on MRP-mediated drug accumulation and efflux.

Multidrug resistance protein-mediated transport of chlorambucil and melphalan conjugated to glutathione

The human multidrug resistance protein (MRP1) confers resistance of cells to a number of different cytostatic drugs and functions as an export pump for glutathione S-conjugates, glucuronides and other amphiphilic anions. The present study details for the first time MRP1-mediated ATP-dependent transport of various glutathione S-conjugates of the bifunctional alkylating agents chlorambucil and melphalan. In membrane vesicles prepared from cells expressing recombinant MRP1, the conjugates were transported at rates in the following order: monoglutathionyl chlorambucil > bisglutathionyl chlorambucil > monohydroxy monoglutathionyl chlorambucil and monoglutathionyl melphalan > monohydroxy monoglutathionyl melphalan. In addition, we show that membranes from chlorambucil-resistant GST-alpha-overexpressing CHO cells as well as from their parental cells express the hamster homologue of MRP1. With both CHO cell membrane preparations, we observed ATP-dependent transport of monoglutathionyl chlorambucil and of leukotriene C4, a glutathione S-conjugate and high-affinity substrate of MRP1. The transport rates measured in the resistant cells were only two- to three-fold higher than those measured in the control cells. These results together with cytotoxicity assays comparing MRP1-overexpressing cell pairs with the CHO cell pair indicate that, although MRP1-mediated transport is active, it may not be the rate-limiting step in chlorambucil resistance in these cell lines.