Potentiation of some anticancer agents by dipyridamole against drug-sensitive and drug-resistant cancer cell lines

Proton Transfer in Bacterial Reaction Centers and Bacteriorhodopsin in the Presence of Dipyridamole

Dipyridamole, 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d]pyrimidine (DIP), a well known vasodilator and inhibitor of membrane peroxidation has recently been shown a potential co-activator (modulator) in the MDR phenomenon in cancer therapy. It inhibits P-glycoprotein (Pgp) which is a efflux pump of anticancer drugs in tumor cells. For the first time it is shown that dipyridamole, markedly slows down the kinetics of the electrogenic phase of the photoelectric response in Rb. sphaeroides chromatophores which is due to the proton transfer from the external medium to the secondary quinone acceptor in the reaction center. In purple membranes from H. salinarium containing bacteriorhodopsin (bR) dipyridamole (in its charged state) significantly slows down the kinetics of the proton transfer to the Schiff base from the primary donor Asp-96 (in wild type bacteria) or from the surrounding (in D96N mutant). Dipyridamole is supposed to affect the proton-transfer via changes in structural dynamics of membrane proteins including modification of their system of hydrogen-bonds.

Preclinical evaluation of a novel pyrimidopyrimidine for the prevention of nucleoside and nucleobase reversal of antifolate cytotoxicity

Antifolates have been used to treat cancer for the last 50 years and remain the mainstay of many therapeutic regimes. Nucleoside salvage, which depends on plasma membrane transport, can compromise the activity of antifolates. The cardiovascular drug dipyridamole inhibits nucleoside transport and enhances antifolate cytotoxicity in vitro, but its clinical activity is compromised by binding to the plasma protein alpha(1)-acid glycoprotein (AGP). We report the development of a novel pyrimidopyrimidine analogue of dipyridamole, NU3153, which has equivalent potency to dipyridamole, remains active in the presence of physiologic levels of AGP, inhibits thymidine incorporation into DNA, and prevents thymidine and hypoxanthine rescue from the multitargeted antifolate, pemetrexed. Pharmacokinetic evaluation of NU3153 suggested that a soluble prodrug would improve the in vivo activity. The valine prodrug of NU3153, NU3166, rapidly broke down to NU3153 in vitro and in vivo. Plasma NU3153 concentrations commensurate with rescue inhibition in vitro were maintained for at least 16 hours following administration of NU3166 to mice at 120 mg/kg. However, maximum inhibition of thymidine incorporation into tumors was only 50%, which was insufficient to enhance pemetrexed antitumor activity in vivo. Comparison with the cell-based studies revealed that pemetrexed enhancement requires substantial (> or =90%) and durable inhibition of nucleoside transport. In conclusion, we have developed non-AGP binding nucleoside transport inhibitors. Pharmacologically active concentrations of the inhibitors can be achieved in vivo using prodrug approaches, but greater potency is required to evaluate inhibition of nucleoside rescue as a therapeutic maneuver.

Multidrug resistance reversal agent from Jatropha elliptica

As part of an ongoing project to identify plant natural products as resistance-modifying agents, bioassay-guided fractionation of an extract of Jatropha elliptica (Pohl) Muell Arg. led to the isolation of a penta-substituted pyridine, namely 2,6-dimethyl-4-phenyl-pyridine-3,5-dicarboxylic acid diethyl ester (8). The structure was established by spectroscopic methods. This known compound was assayed for in vitro antibacterial and resistance-modifying activities against strains of Staphylococcus aureus possessing the MsrA and NorA resistance efflux mechanisms. Antibiotic efflux studies indicated that (8) acts as an inhibitor of the NorA efflux pump and restores the level of intracellular drug concentration.

Selection of drugs to test the specificity of the Tg.AC assay by screening for induction of the gadd153 promoter in vitro

Short-term assays for carcinogenicity testing of chemicals that use transgenic mice designed to have altered expression of genes mechanistically relevant to carcinogenesis are attractive alternatives to two-year dosing studies in rodents. The models that have been the received the greatest level of performance evaluation include p53(+/-), rasH2, Xpa/p53(+/-), and Tg.AC mice. For use of these models in a regulatory setting to evaluate the carcinogenic potential of pharmaceuticals, it is important to establish an assurance of assay specificity and positive predictivity based on studies using drugs with a wide spectrum of pharmacologic activity. For this purpose, 99 noncarcinogenic drugs were prioritized based on their activity in an in vitro induction assay correlative with a positive response in the Tg.AC assay (induction of the gadd153 promoter in HepG2 cells). Activities in two assays less predictive of Tg.AC activity (induction of c-fos and zeta-globin gene promoters) were also measured. Nine percent of the screened drugs induced the gadd153 promoter by at least fourfold. Several criteria were used to select candidates for subsequent in vivo testing in the Tg.AC assay: (1) sufficient drug solubility in appropriate skin paint vehicles to elicit systemic toxicity, (2) the level of induction of the gadd153 promoter by the drug, (3) the in vitro potency of the drug, and (4) the cost of the drug required for a 6-month study. Based on these criteria, amiloride, dipyridamole, and pyrimethamine were selected from 99 rodent noncarcinogens in a drug database for testing the specificity of the Tg.AC assay.

Methylation-dependent silencing of the reduced folate carrier gene in inherently methotrexate-resistant human breast cancer cells

The molecular basis of methotrexate resistance was studied in human MDA-MB-231 breast cancer cells, which are inherently defective in methotrexate uptake and lack expression of the reduced folate carrier (RFC). Transfection of MDA-MB-231 cells with RFC cDNA restored methotrexate uptake and increased methotrexate sensitivity by approximately 50-fold. A CpG island in the promoter region of RFC was found to be methylated in MDA-MB-231 cells, but was unmethylated in RFC expressing, methotrexate-sensitive MCF-7 breast cancer cells. Chromatin immunoprecipitation with antibodies against acetylated histones H3 and H4 showed that the RFC promoter was enriched for acetylated histones on expressed, unmethylated alleles only. Treatment of MDA-MB-231 cells with 5-aza-2'-deoxycytidine restored RFC expression but also led to increased methotrexate efflux and did not reverse methotrexate resistance. This suggests that 5-aza-2'-deoxycytidine up-regulates both methotrexate uptake and some methotrexate-resistance mechanism(s). Reverse transcription-polymerase chain reaction analysis showed increased expression levels of several ATP-dependent efflux pumps in response to 5-aza-2'-deoxycytidine treatment, including P-glycoprotein and members of the multidrug resistance-associated protein family. Up-regulation of P-glycoprotein in response to 5-aza-2'-deoxycytidine was associated with demethylation of a CpG island in the MDR1 promoter, whereas the mechanism(s) for 5-aza-2'-deoxycytidine-induced up-regulation of multidrug resistance-associated proteins is probably indirect. Dipyridamole inhibited methotrexate efflux and reversed methotrexate resistance in 5-aza-2'-deoxycytidine-treated MDA-MB-231 cells.

Reversal of vinblastine resistance in human leukemic cells by haloperidol and dihydrohaloperidol

Haloperidol, an antipsychotic, was investigated in cells overexpressing P-glycoprotein to detemine whether it was a clinically effective drug to reverse for reversing multidrug resistance (MDR) mediated by P-glycoprotein. A nontoxic concentration of haloperidol (1-30 microM) enhanced the cytotoxic effects of vinblastine (VBL) concentration-dependently in VBL-resistant human leukemia (K562/VBL) cells, but had no effect in the parent cells. Haloperidol also enhanced the cytotoxicities of epirubicin, doxorubicin and actinomycin D in the K562/VBL cells, but not those of idarubicin or cisplatin; this enhancement was less than that of the VBL toxicity in the VBL-resistant tumor line. Haloperidol increased the intracellular accumulation of VBL in the K562/VBL cells, and the binding of [3H]-azidopine to the cell-surface protein, P-glycoprotein, was inhibited by haloperidol in a concentration-dependent manner. Haloperidol was less potent than verapamil. Thus, haloperidol appeared to potentiate anticancer agents through the reversal of MDR by competitively inhibiting drug-binding to P-glycoprotein. In contrast, the main metabolite of haloperidol, dihydrohaloperidol, without antipsychotic activity, had less of an effect. Therefore, haloperidol might be useful in reversing drug-resistance.

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.

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

Dipyridamole (DP) has been shown to reverse multidrug resistance (MDR) via interactions with P-glycoprotein (P-gp). The effect of DP on VP16 growth inhibition was investigated in parental (CHO-K1) and MDR (CHO-Adr(r)) Chinese hamster ovary cells. CHO-Adr(r) cells were 18-fold resistant to VP16 and intracellular accumulation was 28% less than in CHO-K1 cells. DP reduced the resistance of CHO-Adr(r) to VP16 by a factor of 2-3 and caused a similar potentiation of VP16 growth inhibition in the parental cells. A time-dependent increase in intracellular VP16 accumulation, which was similar in both cell lines, was caused by DP. The intracellular retention of VP16 was increased 2- to 3-fold by DP in both cell lines. The magnitude of the effect of DP on all three parameters measured was similar (2- to 4-fold), suggesting that the increased growth inhibition was related to increased intracellular exposure to VP16 owing to the inhibition of the efflux of VP16 by DP. However, since the effect of DP was similar in both parental and P-gp-overexpressing cells it is unlikely that the potentiation of VP16 by DP is mediated via an interaction with P-gp.

Biochemical modulation of 'classical' multidrug resistance by BIBW22BS, a potent derivative of dipyridamole

BACKGROUND

Modulators of the 'classical' multidrug resistance (mdr) phenotype have low efficacy in patients with solid tumors. We analyzed BIBW22BS, 4-[N-(2-hydroxy-2-met- hyl-propyl)-ethanolamino]-2,7-bis(cis-2,6-dimethyl-morpho- lino)-6-phenylpteridine, a derivative of dipyridamole, for its higher potential to modulate mdr.

MATERIALS AND METHODS

Four human malignant cell lines: BRO, A2780, GLC4, SW1573, the Pgp-positive sublines: BRO/mdr1.1, 2780AD and the non-Pgp sublines: GLC4/ADR, SW1573/2R120 were used in vitro to investigate BIBW22BS as a modulator of the antiproliferative effects of vincristine and doxorubicin and to compare the potency of BIBW22BS with that of dipyridamole, verapamil, bepridil and flunarizine. BRO/mdr1.1 s.c. well-established xenografts in nude mice were used to study the modulating properties of BIBW22BS 50 mg/kg i.v. followed after one h by vincristine 1 mg/kg i.p. or doxorubicin 8 mg/kg i.p. weekly x 2.

RESULTS

BIBW22BS was 20- to 100-fold more potent than dipyridamole in the reversal of resistance in the Pgp-positive sublines. Reversal of resistance was obtained in a dose-dependent manner and was complete at concentrations of 0.5-2.5 microM. At non-toxic, equimolar concentrations of 1.0 microM BIBW22BS showed higher modulating potency than the calcium-channel blockers. BIBW22BS did not affect resistance in the non-Pgp sublines. BRO/mdr1.1 s.c. xenografts have stable multidrug-resistance characteristics upon serial transplantation. BIBW22BS, vincristine, or doxorubicin as single agents were not effective in vivo, while the addition of BIBW22BS could significantly reduce the tumor growth expressed as the T/C% of vincristine from 109% to 48% and that of doxorubicin from 55% to 32%. However, reversal of vincristine resistance in BRO/mdr1.1 xenografts was not complete when compared to the efficacy of vincristine in BRO xenografts.

CONCLUSION

The results encourage the further preclinical development of BIBW22BS as a modulator of 'classical' multidrug resistance in cancer patients.

Anthracycline antibiotics in cancer therapy. Focus on drug resistance

30 years ago an anthracycline antibiotic was shown to have antineoplastic activity. This led to the development of well over 1000 analogues with a vast spectrum of biochemical characteristics. Many biological actions have been described. The original anthracyclines are active against many types of cancer and are an integral part of several curative combinations. They are ineffective against other tumours. Although some analogues show an altered spectrum of activity or an improved therapeutic index relative to the older agents, it is not clear that cardiotoxicity can be totally avoided with these agents. Primary and secondary resistance to anthracyclines remain major clinical problems. Pharmacokinetic studies have been of limited help in explaining this. Overexpression of a surface-membrane permeability glycoprotein (Pgp) was identified in ovarian cancer of patients who had clinical multidrug resistance in 1985. This led the way for the discovery of a number of resistance mechanisms in vitro. Some of these have been found in more than 1 type of cell line, and more than 1 mechanism may exist in a single cell. Additional resistance proteins have been identified, qualitative and quantitative alterations of topoisomerase II have been described, and some mechanisms in other systems have not yet been identified. Some of these may prove to be important in clinical drug resistance. Drugs such as calcium antagonists and cyclosporin, studied initially for their ability to block the Pgp pump, appear to be heterogeneous in this capacity and may have additional sites of action. It will be critical for clinical studies to define the precise resistance mechanism(s) that must be reversed. To date this has been difficult, even in trials ostensibly dealing with the original Pgp. Liposomes can potentially alter toxicity and target drug delivery to specific sites. In addition, they may permit the use of lipophilic drugs that would otherwise be difficult to administer systemically. Resistant tumours may be sensitive to anthracyclines delivered by liposomes. To reduce cardiac toxicity, administering doxorubicin (adriamycin) by slow infusion through a central-venous line should be considered whenever feasible. Monitoring of cardiac ejection fraction and the use of endomyocardial biopsy will permit patients to be treated safely after they reach the dose threshold at which heart failure begins to be a potential risk. A number of structurally modified anthracyclines with the potential advantages of decreased cardiotoxicity and avoidance of multidrug resistance mechanisms are entering clinical trials. Meanwhile, the vast weight of clinical experience leaves doxorubicin as a well tolerated and effective choice for most potentially anthracycline-sensitive tumours.

Dipyridamole modulates multidrug resistance and intracellular as well as nuclear levels of doxorubicin in B16 melanoma cells

Simultaneous occurrence of resistance to many chemotherapeutic agents, termed multidrug resistance (MDR), is a complex phenotype. MDR occurs due to several reasons, including over-expression of a 170-kDa membrane-bound protein, called P-glycoprotein (P-gp), which apparently participates in active drug efflux. Multidrug-resistant cells also frequently exhibit an altered pattern of intracellular drug distribution, resulting in a reduction in the nuclear level of drugs such as doxorubicin (DOX). In this study, the effect of dipyridamole (DP) on drug resistance and on intracellular as well as nuclear levels of DOX in multidrug-resistant melanoma cells has been examined. For this purpose, drug-sensitive murine melanoma cells (B16V) and their multidrug-resistant variant cells, (B16VDXR; selected for resistance to DOX) which over-produce P-gp, were employed. B16VDXR cells were cross-resistant to several anti-cancer agents including etoposide (VP-16) and mitoxantrone (Mitox). DP (10 microM) significantly potentiated the cytotoxicity of DOX, VP-16 and Mitox towards multidrug-resistant B16VDXR cells but not in parental drug-sensitive B16V cells. The presence of DP resulted in a 3.7-fold increase in the total cellular level and a 4.2-fold increase in the nuclear content of DOX in the resistant cells. Isobologram analysis indicates that DP at several pharmacologically relevant concentrations synergistically potentiates the activity of DOX in B16VDXR cells.

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.

Regulation of initial vinblastine influx by P-glycoprotein

P-glycoprotein (PGP) is an energy-dependent efflux pump that serves to protect cells against the cytotoxicity of many natural product drugs including vinblastine (VBL). In this study we investigated the role of PGP in regulating initial VBL influx. The apparent influx of VBL, measured over the first 20 s, was 2-fold lower in KB-GRC1 cells expressing a transfected mdr1 gene at high level than in non-expressing parental KB-3-1 cells. Inhibition of PGP efflux function with dipyridamole increased the influx rate constant by 4.0-fold in the KB-GRC1 cells but only 2.1-fold in the KB-3-1 cells. Verapamil, another inhibitor of PGP-mediated efflux, increased the initial influx rate constant by 2.7-fold in the KB-GRC1 cells but only 1.4-fold in the KB-3-1 cells. Inhibition of PGP function by depletion of ATP increased influx by 6.8-fold and 2.2-fold in the two cell types, respectively. Mutation of PGP at both ATP binding sites abolished its ability to limit initial influx. Thus, VBL is serving as an efficient substrate for the efflux pump even within the first few seconds of drug exposure, consistent with the hypothesis that PGP may directly efflux drug from the cell membrane.

Pharmacologic circumvention of multidrug resistance

The ability of malignant cells to develop resistance to chemotherapeutic drugs is a major obstacle to the successful treatment of clinical tumors. The phenomenon multidrug resistance (MDR) in cancer cells results in cross-resistance to a broad range of structurally diverse antineoplastic agents, due to outward efflux of cytotoxic substrates by the mdr1 gene product, P-glycoprotein (P-gp). Numerous pharmacologic agents have been identified which inhibit the efflux pump and modulate MDR. The biochemical, cellular and clinical pharmacology of agents used to circumvent MDR is analyzed in terms of their mechanism of action and potential clinical utility. MDR antagonists, termed chemosensitizers, may be grouped into several classes, and include calcium channel blockers, calmodulin antagonists, anthracycline and Vinca alkaloid analogs, cyclosporines, dipyridamole, and other hydrophobic, cationic compounds. Structural features important for chemosensitizer activity have been identified, and a model for the interaction of these drugs with P-gp is proposed. Other possible cellular targets for the reversal of MDR are also discussed, such as protein kinase C. Strategies for the clinical modulation of MDR and trials combining chemosensitizers with chemotherapeutic drugs in humans are reviewed. Several novel approaches for the modulation of MDR are examined.

Phosphoprotein B23 translocation and modulation of actinomycin D and doxorubicin cytotoxicity by dipyridamole in HeLa cells

During continuous exposure, cells were more responsive to doxorubicin (DOX) in the presence of dipyridamole (DPM). Translocation of nucleolar phosphoprotein B23 and inhibition of cell growth occurred with a lower dose of DOX and in a shorter incubation time in the presence of DPM. DPM did not change translocation induced by actinomycin D (Act-D). Short exposure of HeLa cells to Act-D induced "reversible" translocation of protein B23 as well as "reversible" inhibition of cell growth. DPM included in the cell culture after removal of Act-D inhibited the recovery of cell growth as well as the corresponding relocalization of protein B23 from the nucleoplasm to nucleoli. DPM administered in the fresh medium after 30 min exposure to DOX had little effect on the potentiation of the induced translocation of protein B23 and inhibition of cell growth. Our results indicated that "B23 translocation" is closely associated with states of cell growth. The potentiation of the inhibition of cell growth by DPM is associated with the extent of enhanced protein B23 translocation. "B23 translocation" may therefore be a simple and rapid method for assessing the inhibition of cell growth and for determining the efficacy of combination cancer chemotherapy.

Dipyridamole enhances an anti-proliferative effect of interferon in various types of human tumor cells

The anti-proliferative activity of human interferon (HuIFN) was enhanced by dipyridamole, 2,6-bis-(diethanolamino)-4,8-dipiperidinopyrimido-[5,4-d]-py rimidine, when tested against various human tumor cell lines, including KT (breast carcinoma), PLC/PRF/5 (hepatoma), MGC-I, U251-SP and T98 (glioma), HAC-2 and SHIN-3 (ovarian carcinoma), and MM-ICB (melanoma). The enhancement occurred irrespective of the kind of HuIFN used (alpha, beta or gamma) and the original degree of susceptibility of the cells to HuIFN. Even low doses down to 0.01 microM of dipyridamole that had no intrinsic anti-proliferative activity could enhance the effect of HuIFN. The enhancement of HuIFN effects seems not to be caused by induction of HuIFN production, because neither anti-viral activity nor HuIFN antigens were detected in culture medium in cells treated with dipyridamole. Mopidamole, a derivative of dipyridamole lacking one piperidine residue, produced little enhancement of the effects of HuIFN. Among ovarian cancer cell lines tested, the enhancement of the activity of HuIFN by dipyridamole for HAC-2 and SHIN-3 cells was equivalent to or greater than that for 3 chemotherapy agents (adriamycin, vincristine, and a camptothecin derivative). However, neither HOC-21 ovarian cancer cells nor HEC-1 endometrial adenocarcinoma cells were susceptible to any combinations. When MGC-1, U251-SP, and HAC-2 cells were injected into nude mice, the growth of tumors was more markedly inhibited by the subcutaneous administration of HuIFN in combination with oral administration of dipyridamole than by the HuIFN alone. Thus, this combination therapy seems to be worth trying for human cancer, although the enhancement of the effects of HuIFN by dipyridamole varied among the cell lines examined.

5-Fluorouracil's cytotoxicity is enhanced both in vitro and in vivo by concomitant treatment with hyperthermia and dipyridamole

We obtained evidence that the cytotoxic effect of 5-fluorouracil (5-FU) is augmented when the drug is given in combination with hyperthermia (HYP) and dipyridamole (DP). Nontoxic levels of DP enhanced the combined cytotoxicity of 5-FU and HYP against B16 melanoma and human tumor cells in vitro as measured by the succinate dehydrogenase inhibition (SDI) test. Growth of B16 melanoma that had been subcutaneously implanted into the feet of C57 BL mice was inhibited by treatment with the combinations of 5-FU and HYP, of 5-FU and DP, and of 5-FU, HYP and DP as compared with the administration of 5-FU alone. Treatment with HYP plus DP did not alter the body weight of mice that received 5-FU. The administration of DP plus HYP seemed to render the tumor cells more sensitive to 5-FU. The combination of 5-FU, HYP and DP shows promise for the treatment of patients suffering from malignant disease.

Modulation of vinblastine sensitivity by dipyridamole in multidrug resistant fibrosarcoma cells lacking mdr1 expression

We examined the ability of dipyridamole (DPM) to act synergistically with vinblastine (VBL) in HT1080 fibrosarcoma cells and a drug-resistant variant, HT1080/DR4, which lacks mdr1 expression, in order to determine whether DPM requires P-glycoprotein to modulate drug sensitivity. Median effect analysis of clonogenic assay was used to produce the combination index (CI50, values less than 1 indicate synergy). DPM was mildly synergistic with VBL producing a CI50 of 0.83 +/- 0.13 for HT1080 cells and 0.80 +/- 0.16 for HT1080/DR4 cells. HT1080 and HT1080/DR4 cells accumulated 6.7 +/- 0.7 and 5.6 +/- 0.9 pmol 3H-VBL mg cells-1 at steady state (Css) and 20 microM DPM elevated the Css by 1.8 and 2.9-fold, respectively. In comparison, the CI50 was 1.1 +/- 0.2 in parental KB-3-1 cells and 0.1 +/- 0.1 in mdr1-expressing KB-GRC1 cells. The KB-3-1 and KB-GRC1 cells had a Css of 3.8 +/- 0.8 and 0.7 +/- 0.2 pmol 3H-VBL mg cells-1, respectively, and DPM elevated the Css by 9.2-fold in KB-GRC1 cells. These studies demonstrate that DPM can produce synergy independently of mdr1 expression but that much greater levels of synergy are achievable in mdr1-expressing tumour cells.

Novel mechanism of N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine in potentiation of antitumor drug action on multidrug-resistant and sensitive Chinese hamster cells

The mechanism of the synthetic isoprenoid N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine (SDB-ethylenediamine) in potentiating antitumor drug action against multidrug-resistant cells was comparatively studied with other potentiators such as verapamil and cepharanthine. SDB-ethylenediamine increased the accumulation of [3H]daunorubicin (DNR) in Chinese hamster V79 (V79/S) and its multidrug-resistant mutant (V79/ADM) cells. Even after SDB-ethylenediamine was removed from the medium, its effect continued. But when verapamil was removed from the medium, its effect disappeared immediately. Unlike verapamil and cepharanthine, SDB-ethylenediamine did not greatly inhibit the efflux of [3H]DNR from V79/ADM, the binding of [3H]vinblastine to membrane vesicles of V79/ADM, or the binding of [3H]azidopine to P-glycoprotein in the cytoplasmic membrane of V79/ADM. It did stimulate the influx of [3H]DNR into the ATP-depleted cells of V79/S and V79/ADM. Thus, SDB-ethylenediamine uniquely potentiates antitumor drugs. The increased intracellular accumulation of antitumor drugs in the presence of SDB-ethylenediamine is due not only to the inhibition of active efflux but also to the stimulation of the influx of antitumor drugs.

Modulation of antitumour drug resistance: experimental laboratory data and results of clinical evaluation

Laboratory studies have provided a number of interesting leads relating to modulation of antitumour drug resistance. Several ideas have been evaluated clinically and phase I studies with certain newer agents are planned.

Transport of the multidrug resistance modulators verapamil and azidopine in wild type and daunorubicin resistant Ehrlich ascites tumour cells

Verapamil has been proposed to modulate the multidrug resistance phenotype by competitive inhibition of an energy dependent efflux of cytotoxic drug. However, the accumulation of both 14C-verapamil and 3H-verapamil was similar in wild type EHR2 and multidrug resistant EHR2/DNR+ Ehrlich ascites cells, and was much less in both cell lines in energy deprived medium than in medium containing glucose. Azidopine accumulation was also similar in both EHR2 and EHR2/DNR+ cells but, in contrast to verapamil, did not differ significantly with changes in cellular energy levels. Azidopine photolabelled a 170 kDa protein in EHR2/DHR+ plasma membrane vesicles which was immunoprecipitated by monoclonal antibody towards P-glycoprotein. Azidopine increased daunorubicin accumulation and modulated vincristine resistance in EHR2/DNR+ cells in a similar fashion to verapamil. Azidopine photolabelling was inhibited by vincristine and verapamil, but not by daunorubicin. Vincristine, but not daunorubicin, was able to increase both azidopine and verapamil accumulation in EHR2/DNR+ cells only. Finally, though both verapamil and azidopine are a substrate for P-glycoprotein in EHR2/DNR+ cells, they do not themselves appear to be transported by the multidrug resistance efflux mechanism to any significant extent in these cells.

Cytocidal activity of a synthetic isoprenoid, N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine, and its potentiation of antitumor drugs against multidrug-resistant and sensitive cells in vitro

A synthetic isoprenoid, N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine (SDB-ethylenediamine), inhibited the colony formation of multidrug-resistant mutant cell lines derived from Chinese hamster V79 (V79/ADM) and human hepatoma PLC/PRF/5 (PLC/COL) cells to a greater extent than that of the parental cells. When combined with other clinically useful antitumor agents, it potentiated the cytotoxic activity of almost all kinds of drugs tested including adriamycin (ADM), actinomycin D, vincristine, cytosine arabinoside, and 5-fluorouracil (5-FU), and the potentiation ratios were higher against V79/ADM cells than against V79/S cells. Among the antitumor agents tested, the activities of bleomycin-group antibiotics were more strongly enhanced by SDB-ethylenediamine and the potentiation was higher in the parental cells than in V79/ADM cells. SDB-ethylenediamine enhanced the uptake of ADM and daunorubicin into V79/ADM and its parental cells, but it did not increase the uptake of 5-FU or peplomycin, indicating that different mechanisms operate for potentiation in the cases of the latter drugs, i.e., not simply an increase of intracellular drug uptake. Two fragments of SDB-ethylenediamine, solanesol (polyprenoid moiety) and the diamine component (verapamil-like moiety), showed neither cytotoxic activity nor potentiator activity, even if they were mixed together, indicating that the steric conformation of intact SDB-ethylenediamine molecule is important for these two activities.