Modulation of Cytarabine Induced Cytotoxicity Using Novel Deoxynucleoside Analogs in the HL60 Cell Line

In order to enhance the cytotoxicity of ara‐C in the HL60 cell line the following deoxynucleoside analogs were used: cladribine, fludarabine and gemcitabine. HL60 cells were co‐incubated with ara‐C and each of the modulators at the ratios of their respective IC50s. Cytotoxicity was determined with the MTT‐assay and drug interactions were evaluated with the combination index (CI) method (Calcusyn; Chou & Talalay). CI < 1, CI ± 1 and > 1 indicate synergism, additive effect and antagonism, respectively. We observed moderate synergism between ara‐C/cladribine and ara‐C/gemcitabine, with CIs of 0.76 ± 0.14 and 0.82 ± 0.04, respectively. The interaction between ara‐C/fludarabine resulted in moderate antagonism (CI = 1.29 ± 0.11). In conclusion, in this in vitro study we showed that the cytotoxicity of ara‐C can be succesfully modulated in the HL60 cell line by cladribine and gemcitabine.     

Micro-Array Analysis of Resistance for Gemcitabine Results in Increased Expression of Ribonucleotide Reductase Subunits

To study in detail the relation between gene expression and resistance against gemcitabine, a cell line was isolated from a tumor for which gemcitabine resistance was induced in vivo. Similar to the in vivo tumor, resistance in this cell line, C 26-G, was not related to deficiency of deoxycytidine kinase (dCK). Micro-array analysis showed increased expression of ribonucleotide reductase (RR) subunits M1 and M2 as confirmed by real time PCR analysis (28- and 2.7-fold, respectively). In cell culture, moderate cross-resistance (about 2-fold) was observed to 1-ß-D-arabinofuranosylcytosine (ara-C), 2-chloro-2’deoxyadenosine (CdA), LY231514 (ALIMTA), and cisplatin (CDDP), and pronounced cross-resistance (>23-fold) to 2′,2′-difluorodeoxyuridine (dFdU) and 2′,2′-difluorodeoxyguanosine (dFdG). Culture in the absence of gemcitabine reduced resistance as well as RRM1 RNA expression, demonstrating a direct relationship of RRM1 RNA expression with acquired resistance to gemcitabine.     

Micro-array analysis of resistance for gemcitabine results in increased expression of ribonucleotide reductase subunits

To study in detail the relation between gene expression and resistance against gemcitabine, a cell line was isolated from a tumor for which gemcitabine resistance was induced in vivo. Similar to the in vivo tumor, resistance in this cell line, C 26-G, was not related to deficiency of deoxycytidine kinase (dCK). Micro-array analysis showed increased expression of ribonucleotide reductase (RR) subunits M1 and M2 as confirmed by real time PCR analysis (28- and 2.7-fold, respectively). In cell culture, moderate cross-resistance (about 2-fold) was observed to 1-ss-D-arabinofuranosylcytosine (ara-C), 2-chloro-2'deoxyadenosine (CdA), LY231514 (ALIMTA), and cisplatin (CDDP), and pronounced cross-resistance (>23-fold) to 2',2'-difluorodeoxyuridine (dFdU) and 2',2'-difluorodeoxyguanosine (dFdG). Culture in the absence of gemcitabine reduced resistance as well as RRM1 RNA expression, demonstrating a direct relationship of RRM1 RNA expression with acquired resistance to gemcitabine.     

Pharmacological Reversal of Histone Methylation Presensitizes Pancreatic Cancer Cells to Nucleoside Drugs: In Vitro Optimization and Novel Nanoparticle Delivery Studies

We evaluated the potential of an investigational histone methylation reversal agent, 3-deazaneplanocin A (DZNep), in improving the chemosensitivity of pancreatic cancer to nucleoside analogs (i.e., gemcitabine). DZNep brought delayed but selective cytotoxicity to pancreatic cancer cells without affecting normal human pancreatic ductal epithelial (HPDE) cells. Co-exposure of DZNep and gemcitabine induced cytotoxic additivity or synergism in both well- and poorly-differentiated pancreatic cell lines by increased apoptosis. In contrast, DZNep exerted antagonism with gemcitabine against HPDE cells with significant reduction in cytotoxicity compared with the gemcitabine-alone regimen. DZNep marginally depended on purine nucleoside transporters for its cytotoxicity, but the transport dependence was circumvented by acyl derivatization. Drug exposure studies revealed that a short priming with DZNep followed by gemcitabine treatment rather than co-treatment of both agents to produce a maximal chemosensitization response in both gemcitabine-sensitive and gemcitabine-resistant pancreatic cancer cells. DZNep rapidly and reversibly decreased trimethylation of histone H3 lysine 27 but increased trimethylation of lysine 9 in an EZH2- and JMJD1A/2C-dependent manner, respectively. However, DZNep potentiation of nucleoside analog chemosensitization was found to be temporally coupled to trimethylation changes in lysine 27 and not lysine 9. Polymeric nanoparticles engineered to chronologically release DZNep followed by gemcitabine produced pronounced chemosensitization and dose-lowering effects. Together, our results identify that an optimized DZNep exposure can presensitize pancreatic cancer cells to anticancer nucleoside analogs through the reversal of histone methylation, emphasizing the promising clinical utilities of epigenetic reversal agents in future pancreatic cancer combination therapies.     

UV mutagenesis, cytotoxicity and split-dose recovery in a human-CHO cell hybrid having intermediate (6-4) photoproduct repair

Somatic cell hybrids constructed between UV-hypersensitive Chinese hamster ovary cell line UV20 and human lymphocytes were used to examine the influence of a human DNA repair gene, ERCC1, on UV photoproduct repair, mutability at several drug-resistance loci, UV cytotoxicity and UV split-dose recovery. In hybrid cell line 20HL21-4, which contains human chromosome 19, UV-induced mutagenesis at the APRT, HPRT and Na+/K+-ATPase loci was comparable to that in repair-proficient CHO AA8 cells, whereas cell line 20HL21-7, a reduced human-CHO hybrid not containing human chromosome 19, exhibited a hypermutable phenotype at all 3 loci indistinguishable from that of UV20 cells. The response of 20HL21-4 cells to UV cytotoxicity reflected substantial but incomplete restoration of wild-type UV cytotoxic response, whereas responses of UV20 and 20HL21-7 cell lines to UV cytotoxicity were essentially the same, reflecting several-fold UV hypersensitivity. Repair of UV-induced (5-6) cyclobutane dimers and (6-4) photoproducts was examined by radioimmunoassay; (6-4) photoproduct repair was deficient in UV20 and 20HL21-7 cell lines, and intermediate in 20HL21-4 cells relative to wild-type CHO AA8 cells. UV split-dose recovery in 20HL21-4 cells was also intermediate relative to AA8 cells. These results show that the human ERCC1 gene on chromosome 19 is responsible for substantial restoration of UV survival and mutation responses in repair-deficient UV20 cells, but only partially restores (6-4) UV photoproduct repair and UV split-dose recovery.     

Radiotherapy combined with gemcitabine and oxaliplatin in pancreatic cancer cells

Clinical evidence suggests that gemcitabine (Gem) plus oxaliplatin (Ox) is superior to gemcitabine alone in advanced pancreatic carcinoma. The addition of radiation to gemcitabine improves response and is a standard treatment for locally advanced disease. We investigated the effect of oxaliplatin on gemcitabine-based chemoradiation by determining whether gemcitabine and oxaliplatin produced synergistic cytotoxicity using median effect analysis and radiosensitization using clonogenic survival assays. We analyzed the effects of gemcitabine and oxaliplatin on cell cycle distribution by DNA content and on radiation-induced DNA damage repair by phosphorylated H2AX (gamma-H2AX). Gemcitabine and oxaliplatin produced schedule-dependent synergistic cytotoxicity in BxPC-3 and Panc-1 cells (combination indices: 0.76 +/- 0.05, 0.61 +/- 0.11). In BxPC-3 cells, oxaliplatin did not affect gemcitabine-mediated radiosensitization (Gem 1.99 +/- 0.27; Gem + Ox 2.38 +/- 0.30). In Panc-1 cells, oxaliplatin significantly enhanced gemcitabine-mediated radiosensitization (Gem 1.31 +/- 0.05; Gem + Ox 2.90 +/- 0.31). Radiosensitization by gemcitabine was accompanied by early S-phase arrest and induction/persistence of gamma-H2AX protein, which were unaltered by oxaliplatin. Addition of oxaliplatin to gemcitabine produces radiosensitization equal to or greater than gemcitabine alone, supporting our clinical investigation of oxaliplatin with gemcitabine-radiation in pancreatic cancer aimed at improving systemic disease control while maintaining local tumor radiosensitization.     

Cytotoxic effects of new trans-2,4-diaryl-r-3-methyl-1,2,3,4-tetrahydroquinolines and their interaction with antitumoral drugs gemcitabine and paclitaxel on cellular lines of human breast cancer

Two tetrahydroquinoline compounds, called DM8 and DM12, from a new series of the cis-2,4-diaryl-r-3-methyl-1,2,3,4-tetrahydroquinolines, were selected for cytotoxic effects studies on cellular lines of human breast cancer. The synergistic, additive and antagonistic effects in combination of these compounds with anticancer drugs, such as paclitaxel and gemcitabine, were studied. The isobolograms and their analysis demonstrated models of synergism, additivity and antagonism of these tetrahydroquinolines in the presence of paclitaxel and gemcitabine. Results showed that compounds DM8 and DM12 individually induced growth inhibition on breast cancer cell lines MCF-7 and SKBR3, and the addition of paclitaxel and gemcitabine intensified their cytotoxic activity on both cell lines at conc. below 1 μg/mL. During these studies the compound DM12 was identified as new, perspective and safe agent for adjuvant therapy.     

Flow-cytometric determination of intracellular pH, esterase activity and cell volume in human leukemic cell lines following in vitro incubation with cytostatic drugs

A recently developed flow cytometric assay method using patient tumor cells allows the determination not only of their sensitivity to cytostatic drugs but also of biochemical and biophysical parameters after treatment, such as esterase concentration and intracellular pH of the living cells. DNA-content of the dead cells and cell volume of living and dead cells. The T-cell lines CEM, Molt4, Jurkat, the B-cell lines RPMI1788, Daudi, Raji and the promyelocytic line HL60 were incubated with: cytosine arabinoside (ara-C), L-asparaginase, daunorubicin, vincristine and prednisone for 48 h. Living cells then stained with esterase and pH-dye 1,4-diacetoxy-2,3-dicyanobenzene (ADB) and dead cells with DNA-dye propidium-iodide (PI). The esterase concentration, an index of metabolic activity, decreased in the T-cell lines under the influence of ara-C, daunorubicin and vincristine, whereas in the B-cell lines smaller changes in esterase concentration were observed (P less than 0.001). A decrease in intracellular pH was seen in the ara-C and daunorubicin-incubated cells Molt4, CEM and HL60, whereas in the B-cell lines no significant change in intracellular pH was found. In all lines except Jurkat the cell volume of the surviving cells increased under the influence of certain drugs (primarily ara-C and daunorubicin); B-cell lines showed a greater swelling than T-cell lines (P = 0.001).     

Cell cycle effect on the activity of deoxynucleoside analogue metabolising enzymes

Deoxynucleoside analogues (dNAs) are cytotoxic towards both replicating and indolent malignancies. The impact of fluctuations in the metabolism of dNAs in relation to cell cycle could have strong implications regarding the activity of dNAs. Deoxycytidine kinase (dCK) and deoxyguanosine kinase (dGK) are important enzymes for phosphorylation/activation of dNAs. These drugs can be dephosphorylated/deactivated by 5'-nucleotidases (5'-NTs) and elevated activities of 5'-NTs and decreased dCK and/or dGK activities represent resistance mechanisms towards dNAs. The activities of dCK, dGK, and three 5'-NTs were investigated in four human leukemic cell lines in relationship to cell cycle progression and cytotoxicity of dNAs. Synchronization of cell cultures to arrest in G0/G1 by serum-deprivation was performed followed by serum-supplementation for cell cycle progression. The activities of dCK and dGK increased up to 3-fold in CEM, HL60, and MOLT-4 cells as they started to proliferate, while the activity of cytosolic nucleotidase I was reduced in proliferating cells. CEM, HL60, and MOLT-4 cells were also more sensitive to cladribine, cytarabine, 9-beta-D-arabinofuranosylguanine and clofarabine than K562 cells which demonstrated lower levels and less alteration of these enzymes and were least susceptible to the cytotoxic effects of most dNAs. The results suggest that, in the cell lines studied, the proliferation process is associated with a general shift in the direction of activation of dNAs by inducing activities of dCK/dGK and reducing the activity of cN-I which is favourable for the cytotoxic effects of cladribine, cytarabine and, 9-beta-D-arabinofuranosylguanine. These results emphasize the importance of cellular proliferation and dNA metabolism by both phosphorylation and dephosphorylation for susceptibility to dNAs. It underscores the need to understand the mechanisms of action and resistance to dNAs in order to increase efficacy of dNAs treatment by new rational.     

Increased Cytotoxicity of 2′,2′‐Difluoro‐2′‐Deoxycytidine in Human Leukemic Cell‐Lines After a Preincubation with Cyclopentenyl Cytosine

The in vitro modulating effect of Cyclopentenyl cytosine (CPEC) on the metabolism of gemcitabine was studied in lymphocytic and myeloid leukemic cell‐lines. In MOLT‐3 cells, that were pretreated with CPEC, the incorporation of 2′,2′‐difluoro‐2′‐deoxycytidine triphosphate (dFdCTP) into DNA was significantly increased by 57–99% in comparison with cells that were only treated with gemcitabine. The increased incorporation of dFdCTP into DNA in CPEC pretreated cells was paralleled by an increase in apoptotic and necrotic cells of 17–34%. In HL‐60 cells that were preincubated with CPEC, increased concentrations of the mono‐/di‐ and triphosphate form of gemcitabine were observed, as well as an increased incorporation of dFdCTP into DNA (+ 773%). This increased incorporation was paralleled by a significant increase in apoptosis and necrosis. We conclude that CPEC enhances the incorporation of dFdCTP into DNA and thus increases the cytotoxicity of gemcitabine in lymphocytic and myeloid leukemic cell‐lines.     

Retinoylation of proteins in leukemia, embryonal carcinoma, and normal kidney cell lines: differences associated with differential responses to retinoic acid

In HL60 cells a nuclear protein of Mr 55,000 is retinoylated, with the formation of a thioester bond. To gain further knowledge on the role of retinoylation we studied it in cell lines with varied responses to retinoic acid (RA). Compared to HL60 the extent of retinoylation (mol/cell) was about fivefold higher in HL60/MRI, a mutant which is more sensitive to RA than HL60. Retinoylation occurred to the same extent and at similar rates in HL60 and in HL60/RA-res, a mutant resistant to differentiation by RA. One-dimensional polyacrylamide gel electrophoresis patterns for the three HL60 cell lines were similar. However, two-dimensional polyacrylamide gel electrophoresis patterns of the three HL60 cell lines were distinct. While we saw the same major retinoylated protein of Mr 55,000 in the three cell lines, the HL60/RA-res cells also contained a high level of a protein with the same Mr and a lower pI. The extent of retinoylation was greater in the RA-sensitive embryonal carcinoma cell line, PCC4.aza1R, than in a RA-resistant cell line, PCC4.(RA)-2. One-dimensional polyacrylamide gel electrophoresis patterns of retinoylated proteins of the embryonal carcinoma cell lines were different from HL60 and from each other. The retinoylation pattern of the normal canine kidney cell line (MDCK) was different from either HL60 or the embryonal carcinoma cells. These results showed the retinoylation was widespread and that the response to RA of different cell types may depend on the retinoylation of specific proteins.     

Synthesis and antiproliferative potency of 9-beta-D-arabinofuranosyl-2-fluoroadenine phospholipid adducts.

Three novel alkylphospholipid and four novel O-alkylglycerophospholipid derivatives of fludarabine (F-ara-AMP), known as a drug for the clinical treatment of chronic lymphocytic leukemia, were synthesized. The antiproliferative activity was determined in comparison to the parent nucleoside fludarabine in an immortalized but nontumorigenic human mammary epithelial cell line (H 184 A1N4), in two human breast tumor cell lines (MaTu and MCF7), and in two leukemic cell lines (HL 60 and Daudi). Fludarabine inhibited the growth of the leucemic cell lines very effectively. The breast tumor cell lines responded with much less sensitivity. The antiproliferative potency of the new compounds strongly depended on the chemical structure of the lipid component, and derivatives with a high effectiveness against one or both of the breast tumor cell lines were described.     

Prostaglandin E2 regulates vitamin D receptor expression, vitamin D-24-hydroxylase activity and cell proliferation in an adherent human myeloid leukemia cell line (Ad-HL60).

The effects of prostaglandin E2, forskolin, and phorbol 12-myristate 13-acetate on cell proliferation, cell surface antigen expression, vitamin D-24-hydroxylase activity and vitamin D receptor (VDR) expression have been studied in an adherent variant (Ad-HL60) of the human HL60 promyelomonocytic leukemia cell line. Ad-HL60 cells have a more differentiated phenotype than the nonadherent HL60 cells from which they were derived and, unlike the parent cell line, constitutively express vitamin D-24-hydroxylase activity. Treatment of Ad-HL60 cells with 1 microM PGE2 resulted in a decrease in the rate of cell proliferation (cell numbers were approximately 23% of control values after 72 h treatment), a change in expression of leukocyte surface antigens (decreased CD13 and CD14, increased CD11b and CD49d expression), an increase in the synthesis of 24,25-dihydroxyvitamin D3 from substrate 25-hydroxyvitamin D3 (control 5.76 +/- 0.17, 72 h PGE2-treated cells 12.10 +/- 1.90 pmol/h/10(6) cells), and an increase in receptors for the active metabolite of vitamin D, 1 alpha,25-dihydroxyvitamin D3, from 3910 to 11285 receptors per cell in control and 7-day treated cells, respectively. Prostaglandin E2 may be acting via a mechanism involving cyclic AMP in these cells, as we have also demonstrated that 10 microM forskolin, an adenylate cyclase activator, has similar effects. Phorbol 12-myristate 13-acetate had little effect on any of the parameters measured in this cell line.     

Cathepsin B synthesis by the HL60 promyelocytic cell line: effects of stimulating agents and anti-inflammatory compounds

Cathepsin B synthesis by the human HL60 promyelocyte cell line was investigated by immunohistochemistry and by the assay of the enzyme in cell lysates using a fluorimetric substrate. HL60 cells were shown to produce cathepsin B in response to treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). Intracellular levels of cathepsin B and immunohistochemical staining of the enzyme were related to time in culture with increasing concentrations of TPA from 1 nmol/1 to 8.0 nmol/1. Synthesis of cathepsin B was associated with TPA-induced phagocytic activity of cells in culture, expression of alpha-naphthyl acetate esterase and reduced cell division. Cathepsin B production was, therefore, related to differentiation of the HL60 promyelocytes into mature macrophage-like cells. Cathepsin B activity in HL60 cell lysates was significantly increased by incubation of the cells with 10 micrograms/ml endotoxin (lipopolysaccharide) from Escherichia coli, but not carrageenan. The production of cathepsin B by TPA-induced HL60 cells was significantly reduced by 0.25 mumol/1 dexamethasone and the non-steroidal anti-inflammatory compound 4-(6-methoxy-2-naphthyl)-butan-2-one but not by indomethacin. The HL60 promyelocytic cell line is a useful model for the study of factors affecting proteinase synthesis by human mononuclear phagocytes.     

Flow cytometric correlation of the c-myc oncoprotein and cell cycle kinetics of HL60 leukaemia during induced maturation with cytosine arabinoside and dimethylsulphoxide

Abstract The c-myc oncogene codes for a DNA binding protein that functions in a cell cycle-related manner. A useful model for studying the relationship of c-myc expression with cell cycle kinetics is the HL60 cell line. HL60 cells constitutively express high levels of c-myc mRNA; however, the level can be down-regulated as the cells are induced to differentiate. We have developed a flow cytometric assay for correlating c-myc oncoprotein levels with DNA content. C-myc oncoprotein levels were additionally correlated with c-myc mRNA levels as determined by slot blot hybridization. Dimethylsulphoxide (DMSO) and cytosine arabinoside were used to induce granulocytic and monocytic maturation respectively. Treatment of HL60 cells with DMSO leads to an increase in the per cent of cells in G₁/G₀ and a decrease in mean c-myc mRNA and oncoprotein levels. The cells with G₁ DNA content show the greatest decrease in c-myc protein. ARA-c treatment of HL60 cells leads to a slowing and an accumulation of cells in S phase with a moderate decrease in mean mRNA and only a slight decrease in mean c-myc protein levels. These data support the hypothesis that c-myc is involved in the switch from G₁ to G₀.     

Cellular resistance against troxacitabine in human cell lines and pediatric patient acute myeloid leukemia blast cells

Troxacitabine is a cytotoxic deoxycytidine analogue with an unnatural L-configuration, which is activated by deoxycytidine kinase (dCK). The configuration is responsible for differences in the uptake and metabolism of troxacitabine compared to other deoxynucleoside analogues. To determine whether troxacitabine has an advantage over other nucleoside analogues several cell lines resistant to cladribine and gemcitabine were exposed to troxacitabine, while blast cells from pediatric leukemia patients were tested for cross-resistance with other deoxynucleoside analogues. The gemcitabine resistant AG6000 (IC50: >3000 nM), and the cladribine resistant CEM (IC50: 150 nM) and HL-60 (IC50: >3000 nM) cell lines, all with no or decreased dCK expression, were less sensitive to troxacitabine than their wild type counterparts (IC50; A2780: 410, CEM: 71 and HL-60: 158 nM). dCK protein expression in CEM was higher than in HL-60, which, in turn, was higher than in A2780. Catalytically inactive p53 seems to increase the sensitivity to troxacitabine. The patient samples showed a large range of sensitivity to troxacitabine, similar to other deoxynucleoside analogues. Cross-resistance with all other deoxynucleoside analogues was observed.     

Flow cytometric correlation of the c-myc oncoprotein and cell cycle kinetics of HL60 leukaemia during induced maturation with cytosine arabinoside and dimethylsulphoxide

The c-myc oncogene codes for a DNA binding protein that functions in a cell cycle-related manner. A useful model for studying the relationship of c-myc expression with cell cycle kinetics is the HL60 cell line. HL60 cells constitutively express high levels of c-myc mRNA; however, the level can be down-regulated as the cells are induced to differentiate. We have developed a flow cytometric assay for correlating c-myc oncoprotein levels with DNA content. C-myc oncoprotein levels were additionally correlated with c-myc mRNA levels as determined by slot blot hybridization. Dimethylsulphoxide (DMSO) and cytosine arabinoside were used to induce granulocytic and monocytic maturation respectively. Treatment of HL60 cells with DMSO leads to an increase in the per cent of cells in G1/G0 and a decrease in mean c-myc mRNA and oncoprotein levels. The cells with G1 DNA content show the greatest decrease in c-myc protein. ARA-c treatment of HL60 cells leads to a slowing and an accumulation of cells in S phase with a moderate decrease in mean mRNA and only a slight decrease in mean c-myc protein levels. These data support the hypothesis that c-myc is involved in the switch from G1 to G0.     

Resistance to 1,25-dihydroxyvitamin D3 of a deoxycytidine kinase-deficient variant of human leukemia HL60 cells.

A deoxycytidine kinase-deficient variant of HL60 cells (HL60-araC), isolated by its resistance to 1-beta-D-arabinofuranosyl cytosine (ara-C), shows cross-resistance to the differentiation-inducing and growth-inhibitory effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). This is not due to the lack of uptake of 1,25(OH)2D3 by HL60-araC cells, shown by an increased rate of intracellular accumulation of [3H]-1,25(OH)2D3, or to the lack of expression of the gene for the vitamin D3 receptor. However, down-modulation of the expression of this gene by 1,25(OH)2D3 is markedly delayed in HL60-araC cells, and the down-regulation of the expression of the c-myc gene is also delayed. In contrast, the expression of the constitutively expressed 16S mitochondrial rRNA gene is unchanged by 1,25(OH)2D3 treatment of either cell subline. These findings suggest that some cases of drug resistance may be associated with defective functioning of a differentiation pathway.     

Cisplatin cytotoxicity is enhanced with zoledronic acid in A549 lung cancer cell line: preliminary results of an in vitro study

We tested whether zoledronic acid, a biphosphonate with proposed apoptotic activity, augmented the cytotoxicity of cisplatin and/or gemcitabine in A549 lung cancer cell line. This cell line was subjected to different concentrations of the above chemotherapeutic agents and zoledronic acid. Cytotoxicity was assessed by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrasodium bromide) assay. Particularly, zoledronic acid in 100 micromolar (microM) concentration augmented the cytotoxicity by cisplatin 1microg/ml from 25% to 70% (Z=3.22, P=0.0072). A significant portion of cells underwent apoptosis with or without zoledronic acid, but more so with the combination treatment as assessed by an Annexin V-FITC apoptosis detection kit. However, 100microM zoledronic acid showed 50% cytotoxicity on its own, but failed to improve cytotoxicity by Gemcitabine. Thus, we show for the first time in a lung cancer cell line that zoledronic acid bears cytotoxic potential on its own and in conjunction with cisplatin. The clinical potential of this finding should be further studied.