Differential Incorporation of Ara-C, Gemcitabine, and Fludarabine Into Replicating and Repairing DNA in Proliferating Human Leukemia Cells

Diagnostic Microdosing Approach to Study Gemcitabine Resistance

Gemcitabine metabolites cause the termination of DNA replication and induction of apoptosis. We determined whether subtherapeutic "microdoses" of gemcitabine are incorporated into DNA at levels that correlate to drug cytotoxicity. A pair of nearly isogenic bladder cancer cell lines differing in resistance to several chemotherapy drugs were treated with various concentrations of ¹⁴C-labeled gemcitabine for 4-24 h. Drug incorporation into DNA was determined by accelerator mass spectrometry. A mechanistic analysis determined that RRM2, a DNA synthesis protein and a known resistance factor, substantially mediated gemcitabine toxicity. These results support gemcitabine levels in DNA as a potential biomarker of drug cytotoxicity.

LC-based targeted metabolomics analysis of nucleotides and identification of biomarkers associated with chemotherapeutic drugs in cultured cell models

Treatment of mammalian cells with chemotherapeutic drugs can result in perturbations of nucleotide pools. Monitoring these perturbations in cultured tumor cells from human sources is useful for assessment of the effect of drug therapy and a better understanding of the mechanism of action of these drugs. In this study, three classes of chemotherapeutic drugs with different mechanisms of action were used in the development of drug-treated cell models. The LC-based targeted metabolomics analysis of nucleotides in cells of the control group and the drug-treated group was carried out. Several data processing methods were combined for the identification of potential biomarkers associated with the action of drugs, including one-way analysis of variance, principal component analysis, and receiver operating characteristic curves. Intriguingly, tumor cells of both the control group and the drug-treated groups can be distinguished from each other, and several variables were recognized as potential biomarkers, such as ATP, GMP, and UDP for antimetabolite agents, ATP, GMP, and CTP for DNA-damaging agents, as well as GMP, ATP, UDP, and GDP for the mitotic spindle agents. Further validation of the potential biomarkers was performed using the receiver operating characteristic curve. Considering their corresponding area under the curve, which was larger than 0.9, it can be concluded that GMP and ATP are the best potential biomarkers for DNA-damaging drugs, as well as GMP, ATP, and UDP for the other two classes of drugs. This limited nucleotide approach cannot completely distinguish the mechanisms of the nine drugs, but it provides preliminary evidence for the role of pharmacometabolomics in the preclinical development of drugs at least.

Preclinical development of the novel Chk1 inhibitor SCH900776 in combination with DNA-damaging agents and antimetabolites

Many anticancer agents damage DNA and arrest cell-cycle progression primarily in S or G(2) phase of the cell cycle. Previous studies with the topoisomerase I inhibitor SN38 have shown the efficacy of the Chk1 inhibitor UCN-01 to overcome this arrest and induce mitotic catastrophe. UCN-01 was limited in clinical trials by unfavorable pharmacokinetics. SCH900776 is a novel and more selective Chk1 inhibitor that potently inhibits Chk1 and abrogates cell-cycle arrest induced by SN38. Like UCN-01, abrogation of SN38-induced arrest enhances the rate of cell death but does not increase overall cell death. In contrast, SCH900776 reduced the growth-inhibitory concentration of hydroxyurea by 20- to 70-fold. A similar magnitude of sensitization was observed with cytarabine. A 5- to 10-fold sensitization occurred with gemcitabine, but no sensitization occurred with cisplatin, 5-fluorouracil, or 6-thioguanine. Sensitization occurred at hydroxyurea concentrations that marginally slowed DNA replication without apparent activation of Chk1, but this led to dependence on Chk1 that increased with time. For example, when added 18 hours after hydroxyurea, SCH900776 induced DNA double-strand breaks consistent with rapid collapse of replication forks. In addition, some cell lines were highly sensitive to SCH900776 alone, and these cells required lower concentrations of SCH900776 to sensitize them to hydroxyurea. We conclude that some tumors may be very sensitive to the combination of SCH900776 and hydroxyurea. Delayed administration of SCH900776 may be more effective than concurrent treatment. SCH900776 is currently in phase I clinical trials, and these results provide the rationale and schedule for future clinical trials.

Dynamic metabolic labeling of DNA in vivo with arabinosyl nucleosides

Commonly used metabolic labels for DNA, including 5-ethynyl-2'-deoxyuridine (EdU) and BrdU, are toxic antimetabolites that cause DNA instability, necrosis, and cell-cycle arrest. In addition to perturbing biological function, these properties can prevent metabolic labeling studies where subsequent tissue survival is needed. To bypass the metabolic pathways responsible for toxicity, while maintaining the ability to be metabolically incorporated into DNA, we synthesized and evaluated a small family of arabinofuranosyl-ethynyluracil derivatives. Among these, (2'S)-2'-deoxy-2'-fluoro-5-ethynyluridine (F-ara-EdU) exhibited selective DNA labeling, yet had a minimal impact on genome function in diverse tissue types. Metabolic incorporation of F-ara-EdU into DNA was readily detectable using copper(I)-catalyzed azide-alkyne "click" reactions with fluorescent azides. F-ara-EdU is less toxic than both BrdU and EdU, and it can be detected with greater sensitivity in experiments where long-term cell survival and/or deep-tissue imaging are desired. In contrast to previously reported 2'-arabino modified nucleosides and EdU, F-ara-EdU causes little or no cellular arrest or DNA synthesis inhibition. F-ara-EdU is therefore ideally suited for pulse-chase experiments aimed at "birth dating" DNA in vivo. As a demonstration, Zebrafish embryos were microinjected with F-ara-EdU at the one-cell stage and chased by BrdU at 10 h after fertilization. Following 3 d of development, complex patterns of quiescent/senescent cells containing only F-ara-EdU were observed in larvae along the dorsal side of the notochord and epithelia. Arabinosyl nucleoside derivatives therefore provide unique and effective means to introduce bioorthogonal functional groups into DNA for diverse applications in basic research, biotechnology, and drug discovery.

An RNA aptamer that specifically binds pancreatic adenocarcinoma up-regulated factor inhibits migration and growth of pancreatic cancer cells

Previously, we reported that a novel secretory protein, pancreatic adenocarcinoma up-regulated factor (PAUF), which is highly expressed in pancreatic cancer and mediates the growth and metastasis of pancreatic cancer cells. In this study, we generated and characterized a 2'-fluoropyrimidine modified RNA aptamer (P12FR2) directed against human PAUF. P12FR2 binds specifically to human PAUF with an estimated apparent K(D) of 77nM. P12FR2 aptamer inhibits PAUF-induced migration of PANC-1, human pancreatic cancer cells, in a wound healing assay. Moreover, intraperitoneal injection of P12FR2 decreased tumor growth by about 60% in an in vivo xenograft model with CFPAC-1 pancreatic cancer cells, without causing a loss of weight in the treated mice. Taken together, we propose here that PAUF-specific RNA aptamer, P12FR2, has the potential to be effective in the therapy of human pancreatic cancer.

Incorporation of gemcitabine and cytarabine into DNA by DNA polymerase beta and ligase III/XRCC1

1-Beta-D-arabinofuranosylcytosine (cytarabine, araC) and 2',2'-difluoro-2'-deoxycytidine (gemcitabine, dFdC), are effective cancer chemotherapeutic agents due to their ability to become incorporated into DNA and then subsequently inhibit DNA synthesis by replicative DNA polymerases. However, the impact of these 3'-modified nucleotides on the activity of specialized DNA polymerases has not been investigated. The role of polymerase beta and base excision repair may be of particular importance due to the increased oxidative stress in tumors, increased oxidative stress caused by chemotherapy treatment, and the variable amounts of polymerase beta in tumors. Here we directly investigate the incorporation of the 5'-triphosphorylated form of araC, dFdC, 2'-fluoro-2'-deoxycytidine (FdC), and cytidine into two nicked DNA substrates and the subsequent ligation. Opposite template dG, the relative k(pol)/K(d) for incorporation was dCTP > araCTP, dFdCTP >> rCTP. The relative k(pol)/K(d) for FdCTP depended on sequence. The effect on k(pol)/K(d) was due largely to changes in k(pol) with no differences in the affinity of the nucleoside triphosphates to the polymerase. Ligation efficiency by T4 ligase and ligase III/XRCC1 was largely unaffected by the nucleotide analogues. Our results show that BER is capable of incorporating araC and dFdC into the genome.

Targeted delivery of antisense oligodeoxynucleotide by transferrin conjugated pH-sensitive lipopolyplex nanoparticles: a novel oligonucleotide-based therapeutic strategy in acute myeloid leukemia

Therapeutic use of oligodeoxynucleotides (ODNs) that hybridize to and downregulate target mRNAs encoding proteins that contribute to malignant transformation has a sound rationale, but has had an overall limited clinical success in cancer due to insufficient intracellular delivery. Here we report a development of formulations capable of promoting targeted delivery and enhanced pharmacologic activity of ODNs in acute myeloid leukemia (AML) cell lines and patient primary cells. In this study, transferrin (Tf) conjugated pH-sensitive lipopolyplex nanoparticles (LPs) were prepared to deliver GTI-2040, an antisense ODN against the R2 subunit of ribonucleotide reductase that has been shown to contribute to chemoresistance in AML. LPs had an average particle size around 110 nm and a moderately positive zeta potential at approximately 10 mV. The ODN encapsulation efficiency of LPs was >90%. These nanoparticles could release ODNs at acidic endosomal pH and facilitate the cytoplasmic delivery of ODNs after endocytosis. In addition, Tf-mediated targeted delivery of GTI-2040 was achieved. R2 downregulation at both mRNA and protein levels was improved by 8-fold in Kasumi-1 cells and 2- to 20-fold in AML patient primary cells treated with GTI-2040-Tf-LPs, compared to free GTI-2040 treatment. Moreover, Tf-LPs were more effective than nontargeted LPs, with 10 to 100% improvement at various concentrations in Kasumi-1 cells and an average of 45% improvement at 3 microM concentration in AML patient primary cells. Treatment with 1 microM GTI-2040-Tf-LPs sensitized AML cells to the chemotherapy agent cytarabine, by decreasing its IC(50) value from 47.69 nM to 9.05 nM. This study suggests that the combination of pH sensitive LP formulation and Tf mediated targeting is a promising strategy for antisense ODN delivery in leukemia therapy.

Gemcitabine and docetaxel in metastatic sarcoma: past, present, and future

Objective. In the era of oral molecular kinase inhibitors, cytotoxic chemotherapy agents are somewhat overlooked, but remain the backbone of treatment for most cancers. Patients with non-gastrointestinal stromal tumor sarcomas, such as leiomyosarcoma, liposarcoma, and undifferentiated high-grade pleomorphic sarcoma (formerly called malignant fibrous histiocytoma), have received doxorubicin and ifosfamide as the backbone of their treatment for over 15 years or more. The goal of this article is to review the data that have led to the use of gemcitabine and docetaxel as a useful combination for patients with metastatic sarcomas, and to comment on possible synergy of the combination. Methods and results. The literature regarding the use of gemcitabine, docetaxel, or both, is reviewed, with emphasis on patients with metastatic sarcoma. Results. Activity of gemcitabine and docetaxel is observed in leiomyosarcoma and undifferentiated high-grade pleomorphic sarcoma. There is apparent schedule dependence of the combination in other cancers; it is unclear if schedule matters in patients with sarcomas. The dose and schedule of gemcitabine and docetaxel examined in phase II studies are probably too high for routine practice. Conclusions. The combination of gemcitabine and docetaxel is an effective option for patients with metastatic sarcoma, increasing the armamentarium for the practicing oncologist in treating this heterogeneous group of diseases. Given the low response rate to docetaxel as a single agent, it is likely that there is true clinical synergy of the combination. Disclosure of potential conflicts of interest is found at the end of this article.

Correlation of nucleoside and nucleobase transporter gene expression with antimetabolite drug cytotoxicity

Antimetabolite drugs that inhibit nucleic acid metabolism are widely used in cancer chemotherapy. Nucleoside and nucleobase transporters are important for the cellular uptake of nucleic acids and their corresponding anticancer analogue drugs. Thus, these transporters may play a role both in antimetabolite drug sensitivity, by mediating the uptake of nucleoside analogues, and in antimetabolite drug resistance, by mediating the uptake of endogenous nucleosides that may rescue cells from toxicity. Therefore, we examined the relation of the expression of nucleoside and nucleobase transporters to antimetabolite cytotoxicity. We measured the RNA levels of all eight known nucleoside and nucleobase transporters in 50 cell lines included in the National Cancer Institute's Anticancer Drug Screen panel. RNA levels of concentrative nucleoside transporters (CNTs), equilibrative nucleoside transporters (ENTs) and nucleobase transporters (NCBTs) were determined by quantitative RT-PCR using real-time fluorescence acquisition. This method was validated by measuring the expression of the MDR1 gene, and correlating our results with independently determined measurements of MDR1 RNA levels and protein function in these cell lines. We then correlated the pattern of RNA levels to the pattern of cytotoxicity of anticancer drugs in the NCI drug screen database using the COMPARE analysis. Several hypothesized relations between transporter gene expression and cytotoxicity, based upon known interactions between certain nucleoside analogues and transporter proteins, were not observed, suggesting that expression of individual transporters may not be a significant determinant of the cytotoxicity of these drugs. The most closely correlated drug cytotoxicity patterns to transporter gene expression patterns (where increased expression corresponds to increase sensitivity) included those between CNT1 and O6-methylguanine and between ENT2 and hydroxyurea. We also observed that p53 status influenced correlations between ENT1 transporter gene RNA levels and sensitivity to the drugs tiazafurin, AZQ and 3-deazauridine. One of three drugs identified by correlation of cytotoxicity patterns with ENT1 RNA levels, 3-deazauridine, inhibited uptake of the classic ENT1 substrate uridine, demonstrating a physical interaction between an identified drug and the transporter. These studies demonstrate that it is possible to correlate genetic information to functional databases to determine the influence of transport gene expression on drug sensitivity and to identify transporter-drug interactions.

Nitric oxide enhancement of fludarabine cytotoxicity for B-CLL lymphocytes

Fludarabine is active but not curative in the treatment of chronic lymphocytic leukemia (B-CLL). Nitric oxide (NO) supplied from exogenous, NO-donating pro-drugs can also induce apoptosis and death of acute leukemia cells. This study investigated combinations of fludarabine with NO-donating pro-drugs for their cytotoxicity against freshly isolated B-CLL lymphocytes following a 72 h exposure in vitro. The median IC(50)for fludarabine was 2.2 microM (n = 85). The nitric oxide donors DETA-NO, PAPA-NO, and MAHMA-NO were also cytotoxic, and their effects were inversely related to rates of NO release. Neither DETA-NO depleted of NO nor DETA itself was effective, indicating that NO was required for cytotoxicity. Drug interactions were evaluated by a modified combination index method. Synergy was observed in combinations of fludarabine or nelarabine (506U78) with DETA-NO in 52% and 88% of samples, respectively. Interestingly, the combination of fludarabine and DETA-NO was more cytotoxic in B-CLL cells less sensitive to fludarabine. DETA-NO did not enhance the activity of other DNA anti-metabolites, topoisomerase I and II inhibitors, or alkylating agents. Finally, the anti-leukemic activity of fludarabine alone or in combination with DETA-NO was found to correlate with inhibition of cellular RNA synthesis. These results indicate that NO donors could enhance fludarabine therapy for B-CLL.

Gemcitabine modulation of alkylator therapy: a phase I trial of escalating gemcitabine added to fixed doses of ifosfamide and doxorubicin

BACKGROUND

The authors investigated the maximum tolerated dose (MTD) and dose limiting toxicity (DLT) associated with the addition of a biomodulating dose of gemcitabine to an established regimen of ifosfamide and doxorubicin as part of a program to explore the potential of low-dose gemcitabine to modulate the activity of alkylating agents.

METHODS

A Phase I trial was carried out in a population of patients with bladder or pelvic carcinoma for whom no standard therapy was available. Doses of ifosfamide and doxorubicin were held fixed at 2 g/m(2) for 4 days and 20 mg/m(2) for 3 days, respectively. Gemcitabine was given on Day 2 and Day 4 at doses of 90 mg/m(2), 150 mg/m(2), and 200 mg/m(2) per dose.

RESULTS

A total of 18 patients received 53 courses of therapy. Myelosuppression was dose limiting. Nonhematologic toxicity also was significant, with 10 of 18 patients experiencing toxicity of Grade 3 or greater. For previously untreated patients with an intact performance status, the MTD for gemcitabine in this context was at least 150 mg/m(2) per dose. According to an intent-to-treat analysis, 11 of 18 patients demonstrated a clinically significant response to this regimen.

CONCLUSIONS

The regimen of ifosfamide and doxorubicin with the addition of gemcitabine was significantly toxic but has promising activity. Based on the observed activity and the generally reversible nature of the toxicity, the authors have initiated a Phase II trial of this regimen in patients with untreated, metastatic urothelial carcinoma.

Nucleoside analogues: mechanisms of drug resistance and reversal strategies

Nucleoside analogues (NA) are essential components of AML induction therapy (cytosine arabinoside), effective treatments of lymphoproliferative disorders (fludarabine, cladribine) and are also used in the treatment of some solid tumors (gemcitabine). These important compounds share some general common characteristics, namely in terms of requiring transport by specific membrane transporters, metabolism and interaction with intracellular targets. However, these compounds differ in regard to the types of transporters that most efficiently transport a given compound, and their preferential interaction with certain targets which may explain why some compounds are more effective against rapidly proliferating tumors and others on neoplasia with a more protracted evolution. In this review, we analyze the available data concerning mechanisms of action of and resistance to NA, with particular emphasis on recent advances in the characterization of nucleoside transporters and on the potential role of activating or inactivating enzymes in the induction of clinical resistance to these compounds. We performed an extensive search of published in vitro and clinical data in which the levels of expression of nucleoside-activating or inactivating enzymes have been correlated with tumor response or patient outcome. Strategies aiming to increase the intracellular concentrations of active compounds are presented.

Collateral sensitivity to gemcitabine (2',2'-difluorodeoxycytidine) and cytosine arabinoside of daunorubicin- and VM-26-resistant variants of human small cell lung cancer cell lines

Multidrug resistance (MDR), characterized by a cross-resistance to many natural toxin-related compounds, may be caused either by overexpression of a drug efflux pump such as P-glycoprotein, (P-gP), multidrug resistance proteins MRP1-3, or BCRP/MXR or, in the case of DNA topoisomerase II active drugs, by a decrease in the enzymatic activity of the target molecule termed altered topoisomerase MDR (at-MDR). However, human small cell lung carcinoma (SCLC) cell lines showed a collateral sensitivity to 2',2'-difluorodeoxycytidine (gemcitabine, dFdC) and 1-beta-D-arabinofuranosylcytosine (ara-C). H69/DAU, a daunorubicin (DAU)-resistant variant of H69 with a P-gP overexpression, and NYH/VM, a VM-26 (teniposide)-resistant variant of NYH with an at-MDR, were both 2-fold more sensitive to gemcitabine and 7- and 2-fold more sensitive to ara-C, respectively. MDR variants had a 4.3- and 2.0-fold increased activity of deoxycytidine kinase (dCK), respectively. dCK catalyzes the first rate-limiting activation step of both gemcitabine and ara-C. In addition, deoxycytidine deaminase, responsible for inactivation of dFdC and ara-C, was 9.0-fold lower in H69/DAU cells. The level of thymidine kinase 2, a mitochondrial enzyme that can also phosphorylate deoxycytidine and gemcitabine, was not significantly different between the variants. These differences most likely caused an increased accumulation of the active metabolites (dFdCTP, 2.1- and 1.6-fold in NYH/VM and H69/DAU cells, respectively) and of ara-CTP (1.3-fold in NYH/VM cells). Ara-CTP accumulation was not detectable in either H69 variant. The pools of all ribonucleoside and deoxyribonucleoside triphosphates were at least 3- to 4-fold higher in the NYH variants compared to the H69 variants; for dCTP and dGTP this difference was even larger. The higher ribonucleotide pools might explain the >10-fold higher accumulation of dFdCTP in NYH compared to H69 variants. Since dCTP is low, H69 cells might not need a high ara-CTP accumulation to inhibit DNA polymerase. This might be related to the lack of ara-CTP in H69 variants. In addition, the increased CTP, ATP, and UTP pools in the MDR variants might explain the increased ara-CTP and dFdCTP accumulation. In conclusion, the MDR variants of the human SCLC cell lines were collaterally sensitive due to an increased dCK activity, and consequently an increased ara-CTP and dFdCTP accumulation.

Activation of caspases and induction of apoptosis by novel ribonucleotide reductase inhibitors amidox and didox

OBJECTIVE

Amidox and didox are two polyhydroxy-substituted benzohydroxamic acid derivatives that belong to a new class of ribonucleotide reductase (RR) inhibitors. RR is the rate-limiting enzyme for de novo deoxyribonucleotide synthesis, and its activity is significantly increased in tumor cells in proportion to the proliferation rate. Therefore, RR is a target for antitumor therapy.

MATERIALS AND METHODS

HL-60 and K562 leukemia cells were treated with increasing doses of amidox and didox. Thereafter, the mode of cytotoxic drug action was determined by Hoechst 33258/propidium iodide (HO/PI) double staining, annexin binding, DNA fragmentation, and caspase activation. This was correlated to the decrease in dNTP levels. Staining with HO/PI and binding of fluorescein isothiocyanate-conjugated annexin V to externalized phosphatidylserine were used to quantify apoptosis.

RESULTS

Low doses of amidox or didox resulted in an increase of apoptotic HL-60 cells within 48 hours. Higher doses (50 microM amidox or 250 microM didox) led to rapid induction of apoptosis, which could be detected as early as 4 hours after treatment. After 48 hours with these concentrations, almost 100% of the HL-60 cells died by apoptosis without an increase in necrosis. K562 cells were found to be resistant to amidox but not to didox. In HL-60 cells, upstream caspase 8 is processed in response to didox, whereas caspases 8 and 9 are processed upon amidox treatment. Didox-induced apoptosis, but not amidox-induced apoptosis, can be correlated with the decrease in dNTP levels. The results suggests that amidox induces several apoptosis mechanisms in HL-60 cells. In contrast, only caspase 9 is activated by didox in K562 cells, and because amidox hardly induces apoptosis in this cell line, no caspase cleavage is observed.

CONCLUSIONS

Didox triggers distinct apoptosis pathways in HL-60 and K562 cells.

Potent inhibition of hemangiosarcoma development in mice by cidofovir

The acyclic nucleoside phosphonate analogue cidofovir is a broad-spectrum anti-DNA virus agent, which also possesses potent inhibitory activity against various tumors associated with papillomaviruses in animal models and patients. Moreover, we recently described the potent inhibition of polyomavirus (PyV)-induced hemangioma formation in rats by cidofovir. This activity could not be explained by an antiviral mechanism. We have now evaluated the effect of cidofovir on the growth of hemangiosarcomas originating from PyV-transformed (PV/2b/35) cells, which do not produce polyomavirus. In vitro, cidofovir proved to be cytostatic for PV/2b/35 cells at a 50% cytostatic concentration (CC(50)) of 2.3 microg/ml. At cidofovir concentrations > or =20 microg/ml, cytotoxicity due to induction of apoptosis was observed. In vivo, intratumoral therapy with cidofovir, at 100 mg/kg 3 times a week, completely inhibited the development and even caused regression of established PV/2b/35 hemangiosarcomas in nude mice. Five days after the start of treatment, few proliferating cells were noted in the cidofovir-treated tumors, whereas control tumors were characterized by high expression of proliferating cell nuclear antigen (PCNA). Moreover, cidofovir induced apoptosis in the hemangiosarcomas, as evidenced by Tunel (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) staining. Also after intraperitoneal administration, cidofovir afforded a prominent protection against the growth of intraperitoneally or intracerebrally inoculated hemangiosarcoma cells in SCID mice. In conclusion, cidofovir possesses a direct antitumor activity, which is mediated by induction of tumor cell apoptosis. Cidofovir should be further explored for its potential in the treatment of fast-growing vascular tumors, like hemangiomas and hemangiosarcomas.

Therapeutic potential of purine analogue combinations in the treatment of lymphoid malignancies

The main purine analogues with activity against lymphoid malignancies are fludarabine, cladribine and pentostatin, all of which are active against slowly proliferating cells through their inhibition of DNA repair and therefore have significant synergistic activity with cytotoxic agents which cause DNA damage. Combinations of purine analogues and alkylating agents or platinum compounds result in markedly increased activity but at the expense of more severe haematological toxicity, while evidence of synergy with anthracyclines/anthracenediones is apparent in the treatment of malignant lymphoma. Interaction between fludarabine or cladribine with deoxycytidine kinase results in a significant enhancement of the activity of cytarabine. Unexpected evidence of clinical synergy is also apparent in combinations of purine analogues and anti-CD20 monoclonal antibodies.

Inhibition of RNA transcription: a biochemical mechanism of action against chronic lymphocytic leukemia cells by fludarabine

Fludarabine is a nucleotide analog effective in the clinical treatment of chronic lymphocytic leukemia (CLL) and other indolent lymphocytic malignancies. Although the incorporation of fludarabine into DNA is a key event in causing cytotoxicity in proliferating leukemia cells, the precise mechanisms by which fludarabine kills CLL cells remain unclear because of the quiescent nature of this malignancy. The present study demonstrated that inhibition of RNA transcription correlated significantly with the cytotoxic action of fludarabine in CLL cells. In contrast, suppression of the low level of DNA synthesis did not affect the survival of the leukemia cells. In addition, inhibition of fludarabine incorporation into cellular DNA through repair synthesis in CLL cells did not alter the cytotoxicity of this drug. Rather, inhibition of RNA synthesis by fludarabine led to a specific diminishment of certain cellular proteins from CLL cells. The combination of fludarabine with another RNA synthesis inhibitor, actinomycin D, or with the protein synthesis inhibitor, puromycin, substantially enhanced the cytotoxic activity against CLL cells. These results suggest that termination of mRNA transcription and the consequent depletion of proteins required for cell survival may be a novel biochemical mechanism of action of fludarabine in CLL cells. Thus, inhibition of RNA/protein synthesis may provide a new therapeutic strategy for the treatment of CLL patients.

Characterization of a dCTP transport activity reconstituted from human mitochondria

A protein fraction of mitochondria from human acute lymphocytic leukemia cells, which could be reconstituted into proteoliposomes to have dCTP transport activity, has been partially purified by hydroxyapatite and blue Sepharose chromatography. The dCTP transport activity in proteoliposomes was time-dependent and could be activated by Ca2+ and to a lesser extent by Mg2+. None of the other divalent cations tested could activate the transport activity. The Km value of dCTP in the presence of Ca2+ was shown to be 3 microM. dCDP but not dCMP or dCyd could inhibit the transport activity. Other deoxynucleoside triphosphates could also inhibit the uptake of dCTP with the potency dGTP = dATP > TTP. Although ATP could competitively inhibit dCTP uptake with a Ki value of 8 microM, the reconstituted dCTP uptake activity was not sensitive to the ATP/ADP carrier inhibitor atractyloside or the sulfhydryl reagent N-ethylmaleimide. This suggests that the dCTP transport system studied is not the same as the ATP/ADP carrier. In conclusion, these studies describe the first functionally reconstituted mitochondrial carrier that displays an efficient transport activity for dCTP.

Combination chemotherapy studies with gemcitabine and etoposide in non-small cell lung and ovarian cancer cell lines

Gemcitabine (2',2'-difluorodeoxycytidine, dFdC) and etoposide (4'-demethylepipodo-phyllo-toxin-9-4,6-O-ethylidene-beta-D-g lucopyranoside, VP-16) are antineoplastic agents with clinical activity against various types of solid tumors. Because of the low toxicity profile of dFdC and the differences in mechanisms of cytotoxicity, combinations of both drugs were studied in vitro. For this purpose, we used the human ovarian cancer cell line A2780, its cis-diammine-dichloroplatinum-resistant and VP-16 cross-resistant variant ADDP, and two non-small cell lung cancer cell lines, Lewis Lung (LL, murine) and H322 (human). The interaction between the drugs was determined with the multiple drug effect analysis (fixed molar ratio) and with a variable drug ratio. In the LL cell line, the combination of dFdC and VP-16 at a constant molar ratio (dFdC:VP-16 = 1:4 or 1:0.125 after 4- or 24-hr exposure, respectively) was synergistic (combination index [CI], calculated at 50% growth inhibition = 0.7 and 0.8, respectively; CI <1 indicating synergism). After 24- and 72-hr exposure to both drugs at a constant ratio, additivity was found in the A2780, ADDP, and H322 cell lines (dFdC:VP-16 = 1:500 for both exposure times in these cell lines). When cells were exposed to a combination of dFdC and VP-16 for 24 or 72 hr, with VP-16 at its IC25 and dFdC in a concentration range, additivity was found in both the LL and H322 cells; synergism was observed in the A2780 and ADDP cells, which are the least sensitive to VP-16. Schedule dependency was found in the LL cell line; when cells were exposed to dFdC 4 hr prior to VP-16 (constant molar ratio, total exposure 24 hr), synergism was found (CI = 0.5), whereas additivity was found when cells were exposed to VP-16 prior to dFdC (CI = 1.6). The mechanism of interaction between the drugs was studied in more detail in the LL cell line; dFdCTP accumulation was 1.2-fold enhanced by co-incubation with VP-16, and was even more pronounced (1.4-fold) when cells were exposed to VP-16 prior to dFdC. dCTP levels were decreased by VP-16 alone as well as by the combination of both compounds, which may favor phosphorylation of dFdC, thereby increasing dFdCTP accumulation. DNA strand break (DSB) formation was increased for exposure to both compounds together compared to exposure to each compound separately, this effect being most pronounced when cells were exposed to VP-16 prior to dFdC (38% and 0% DSB for dFdC and VP-16 alone, respectively and 97% DSB for the combination). The potentiation in DSB formation might be a result of the inhibition of DNA repair by dFdC. Provided the right schedule is used, VP-16 is certainly a compound eligible for combination with dFdC.