Modulation of deoxycytidylate deaminase in intact human leukemia cells. Action of 2',2'-difluorodeoxycytidine

A new technique for the analysis of metabolic pathways of cytidine analogues and cytidine deaminase activities in cells

Deoxycytidine analogues (dCas) are widely used for the treatment of malignant diseases. They are commonly inactivated by cytidine deaminase (CDD), or by deoxycytidine monophosphate deaminase (dCMP deaminase). Additional metabolic pathways, such as phosphorylation, can substantially contribute to their (in)activation. Here, a new technique for the analysis of these pathways in cells is described. It is based on the use of 5-ethynyl 2'-deoxycytidine (EdC) and its conversion to 5-ethynyl 2'-deoxyuridine (EdU). Its use was tested for the estimation of the role of CDD and dCMP deaminase in five cancer and four non-cancer cell lines. The technique provides the possibility to address the aggregated impact of cytidine transporters, CDD, dCMP deaminase, and deoxycytidine kinase on EdC metabolism. Using this technique, we developed a quick and cheap method for the identification of cell lines exhibiting a lack of CDD activity. The data showed that in contrast to the cancer cells, all the non-cancer cells used in the study exhibited low, if any, CDD content and their cytidine deaminase activity can be exclusively attributed to dCMP deaminase. The technique also confirmed the importance of deoxycytidine kinase for dCas metabolism and indicated that dCMP deaminase can be fundamental in dCas deamination as well as CDD. Moreover, the described technique provides the possibility to perform the simultaneous testing of cytotoxicity and DNA replication activity.

Intracellular Pharmacokinetics of Pyrimidine Analogues used in Oncology and the Correlation with Drug Action

Pyrimidine analogues can be considered as prodrugs, like their natural counterparts, they have to be activated within the cell. The intracellular activation involves several metabolic steps including sequential phosphorylation to its monophosphate, diphosphate and triphosphate. The intracellularly formed nucleotides are responsible for the pharmacological effects. This review provides a comprehensive overview of the clinical studies that measured the intracellular nucleotide concentrations of pyrimidine analogues in patients with cancer. The objective was to gain more insight into the parallels between the different pyrimidine analogues considering their intracellular pharmacokinetics. For cytarabine and gemcitabine, the intracellular pharmacokinetics have been extensively studied over the years. However, for 5-fluorouracil, capecitabine, azacitidine and decitabine, the intracellular pharmacokinetics was only very minimally investigated. This is probably owing to the fact that there were no suitable bioanalytical assays for a long time. Since the advent of suitable assays, the first exploratory studies indicate that the intracellular 5-fluorouracil, azacitidine and decitabine nucleotide concentrations are very low compared with the intracellular nucleotide concentrations obtained during treatment with cytarabine or gemcitabine. Based on their pharmacology, the intracellular accumulation of nucleotides appears critical to the cytotoxicity of pyrimidine analogues. However, not many clinical studies have actually investigated the relationship between the intracellular nucleotide concentrations in patients with cancer and the anti-tumour effect. Only for cytarabine, a relationship was demonstrated between the intracellular triphosphate concentrations in leukaemic cells and the response rate in patients with AML. Future clinical studies should show, for the other pyrimidine analogues, whether there is a relationship between the intracellular nucleotide concentrations and the clinical outcome of patients. Research that examined the intracellular pharmacokinetics of cytarabine and gemcitabine focused primarily on the saturation aspect of the intracellular triphosphate formation. Attempts to improve the dosing regimen of gemcitabine were aimed at maximising the intracellular gemcitabine triphosphate concentrations. However, this strategy does not make sense, as efficient administration also means that less gemcitabine can be administered before dose-limiting toxicities are achieved. For all pyrimidine analogues, a linear relationship was found between the dose and the plasma concentration. However, no correlation was found between the plasma concentration and the intracellular nucleotide concentration. The concentration-time curves for the intracellular nucleotides showed considerable inter-individual variation. Therefore, the question arises whether pyrimidine analogue therapy should be more individualised. Future research should show which intracellular nucleotide concentrations are worth pursuing and whether dose individualisation is useful to achieve these concentrations.

Overexpression of microRNA-620 facilitates the resistance of triple negative breast cancer cells to gemcitabine treatment by targeting DCTD

Patients with triple negative breast cancer (TNBC) have a poor survival rate following chemotherapy due to drug resistance. Notably, the molecular mechanism of drug resistance remains elusive. Between December 2011 and December 2014, 36 TNBC samples were obtained from Liaocheng People's Hospital. Three gemcitabine-resistant MDA-MB-231 cell lines (MDA-MB-231rGEM1, MDA-MB-231rGEM2 and MDA-MB-231rGEM3) were obtained by exposure of MDA-MB-231 cells to increasing concentrations of gemcitabine for >12 months. Reverse transcription-quantitative polymerase chain reaction was performed to detect the expression levels of specific genes, including microRNA (miR)-620, ATP-binding cassette sub-family B member 1 (ABCB1), ABCC10, cytidine monophosphate kinase, deoxycytidine monophosphate deaminase (DCTD), nucleoside diphosphate kinase 1 (NME1), ribonucleoside-diphosphate reductase large subunit (RRM1) and RRMB2. Western blot analysis was performed to assess the protein expression levels of DCTD. Furthermore, cell proliferation was assessed using a Cell Counting Kit-8 assay and cell apoptosis was detected using an Annexin V/Dead Cell Apoptosis kit. Interactions between miR-620 and DCTD were predicted using TargetScan and detected with the dual luciferase reporter assay. Elevation of miR-620 expression levels were detected in two of the assessed gemcitabine-resistant MDA-MB-231 cell lines compared with MDA-MB-231 cells. Gemcitabine induced significant elevation of miR-620 in MDA-MB-231 cells. An increase of DCTD at mRNA and protein expression levels in MDA-MB-231rGEM1 cells was observed compared with those in MDA-MB-231 cells. Results suggested that DCTD was directly regulated by miR-620. Inhibition of miR-620 and overexpression of DCTD reversed gemcitabine resistance in MDA-MB-231rGEM1 cells via inducing cell apoptosis and cell growth arrest. A negative correlation was identified between miR-620 and DCTD mRNA expression levels in patients with TNBC. The present results demonstrated that overexpression of miR-620 could contribute to the development of gemcitabine resistance in patients with TNBC via the direct downregulation of DCTD.

Intracellular pharmacokinetics of gemcitabine, its deaminated metabolite 2',2'-difluorodeoxyuridine and their nucleotides

AIMS

Gemcitabine (2',2'-difluoro-2'-deoxycytidine; dFdC) is a prodrug that has to be phosphorylated within the tumour cell to become active. Intracellularly formed gemcitabine diphosphate (dFdCDP) and triphosphate (dFdCTP) are considered responsible for the antineoplastic effects of gemcitabine. However, a major part of gemcitabine is converted into 2',2'-difluoro-2'-deoxyuridine (dFdU) by deamination. In the cell, dFdU can also be phosphorylated to its monophosphate (dFdUMP), diphosphate (dFdUDP) and triphosphate (dFdUTP). In vitro data suggest that these dFdU nucleotides might also contribute to the antitumour effects, although little is known about their intracellular pharmacokinetics (PK). Therefore, the objective of the present study was to gain insight into the intracellular PK of all dFdC and dFdU nucleotides formed during gemcitabine treatment.

METHODS

Peripheral blood mononuclear cell (PBMC) samples were collected from 38 patients receiving gemcitabine, at multiple time points after infusion. Gemcitabine, dFdU and their nucleotides were quantified in PBMCs. In addition, gemcitabine and dFdU plasma concentrations were monitored. The individual PK parameters in plasma and in PBMCs were determined.

RESULTS

Both in plasma and in PBMCs, dFdU was present in higher concentrations than gemcitabine [mean intracellular area under the concentration-time curve from time zero to 24 h (AUC_0-24 h ) 1650 vs. 95 μM*h]. However, the dFdUMP, dFdUDP and dFdUTP concentrations in PBMCs were much lower than the dFdCDP and dFdCTP concentrations. The mean AUC_0-24 h for dFdUTP was 312 μM*h vs. 2640 μM*h for dFdCTP.

CONCLUSIONS

The study provides the first complete picture of all nucleotides that are formed intracellularly during gemcitabine treatment. Low intracellular dFdU nucleotide concentrations were found, which calls into question the relevance of these nucleotides for the cytotoxic effects of gemcitabine.

Dr Jekyll and Mr Hyde: a strange case of 5-ethynyl-2'-deoxyuridine and 5-ethynyl-2'-deoxycytidine

5-Ethynyl-2′-deoxyuridine (EdU) and 5-ethynyl-2′-deoxycytidine (EdC) are mainly used as markers of cellular replicational activity. Although EdU is employed as a replicational marker more frequently than EdC, its cytotoxicity is commonly much higher than the toxicity of EdC. To reveal the reason of the lower cytotoxicity of EdC, we performed a DNA analysis of five EdC-treated human cell lines. Surprisingly, not a single one of the tested cell lines contained a detectable amount of EdC in their DNA. Instead, the DNA of all the cell lines contained EdU. The content of incorporated EdU differed in particular cells and EdC-related cytotoxicity was directly proportional to the content of EdU. The results of experiments with the targeted inhibition of the cytidine deaminase (CDD) and dCMP deaminase activities indicated that the dominant role in the conversion pathway of EdC to EdUTP is played by CDD in HeLa cells. Our results also showed that the deamination itself was not able to effectively prevent the conversion of EdC to EdCTP, the conversion of EdC to EdCTP occurs with much lesser effectivity than the conversion of EdU to EdUTP and the EdCTP is not effectively recognized by the replication complex as a substrate for the synthesis of nuclear DNA.

Genomic signatures for paclitaxel and gemcitabine resistance in breast cancer derived by machine learning

Increasingly, the effectiveness of adjuvant chemotherapy agents for breast cancer has been related to changes in the genomic profile of tumors. We investigated correspondence between growth inhibitory concentrations of paclitaxel and gemcitabine (GI50) and gene copy number, mutation, and expression first in breast cancer cell lines and then in patients. Genes encoding direct targets of these drugs, metabolizing enzymes, transporters, and those previously associated with chemoresistance to paclitaxel (n = 31 genes) or gemcitabine (n = 18) were analyzed. A multi-factorial, principal component analysis (MFA) indicated expression was the strongest indicator of sensitivity for paclitaxel, and copy number and expression were informative for gemcitabine. The factors were combined using support vector machines (SVM). Expression of 15 genes (ABCC10, BCL2, BCL2L1, BIRC5, BMF, FGF2, FN1, MAP4, MAPT, NFKB2, SLCO1B3, TLR6, TMEM243, TWIST1, and CSAG2) predicted cell line sensitivity to paclitaxel with 82% accuracy. Copy number profiles of 3 genes (ABCC10, NT5C, TYMS) together with expression of 7 genes (ABCB1, ABCC10, CMPK1, DCTD, NME1, RRM1, RRM2B), predicted gemcitabine response with 85% accuracy. Expression and copy number studies of two independent sets of patients with known responses were then analyzed with these models. These included tumor blocks from 21 patients that were treated with both paclitaxel and gemcitabine, and 319 patients on paclitaxel and anthracycline therapy. A new paclitaxel SVM was derived from an 11-gene subset since data for 4 of the original genes was unavailable. The accuracy of this SVM was similar in cell lines and tumor blocks (70-71%). The gemcitabine SVM exhibited 62% prediction accuracy for the tumor blocks due to the presence of samples with poor nucleic acid integrity. Nevertheless, the paclitaxel SVM predicted sensitivity in 84% of patients with no or minimal residual disease.

Gemcitabine: metabolism and molecular mechanisms of action, sensitivity and chemoresistance in pancreatic cancer

Gemcitabine is the first-line treatment for pancreatic adenocarcinoma, but is increasingly used to treat breast, bladder, and non-small cell lung cancers. Despite such broad use, intrinsic and acquired chemoresistance is common. In general, the underlying mechanisms of chemoresistance are poorly understood. Here, current knowledge of gemcitabine metabolism, mechanisms of action, sensitivity and chemoresistance reported over the past two decades are reviewed; and we also offer new perspectives to improve gemcitabine efficacy with particular reference to the treatment of pancreatic cancer.

Nucleoside-catabolizing enzymes in mycoplasma-infected tumor cell cultures compromise the cytostatic activity of the anticancer drug gemcitabine

The intracellular metabolism and cytostatic activity of the anticancer drug gemcitabine (2',2'-difluoro-2'-deoxycytidine; dFdC) was severely compromised in Mycoplasma hyorhinis-infected tumor cell cultures. Pronounced deamination of dFdC to its less cytostatic metabolite 2',2'-difluoro-2'-deoxyuridine was observed, both in cell extracts and spent culture medium (i.e. tumor cell-free but mycoplasma-containing) of mycoplasma-infected tumor cells. This indicates that the decreased antiproliferative activity of dFdC in such cells is attributed to a mycoplasma cytidine deaminase causing rapid drug catabolism. Indeed, the cytostatic activity of gemcitabine could be restored by the co-administration of tetrahydrouridine (a potent cytidine deaminase inhibitor). Additionally, mycoplasma-derived pyrimidine nucleoside phosphorylase (PyNP) activity indirectly potentiated deamination of dFdC: the natural pyrimidine nucleosides uridine, 2'-deoxyuridine and thymidine inhibited mycoplasma-associated dFdC deamination but were efficiently catabolized (removed) by mycoplasma PyNP. The markedly lower anabolism and related cytostatic activity of dFdC in mycoplasma-infected tumor cells was therefore also (partially) restored by a specific TP/PyNP inhibitor (TPI), or by exogenous thymidine. Consequently, no effect on the cytostatic activity of dFdC was observed in tumor cell cultures infected with a PyNP-deficient Mycoplasma pneumoniae strain. Because it has been reported that some commensal mycoplasma species (including M. hyorhinis) preferentially colonize tumor tissue in cancer patients, our findings suggest that the presence of mycoplasmas in the tumor microenvironment could be a limiting factor for the anticancer efficiency of dFdC-based chemotherapy. Accordingly, a significantly decreased antitumor effect of dFdC was observed in mice bearing M. hyorhinis-infected murine mammary FM3A tumors compared with uninfected tumors.

Algorithmic modeling quantifies the complementary contribution of metabolic inhibitions to gemcitabine efficacy

Gemcitabine (2,2-difluorodeoxycytidine, dFdC) is a prodrug widely used for treating various carcinomas. Gemcitabine exerts its clinical effect by depleting the deoxyribonucleotide pools, and incorporating its triphosphate metabolite (dFdC-TP) into DNA, thereby inhibiting DNA synthesis. This process blocks the cell cycle in the early S phase, eventually resulting in apoptosis. The incorporation of gemcitabine into DNA takes place in competition with the natural nucleoside dCTP. The mechanisms of indirect competition between these cascades for common resources are given with the race for DNA incorporation; in clinical studies dedicated to singling out mechanisms of resistance, ribonucleotide reductase (RR) and deoxycytidine kinase (dCK) and human equilibrative nucleoside transporter1 (hENT1) have been associated to efficacy of gemcitabine with respect to their roles in the synthesis cascades of dFdC-TP and dCTP. However, the direct competition, which manifests itself in terms of inhibitions between these cascades, remains to be quantified. We propose an algorithmic model of gemcitabine mechanism of action, verified with respect to independent experimental data. We performed in silico experiments in different virtual conditions, otherwise difficult in vivo, to evaluate the contribution of the inhibitory mechanisms to gemcitabine efficacy. In agreement with the experimental data, our model indicates that the inhibitions due to the association of dCTP with dCK and the association of gemcitabine diphosphate metabolite (dFdC-DP) with RR play a key role in adjusting the efficacy. While the former tunes the catalysis of the rate-limiting first phosphorylation of dFdC, the latter is responsible for depletion of dCTP pools, thereby contributing to gemcitabine efficacy with a dependency on nucleoside transport efficiency. Our simulations predict the existence of a continuum of non-efficacy to high-efficacy regimes, where the levels of dFdC-TP and dCTP are coupled in a complementary manner, which can explain the resistance to this drug in some patients.

Deoxyuridine analog nucleotides in deoxycytidine analog treatment: secondary active metabolites?

Deoxycytidine analogs (dCa's) are nucleosides widely used in anticancer and anti (retro) viral therapies. Intracellularly phosphorylated dCa anabolites are considered to be their main active metabolites. This article reviews the literature on the formation and pharmacological activity of deaminated dCa nucleotides. Most dCa's are rapidly deaminated into deoxyuridine analogs (dUa's) which are only slowly phosphorylated and therefore relatively inactive. dUa nucleotides are, however, also formed via deamination of dCa monophosphates by deoxycytidine monophosphate deaminase (dCMPD). dUa-monophosphates can interact with thymidylate synthase (TS), whereas dUa-triphosphates are incorporated into nucleic acids and interfere with polymerases. Administration of dCa's as monophosphate prodrugs or co-administration of the cytidine deaminase inhibitor tetrahydrouridine (THU) does not prevent dUa nucleotide formation which is, on the other hand, influenced by the dose and dCMPD activity. Taken together, these observations show that the formation of dUa nucleotides is a common phenomenon in treatment with dCa's and these compounds may play a role in treatment outcome. We conclude that more attention should be given to these relatively unknown, but potentially important metabolites.

Human equilibrative nucleoside transporter 1 levels predict response to gemcitabine in patients with pancreatic cancer

BACKGROUND & AIMS

The human equilibrative nucleoside transporter (hENT1) protein transports gemcitabine into cells. Small retrospective studies in pancreatic cancer suggest that levels of hENT1 protein or messenger RNA may have prognostic value. We studied the predictive value of hENT1 levels in a cohort of pancreatic adenocarcinoma patients from the large prospective randomized adjuvant treatment trial RTOG9704.

METHODS

In RTOG9704, 538 patients were assigned randomly, after surgical resection, to groups that were given either gemcitabine or 5-fluorouracil (5-FU). Immunohistochemistry for hENT1 was performed on a tissue microarray of 229 resected pancreatic tumors from RTOG9704 and scored as having no staining, low staining, or high staining. Associations between hENT1 protein and treatment outcome were analyzed by unconditional logistic regression analysis using the chi-square test and the Cox proportional hazards model.

RESULTS

HENT1 expression was associated with overall and disease-free survival in a univariate (hazard ratio [HR], 0.51; 95% confidence interval [CI], 0.29-0.91; P= .02; and HR, 0.57; 95% CI, 0.32-1.00; P= .05) and multivariate model in the group given gemcitabine (HR, 0.40; 95% CI, 0.22-0.75; P= .004; and HR, 0.39; 95% CI, 0.21-0.73; P= .003). hENT1 expression was not associated with survival in the group given 5-FU.

CONCLUSIONS

In this prospective randomized trial, hENT1 protein expression was associated with increased overall survival and disease-free survival in pancreatic cancer patients who received gemcitabine, but not in those who received 5-FU. These findings are supported by preclinical data; the gemcitabine transporter hENT1 is therefore a molecular and mechanistically relevant predictive marker of benefit from gemcitabine in patients with resected pancreatic cancer.

Preclinical relevance of dosing time for the therapeutic index of gemcitabine-cisplatin

The relevance of gemcitabine timing for chronotherapeutic optimisation was investigated. Healthy mice received multiple doses of gemcitabine (120, 160 or 200 mg kg⁻¹ injection (inj)⁻¹) at one of six circadian times (3, 7, 11, 15, 19 or 23 h after light onset – HALO) on days 1, 4, 7 and 10 or a single dose of gemcitabine (400 mg kg⁻¹) at 11 or 23 HALO±cisplatin (5 mg kg⁻¹ at 1 min, 4 or 8 h later). Mice bearing Glasgow osteosarcoma received multiple doses of gemcitabine (200 mg kg⁻¹ inj⁻¹) at 11 or 23 HALO±cisplatin (5 mg kg⁻¹ inj⁻¹ at 1 min or 4 h later) on days of 10, 13, 16 and 19 following tumour inoculation. A circadian rhythm in body weight loss was statistically validated, with 1030 HALO corresponding to the least toxic time (95% CL, 0800 to 1300). Gemcitabine dosing produced least body weight loss and least neutropenia after injection at 11 vs 23 HALO, whether the drug was given alone or with cisplatin (P=0.001). Gemcitabine–cisplatin tolerability was improved by dosing gemcitabine at 11 HALO and CDDP at 15 HALO (P<0.001). The administration of this schedule to tumour-bearing mice increased median survival three-fold as compared to treatments where both drugs were given simultaneously at 11 or 23 HALO (P=0.02). The optimal schedule would correspond to the delivery of gemcitabine upon awakening and cisplatin near mid-activity in cancer patients.

Gemcitabine monotherapy in recurrent ovarian cancer: from the bench to the clinic

Gemcitabine (2'2'-difluorodeoxycytidine [dFdC]) is a synthetic analog of deoxycytidine with two fluorine atoms at the 2' position of the carbohydrate. As a hydrophobic molecule, dFdC competes for intracellular access via membrane-associated nucleoside transporter proteins. Following intracellular transport, dFdC is phosphorylated sequentially by deoxycytidine kinase to gemcitabine triphosphate, which inhibits ribonucleotide metabolism, hinders DNA processing, and increases accumulation of intrastrand adducts and interstrand cross-links, thereby leading to a G1 block in the cell cycle. dFdC monotherapy has been extensively evaluated at doses of 800-1250 mg/m2. dFdC is generally well tolerated, with the most frequently occurring dose-limiting toxicities being hematologic, noncumulative, and easily managed by dose alteration. Several studies involving treatment of recurrent ovarian cancer patients with dFdC monotherapy, most of whom had platinum-resistant disease and/or prior exposure to paclitaxel, led to overall response rates of 14-22% and a median duration of response of 4.0-10.6 months. An additional one third of the participants experienced stable disease for an overall clinical benefit in approximately one half of the treated patients. Tumor cells with a multidrug resistance phenotype have increased sensitivity to dFdC (collateral sensitivity). As dFdC is unaffected by platinum resistance, and not susceptible to classic multidrug resistance, it could be particularly beneficial to administer following treatment with agents that induce multidrug resistance. Integration of dFdC with platinum and/or radiation should also be investigated.

The absence of human equilibrative nucleoside transporter 1 is associated with reduced survival in patients with gemcitabine-treated pancreas adenocarcinoma

PURPOSE

Gemcitabine monotherapy is the standard palliative chemotherapy for pancreatic adenocarcinoma. Gemcitabine requires plasma membrane nucleoside transporter proteins to efficiently enter cells and exert it cytotoxicity. In vitro studies have demonstrated that deficiency of human equilibrative nucleoside transporter 1 (hENT1), the most widely abundant and distributed nucleoside transporter in human cells, confers resistance to gemcitabine toxicity, but the distribution and abundance of nucleoside transporters in normal and malignant pancreatic tissue is unknown.

EXPERIMENTAL DESIGN

We studied tumor blocks from normal pancreas and 21 Alberta patients with gemcitabine-treated pancreatic cancer. Immunohistochemistry on the formalin-fixed, paraffin-embedded tissues was performed with specific hENT1 and human Concentrative Nucleoside Transporter 3 monoclonal antibodies and scored by a pathologist blinded to clinical outcomes.

RESULTS

hENT1 was detected in normal Langerhan cells and lymphocytes but not in normal glandular elements. Patients in whom all adenocarcinoma cells had detectable hENT1 had significantly longer median survivals from gemcitabine initiation than those for whom hENT1 was absent in a proportion (10 to 100%) of adenocarcinoma cells (median survival, 13 versus 4 months, P = 0.01). Immunohistochemistry for human Concentrative Nucleoside Transporter 3 revealed moderate to high-intensity staining in all adenocarcinoma tissue samples.

CONCLUSIONS

Patients with pancreatic adenocarcinoma with uniformly detectable hENT1 immunostaining have a significantly longer survival after gemcitabine chemotherapy than tumors without detectable hENT1. Immunohistochemistry for hENT1 shows promise as a molecular predictive assay to appropriately select patients for palliative gemcitabine chemotherapy but requires formal validation in prospective, randomized trials.

Randomized phase II comparison of dose-intense gemcitabine: thirty-minute infusion and fixed dose rate infusion in patients with pancreatic adenocarcinoma

PURPOSE

To conduct a randomized phase II trial of dose-intense gemcitabine using a standard 30-minute infusion or the fixed dose rate (FDR) infusion (10 mg/m2/min) in patients with pancreatic adenocarcinoma.

PATIENTS AND METHODS

In this prospective trial, patients with locally advanced and metastatic pancreatic adenocarcinoma were treated with 2,200 mg/m2 gemcitabine over 30 minutes (standard arm) or 1,500 mg/m2 gemcitabine over 150 minutes (FDR arm) on days 1, 8, and 15 of every 4-week cycle. The primary end point of this trial was time to treatment failure. Secondary end points included time to progression, median survival, safety, and pharmacokinetic studies of gemcitabine.

RESULTS

Ninety-two patients were enrolled onto this study; 91% of the patients had metastatic disease. Time to treatment failure was comparable in both treatment groups; however, the median survival for all patients was 5.0 months in the standard arm and 8.0 months in the FDR arm (P =.013). For patients with metastases, the median survival was 4.9 months in the standard arm and 7.3 months in FDR arm (P =.094). The 1- and 2-year survival rates for all patients were 9% (standard arm) versus 28.8% (FDR; P =.014) and 2.2% (standard arm) versus 18.3% (FDR; P =.007), respectively. Patients in the FDR infusion arm experienced consistently more hematologic toxicity. Pharmacokinetic analyses demonstrated a two-fold increase in intracellular gemcitabine triphosphate concentration in the FDR arm (P =.046).

CONCLUSION

Pharmacokinetic and clinical data in this trial supports the continued evaluation of the FDR infusion strategy with gemcitabine.

New chemotherapy agents in veterinary medicine

Several anticancer drugs have been added to the therapeutic armamentarium in recent years. Some of these agents are traditional drugs with a long history of use in human oncology. Increased sophistication in clinical trial design in veterinary oncology has allowed the incorporation of agents previously viewed as excessively toxic. Other agents have been developed more recently. This article summarizes the veterinary experience with two older alkylating agents, lomustine and streptozocin, and newer compounds ifosfamide and gemcitabine. The published literature regarding veterinary use of these agents is limited, and the reader is advised to contact a veterinary oncologist for current guidelines when contemplating use of these agents.

Nucleoside analogues and nucleobases in cancer treatment

Cytotoxic nucleoside analogues and nucleobases were among the first chemotherapeutic agents to be introduced for the medical treatment of cancer. This family of compounds has grown to include a variety of purine and pyrimidine nucleoside derivatives with activity in both solid tumours and malignant disorders of the blood. These agents behave as antimetabolites, compete with physiological nucleosides, and interact with a large number of intracellular targets to induce cytotoxicity. Progress has recently been made in the identification and characterisation of nucleoside transporters and the enzymes of nucleoside metabolism. In addition, there is now greater understanding of the molecular mechanisms of anticancer nucleoside activity, which provides opportunities for potentiating their antitumour effects. Strategies to optimise intracellular analogue accumulation and to enhance cancer-cell selectivity are proving beneficial in clinical trials.

Overview: gemcitabine as single-agent therapy for advanced breast cancer

Breast cancer is one of the most common cancers worldwide. Various therapies, such as hormonal therapy, chemotherapy, and biologic therapies, can increase cure rates in the early-stage setting and improve survival and quality of life in specific advanced-disease settings. For advanced disease, the optimal timing, type, and combination of drugs remain to be defined. Gemcitabine is a newer agent with a unique mode of action that involves DNA chain termination and mechanisms that result in self-potentiation. This results in a broad spectrum of activity in many types of solid tumors including breast cancer. As a single agent, gemcitabine yields response rates ranging from 14%-37% as first-line therapy for advanced breast cancer and 23%-42% as salvage therapy. However, these were small studies with large confidence intervals around all the indices of benefit including response rate, response duration, and time to disease progression. Gemcitabine was associated with higher response rates when used in combination with other agents. The side-effect profile of gemcitabine has been favorable compared to that of commonly used cytotoxic drugs, and includes myelosuppression and fatigue, with a notable absence of alopecia and gastrointestinal symptoms. Larger ongoing studies will help define the utility of gemcitabine in advanced breast cancer

Determinants of resistance to 2',2'-difluorodeoxycytidine (gemcitabine)

The inherent or induced resistance of tumors to cytostatic agents is a major clinical problem. In this review, we summarize the pre-clinical mechanisms of acquired and inherent resistance to the fluorinated deoxycytidine analog gemcitabine (2',2'-difluorodeoxycytidine, dFdC, Gemzar((R))), which has proven activity in non-small cell lung carcinoma, pancreatic and bladder cancer. Extensive research has been performed to elucidate the complex mechanism of action of this relatively new drug. Gemcitabine requires phosphorylation to mono-, di- and triphosphates to be active. Similar to the structurally and functionally related deoxycytidine analog ara-C, the first, crucial step in phosphorylation is catalyzed by deoxycytidine kinase (dCK). However, in contrast to ara-C, gemcitabine has multiple intracellular targets; up- or down-regulation of these targets may confer resistance to this drug. Resistance is associated with altered activities of enzymes involved in the metabolism of the drug, of target enzymes, and of enzymes involved in programmed cell death. However, the only strong correlations with gemcitabine sensitivity are dCK activity and dFdCTP pools, with a potential important role for ribonucleotide reductase.

Gemcitabine in hematologic malignancies

Gemcitabine is a pyrimidine analogue that showed significant activity in solid malignancies. Gemcitabine acts by inhibiting DNA synthesis through chain termination and ribonucleotide reductase inhibition. During initial phase I and II studies, gemcitabine had a low toxicity profile and was well tolerated as a single agent and in combination therapy. Recently, there has been more interest in studying the activity of gemcitabine in hematologic malignancies. Gemcitabine demonstrated good activity in refractory Hodgkin disease patients, non-Hodgkin lymphoma, cutaneous T-cell lymphoma, and acute leukemias. There is a preponderance of evidence on the activity of gemcitabine in vitro in myeloma and leukemic cell lines. The activity of gemcitabine in these disorders will pave the way for incorporating this agent into the early phases of therapy.

Differential effects of gemcitabine on ribonucleotide pools of twenty-one solid tumour and leukaemia cell lines

To gain a more detailed insight into the metabolism of 2', 2'-difluoro-2'-deoxycytidine (dFdC, gemcitabine, Gemzar) and its effect on normal ribonucleotide (NTP) metabolism in relation to sensitivity, we studied the accumulation of dFdCTP and the changes in NTP pools after dFdC exposure in a panel of 21 solid tumour and leukaemia cell lines. Both sensitivity to dFdC and accumulation of dFdCTP were clearly cell line-dependent: in this panel of cell lines, the head and neck cancer (HNSCC) cell line 22B appeared to be the most sensitive, whereas the small cell lung cancer (SCLC) cell lines were the least sensitive to dFdC. The human leukaemia cell line CCRF-CEM accumulated the highest concentration of dFdCTP, whereas the non-SCLC cell lines accumulated the least. Not only the amount of dFdCTP accumulation was clearly related to the sensitivity for dFdC (R=-0.61), but also the intrinsic CTP/UTP ratio (R=0.97). NTP pools were affected considerably by dFdC treatment: in seven cell lines dFdC resulted in a 1.7-fold depletion of CTP pools, in two cell lines CTP pools were unaffected, but in 12 cell lines CTP pools increased about 2-fold. Furthermore, a 1.6-1.9-fold rise in ATP, UTP and GTP pools was shown in 20, 19 and 20 out of 21 cell lines, respectively. Only the UTP levels after treatment with dFdC were clearly related to the amount of dFdCTP accumulating in the cell (R=0.64 (P<0.01)), but not to the sensitivity to dFdC treatment. In conclusion, we demonstrate that besides the accumulation of dFdCTP, the CTP/UTP ratio was clearly related to the sensitivity to dFdC. Furthermore, the UTP levels and the CTP/UTP ratio after treatment were related to dFdCTP accumulation. Therefore, both the CTP and UTP pools appear to play an important role in the sensitivity to dFdC.

Radiosensitizing potential of gemcitabine (2',2'-difluoro-2'-deoxycytidine) within the cell cycle in vitro

PURPOSE

Gemcitabine (2',2'-difluorodeoxycytidine; dFdCyd) is a new deoxycitidine analog which exhibits substantial activity against solid tumors and radiosensitizing properties in vitro. To examine cell cycle-specific effects of a combined treatment with gemcitabine and radiation, the in vitro clonogenic survival of two different cell lines was measured for cells from log-phase culture, G1 and S-phase cells.

METHODS AND MATERIALS

Chinese hamster (V79) and human colon carcinoma (Widr) cells were exposed to different radiation doses and for different points of time relative to gemcitabine treatment (2 h). Experiments were also carried out with different cell-cycle populations obtained after mitotic selection (V79) or after serum stimulation of plateau-phase cells (Widr). The resulting survival curves were analyzed according to the LQ model, and mean inactivation doses (MID) and the cell cycle-specific enhancement ratios (ER) were calculated from the survival curve parameters.

RESULTS

Effectiveness of combined treatment of log-phase cells was greatest when cells were irradiated at the end of the gemcitabine exposure [ER: 1.28 (V79), 1.24 (Widr)]. For later times after the removal of the drug, radiosensitization declined, approaching independent toxicity. From the time course of interactive-type damage decay half-life values of 75 min (V79) and 92 min (Widr) were derived. Gemcitabine did not radiosensitize G1 Widr cells or V79 cells from the G1/S border, but substantial radiosensitization was observed for the S-phase cell preparations [ER: 1.45 (V79-lateS), 1.57 (Widr)].

CONCLUSIONS

Treatment of cells with gemcitabine immediately before irradiation eliminates, or at least greatly reduces, the variation in radiosensitivity during the cell cycle that is manifested by radioresistance during S phase. This reversal of S-phase radioresistance could imply that gemcitabine interferes with the potentially lethal damage repair/fixation pathway. Other approaches have been taken to overcome S-phase radioresistance, such as hyperthermia or densely ionizing radiation, and combined treatments with dFdCyd could prove of value to complement such efforts.

Deoxycytidine kinase can be also potentiated by the G-protein activator NaF in cells

Recently, it has been shown, that 2-Chloro-deoxyadenosine (1), a series of analogues, and other DNA synthesis inhibitors, increased the deoxycytidine kinase (dCK) enzyme activity in different cells, without influencing thymidine kinase isoenzymes (TK1, TK2), dCMP-deaminase and thymidylate synthase (TS) activities (2,3). The dCK activity was 2-4 times higher in analogue treated cells, than in controls, which can not be explained by metabolic pool imbalance induced by the drugs. New mRNA and protein synthesis of dCK could not be detected, thus post-translational modification has been suggested for potentiation the activity of the dCK (1). Because secondary modifications of enzymes usually involve the signalling processes in cells, the universal G-protein activator fluorine ions were tested. dCK activity of human lymph node lymphocytes were increased 2-times, if cells were incubated in the presence of NaF for 1-2 hrs in cultures, while TK activity was not changed. The formation of dUTP from dCyd, was also enhanced by NaF, in parallel of dCK potentiation.

Activation of deoxycytidine kinase during inhibition of DNA synthesis in human lymphocytes

Deoxycytidine kinase was shown to be activated during 2-chlorodeoxyadenosine (CdA) treatment of human lymphocytes, under the conditions when the DNA synthesis is inhibited. As the increase of dCK activity was shown in crude protein extracts, without an increase in the amount of dCK protein, shown by immunostaining after SDS-PAGE, a secondary modification of the protein structure was considered. NaF treatment of cells in the concentration range of 5-20 mM gave a similar activation of dCK, suggesting a possible role of phosphatases and/or a possibility of a G-protein related phenomenon. Using the same conditions, no effect of CdA or NaF was found on the thymidine kinase activity of cell extracts. Alternatively, activation of catabolic pathways could be considered, however, the increase in dCK activity was not influenced either by the removal of 5'-nucleotidases, or by the inhibition of deaminases.

Similar changes were induced by Cladribine and by gemcitabine, in the deoxypyrimidine salvage, during short-term treatments

Short term treatments (1-2 hrs) of human tonsillar lymphocytes by Cladribine (2-Chloro-deoxyadenosine, CdA) have suggested a new target for CdA, the inhibition of dCMP deaminase (Sasvári et al. 1994; BBRC 203, 1378). Further investigations have shown, that the dCMP-deaminase activity could be inhibited by 2-Cl-dAMP in cell free extracts of lymphocytes. The pool size of dUMP (measured by an antibody against dUMP) was also decreased in WiDr colon cancer cells by CdA. The new antimetabolite against solid tumours, Gemcitabine (2',2'-difluoro-deoxycytidine, dFdC), had similar effects on the salvage of thymidine (dThd) and deoxycytidine (dCyd) as CdA. The Ki values for 3H-dThd and 3H-dCyd incorporation into DNA were 0.16 uM and 1.0 uM dFdC, respectively. The labeling of the TTP pool increased 6-7 times, while of dCTP pool only 1.5-1.7 times, suggesting a decrease of the size of corresponding pools. Similarly to CdA, the labeling as well as the concentration of dUMP was also decreased by dFdC. Both analogues are able to increase the deoxycytidine kinase activity, necessary for their phosphorylation and therapeutic action in cells. The target(s) for the two different drugs seems to be common.

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

The major actions of nucleoside analogs such as arabinosylcytosine (ara-C) and fludarabine occurs after their incorporation into DNA, during either replication or repair synthesis. The metabolic salvage and DNA incorporation of the normal nucleoside, deoxycytidine, is functionally compartmentalized toward repair synthesis in a process regulated by ribonucleotide reductase. The aim of this study was to investigate the metabolic pathways by which nucleoside analogs that do (fludarabine, gemcitabine) or do not (ara-C) affect ribonucleotide reductase are incorporated into DNA in proliferating human leukemia cells. Using alkaline density-gradient centrifugation to separate repaired DNA from replicating DNA and unreplicated parental DNA strands, approximately 60% of ara-C nucleotide in DNA was incorporated by repair synthesis in CCRF-CEM cells; the remainder was incorporated by replication. In contrast, fludarabine and gemcitabine, nucleosides that inhibit ribonucleotide reductase and decreased deoxynucleotide pools, were incorporated mainly within replicating DNA. Hydroxyurea also depleted deoxynucleotide pools and increased the incorporation of ara-C into DNA by replicative synthesis. Stimulation of DNA repair activity by UV irradiation selectively enhanced the incorporation of all nucleosides tested through repair synthesis. These findings suggest that the pathways by which therapeutically useful nucleoside analogs are incorporated into DNA are affected by cellular dNTP pools from de novo synthesis and by the relative activities of DNA repair and replication. The antitumor activity of these drugs may be enhanced by combination with either ribonucleotide reductase inhibitors to increase their incorporation into replicating DNA or with agents that induce DNA damage and evoke the DNA repair process.

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

The major actions of nucleoside analogs such as arabinosylcytosine (ara-C) and fludarabine occurs after their incorporation into DNA, during either replication or repair synthesis. The metabolic salvage and DNA incorporation of the normal nucleoside, deoxycytidine, is functionally compartmentalized toward repair synthesis in a process regulated by ribonucleotide reductase. The aim of this study was to investigate the metabolic pathways by which nucleoside analogs that do (fludarabine, gemcitabine) or do not (ara-C) affect ribonucleotide reductase are incorporated into DNA in proliferating human leukemia cells. Using alkaline density-gradient centrifugation to separate repaired DNA from replicating DNA and unreplicated parental DNA strands, approximately 60% of ara-C nucleotide in DNA was incorporated by repair synthesis in CCRF-CEM cells; the remainder was incorporated by replication. In contrast, fludarabine and gemcitabine, nucleosides that inhibit ribonucleotide reductase and decreased deoxynucleotide pools, were incorporated mainly within replicating DNA. Hydroxyurea also depleted deoxynucleotide pools and increased the incorporation of ara-C into DNA by replicative synthesis. Stimulation of DNA repair activity by UV irradiation selectively enhanced the incorporation of all nucleosides tested through repair synthesis. These findings suggest that the pathways by which therapeutically useful nucleoside analogs are incorporated into DNA are affected by cellular dNTP pools from de novo synthesis and by the relative activities of DNA repair and replication. The antitumor activity of these drugs may be enhanced by combination with either ribonucleotide reductase inhibitors to increase their incorporation into replicating DNA or with agents that induce DNA damage and evoke the DNA repair process.

Gemcitabine and radiosensitization in human tumor cells

Gemcitabine is a nucleoside analogue with excellent clinical activity against solid tumors. Within the cell, gemcitabine is rapidly phosphorylated to its active di- and triphosphate metabolites. Cytotoxicity with gemcitabine appears to be related to multiple effects on DNA replication, where gemcitabine triphosphate can serve as both an inhibitor and substrate for DNA synthesis. Gemcitabine diphosphate inhibits ribonucleotide reductase, producing decreases in cellular dNTP pool levels in a cell-specific manner. These two major characteristics of gemcitabine, reduction in cellular dNTP pools and incorporation into DNA, are features of other antimetabolites antitumor agents which also exhibit radiosensitizing properties. Based on these favorable metabolic characteristics and the clinical activity of gemcitabine in tumor types which are commonly treated with radiation, the ability of gemcitabine to enhance X-radiation induced cytotoxicity was evaluated. Gemcitabine has been shown to be a potent radiosensitizer in a variety of tumor cell lines, including HT-29 colorectal carcinoma, pancreatic cancer, breast, non-small cell lung and head and neck cancer cell lines. Gemcitabine was most effective as a radiosensitizer when administered at least 2 hours prior to irradiation. For most cell lines, radiosensitization was evident at non-cytotoxic concentrations. The extent of radiosensitization increased with both increasing gemcitabine concentration and duration of exposure. Radiosensitization did not require redistribution of cells into a more radiosensitive phase of the cell cycle. The major metabolic effects observed under radiosensitizing conditions were the accumulation of high levels of gemcitabine triphosphate, and a selective decrease in the cellular dATP pool. The pattern of dATP decrease paralleled the increase in radiosensitization, whereas the level of gemcitabine triphosphate was not associated with the enhanced sensitivity to radiation. Compared to other radiosensitizers, the advantage of gemcitabine is that is can induce radiosensitization at concentrations that are 1000 times lower than typical plasma levels obtained with this drug. These studies will be used as guidelines for developing clinical trials of gemcitabine with radiation.

A phase II trial of gemcitabine in patients with 5-FU-refractory pancreas cancer

PURPOSE

To assess the effect of gemcitabine in patients with metastatic pancreas cancer that had progressed despite prior treatment with 5-FU.

PATIENTS AND METHODS

Seventy-four patients were enrolled in this multicenter trial. Alleviation of cancer-related symptoms was the primary endpoint. Sixty-three patients completed a pain stabilization period and were treated with gemcitabine. Clinical Benefit Response was defined as a > or = 50% reduction in pain intensity, > or = 50% reduction in daily analgesic consumption, or > or = 20 point improvement in KPS that was sustained for > or = 4 consecutive weeks.

RESULTS

Seventeen of 63 pts (27.0%) attained a Clinical Benefit Response (95% CI: 16.0%-38.0%). The median duration of Clinical Benefit Response was 14 weeks (range: 4-69 weeks). Median survival for patients treated with gemcitabine was 3.85 months (range: 0.3-18.0+ months). Therapy was generally well-tolerated with a low incidence of grade 3 or 4 toxicities.

CONCLUSION

Systematic assessment of subjective outcomes can be used to evaluate the clinical impact of new therapies for pancreas cancer, a highly symptomatic disease. Our findings suggest that gemcitabine is a useful palliative agent in patients with 5-FU-refractory pancreas cancer.

Activity of gemcitabine in patients with non-small cell lung cancer: a multicentre, extended phase II study

Gemcitabine is a novel nucleoside analogue with activity in solid tumours. This study assessed the objective response rate to gemcitabine given weekly intravenously at a dose of 1250 mg/m2 for 3 weeks followed by 1 week of rest (one cycle) in chemonaive patients with inoperable non-small cell lung cancer (NSCLC). 161 patients with NSCLC were recruited from 10 sites in nine countries. Most patients had stage IIIb (31.3%) or IV (64.6%) disease, and 93.8% had a performance status of 0 or 1 according to the WHO scale. Of 151 evaluable patients, there were 3 complete responses and 30 partial responses lasting at least 4 weeks for an objective response rate of 21.8% (95% CI 15.5-29.3%). All responses were validated by an extramural Oncology Review Board. The mean duration of response was 8.8 months. The mean survival for all patients (16.1% of patients still alive 26 months after last patient started treatment) was 11.5 months. Improvements were also observed in secondary efficacy parameters such as performance status, weight, analgesic requirement, pain, and other disease-related symptoms including cough, dyspnoea, haemoptysis, anorexia, somnolence and hoarseness. Haematological and non-haematological toxicity was mild given the biological activity of gemcitabine. This study confirms gemcitabine as one of the most active agents in NSCLC with the added benefit of a modest toxicity profile and ease of administration on an out-patient basis. Gemcitabine is a suitable candidate for combination chemotherapy in patients with NSCLC.

Phase II study of gemcitabine in patients with advanced pancreatic cancer

The efficacy and safety of gemcitabine at a starting dose of 800 mg m2 administered once a week for 3 weeks with 1 week's rest was investigated in chemonaive patients with advanced and/or metastatic pancreatic cancer. Of 34 patients, 32 were evaluable for efficacy, 20 patients had metastatic stage IV disease, 25 had a performance status of 1 and 26 (76%) patients has significant pain on presentation. All responses were independently validated by an external oncology review board: two patients achieved a partial response that lasted 5.8 and 5.2 months (6.3%) and six patients were stable for at least 4 weeks. The median duration of survival for evaluable patients was 6.3 months (range 1.6-19.2 months). The tumour markers, CEA, CA 19-9 and CA 195 were serially measured in 16 patients. There was a good correlation with tumour response when all three markers were significantly decreased. In 4 of 16 patients, tumour marker levels decreased by > or = 60%, including the two responders, one patient who survived for 12 months and one patient who showed objective tumour shrinkage but was deemed ineligible for response evaluation because the disease was considered not to be bidimensionally measurable. Symptomatic benefits included improvement in performance status (17.2%), analgesic requirement (7.4%), pain score (28.6%) and nausea (27.3%). The mean number of cycles administered was 2.5 and the mean dosage received was 890 mg m2 per injection. Seventy-four per cent of dose administrations were given on schedule. Toxicity, particularly haematological toxicity, reported as the maximum WHO grade experienced by patients was mild. Infective episodes were rare and limited to WHO grade 2 (6.7%). Nausea and vomiting was generally modest (WHO grade 3, 26.7%). Other side-effects included mild transient flu-like symptoms (seven patients) and peripheral oedema (three patients), which was not associated with abnormal cardiac hepatic or renal function. Gemcitabine has modest activity in pancreatic cancer, a limited positive improvement on a range of patient benefit parameters and has a mild toxicity profile. For these reasons and because of its novel mode of action, gemcitabine warrants further investigation in combination studies in pancreatic cancer.

Improved synthesis of zebularine [1-(beta-D-ribofuranosyl)-dihydropyrimidin-2-one] nucleotides as inhibitors of human deoxycytidylate deaminase

The 2'-deoxy (2a) and 2'-ara-fluoro (3a) derivatives of zebularine [1-(beta-D-ribofuranosyl)-dihydropyrimidin-2-one, 1a] were phosphorylated in high yield to the 5'-nucleotides 2b and 3b, respectively, and characterized by HPLC, enzyme degradation, 1H, 13C and 31P NMR, and high resolution mass spectral analysis. Their inhibitory activity against partially purified MOLT-4 deoxycytidylate deaminase (dCMPD) in the presence of the allosteric effector deoxycytidine triphosphate (dCTP) and Mg+2 ion was examined. Compounds 2b and 3b inhibited dCMPD with Ki values of 2.1 x 10(-8) M and 1.2 x 10(-8) M, respectively. The parent nucleotide, zebularine monophosphate 1b was ineffective at concentrations > 100 mumol. The effect of the nucleosides, 1a-3a, as well as tetrahydrouridine (THU) and 2'-deoxy THU (dTHU), on the cellular production of DNA precursors was examined in human MOLT-4 peripheral lymphoblasts. It was shown that 1a, 2a and 3a all elevated intracellular dCTP and TTP levels in whole cells with the most powerful effect elicited by 1a. The 2'-fluoro derivative 3a was chemically phosphorylated much more cleanly and higher yield than 2a, without the formation of diphosphorylated by-products. This compound was found to be infinitely less sensitive to acid-catalyzed degradation than 2a. Since the substitution of fluorine for hydrogen had a slight potentiating effect on the dCMPD inhibitory activity while stabilizing the compound toward acid-catalyzed and enzymatic depyrimidination, compound 3b emerges as a very attractive tool for the pharmacological modulation of pyrimidine deaminase activity.

Schedule dependence of sensitivity to 2',2'-difluorodeoxycytidine (Gemcitabine) in relation to accumulation and retention of its triphosphate in solid tumour cell lines and solid tumours

2',2'-Difluorodeoxycytidine (Gemcitabine, dFdC) is a relatively new deoxycytidine antimetabolite, with established activity against ovarian cancer and non-small-cell lung cancer. dFdC is assumed to exert its antitumour effect mainly by incorporation of the triphosphate dFdCTP into DNA. We determined the sensitivity to dFdC of six cell lines derived from solid tumours; two ovarian carcinoma (A2780 and OVCAR-3), two colon carcinoma (WiDr and C26-10) and two squamous cell carcinoma cell lines (UM-SCC-14C and UM-SCC-22B). In vitro sensitivity to dFdC was strongly time dependent. Under all conditions A2780 was the most sensitive cell line with an IC50 (the concentration of dFdC causing 50% growth inhibition) of 31 and 0.6 nM at 1 and 48 hr exposure, respectively. WiDr and C26-10 cells were relatively insensitive, with IC50s of 468 and 1133 nM, respectively, at 1 hr exposure, but of 11 and 6 nM at 48 hr exposure. Accumulation of the triphosphate dFdCTP was also time dependent. After 4 hr exposure to 10 microM dFdC, A2780, WiDr and C26-10 cells accumulated 223, 136 and 267 pmol/10(6) cells, respectively; after 24 hr exposure they accumulated 1045, 619 and 617 pmol/10(6) cells, respectively. A2780 cells retained the high dFdCTP concentration longer than 24 hr. For comparison purposes we also studied dFdCTP kinetics in the corresponding solid tumours, showing the same sensitivity pattern as the cell lines. In general, sensitivity to dFdC in vitro related with dFdCTP accumulation and retention, but in vivo this relation was less clear. Unexpectedly, remarkable in vitro and in vivo changes were observed in the ribonucleotide pools. The most predominant in vitro cell line dependent changes were a decrease in CTP concentrations, accompanied by an increase in UTP and GTP concentrations. In vivo CTP, UTP and GTP pools increased in all tumours. In conclusion, in this study we demonstrate that dFdCTP is accumulated and retained in solid tumours and cell lines. dFdCTP is not only important as a DNA precursor, but also appears to interfere with normal ribonucleotide metabolism.

Inhibition of ribonucleotide reductase by 2'-substituted deoxycytidine analogs: possible application in AIDS treatment

After phosphorylation to the corresponding diphosphates, 2'-azido-2'-deoxycytidine and 2'-difluorocytidine act as powerful inhibitors of ribonucleotide reductase. Phosphorylation requires deoxycytidine kinase, an enzyme with particularly high activity in lymphoid cells. Therefore, the deoxycytidine analogs can be expected to inhibit the reductase with some specificity for the lymphoid system. Pretreatment of human CEM lymphoblasts with the analogs considerably increased the phosphorylation of 3'-deoxy-3'-azidothymidine (AzT). The increased phosphorylation of AzT is caused by a prolongation of the S phase of the cell cycle. Our results suggest the possibility of a combination of 2'-substituted deoxycytidine analogs with AzT in the treatment of AIDS. Gao et al. [Gao, W.-Y., Cara, A., Gallo, R. C. & Lori, F. (1993) Proc. Natl. Acad. Sci. USA 90, 8925-8928] have suggested the use of the ribonucleotide reductase inhibitor hydroxyurea for this purpose, since the resulting decrease in the size of deoxyribonucleotide pools decreases the processivity of the HIV reverse transcriptase. From our results it would appear that the 2'-substituted deoxycytidine analogs might be preferable to hydroxyurea.

New targets for pyrimidine antimetabolites for the treatment of solid tumours. 2: Deoxycytidine kinase

Deoxycytidine kinase is an enzyme required for the activation of, for example, cytarabine, the most widely used agent for the chemotherapy of haematological malignancies. However, deoxycytidine kinase also plays an important role in the activation of several new agents used in the treatment of leukaemia, such as cladribine. Recently, a new cytidine analogue, gemcitabine, has shown impressive activity as a single agent against several solid malignancies (ovarian cancer, non-small cell lung cancer), demonstrating that in solid tumours deoxycytidine kinase can be an important target for the activation of antimetabolites. Studies on the regulation of deoxycytidine kinase have shown that the enzyme has a complicated regulation (feedback inhibition by the product and regulation by ribonucleotides). Modulation of deoxycytidine kinase activity has already been shown to be an effective way to improve the effect of cytarabine and will probably be a target for new therapies.