Thymidylate synthase (TS) and ribonucleotide reductase (RNR) may be involved in acquired resistance to 5-fluorouracil (5-FU) in human cancer xenografts in vivo

Synthesis of Fluorinated Nucleosides/Nucleotides and Their Antiviral Properties

The FDA has approved several drugs based on the fluorinated nucleoside pharmacophore, and numerous drugs are currently in clinical trials. Fluorine-containing nucleos(t)ides offer significant antiviral and anticancer activity. The insertion of a fluorine atom, either in the base or sugar of nucleos(t)ides, alters its electronic and steric parameters and transforms the lipophilicity, pharmacodynamic, and pharmacokinetic properties of these moieties. The fluorine atom restricts the oxidative metabolism of drugs and provides enzymatic metabolic stability towards the glycosidic bond of the nucleos(t)ide. The incorporation of fluorine also demonstrates additional hydrogen bonding interactions in receptors with enhanced biological profiles. The present article discusses the synthetic methodology and antiviral activities of FDA-approved drugs and ongoing fluoro-containing nucleos(t)ide drug candidates in clinical trials.

The epigenetic modifier HDAC2 and the checkpoint kinase ATM determine the responses of microsatellite instable colorectal cancer cells to 5-fluorouracil

The epigenetic modifier histone deacetylase-2 (HDAC2) is frequently dysregulated in colon cancer cells. Microsatellite instability (MSI), an unfaithful replication of DNA at nucleotide repeats, occurs in about 15% of human colon tumors. MSI promotes a genetic frameshift and consequently a loss of HDAC2 in up to 43% of these tumors. We show that long-term and short-term cultures of colorectal cancers with MSI contain subpopulations of cells lacking HDAC2. These can be isolated as single cell-derived, proliferating populations. Xenografted patient-derived colon cancer tissues with MSI also show variable patterns of HDAC2 expression in mice. HDAC2-positive and HDAC2-negative RKO cells respond similarly to pharmacological inhibitors of the class I HDACs HDAC1/HDAC2/HDAC3. In contrast to this similarity, HDAC2-negative and HDAC2-positive RKO cells undergo differential cell cycle arrest and apoptosis induction in response to the frequently used chemotherapeutic 5-fluorouracil, which becomes incorporated into and damages RNA and DNA. 5-fluorouracil causes an enrichment of HDAC2-negative RKO cells in vitro and in a subset of primary colorectal tumors in mice. 5-fluorouracil induces the phosphorylation of KAP1, a target of the checkpoint kinase ataxia-telangiectasia mutated (ATM), stronger in HDAC2-negative cells than in their HDAC2-positive counterparts. Pharmacological inhibition of ATM sensitizes RKO cells to cytotoxic effects of 5-fluorouracil. These findings demonstrate that HDAC2 and ATM modulate the responses of colorectal cancer cells towards 5-FU.

Review of 5-FU resistance mechanisms in colorectal cancer: clinical significance of attenuated on-target effects

The emergence of chemoresistant disease during chemotherapy with 5-Fluorouracil-based (5-FU-based) regimens is an important factor in the mortality of metastatic CRC (mCRC). The causes of 5-FU resistance are multi-factorial, and besides DNA mismatch repair deficiency (MMR-D), there are no widely accepted criteria for determining which CRC patients are not likely to be responsive to 5-FU-based therapy. Thus, there is a need to systematically understand the mechanistic basis for 5-FU treatment failure and an urgent need to develop new approaches for circumventing the major causes of 5-FU resistance. In this manuscript, we review mechanisms of 5-FU resistance with an emphasis on: (1) altered anabolic metabolism limiting the formation of the primary active metabolite Fluorodeoxyuridylate (5-Fluoro-2'-deoxyuridine-5'-O-monophosphate; FdUMP); (2) elevated expression or activity of the primary enzymatic target thymidylate synthase (TS); and (3) dysregulated programmed cell death as important causes of 5-FU resistance. Importantly, these causes of 5-FU resistance can potentially be overcome through the use of next-generation fluoropyrimidine (FP) polymers (e.g., CF10) that display reduced dependence on anabolic metabolism and more potent TS inhibitory activity.

Regulation of thymidylate synthase: an approach to overcome 5-FU resistance in colorectal cancer

Thymidylate synthase is the rate-limiting enzyme required for DNA synthesis and overexpression of this enzyme causes resistance to cancer cells. Long treatments with 5-FU cause resistance to Thymidylate synthase targeting drugs. We have also compiled different mechanisms of drug resistance including autophagy and apoptosis, drug detoxification and ABC transporters, drug efflux, signaling pathways (AKT/PI3K, RAS-MAPK, WNT/β catenin, mTOR, NFKB, and Notch1 and FOXM1) and different genes associated with resistance in colorectal cancer. We can overcome 5-FU resistance in cancer cells by regulating thymidylate synthase by natural products (Coptidis rhizoma), HDAC inhibitors, mTOR inhibitors, Folate antagonists, and several other drugs which have been used in combination with TS inhibitors. This review is a compilation of different approaches reported for the regulation of thymidylate synthase to overcome resistance in colorectal cancer cells.

Issues and limitations of available biomarkers for fluoropyrimidine-based chemotherapy toxicity, a narrative review of the literature

Fluoropyrimidine-based chemotherapies are widely used to treat gastrointestinal tract, head and neck, and breast carcinomas. Severe toxicities mostly impact rapidly dividing cell lines and can occur due to the partial or complete deficiency in dihydropyrimidine dehydrogenase (DPD) catabolism. Since April 2020, the European Medicines Agency (EMA) recommends DPD testing before any fluoropyrimidine-based treatment. Currently, different assays are used to predict DPD deficiency; the two main approaches consist of either phenotyping the enzyme activity (directly or indirectly) or genotyping the four main deficiency-related polymorphisms associated with 5-fluorouracil (5-FU) toxicity. In this review, we focused on the advantages and limitations of these diagnostic methods: direct phenotyping by evaluation of peripheral mononuclear cell DPD activity (PBMC-DPD activity), indirect phenotyping assessed by uracil levels or UH2/U ratio, and genotyping DPD of four variants directly associated with 5-FU toxicity. The risk of 5-FU toxicity increases with uracil concentration. Having a pyrimidine-related structure, 5-FU is catabolised by the same physiological pathway. By assessing uracil concentration in plasma, indirect phenotyping of DPD is then measured. With this approach, in France, a decreased 5-FU dose is systematically recommended at a uracil concentration of 16 ng/ml, which may lead to chemotherapy under-exposure as uracil concentration is a continuous variable and the association between uracil levels and DPD activity is not clear. We aim herein to describe the different available strategies developed to improve fluoropyrimidine-based chemotherapy safety, how they are implemented in routine clinical practice, and the possible relationship with inefficacy mechanisms.

Pharmacogenomics of 5-fluorouracil in colorectal cancer: review and update

BACKGROUND

Colorectal cancer (CRC) is a disease with high morbidity and mortality rates. 5-fluorouracil (5-FU) is the first-line recommended drug for chemotherapy in patients with CRC, and it has a good effect on a variety of other solid tumors as well. Unfortunately, however, due to the emergence of drug resistance the effectiveness of treatment may be greatly reduced. In the past decade, major progress has been made in the field of 5-FU drug resistance in terms of molecular mechanisms, pre-clinical (animal) models and clinical trials.

CONCLUSIONS

In this article we systematically review and update current knowledge on 5-FU pharmacogenomics related to drug uptake and activation, the expression and activity of target enzymes (DPD, TS and MTHFR) and key signaling pathways in CRC. Furthermore, a summary of drug combination strategies aimed at targeting specific genes and/or pathways to reverse 5-FU resistance is provided. Based on this, we suggest that causal relationships between genes, pathways and drug sensitivity should be systematically considered from a multidimensional perspective. In the design of research methods, emerging technologies such as CRISPR-Cas, TALENS and patient-derived xenograft models should be applied as far as possible to improve the accuracy of clinically relevant results.

5-fluorouracil and other fluoropyrimidines in colorectal cancer: Past, present and future

5-Fluorouracil (5-FU) is an essential component of systemic chemotherapy for colorectal cancer (CRC) in the palliative and adjuvant settings. Over the past four decades, several modulation strategies including the implementation of 5-FU-based combination regimens and 5-FU pro-drugs have been developed and tested to increase the anti-tumor activity of 5-FU and to overcome the clinical resistance. Despite the encouraging progress in CRC therapy to date, the patients' response rates to therapy continue to remain low and the patients' benefit from 5-FU-based therapy is frequently compromised by the development of chemoresistance. Inter-individual differences in the treatment response in CRC patients may originate in the unique genetic and epigenetic make-up of each individual. The critical element in the current trend of personalized medicine is the proper comprehension of causes and mechanisms contributing to the low or lack of sensitivity of tumor tissue to 5-FU-based therapy. The identification and validation of predictive biomarkers for existing 5-FU-based and new targeted therapies for CRC treatment will likely improve patients' outcomes in the future. Herein we present a comprehensive review summarizing options of CRC treatment and the mechanisms of 5-FU action at the molecular level, including both anabolic and catabolic ways. The main part of this review comprises the currently known molecular mechanisms underlying the chemoresistance in CRC patients. We also focus on various 5-FU pro-drugs developed to increase the amount of circulating 5-FU and to limit toxicity. Finally, we propose future directions of personalized CRC therapy according to the latest published evidence.

A Phase II study of S-1 plus oral leucovorin in heavily treated metastatic colorectal cancer patients

Purpose

Fewer treatment options are available for refractory metastatic colorectal cancer (mCRC). In early trials, S-1 monotherapy was effective for mCRC patients after chemotherapy failure and its combination with oral leucovorin therapy offers promising results in untreated mCRC. Hence, we conduct a Phase II trial to assess the efficacy of S-1 plus oral leucovorin (SL) in refractory mCRC that progressed after multiple prior standard therapies.

Methods

In this open-label, single-arm study, we enrolled the refractory mCRC patients who received fluoropyrimidine, oxaliplatin, and irinotecan treatment and at least one targeted therapy previously. The doses of SL were 40–60 and 30 mg twice daily separately. They were administered for 7 days in a 2-week cycle. Treatment was continued until disease progression.

Results

Of the 41 enrolled patients, 36 patients were evaluable with 61.1% disease control rate. The median progression-free survival and overall survival were 2.55 and 7.63 months, respectively. Regression change in tumor size stayed 10%–20% in five patients (13.9%) through 18 weeks after treatment, and two patients continued free from tumor progression at 30 and 42 weeks. Compared with moderate heavily pretreated mCRC patient subgroup (≤4 prior regimens), the severe heavily pretreated subgroup (≥5 prior regimens) showed similar disease control rate and survival benefit. Grade 3 or higher toxicities were documented only in 11 patients (26.8%).

Conclusion

SL shows potential as a salvage regimen in refractory mCRC patients especially in the severe heavily pretreated setting and is well tolerated in these patients.

Thymidine phosphorylase: the unforeseen driver in colorectal cancer treatment?

5-Fluorouracil- and leucovorin-based chemotherapy regimens are the backbone of colorectal cancer treatment. The addition of oxaliplatin, irinotecan and monoclonal antibodies to this backbone has largely improved clinical outcomes, but has also led to new questions, with conflicting data frequently reported in studies. Thymidine phosphorylase (TP) is a nucleoside-metabolizing enzyme involved in 5-fluorouracil pharmacokinetics, as well as inflammatory responses, neoangiogenesis and apoptosis. TP expression is regulated by hypoxia, inflammatory cytokines and antitumoral agents. We hypothesize that TP could be the unforeseen driver in the conflicting data observed with different regimens commonly used in colorectal cancer treatment. Greater comprehension of the role of this enzyme in tumor progression and pyrimidine metabolism may lead to more accurate, patient-tailored therapy.

Correlation of Expression Levels of Copper Transporter 1 and Thymidylate Synthase with Treatment Outcomes in Patients with Advanced Non-small Cell Lung Cancer Treated with S-1/Carboplatin Doublet Chemotherapy

Background: Copper transporter 1 (CTR1) is a critical determinant of the uptake and cytotoxic effect of the platinum drugs carboplatin and cisplatin. Thymidylate synthase (TS) is an enzyme involved in DNA synthesis and is associated with resistance of tumor cells to 5-fluorouracil. We investigated the correlation between CTR1 and TS expression levels and treatment outcomes in patients with advanced non-small-cell lung cancer (NSCLC) treated with S-1/carboplatin doublet chemotherapy. Methods: Twenty-nine patients were enrolled in this study. Tumor expression of CTR1 and TS was measured immunohistochemically and analyzed for correlation with tumor response, progression-free survival (PFS), and overall survival (OS). Results: Tumor response was significantly better in patients with CTR1High tumors than in patients with CTR1Low tumors (64% vs. 18%, P = 0.02). Patients with TSLow tumors had a significantly longer OS (median 21.2 vs. 8.5 months, P = 0.02), but not PFS, than patients with TSHigh tumors. When CTR1 and TS co-expression was analyzed, patients with either CTR1High or TSLow tumors showed a significantly better tumor response (50% vs. 0%, P = 0.01), longer PFS (median 4.2 vs. 2.1 months, P = 0.03), and longer OS (median 21.2 vs. 8.5 months, P = 0.01) than patients with both CTR1Low and TSHigh tumors. Conclusions: Our study suggests that combined CTR1/TS expression status has the potential to be an important predictor of good treatment outcomes in patients with advanced NSCLC treated with S-1/carboplatin doublet chemotherapy.

A noncanonical poly(A) RNA polymerase gene affects morphology in Phoma medicaginis

Phoma medicaginis (syn. Ascochyta medicaginicola Qchen & L. Cai) causes spring black stem and leaf spot, an important disease of alfalfa and annual medics. P. medicaginis forms uninucleate conidia in melanized pycnidia and is genetically tractable using Agrobacterium mediated transformation (ATMT), resulting in random integration of T-DNA that occasionally generates pycnidial mutants. The T-DNA tagged mutant, P265 displayed smaller pycnidia and more aerial hyphae than the wild type. A single T-DNA disrupted a putative noncanonical poly(A) RNA polymerase gene, Pmncpap1, which in yeast interacts with ribonucleotide reductase (RNR). As in yeast mutants, P265 showed sensitivity to hydroxyurea (HU), a RNR inhibitor. To characterize the role of Pmncpap1, targeted ΔPmncpap1 mutants were created using a hygromycin selectable marker flanked by 1 Kbp regions of Pmncpap1. ΔPmncpap1 mutants possessed similar morphological features to those of P265. The plasmid for rescue of PmncPAP1, pCAM-Nat1 (nourseothricin selection) was constructed and used to introduce full-length PmncPAP1 into mutants. Rescued P265 showed partial recovery of wild type and the original T-DNA was lost due to homologous integration. To our knowledge, this is the first ncPAP to be examined in a filamentous fungus.

Targeting One Carbon Metabolism with an Antimetabolite Disrupts Pyrimidine Homeostasis and Induces Nucleotide Overflow

Antimetabolites that affect nucleotide metabolism are frontline chemotherapy agents in several cancers and often successfully target one carbon metabolism. However, the precise mechanisms and resulting determinants of their therapeutic value are unknown. We show that 5-fluorouracil (5-FU), a commonly used antimetabolite therapeutic with varying efficacy, induces specific alterations to nucleotide metabolism by disrupting pyrimidine homeostasis. An integrative metabolomics analysis of the cellular response to 5-FU reveals intracellular uracil accumulation, whereas deoxyuridine levels exhibited increased flux into the extracellular space, resulting in an induction of overflow metabolism. Subsequent analysis from mice bearing colorectal tumors treated with 5-FU show specific secretion of metabolites in tumor-bearing mice into serum that results from alterations in nucleotide flux and reduction in overflow metabolism. Together, these findings identify a determinant of an antimetabolite response that may be exploited to more precisely define the tumors that could respond to targeting cancer metabolism.

A single-arm phase II trial of combined chemotherapy with S-1, oral leucovorin, and bevacizumab in heavily pre-treated patients with metastatic colorectal cancer

BACKGROUND

The mean 5-6-month survival after failed standard chemotherapy for metastatic colorectal cancer (mCRC) necessitates more effective treatments for refractory mCRC. For untreated mCRC, S-1 + oral leucovorin (SL) therapy offers promising results without severe toxicity. The ML18147 trial demonstrated that bevacizumab (Bev) prolongs overall survival after mCRC progression. We conducted a single-centre phase-II trial to evaluate the safety and efficacy of SL/Bev combination chemotherapy as mCRC salvage therapy.

METHODS

Major eligibility criteria were confirmed adenocarcinoma diagnosis; age >20 years; Eastern Cooperative Oncology Group performance status, 0-2; and progression after administration/intolerance of/to approved drugs for mCRC. (5-FU, oxaliplatin, irinotecan, Bev, and anti-EGFR antibody, if KRAS wild-type). S-1 (80-120 mg/body) and leucovorin (25 mg) were orally administered in a 1-week-on/1-week-off schedule. Bev (5 mg/kg) was administered on day 1 of every 2-week cycle. The primary endpoint was disease control rate (DCR).

RESULTS

A total of 31 patients were enrolled. DCR was 65% [95% confidence interval (CI), 48-100%] and the response rate was 7% (95% CI, 0.7-22%). One patient showing partial response to SL/Bev had a BRAF-mutant tumor. Median progression-free survival and overall survivals were 5.3 [95% CI, 2.1-9.3] and 9.9 [95% CI, 7.4-NA] months, respectively. The most-frequent grade-3/4 adverse events were mucositis (26%) and diarrhea (11%), which were manageable by dose reduction/interruption.

CONCLUSIONS

SL/Bev showed impressive activity in refractory mCRC and was tolerable, suggesting its potential as an alternative chemotherapy for refractory mCRC.

TRIAL REGISTRATION

This study has been registered in University Hospital Medical Information Network (UMIN) Clinical Trials Registry ( ID UMIN000009083 ) on 11 October 2012.

In vivo 5FU-exposed human medullary thyroid carcinoma cells contain a chemoresistant CD133+ tumor-initiating cell subset

BACKGROUND

The hierarchical model of solid tumor proposes the existence of rare tumor cell subpopulations with stem-cell properties. The glycoprotein prominin-1 (CD133) represents one of the cancer stem-cell markers in several tumor types. The CD133+ cell subpopulation was shown to be enriched for tumor-initiating and highly chemoresistant cells in human cancer(s).

METHODS

We investigated whether CD133+ cells derived from human medullary thyroid carcinoma (MTC) possess tumor-initiating properties in vivo and exhibit differential responses to chemotherapeutic agents. We demonstrated that separated CD133+ cells from the human MTC cell line TT are enriched for tumor-initiating cells as demonstrated by tumor formation in vivo. Nevertheless, TT CD133+ cells do not exhibit increased chemoresistance in comparison to parental cells. However, when MTC xenotransplants were treated with the chemotherapeutic drug 5-fluorouracil (5FU) in vivo, CD133 expression increased in MTC cells.

RESULTS

This cell line, designated FTTiv isolated from the drug-exposed xenotransplants, exhibits a significantly different response to 5FU associated with the substantial change in the expression profile of genes involved in 5FU metabolism and drug resistance. Moreover, the CD133+ tumor-initiating subpopulation derived from these drug-exposed FTTiv cells is significantly more resistant to 5FU and retains the chemoresistant properties upon FTTiv culture propagation.

CONCLUSIONS

These data suggest that the chemoresistant phenotype and the CD133+ MTC subpopulation emerged in response to chemotherapy in vivo.

Enhancement of 5-fluorouracil-induced cytotoxicity by leucovorin in 5-fluorouracil-resistant gastric cancer cells with upregulated expression of thymidylate synthase

BACKGROUND

Elucidation of the mechanisms by which gastric cancer cells acquire resistance to 5-fluorouracil (5FU) may provide important clues to the development of effective chemotherapy for 5FU-resistant gastric cancer

METHODS

Four 5FU-resistant cell lines (MKN45/5FU, MKN74/5FU, NCI-N87/5FU, and KATOIII/5FU) were established by continuous exposure of the cells to progressively increasing concentrations of 5FU for about 1 year. Then, mRNA expression levels of four genes associated with 5FU metabolism, i.e., thymidylate synthase (TS), dihydropyrimidine dehydrogenase, thymidine phosphorylase, and orotate phosphoribosyltransferase, were quantitatively evaluated by real-time reverse transcriptase-polymerase chain reaction. In addition, TS protein expression was measured by Western blot analysis.

RESULTS

As compared with the parent cell lines, the 5FU-resistant cell lines showed 3.8- to 11.6-fold higher resistance to 5FU, as well as 1.9- to 3.5-fold higher TS mRNA expression and 1.6- to 7.1-fold higher TS protein expression. In contrast, the expressions of other genes did not differ significantly among the cell lines. The cytotoxicity of 5FU was enhanced 2.3- to 2.8 fold by leucovorin (LV) against three of the four 5FU-resistant cell lines.

CONCLUSIONS

Collectively, LV enhanced the cytotoxicity of 5FU not only against the parent gastric cancer cell lines, but also against the 5FU-resistant cell lines, even those with elevated TS expression levels. These results suggest that clinical studies of a combination of 5FU and LV are warranted in patients who have recurrent gastric cancer after 5FU-based therapy.

Phase I study of GTI-2040, a ribonucleotide reductase antisense, with high dose cytarabine in patients with relapsed/refractory acute myeloid leukemia

We hypothesized that GTI-2040, a 20-mer oligonucleotide complementary to the R2 subunit mRNA of ribonucleotide reductase, combined with high dose cytarabine (HiDAC) would result in enhanced cytotoxicity by favoring Ara-CTP DNA incorporation. In a phase I dose escalation trial, adults (≥ 60 years) with refractory or relapsed acute myeloid leukemia (AML) received daily HiDAC plus infusional GTI-2040. Using a novel assay, evidence of intracellular drug accumulation and target R2 down-regulation was observed. GTI-2040/HiDAC can be administered safely. However, with no complete remissions observed, alternative doses and schedules may need to be investigated to achieve clinical activity in older patients with AML.

Serine, glycine and one-carbon units: cancer metabolism in full circle

One-carbon metabolism involving the folate and methionine cycles integrates nutritional status from amino acids, glucose and vitamins, and generates diverse outputs, such as the biosynthesis of lipids, nucleotides and proteins, the maintenance of redox status and the substrates for methylation reactions. Long considered a 'housekeeping' process, this pathway has recently been shown to have additional complexity. Genetic and functional evidence suggests that hyperactivation of this pathway is a driver of oncogenesis and establishes a link to cellular epigenetic status. Given the wealth of clinically available agents that target one-carbon metabolism, these new findings could present opportunities for translation into precision cancer medicine.

The upregulation of dihydropyrimidine dehydrogenase in liver is involved in acquired resistance to 5-fluorouracil

BACKGROUND

Acquired resistance to 5-fluorouracil (5-FU) is one of the important reasons for failure in 5-FU-based chemotherapy. The upregulation of dihydropyrimidine dehydrogenase (DPD) in tumours was reported as an important factor for acquired 5-FU resistance. The aim of this study is to examine whether intra-hepatic DPD was involved in acquired 5-FU resistance.

METHODS

HT-29 human colorectal xenograft tumours were established in nude mice. After long-term exposure to 5-FU, some of the tumour became "resistant" and the others remained "sensitive" to 5-FU. DPD expression levels in the livers and tumours of "resistant", "sensitive" or untreated mice were examined, and pharmacokinetics of 5-FU in rats' plasma were investigated. Gimeracil, a DPD inhibitor, was checked whether it could reverse the reduced bioavailability of 5-FU.

RESULTS

DPD expression was upregulated obviously in tumours of "resistant" mice as reported previously. Importantly, DPD expression was also upregulated significantly in livers of "resistant" mice, compared with those of "sensitive" or untreated mice. Furthermore, the upregulation of DPD expression in livers led to accelerated metabolism of 5-FU. Gimeracil was found to reverse the reduced serum 5-FU concentration. The cultured tumour cells from 5-FU treated mice showed relative sensitivity to higher concentration of 5-FU, even the "resistant" tumour cells.

CONCLUSION

Our study suggested that the upregulation of DPD in liver may be involved in acquired resistance to 5-FU, and DPD inhibitors or increasing 5-FU dosage may have potential application in overcoming 5-FU acquired resistance.

MEK2 regulates ribonucleotide reductase activity through functional interaction with ribonucleotide reductase small subunit p53R2

The p53R2 protein, a newly identified member of the ribonucleotide reductase family that provides nucleotides for DNA damage repair, is directly regulated by p53. We show that p53R2 is also regulated by a MEK2 (ERK kinase 2/MAP kinase kinase 2)-dependent pathway. Increased MEK1/2 phosphorylation by serum stimulation coincided with an increase in the RNR activity in U2OS and H1299 cells. The inhibition of MEK2 activity, either by treatment with a MEK inhibitor or by transfection with MEK2 siRNA, dramatically decreased the serum-stimulated RNR activity. Moreover, p53R2 siRNA, but not R2 siRNA, significantly inhibits serum-stimulated RNR activity, indicating that p53R2 is specifically regulated by a MEK2-dependent pathway. Co-immunoprecipitation analyses revealed that the MEK2 segment comprising amino acids 65-171 is critical for p53R2-MEK2 interaction, and the binding domain of MEK2 is required for MEK2-mediated increased RNR activity. Phosphorylation of MEK1/2 was greatly augmented by ionizing radiation, and RNR activity was concurrently increased. Ionizing radiation-induced RNR activity was markedly attenuated by transfection of MEK2 or p53R2 siRNA, but not R2 siRNA. These data show that MEK2 is an endogenous regulator of p53R2 and suggest that MEK2 may associate with p53R2 and upregulate its activity.

Role of miR-19b and its target mRNAs in 5-fluorouracil resistance in colon cancer cells

BACKGROUND

Drug resistance in colorectal cancers is assumed to be mediated by changes in the expression of microRNAs, but the specific identities and roles of microRNAs are largely unclear. We examined the effect of 5-fluorouracil (5-FU) resistance on microRNA expression.

METHODS

Two types of 5-FU-resistant colon cancer cells were derived from the DLD-1 and KM12C cell lines. The expressions of microRNAs were profiled with a microarray containing 723 microRNAs and validated by quantitative real-time polymerase chain reaction (qRT-PCR). To survey the downstream mediators of microRNA, we used a microRNA:mRNA immunoprecipitation (RIP)-Chip and pathway analysis tool to identify potential direct targets of microRNA.

RESULTS

In response to 5-FU, miR-19b and miR-21 were over-expressed in 5-FU-resistant cells. Of note, miR-19b was up-regulated 3.47-fold in the DLD-1 resistant cells, which exhibited no alteration in cell cycle profiles despite exposure to 5-FU. After transfection of miR-19b, specific mRNAs were recruited to microRNA:mRNA complexes isolated with Ago2 antibody and subjected to whole-genome transcriptional analysis. In this analysis, 66 target mRNAs were enriched by at least 5.0-fold in the microRNA:mRNA complexes from DLD-1 resistant cells. Ingenuity pathway analysis of mRNA targets significantly (P < 0.05) indicated the category "Cell Cycle" as a probable area of the molecular and cellular function related with 5-FU resistance. Among candidate mRNA targets, SFPQ and MYBL2 have been linked to cell cycle functions.

CONCLUSIONS

We revealed up-regulation of miR-19b in response to 5-FU and potential targets of miR-19b mediating the cell cycle under treatment with 5-FU. Our study provides an important insight into the mechanism of 5-FU resistance in colorectal cancers.

Combination therapy using oral S-1 and targeted agents against human tumor xenografts in nude mice

In this study, combination therapies using the oral fluoropyrimidine tegafur-gimeracil-oteracil (S-1) with several targeted agents or antibodies, were evaluated. First, the effects of tyrosine kinase inhibitors (erlotinib hydrochloride, sorafenib tosilate and sunitinib malate) against human non-small cell lung cancer (NSCLC), breast cancer and colorectal cancer were evaluated in vivo. The effects of the combination of S-1 and targeted antibodies (bevacizumab and cetuximab) against human colorectal cancers was also evaluated in vivo. S-1 and the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, erlotinib, showed a significant inhibition of growth in human NSCLC (Lu-99 and PC-9 cell lines). The antitumor activity of the combination of S-1 and erlotinib against Lu-99 and PC-9 cancer cell lines was significantly superior to either monotherapy (P<0.05). Combination therapy using the multi-tyrosine kinase inhibitors, sorafenib or sunitinib, with S-1 against breast cancer (MX-1 cell line) and NSCLC (NCI-H460 cell line) was significantly superior to either monotherapy (P<0.01). The combination of the anti-vascular endothelial growth factor antibody bevacizumab or the anti-EGFR antibody, cetuximab, with S-1 against human colorectal cancer [Col-1, KM20C (bevacizumab) and DLD-1 (cetuximab) cell lines] and a 5-fluorouracil (5-FU)-resistant cell line (KM12C/5-FU) was significantly superior to either monotherapy (p<0.01). In particular, the growth of the Col-1 cells was completely inhibited by the combination of S-1 and bevacizumab. No toxic mortalities and no significant difference in the body weight changes of the animals treated with S-1 combined with the targeted agents or with the mono-therapies were observed; therefore, the treatments appeared to be well-tolerated. Our preclinical findings indicate that the combination therapies of S-1 and targeted agents are promising treatment options.

Establishment and characterization of two 5-fluorouracil-resistant hepatocellular carcinoma cell lines

5-Fluorouracil (5-FU) chemotherapy is the first choice treatment for advanced hepatocellular carcinoma (HCC), and resistance is the major obstacle to successful treatment. Recent studies have reported that epithelial-to-mesenchymal transition (EMT) is associated with chemoresistance in cancers. We speculated that EMT and 5-FU metabolism are related to the mechanism of 5-FU resistance. First, two 5-FU-resistant cell lines, HLF-R4 and HLF-R10, were established from the HLF undifferentiated human HCC cell line. Whereas cell growth was similar in the HLF and HLF-R cell lines, HLF-Rs are about 4- and 10-fold more resistant compared with the HLF cells; thus, we named these cell lines HLF-R4 and HLF-R10, respectively. The terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay also showed a dramatically decreased number of apoptotic cells in the HLF-Rs after treatment with 5-FU. We next assessed the characteristics of the HLF, HLF-R4 and HLF-R10 cells. Consistent with our hypothesis, the HLF-Rs had typical morphologic phenotypes of EMT, loss of cell-cell adhesion, spindle-shaped morphology and increased formation of pseudopodia. Real-time quantitative reverse transcriptase polymerase chain reaction data showed downregulated E-cadherin and upregulated Twist-1 and also indicated that EMT changes occurred in the HLF-Rs. We also found decreased ribonucleotide reductase and increased multidrug resistance protein 5 genes in the HLF-R cells. Our results suggested that the metabolism of EMT and 5-FU has important roles in 5-FU chemoresistance in the HLF-R cells, and that the HLF-R cells would be useful in vitro models for understanding the 5-FU-resistant mechanisms in HCC.

Randomized phase II trial of first-line treatment with tailored irinotecan and S-1 therapy versus S-1 monotherapy for advanced or recurrent gastric carcinoma (JFMC31-0301)

The pharmacokinetics of irinotecan vary markedly between individuals. This study sought to compare tailored irinotecan and S-1 therapy with S-1 monotherapy for the treatment of patients with advanced/recurrent gastric cancer. Patients with advanced/recurrent gastric cancer were randomized to receive tailored irinotecan and S-1 (arm A) therapy or S-1 therapy alone (arm B). Arm A received S-1 (80-120 mg/m(2)/day) for 14 days, with irinotecan on days 1 and 15. The initial irinotecan dose of 75 mg/m(2) (level 0) was adjusted for toxicity during an earlier course. In arm B, S-1 (80-120 mg/day) was administered alone for 28 days, followed by 14 days without therapy. Ninety-five patients were randomized (48 patients to arm A and 47 patients to arm B). The response rate of the primary tumor (Japanese criteria) was 25.0% in arm A (12 of 48 patients) and 14.9% in arm B (seven of 47 patients), whereas the response rates according to Response Evaluation Criteria In Solid Tumors were 27.8% (10 of 36) versus 21.9% (seven of 32). Hematological toxicity, anorexia, and diarrhea were significantly more common in arm A, but both arms had similar grades 3-4 toxicities. These findings suggest the usefulness of tailored irinotecan and S-1 therapy for gastric cancer.

A common telomeric gene silencing assay is affected by nucleotide metabolism

Telomere-associated position-effect variegation (TPEV) in budding yeast has been used as a model for understanding epigenetic inheritance and gene silencing. A widely used assay to identify mutants with improper TPEV employs the URA3 gene at the telomere of chromosome VII-L that can be counterselected with 5-fluoroorotic acid (5-FOA). 5-FOA resistance has been inferred to represent lack of transcription of URA3 and therefore to represent heterochromatin-induced gene silencing. For two genes implicated in telomere silencing, POL30 and DOT1, we show that the URA3 telomere reporter assay does not reflect their role in heterochromatin formation. Rather, an imbalance in ribonucleotide reductase (RNR), which is induced by 5-FOA, and the specific promoter of URA3 fused to ADH4 at telomere VII-L are jointly responsible for the variegated phenotype. We conclude that metabolic changes caused by the drug employed and certain mutants being studied are incompatible with the use of certain prototrophic markers for TPEV.

Essential role of Tip60-dependent recruitment of ribonucleotide reductase at DNA damage sites in DNA repair during G1 phase

A balanced deoxyribonucleotide (dNTP) supply is essential for DNA repair. Here, we found that ribonucleotide reductase (RNR) subunits RRM1 and RRM2 accumulated very rapidly at damage sites. RRM1 bound physically to Tip60. Chromatin immunoprecipitation analyses of cells with an I-SceI cassette revealed that RRM1 bound to a damage site in a Tip60-dependent manner. Active RRM1 mutants lacking Tip60 binding failed to rescue an impaired DNA repair in RRM1-depleted G1-phase cells. Inhibition of RNR recruitment by an RRM1 C-terminal fragment sensitized cells to DNA damage. We propose that Tip60-dependent recruitment of RNR plays an essential role in dNTP supply for DNA repair.

A mathematical feasibility argument for the use of aptamers in chemotherapy and imaging

A challenge for drug design is to create molecules with optimal functions that also partition efficiently into the appropriate in vivo compartment(s). This is particularly true in cancer treatments because cancer cells upregulate their expression of multidrug resistance transporters, which necessitates a higher concentration of extracellular drug to promote sufficiently high intracellular concentrations for cell killing. Pharmacokinetics can be improved by ancillary molecules, such as cyclodextrins, that increase the effective concentrations of hydrophobic drugs in the blood by providing hydrophobic binding pockets. However, the extent to which the extracellular concentration of drug can be increased is limited. A second approach, different from the 'push' mechanism just discussed, is a 'pull' mechanism by which the effective intracellular concentrations of a drug is increased by a molecule with an affinity for the drug that is located inside the cell. Here we propose and give a proof in principle that intracellular RNA aptamers might perform this function. The mathematical model considers the following: Suppose I denotes a drug (inhibitor) that must be distributed spatially throughout a cell, but that tends to remain outside the cell due the transport properties of the cell membrane. Suppose that E, an enzyme that binds to I, is expressed by the cell and remains in the cell. It may be that the equilibrium E+I[right arrow over left arrow]{k(-1)k(1)}P is not sufficiently far enough to the right to drive enough free inhibitor into the cell to completely inhibit the enzyme. Here we evaluate the use of an intracellular aptamer with affinity for the inhibitor (I) to increase the efficiency of inhibitor transport across the cell membrane and thus drive the above equilibrium further to the right than would ordinarily be the case. We show that this outcome will occur if: (1) the aptamer neither binds too tightly nor too weakly to the inhibitor than the enzyme and (2) the aptamer is much more diffusible in the cell cytoplasm than the enzyme. Thus, we propose and show by simulation that an intracellular aptamer can be enlisted for an integrated approach to increasing inhibitor effectiveness and imaging aptamer-expressing cells.

Clinical implications of thymidylate synthetase, dihydropyrimidine dehydrogenase and orotate phosphoribosyl transferase activity levels in colorectal carcinoma following radical resection and administration of adjuvant 5-FU chemotherapy

BACKGROUND

A number of studies have investigated whether the activity levels of enzymes involved in 5-fluorouracil (5-FU) metabolism are prognostic factors for survival in patients with colorectal carcinoma. Most reports have examined thymidylate synthetase (TS) and dihydropyrimidine dehydrogenase (DPD) in unresectable or metastatic cases, therefore it is unclear whether the activity of these enzymes is of prognostic value in colorectal cancer patients treated with radical resection and adjuvant chemotherapy with 5-FU.

METHODS

This study examined fresh frozen specimens of colorectal carcinoma from 40 patients who had undergone curative operation and were orally administered adjuvant tegafur/uracil (UFT) chemotherapy. TS, DPD and orotate phosphoribosyl transferase (OPRT) activities were assayed in cancer tissue and adjacent normal tissue and their association with clinicopathological variables was investigated. In addition, the relationships between TS, DPD and OPRT activities and patient survival were examined to determine whether any of these enzymes could be useful prognostic factors.

RESULTS

While there was no clear relationship between pathological findings and TS or DPD activity, OPRT activity was significantly lower in tumors with lymph node metastasis than in tumors lacking lymph node metastasis. Postoperative survival was significantly better in the groups with low TS activity and/or high OPRT activity.

CONCLUSION

TS and OPRT activity levels in tumor tissue may be important prognostic factors for survival in Dukes' B and C colorectal carcinoma with radical resection and adjuvant chemotherapy with UFT.

Phase I study of GTI-2040, an antisense to ribonucleotide reductase, in combination with high-dose cytarabine in patients with acute myeloid leukemia

PURPOSE

Inhibition of ribonucleotide reductase reduces the availability of the endogenous pool of deoxycytidine and may increase cytarabine (AraC) cytotoxicity. We performed a phase I dose escalation trial of AraC combined with GTI-2040, a 20-mer antisense oligonucleotide shown in preclinical studies to decrease levels of the R2 subunit of ribonucleotide reductase, to determine the maximum tolerated dose in adults with relapsed/refractory acute myeloid leukemia.

EXPERIMENTAL DESIGN

Twenty-three adults (ages 18-59 years) were enrolled in this dose escalation phase I trial, receiving high-dose AraC twice daily combined with infusional GTI-2040. An ELISA-based assay measured plasma and intracellular concentrations of GTI-2040. R2 protein changes were evaluated by immunoblotting in pretreatment and post-treatment bone marrow samples.

RESULTS

The maximum tolerated dose was 5 mg/kg/d GTI-2040 (days 1-6) and 3 g/m2/dose AraC every 12 hours for 8 doses. Neurotoxicity was dose limiting. Eight patients (35%) achieved complete remission. Mean bone marrow intracellular concentration of GTI-2040 were higher at 120 hours than at 24 hours from the start of GTI-2040 (P = 0.002), suggesting intracellular drug accumulation over time. Reductions in bone marrow levels of R2 protein (>50%) were observed at 24 and 120 hours. Higher baseline R2 protein expression (P = 0.03) and reductions after 24 hours of GTI-2040 (P = 0.04) were associated with complete remission.

CONCLUSIONS

GTI-2040 and high-dose AraC were coadministered safely with successful reduction of the intended R2 target and encouraging clinical results. The clinical efficacy of this combination will be tested in an upcoming phase II study.

Genome-wide screening of loci associated with drug resistance to 5-fluorouracil-based drugs

Resistance to chemotherapeutic agents represents the chief cause of mortality in cancer patients with advanced disease. Chromosomal aberration and altered gene expression are the main genetic mechanisms of tumor chemoresistance. In this study, we have established an algorithm to calculate DNA copy number using the Affymetrix 10K array, and performed a genome-wide correlation analysis between DNA copy number and antitumor activity against 5-fluorouracil (5-FU)-based drugs (S-1, tegafur + uracil [UFT], 5'-DFUR and capecitabine) to screen for loci influencing drug resistance using 27 human cancer xenografts. A correlation analysis confirmed that the single nucleotide polymorphism (SNP) showing significant associations with drug sensitivity were concentrated in some cytogenetic regions (18p, 17p13.2, 17p12, 11q14.1, 11q11 and 11p11.12), and we identified some genes that have been indicated their relations to drug sensitivity. Among these regions, 18p11.32 at the location of the thymidylate synthase gene (TYMS) was strongly associated with resistance to 5-FU-based drugs. A change in copy number of the TYMS gene was reflected in the TYMS expression level, and showed a significant negative correlation with sensitivity against 5-FU-based drugs. These results suggest that amplification of the TYMS gene is associated with innate resistance, supporting the possibility that TYMS copy number might be a predictive marker of drug sensitivity to fluoropyrimidines. Further study is necessary to clarify the functional roles of other genes coded in significant cytogenetic regions. These promising data suggest that a comprehensive DNA copy number analysis might aid in the quest for optimal markers of drug response.

Pretreatment with insulin enhances anticancer functions of 5-fluorouracil in human esophageal and colonic cancer cells

AIM

To investigate the effects of insulin on enhancing 5-fluorouracil (5-FU) anticancer functions and its mechanisms in the human esophageal cancer cell line (Eca 109) and human colonic cancer cell line (Ls-174-t).

METHODS

The effect of insulin/5-FU combination treatment on the growth of Eca 109 and Ls-174-t cells was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. After insulin treatment or insulin/5-FU treatment, cell cycle distribution of both cell lines was analyzed by flow cytometry. Western blot assay was used to assess the expression of caspase-3 and thymidylate synthase (TS). Apoptosis was detected by flow cytometry, DNA fragmentation assay, and terminal transferase dUTP nick end labeling assay (TUNEL). Moreover, the changes of 5-FU uptake after insulin pretreatment were detected by HPLC assay and Western blot analysis.

RESULTS

We found that insulin enhanced the inhibitory effect of 5- FU on cell proliferation when Eca 109 cells and Ls-174-t cells were pretreated with insulin for the appropriate time. Insulin increased the cell number of the S phase and the uptake of 5-FU. Insulin/5-FU treatment enhanced apoptosis of tumor cells and upregulated the expression of cleaved caspase-3 compared with 5-FU treatment. Moreover, insulin/5-FU treatment induced the changes of free TS and the TS ternary complex level compared with 5-FU treatment in Eca 109 and Ls-174-t cells.

CONCLUSION

These data suggest that insulin enhances anticancer functions of 5- FU when it is treated before 5-FU for the appropriate time in human esophageal and colonic cancer cell lines.

Upregulation of Enzymes Metabolizing 5-Fluorouracil in Colorectal Cancer

BACKGROUND

The purpose of this study was to compare the activities of various enzymes, participating in the metabolism of 5-fluorouracil, between colorectal cancer and nontumor tissues and to investigate the association of the enzyme activities with clinicopathological backgrounds.

METHODS

Activities of seven enzymes involved in nucleic acid metabolism--orotate phosphoribosyltransferase (OPRT), ribonucleotide reductase (RNR), thymidylate synthase, dihydropyrimidine dehydrogenase, thymidine phosphorylase, uridine phosphorylase and thymidine kinase (TK)--were measured in tumor and nontumor tissues from 28 patients who were operated on for colorectal cancers.

RESULTS

OPRT, thymidylate synthase, RNR, thymidine phosphorylase, uridine phosphorylase and thymidine kinase activities were significantly higher in tumor areas than in nontumor areas. OPRT showed the highest T/N ratio (the ratio of each enzyme activity in tumor areas to that in nontumor areas). The T/N ratio of RNR activity showed a tendency to be associated with lymph node metastasis and Dukes classification.

CONCLUSION

The results suggest that OPRT is a main enzyme participating in the phosphorylation of 5-fluorouracil and has an important role in tumor growth. The T/N ratio of RNR may be predictive of tumor progression.

Gene expression analysis using human cancer xenografts to identify novel predictive marker genes for the efficacy of 5-fluorouracil-based drugs

The development of a diagnostic method for predicting the therapeutic efficacy or toxicity of anticancer drugs is a critical issue. We carried out a gene expression analysis to identify genes whose expression profiles were correlated with the sensitivity of 30 human tumor xenografts to 5-fluorouracil (5-FU)-based drugs (tegafur + uracil [UFT], tegafur + gimeracil + oteracil [S-1], 5'-deoxy-5-fluorouridine [5'-DFUR], and N4-pentyloxycarbonyl-5'-deoxy-5-fluorocytidine [capecitabine]), as well as three other drugs (cisplatin [CDDP], irinotecan hydrochloride [CPT-11], and paclitaxel) that have different modes of action. In the present study, we focused especially on the fluoropyrimidines. The efficacy of all anticancer drugs was assayed using human tumor xenografts in nude mice. The mRNA expression profile of each of these xenografts was analyzed using a Human Focus array. Correlation analysis between the gene expression profiles and the chemosensitivities of seven drugs identified 39 genes whose expression levels were correlated significantly with multidrug sensitivity, and we suggest that the angiogenic pathway plays a pivotal role in resistance to fluoropyrimidines. Furthermore, many genes showing specific correlations with each drug were also identified. Among the candidate genes associated with 5-FU resistance, the dihydropyrimidine dehydrogenase mRNA expression profiles of the tumors showed a significant negative correlation with chemosensitivity to all of the 5-FU based drugs except for S-1. Therefore, the administration of S-1 might be an effective strategy for the treatment of high dihydropyrimidine dehydrogenase-expressing tumors. The results of the present study may enhance the prediction of tumor response to anticancer drugs and contribute to the development of tailor-made chemotherapy.

Predicting 5-fluorouracil chemosensitivity of liver metastases from colorectal cancer using primary tumor specimens: Three-gene expression model predicts clinical response

We identified genes related to 5-fluorouracil (5-FU) sensitivity in colorectal cancer and utilized these genes for predicting the 5-FU sensitivity of liver metastases. Eighty-one candidate genes involved in 5-FU resistance in gastric and colon cancer cell lines were previously identified using a cDNA microarray. In this study, the mRNA expression levels of these 81 selected genes and the genes of 5-FU-related enzymes, including thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD) and orotate phosphoribosyltransferase (OPRT), were measured using real-time quantitative RT-PCR assays of surgically resected materials from primary colorectal tumors in 22 patients. Clinical responses were estimated by evaluating the effects of 5-FU-based hepatic artery injection (HAI) chemotherapy for synchronous liver metastases. Four genes (TNFRSF1B, SLC35F5, NAG-1 and OPRT) had significantly different expression profiles in 5-FU-nonresponding and responding tumors (p < 0.05). A "Response Index" system using three genes (TNFRSF1B, SLC35F5 and OPRT) was then developed using a discriminate analysis; the results were well correlated with the individual chemosensitivities. Among the 11 cases with positive scores in our response index, 9 achieved a reduction in their liver metastases after 5-FU-based chemotherapy, whereas only 1 of the 11 cases with negative scores responded well to chemotherapy. Our "Response Index" system, consisting of TNFRSF1B, SLC35F5 and OPRT, has great potential for predicting the efficacy of 5-FU-based chemotherapy against liver metastases from colorectal cancer.

Synthesis and antitumor activity of 5-bromo-1-mesyluracil

Large-scale preparation of 5-bromo-1-mesyluracil (BMsU) 4 has been optimized. BMsU was synthesized by condensation of silylated 5-bromouracil and MsCl in acetonitrile or by the reaction of 5-bromouracil with MsCl in pyridine. The same product was obtained by bromination of 1-mesyluracil. The purpose of this study was to elucidate the effects of BMsU on the biosynthetic activity of tumor cell enzymes involved in DNA, RNA and protein syntheses, and in de novo and salvage pyrimidine and purine syntheses. Investigations were performed in vitro on human cervix carcinoma cells (HeLa). BMsU displayed inhibitory effects on DNA and RNA syntheses in HeLa cells after 24 h of treatment. De nova biosynthesis of pyrimidine and purine was also affected. Antitumor activity of BMsU is closely associated with its inhibitory activity on the enzymes that play an important role in the metabolism of tumor cells. In vivo antitumor activity of BMsU was also investigated. The model used in investigations was a mouse anaplastic mammary carcinoma transplanted into the thigh of the right leg of CBA mice. Significant reduction in tumor growth time was achieved with BmsU administered at a dose of 50 mg/kg.

Genetic factors influencing pyrimidine-antagonist chemotherapy

Pyrimidine antagonists, for example, 5-fluorouracil (5-FU), cytarabine (ara-C) and gemcitabine (dFdC), are widely used in chemotherapy regimes for colorectal, breast, head and neck, non-small-cell lung cancer, pancreatic cancer and leukaemias. Extensive metabolism is a prerequisite for conversion of these pyrimidine prodrugs into active compounds. Interindividual variation in the activity of metabolising enzymes can affect the extent of prodrug activation and, as a result, act on the efficacy of chemotherapy treatment. Genetic factors at least partly explain interindividual variation in antitumour efficacy and toxicity of pyrimidine antagonists. In this review, proteins relevant for the efficacy and toxicity of pyrimidine antagonists will be summarised. In addition, the role of germline polymorphisms, tumour-specific somatic mutations and protein expression levels in the metabolic pathways and clinical pharmacology of these drugs are described. Germline polymorphisms of uridine monophosphate kinase (UMPK), orotate phosphoribosyl transferase (OPRT), thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD) and methylene tetrahydrofolate reductase (MTHFR) and gene expression levels of OPRT, UMPK, TS, DPD, uridine phosphorylase, uridine kinase, thymidine phosphorylase, thymidine kinase, deoxyuridine triphosphate nucleotide hydrolase are discussed in relation to 5-FU efficacy. Cytidine deaminase (CDD) and 5'-nucleotidase (5NT) gene polymorphisms and CDD, 5NT, deoxycytidine kinase and MRP5 gene expression levels and their potential relation to dFdC and ara-C cytotoxicity are reviewed.

In vivo induction of resistance to gemcitabine results in increased expression of ribonucleotide reductase subunit M1 as the major determinant

Gemcitabine is a deoxycytidine (dCyd) analogue with activity against several solid cancers. Gemcitabine is activated by dCyd kinase (dCK) and interferes, as its triphosphate dFdCTP, with tumor growth through incorporation into DNA. Alternatively, the metabolite gemcitabine diphosphate (dFdCDP) can interfere with DNA synthesis and thus tumor growth through inhibition of ribonucleotide reductase. Gemcitabine can be inactivated by the enzyme dCyd deaminase (dCDA). In most in vitro models, resistance to gemcitabine was associated with a decreased dCK activity. In all these models, resistance was established using continuous exposure to gemcitabine with increasing concentrations; however, these in vitro models have limited clinical relevance. To develop in vivo resistance to gemcitabine, we treated mice bearing a moderately sensitive tumor Colon 26-A (T/C = 0.25) with a clinically relevant schedule (120 mg/kg every 3 days). By repeated transplant of the most resistant tumor and continuation of gemcitabine treatment for >1 year, the completely resistant tumor Colon 26-G (T/C = 0.96) was created. Initial studies focused on resistance mechanisms known from in vitro studies. In Colon 26-G, dCK activity was 1.7-fold decreased; dCDA and DNA polymerase were not changed; and Colon 26-G accumulated 1.5-fold less dFdCTP, 6 hours after a gemcitabine injection, than the parental tumor. Based on in vitro studies, these relative minor changes were considered insufficient to explain the completely resistant phenotype. Therefore, an expression microarray was done with Colon 26-A versus Colon 26-G. Using independently grown nonresistant and resistant tumors, a striking increase in expression of the RRM1 subunit gene was found in Colon 26-G. The expression of RRM1 mRNA was 25-fold increased in the resistant tumor, as measured by real-time PCR, which was confirmed by Western blotting. In contrast, RRM2 mRNA was 2-fold decreased. However, ribonucleotide reductase enzyme activity was only moderately increased in Colon 26-G. In conclusion, this is the first model with in vivo induced resistance to gemcitabine. In contrast to most in vitro studies, dCK activity was not the most important determinant of gemcitabine resistance. Expression microarray identified RRM1 as the gene with the highest increase in expression in the Colon 26-G, which might clarify its complete gemcitabine-resistant phenotype. This study is the first in vivo evidence for a key role for RRM1 in acquired gemcitabine resistance.

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Possible antitumor activity of 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd, TAS-106) against an established gemcitabine (dFdCyd)-resistant human pancreatic cancer cell line

We established a variant of MIAPaCa-2 human pancreatic cancer cells that is resistant to 2',2'-difluorodeoxycytidine (gemcitabine, dFdCyd), MIAPaCa-2/dFdCyd, and elucidated the biochemical characteristics and mechanism of dFdCyd-resistance in these cells. We also evaluated 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd, TAS-106, RNA polymerase inhibitor), a new anticancer ribonucleoside, for antitumor activity against the resistant cells in vitro and in vivo. MIAPaCa-2/dFdCyd cells were 2541-fold more resistant to dFdCyd than parental MIAPaCa-2 cells, and the major mechanism of the dFdCyd-resistance was found to be a decrease in the intracellular pool of dFdCyd and its active metabolites, which would result in a decrease in incorporation of dFdCyd triphosphate into DNA. This finding was confirmed by the discovery of decreased deoxycytidine kinase activity, increased cytidine deaminase and ribonucleotide reductase activity, and increased 5'-nucleotidase mRNA expression in the MIAPaCa-2/dFdCyd cells. The cytotoxicity of TAS-106 as an antitumor nucleoside analog was similar in both parental and dFdCyd-resistant cells, with IC(50) values of 6.25 and 6.27 nM, respectively, and this finding was supported by similar intracellular uptake and metabolism of TAS-106 in both cell lines. We also evaluated the in vivo antitumor activity of TAS-106 against MIAPaCa-2 and dFdCyd-resistant MIAPaCa-2/dFdCyd tumors implanted into nude mice. The tumor growth inhibition rate of weekly additions of TAS-106 (7 mg/kg, iv) against parental and dFdCyd-resistant tumors was 73% and 76%, respectively, while that of dFdCyd administered twice a week (240 mg/kg, iv) was 84% and 34%, respectively. These results suggest that TAS-106 would contribute to the treatment of patients with advanced pancreatic carcinomas in whom dFdCyd-based chemotherapy has failed.

Metabolic effects of novel N-1-sulfonylpyrimidine derivatives on human colon carcinoma cells

Novel N-1-sulfonylpyrimidine derivatives have a strong antiproliferative activity and an ability to induce apoptosis in treated tumor cells. The purpose of this study was to elucidate the effects of two N-1-sulfonylpyrimidine nucleobases on catalytic activity of tumor cells' enzymes involved in DNA and RNA synthesis, and in de novo and salvage pyrimidine and purine syntheses. Investigations were performed in vitro on colon carcinoma cells (Caco2). The biosynthetic activity of the tumor cells' enzymes was determined using sensitive radio-assays. Enzyme activity in treated cells was calculated relative to untreated control cells. Both of the investigated compounds, 1-(p-toluenesulfonyl) cytosine (TsC) and 5-bromo-1-(methanesulfonyl) uracil (BMsU) inhibited activities of specific enzymes involved in nucleic acid synthesis. BMsU strongly inhibited activities of DNA polymerase alpha (53%), thymidine kinase (68%), thymidilate synthase (43%), and ribonucleotide reductase (46%). De novo biosynthesis of pyrimidine and purine was reduced by 20%. TsC was able to inhibit RNA polymerase (37%), orotate phosphoribosyltransferase (39%), uridine kinase (44%), ribonucleotid reductase (47%), and de novo purine synthesis (61%). Antitumor activity of 1-(p-toluenesulfonyl) cytosine (TsC) and 5-bromo-1-(methanesulfonyl) uracil (BMsU) is closely associated with their inhibitory activity on enzymes that play an important role in the metabolism of tumor cells.

Modulation of 5-fluorouracil cytotoxicity through thymidylate synthase and NF-κB down-regulation and its application on the radiolabelled iododeoxyuridine therapy on human hepatoma cell

The inhibition of thymidylate synthase (TS) by 5-fluorouracil (5-FU) was known to increase the incorporation of radiolabelled iododeoxyuridine (IdUrd) into DNA. The relatively non-toxic compounds such as thiol-containing antioxidant pyrrolidinodithiocarbamte (PDTC) or aromatic fatty acid phenylbutyrate (PB) had been reported to enhance the cytotoxic efficacy of 5-FU. We designed a novel strategy through triplet combination of PB, PDTC and 5-FU to increase the radiolabelled IdUrd uptake and investigated the underlying mechanisms. The growth inhibition and [(125)I]IdUrd-DNA incorporation by PB, PDTC, 5-FU in different combinations were tested on parent or p21(Waf1) transfected Hep3B cells. The combination of PB and PDTC was more effective in enhancing 5-FU cytotoxicity than either drug alone. The combination of PB/PDTC and 5-FU blocked cells in S-phase and resulted in 8.5-fold increase of radiolabelled IdUrd-DNA incorporation. The transfection of p21(Waf1) did not change the general pattern of enhancement. Intriguingly, the combination of PB and PDTC effectively down-regulated NF-kappaB and TS and prevented their up-regulation from 5-FU treatment than either drug alone through a p21(Waf1)-independent mechanism. Based on this strategy, the 3-drug combination offered potential for improved radiolabelled IdUrd molecular radiotherapy for hepatoma treatment.

Inhibition of SRC tyrosine kinase impairs inherent and acquired gemcitabine resistance in human pancreatic adenocarcinoma cells

PURPOSE

We tested the hypotheses that Src tyrosine kinase overactivity represents a chemoresistance mechanism and that Src inhibition may enhance gemcitabine cytotoxicity in pancreatic adenocarcinoma cells.

EXPERIMENTAL DESIGN

Pancreatic adenocarcinoma cells PANC1, MiaPaCa2, Capan2, BxPC3, and PANC1(GemRes), a stably gemcitabine-resistant subline of PANC1, were exposed to combinations of gemcitabine and Src tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Src expression, phosphorylation (Tyr-416), and activity were analyzed by immunoblotting and in vitro kinase assay. Expression of the M2 subunit of ribonucleotide reductase (RRM2), a putative chemoresistance enzyme, was quantified by Northern and Western blot. Cellular proliferation was quantified by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Apoptosis was characterized by YO-PRO-1/propidium iodide staining, fluorometric caspase profiling, and caspase inhibition (Z-Val-Ala-Asp-fluoromethyl ketone). The effects of constitutively active and dominant negative Src were determined. The therapeutic efficacy of PP2 in combination with gemcitabine was tested in nude mice orthotopically xenografted with PANC1(GemRes).

RESULTS

Greater gemcitabine resistance was associated with higher Src phosphorylation and activity, both of which were higher in PANC1(GemRes), relative to PANC1; total Src levels were alike. PANC1(GemRes) overexpressed RRM2. PP2 enhanced inherent gemcitabine chemosensitivity and attenuated gemcitabine resistance in PANC1(GemRes). Constitutively active Src increased gemcitabine chemoresistance; dominant negative Src impaired gemcitabine chemoresistance. PP2 augmented gemcitabine-induced caspase-mediated apoptosis, suppressed RRM2 expression, and decreased activity of the RRM2-regulating transcription factor E2F1 in PANC1(GemRes). PP2 and gemcitabine in combination substantially decreased tumor growth and inhibited metastasis in vivo.

CONCLUSIONS

Increased Src tyrosine kinase activity represents a potential chemoresistance mechanism and a promising therapeutic target warranting further investigation in gemcitabine-resistant pancreatic adenocarcinoma.

Silencing of the p53R2 gene by RNA interference inhibits growth and enhances 5-fluorouracil sensitivity of oral cancer cells

The p53R2 gene encodes the ribonucleotide reductase (RR) small subunit 2 homologue, and is induced by several stress signals activating p53, such as DNA-damaging agents. The p53R2 gene product causes an increase in the deoxynucleotide triphosphate (dNTP) pool in the nucleus, which facilitates DNA repair and synthesis. We hypothesized that p53R2 would be a good molecular target for cancer gene therapy. In this study, three human oral cancer cell lines (SAS, HSC-4 and Ca9-22), a human breast cancer cell line MCF-7, and a normal human fibroblast cell line NHDF were tested. We silenced the expression of p53R2 with the highly specific post-transcriptional suppression of RNA interference (RNAi). We investigated p53R2 expression with the reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting. The sensitivity to anticancer agents was evaluated by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The expression of p53R2 showed no association with the mutational status of p53. The cancer cell lines with higher p53R2 expression were more resistant to 5-FU. RNAi-mediated p53R2 reduction selectivity inhibited growth and enhanced chemosensitivity in cancer cell lines but not in normal fibroblasts. These results suggest that basal transcription of p53R2 could be associated with the sensitivity to anticancer agents. Moreover, we assessed the possibility that p53R2 would be a good molecular target, and report that RNAi targeting of p53R2 could be useful for oral cancer gene therapy.

Superior antitumour activity of S-1 in tumours with a high dihydropyrimidine dehydrogenase activity

To elucidate the mechanism of the enhanced antitumour activity of S-1 (1 M tegafur, 0.4 M 5-chloro-2, 4-dihydroxypyridine, and 1 M potassium oxonate) in terms of the phosphorylation and degradation pathways of 5-fluorouracil (5-FU) metabolism, we investigated tumoral thymidylate synthase (TS) content, dihydropyrimidine dehydrogenase (DPD) activity, the TS inhibition rate (TS-IR), and 5-FU incorporated into RNA (F-RNA) in four human gastric cancer xenografts (MKN-28, MKN-74, GCIY and GT3TKB) and compared the results obtained with S-1 with those obtained with 5-FU and UFT (1 M tegafur, 4 M uracil). 5-FU was administered intraperitoneally (i.p.) to mice at a dose of 50 mg/kg, three times, on days 0, 4 and 8. S-1 and UFT were administered orally at doses of 10 and 24 mg/kg, respectively, once a day, for 9 consecutive days. Antitumour activity was evaluated as the maximum inhibition of tumour growth in each animal. S-1 showed a better antitumour activity than 5-FU and UFT in tumours with a high DPD activity (GCIY and GT3TKB). There were inverse correlations between the antitumour activity and both TS content and DPD activity in the 5-FU and UFT groups. However, no such correlations were observed in the S-1 group. In GCIY and GT3TKB xenografts, TS-IR was significantly higher in the S-1 group than in the 5-FU or UFT groups. In GT3TKB xenografts, the F-RNA level was significantly higher in the S-1 group than in the 5-FU or UFT groups. The superior cytotoxicity of S-1 appears to be attributable to both an increased inhibition of DNA synthesis and an enhanced blockade of RNA function against tumours with a high DPD activity.