Acute effect of 5-fluorouracil on cytoplasmic and nuclear dihydrofolate reductase messenger RNA metabolism

Topical Chemotherapy for Treating Ocular Surface Squamous Neoplasia with a Combination of Interferon α-2b and 5-Fluorouracil

Introduction

This study evaluated the efficacy of combined interferon α-2b (IFNα2b) and 5-fluorouracil (5-FU) as primary treatment for ocular surface squamous neoplasia (OSSN).

Methods

In this retrospective study, 27 eyes with OSSN followed by topical application of combined IFNα2b and 5-FU were examined. Reported outcome measures were tumor response, visual acuity, time to complete resolution, recurrence and treatment complications.

Results

Twenty-six patients (17 male, 9 female) had a mean age of 63.9 (median, 67; range 22–83) years. Complete tumor response was observed in 24 eyes (88.9%). Three eyes (11.1%) showed partial response to the chemotherapy agents and later underwent surgical tumor removal. The median time to complete resolution was 6 (mean, 6.1; range, 3–11) weeks. Of these, the patients received between one to three cycles of 5-FU therapy (median, 2; mean, 1.8). Complications noted were transient irritation at 5-FU cycle (11 eyes, 40.7%). There was no tumor recurrence at mean follow-up of 16.1 (median, 12; range 6–38) months.

Conclusions

Combination therapy of IFNα2b and 5-FU was a safe and effective treatment, inducing a short duration of administration and low recurrence rate for OSSN.

Trial Registration

Retrospectively registered, UHCT22048.

Downregulation of KRAB zinc finger proteins in 5-fluorouracil resistant colorectal cancer cells

Radio-chemotherapy with 5-flu orouracil (5-FU) is the standard of care treatment for patients with colorectal cancer, but it is only effective for a third of them. Despite our understanding of the mechanism of action of 5-FU, drug resistance remains a significant limitation to the clinical use of 5-FU, as both intrinsic and acquired chemoresistance represents the major obstacles for the success of 5-FU-based chemotherapy. In order to identify the mechanism of acquired resistance, 5-FU chemoresistance was induced in CRC cell lines by passaging cells with increasing concentrations of 5-FU. To study global molecular changes, quantitative proteomics and transcriptomics analyses were performed on these cell lines, comparing the resistant cells as well as the effect of chemo and radiotherapy. Interestingly, a very high proportion of downregulated genes were annotated as transcription factors coding for Krüppel-associated box (KRAB) domain-containing zinc-finger proteins (KZFPs), the largest family of transcriptional repressors. Among nearly 350 KRAB-ZFPs, almost a quarter were downregulated after the induction of a 5-FU-resistance including a common one between the three CRC cell lines, ZNF649, whose role is still unknown. To confirm the observations of the proteomic and transcriptomic approaches, the abundance of 20 different KZFPs and control mRNAs was validated by RT-qPCR. In fact, several KZFPs were no longer detectable using qPCR in cell lines resistant to 5-FU, and the KZFPs that were downregulated only in one or two cell lines showed similar pattern of expression as measured by the omics approaches. This proteomic, transcriptomic and genomic analysis of intrinsic and acquired resistance highlights a possible new mechanism involved in the cellular adaptation to 5-FU and therefore identifies potential new therapeutic targets to overcome this resistance.

Early 5-Fluorouracil-Induced Changes of Poly(A) Polymerase in Hela and Wish Cells

5-Fluorouracil (5-FU), a drug with numerous mechanisms of action which has a long-term suppressive effect on human cancer cell proliferation, mediates both partial dephosphorylation and inactivation of poly(A) polymerase (PAP) [EC. 2.7.7.19] as detected by immunoblotting analysis and non-specific enzyme assay, respectively, in human carcinoma HeLa and diploid WISH cells at a concentration of 100 μM. When the same experiment is done in the presence of phosphatase inhibitors, 5-FU-induced partial PAP dephosphorylation is abolished. Moreover, a cell type-modulated, differential response of HeLa cells (5-FU chemosensitive cells) versus WISH cells (drug-resistant diploid cells) is observed. These results suggest that 5-FU induces early direct or indirect changes in the structure and function of PAP and may regulate premRNA cleavage-polyadenylation.

Human dihydrofolate reductase and thymidylate synthase form a complex in vitro and co-localize in normal and cancer cells

Enzymes involved in thymidylate biosynthesis, thymidylate synthase (TS), and dihydrofolate reductase (DHFR) are well-known targets in cancer chemotherapy. In this study, we demonstrated for the first time, that human TS and DHFR form a strong complex in vitro and co-localize in human normal and colon cancer cell cytoplasm and nucleus. Treatment of cancer cells with methotrexate or 5-fluorouracil did not affect the distribution of either enzyme within the cells. However, 5-FU, but not MTX, lowered the presence of DHFR-TS complex in the nucleus by 2.5-fold. The results may suggest the sequestering of TS by FdUMP in the cytoplasm and thereby affecting the translocation of DHFR-TS complex to the nucleus. Providing a strong likelihood of DHFR-TS complex formation in vivo, the latter complex is a potential new drug target in cancer therapy. In this paper, known 3D structures of human TS and human DHFR, and some protozoan bifunctional DHFR-TS structures as templates, are used to build an in silico model of human DHFR-TS complex structure, consisting of one TS dimer and two DHFR monomers. This complex structure may serve as an initial 3D drug target model for prospective inhibitors targeting interfaces between the DHFR and TS enzymes.

Molecular basis of cancer-therapy-induced cardiotoxicity: introducing microRNA biomarkers for early assessment of subclinical myocardial injury

Development of reliable biomarkers for early clinical assessment of drug-induced cardiotoxicity could allow the detection of subclinical cardiac injury risk in vulnerable patients before irreversible damage occurs. Currently, it is difficult to predict who will develop drug-induced cardiotoxicity owing to lack of sensitivity and/or specificity of currently used diagnostics. miRNAs are mRNA regulators and they are currently being extensively profiled for use as biomarkers due to their specific tissue and disease expression signature profiles. Identification of cardiotoxicity-specific miRNA biomarkers could provide clinicians with a valuable tool to allow prognosis of patients at risk of cardiovascular injury, alteration of a treatment regime or the introduction of an adjunct therapy in order to increase the long-term survival rate of patients treated with cardiotoxic drugs.

Use of adenoviral vectors to target chemotherapy to tumor vascular endothelial cells suppresses growth of breast cancer and melanoma

To target chemotherapy to tumor vascular endothelial cells (TVECs), we created the AdTie2RprCDFib(knob-RGD+) vector by inserting into an AdEasy adenoviral vector (Ad) backbone: (i) the cytosine deaminase (CD) gene driven by the Tie2 receptor promoter (Tie2Rpr) into the E1 region of Ad; (ii) mutations that reduce binding of the fiber knob to the Coxsackie adenovirus receptor (CAR); and (iii) the RGD peptide into the H1 loop of fiber for binding to the alpha(V)beta(3) integrin receptors on TVECs. To reduce uptake of the AdTie2RprCDFib(knob-RGD+) by reticuloendothelial (RE) and liver cells, we intravenously (i.v.) injected Hetastarch and low-dose Ad (one million vector particles (VPs)) prior to i.v. injection of a therapeutic dose (one billion VPs) of the AdTie2RprCDFib(knob-RGD+) vector. This treatment induced regressions of N202 breast cancer and B16 melanoma without toxicity to normal tissues. We showed that the tumor regression was induced by infection of the TVECs and not by the infection of tumor cells by the AdTie2RprCDFib(knob-RGD+) vector.

Methods for delivery of adenoviral vectors to tumor vasculature

The discovery that the luminal membrane of tumor vascular endothelial cells contains antigens different from those on the luminal membrane of endothelial cells in the vessels of normal tissues has opened the door to the use of adenoviral vectors for tumor vascular targeting as a form of cancer treatment. Other laboratories have shown that introduction of the RGD peptide increases binding of the adenoviral vector to dividing endothelial cells and to tumor cells. The major obstacle to achieving delivery of intravenously administered adenoviral vectors to tumor vascular endothelial cells and tumor cells is the nonspecific uptake of adenoviral vectors in the liver and other organs. Another obstacle is the low level of the coxsackievirus-adenovirus receptor, to which the adenoviral fiber protein binds, on tumor vascular endothelial cells and tumor cells. We therefore introduced the RGD peptide into the adenoviral vector fiber protein and then tested the effect of intravenous 6% hetastarch on the delivery to adenoviral vector to tumor tissue. Our results show that pretreatment with hetastarch increases the delivery of the adenoviral vector to tumor cells and their vasculature, reduces up-take by normal tissues, reduces vector-mediated toxicity to the liver, and intensifies vector-induced suppression of tumor cell growth.

Tumor-specific therapeutic effect induced by an oncolytic adenoviral vector containing heat shock protein 70 and prodrug activation genes

We constructed a melanoma-specific oncolytic adenoviral vector Ad.MCDIRESE1.71Hsp3, in which the cytosine deaminase and adenoviral E1A genes linked by the IRES sequence were under the control of a mouse tyrosinase enhancer/promoter transcriptional element in the E1 region of the vector. We also inserted the human heat shock protein 70 (Hsp70) gene driven by the cytomegalovirus promoter into the E3 region of this vector. The RGD-4C peptide was inserted into the HI loop of the fiber knob domain of the Ad.MCDIRESE1.71Hsp3 vector to increase the transduction efficiency of this vector to tumor cells. The Ad.MCDIRESE1.71Hsp3 vector replicates specifically in melanoma cells, and it has a melanoma-specific cytotoxic effect in the presence of 5-fluorocytosine in vitro and in vivo. Moreover, the in vivo killing of tumor cells associated with the overexpression of Hsp70 generated by the intratumoral injection of the Ad.MCDIRESE1.71Hsp3 vector into established subcutaneous tumors can lead to the suppression of tumor growth and potent melanoma-specific systemic immune responses.

Vector targeting makes 5-fluorouracil chemotherapy less toxic and more effective in animal models of epithelial neoplasms

PURPOSE

5-fluorouracil (5-FU) has been combined in the past with other drugs for the combination chemotherapy for cancers of the breast, ovary, and colon. These drug regimens were limited by the fact that 5-FU fails to kill nondividing cancer cells at the doses that are safe to deliver. The goal of the present study is to test the feasibility of replacing 5-FU in established 5-FU combination chemotherapy with the Ad-LpCDIRESE1A/5-fluorocytosine (5-FC) system for the purpose of reducing toxicity and increasing efficacy.

EXPERIMENTAL DESIGN

We have replaced 5-FU in the weekly combination of CPT-11, folinic acid (FA) and 5-FU chemotherapy by 5-FC and an adenoviral vector that carries the L-plastin (Lp) tumor-specific promoter-driven transcription unit encoding the cytosine deaminase gene linked to the E1A gene by an internal ribosomal entry site element. This combination is called "genetic combination therapy." The goal of using the vector was to decrease the toxicity to normal tissue and to increase the efficacy of therapy in the cancer cells by increasing the concentration of 5-FU sufficiently high that even nondividing cancer cells would be killed by 5-FU through its incorporation into mRNA and consequent inhibition of synthesis of functional proteins. We compared the in vivo efficacy of the genetic combination therapy with the conventional combination chemotherapy in a mouse colon cancer model.

RESULTS

Both replication-competent and -noncompetent adenoviral vectors carrying an L-plastin-driven cytosine deaminase transcription unit when combined with 5-FC, CPT-11, and FA showed increased in vitro therapeutic activity that was significantly higher than that of the conventional chemotherapy combination. Tumor-bearing mice treated with the genetic combination therapy showed a statistically significant advantage in terms of increased response rate, response duration, survival, and reduced toxicity when compared with tumor-bearing mice treated with the conventional combination chemotherapy.

CONCLUSIONS

Replacement of 5-FU in 5-FU-based combination chemotherapy with the Ad-LpCDIRESE1A vector and 5-FU reduces toxicity and increases efficacy. This is a concept that could be potentially applied widely for many forms of cancer treatment.

5-Fluorouracil incorporation into RNA and DNA in relation to thymidylate synthase inhibition of human colorectal cancers

BACKGROUND

The mechanism of action of 5-fluorouracil (5-FU) has been associated with inhibition of thymidylate synthase (TS) and incorporation of 5-FU into RNA and DNA, but limited data are available in human tumor tissue for the latter. We therefore measured incorporation in human tumor biopsy specimens after administration of a test dose of 5-FU alone or with leucovorin.

PATIENTS AND METHODS

Patients received 5-FU (500 mg/m(2)) with or without high-dose leucovorin, low-dose leucovorin or l-leucovorin, and biopsy specimens were taken after approximately 2, 24 or 48 h. Tissues were pulverized and extracted for nucleic acids. 5-FU incorporation was measured using gas chromatography/mass spectrometry after complete degradation to bases of isolated RNA and DNA.

RESULTS

Maximal incorporation into RNA (1.0 pmol/micrograms RNA) and DNA (127 fmol/micrograms DNA) of 59 and 46 biopsy specimens, respectively, was found at 24 h after 5-FU administration. Incorporation into RNA but not DNA was significantly correlated with intratumoral 5-FU levels. However, DNA incorporation was significantly correlated with the RNA incorporation. Primary tumor tissue, liver metastasis and normal mucosa did not show significant differences, while leucovorin had no effect. Neither for RNA (30 patients) nor DNA (24 patients) incorporation was a significant correlation with response to 5-FU therapy found. However, in the same group of patients, response was significantly correlated to TS inhibition (mean TS in responding and non-responding groups 45 and 231 pmol/h/mg protein, respectively; P=0.001).

CONCLUSIONS

5-FU is incorporated at detectable levels into RNA and DNA of human tumor tissue, but no relation between the efficacy of 5-FU treatment and incorporation was found, in contrast to TS.

Cytosine Deaminase Adenoviral Vector and 5-Fluorocytosine Selectively Reduce Breast Cancer Cells 1 Million-Fold When They Contaminate Hematopoietic Cells: A Potential Purging Method for Autologous Transplantation

Ad.CMV-CD is a replication incompetent adenoviral vector carrying a cytomegalovirus (CMV)-driven transcription unit of the cytosine deaminase (CD) gene. The CD transcription unit in this vector catalyzes the deamination of the nontoxic pro-drug, 5-fluorocytosine (5-FC), thus converting it to the cytotoxic drug 5-fluorouracil (5-FU). This adenoviral vector prodrug activation system has been proposed for use in selectively sensitizing breast cancer cells, which may contaminate collections of autologous stem cells products from breast cancer patients, to the toxic effects of 5-FC, without damaging the reconstitutive capability of the normal hematopoietic cells. This system could conceivably kill even the nondividing breast cancer cells, because the levels of 5-FU generated by this system are 10 to 30 times that associated with systemic administration of 5-FU. The incorporation of 5-FU into mRNA at these high levels is sufficient to disrupt mRNA processing and protein synthesis so that even nondividing cells die of protein starvation. To test if the CD adenoviral vector sensitizes breast cancer cells to 5-FC, we exposed primary explants of normal human mammary epithelial cells (HMECs) and the established breast cancer cell (BCC) lines MCF-7 and MDA-MB-453 to the Ad.CMV-CD for 90 minutes. This produced a 100-fold sensitization of these epithelial cells to the effects of 48 hours of exposure to 5-FC. We next tested the selectivity of this system for BCC. When peripheral blood mononuclear cells (PBMCs), collected from cancer patients during the recovery phase from conventional dose chemotherapy-induced myelosuppression, were exposed to the Ad.CMV-CD for 90 minutes in serum-free conditions, little or no detectable conversion of 5-FC into 5-FU was seen even after 48 hours of exposure to high doses of 5-FC. In contrast, 70% of 5-FC was converted into the cytotoxic agent 5-FU when MCF-7 breast cancer cells (BCCs) were exposed to the same Ad.CMV-CD vector followed by 5-FC for 48 hours. All of the BCC lines tested were shown to be sensitive to infection by adenoviral vectors when exposed to a recombinant adenoviral vector containing the reporter gene betagalactosidase (Ad.CMV-βgal). In contrast, less than 1% of the CD34-selected cells and their more immature subsets, such as the CD34+CD38− or CD34+CD33− subpopulations, were positive for infection by the Ad.CMV-βgal vector, as judged by fluorescence-activated cell sorting (FACS) analysis, when exposed to the adenoviral vector under conditions that did not commit the early hematopoietic precursor cells to maturation. When artificial mixtures of hematopoietic cells and BCCs were exposed for 90 minutes to the Ad.CMV-CD vector and to 5-FC for 10 days or more, a greater than 1 million fold reduction in the number of BCCs, as measured by colony-limiting dilution assays, was observed. To test if the conditions were damaging for the hematopoietic reconstituting cells, marrow cells collected from 5-FU–treated male donor mice were incubated with the cytosine deaminase adenoviral vector and then exposed to 5-FC either for 4 days in vitro before transplantation or for 14 days immediately after transplantation in vivo. There was no significant decrease in the reconstituting capability of the male marrow cells, as measured by their persistence in female irradiated recipients for up to 6 months after transplantation. These observations suggest that adenovirus-mediated gene transfer of the Escherichia coli cytosine deaminase gene followed by exposure to the nontoxic pro-drug 5-FC may be a potential strategy to selectively reduce the level of contaminating BCCs in collections of hematopoietic cells used for autografts in breast cancer patients.

A comparison of the in vivo and in vitro response of rat embryos to 5-fluorouracil

This study serves to further define the capabilities of the whole embryo culture system using the well-known teratogen, 5-fluorouracil (5-FU), an antineoplastic agent. An initial in vivo study was performed whereby pregnant rats were injected intraperitoneally with 10-30 mg/kg 5-FU on day 9 of gestation. On day 20 of gestation, the effects of this drug on the growth and development of embryos were evaluated. The number of externally malformed fetuses increased in a dose-related manner, and the most common defect was micro-/anophthalmos in fetuses of dams treated with 5-FU. Growth retardation was also noted in the 5-FU treated groups. An in vitro study was performed in which drug concentrations were varied (0.15-0.30 microg/ml). Externally abnormal embryos were observed in whole embryo culture system from embryonic day 9 to 11. The most common defect was hypoplastic optic vesicles. In the whole embryo culture system, crown-rump length, somite number, protein contents, and morphological score were decreased in a dose-dependent fashion. Finally, histological evaluation and observation of the pattern of cell death of the optic vesicle of 11-day-old embryos in in vivo and in vitro were performed. These parameters revealed no differences in response between in vivo and in vitro embryos treated with 5-FU, suggesting that the whole embryo culture system was an appropriate model for developmental toxicity studies of 5-FU.

An alternative molecular mechanism of action of 5-fluorouracil, a potent anticancer drug

It is assumed that the primary mode of action of 5-fluorouracil (5-FUra) is mediated via inhibition of thymidylate synthetase. Persistent inhibition of cellular proliferation after treatment of the 5-FUra-inhibited cells with exogenous thymidine do not support the notion that the anti-proliferitive action of 5-FUra is due exclusively to inhibition of DNA replication. Our studies have revealed an alternative mechanism of action at the level of pre-ribosomal RNA (pre-rRNA) processing. Pre-rRNA processing was inhibited completely in vitro as well as in S-100 extract from the mouse lymphosarcoma P1798 cells that were treated with 5-FUra. Under this condition, the 5-FUra-substituted pre-rRNA substrate was processed efficiently at the primary processing site. This study showed that the activity and/or the synthesis of a factor potentially involved in pre-rRNA processing is blocked in cells treated with the fluoropyrimidine. UV-cross-linking study showed that a 200 kDa polypeptide designated ribosomal RNA binding protein (RRBP) was absent in the S-100 extract from the drug-treated mouse lymphosarcoma cells. Since a polypeptide that cross-links to a processing site on RNA is usually involved in the RNA processing, RRBP may have a direct role in pre-rRNA processing. A key molecular mechanism far the antiproliferative action of 5-FUra may be due to its interference with the activity and/or synthesis of RRBP. Exposure of cells to 5-FUra did not inhibit the interaction between U3 small nucleolar RNA (snoRNA) and pre-rRNA at the primary processing site (a key step in the processing reaction) and the formation of U3 small nucleolar ribonucleoprotein (snoRNP). Treatment of cells with the fluoropyrimidine did not block the 3' end processing of pre-messenger RNA (pre-mRNA). This article also discusses the effects of 5-FUra on pre-mRNA splicing and mRNA translation, and proposes other avenues of research to explore further the mechanism of action of this important pyrimidine analog.

Altered RNA turnover in carcinogenesis. The diagnostic potential of modified base excretion

Excretion of urinary modified nucleosides is frequently elevated in patients with oncogenic disease. Increases of urinary pseudouridine excretion are now demonstrated in patients with a variety of brain tumors. The potential use of urinary modified base excretion as a cancer marker is discussed and possible sources of the elevated nucleosides are detailed. The specific steps in RNA metabolism that result in increased levels of RNA nucleoside excretion are poorly understood. This knowledge will be necessary to understand the molecular mechanism and the clinical significance of urinary nucleoside excretion in treatment and diagnosis of oncogenic disease.

The genetic toxicology of 5-fluoropyrimidines and 5-chlorouracil

The halogenated pyrimidines were synthesized in the 1950s as potential anti-tumor agents after the discovery that certain tumors preferentially incorporated uracil rather than thymine into the DNA. The fluorinated derivatives are widely recognized today as effective treatment modalities, especially with tumors of the head, neck and breast. Mechanistically, efficacy of the fluorinated pyrimidines results from the ability of these compounds to incorporate into RNA and inhibit its maturation to those forms necessary for cellular metabolism and from the inhibition of the enzyme, thymidylate synthetase, which controls the biosynthesis of thymine and DNA synthesis. The 5-fluoropyrimidines can incorporate into DNA, but the contribution of this phenomenon to the overall efficacy of this class of chemotherapeutic agents is not totally resolved. Evidence exists that this class of compounds possesses the properties to induce genotoxic effects, both in bacterial and eukaryotic cells. Most notably, these effects include the induction of cellular toxicity and the induction of chromosome aberrations. The biology and chemistry of the chlorinated pyrimidines were first explored as a possible means of sensitizing the DNA to ionizing radiation in a manner similar to the sensitization observed when DNA incorporates bromodeoxyuridine. This approach was not utilized clinically. The genetic toxicology of this compound became important with the discovery of the ribonucleoside in the effluents of sewage treatment plants. Evidence is now available that the chlorinated pyrimidines, upon conversion to deoxyribonucleosides, are effective mutagens, clastogens and toxicants, as well as extremely effective inducers of sister-chromatid exchanges.

Biochemical modulation of 5-fluorouracil with leucovorin or delayed uridine rescue. Correlation of antitumor activity with dosage and FUra incorporation into RNA

Two strategies for modulation of 5-fluorouracil (FUra) activity were compared in vivo in advanced murine CD8F1 breast tumors with regard to three parameters: chemotherapeutic activity, inhibition of thymidylate synthase (TSase) activity, and incorporation of FUra into RNA, (FU)RNA. Inhibition of TSase by FUra was modulated by leucovorin (LV), and the incorporation of FUra into RNA was increased by the administration of otherwise lethal doses of FUra followed by uridine "rescue". Thymidylate synthase activity was inhibited substantially (49%) by low-dose FUra at 25 mg/kg, but was not further enhanced (48%) by repeated daily treatments at the same dose (FUra25 x 4). Inhibition of TSase was somewhat enhanced (55%) by the addition of LV to FUra25 x 4, and a greater therapeutic effect was obtained with FUra25 x 4 + LV over FUra25 x 4 alone. FUra as a single agent at the maximum tolerated weekly dose of 100 mg/kg inhibited TSase activity 66-73%. This inhibition was further enhanced slightly by the addition of LV (71-82%), and its therapeutic efficacy was greater than with FUra25 x 4 with or without LV. However, in contrast to low dose FUra25 x 4, the antitumor effect of FUra100 was not enhanced by LV. (FU)RNA increased with FUra dose from 0.4 (FUra25) to 2.2 nmol/mg DNA (FUra100). At a very-high-dose of FUra (200-225 mg/kg) followed by uridine "rescue", TSase inhibition was not further enhanced, but both (FU)RNA (4.8 nmol/mg DNA) and the therapeutic efficacy were increased. Since TSase could not be further inhibited at doses above FUra100, the increased chemotherapeutic efficacy correlated with increased (FU)RNA.

Thyroid hormone and androgen regulation of nerve growth factor gene expression in the mouse submandibular gland

The nerve growth factor (NGF) content of the mouse submandibular gland (SMG) is under hormonal control and is modulated by both thyroid hormones (TH) and androgens. The sexual dimorphism of the gland is well documented. In the adult male mouse, the SMG contains 10 times more NGF compared to the female. Conversely, castration of male mice reduces the SMG NGF levels to those found in control females. In order to determine the locus at which androgens and TH exert their effect on NGF gene expression in the SMG, steady-state NGF mRNA levels were determined. Daily treatment of adult female mice with TH for 1 week increased NGF mRNA levels 6-fold. Androgen treatment produced a 20-fold increase in SMG NGF mRNA, which was comparable to levels detected in the control adult male SMG. The effect of TH on NGF mRNA levels was time-dependent and coincided with the increase in NGF protein concentrations. At 48 h after a single TH injection, NGF mRNA levels (measured in SMG total RNA) increased 2-4-fold, while heteronuclear (hn) RNA levels were increased 1.5-2-fold. The NGF gene transcription rate was determined by run-on assay following TH treatment. A small but significant 2-fold induction by TH of NGF gene transcription was found at 24-48 h. Cytoplasmic RNA prepared from the same SMGs used in the run-on experiments was tested by S1 nuclease protection; NGF cytoplasmic RNA was increased 7-fold in the SMGs of females treated with TH 48 h previously. These results demonstrate that the effect of TH on NGF gene expression is due in part to an induction of NGF gene transcription. The discrepancies observed between transcription rate and mRNA levels suggest that the major effect of TH is at the post-transcriptional level, possibly mRNA stabilization. The time required to observe an induction of TH on NGF gene transcription is suggestive of an indirect effect, possibly through the induction by TH of another protein which in turn activates the NGF gene.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

Ribose-transfer activity from uridine to 5-fluorouracil in Ehrlich ascites tumor cells

Anabolism of 5-fluorouracil (5-FU) in the presence of uracil was examined using the cell-free extract of Ehrlich ascites tumor cells. FU-nucleoside formation from 5-FU with ribose 1-phosphate (R-1-P) or 2'-deoxyribose 1-phosphate was not readily inhibited even by the addition of uracil at 100 times higher concentration than 5-FU. FU-nucleotide formation from 5-FU with R-1-P and adenosine 5'-triphosphate or with 5-phosphoribosyl 1-pyrophosphate was slightly reduced as the concentration of uracil was increased. It was also found that 5-fluorouridine (5-FUR) was produced by "nucleoside N-ribosyltransferase," transferring a ribose moiety from uridine (UR) to 5-FU directly. This activity might play a role in the preferential formation of 5-FUR. However, 5-fluoro-2'-deoxyuridine was not produced by directly transferring a deoxyribose moiety. On the basis of several column chromatographies and characterization of kinetics, pH dependency, and response to inhibitors, the enzyme protein of the ribosyltransferase could not be distinguished from that of the phosphorylase.

Metabolism and mechanism of action of 5-fluorouracil

This is a review on the mechanism of action of FUra. Three main areas are addressed: metabolism, RNA-directed actions of FUra, and DNA-directed actions of FUra. Key words for bibliographic purposes: metabolism, RNA, rRNA, mRNA, tRNA, DNA primase, DNA, thymidylate synthetase, uracil N-glycosylase, FUra, FUrd, FdUrd, FdUMP, RNA splicing, 5,10-methylene tetrahydrofolate, FUTP.

The in vitro embryotoxicity of 5-fluorouracil in rat embryos

The fluorinated pyrimidine 5-fluorouracil (5-FU) is an effective chemotherapeutic agent that is teratogenic in a number of species. The mechanism for the embryopathic effect of the drug is unknown. We examined the effects of this compound on gestation day 10.5 rat embryos cultured for 48 hours in a rodent whole embryo culture system. Embryos were exposed for 1-4 hours to various doses of 5-FU. Embryolethality was minimal in all treatment groups. The malformation frequency increased with higher doses; within a dose, the malformation frequency increased with longer exposure to the drug. The tail and hindlimb bud were the most commonly affected structures in vitro; tail and leg defects are produced in several species by exposure to the drug in vivo. The embryopathic drug concentration in the culture media (2-8 micrograms/ml) is similar to the plasma level of 2-17 micrograms/ml, which is associated with embryopathy in vivo. Results from this study suggest that the whole embryo culture system is an appropriate model for developmental toxicity studies of 5-FU.

Correlation between ribosomal RNA production and RNA-directed fluoropyrimidine cytotoxicity

The relationship between cytotoxicity and fluoropyrimidine effects on the production of mature cytoplasmic 28S and 18S ribosomal RNA was studied in S-180 cells for the fluoropyrimidines: 5-fluorouracil (FUra), 5-fluorouridine (FUrd), 5-fluorodeoxyuridine (FdUrd), and 5'-deoxy-5-fluorouridine (5'-dFUrd). After a 6-hr drug exposure, the total cytotoxicity in the absence of added thymidine (dThd) was determined by soft-agar cloning and resulted in LC90 (lethal concentration to 90% of cells) values of 0.6 microM FdUrd, 0.7 microM FUrd, 5.3 microM FUra and 93 microM 5'-dFUrd. The RNA-directed (dThd-nonreversible) cytotoxicity was assessed by cloning the cells in the presence of 10 microM dThd. This resulted in an altered order of potency and increased LC90 values to 5.5 microM FUrd, 20 microM FUra, 265 microM FdUrd and 870 microM 5'-dFUrd. The production of mature cytoplasmic rRNA was determined by measuring the amount of [3H]cytidine incorporated into the 28S and 18S rANA species following their separation by agarose gel electrophoresis, compared with the level of [3H]cytidine incorporated into the nuclear rRNA. When all four fluoropyrimidines were compared together, the degree of inhibition of cytoplasmic rRNA production was poorly predictive of the total cytotoxicity in the absence of dThd (correlation coefficient, r = 0.77). FdUrd, in particular, had a very minor effect on rRNA production even at very toxic drug concentrations. When toxicity was assessed in the presence of dThd, however, there was a strong and significant correlation between rRNA production and RNA-directed cytotoxicity (r = 0.95, P less than 0.001), for all the fluoropyrimidines tested, including FdUrd. Thus, when the inhibition of thymidylate formation was eliminated as a site of drug action and only RNA-directed cytotoxicity was assessed, the impaired production of cytoplasmic rRNA was strongly associated with cytotoxicity. These results demonstrate that the inhibition of mature cytoplasmic rRNA production may be an important common mechanism of RNA-directed cytotoxicity for all the fluoropyrimidines, and not limited to FUrd or FUra.