Upregulation of cytidine deaminase in NAT1 knockout breast cancer cells

PURPOSE

Arylamine N-acetyltransferase 1 (NAT1), a phase II metabolic enzyme, is frequently upregulated in breast cancer. Inhibition or depletion of NAT1 leads to growth retardation in breast cancer cells in vitro and in vivo. A previous metabolomics study of MDA-MB-231 breast cancer cells suggests that NAT1 deletion leads to a defect in de novo pyrimidine biosynthesis. In the present study, we observed that NAT1 deletion results in upregulation of cytidine deaminase (CDA), which is involved in the pyrimidine salvage pathway, in multiple breast cancer cell lines (MDA-MB-231, MCF-7 and ZR-75-1). We hypothesized that NAT1 KO MDA-MB-231 cells show differential sensitivity to drugs that either inhibit cellular pyrimidine homeostasis or are metabolized by CDA.

METHODS

The cells were treated with (1) inhibitors of dihydroorotate dehydrogenase or CDA (e.g., teriflunomide and tetrahydrouridine); (2) pyrimidine/nucleoside analogs (e.g., gemcitabine and 5-azacytidine); and (3) naturally occurring, modified cytidines (e.g., 5-formyl-2'-deoxycytidine; 5fdC).

RESULTS

Although NAT1 KO cells failed to show differential sensitivity to nucleoside analogs that are metabolized by CDA, they were markedly more sensitive to 5fdC which induces DNA damage in the presence of high CDA activity. Co-treatment with 5fdC and a CDA inhibitor, tetrahydrouridine, abrogated the increase in 5fdC cytotoxicity in NAT1 KO cells, suggesting that the increased sensitivity of NAT1 KO cells to 5fdC is dependent on their increased CDA activity.

CONCLUSIONS

The present findings suggest a novel therapeutic strategy to treat breast cancer with elevated NAT1 expression. For instance, NAT1 inhibition may be combined with cytotoxic nucleosides (e.g., 5fdC) for breast cancer treatment.

A pilot clinical trial of oral tetrahydrouridine/decitabine for noncytotoxic epigenetic therapy of chemoresistant lymphoid malignancies

One mechanism by which lymphoid malignancies resist standard apoptosis-intending (cytotoxic) treatments is genetic attenuation of the p53/p16-CDKN2A apoptosis axis. Depletion of the epigenetic protein DNA methyltransferase 1 (DNMT1) using the deoxycytidine analog decitabine is a validated approach to cytoreduce malignancy independent of p53/p16. In vivo decitabine activity, however, is restricted by rapid catabolism by cytidine deaminase (CDA). We, therefore, combined decitabine with the CDA-inhibitor tetrahydrouridine and conducted a pilot clinical trial in patients with relapsed lymphoid malignancies: the doses of tetrahydrouridine/decitabine used (∼10/0.2 mg/kg orally (PO) 2×/week) were selected for the molecular pharmacodynamic objective of non-cytotoxic, S-phase dependent, DNMT1-depletion, guided by previous Phase 1 studies. Patients with relapsed/refractory B- or T-cell malignancies (n = 7) were treated for up to 18 weeks. Neutropenia without concurrent thrombocytopenia is an expected toxicity of DNMT1-depletion and occurred in all patients (Grade 3/4). Subjective and objective clinical improvements occurred in 4 of 7 patients, but these responses were lost upon treatment interruptions and reductions to manage neutropenia. We thus performed parallel experiments in a preclinical in vivo model of lymphoma to identify regimen refinements that might sustain DNMT1-targeting in malignant cells but limit neutropenia. We found that timed-alternation of decitabine with the related molecule 5-azacytidine, and combination with inhibitors of CDA and de novo pyrimidine synthesis could leverage feedback responses of pyrimidine metabolism to substantially increase lymphoma cytoreduction but with less neutropenia. In sum, regimen innovations beyond incorporation of a CDA-inhibitor are needed to sustain decitabine DNMT1-targeting and efficacy against chemo-resistant lymphoid malignancy. Such potential solutions were explored in preclinical in vivo studies.

Oral tetrahydrouridine and decitabine for non-cytotoxic epigenetic gene regulation in sickle cell disease: A randomized phase 1 study

BACKGROUND

Sickle cell disease (SCD), a congenital hemolytic anemia that exacts terrible global morbidity and mortality, is driven by polymerization of mutated sickle hemoglobin (HbS) in red blood cells (RBCs). Fetal hemoglobin (HbF) interferes with this polymerization, but HbF is epigenetically silenced from infancy onward by DNA methyltransferase 1 (DNMT1).

METHODS AND FINDINGS

To pharmacologically re-induce HbF by DNMT1 inhibition, this first-in-human clinical trial (NCT01685515) combined 2 small molecules-decitabine to deplete DNMT1 and tetrahydrouridine (THU) to inhibit cytidine deaminase (CDA), the enzyme that otherwise rapidly deaminates/inactivates decitabine, severely limiting its half-life, tissue distribution, and oral bioavailability. Oral decitabine doses, administered after oral THU 10 mg/kg, were escalated from a very low starting level (0.01, 0.02, 0.04, 0.08, or 0.16 mg/kg) to identify minimal doses active in depleting DNMT1 without cytotoxicity. Patients were SCD adults at risk of early death despite standard-of-care, randomized 3:2 to THU-decitabine versus placebo in 5 cohorts of 5 patients treated 2X/week for 8 weeks, with 4 weeks of follow-up. The primary endpoint was ≥ grade 3 non-hematologic toxicity. This endpoint was not triggered, and adverse events (AEs) were not significantly different in THU-decitabine-versus placebo-treated patients. At the decitabine 0.16 mg/kg dose, plasma concentrations peaked at approximately 50 nM (Cmax) and remained elevated for several hours. This dose decreased DNMT1 protein in peripheral blood mononuclear cells by >75% and repetitive element CpG methylation by approximately 10%, and increased HbF by 4%-9% (P < 0.001), doubling fetal hemoglobin-enriched red blood cells (F-cells) up to approximately 80% of total RBCs. Total hemoglobin increased by 1.2-1.9 g/dL (P = 0.01) as reticulocytes simultaneously decreased; that is, better quality and efficiency of HbF-enriched erythropoiesis elevated hemoglobin using fewer reticulocytes. Also indicating better RBC quality, biomarkers of hemolysis, thrombophilia, and inflammation (LDH, bilirubin, D-dimer, C-reactive protein [CRP]) improved. As expected with non-cytotoxic DNMT1-depletion, platelets increased and neutrophils concurrently decreased, but not to an extent requiring treatment holds. As an early phase study, limitations include small patient numbers at each dose level and narrow capacity to evaluate clinical benefits.

CONCLUSION

Administration of oral THU-decitabine to patients with SCD was safe in this study and, by targeting DNMT1, upregulated HbF in RBCs. Further studies should investigate clinical benefits and potential harms not identified to date.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT01685515.

Epigenetic regulation by decitabine of melanoma differentiation in vitro and in vivo

Apoptosis genes, such as TP53 and p16/CDKN2A, that mediate responses to cytotoxic chemotherapy, are frequently nonfunctional in melanoma. Differentiation may be an alternative to apoptosis for inducing melanoma cell cycle exit. Epigenetic mechanisms regulate differentiation, and DNA methylation alterations are associated with the abnormal differentiation of melanoma cells. The effects of the deoxycytidine analogue decitabine (5-aza-2'-deoxycytidine), which depletes DNA methyl transferase 1 (DNMT1), on melanoma differentiation were examined. Treatment of human and murine melanoma cells in vitro with concentrations of decitabine that did not cause apoptosis inhibited proliferation accompanied by cellular differentiation. A decrease in promoter methylation, and increase in expression of the melanocyte late-differentiation driver SOX9, was followed by increases in cyclin-dependent kinase inhibitors (CDKN) p27/CDKN1B and p21/CDKN1A that mediate cell cycle exit with differentiation. Effects were independent of the TP53, p16/CDKN2A and also the BRAF status of the melanoma cells. Resistance, when observed, was pharmacologic, characterized by diminished ability of decitabine to deplete DNMT1. Treatment of murine melanoma models in vivo with intermittent, low-dose decitabine, administered sub-cutaneously to limit high peak drug levels that cause cytotoxicity and increase exposure time for DNMT1 depletion, and with tetrahydrouridine to decrease decitabine metabolism and further increase exposure time, inhibited tumor growth and increased molecular and tumor stromal factors implicated in melanocyte differentiation. Modification of decitabine dose, schedule and formulation for differentiation rather than cytotoxic objectives inhibits the growth of melanoma cells in vitro and in vivo.

Tetrahydrouridine inhibits cell proliferation through cell cycle regulation regardless of cytidine deaminase expression levels

Tetrahydrouridine (THU) is a well characterized and potent inhibitor of cytidine deaminase (CDA). Highly expressed CDA catalyzes and inactivates cytidine analogues, ultimately contributing to increased gemcitabine resistance. Therefore, a combination therapy of THU and gemcitabine is considered to be a potential and promising treatment for tumors with highly expressed CDA. In this study, we found that THU has an alternative mechanism for inhibiting cell growth which is independent of CDA expression. Three different carcinoma cell lines (MIAPaCa-2, H441, and H1299) exhibited decreased cell proliferation after sole administration of THU, while being unaffected by knocking down CDA. To investigate the mechanism of THU-induced cell growth inhibition, cell cycle analysis using flow cytometry was performed. This analysis revealed that THU caused an increased rate of G1-phase occurrence while S-phase occurrence was diminished. Similarly, Ki-67 staining further supported that THU reduces cell proliferation. We also found that THU regulates cell cycle progression at the G1/S checkpoint by suppressing E2F1. As a result, a combination regimen of THU and gemcitabine might be a more effective therapy than previously believed for pancreatic carcinoma since THU works as a CDA inhibitor, as well as an inhibitor of cell growth in some types of pancreatic carcinoma cells.

Is the resistance of gemcitabine for pancreatic cancer settled only by overexpression of deoxycytidine kinase?

The prognosis of pancreatic cancer remains poor, and the standard first-line chemotherapy with gemcitabine (GEM) has a response rate of less than 20%. Since expression of deoxycytidine kinase (dCK) seems important for improvement of GEM sensitivity, overexpression of dCK was investigated using pancreatic cancer cell lines (Panc-1, MIAPaCa-2 and BxPC-3). dCK gene was introduced into the cell lines by retrovirus and changes in IC50 were examined. Sensitivity of two pancreatic cancer cell lines to GEM elevated dramatically in comparison with control cells, but change of sensitivity remained at 1.8 times in BxPC-3. Since addition of tetrahydro uridine (THU), an inhibitor of deoxycytidine deaminase (CDA), increased the sensitivity 54-fold, overexpression of CDA seems to be the mechanism for improvement of the sensitivity. In conclusion, dCK is a key enzyme of GEM, but resistance of GEM is not improved in all pancreatic cancer cells by overexpression of dCK. Combination treatment based on expression of GEM metabolism-related gene may become an effective therapy in the future.

The 1.48 A resolution crystal structure of the homotetrameric cytidine deaminase from mouse

Cytidine deaminase (CDA) is a zinc-dependent enzyme that catalyzes the deamination of cytidine or deoxycytidine to form uridine or deoxyuridine. Here we present the crystal structure of mouse CDA (MmCDA), complexed with either tetrahydrouridine (THU), 3-deazauridine (DAU), or cytidine. In the MmCDA-DAU complex, it clearly demonstrates that cytidine is distinguished from uridine by its 4-NH(2) group that acts as a hydrogen bond donor. In the MmCDA-cytidine complex, cytidine, unexpectedly, binds as the substrate instead of the deaminated product in three of the four subunits, and in the remaining subunit it binds as the product uridine. Furthermore, the charge-neutralizing Arg68 of MmCDA has also exhibited two alternate conformations, I and II. In conformation I, the only conformation observed in the other structurally known homotetrameric CDAs, Arg68 hydrogen bonds Cys65 and Cys102 to modulate part of their negative charges. However, in conformation II the side chain of Arg68 rotates about 130 degrees around the Cgamma-Cdelta bond and abolishes these hydrogen bonds. The lack of hydrogen bonding may indirectly weaken the zinc-product interaction by increased electron donation from cysteine to the zinc ion, suggesting a novel product-expelling mechanism. On the basis of known structures, structural analysis further reveals two subclasses of homotetrameric CDAs that can be identified according to the position of the charge-neutralizing arginine residue. Implications for CDA-RNA interaction have also been considered.

Relatively small increases in the steady-state levels of nucleobase deamination products in DNA from human TK6 cells exposed to toxic levels of nitric oxide

Nitric oxide (NO) is a physiologically important molecule that has been implicated in the pathophysiology of diseases associated with chronic inflammation, such as cancer. While the complicated chemistry of NO-mediated genotoxicity has been extensively study in vitro, neither the spectrum of DNA lesions nor their consequences in vivo have been rigorously defined. We have approached this problem by exposing human TK6 lymphoblastoid cells to controlled steady-state concentrations of 1.75 or 0.65 microM NO along with 186 microM O2 in a recently developed reactor that avoids the anomalous gas-phase chemistry of NO and approximates the conditions at sites of inflammation in tissues. The resulting spectrum of nucleobase deamination products was defined using a recently developed liquid chromatography/mass spectrometry (LC/MS) method, and the results were correlated with cytotoxicity and apoptosis. A series of control experiments revealed the necessity of using dC and dA deaminase inhibitors to avoid adventitious formation of 2'-deoxyuridine (dU) and 2'-deoxyinosine (dI), respectively, during DNA isolation and processing. Exposure of TK6 cells to 1.75 microM NO and 186 microM O2 for 12 h (1260 microM x min dose) resulted in 32% loss of cell viability measured immediately after exposure and 87% cytotoxicity after a 24 h recovery period. The same exposure resulted in 3.5-, 3.8-, and 4.1-fold increases in dX, dI, and dU, respectively, to reach the following levels: dX, 7 (+/- 1) per 10(6) nt; dI, 25 (+/- 2.1) per 10(6) nt; and dU, 40 (+/- 3.8) per 10(6) nt. dO was not detected above the limit of detection of 6 lesions per 10(7) nt in 50 microg of DNA. A 12 h exposure to 0.65 microM NO and 190 microM O2 (468 microM x min dose) caused 1.7-, 1.8-, and 2.0-fold increases in dX, dI, and dU, respectively, accompanied by a approximately 15% (+/- 3.6) reduction in cell viability immediately after exposure. Again, dO was not detected. These results reveal modest increases in the steady-state levels of DNA deamination products in cells exposed to relatively cytotoxic levels of NO. This could result from limited nitrosative chemistry in nuclear DNA in cells exposed to NO or high levels of formation balanced by rapid repair of nucleobase deamination lesions in DNA.

Molecular characterization of thymidine kinase from Ureaplasma urealyticum: nucleoside analogues as potent inhibitors of mycoplasma growth

Ureaplasma urealyticum (U. urealyticum), belonging to the class Mollicutes, is a human pathogen colonizing the urogenital tract and causes among other things respiratory diseases in premature infants. We have studied the salvage of pyrimidine deoxynucleosides in U. urealyticum and cloned a key salvage enzyme, thymidine kinase (TK) from U. urealyticum. Recombinant Uu-TK was expressed in E. coli, purified and characterized with regards to substrate specificity and feedback inhibition. Uu-TK efficiently phosphorylated thymidine (dThd) and deoxyuridine (dUrd) as well as a number of pyrimidine nucleoside analogues. All natural ribonucleoside/deoxyribonucleoside triphosphates, except dTTP, served as phosphate donors, while dTTP was a feedback inhibitor. The level of Uu-TK activity in U. urealyticum extracts increased upon addition of dUrd to the growth medium. Fluoropyrimidine nucleosides inhibited U. urealyticum and M. pneumoniae growth and this inhibitory effect could be reversed by addition of dThd, dUrd or deoxytetrahydrouridine to the growth medium. Thus, the mechanism of inhibition was most likely the depletion of dTTP, either via a blocked thymidine kinase reaction and/or thymidylate synthesis step and these metabolic reactions should be suitable targets for antimycoplasma chemotherapy.

Epimer interconversion, isomerization, and hydrolysis of tetrahydrouridine: Implications for cytidine deaminase inhibition

Tetrahydrouridine (THU) is an inhibitor of cytidine deaminase (CDA), the enzyme responsible for the deactivation of ara-C and other cytidine analogues in vivo, and therefore is capable of improving the therapeutic efficacy of these antitumor agents. In aqueous solution formulations, THU exists as a mixture of epimers differing in stereochemistry of the 4-OH substituent. The aims of this study were to investigate the interconversion kinetics of the epimers of THU, the CDA inhibitory effects of these epimers, and the stability and degradation mechanisms of THU epimer mixtures in aqueous solution with the ultimate goal of developing optimal conditions for a parenteral formulation of THU. A stability indicating HPLC assay utilizing a derivatized beta-cyclodextrin column was developed to separate the two epimers of THU and to monitor their reversible isomerization to their beta-ribopyranosyl counterparts and their hydrolysis to form N-glycosidic bond cleavage products. MS and one- and two-dimensional (1)H- and (13)C-NMR measurements were conducted to identify THU epimers and degradation products and to quantitatively model the degradation kinetics. The interconversion reaction between the two THU epimers is acid catalyzed with a first-order rate constant for conversion of epimer 1(1) to epimer 1(2) of (7.4 +/- 0.3) x 10(-3) h(-1) and an equilibrium constant ([1(2)]/[1(1)] of 1.7 +/- 0.1 at pH 7.4 and 25 degrees C. Epimer interconversion was therefore sufficiently slow at pH 7.4 to allow the isolation of each and evaluation of their CDA inhibitory activities utilizing 1% (w/v) mouse kidney homogenates as a source for cytidine deaminase and cytidine as a substrate. Inhibition constants for the two THU epimers (1(1) and 1(2)) were determined to be 8 +/- 1 x 10(-7) M and 6.2 +/- 0.2 x 10(-8) M, respectively. Studies at elevated temperature suggested that THU degradation from epimer mixtures is biphasic with the initial rate of disappearance being acid catalyzed and first order in initial THU concentration, thus ruling out dimerization as a potential reaction mechanism. NMR/MS analyses revealed that the major degradation products included the beta-ribopyranosyl THU isomers (two epimers), the reduced pyrimidinone base (tetrahydrouracil), and various anomers of D-ribose formed through N-glycosidic bond cleavage, and the products of subsequent reactions of the base. Kinetic modeling of the data obtained from both HPLC and NMR measurements indicated that in an acidic solution THU beta-ribofuranosyl --> beta-ribopyranosyl isomerization is a rapid equilibrium reaction, which proceeds through an intermediate observable in 1H-NMR, and is followed by slower N-glycosidic bond hydrolysis. All the reactions between THU, its ribopyranosyl isomers, the intermediate, and the base are acid catalyzed and appear to proceed through the same sugar ring-opened intermediate (carbinolamine), consistent with previous literature.

Drug resistance to 5-aza-2'-deoxycytidine, 2',2'-difluorodeoxycytidine, and cytosine arabinoside conferred by retroviral-mediated transfer of human cytidine deaminase cDNA into murine cells

PURPOSE

The hematopoietic toxicity produced by the cytosine nucleoside analogs is a critical problem that limits their effectiveness in cancer therapy. One strategy to prevent this dose-limiting toxicity would be to insert a gene for drug resistance to these analogs into normal bone marrow cells. Cytidine (CR) deaminase can deaminate and thus inactivate 5-aza-2'-deoxycytidine (5-AZA-CdR), 2',2'-difluorodeoxycytidine (dFdC) and cytosine arabinoside (ARA-C). The aim of this study was to determine if gene transfer of CR deaminase into murine fibroblast cells confers drug resistance to these cytosine nucleoside analogs and if this resistance can be prevented by the CR deaminase inhibitor, 3,4,5,6-tetrahydrouridine (THU).

METHODS

NIH 3T3 murine fibroblast cells were transduced with retroviral particles containing the human CR deaminase cDNA. Assays measuring CR deaminase activity as well as the inhibitory action of 5-AZA-CdR, dFdC and ARA-C on colony formation, were performed in the presence of different concentrations of THU.

RESULTS

Retroviral-mediated transfer of the CR deaminase gene into 3T3 fibroblasts produced a considerable increase in CR deaminase activity. The transduced cells also showed significant drug resistance to 5-AZA-CdR, dFdC and ARA-C, as demonstrated by a clonogenic assay. This drug resistance phenotype and elevated CR deaminase activity were reversed by THU.

CONCLUSIONS

These findings indicate that the CR deaminase gene can potentially be used in cancer gene therapy for protecting normal cells against the cytotoxic actions of different cytosine nucleoside analogs. In addition, the CR deaminase-transduced cells can be used as a model for screening different CR deaminase inhibitors in an intact cellular system.

Induction of resistance to 1-beta-D-arabinofuranosylcytosine in human H9 cell line by simian immunodeficiency virus

We examined drug sensitivity of human T cell acute lymphoblastic leukemia H9 cells chronically infected with simian immunodeficiency virus (SIVmac) and found that the retrovirus-infected H9 cells showed 8.2-fold resistance to 1-beta-D-arabinofuranosylcytosine (Ara-C). In the infected cells, Ara-CTP levels decreased to 20% of that found in uninfected H9 cells after 3 h incubation at Ara-C concentration of 1 microM, and 8.1-fold increase of cytidine deaminase activity was observed in the infected H9 cells. A competitive inhibitor of cytidine deaminase, 3, 4, 5, 6-tetrahydrouridine (THU), at 100 microM reversed Ara-C resistance in the infected cells. These results indicate that inducing increased cytidine deaminase activity by SIVmac infection conferred Ara-C resistance to H9 cells. An understanding of these cellular differences in drug sensitivity may aid in the development of therapeutic strategies against retrovirus-infected cells.

Kinetic studies on 2',2'-difluorodeoxycytidine (Gemcitabine) with purified human deoxycytidine kinase and cytidine deaminase

Phosphorylation of cytosine analogs by deoxycytidine kinase (dCK) and deamination by cytidine deaminase (CDA) are two important processes in the activation and elimination of these drugs. We have investigated the kinetic parameters of 2',2'-difluorodeoxycytidine (dFdC) using purified enzymes from human cells. Deoxycytidine (CdR) and dFdC had Km values of 1.5 and 4.6 microM for dCK, respectively. Feedback inhibition of dCK by deoxycytidine 5'-triphosphate (dCTP) was also studied. Our results show that dCTP produced a greater inhibition of the phosphorylation of dFdC than CdR with concentrations of dCTP ranging from 1 to 25 microM. dFdC was a good substrate for CDA. Kinetic studies with this enzyme gave Km values for CdR and dFdC of 46.3 and 95.7 microM, respectively. The effect of competitive inhibitors of CDA on the deamination of dFdC was also investigated. Diazepinone riboside was a more potent inhibitor than tetrahydrouridine using either CdR or dFdC as the substrate. Inhibitors of CDA could be useful in clinical trials in patients with cancer to increase the chemotherapeutic effectiveness of dFdC.

Human immunodeficiency virus type 1 induces 1-beta-D-arabinofuranosylcytosine resistance in human H9 cell line

We have found that chronically HIV-1(IIIB)-infected H9 cells showed 21-fold resistance to 1-beta-D-arabinofuranosylcytosine (ARA-C) compared with uninfected H9 cells. In the infected H9 cells, a 37% increase of dCTP pool and a 34% increase of dATP were observed, and no alteration of dTTP and dGTP was observed, compared with the uninfected H9 cells. A marked decrease of ARA-CTP generation was observed in the infected H9 cells after 3-h incubation with 0.1-10 microM ARA-C. The level of deoxycytidine kinase activity with ARA-C as substrate was similar in both the infected and the uninfected cells; however, a 37-fold increase of cytidine deaminase activity was observed in the infected H9 cells. These results indicate that the induction of cytidine deaminase activity by HIV-1(IIIB) infection conferred ARA-C resistance to H9 cells. This conclusion was supported by the observation that a marked reversal of ARA-C resistance in the infected H9 cells occurred after treatment with the inhibitor of cytidine deaminase, 3,4,5,6-tetrahydrouridine. The understanding of these cellular alterations in drug sensitivity may facilitate the development of effective therapeutic strategies against HIV-1-infected cells.

Activity of 2',2'-difluorodeoxycytidine (Gemcitabine) against human tumor colony forming units

2',2'-Difluorodeoxycytidine (LY 188011, Gemcitabine) is a novel pyrimidine antimetabolite with promising activity in preclinical models for leukemia and solid tumors. Phase I clinical trials with the agent are ongoing. In order to better define types of tumors with clinical sensitivity to Gemcitabine (to help target phase II trials), we have studied the antitumor effects of this agent against a variety of freshly explanted human tumor specimens using an in vitro capillary soft agar cloning system. Final concentrations of 2.0-200 micrograms/ml were used for short-term (1 h) and continuous incubations experiments. Using a short-term incubation, 94/215 (44%) tumor specimens were evaluable for the determination of antitumor activity. The most common tumor types studied included colorectal, breast, non-small cell lung, ovarian cancer, kidney and melanoma. A concentration-dependent increase in the frequency of inhibited tumor specimens was noted (2 micrograms/ml: 6/94 specimens, 20 micrograms/ml: 13/94 specimens, 200 micrograms/ml:33/94 specimens; p less than 0.0001). A similar increase in tumor growth inhibition was found using a continuous incubation (2 micrograms/ml: 0/14 specimens, 20 micrograms/ml: 1/14 specimens, 200 micrograms/ml: 7/14 specimens; p less than 0.001). We conclude that Gemcitabine is an active antitumor agent against tumor colony forming units from a variety of human malignancies if sufficiently high concentrations can be achieved. The agent should be evaluated for Phase II clinical activity against those tumor types.

Effect of tetrahydrouridine on metabolism and transport of 1-beta-D-arabinofuranosylcytosine in human cells

Deamination of cytosine arabinoside (ara-C) by cytidine deaminase is the main mode of inactivation of this drug which can be responsible for ara-C resistance. The present study was undertaken to determine the effect of tetrahydrouridine (THU; a potent inhibitor of cytidine deaminase) on ara-C transport and metabolism in human cells. A rapid transport of ara-C into freshly isolated hepatocytes and an increased intracellular accumulation of the unchanged drug were observed in the presence of 50 micrograms/ml THU. THU inhibited the intracellular deamination of ara-C by 80% and slowed elimination of the compound extracellularly. The intracellular ara-C concentrations achieved after incubation with 1 micrograms/ml ara-C plus 50 micrograms/ml THU are similar to those attained with ara-C (10 micrograms/ml) alone. Treatment of leukemic K562 cells with the combination of THU (50 micrograms/ml) and ara-C (1 micrograms/ml) led to an augmentation of intracellular ara-C triphosphate formation up to twofold.

Potent inhibitors for the deamination of cytosine arabinoside and 5-aza-2'-deoxycytidine by human cytidine deaminase

Deamination of the nucleoside analogues ARA-C and 5-AZA-CdR by CR deaminase results in a loss of antileukemic activity. To prevent the inactivation of these analogues, inhibitors of CR deaminase may prove to be useful agents. In the present study we investigated the effects of the deaminase inhibitors Zebularine, 5-F-Zebularine, and diazepinone riboside on the deamination of CR, ARA-C, and 5-AZA-CdR using highly purified human CR deaminase (EC 3.5.4.5). These inhibitors produced a competitive type of inhibition with each substrate, the potency of which followed the patterns diazepinone riboside greater than 5-F-Zebularine and THU greater than Zebularine. 5-AZA-CdR was more sensitive than ARA-C to the inhibition produced by these deaminase inhibitors. The inhibition constants for diazepinone riboside lay in the range of 5-15 nM, suggesting that this inhibitor could be an excellent candidate for use in combination chemotherapy with either ARA-C or 5-AZA-CdR in patients with leukemia.

Deoxyribonucleoside triphosphate pools of herpes simplex virus infected cells: the influence of selective antiherpes agents and the role of the deaminase pathway

The effect of 5-methoxymethyl-2'-deoxycytidine (MMdCyd), in combination with tetrahydrodeoxyuridine (H4dUrd) and 5-methoxymethyl-2'-deoxyuridine (MMdUrd) on deoxyribonucleoside triphosphate pools was assessed. The dNTP pool content was almost 5 times as high in herpes simplex virus (HSV) infected VERO cells compared with mock-infected cells. Significant differences in dNTP pool sizes were observed with the different treatments. Treatment of HSV-infected cells with MMdCyd and MMdUrd resulted in a massive expansion of the dTTP pool, whereas pools of dCTP and dGTP were not affected substantially. MMdUrd and MMdCyd produced dATP pools that were 4 and 2.5 times that of the controls, respectively. Treatment with H4dUrd resulted in the dCTP pool increasing 12 times and barely detectable levels of dTTP. MMdCyd in combination with H4dUrd resulted in a marked reduction of the total deoxyribonucleoside triphosphate level. These results indicate that during viral replication the bulk of the thymidine nucleotides are derived from the dCyd/dCMP deaminase de novo pathway.

Regulatory effects of deoxyribonucleosides on the activity of 5-methoxymethyl-2'-deoxycytidine: modulation of antiherpes activity by deoxyguanosine and tetrahydrodeoxyuridine

The effect of purine and pyrimidine deoxyribonucleosides on the activity of 5-methoxymethyl-2'-deoxycytidine (MMdCyd) against herpes simplex virus type 1 (HSV-1) was investigated. The antiviral activity of MMdCyd was decreased by deoxythymidine, deoxyuridine and deoxycytidine. Deoxyadenosine had no effect at concentrations up to 500 microM. In contrast, deoxyguanosine (dGuo) potentiated MMdCyd activity. The mean ED50 (1.5 microM) for the combination (MMdCyd plus 100 microM dGuo) was approximately 20-fold lower than that of MMdCyd (ED50 26 microM). When tetrahydrodeoxyuridine (H4dUrd, 540 microM) was added along with MMdCyd and dGuo, anti-HSV-1 activity of MMdCyd was further potentiated by 25-fold (ED50 0.06 microM). The inhibition of virus replication, as determined by the plaque reduction assay, was further confirmed by virus yield studies and by parallel observations on virus-induced cytopathogenicity. The order of decreasing effectiveness for reducing the production of infectious virus particles (virus yield) by different treatments was: MMdCyd + dGuo + H4dUrd greater than MMdCyd + DGuo greater than MMdCyd + H4dUrd greater than MMdCyd greater than dGuo + H4dUrd greater than dGuo greater than H4dUrd. The effect of dGuo and dGuo in combination with H4dUrd on deoxyribonucleoside triphosphate (dNTP) pools was determined in Vero cells infected with multiplicity of infection of 5 PFU/cell. In the presence of 100 microM dGuo, there was approximately a 3-fold, 2-fold and 12-fold increase in dCTP, dTTP and dGTP pool sizes respectively, as compared to control (untreated) cells. Treatment with H4dUrd (1.06 mM) in combination with dGuo (100 microM), resulted in an increase of the dCTP pool and a marked fall in the dTTP and dGTP pool. The possible mechanisms for potentiation of MMdCyd activity by dGuo and H4dUrd are discussed.

Effect of tetrahydrouridine and deoxytetrahydrouridine on the interaction between 2'-deoxycytidine and 1-beta-D-arabinofuranosylcytosine in human leukemia cells

The interaction between 2'-deoxycytidine (dCyd) and 1-beta-D-arabinofuranosylcytosine (ara-C), administered at pharmacologically achievable concentrations, was examined in four continuously cultured human leukemia cell lines, HL-60, KG-1, K-562, and CCRF-CEM. In three of the cell lines (HL-60, K-562, and CCRF-CEM), co-administration of 20 or 50 microM dCyd with 10 microM ara-C reduced ara-CTP formation by at least 90% and incorporation of ara-C into DNA by at least 80%. In contrast, KG-1 cells exhibited substantially smaller reductions in both ara-CTP formation and incorporation of ara-C into DNA under identical conditions. KG-1 cells were distinguished by the highest activity of the enzyme cytidine deaminase of the four lines assayed, and exhibited the smallest increments in the intracellular accumulation of both dCyd and deoxycytidine triphosphate (dCTP) in response to exogenous dCyd. Co-administration of 1 mM tetrahydrouridine (THU) or 0.5 mM deoxy-tetrahydrouridine (dTHU) had little effect on the ability of dCyd to antagonize ara-C metabolism in HL-60, KG-1 and K-562 cells. In contrast, these deaminase inhibitors substantially increased the intracellular accumulation of dCTP as well as the ability of dCyd to antagonize ara-CTP formation and incorporation of ara-C into DNA in KG-1 cells. THU and dTHU also permitted dCyd to antagonize ara-C growth inhibitory effects in KG-1 cells to the extent observed in the other leukemic cell lines. These studies suggest that the intracellular deamination of exogenous deoxycytidine may influence the degree to which this nucleoside antagonizes ara-C metabolism and toxicity in some leukemic cells. They also raise the possibility that deaminase inhibitors may be employed to modulate, and perhaps to improve, the therapeutic selectivity of pharmacologically relevant concentrations of ara-C and dCyd in the treatment of acute leukemia in man.

Antiherpes virus activity and effect on deoxyribonucleoside triphosphate pools of (E)-5-(2-bromovinyl)-2'-deoxycytidine in combination with deaminase inhibitors

The antiviral activity and cytotoxicity of (E)-5-(2-bromovinyl)-2'-deoxycytidine (BrVdCyd) against herpes simplex virus type 1 (HSV-1), singly and in combination with deaminase inhibitors was determined using rabbit kidney (RK-13), HEP-2, BHK-21 and VERO cells. BrVdCyd was a potent inhibitor of HSV-1 replication with ED50 values of 0.30 to 1.20 microM depending on the cell line used. In the presence of tetrahydrouridine or tetrahydrodeoxyuridine (H4dUrd), potency of BrVdCyd increased approximately two fold (ED50: 0.54 microM) in HSV-infected VERO cells. The combination of BrVdCyd and H4dUrd was also effective in decreasing virus yield. Dihydrodeoxyuridine (H2dUrd) reversed the activity of BrVdCyd (ED50: 6 to 7 microM). The effect of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BrVdUrd), BrVdCyd and BrVdCyd in combination with H4dUrd on deoxyribonucleoside triphosphate (dNTP) pools was assessed in VERO cells infected with a high multiplicity of infection (10 PFU/cell). Significant differences in dNTP poll sizes (pmol/10(6) cell) were observed with different treatments. BrVdUrd and BrVdCyd treatment resulted in marked expansion of the dTTP pool (greater than 1200 pmol) compared to HSV-infected VERO cells (303 pmol). Exposure to H4dUrd resulted in a 12-fold expansion of the dCTP pool (326 pmol) and barely detectable levels of dTTP (less than 1.0 pmol). BrVdCyd plus H4dUrd treatment resulted in a slight expansion of the dTTP pool (515 pmol). These results indicate: (i) H4dUrd inhibits de novo dCyd/dCMP deaminase pathway and (ii) exposure to BrVdCyd plus H4dUrd puts a strain on viral DNA synthesis to such an extent that even though dTTP is being formed from alternative pathways, its eventual utilization as a substrate is reduced and hence it builds up.

Modulation of deoxynucleotide metabolism by the deoxycytidylate deaminase inhibitor 3,4,5,6-tetrahydrodeoxyuridine

Tetrahydrodeoxyuridine (dTHU) inhibits deoxycytidine deaminase and, after intracellular phosphorylation to the active 5'-monophosphate, also inhibits deoxycytidylate deaminase (dCMPD). Because in vitro studies have shown that dCMPD may regulate pyrimidine deoxynucleotide metabolism, the objective of this study was to investigate the effects of dTHU on deoxynucleotide metabolism in whole cells. Nearly complete inhibition of dCMPD, measured in intact CCRF-CEM cells by incorporation of [14C]dCyd into dTTP, occurred after a 45-min incubation with 100 microM dTHU. This was accompanied by an 8-fold dCTP pool expansion, although dATP, dTTP, dGTP, and ribonucleoside triphosphate pools were unaffected. Tetrahydrouridine, which inhibits deoxycytidine deaminase exclusively, had no effect on nucleotide pools. The dCTP pool expansion was directly proportional to the dTHU concentration (3-100 microM) and reached a maximum after 2 hr. Inhibition of ribonucleotide reductase by hydroxyurea completely prevented the dTHU-induced dCTP pool expansion, indicating that the substrate of dCMPD was derived from the ribonucleotide pool and that CDP was the predominant precursor of dCTP. dTHU-mediated inhibition of dCMPD appeared reversible. Exposure of cells to 100 microM dTHU followed by washing into fresh medium resulted in a linear decrease of the dCTP pool and an increase in the dTTP pool. The increased dCTP concentration after preincubation with dTHU was associated with an inhibition of deoxycytidine kinase, as indicated by a reduced capacity of cells to phosphorylate ara-C. dTHU is a useful new tool for investigating the role of dCMPD in the regulation of deoxynucleotide metabolism in whole cells.

Selective radiosensitization and cytotoxicity of human melanoma cells using halogenated deoxycytidines and tetrahydrouridine

The halogenated pyrimidines 5-chloro-2'-deoxycytidine (CldCyd) and 5-bromo-2'-deoxycytidine (BrdCyd) can act as radiosensitizers and cytotoxic agents. It was hypothesized that tumor cells and normal cells might use different metabolic pathways to incorporate these halogenated deoxycytidines into DNA. This difference could potentially be exploited to produce selective radiosensitization and cytotoxicity of human tumor cells compared to normal human fibroblasts. This hypothesis was tested using two human melanoma cell lines and two normal fibroblast cell lines. Either CldCyd or BrdCyd alone caused both cytotoxicity and radiosensitization of tumor and normal cells. The addition of the cytidine deaminase inhibitor tetrahydrouridine (H4U) significantly protected the normal cells but had relatively little effect on the tumor cells. These data indicate that it may be possible to exploit differences between the pyrimidine metabolism of normal cells and melanoma cells to improve the therapeutic index of halogenated pyrimidines both as radiosensitizers and as cytotoxic agents.

Treatment of FeLV-induced immunodeficiency syndrome (FeLV-FAIDS) with controlled release capsular implantation of 2',3'-dideoxycytidine

2',3'-dideoxycytidine (ddC) inhibits replication of the immunodeficiency inducing strain of feline leukemia virus (FeLV-FAIDS) in vitro at concentrations ranging from 1-10 micrograms/ml. Additive antiviral effect is achieved when ddC is combined with either human recombinant alpha interferon (IFN alpha) or tumor necrosis factor (TNF) plus IFN alpha. Initial in vivo pharmacokinetic studies in cats, utilizing bolus intravenous administration of ddC (20 mg/kg), resulted in peak plasma concentrations of 15 micrograms/ml 1 min after administration and a half-life of approximately 1 h. These values could not be augmented with high levels of the deaminase blocker tetrahydrouridine administered prior to or concurrently with ddC. In vivo trials utilizing multiple, daily intravenous injections of ddC could not prevent the development of persistent viremia in cats infected with FeLV-FAIDS. To enhance ddC pharmacokinetics and antiviral activity, controlled release capsular implants were developed by blending ddC with a copolymer consisting of DL-lactide glycolide and hydroxypropyl cellulose, which was melt-spun into fibers and encapsulated in a sheath of polyethylene glycol for subcutaneous implantation. Pharmacokinetic studies, conducted in cats receiving an average dose of 600 mg of ddC, indicated an average peak plasma concentration of 17 micrograms/ml achieved at 6 h post implantation with 3.5 micrograms/ml noted at 48 h; and an extension of plasma half-life from 1.5 (bolus subcutaneous injection) to 20 h. sustained plasma concentrations of 1.5 to 10 micrograms/ml, equivalent to ddC levels previously shown to have anti-FeLV activity in vitro, were maintained throughout a 72 h period. Implantation devices could be replenished every 48 h and elevated plasma levels were sustained for four weeks without signs of clinical toxicity, sepsis or significant alterations in the hemogram. Initial clinical trials employing controlled release capsular ddC implants in vivo indicate significant retardation of FeLV-FAIDS replication throughout a four week treatment period.

Effect of tetrahydrouridine on the clinical pharmacology of 1-beta-D-arabinofuranosylcytosine when both drugs are coinfused over three hours

When 1-beta-D-arabinofuranosylcytosine (ara-C), 25 mg/m2, is infused over 3 h together with tetrahydrouridine (THU) at 10 to 350 mg/m2 to heavily pretreated patients with solid tumors, Michaelis-Menten type kinetic values are observed with leveling off of delta area under the curve, delta ara-C levels at 3 h, and delta total body clearance after 150 mg/m2 of THU. When the ara-C dose was increased to 50, 75, and 100 mg/m2 coinfusion of 250 or 350 mg/m2 of THU significantly increased plasma ara-C at peak and area under the curve. In contrast, total body clearance and volume of distribution decreased significantly. At 100 mg/m2 of ara-C coinfused with high doses of THU, i.e., at 350 mg/m2, the pharmacokinetics of plasma ara-C was changed from a biphasic decay of plasma ara-C at peaks (control) to a curve similar or identical to a monophasic curve, indicating that THU not only inhibits deamination but also changes the distribution of ara-C. This combination provides plasma ara-C levels (greater than or equal to 10 microM) comparable to high dose ara-C at 1 g/m2. Such plasma ara-C levels are considered to be sufficient for saturation of the kinases catalyzing the production of 1-beta-D-arabinofuranosylcytosine 5'-triphosphate. This reduced ara-C dose necessary to achieve saturation of kinases also reduces plasma 1-beta-D-arabinofuranosyluracil levels substantially. Toxicity of this combination was predominantly confined to bone marrow and gastrointestinal toxicity.

Protective, tumor-selective dual pathway activation of 5-fluoro-2'-deoxycytidine provided by tetrahydrouridine in mice bearing mammary adenocarcinoma-755

Treatment of C57BL X DBA/2 F (hereafter called BD2F) mice bearing ascitic mammary adenocarcinoma-755 (ADC-755) with [3H]-5-fluoro-2'-deoxycytidine ([3H]FdCyd) plus tetrahydrouridine (H4Urd) resulted in antimetabolite pool sizes indicative of a tumor-selective, dual pathway metabolism of FdCyd via both cytidine deaminase and deoxycytidine kinase. In contrast to the high levels of all RNA- and DNA-level antimetabolites (as assayed by high performance liquid chromatography) derived from FdCyd found in tumor tissue, normal tissues (bone marrow, intestine, liver, and spleen) and serum metabolized FdCyd to only a small extent following FdCyd plus H4Urd treatment. RNA-level antimetabolite pools and 5-fluoro-2'-deoxyuridine (FdUrd) were generally 100-fold lower in normal than in tumor tissue, and 5-fluoro-2'-deoxyuridylate was 10- to 15-fold lower in normal than in tumor tissue. The use of [3H]FdUrd, on the other hand, resulted in the formation of higher levels (10- to 40-fold) of DNA- and RNA-level antimetabolites in normal tissue and lower levels (1/8) of 5-fluoro-2'-deoxyuridylate in tumor tissue. Both [3H]FdCyd plus H4Urd and [3H]FdUrd were utilized at their optimal drug doses. FdUrd- and FdCyd-derived metabolic products incorporated into the RNA and DNA of normal and tumor tissue of BD2F mice bearing ADC-755 were also examined. The drug combination [3H]FdCyd plus H4Urd resulted in the selective incorporation of antimetabolites into tumor RNA and DNA; only a very small extent of antimetabolites incorporated into normal tissue RNA and DNA. FdCyd was incorporated 5- to 10-fold greater in tumor than intestine, liver, or spleen following FdCyd plus H4Urd administration. FdCyd incorporation was 190-fold greater in tumor than in bone marrow. Mice bearing ADC-755 treated with [3H]-FdUrd resulted in only marginal selectivity in terms of antimetabolite incorporation in tumor tissue. Deoxycytidylate and cytidine deaminase enzyme assays have confirmed that H4Urd administration effectively inhibited normal cytidine deaminase activities, while only weakly inhibiting the elevated levels found in tumor tissue. Thymidine kinase, deoxycytidine kinase, deoxycytidylate deaminase, and cytidine deaminase have been shown previously to be significantly elevated in the mouse tumor model used; these enzymatic elevations are also characteristic of many human tumors. Treatment with FdCyd plus H4Urd resulted in 17 of 30 cures against ADC-755 compared to 4 of 20 and 0 of 20 for 5-fluorouracil and 5-fluoro-2'-deoxyuridine treatments, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Furanose-pyranose isomerization of reduced pyrimidine and cyclic urea ribosides

Tetrahydrouridine (THU, 2) and other fully reduced cyclic urea ribofuranosyl nucleosides undergo a rapid, acid-catalyzed isomerization to their more stable ribopyranosyl form. This isomerization is characterized by a change in spectral properties and by a greater than 10-fold decrease in potency for those nucleosides that act as potent inhibitors of cytidine deaminase in their ribofuranose form. 1-(beta-D-Ribopyranosyl)hexahydropyrimidin-2-one (7) was synthesized and used in conjunction with its furanose isomer 6 as a model compound for more extensive 1H and 13C NMR, mass spectral, and kinetic studies of this isomerization. The 0.4 delta upfield shift and 4-Hz increase in the J1',2' coupling constant for the pyranose anomeric proton in the 1H NMR spectrum is indicative of a pyranose beta-CI conformation in which the aglycon and C-2' and C-4' hydroxyls are equatorial. The mass spectra of trimethylsilylated pyranose nucleosides also show a characteristic large shift in the m/z 204-217 abundance and the appearance of two new rearrangement ions at M-133 and M-206. For furanose 6 the rate of isomerization is pH and temperature dependent with pyranose 7 predominating by a factor of 6-9 equilibrium. At pH 1 and 37 degrees C, furanose 6 has an initial half-life of less than 12 min. Accordingly, this isomerization may explain the observed lack of enhanced ara-C levels in studies evaluating the oral administration of an ara-C and THU combination to species with an acidic stomach content.

Deoxycytidine stimulates the in vitro growth of normal CFU-GM and reverses the negative regulatory effects of acidic isoferritin and prostaglandin E1

We have examined the effect of supraphysiologic concentrations of the naturally occurring nucleoside deoxycytidine (dCyd) on the in vitro growth of normal (CFU-GM) and leukemic (L-CFU) myeloid progenitor cells. Bone marrow samples obtained from 34 consecutive patients undergoing routine diagnostic bone marrow aspirations for nonmalignant hematologic disorders exhibited nearly a twofold increment in CFU-GM when continuously cultured in the presence of 10(-4) mol/L dCyd. Higher dCyd concentrations were associated with a smaller degree of enhancement of colony formation. In contrast, the growth of leukemic blast progenitors obtained from patients with acute nonlymphocytic leukemia were not enhanced by any of the dCyd concentrations tested. Coadministration of 10(-3) mol/L tetrahydrouridine (THU), a cytidine deaminase inhibitor, failed to alter the relative inability of dCyd to enhance L-CFU colony growth. The stimulatory effect of dCyd on normal CFU-GM was not mediated by the adherent mononuclear cell population of the marrow, nor was it restricted to the subpopulation of CFU-GM in S phase at the time of initial exposure. Moreover, treatment of normal bone marrow cells with dCyd at concentrations ranging from 10(-6) to 5 X 10(-3) mol/L for 24 hours had only a minor effect on the fraction of CFU-GM in S phase. Coadministration of 10(-4) mol/L dCyd was able to reverse the inhibitory effects of several putative regulators of normal myelopoiesis, including leukemia inhibitory activity (LIA), acidic isoferritins (AIF), and prostaglandin E1 (PGE1). Leukemic myeloblasts exposed to 10(-4) mol/L dCyd exhibited substantial expansion of intracellular pools of dCyd triphosphate (dCTP), demonstrating that inability to metabolize dCyd could not be solely responsible for the absence of growth potentiation in these cells. These studies suggest that supraphysiologic concentrations of dCyd may confer a selective in vitro growth advantage upon normal v leukemic myeloid progenitor cells, and may free the former from the inhibitory effects of several potential negative regulators of myelopoiesis.

Synthesis and release of deoxycytidine by human B and T lymphoblasts

Deoxycytidine kinase activity is abundant in human T and B lymphocytes. However, the role of the enzyme in endogenous deoxynucleoside metabolism has not been established. The present experiments show that dividing human B lymphoblasts, but not T lymphoblasts, release substantial amounts of deoxycytidine (dCyd) into the medium, and have an active dCyd-dCMP (deoxycytidine-deoxycytidine 5'-phosphate) substrate cycle. Exogenous dCyd has been shown to protect human lymphocytes from the toxic effects of deoxyadenosine, deoxyguanosine and related compounds. Thus, the differential rates of dCyd release by T and B lymphocytes may affect the sensitivities of the two cell types to the growth inhibitory effects of exogenous deoxynucleosides.

Incorporation of 5-fluorodeoxycytidine and metabolites into nucleic acids of human MCF-7 breast carcinoma cells

Several mechanisms of action have been proposed for the antitumor agents, 5-fluorouracil (FUra) and 5-fluorodeoxyuridine (FdUrd), including their incorporation into both cellular RNA and DNA. Another fluorinated pyrimidine, 5-fluorodeoxycytidine (FdCyd), has been shown to be even more active than FdUrd against certain experimental tumors. Although FdCyd is deaminated to FdUrd, the precise mechanism of action of this agent has remained unclear. We have therefore monitored the incorporation of FdCyd and its metabolites into the nucleic acids of human MCF-7 breast carcinoma cells. The results demonstrate the internucleotide incorporation of FdCyd in MCF-7 DNA. The results also demonstrate that FUra residues are detectable in both MCF-7 DNA and RNA following treatment with FdCyd. Cytidine and deoxycytidylate deaminase inhibitors increased the extent of (FdCyd) DNA synthesis, but they had little if any effect on formation of (FUra) RNA. In contrast, deoxyuridine increased incorporation of FdCyd into DNA and blocked the formation of FUra RNA. Deoxyuridine also enhanced the cytotoxicity associated with FdCyd treatment. The present results further demonstrate that FdCyd inhibits postsynthetic methylation of MCF-7 DNA. These findings would suggest that FdCyd has multiple mechanisms of action and that incorporation of this agent into DNA distinguishes its effects from those of FUra and FdUrd.

Sensitization to X ray by 5-chloro-2'-deoxycytidine co-administered with tetrahydrouridine in several mammalian cell lines and studies of 2'-chloro derivatives

5-Chloro-2'-deoxycytidine (CldC) + tetrahydrouridine (H4U) sensitizes mammalian cells (HEp-2, RIF-1, S-180) to X ray. This sensitization, as demonstrated previously with HEp-2 cells, is heightened when cells are pre-incubated with inhibitors of pyrimidine synthesis. CHO cells, which intrinsically lack both cytidine deaminase (CD) and deoxycytidylate deaminase (dCMPD), are sensitized to X ray by 5-chlorodeoxyuridine (CldU) but display no significant sensitization with CldC + H4U. The presence and level of these deaminases appears to correlate with X ray sensitization in cell culture. From experiments in cell culture, it can be inferred that one pathway of conversion, deoxycytidine kinase----dCMPD, or CD----thymidine kinase, may be sufficient for metabolizing CldC to a radiosensitizer. However, if both pathways are blocked, as in CHO cells, no X ray sensitization results. In addition to HEp-2 cells, which are extremely elevated in both CD and dCMPD activities, we have examined the sensitization of S-180 and RIF-1 cells to X ray by CldC + H4U. Both cell lines possess an enzymatic profile consistent with their sensitization to X ray by CldC + H4U. Dose enhancement ratios of 1.5 to 1.9 for cells treated with CldC + H4U and ratios of 2.0-2.7 for cells pre-treated with inhibitors of pyrimidine synthesis prior to CldC + H4U have been obtained. Based on reports of the marked X ray sensitization of bacteria by 2'-chloro-2'-deoxythymidine, we obtained 2',5-dichloro-2'-deoxycytidine and 5-bromo-2'-chloro-2-deoxyuridine and found these analogs to be X ray sensitizers of mammalian cells. The strategy that we propose with CldC + H4U and the related 2'-chloro derivatives, based on the elevation of CD and dCMPD in human tumors, offers a degree of selectivity that is not necessarily related to differences in cell kinetics; such that malignancies other than brain tumors may be amenable to this therapy.

Synthesis of pyrimidin-2-one nucleosides as acid-stable inhibitors of cytidine deaminase

One of the problems encountered in the use of tetrahydrouridine (THU, 2) and saturated 2-oxo-1,3-diazepine nucleosides as orally administered cytidine deaminase (CDA) inhibitors is their acid instability. Under acid conditions these compounds are rapidly converted into inactive ribopyranoside forms. A solution this problem was sought by functionalizing the acid-stable but less potent CDA inhibitor 1-beta-D-ribofuranosyl-2(1H)-pyrimidinone (1) with the hope of increasing its potency to the level achieved with THU. The selection of the hydroxymethyl substituent at C-4, which led to the synthesis of 4-(hydroxymethyl)-1-beta-D-ribofuranosyl-2(1H)-pyrimidinone (10), 3,4-dihydro-4-(hydroxymethyl)-1-beta-D-ribofuranosyl-2(1H)-pyrimidinone (7), and 3,4,5,6-tetrahydro-4-(dihydroxymethyl)-1-beta-D-ribofuranosyl-2(1H)-p yrimidinone (28) was based on the transition-state (TS) concept. The key intermediate precursor, 4-[(benzoyloxy)methyl]-1-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)-2(H) -pyrimidinone (24), was obtained via the classical Hilbert-Johnson reaction between 2-methoxy-4-[(benzoyloxy)methyl]pyrimidine (20) and 2,3,5-tri-O-benzoyl-1-D-ribofuranosyl bromide (21). Deprotection of 24 afforded compound 10, while its sodium borohydride reduction products afforded compounds 7 and 28 after removal of the blocking groups. Syntheses of 3,4-dihydro-1-beta-D-ribofuranosyl-2(1H)-pyrimidinone (9) and 3,6-dihydro-1-beta-D-ribofuranosyl-2(1H)-pyrimidinone (8), which lack the hydroxymethyl substituent, was accomplished in a similar fashion. The new compounds bearing the hydroxymethyl substituent were more acid stable than THU, and their CDA inhibitory potency, expressed in terms of Ki values, spanned from 10(-4) to 10(-7) M in a manner consistent with the TS theory. Compound 7, in particular, was superior to its parent 1 and equipotent to THU (Ki = 4 X 10(-7) M) when examined against mouse kidney CDA. The superior acid stability of this compound coupled to its potent inhibitory properties against CDA should provide a means of testing oral combinations of rapidly deaminated drugs, viz. ara-C, without the complications associated with the acid instability of THU.

In vitro and in vivo radiation sensitization by the halogenated pyrimidine 5-chloro-2'-deoxycytidine

5-Chloro-2'-deoxycytidine (Cld/Cyd) is hypothesized to have preferential incorporation into tumor DNA on the basis of elevated deoxycytidine-5'-phosphate deaminase and deoxycytidine kinase levels in tumors. Radiosensitization by Cld/Cyd was evaluated in exponentially growing Chinese hamster ovary cells by determining the ratio of radiation doses in control and treated cells to produce the same degree of cell killing (sensitizer enhancement ratio). Sensitizer enhancement ratios of 1.2-1.8 are seen at Cld/Cyd concentrations of 3-100 microM, 64 h incubation, and 200-600 cGy irradiation. Coincubation with tetrahydrouridine (H4Urd), a proposed inhibitor of Cld/Cyd catabolism by plasma cytidine deaminase resulted in no enhanced drug or radiation cytotoxicity. C3H mice given implants of RIF-1 tumors received 72-h continuous i.p. infusions of Cld/Cyd with or without H4Urd, or 5-bromo-2'-deoxyuridine (BrdUrd). Excised tumors were irradiated as single cell suspensions in vitro. Infusions of equimolar (0.4 mmol/kg/day) Cld/Cyd or BrdUrd resulted in greater radiosensitization by BrdUrd with no potentiation of Cld/Cyd by coinfusion with 0.8 mmol/kg/day H4Urd. Infusions with equitoxic doses of Cld/Cyd (0.8 mmol/kg/day) or BrdUrd (0.4 mmol/kg/day) yielded equal BrdUrd and Cld/Cyd sensitizer enhancement ratios of 1.6, without H4Urd potentiation of Cld/Cyd. Fluorescence-activated cell sorter analysis of tumor cell suspensions using a monoclonal antibody reactive with BrdUrd and Cld/Cyd disclosed a population of noncycling cells in tumors treated with Cld/Cyd/H4Urd that is not seen in tumors exposed to either BrdUrd or Cld/Cyd alone.

Enhanced serum concentrations of Ara-C using suppositories containing tetrahydrouridine as a deamination inhibitor of Ara-C

Rectal bioavailability of Ara-C (serum AUC 4 h: 65 micrograms h-1 ml-1) administered in a suppository formulation containing tetrahydrouridine (a deamination inhibitor) and sodium salicylate (an adjuvant) to dogs was better than that from a suppository formulation without tetrahydrouridine (serum AUC 4 h: 18 micrograms h-1 ml-1).

Metabolic channeling of 5-fluoro-2'-deoxycytidine utilizing inhibitors of its deamination in cell culture

The metabolism of 5-fluoro-2'-deoxycytidine (FdC) with and without tetrahydrouridine (H4U) or 2'-deoxytetrahydrouridine (dH4U) was examined in log phase HEp-2 cells using HPLC and TLC methods which quantified: the incorporation of FdC-related antimetabolites into RNA and DNA and pool size levels of FdC-related antimetabolites. [3H]-FdC administered to log phase HEp-2 cells at a concentration of 0.01 microM for 24 hr resulted in the incorporation of 5.22 X 10(-8) mol of FdC/mol of DNA phosphate, a 0.021% substitution of FdC for dC. Coadministration of 1.0 mM H4U or dH4U resulted in 2- and 25-fold increases in the incorporation of FdC, respectively. No detectable incorporation of 5-fluoro-2'-deoxyuridine (FdU) into HEp-2 DNA resulted (detection limit, approximately 5 fmol). In contrast, treatment of HEp-2 cells with 0.1 microM FdU resulted in the incorporation of 1.83 X 10(-9) mol of FdU (74.7 fmol detected)/mol of DNA phosphate. A linear incorporation of FdC into the DNA of HEp-2 cells was found with increasing concentrations of FdC and 1.0 mM dH4U . 0.1 microM FdC resulted in the incorporation of 2.39 X 10(-6) mol of FUMP/mol of cytoplasmic RNA phosphate and 2.23 X 10(-5) mol of FUMP/mol of nuclear RNA phosphate. Similarly, HEp-2 cells treated with 0.1 microM FdU resulted in the incorporation of 1.10 X 10(-5) mol of FUMP/mol of nuclear RNA phosphate and 9.44 X 10(-7) mol of FUMP/mol of cytoplasmic RNA phosphate. In contrast, no detectable FUMP incorporation into either nuclear or cytoplasmic RNAs of HEp-2 cells resulted when H4U or dH4U was coadministered with 0.1 microM FdC. Pool size analyses of log phase HEp-2 cells following a 30-min exposure to FdU or FdC with and without H4U or dH4U were also performed; 0.1 microM FdC treatment resulted in the formation of 169 fmol of FUMP/1.0 X 10(6) viable HEp-2 cells. Treatment with 0.1 microM FdU produced 253 fmol of FUMP/1.0 X 10(6) viable HEp-2 cells. In contrast, no detectable FUMP pools were formed when H4U or dH4U was coadministered with 0.1 microM FdC (detection limit, approximately 5 fmol). Pool levels of FdUMP, the inhibitor of thymidylate synthetase, were also assayed; 36.9 fmol of FdUMP/1.0 X 10(6) viable HEp-2 cells were detected upon administration of 0.1 microM FdC.(ABSTRACT TRUNCATED AT 400 WORDS)

Tetrahydrouridine, cytidine analogues, and hemoglobin F

5-Azacytidine (azaC) has previously been shown to raise Hb F levels in the repeatedly phlebotomized baboon (PCV: around 20%). The administration of tetrahydrouridine (THU), an inhibitor of the enzymatic conversion of azaC to 5-azauridine, made it possible to reduce the amount of azaC and also of 2-deoxy-5-azacytidine (d-azaC) by more than 90% and still achieve maximal Hb F elevations. However, the granulocytopenia, usually occurring after 5-azaC, was not altered by the lowering of the dosages in the presence of THU. Thus, the granulocytopenia is not due to 5-azauridine or other catabolic products resulting from deamination. It is also unlikely that it is caused by a direct influence of azaC on RNA since d-azaC also causes granulocytopenia. The persistence of reticulocytosis throughout the treatment with azaC or d-azaC makes it appear likely that the observed increase in Hb F levels to more than 60% of total hemoglobin is not due to a cytotoxic effect on erythropoiesis resulting in a shift of cell populations toward greater immaturity, but to a direct influence of the drug on the regulation of gamma globin chain production.

Metabolism of cytosine arabinoside in Tetrahymena pyriformis

The metabolism of cytosine arabinoside (araC) in Tetrahymena pyriformis amicronucleate strain W was studied. araC inhibited cell multiplication and protein synthesis at concentrations higher than 0.1 and 0.25 M respectively. araC had no effect on protein synthesis. araC was converted to araCMP, araCDP and araCTP by homogenized cell preparations. A deaminase activity converted araC to uracil arabinoside. The deaminase activity totally inhibited by tetrahydrouridine (THU) at a concn of 4 X 10(-6) M. The Ki for THU was 8 X 10(-8) M.

Inhibition of Escherichia coli cytidine deaminase by a phosphapyrimidine nucleoside

The nature of the interaction between Escherichia coli cytidine deaminase and the phosphapyrimidine nucleoside 1 has been studied kinetically and spectrophotometrically. Compound 1 was designed as a transition-state analog, and is a potent, slow-binding inhibitor of cytidine deaminase (Ashley, G. W., and Bartlett, P. A. (1982) Biochem. Biophys. Res. Commun. 108, 1467-1474). We present evidence that the binding of 1 is reversible, with no covalent linkage between the enzyme and 1. At pH 6, the rate of recovery of enzyme activity from dissociation of the E X I complex is strongly dependent on the concentration of E X I, indicating that the inhibitor dissociates reversibly. UV difference spectroscopy reveals that the chromophore of 1 is unaltered on binding to the enzyme, thus eliminating the possibility of reversible, covalent modification of the enzyme. For the binding of the active beta-anomers of 1 to cytidine deaminase, the following kinetic parameters were determined at pH 6: kon = 8300 M-1 S-1, koff = 7.8 X 10(-6) S-1, Ki = 0.9 nM. We were also able to observe and characterize time-dependent inhibition of E. coli cytidine deaminase by tetrahydrouridine, 3. This interaction involves involves initial formation of a loose complex (KD = 1.2 microM), followed by isomerization in a slow step to give a more tightly bound complex (Ki = 0.24 microM) with forward and reverse rate constants kf = 3.81 min-1 and kr = 0.95 min-1, respectively.

Reversal of deamination-related cytotoxicity of 5-methyl-2'-deoxycytidine by tetrahydrouridine in human leukemia cells

The present experiments were conducted to test the effects of the potent cytidine deaminase inhibitor tetrahydrouridine (THU) on the metabolism and cytotoxicity of 5-methyl-2'-deoxycytidine (5-Med-Cyd) in several human leukemia cell lines in vitro. It was observed that 5-Med-Cyd exerts its effects via deamination to thymidine, which is particularly toxic to human promyelocytic (HL-60) and T-cell (JM) leukemia cell lines in vitro. The deamination and the cytotoxicity of 5-Med-Cyd were effectively hindered by 10(-3) M THU in 3-day cultures of HL-60 cells. Although the catabolism of [14C]5-Med-Cyd in the HL-60 cell cultures was blocked by THU, no radioactive 5-Med-Cyd was incorporated into DNA. The cytotoxicity and DNA incorporation of fluorodeoxycytidine are enhanced by THU. Unlike that compound 5-Med-Cyd resembled more bromodeoxycytidine and iododeoxycytidine; THU decreases the toxicity of both of these deoxycytidine analogues.

Marked radiosensitization of cells in culture to X ray by 5-chlorodeoxycytidine coadministered with tetrahydrouridine, and inhibitors of pyrimidine biosynthesis

Our approach to overcome the problem of rapid catabolism and general toxicity encountered with 5-halogenated analogues of deoxyuridine (5-bromo, chloro or iododeoxyuridine), which has limited their use as tumor radiosensitizers, is to utilize 5-chlorodeoxycytidine (CldC) with tetrahydrouridine (H4U). We propose that CldC, coadministered with H4U, is metabolized in the following manner: CldC----CldCMP----CldUMP---- ----CldUTP----DNA. All the enzymes of this pathway are elevated in many human malignant tumors and in HEp-2 cells. In X irradiation studies with HEp-2 cells, limited to 1 or 2 radiation doses, we have obtained 3.0 to 3.8 apparent dose enhancement ratios (these represent upper limits) when cells are preincubated with inhibitors of pyrimidine biosynthesis: N-(Phosphonacetyl)-L-aspartate (PALA) and 5-fluorodeoxyuridine (FdU) or 5-fluorodeoxycytidine (FdC) + H4U. Optimum conditions for radiosensitization are: PALA (0.1 mg/ml) 18-20 hr prior to FdU (0.1 microM) or FdC (0.02 microM) + H4U (0.1 mM) followed 6 hr later by CldC (0.1-0.2 mM) + H4U (0.1 mM) for 56-68 hr. Viabilities of 10 +/- 4% to 15 +/- 1% (+/- S.E.) were obtained for drug-treated unirradiated cells. Enzymatic studies indicate that this toxicity may be tumor selective. CldC + H4U alone (at these concentrations) results in 20% substitution of CldU for thymidine in DNA (determined by HPLC analysis). Preliminary toxicity studies indicate that mice will tolerate treatment protocols involving a single dose of PALA (200 mg/kg) followed by a dose of FdU (50 mg/kg) and 3 cycles of CldC (500 mg/kg) + H4U (100 mg/kg) at 10 hour intervals, with marginal weight loss (4%). In this approach we seek to obtain preferential conversion of CldC to CldUTP at the tumor site by taking advantage of quantitative differences in enzyme levels between tumors and normal tissues.

Labelling of the thymidine and deoxycytidine bases of DNA by [2-14C]deoxycytidine in cultured L1210 cells

Exposure of cultured L1210 cells to [2-14C]deoxycytidine and analysis or radioactivity incorporated into DNA-pyrimidines revealed that 2.7--5.5-fold more radioactivity is incorporated into DNA-thymine than into cytosine bases. Thus the pathway involving deamination of deoxycytidylate to deoxyuridylate and methylation to thymidylate is highly favored over successive phosphorylation to dCTP. Several modified and endogenous pyrimidines altered the labelling of DNA-thymine and DNA-cytosine with [2-14C]deoxycytidine. 3-Deazauridine at 0.1 mM caused a 56% increase in the labelling of DNA-thymine. Both thymidine and 3-deazauridine (greater than or equal to 10 microM) increased the specific activity of DNA-cytosine by 4-fold. Cytosine arabinoside (ara-C) (greater than or equal to 10 microM) reduced the labelling of both DNA-cytosine and DNA-thymine. Excess cytidine (0.1 mM) reduced the labelling of DNA-cytosine by 40%. Tetrahydrouridine at concentrations up to 1 mM had no effect.

Influence of tetrahydrouridine on the pharmacokinetics of intrathecally administered 1-beta-D-arabinofuranosylcytosine

Tetrahydrouridine (THU), a potent inhibitor of cytidine deaminase, has been shown to increase the antitumor activity of 1-beta-D-arabinofuranosylcytosine (ara-C) both in vitro and in vivo. In initial studies, which examined the cerebrospinal fluid (CSF) pharmacokinetics of intrathecally (i.t.) administered THU, the drug was found to be slowly cleared from the CSF with alpha and beta half-lives of 1 and 8 hr, respectively. In subsequent experiments, both i.v. pretreatment with THU and concomitant i.t. injection of THU were found to retard the disappearance of i.t. ara-C from the CSF, although the effect of i.t. THU was more profound. ara-C given alone was cleared from CSF with alpha and beta half-lives of 27.5 +/- 6.7 and 115.6 +/- 0.4 (S.D.) min, respectively. Pretreatment with i.v. THU resulted in alpha and beta half-lives of 10.4 +/- 1.5 and 85.7 +/- 11.1, respectively, whereas concomitant administration of i.t. THU resulted in a single half-life of 96 +/- 0.7. The mean calculated clearance rates for ara-C alone, ara-C plus i.v. THU, and ara-C plus i.t. THU were 7.5, 6.2, and 4.2 ml/hr, respectively. This effect appeared to be primarily due to THU inhibition of ara-C deamination, since a decrease in formation of 1-beta-D-arabinofuranosyluracil in the CSF was observed when ara-C was given in the presence of THU (either i.t. or i.v.). No acute neurotoxicity was noted after administration of either i.t. THU alone or i.t. THU with ara-C. The ability of THU to alter CSF ara-C pharmacokinetics may have potential therapeutic value.

Cytosine arabinoside triphosphate production in human leukaemic myeloblasts: interactions with deoxycytidine

The effect of 1 mu M deoxycytidine (dC) on Ara-C conversion to Ara-CTP and on inhibition of DNA synthesis by Ara-C was measured in intact leukaemic myeloblasts. dC decreased Ara-CTP production in blasts with high Ara-C phosphorylation, but not those with low activity. The Ki for dC was similar to values found with partially purified deoxycytidine kinase. The change in Ara-CTP concentration was associated with a proportional reduction in inhibition of DNA synthesis. dC decreased the effects of Ara-C by inhibition of Ara-CTP production, rather than by production of dCTP and competition with Ara-CTP. Since low Ara-CTP production in patients' blasts is a predictor of poor therapeutic response to Ara-C, the use of dC with Ara-C may improve the therapeutic index in this group of patients.

Tetrahydrouridine specifically facilitates deoxycytidine incorporation into herpes simplex virus DNA

As reported by Jamieson and Subak-Sharpe (J. Gen. Virol. 31:303-313, 1976), exogenous deoxycytidine is very poorly incorporated into herpes simplex virus DNA. Here it is shown that this incorporation was dramatically increased in the presence of tetrahydrouridine (THU), a specific inhibitor of cytidine-deoxycytidine deaminase. Thus, the exclusion of deoxycytidine from herpes simplex virus DNA probably results from massive degradation by the deaminase, which is consistent with the observation that in the absence of THU, most of the nucleotides formed from exogenous deoxycytidine are dUMP. The effect of tHU upon deoxycytidine incorporation was specific for herpes simplex virus-infected cells; THU did not increase deoxycytidine incorporation into DNA of uninfected cells. Therefore, one might expect THU to enhance the antiviral activity of 1-beta-D-arabinofuranasylcytosine since this analog is also readily deaminated. However, THU increased both the antiviral activity and the cell toxicity only slightly and to about the same extent. Therefore, the metabolism of 1-beta-D-arabnofuranosylcytosine is different from that of deoxycytidine in herpes simplex virus-infected cells.