Action of the active metabolites of tiazofurin and ribavirin on purified IMP dehydrogenase

Ribavirin inhibits Chandipura virus replication in Vero cells

Chandipura virus (CHPV) is an emerging tropical pathogen in India. The virus has been reported to be associated with an acute encephalitis syndrome in young children with a case fatality rate of 55% to 75%. Clinical management with symptomatic treatment is the only option available to treat infected patients. No vaccines are available for prophylaxis. In light of the prophylactic limitations, antiviral therapy would play an important role in control of CHPV infection. In the present study, ribavirin (RBV), an antiviral drug widely accepted for human use and having an antiviral effect on many RNA and DNA viruses, was tested against the CHPV. A screening assay that scores for the virus-mediated plaque formation in the cultured Vero cells was used. RBV exhibited 50% inhibitory concentration (IC50 ) at 89.84 ± 1.8 µM. The drug was very effective when the cells were treated either within an hour postinfection or 4 to 6 hours before infection. The plaque morphology was different in RBV treated cells; the plaques were smaller in size as compared with the plaques in the virus infected cells. The study reports the antiviral activity of RBV against CHPV, and hence, suggests the possible utility of RBV in CHPV infected patients to mitigate the disease. A further clinical trial is needed before introducing the drug for human use against CHPV infection.

A Blood Pact: the Significance and Implications of eIF4E on Lymphocytic Leukemia

Elevated levels of eukaryotic initiation factor 4E (eIF4E) are implicated in neoplasia, with cumulative evidence pointing to its role in the etiopathogenesis of hematological diseases. As a node of convergence for several oncogenic signaling pathways, eIF4E has attracted a great deal of interest from biologists and clinicians whose efforts have been targeting this translation factor and its biological circuits in the battle against leukemia. The role of eIF4E in myeloid leukemia has been ascertained and drugs targeting its functions have found their place in clinical trials. Little is known, however, about the pertinence of eIF4E to the biology of lymphocytic leukemia and a paucity of literature is available in this regard that prospectively evaluates the topic to guide practice in hematological cancer. A comprehensive analysis on the significance of eIF4E translation factor in the clinical picture of leukemia arises, therefore, as a compelling need. This review presents aspects of eIF4E involvement in the realm of the lymphoblastic leukemia status; translational control of immunological function via eIF4E and the state-of-the-art in drugs will also be outlined.

Efficacy of ribavirin against malignant glioma cell lines

Ribavirin (1-β-D-ribofuranosy-1,2,4-triazole-3-carboxamide) has been widely administered as an antiviral agent against RNA and DNA viruses. Ribavirin, in combination with interferon, has predominantly been applied in the treatment of the hepatitis C virus infection and its potential antitumor efficacy has recently become a point of interest. The aim of the present study was to evaluate the effect of ribavirin on the growth of malignant glioma cells, to identify novel predictive genes in malignant glioma cells (by analyzing gene expression profiles) and to assess the influence of ribavirin on the cell cycle of malignant glioma cells. The present study evaluated the antitumor efficacy of ribavirin against various malignant glioma cell lines (A-172, AM-38, T98G, U-87MG, U-138MG, U-251MG and YH-13). After culturing the cells in ribavirin-containing culture medium (final concentration, 0–1,000 μM) for 72 h, the viable proliferated cells were harvested and counted. The half maximal inhibitory concentration of ribavirin, with regard to the growth of the malignant glioma cell lines, was determined from the concentration of ribavirin required for 50% growth inhibition in comparison to the untreated control cells. Furthermore, the current study identified the genes in which the gene expression levels correlated with the ribavirin sensitivity of the malignant glioma cells lines, using a high-density oligonucleotide array. Finally, cell cycle analysis was performed on the U-87MG cell line. It was identified that ribavirin inhibited the growth of all of the malignant glioma cell lines in a dose-dependent manner, although the ribavirin sensitivity varied between each cell line. Of the extracted genes, PDGFRA demonstrated the strongest positive correlation between gene expression level and ribavirin sensitivity. Cell cycle analysis of the U-87MG cell line demonstrated that ribavirin treatment induces G0/G1 arrest and thus may be an effective agent for inhibiting malignant glioma cell growth. Therefore, the results of the current study indicate that ribavirin may have potential as a therapeutic agent in the treatment of malignant gliomas.

4-Pyridone-3-carboxamide-1-β-D-ribonucleoside triphosphate (4PyTP), a novel NAD metabolite accumulating in erythrocytes of uremic children: a biomarker for a toxic NAD analogue in other tissues?

We have identified a novel nucleotide, 4-pyridone 3/5-carboxamide ribonucleoside triphosphate (4PyTP), which accumulates in human erythrocytes during renal failure. Using plasma and erythrocyte extracts obtained from children with chronic renal failure we show that the concentration of 4PyTP is increased, as well as other soluble NAD⁺ metabolites (nicotinamide, N¹-methylnicotinamide and 4Py-riboside) and the major nicotinamide metabolite N¹-methyl-2-pyridone-5-carboxamide (2PY), with increasing degrees of renal failure. We noted that 2PY concentration was highest in the plasma of haemodialysis patients, while 4PyTP was highest in erythrocytes of children undergoing peritoneal dialysis: its concentration correlated closely with 4Py-riboside, an authentic precursor of 4PyTP, in the plasma. In the dialysis patients, GTP concentration was elevated: similar accumulation was noted previously, as a paradoxical effect in erythrocytes during treatment with immunosuppressants such as ribavirin and mycophenolate mofetil, which deplete GTP through inhibition of IMP dehydrogenase in nucleated cells such as lymphocytes. We predict that 4Py-riboside and 4Py-nucleotides bind to this enzyme and alter its activity. The enzymes that regenerate NAD⁺ from nicotinamide riboside also convert the drugs tiazofurin and benzamide riboside into NAD⁺ analogues that inhibit IMP dehydrogenase more effectively than the related ribosides: we therefore propose that the accumulation of 4PyTP in erythrocytes during renal failure is a marker for the accumulation of a related toxic NAD⁺ analogue that inhibits IMP dehydrogenase in other cells.

Ribavirin as an anti-cancer therapy: acute myeloid leukemia and beyond?

Ribavirin was discovered nearly 40 years ago as a broad-spectrum antiviral drug. Recent data suggest that ribavirin may also be an effective cancer therapy. In this case, ribavirin targets an oncogene, the eukaryotic translation initiation factor eIF4E, elevated in approximately 30% of cancers including many leukemias and lymphomas. Specifically, ribavirin impedes eIF4E mediated oncogenic transformation by acting as an inhibitor of eIF4E. In a phase II clinical trial, ribavirin treatment led to substantial clinical benefit in patients with poor-prognosis acute myeloid leukemia (AML). Here molecular targeting of eIF4E correlated with clinical response. Ribavirin also targets a key enzyme in the guanosine biosynthetic pathway, inosine monophosphate dehydrogenase (IMPDH), and also modulates immunity. Parallels with known antiviral mechanisms could be informative; however, after 40 years, these are not entirely clear. The antiviral effects of ribavirin appear cell-type specific. This variation likely arises for many reasons, including cell specific variations in ribavirin metabolism as well as virus specific factors. Thus, it seems that the mechanisms for ribavirin action in cancer therapy may also vary in terms of the cancer/tissue under study. Here we review the anticancer activities of ribavirin and discuss the possible utility of incorporating ribavirin into diverse cancer therapeutic regimens.

The immunosuppressor mycophenolic acid kills activated lymphocytes by inducing a nonclassical actin-dependent necrotic signal

Mycophenolate mofetil (MMF) is an immunosuppressive agent used in transplantation. Over the last decade, MMF has also emerged as an alternative therapeutic regimen for autoimmune diseases, mainly for patients refractory to other therapies. The active compound of MMF, mycophenolic acid (MPA), depletes the intracellular pool of guanosine tri-phosphate through inosine monophosphate dehydrogenase blockade. The molecular mechanism involved in the elimination of T and B lymphocytes upon inhibition of inosine monophosphate dehydrogenase remains elusive. In this study, we showed that in contrast to the immunosuppressors azathioprine, cyclosporin A, and tacrolimus, MPA killed lymphocytes through the activation of a caspase-independent necrotic signal. Furthermore, the MPA-mediated necrotic signal relied on the transmission of a novel intracellular signal involving Rho-GTPase Cdc42 activity and actin polymerization. In addition to its medical interest, this study sheds light on a novel and atypical molecular mechanism leading to necrotic cell death.

Therapeutic effects of combined treatment with ribavirin and tiazofurin on experimental autoimmune encephalomyelitis development: clinical and histopathological evaluation

Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis (MS) and the helpful tool in preclinical testing of various substances considered for treatment of this human CNS disease. Ribavirin (R) and tiazofurin (T) are purine nucleoside analogues, with the broad spectrum of anti-viral, anti-tumoral and anti-inflammatory properties. We proposed that combined treatment with RT, administrated during the effector phase of EAE, would attenuate disease severity, both clinically and pathologically. Ribavirin was given daily at a dosage of 30 mg/kg and tiazofurin was given at a dosage of 10 mg/kg every other day for 15 days. We detected amelioration of clinical signs and faster recovery in the RT group compared to the control group. Immunohistochemical analyses revealed that RT treatment decrease the number of T cells, macrophages and microglia. In the controls, we detected reactive type of microglia, while in the RT group we noticed ramified/resting form. Demyelination areas and axonal damage were not recorded in the RT group, in contrast to the control group where multiple areas of demyelination zones and axonal loss were found. RT combination treatment suppresses ongoing EAE, prevents demyelination and axonal loss, and therefore may well be the potential therapy for the treatment of MS.

Combination of nucleoside analogues tiazofurin and ribavirin downregulates experimental autoimmune encephalomyelitis

The effect of combined treatment with ribavirin and tiazofurin on the development of experimental autoimmune encephalomyelitis, the best characterized animal model for human autoimmune disease multiple sclerosis, was investigated. The disease was induced in highly susceptible Dark Agouti rats with spinal cord homogenate in complete Freund's adjuvant. Although ribavirin or tiazofurin alone reduced the clinical and histopathological signs of experimental autoimmune encephalomyelitis, the combination of drugs achieved the same effect with significantly lower doses.

Therapeutic effect of nucleoside analogs on experimental autoimmune encephalomyelitis in dark agouti rats

Experimental autoimmune encephalomyelitis (EAE) is a commonly used animal model of the human neurological disorder multiple sclerosis. The purpose of the present study was to investigate the effect of combined treatment with two nucleoside analogs, ribavirin and tiazofurin, on development of EAE actively induced in highly susceptible dark agouti rats. The obtained results showed that ribavirin and tiazofurin applied either separately or in combination from the onset of the firstsymptoms of EAE after its induction (therapeutic treatment) significantly suppressed EAE?s clinical symptoms. However, the most pronounced effect was gained with combined treatment, probably as a result of synergistic/additive action.

Metabolism and antiviral activity of ribavirin

Thirty years after its synthesis, the mechanism of action of ribavirin is still not completely understood. Although much is known about the metabolism and biochemical effects of ribavirin in human cells, there is still much to be learned about the precise mechanism of action of ribavirin with the various viruses. New information about its ability to induce mutations in viral genomes has led to new questions about its mechanism of action. There is considerable evidence that indicates that ribavirin triphosphate (RTP) can interact with the various viral RNA polymerases, and it seems likely that this interaction is important to the mechanism of action of ribavirin. It seems likely that ribavirin will not have one universal mechanism of action, but will inhibit different viruses in different ways. In some cases, inhibition of IMP dehydrogenase may be sufficient for antiviral activity. Whereas, in other cases, inhibition of viral RNA polymerases by RTP may be more important. It is also likely that RTP will interact with the different viral RNA polymerases in different ways leading to different mechanisms of actions. More comprehensive studies are needed that address all aspects of ribavirin metabolism and biochemical actions to gain a thorough understanding of the activity of this agent. Finally, the differences in the metabolism and biochemical actions of ribavirin, selenazofurin, and tiazofurin indicate that small structural changes can have profound effects on biological activity. This observation is well known by investigators familiar with nucleoside analogs, but indicate that one should not assume that agents of similar structure have identical activities.

Detection of the mycophenolate-inhibited form of IMP dehydrogenase in vivo

IMP dehydrogenase (IMPDH) is the rate-limiting enzyme for de novo GMP synthesis. Its activity is correlated with cell growth, and it is the target of a number of proven and experimental drug therapies including mycophenolic acid (MPA). MPA inhibits the enzyme by trapping a covalent nucleotide-enzyme intermediate. Saccharomyces cerevisiae has four IMPDH genes called IMD1-IMD4. IMD2 is transcriptionally regulated and is the only one that enables yeast to grow in the presence of MPA. We show here that de novo synthesis of the IMD2-encoded protein is strongly induced upon MPA treatment. We also monitor the in vivo formation of a covalent nucleotide-enzyme intermediate for Imd2, Imd3, and Imd4 that accumulates in the presence of MPA. Complete formation of the Imd2 intermediate requires drug concentrations manyfold higher than that required to quantitatively trap the Imd3- or Imd4-nucleotide adducts. Purification of the tagged IMD gene products reveals that the family of polypeptides coassemble to form heteromeric IMPDH complexes, suggesting that they form mixed tetramers. These data demonstrate that S. cerevisiae harbor multiple IMPDH enzymes with varying drug sensitivities and offer an assay to monitor the inhibition of IMPDH in living cells. They also suggest that mixed inhibition profiles may result from heteromeric complexes in cell types that contain multiple IMPDH gene products. The mobility shift assay could serve as a tool for the detection of drug-inactivated IMPDH in the cells of patients receiving MPA therapy.

Inhibition of caffeine biosynthesis in tea (Camellia sinensis) and coffee (Coffea arabica) plants by ribavirin

The effects of ribavirin, an inhibitor of inosine-5'-monophosphate (IMP) dehydrogenase, on [8-(14)C]inosine metabolism in tea leaves, coffee leaves and coffee fruits were investigated. Incorporation of radioactivity from [8-(14)C]inosine into purine alkaloids, such as theobromine and caffeine, guanine residues of RNA, and CO(2) was reduced by ribavirin, while incorporation into nucleotides, including IMP and adenine residues of RNA, was increased. The results indicate that inhibition of IMP dehydrogenase by ribavirin inhibits both caffeine and guanine nucleotide biosynthesis in caffeine-forming plants. The use of IMP dehydrogenase-deficient plants as a potential source of good quality caffeine-deficient tea and coffee plants is discussed.

Inhibitors of de novo nucleotide biosynthesis as drugs

Potent inhibitors of enzymes catalyzing reactions in the de novo pathways for biosynthesis of purine and pyrimidine nucleotides are synthetic or natural-product analogues of pathway intermediates or, more recently, inhibitors rationally designed from a knowledge of the catalytic mechanism. Such inhibitors may be effective drugs against cancer, inflammatory disorders, or various infections. For human cancer, the purine pathway may be a better target for inhibition than the pyrimidine pathway, where toxic side effects are more apparent. Drugs such as methotrexate and 6-mercaptopurine have multiple sites of action, making it difficult to quantitatively predict their effects upon cells. Rational design of inhibitors based upon the X-ray structure of the target enzyme has the prospect of yielding drugs with only one site of action in human cells. Such a drug is VX-497, a potent inhibitor of the purine enzyme, IMP dehydrogenase.

Interferon-alpha-2b plus ribavirin: a review of its use in the management of chronic hepatitis C

Relatively few patients (< or =20%) with chronic hepatitis C achieve a sustained virological response after interferon-alpha monotherapy. Hence, alternative treatment strategies such as the addition of the broad spectrum antiviral agent ribavirin to interferon-alpha-2b have been investigated. Combination therapy with subcutaneous interferon-alpha-2b [3 million units (MU) three times per week] plus oral ribavirin (1000 to 1200 mg/day) has proven effective in several well designed trials of 24 to 48 weeks' duration in adult patients with compensated chronic hepatitis C. Compared with interferon-alpha-2b (3 or 6 MU three times per week) with or without placebo, combination treatment with interferon-alpha-2b plus ribavirin significantly enhanced end-of-treatment and sustained virological and biochemical response rates in treatment-naive and treatment-experienced patients [sustained virological response rates in treatment-naive recipients (6 to 19% vs 31 to 43% of patients); sustained overall (virological plus biochemical) response rates in nonresponders to (1 vs 14%) or relapsers (4 to 5% vs 30 to 44%) after previous interferon-alpha monotherapy]. Forty-eight weeks of combination therapy was superior to 24 weeks in treatment-naive patients infected with hepatitis virus C (HCV) genotype 1, whereas response rates were similar at 24 and 48 weeks in those infected with other HCV genotypes. Furthermore, there were marked improvements in histological inflammatory scores in patients who responded to treatment with either interferon-alpha-2b plus ribavirin or interferon-alpha-2b alone. Although adverse events associated with either drug during combination therapy occurred frequently, these were generally mild to moderate in intensity and were consistent with those reported for each individual agent. Twenty-six percent of patients required dosage modifications of one or both drugs during combination therapy.

CONCLUSIONS

Interferon-alpha-2b plus ribavirin is an efficacious first- and second-line therapy in adult patients with compensated chronic hepatitis C, significantly improving sustained virological and biochemical responses versus interferon-alpha-2b monotherapy. The tolerability profile of interferon-alpha-2b plus ribavirin therapy is consistent with the individual profiles of these agents with no evidence of additive effects. The place of interferon-alpha-2b plus ribavirin combination therapy in relation to newer agents, including pegylated interferons-alpha and other multidrug regimens, remains to be determined in this rapidly evolving area of therapeutic management. Currently, combination therapy with interferon-alpha-2b plus ribavirin is recommended as first-line therapy for patients with chronic hepatitis C and compensated liver disease, and is an option for use as second-line therapy in those who have relapsed after, or failed to respond to, previous treatment with interferon-alpha.

Drug selectivity is determined by coupling across the NAD+ site of IMP dehydrogenase

Drug resistance often results from mutations that are located far from the drug-binding site. The effects of these mutations are perplexing. The inhibition of IMPDH by MPA is an example of this phenomenon. Mycophenolic acid (MPA) is a species-specific inhibitor of IMPDH; mammalian IMPDHs are very sensitive to MPA, while the microbial enzymes are resistant to the inhibitor. MPA traps the covalent intermediate E-XMP and binds in the nicotinamide half of the dinucleotide site. Previous results indicated that about half of the difference in sensitivity derives from residues in the MPA-binding site [Digits, J. A., and Hedstrom, L. (1999) Biochemistry 38, 15388-15397]. The remainder must be attributed to regions outside the MPA-binding site. The adenosine subsite of the NAD+ site is not conserved among IMPDHs and is, therefore, a likely candidate. Our goal is to examine the coupling between the nicotinamide and adenosine sites in order to test this hypothesis. We performed multiple inhibitor experiments with the Tritrichomonas foetus and human type 2 IMPDHs using tiazofurin and ADP, which bind in the nicotinamide and adenosine subsites, respectively. For T. foetus IMPDH, tiazofurin and ADP are extraordinarily synergistic. In contrast, these inhibitors are virtually independent for the human type 2 enzyme. We suggest that the difference in coupling of the nicotinamide and adenosine subsites accounts for the remaining difference in MPA affinity between T. foetus and human IMPDH.

Inosine-5'-monophosphate dehydrogenase: regulation of expression and role in cellular proliferation and T lymphocyte activation

Guanine nucleotide synthesis is essential for the maintenance of normal cell growth and function, as well as for cellular transformation and immune responses. The expression of two genes encoding human inosine-5'-monophosphate dehyrogenase (IMPDH) type I and type II results in the translation of catalytically indistinguishable enzymes that control the rate-limiting step in the de novo synthesis of guanine nucleotides. Cellular IMPDH activity is increased more than 10-fold in activated peripheral blood T lymphocytes and is attributable to the increased expression of both the type I and type II enzymes. In contrast, abrogation of cellular IMPDH activity by selective inhibitors prevents T lymphocyte activation and establishes a requirement for elevated IMPDH activity in T lymphocytic responses. In order to assess the molecular mechanisms governing the expression of the IMPDH type I and type II genes in resting and activated peripheral blood T lymphocytes, we have cloned the human IMPDH type I and type II genes and characterized their genomic organization and their respective 5'-flanking regions. Both genes contain 14 highly conserved exons that vary in size from 49 to 207 base pairs. However, the intron structures are completely divergent, resulting in disparities in gene length (18 kilobases for type I and 5.8 kilobases for type II). In addition, the 5'-regulatory sequences are highly divergent; expression of the IMPDH type I gene is controlled by three distinct promoters in a tissue specific manner while the type II gene is regulated by a single promoter and closely flanked in the 5' region by a gene of unknown function. The conservation of the IMPDH type I and type II coding sequence in the presence of highly divergent 5'-regulatory sequences points to a multifactorial control of enzyme expression and suggests that tissue-specific and/or developmentally specific regulation of expression may be important. Delineation of these regulatory mechanisms will aid in the elucidation of the signaling events that ultimately lead to the synthesis of guanine nucleotides required for cellular entry into S phase and the initiation of DNA replication.

Ribavirin inhibits protein synthesis and cell proliferation induced by mitogenic factors in primary human and rat hepatocytes

Ribavirin, a guanosine analog, used in combination with interferon alpha (IFN-alpha) in the treatment of chronic hepatitis induced by hepatitis C virus (HCV) infection, has been shown to improve liver histology and to decrease transaminases even when administered alone. We analyzed the direct effects of ribavirin on the liver by using primary cultures of human and rat hepatocytes. Between 10 to 60 micromol/L, ribavirin was found to inhibit both the synthesis and secretion of whole proteins in a time- and dose-dependent fashion. Such an effect was confirmed by the measurement of albumin and haptoglobin secretion rates. [3H]-Thymidine incorporation was suppressed both in hepatocyte growth factor-stimulated human hepatocytes and in epidermal growth factor (EGF)-stimulated rat hepatocytes in the presence of ribavirin. The inhibitory effect on DNA synthesis was associated with a delayed progression to S phase of the cell cycle, as determined by flow cytometry and detection of cyclin A and cdc2 which are two proteins expressed during the S phase. The inhibition of DNA synthesis, caused by 50 micromol/L ribavirin, was completely restored by the addition of 80 micromol/L guanosine. These observations demonstrate that ribavirin at concentrations close to those found in plasma of treated patients can directly affect hepatic functions in vitro. Its effects could, however, be reduced in vivo by guanosine salvage supply.

Expression, purification, and characterization of inosine 5'-monophosphate dehydrogenase from Borrelia burgdorferi

Inosine 5'-monophosphate dehydrogenase (IMPDH) is the rate-limiting enzyme in de novo guanine nucleotide biosynthesis. IMPDH converts IMP to xanthosine 5'-monophosphate with concomitant conversion of NAD+ to NADH. All IMPDHs characterized to date contain a 130-residue "subdomain" that extends from an N-terminal loop of the alpha/beta barrel domain. The role of this subdomain is unknown. An IMPDH homolog has been cloned from Borrelia burgdorferi, the causative agent of Lyme disease (Margolis, N., Hogan, D., Tilly, K., and Rosa, P. A. (1994) J. Bacteriol. 176, 6427-6432). This homolog has replaced the subdomain with a 50-residue segment of unrelated sequence. We have expressed and characterized the B. burgdorferi IMPDH homolog. This protein has IMPDH activity, which unequivocally demonstrates that the subdomain is not required for catalytic activity. The monovalent cation and dinucleotide binding sites of B. burgdorferi IMPDH are significantly different from those of human IMPDH. Therefore, these sites are targets for the design of specific inhibitors for B. burgdorferi IMPDH. Such inhibitors might be new treatments for Lyme disease.

The practical synthesis of a methylenebisphosphonate analogue of benzamide adenine dinucleotide: inhibition of human inosine monophosphate dehydrogenase (type I and II)

beta-Methylene-BAD (8), a nonhydrolyzable analogue of benzamide adenine dinucleotide (BAD), was synthesized as potential inhibitor of human inosine monophosphate dehydrogenase (IMPDH). Treatment of 2',3'-O-isopropylideneadenosine 5'-methylenebisphosphonate (15) with DCC afforded P1,P4-bis(2',3'-O-isopropylideneadenosine) 5'-P1,P2:P3,P4-dimethylenetetrakisphosphonate (17). This compound was further converted with DCC to an active intermediate 18 which upon reaction with 3-(2',3'-O-isopropylidene-beta-D-ribofuranosyl)benzamide (19) gave, after hydrolysis and deisopropylidenation, the desired beta-methylene-BAD (8) in 95% yield. In a similar manner, treatment of 18 with 2',3'-O-isopropylidenetiazofurin (21) followed by hydrolysis and deprotection afforded beta-methylene-TAD (5) in 91% yield. Compound 8 (IC50 = 0.665 microM) was found to be a 6-8 times less potent inhibitor of IMPDH than 5 (IC50 = 0.107 microM) and was almost equally potent against IMPDH type I and type II. Although TAD and beta-methylene-TAD were bound by LADH with the same affinity, the binding affinity of 8 toward LADH (Ki = 333 microM) was found to be 50-fold lower than that of the parent pyrophosphate 7 (Ki = 6.3 microM).

IMP dehydrogenase from Pneumocystis carinii as a potential drug target

Mycophenolic acid, a specific inhibitor of IMP dehydrogenase (IMPDH; EC 1.1.1.205), is a potent inhibitor of Pneumocystis carinii growth in culture, suggesting that IMPDH may be a sensitive target for chemotherapy in this organism. The IMPDH gene was cloned as a first step to characterizing the enzyme and developing selective inhibitors. A 1.3-kb fragment containing a portion of the P. carinii IMPDH gene was amplified by PCR with two degenerate oligonucleotides based on conserved sequences in IMPDH from humans and four different microorganisms. Northern hybridization analysis showed the P. carinii IMPDH mRNA to be approximately 1.6 kb. The entire cDNA encoding P. carinii IMPDH was isolated and cloned. The deduced amino acid sequence of P. carinii IMPDH shared homology with bacterial (31 to 38%), protozoal (48 to 59%), mammalian (60 to 62%), and fungal (62%) IMPDH enzymes. The IMPDH cDNA was expressed by using a T7 expression system in an IMPDH-deficient strain of Escherichia coli (strain S phi 1101). E. coli S phi 1101 cells containing the P. carinii IMPDH gene were able to grow on medium lacking guanine, implying that the protein expressed in vivo was functional. Extracts of these E. coli cells contained IMPDH activity that had an apparent Km for IMP of 21.7 +/- 0.3 microM and an apparent Km for NAD of 314 +/- 84 microM (mean +/- standard error of the mean; n = 3), and the activity was inhibited by mycophenolic acid (50% inhibitory concentration, 24 microM; n = 2).

The conformation of inosine 5'-monophosphate (IMP) bound to IMP dehydrogenase determined by transferred nuclear overhauser effect spectroscopy

IMP dehydrogenase (IMPDH) catalyzes the NAD-dependent synthesis of xanthosine 5'-monophosphate which is the rate-limiting step in guanine nucleotide biosynthesis. Although IMPDH is the target of numerous chemotherapeutic agents, nothing has been known about the conformation of the enzyme-bound substrates. The conformation of IMP bound to human type II IMP dehydrogenase has been determined by two-dimensional transferred nuclear Overhauser effect NMR spectroscopy at 600 MHz. NOE buildup rates were determined by recording NOESY spectra at numerous mixing times. The cross-relaxation rates determined from the initial NOE build-up rates were used to calculate inter-proton distances of bound IMP. The conformation of the enzyme-bound IMP was obtained by molecular modeling with energy minimization using the experimentally determined inter-proton distance constraints. The glycosidic torsion angle of the bound nucleotide is anti and the sugar is in the C2-endo-conformation. This conformation places H2 of IMP, which is transferred to NAD in the reaction, in a position clear of the rest of the molecule in order to facilitate the reaction.

Conformational changes and stabilization of inosine 5'-monophosphate dehydrogenase associated with ligand binding and inhibition by mycophenolic acid

The effects of substrate, product, and inhibitor (mycophenolic acid) binding on the conformation and stability of hamster type II inosine 5'-monophosphate dehydrogenase (IMPDH) have been examined. The protein in various states of ligand occupancy was compared by analyzing susceptibility to in vitro proteolysis, the degree of binding of a hydrophobic fluorescent dye, secondary structure content as determined by far-UV circular dichroism spectra, and urea-induced denaturation curves. These analysis methods revealed consistent evidence that IMPDH undergoes a local reorganization when IMP or XMP bind. NAD+ produced no such effect. In fact, no evidence was found for NAD+ binding independently of IMP. It is proposed that IMPDH adopts an open conformation around its nucleotide binding sites in the absence of substrates and that binding of IMP stabilizes a closed conformation that has a higher affinity for NAD+. The data also suggest the enzyme remains in the closed configuration throughout the catalytic steps and then reverts to the open conformation with XMP release, thereby consummating the enzyme cycle. Mycophenolic acid inhibition appeared to impart even greater stability. We propose that localized conformational changes occur during the normal and mycophenolic acid-inhibited reaction sequences of IMPDH.

Nuclear magnetic resonance and molecular modeling study on mycophenolic acid: implications for binding to inosine monophosphate dehydrogenase

The conformation of the sodium salt of mycophenolic acid (MPA), a potent inhibitor of inosine monophosphate dehydrogenase (IMPD), derived from 1D DIFNOE and 2D ROESY experiments in water and molecular dynamics (MD) is described. The hexenoic acid side chain conformation consistent with the NMR data was similar to that seen in the X-ray structure of MPA. The solution conformation was applied in a molecular modeling study in order to explore the potential features of enzyme binding. Our results, based on striking similarities in molecular volume and electrostatic isopotential between MPA and cofactor NAD+, lead to the suggestion that MPA is capable of binding to the nicotinamide site of IMPD and mimicking the NAD+ inverse regulation of the enzyme. In addition, our proposed model is in good agreement with the observed high affinity of the dinucleotide analogues thiazole- and selenazole-4-carboxamide adenine dinucleotide to IMPD.

Monovalent cation activation and kinetic mechanism of inosine 5'-monophosphate dehydrogenase

Human type II inosine 5'-monophosphate dehydrogenase has been purified to homogeneity from an Escherichia coli strain that express large quantities of the enzyme from the cloned gene. Steady state kinetic studies have been used to characterize the activation by monovalent cations, including Li+, Na+, K+, Rb+, Cs+, Tl+, NH4+, and N(CH3)4+. The enzyme has less than 1% of the maximal activity in the absence of an added monovalent cation, such as K+, Na+, Rb+, Tl+, or NH4+. The enzyme is activated by K+ and Tl+ at lower concentrations than those of other monovalent cations. Li+ and N(CH3)4+ do not activate the enzyme, nor do they inhibit the K(+)-activated enzyme, implying that ionic radius is important in binding selectivity. The Km values for both substrates and Vmax differ with different monovalent cations. Initial velocity and product inhibition kinetic data are consistent with an ordered steady state mechanism in which the enzyme binds K+ first, TMP second, and then NAD; the product NADH is released before xanthosine 5'-monophosphate. Substrate and product binding experiments support this mechanism and show the presence of one substrate binding site per subunit. Several rate constants were obtained from a computer simulation of the complete steady state rate equation.

Synthesis and modeling studies with monocyclic analogues of mycophenolic acid

Two stepwise procedures, developed for the introduction of the (E)-4-methyl-4-hexenoic acid side chain of mycophenolic acid, were used in the synthesis of monocyclic mycophenolic acid analogues 2a-i. The derivatives with a methyl group or hydrogen at C-4 and lacking the lactone moiety were much less cytotoxic than mycophenolic acid. The monocyclic analogues with a C-4 chloro group did show some activity, albeit much less than mycophenolic acid. The observed differences in potency are rationalized by semiempirical calculations of intramolecular H-bonds.

Recombinant human inosine monophosphate dehydrogenase type I and type II proteins. Purification and characterization of inhibitor binding

Inosine monophosphate dehydrogenase (IMPDH) activity results from the expression of two separate genes, and the resulting proteins (type I and type II) are 84% identical at the amino acid level. Although the type II mRNA is expressed at higher levels in proliferating cells, both mRNAs, and by extrapolation both proteins, are present in normal and malignant cells. Since IMPDH is an important target for the development of drugs with both chemotherapeutic and immunosuppressive activity, we have compared the kinetic and physical properties of the two human enzymes expressed in and purified from Escherichia coli. Type I and II IMPDH had kcat values of 1.8 and 1.4 sec-1, respectively, with Km values for IMP of 14 and 9 microM and Km values for NAD of 42 and 32 microM. The two enzymes were inhibited competitively by the immunosuppressive agent mizoribine 5'-monophosphate (MMP) with Ki values of 8 and 4 nM and inhibited uncompetitively by mycophenolic acid with Ki values of 11 and 6 nM. The association of MMP to either isozyme, as monitored by fluorescence quenching, was relatively slow with kon values of 3-8 x 10(4) M-1 sec-1 and koff values of 3 x 10(-4) sec-1 (half-lives of 36-43 min). Thus, MMP is a potent, tight-binding competitive inhibitor that does not discriminate between the two IMPDH isozymes.

Role of differentiation induction in action of purine antimetabolites

In cancer cells, particularly in leukaemic cells, guanylate biosynthesis is up-regulated as shown by the increased activities of IMP dehydrogenase, the rate-limiting enzyme of de novo GTP biosynthesis, and of the salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGPRT). In enzyme pattern-targeted chemotherapy, tiazofurin inhibits IMP dehydrogenase activity in cancer cells and allopurinol-induced high serum hypoxanthine levels inhibit HGPRT activity. A triad of responses was observed in the blast cells of patients treated with tiazofurin infusions: chemotherapy, induced differentiation, and down-regulation of c-Ki-ras and c-myc oncogenes. Tiazofurin was synergistic in cytotoxicity and in causing differentiation with ribavirin, retinoic acid, and gemcitabine [corrected]. Induced differentiation plays an important role in the overall impact of antipurine agents.

Targeted and non-targeted actions of anti-cancer drugs

(1) The currently used clinical anti-metabolites are targeted against-key enzymes of de novo purine and pyrimidine biosynthesis. However, the activities of salvage enzymes in each of the biosynthetic segments are markedly higher than those of the rate-limiting enzymes of de novo biosynthesis. Enzyme-pattern-targeted chemotherapy has been suggested to overcome the circumvention activity of salvage. Combination of inhibition of de novo and salvage pathways does provide a synergistic impact. Examples that enzyme-pattern-targeted drug treatment yields synergism include the following: tiazofurin (against IMP DH) and allopurinol (by raising serum hypoxanthine levels it inhibits GPRT); methotrexate or 5-FU lead to inhibition of the dTMP synthase reaction and AZT (a competitive inhibitor of thymidine kinase) or dipyridamole (a nucleoside transport inhibitor); acivicin, an inhibitor and inactivator of glutamine-utilizing enzymes in the de novo pathways of purine and pyrimidine biosynthesis, and dipyridamole. (2) Administration of MTX, 5-FU, tiazofurin or acivicin causes inhibition and/or inactivation of target enzymes. That these drugs are effective in spite of the presence of highly active salvage enzymes is now accounted for, at least in part, by new observations showing that these drugs markedly reduce (but do not eliminate) the activities (amounts) of CdR and TdR kinases, dTMP synthase and GPRT. This action is attributed to the rapid decay rate of these enzymes. (3) Studies on the bone marrow enzymic programs indicate that there is a window of opportunity for strengthening therapy and for the protection of bone marrow by administering salvage metabolites when the salvage enzymes are still present in high enough activities, i.e., 2-6 hr after administration of the blockers of de novo enzyme activities. (4) These results are a strong argument for discovering and utilizing inhibitors of purine and pyrimidine salvage enzymes to achieve more successful enzyme-pattern-targeted chemotherapy and to avoid development of resistant clones of cancer cells. (5) These approaches provide greater explanatory coherence than the previous accounts because recognition of (a) the importance of salvage and (b) rapid decay of key and salvage enzymes reveals a paradigm shift. The problem-solving process in chemotherapy should now be not only data-driven but also explanation-driven.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

Pharmacodynamics of the inhibition of GTP synthesis in vivo by mycophenolic acid

Mycophenolic acid is effective against a wide range of experimental tumors in rodents when given orally, despite rapid metabolism to the inactive glucuronide derivative and rapid clearance from plasma. In the light of this, the pharmacodynamic action of mycophenolic acid on the radiolabelling of GTP and ATP by [14C]hypoxanthine in spleen and heart has been investigated in vivo in the rat as a preliminary to studies in tumor tissue. The data indicate that inhibition of GTP, and more surprisingly, ATP synthesis in spleen was sustained for at least 24.25 hr after single oral doses of the disodium salt of mycophenolic acid, indicating that the inhibitor is retained in sensitive cells for considerably longer than might be expected from the pharmacokinetic profile in the plasma in this species. GTP and ATP levels became depressed in rat spleen subsequent to the inhibition of nucleotide radiolabelling. The persistence of mycophenolic acid in proliferating cells may account for the effectiveness of once daily dosing against rapidly growing experimental tumors. In contrast with spleen, there was no incorporation of radiolabel from [14C]hypoxanthine into either GTP or ATP in rat heart and mycophenolic acid had no effect on the cardiac content of either nucleotide. The lack of effect of mycophenolic acid on cardiac GTP levels is consistent with the absence of any pharmacological action on cardiac functions associated with receptor-G-protein-GTP interactions. The ability of the morpholinoethyl ester of mycophenolic acid (a clinically effective immunosuppressive agent) to inhibit GTP synthesis and depress GTP levels in rat spleen in vivo was compared with that of mycophenolic free acid and its disodium salt. The ester derivative was clearly more effective than the poorly water-soluble free acid but showed comparable activity with the freely soluble disodium salt.

Synthesis of thiazole-4-carboxamide-adenine difluoromethylenediphosphonates substituted with fluorine at C-2' of the adenosine

Synthesis of an analogue 3 of thiazole-4-carboxamide adenine-dinucleotide (TAD) in which the beta-oxygen atom of the pyrophosphate bridge is replaced by a difluoromethylene group has been achieved. Likewise, 2'-deoxy-2'-fluoroadenosine containing analogues of TAD (4) and its difluoromethylenediphosphonate congener (5) have been synthesized. Adenosine 5'-difluoromethylenediphosphonate (8) was prepared from 5'-O-tosyladenosine (6) and tris(tetra-n-butylammonium)difluoromethylenediphosphonate (7) by a modified procedure of Poulter's. Compound 8 was converted into the 2',3'-cyclic carbonate 9 by treatment with triethyl orthoformate. Treatment of 9 with 2',3'-O-isopropylidenetiazofurin (10) in pyridine in the presence of DCC gave a mixture of dinucleotide 11 and the isopropylidene-protected diadenosine tetraphosphonate 12. After deprotection of 11, the desired beta-difluoromethylene TAD (3) was separated by HPLC as the minor product. The diadenosine tetraphosphonate 12, an analogue of Ap4A, was obtained as the major component. Alternatively, 2',3'-O-isopropylidenetiazofurin (10) was tosylated, and the product 13 was further converted into the corresponding difluoromethylenediphosphonate 14 by coupling with 7. DCC-catalyzed coupling of 14 with 2'-deoxy-2'-fluoroadenosine (15) followed by deisopropylidenation afforded the analogue 5. Again the corresponding tetraphosphonate analogue of tiazofurin 17 was the predominant product. Dinucleotide 4 was obtained by coupling of the carbonyldiimidazole-activated tiazofurin 5'-monophosphate with 2'-deoxy-2'-fluoroadenosine 5'-monophosphate. 2'-Deoxy-2'-fluoroadenosine (15) was prepared efficiently from the known N6-benzoyl-3'-O-tetrahydropyranyladenosine (18), which was converted into 3'-O-tetrahydropyranyl-2'-O-triflyl-5'-O-trityladenosine (20) by tritylation and triflation. Treatment of 20 with sodium acetate in hexamethylphosphoric triamide, followed by deacetylation afforded 9-(3-O-tetrahydropyranyl-5-O-trityl-beta-D- arabinofuranosyl)-N6-benzoyladenine (22), which was then treated with DAST. After deprotection of the product, 15 was obtained in good yield.

Inosine monophosphate dehydrogenase from porcine (Sus scrofa domestica) thymus: purification and properties

1. IMP dehydrogenase (EC 1.1.1.205) from porcine thymus glands has been purified to homogeneity. 2. The enzyme has a subunit MW of 57 kDa and an amino acid composition similar to those obtained from other normal and cancerous mammalian cells. 3. The apparent Km values at pH 8.0 for IMP and NAD+ are 7 and 16 microM, respectively. 4. GMP, XMP and AMP are competitive inhibitors towards IMP and Ki values of 50, 85 and 282 microM, respectively. 5. The effectiveness of nucleotides to protect inactivation by CI-IMP is IMP > GMP > XMP > AMP.

Guanine nucleotide is essential and Ca2+ is a modulator in the exocytotic reaction of permeabilized rat mast cells

Exocytosis from metabolically depleted permeabilized rat mast cells was measured in response to provision of Ca2+ and guanine nucleotide [GTP or guanosine 5'-[gamma-thio]triphosphate (GTP[S])]. For cells permeabilized in simple salt solutions (NaCl), both of these effectors were required to induce secretion. Exclusion of Mg2+ caused an increase in both the sensitivity of the system to GTP and the extent of secretion elicited, while having no such effects on secretion induced by GTP[S]. The effect of Mg2+ depletion on the ability of GTP to stimulate secretion is probably due to the dependence on Mg2+ of the GTPase activity of GE (a postulated GTP-binding protein which mediates exocytosis). This argues that a persistent stimulus to the G-protein is required to support secretion. Affinity for both GTP[S] and GTP is enhanced when the cells are permeabilized in zwitterionic electrolytes (glutamate, gamma-aminobutyric acid, glycine) instead of NaCl. Under these conditions, secretion occurs in response to provision of either GTP[S] [in the effective absence of Ca2+ (pCa 9)] or Ca2+ (in the absence of guanine nucleotide). Secretion induced by GTP[S] is strongly promoted by the presence of Mg2+ at concentrations in the millimolar range; this promotion by Mg2+ declines as the concentration of Ca2+ is elevated towards pCa 7. At pCa 6, Mg2+ is without effect. Ca(2+)-induced secretion requires the provision of MgATP. Since this is further enhanced by low concentrations (< 100 microM) and then inhibited by high concentrations of GDP, the essential role of ATP is likely to be in the maintenance of GTP via transphosphorylation by a nucleoside diphosphate kinase reaction. Thus, under conditions of high affinity (glutamate environment), GTP[S] alone is capable of inducing exocytosis. Ca2+ acts in concert with guanine nucleotides: it enhances the rate and extent of secretion and increases the affinity for Mg2+ and guanine nucleotides in the activation of the GTP-binding protein (GE) which regulates exocytosis.

A study of deoxyribonucleotide metabolism and its relation to DNA synthesis. Supercomputer simulation and model-system analysis

A model system (1) was established to analyze purine and pyrimidine metabolism. This system has been expanded to include macrosimulation of DNA synthesis and the study of its regulation by terminal deoxynucleoside triphosphates (dNTPs) via a complex set of interactions. Computer experiments reveal that our model exhibits adequate and reasonable sensitivity in terms of dNTP pool levels and rates of DNA synthesis when inputs to the system are varied. These simulation experiments reveal that in order to achieve maximum DNA synthesis (in terms of purine metabolism), a proper balance is required in guanine and adenine input into this metabolic system. Excessive inputs will become inhibitory to DNA synthesis. In addition, studies are carried out on rates of DNA synthesis when various parameters are changed quantitatively. The current system is formulated by 110 differential equations.

Metabolism and action of purine nucleoside analogs

Recent investigations have identified many new purine nucleoside analogs that act as antimetabolites. This article focuses on the metabolism and mechanisms of action of tiazofurin, 3-deazaguanosine, neplanocin A, arabinosyladenine in combination with inhibitors of adenosine deaminase, arabinosyl-2-fluoroadenine, and 2-chloro-2'-deoxyadenosine, drugs that are either currently being evaluated in clinical trials or are close to that stage. The diverse metabolic requirements for activation, unique mechanisms of action, and differential biological activities of these compounds are characterized and evaluated for prospective therapeutic application.