Evidence Favoring a Positive Feedback Loop for Physiologic Auto Upregulation of hnRNP-E1 during Prolonged Folate Deficiency in Human Placental Cells.

Background: Previously, we determined that heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) functions as an intracellular physiologic sensor of folate deficiency. In this model, l-homocysteine, which accumulates intracellularly in proportion to the extent of folate deficiency, covalently binds to and thereby activates homocysteinylated hnRNP-E1 to interact with folate receptor-α mRNA; this high-affinity interaction triggers the translational upregulation of cell surface folate receptors, which enables cells to optimize folate uptake from the external milieu. However, integral to this model is the need for ongoing generation of hnRNP-E1 to replenish homocysteinylated hnRNP-E1 that is degraded.Objective: We searched for an interrelated physiologic mechanism that could also maintain the steady-state concentration of hnRNP-E1 during prolonged folate deficiency.Methods: A novel RNA-protein interaction was functionally characterized by using molecular and biochemical approaches in vitro and in vivo.Results: l-homocysteine triggered a dose-dependent high-affinity interaction between hnRNP-E1 and a 25-nucleotide cis element within the 5'-untranslated region of hnRNP-E1 mRNA; this led to a proportionate increase in these RNA-protein complexes, and translation of hnRNP-E1 both in vitro and within placental cells. Targeted perturbation of this RNA-protein interaction either by specific 25-nucleotide antisense oligonucleotides or mutation within this cis element or by small interfering RNA to hnRNP-E1 mRNA significantly reduced cellular biosynthesis of hnRNP-E1. Conversely, transfection of hnRNP-E1 mutant proteins that mimicked homocysteinylated hnRNP-E1 stimulated both cellular hnRNP-E1 and folate receptor biosynthesis. In addition, ferrous sulfate heptahydrate [iron(II)], which also binds hnRNP-E1, significantly perturbed this l-homocysteine-triggered RNA-protein interaction in a dose-dependent manner. Finally, folate deficiency induced dual upregulation of hnRNP-E1 and folate receptors in cultured human cells and tumor xenografts, and more selectively in various fetal tissues of folate-deficient dams.Conclusions: This novel positive feedback loop amplifies hnRNP-E1 during prolonged folate deficiency and thereby maximizes upregulation of folate receptors in order to restore folate homeostasis toward normalcy in placental cells. It will also functionally impact several other mRNAs of the nutrition-sensitive, folate-responsive posttranscriptional RNA operon that is orchestrated by homocysteinylated hnRNP-E1.

From the covalent linkage of drugs to novel inhibitors of ribonucleotide reductase: synthesis and biological evaluation of valproic esters of 3'-C-methyladenosine.

We synthesized a series of serum-stable covalently linked drugs derived from 3'-C-methyladenosine (3'-Me-Ado) and valproic acid (VPA), which are ribonucleotide reductase (RR) and histone deacetylase (HDAC) inhibitors, respectively. While the combination of free VPA and 3'-Me-Ado resulted in a clear synergistic apoptotic effect, the conjugates had lost their HDAC inhibitory effect as well as the corresponding apoptotic activity. Two of the analogs, 2',5'-bis-O-valproyl-3'-C-methyladenosine (A160) and 5'-O-valproyl-3'-C-methyladenosine (A167), showed promising cytotoxic activities against human hematological and solid cancer cell lines. A167 was less potent than A160 but had interesting features as an RR inhibitor. It inhibited RR activity by competing with ATP as an allosteric effector and concomitantly reduced the intracellular deoxyribonucleoside triphosphate (dNTP) pools. A167 represents a novel lead compound, which in contrast to previously used RR nucleoside analogs does not require intracellular kinases for its activity and therefore holds promise against drug resistant tumors with downregulated nucleoside kinases.

Influence of physiologic folate deficiency on human papillomavirus type 16 (HPV16)-harboring human keratinocytes in vitro and in vivo.

Although HPV16 transforms infected epithelial tissues to cancer in the presence of several co-factors, there is insufficient molecular evidence that poor nutrition has any such role. Because physiological folate deficiency led to the intracellular homocysteinylation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and activated a nutrition-sensitive (homocysteine-responsive) posttranscriptional RNA operon that included interaction with HPV16 L2 mRNA, we investigated the functional consequences of folate deficiency on HPV16 in immortalized HPV16-harboring human (BC-1-Ep/SL) keratinocytes and HPV16-organotypic rafts. Although homocysteinylated hnRNP-E1 interacted with HPV16 L2 mRNA cis-element, it also specifically bound another HPV16 57-nucleotide poly(U)-rich cis-element in the early polyadenylation element (upstream of L2L1 genes) with greater affinity. Together, these interactions led to a profound reduction of both L1 and L2 mRNA and proteins without effects on HPV16 E6 and E7 in vitro, and in cultured keratinocyte monolayers and HPV16-low folate-organotypic rafts developed in physiological low folate medium. In addition, HPV16-low folate-organotypic rafts contained fewer HPV16 viral particles, a similar HPV16 DNA viral load, and a much greater extent of integration of HPV16 DNA into genomic DNA when compared with HPV16-high folate-organotypic rafts. Subcutaneous implantation of 18-day old HPV16-low folate-organotypic rafts into folate-replete immunodeficient mice transformed this benign keratinocyte-derived raft tissue into an aggressive HPV16-induced cancer within 12 weeks. Collectively, these studies establish a likely molecular linkage between poor folate nutrition and HPV16 and predict that nutritional folate and/or vitamin-B(12) deficiency, which are both common worldwide, will alter the natural history of HPV16 infections and also warrant serious consideration as reversible co-factors in oncogenic transformation of HPV16-infected tissues to cancer.

Variability of apoptosis and response in N1-S1 rodent hepatomas to benzamide riboside and correlation to early changes in water apparent diffusion coefficient and sodium MR imaging.

PURPOSE: This pilot trial assesses variability of apoptosis and response 1 day after hepatic intraarterial (IA) benzamide riboside (BR) in rodent hepatomas and its correlation to water apparent diffusion coefficient (ADC) and single-quantum (SQ) and triple-quantum-filtered (TQF) sodium-23 ((23)Na) magnetic resonance (MR) imaging. MATERIALS AND METHODS: Sprague-Dawley rats (n = 8) were inoculated with 10(6) N1-S1 cells. IA BR (20 mg/kg) was infused after 14 days. Animals were killed 1 day (n = 4) or 21 days (n = 4) after therapy. Imaging was performed 1 day before and after treatment. Volume was assessed over 2 weeks. Percentage apoptosis was counted from terminal deoxynucleotidyl transferase dUTP nick-end labeling-stained slides at 400×magnification. Kruskal-Wallis tests were used to compare apoptosis, and Wilcoxon signed-rank tests were used to compare MR signal intensity (SI). RESULTS: Apoptosis was marginally greater in tumor than in nontumor (6.7% vs 1.3%; P = .08), varying from 2% to 10%. Before treatment, MR SI was greater in tumor than in nontumor (ADC, 1.18 vs 0.76 [P = .0078]; SQ, 1.20 vs 1.04 [P = .03]; TQF, 0.55 vs 0.34 [P = .03]). After treatment, tumors increased in volume (0.62 vs 0.33; P = .016) variably over 2 weeks. MR SI remained greater in tumor than in nontumor (ADC, 1.20 vs 0.77 [P = .0078]; SQ, 1.76 vs 1.15 [P = .016]; TQF, 0.84 vs 0.49 [P = .03]). SQ and TQF SI increased by 47% (P = .016) and 53% (P = .016) in tumors, whereas ADC did not change. CONCLUSIONS: Apoptosis was marginal and varied from 2% to 10%. Water ADC, SQ, and TQF MR imaging distinguished tumor from nontumor. Changes in water ADC and sodium MR imaging correlated to apoptosis and volume in select cases, but additional animals are needed to validate this trend against tumor growth.

Incrimination of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) as a candidate sensor of physiological folate deficiency.

The mechanism underlying the sensing of varying degrees of physiological folate deficiency, prior to adaptive optimization of cellular folate uptake through the translational up-regulation of folate receptors (FR) is unclear. Because homocysteine, which accumulates intracellularly during folate deficiency, stimulated interactions between heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and an 18-base FR-α mRNA cis-element that led to increased FR biosynthesis and net up-regulation of FR at cell surfaces, hnRNP-E1 was a plausible candidate sensor of folate deficiency. Accordingly, using purified components, we evaluated the physiological basis whereby L-homocysteine triggered these RNA-protein interactions to stimulate FR biosynthesis. L-homocysteine induced a concentration-dependent increase in RNA-protein binding affinity throughout the range of physiological folate deficiency, which correlated with a proportionate increase in translation of FR in vitro and in cultured human cells. Targeted reduction of newly synthesized hnRNP-E1 proteins by siRNA to hnRNP-E1 mRNA reduced both constitutive and L-homocysteine-induced rates of FR biosynthesis. Furthermore, L-homocysteine covalently bound hnRNP-E1 via multiple protein-cysteine-S-S-homocysteine mixed disulfide bonds within K-homology domains known to interact with mRNA. These data suggest that a concentration-dependent, sequential disruption of critical cysteine-S-S-cysteine bonds by covalently bound L-homocysteine progressively unmasks an underlying RNA-binding pocket in hnRNP-E1 to optimize interaction with FR-α mRNA cis-element preparatory to FR up-regulation. Collectively, such data incriminate hnRNP-E1 as a physiologically relevant, sensitive, cellular sensor of folate deficiency. Because diverse mammalian and viral mRNAs also interact with this RNA-binding domain with functional consequences to their protein expression, homocysteinylated hnRNP-E1 also appears well positioned to orchestrate a novel, nutrition-sensitive (homocysteine-responsive), posttranscriptional RNA operon in folate-deficient cells.

Cofactor-type inhibitors of inosine monophosphate dehydrogenase via modular approach: targeting the pyrophosphate binding sub-domain.

Cofactor-type inhibitors of inosine monophosphate dehydrogenase (IMPDH) that target the nicotinamide adenine dinucleotide (NAD) binding domain of the enzyme are modular in nature. They interact with the three sub-sites of the cofactor binding domain; the nicotinamide monophosphate (NMN) binding sub-site (N sub-site), the adenosine monophosphate (AMP) binding sub-site (A sub-site), and the pyrophosphate binding sub-site (P sub-site or P-groove). Mycophenolic acid (MPA) shows high affinity to the N sub-site of human IMPDH mimicking NMN binding. We found that the attachment of adenosine to the MPA through variety of linkers afforded numerous mycophenolic adenine dinucleotide (MAD) analogues that inhibit the two isoforms of the human enzyme in low nanomolar to low micromolar range. An analogue 4, in which 2-ethyladenosine is attached to the mycophenolic alcohol moiety through the difluoromethylenebis(phosphonate) linker, was found to be a potent inhibitor of hIMPDH1 (K(i)=5 nM), and one of the most potent, sub-micromolar inhibitor of leukemia K562 cells proliferation (IC(50)=0.45 µM). Compound 4 was as potent as Gleevec (IC(50)=0.56 µM) heralded as a 'magic bullet' against chronic myelogenous leukemia (CML). MAD analogues 7 and 8 containing an extended ethylenebis(phosphonate) linkage showed low nanomolar inhibition of IMPDH and low micromolar inhibition of K562 cells proliferation. Some novel MAD analogues described herein containing linkers of different length and geometry were found to inhibit IMPDH with K(i)'s lower than 100 nM. Thus, such linkers can be used for connection of other molecular fragments with high affinity to the N- and A-sub-site of IMPDH.

Dual inhibitors of inosine monophosphate dehydrogenase and histone deacetylase based on a cinnamic hydroxamic acid core structure.

Small molecules that act on multiple biological targets have been proposed to combat the drug resistance commonly observed for cancer chemotherapy. By combining the structural features of known inhibitors of inosine monophosphate dehydrogense (IMPDH) and histone deacetylase (HDAC), dual inhibitors of IMPDH and HDAC based on the scaffold of cinnamic hydroxamic acid (CHA) have been designed, synthesized, and evaluated in biological assays. Key features, including the linker length, linker functionality, substitution position, and interacting groups, have been explored. Their individual contribution to the inhibitory activities against human IMPDH1 and IMPDH2 as well as HDAC has been assessed.

Effects of WY-14,643 on the phosphorylation and activation of AMP-dependent protein kinase.

BACKGROUND: AMP-dependent protein kinase (AMPK) and peroxisome proliferator-activated receptor (PPAR) alpha facilitate fatty acid oxidation. We have shown that treatment of hepatoma cells with ethanol or feeding ethanol-containing diets to mice inhibited both PPARalpha and AMPK activity. Importantly, WY-14,643 reversed the development of fatty liver in alcohol-fed mice. Whether WY-14,643, a PPARalpha agonist, has any effects on AMPK is not known. The aim of this study was to investigate the effect of WY-14,643 on AMPK activity. METHODS: The effect of WY-14,643 on AMPK phosphorylation and activity were examined in rat hepatoma cells (H4IIEC3). The effect of WY-14,643 on upstream kinases of AMPK, PKC-zeta/LKB1, intracellular AMP:ATP ratio, oxidative stress, and AMPK gene expression were studied. RESULTS: Treatment of the H4IIEC3 cells with WY-14,643 for 24h led to 60% increase in the phosphorylation of AMPK. The effect of WY-14,643 on AMPK phosphorylation is PKC-zeta/LKB1 independent. WY-14,643 did not alter the levels of intracellular AMP:ATP ratio and it did not increase the levels of reactive oxygen species at 24-h of treatment. WY-14,643-induced AMPK alpha subunit expression by 2- to 2.5-fold, but there was no change in AMPKalpha subunit protein at 24h. The effect of WY-14,643 on AMPK phosphorylation did not altered by the presence of an NADPH oxidase inhibitor. CONCLUSIONS: WY-14,643 induced AMPKalpha subunit phosphorylation and the activity of the enzyme. This was associated with induction of AMPKalpha1 and alpha2 mRNA, but the mechanism for this activation is uncertain.

Technical aspects of imaging and transfemoral arterial treatment of N1-S1 tumors in rats: an appropriate model to test the biology and therapeutic response to transarterial treatments of liver cancers.

The present study was undertaken to assess the technical feasibility of transfemoral hepatic artery catheterization in rats and to describe the imaging techniques that can be used on tumors in rats. A total of 106 N1-S1 cells were inoculated into the left lobes of 74 rats. In 17, transfemoral angiography was attempted. Tumor volumes for 2 weeks before angiography were measured with magnetic resonance imaging in 40 animals. Tumors grew in 63 animals. Angiography was successful in 16 rats. Mean tumor volumes were 0.13 mL and 0.9 mL after 1 and 2 weeks, respectively. In conclusion, transfemoral hepatic artery catheterization is feasible in this animal model.

Mycophenolic acid analogs with a modified metabolic profile.

Mycophenolic acid (MPA), a clinically used immunosuppressant, is extensively metabolized into an inactive C7-glucuronide and removed from circulation. To circumvent the metabolic liability imposed by the C7-hydroxyl group, we have designed a series of hybrid MPA analogs based on the pharmacophores present in MPA and new generations of inosine monophosphate dehydrogenase (IMPDH) inhibitors. The synthesis of MPA analogs has been accomplished by an allylic substitution of a common lactone. Biological evaluations of these analogs and a preliminary structure-activity relationship (SAR) are presented.

Translational upregulation of folate receptors is mediated by homocysteine via RNA-heterogeneous nuclear ribonucleoprotein E1 interactions.

Cellular acquisition of folate is mediated by folate receptors (FRs) in many malignant and normal human cells. Although FRs are upregulated in folate deficiency and downregulated following folate repletion, the mechanistic basis for this relationship is unclear. Previously we demonstrated that interaction of an 18-base cis-element in the 5'-untranslated region of FR mRNA and a cystolic trans-factor (heterogeneous nuclear ribonucleoprotein E1 [hnRNP E1]) is critical for FR synthesis. However, the molecular mechanisms controlling this interaction, especially within the context of FR regulation and folate status, have remained obscure. Human cervical carcinoma cells exhibited progressively increasing upregulation of FRs after shifting of folate-replete cells to low-folate media, without a proportionate rise in FR mRNA or rise in hnRNP E1. Translational FR upregulation was accompanied by a progressive accumulation of the metabolite homocysteine within cultured cells, which stimulated interaction of the FR mRNA cis-element and hnRNP E1 as well as FR biosynthesis in a dose-dependent manner. Abrupt reversal of folate deficiency also led to a rapid parallel reduction in homocysteine and FR biosynthesis to levels observed in folate-replete cells. Collectively, these results suggest that homocysteine is the key modulator of translational upregulation of FRs and establishes the linkage between perturbed folate metabolism and coordinated upregulation of FRs.

Dual inhibitors of inosine monophosphate dehydrogenase and histone deacetylases for cancer treatment.

Mycophenolic acid (MPA), an inhibitor of IMP-dehydrogenase (IMPDH), is used worldwide in transplantation. Recently, numerous studies showed its importance in cancer treatment. Consequently, MPA entered clinical trials in advanced multiple myeloma patients. Suberoylanilide hydroxamic acid (SAHA), a potent differentiation agent acting through inhibition of histone deacetylases (HDACs), was recently approved for treatment of cutaneous T cell lymphoma. We report herein the synthesis of dual inhibitors of IMPDH and HDACs. We found that mycophenolic hydroxamic acid (9, MAHA) inhibits both IMPDH (Ki=30 nM) and HDAC (IC50=5.0 microM). A modification of SAHA with groups known to interact with IMPDH afforded a SAHA analogue 14, which inhibits IMPDH (Ki=1.7 microM) and HDAC (IC50=0.06 microM). Both MAHA (IC50=4.8 microM) and SAHA analogue 14 (IC50=7.7 microM) were more potent than parent compounds as antiproliferation agents. They were also significantly more potent as differentiation inducers.

Probing binding requirements of type I and type II isoforms of inosine monophosphate dehydrogenase with adenine-modified nicotinamide adenine dinucleotide analogues.

Novel tiazofurin adenine dinucleotide (TAD) analogues 25-33 containing a substituent at C2 of the adenine ring have been synthesized as inhibitors of the two isoforms of human IMP-dehydrogenase. The 2-ethyl TAD analogue 33 [Ki = 1 nM (type I), Ki = 14 nM (type II)] was found to be the most potent. It did not inhibit three other cellular dehydrogenases up to 50 microM. Mycophenolic adenine bis(phosphonate)s containing a 2-phenyl (37) or 2-ethyl group (38), were prepared as metabolically stable compounds, both nanomolar inhibitors. Compound 38 [Ki = 16 nM (type I), Ki = 38 nM (type II)] inhibited proliferation of leukemic K562 cells (IC50 = 1.1 microM) more potently than tiazofurin (IC50 = 12.4 microM) or mycophenolic acid (IC50 = 7.7 microM).

Efficient synthesis of benzamide riboside, a potential anticancer agent.

An efficient five step synthesis of benzamide riboside (BR) amenable for a large scale synthesis has been developed. It allows for extensive pre-clinical studies of BR as a potential anticancer agent.

Antitumor activity of C-methyl-beta-D-ribofuranosyladenine nucleoside ribonucleotide reductase inhibitors.

A series of adenosine derivatives substituted at the 1'-, 2'-, or 3'-position of the ribose ring with a methyl group was synthesized and evaluated for antitumor activity. From this study 3'-C-methyladenosine (3'-Me-Ado) emerged as the most active compound, showing activity against human myelogenous leukemia K562, multidrug resistant human leukemia K562IU, human promyelocytic leukemia HL-60, human colon carcinoma HT-29, and human breast carcinoma MCF-7 cell lines with IC(50) values ranging from 11 to 38 muM. Structure-activity relationship studies showed that the structure of 3'-Me-Ado is crucial for the activity. Substitution of a hydrogen atom of the N(6)-amino group with a small alkyl or cycloalkyl group, the introduction of a chlorine atom in the 2-position of the purine ring, or the moving of the methyl group from the 3'-position to other ribose positions brought about a decrease or loss of antitumor activity. The antiproliferative activity of 3'-Me-Ado appears to be related to its ability to deplete both intracellular purine and pyrimidine deoxynucleotides through ribonucleotide reductase inhibition.

Characterization of human brain nicotinamide 5'-mononucleotide adenylyltransferase-2 and expression in human pancreas.

NMNAT (nicotinamide 5'-mononucleotide adenylyltransferase; EC 2.7.7.1) catalyses the transfer of the adenylyl group from ATP to NMN to form NAD. We have cloned a novel human NMNAT cDNA, designated hNMNAT-2, from human brain. The cDNA contains a 924 bp open reading frame that encodes a 307 amino acid peptide that was expressed as a histidine-patch-containing thioredoxin fusion protein. Expressed hNMNAT-2 shared only 35% amino acid sequence homology with the human NMNAT enzyme (hNMNAT-1), but possessed enzymic activity comparable with hNMNAT-1. Using human genomic databases, hNMNAT-2 was localized to chromosome 1q25 within a 171 kb gene, whereas hNMNAT-1 is on chromosome 1p32-35. Northern blot analysis revealed highly restricted expression of hNMNAT-2 to brain, heart and muscle tissues, which contrasts with the wide tissue expression of hNMNAT-1; different regions of the brain exhibited differential expression of hNMNAT-2. Substitution mutations of either of two invariant residues, His-24 or Trp-92, abolished enzyme activity. Anti-peptide antibody to a unique epitope within hNMNAT-2 was produced, and immunohistochemical analysis of sections of normal adult human pancreas revealed that hNMNAT-2 protein was markedly expressed in the islets of Langerhans. However, the pancreatic exocrine cells exhibited weak expression of hNMNAT-2 protein. Sections of pancreas from insulinoma patients showed strong expression of hNMNAT-2 protein in the insulin-producing tumour cells, whereas acinar cells exhibited relatively low expression of hNMNAT-2 protein. These data suggest that the unique tissue-expression patterns of hNMNAT-2 reflect distinct functions for the isoforms in the regulation of NAD metabolism.

Modulation of cytotoxicity of benzamide riboside by expression of NMN adenylyltransferase.

Benzamide riboside (BR) is a nucleoside prodrug that is phosphorylated to its 5'-monophosphate (BRMP) and then converted to its active metabolite, BAD (benzamide adenine dinucleotide), an analogue of NAD by the action of NMN adenylyltransferase (NMNAT). BAD is a potent, reversible, and noncompetitive inhibitor of inosine 5'-monophosphate dehydrogenase (IMPDH) resulting in depletion of guanylates (GTP and dGTP). IMPDH inhibitors such as BR induce differentiation and apoptosis as a consequence of GTP depletion. Tiazofurin (TR) and selenazofurin (SR) require similar metabolism by NMNAT. NMNAT is the rate-limiting step in the synthesis of NAD and NAD analogues. BR- and TR-sensitive leukemic cells contain high NMNAT activity, whereas resistant clones have greatly downregulated NMNAT activity (<0.1% of wild type). Perhaps the applicability of BR and analogues could be enhanced if combined with NMNAT gene expression in BR-resistant leukemic blasts. NAD has important regulatory role in repair of DNA damage and cell growth since it is a substrate for poly(ADP-ribose) polymerase (PARP). PARP appears to direct short-patch base excision repair and induce p53 upregulation leading to apoptosis. BR inhibits PARP at high concentrations when assayed in permeabilized leukemic cells. Several other IMPDH inhibitors (TR, mycophenolic acid, and ribavirin) exhibit similar PARP inhibitory activity. Although this inhibition was reversible, it was not prevented by the addition of guanosine, GTP, or its nonhydrolyzable analog gamma-S-GTP. Therefore, it can be concluded that IMPDH inhibitors directly inhibit PARP. Presumably, the shared IMP-NAD active site of IMPDH has a similar architecture to the NAD-binding pocket of PARP.

Studies on the mechanism of action of benzamide riboside: a novel inhibitor of IMP dehydrogenase.

Benzamide is a well known inhibitor of poly(ADP-ribose)polymerase, an enzyme involved in DNA repair. However, benzamide exhibited neuotoxicity in animals and hence, in the hope of overcoming this problem, benzamide riboside (BR) was synthesized. Our mechanism of action studies on BR suggested that the agent was being metabolized to its 5'-monophosphate and then to its NAD analogue (BAD, benzamide adenine dinucleotide) that inhibits Inosine 5'-monophosphate dehydrogenase (IMPDH). IMPDH is the rate-limiting enzyme of the branched purine nucleotide synthetic pathway that provides guanylates including GTP and dGTP. There are two isoforms of IMPDH, type I that is constitutively present in all cells, and type II that is inducible and is present in highly proliferating cells such as cancer. Ongoing studies with BR analogues suggest that they are more selective in inhibiting IMPDH type II. Our studies have characterized the metabolites of BR, especially its NAD analogue, BAD, by synthesizing this active metabolite by enzymatic means, and identifying its structure by NMR and mass spectrometry. We have partially purified IMPDH from tumor cells and have examined the kinetics of inhibition of IMPDH by BAD. We have also compared biochemical and cytotoxic activities of BR with tiazofurin and selenazofurin, that share similar mechanisms of action with BR. Our studies demonstrated that 2-3-fold more BAD is formed compared to TAD and SAD, the active metabolites of tiazofurin and selenazofurin, respectively. BR has demonstrated potent cytotoxic activity in a diverse group of human tumor cells, specifically more active in sarcomas and CNS neoplasms compared to tiazofurin or selenazofurin. Future in vivo animal studies should set a stage for determining its effectiveness in clinical Phase I studies.

Toxicity and efficacy of benzamide riboside in cancer chemotherapy models.

Benzamide riboside (BR), a synthetic C-nucleoside, acts as a strong growth inhibitor of cancer cells in vitro and in vivo. BR, like TR and related nucleoside prodrugs, act by anabolism to NAD analogs. These analogs selectively inhibit IMPDH, leading to depletion of cellular GTP, growth cessation, and cell differentiation. To date only preclinical studies have been carried out. However, in tiazofurin (TR), a related drug, phase I/II clinical trials have been conducted in patients with acute leukemia and shown to be a very promising agent with a response rate of 85% in 26 patients in one of the trials. Tiazofurin is now undergoing phase III clinical trials as a result. Dose limiting toxicity of tiazofurin was headache, somnolence and nausea with no myelosuppression noted. By contrast, BR showed skeletal muscle toxicity, hepatotoxicity and myelosuppression in preclinical data. Skeletal muscle toxicity was noted in the paraspinal muscles and may represent dose-limiting toxicity. Since BR does exhibit myelosuppression, the most common chemotherapy-related side effect in humans, careful judgment is warranted should BR be included in multidrug regimens, although BR's potent cytotoxicity to tumor cells in preclinical models still makes it a promising drug.

The chemistry of nicotinamide adenine dinucleotide (NAD) analogues containing C-nucleosides related to nicotinamide riboside.

Oncolytic C-nucleosides, tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide) and benzamide riboside (3-beta-D-ribofuranosylbenzamide) are converted in cell into active metabolites thiazole-4-carboxamide- and benzamide adenine dinucleotide, TAD and BAD, respectively. TAD and BAD as NAD analogues were found to bind at the nicotinamide adenine dinucleotide (cofactor NAD) site of inosine monophosphate dehydrogenase (IMPDH), an important target in cancer treatment. The synthesis and evaluation of anticancer activity of a number of C-nucleosides related to tiazofurin and nicotinamide riboside then followed and are reviewed herein. Interestingly, pyridine C-nucleosides (such as C-nicotinamide riboside) are not metabolized into the corresponding NAD analogues in cell. Their conversion by chemical methods is described. As dinucleotides these compounds show inhibition of IMPDH in low micromolar level. Also, the synthesis of BAD in metabolically stable bis(phosphonate) form is discussed indicating the usefulness of such preformed inhibitors in drug development. Among tiazofurin analogues, Franchetti and Grifantini found, that the replacement of the sulfur by oxygen (as in oxazafurin) but not the removal of nitrogen (tiophenfurin) of the thiazole ring resulted in inactive compounds. The anti cancer activity of their synthetic dinucleotide analogues indicate that inactive compounds are not only poorly metabolized in cell but also are weak inhibitors of IMPDH as dinucleotides.