Role of pH in Regulating Cancer Pyrimidine Synthesis

Replication is a fundamental aspect of cancer, and replication is about reproducing all the elements and structures that form a cell. Among them are DNA, RNA, enzymes, and coenzymes. All the DNA is doubled during each S (synthesis) cell cycle phase. This means that six billion nucleic acids must be synthesized in each cycle. Tumor growth, proliferation, and mutations all depend on this synthesis. Cancer cells require a constant supply of nucleotides and other macromolecules. For this reason, they must stimulate de novo nucleotide synthesis to support nucleic acid provision. When deregulated, de novo nucleic acid synthesis is controlled by oncogenes and tumor suppressor genes that enable increased synthesis and cell proliferation. Furthermore, cell duplication must be achieved swiftly (in a few hours) and in the midst of a nutrient-depleted and hypoxic environment. This also means that the enzymes participating in nucleic acid synthesis must work efficiently. pH is a critical factor in enzymatic efficiency and speed. This review will show that the enzymatic machinery working in nucleic acid synthesis requires a pH on the alkaline side in most cases. This coincides with many other pro-tumoral factors, such as the glycolytic phenotype, benefiting from an increased intracellular pH. An increased intracellular pH is a perfect milieu for high de novo nucleic acid production through optimal enzymatic performance.

Synthesis and biological evaluation of 6-substituted-5-fluorouridine ProTides

A new family of thirteen phosphoramidate prodrugs (ProTides) of different 6-substituted-5-fluorouridine nucleoside analogues were synthesized and evaluated as potential anticancer agents. In addition, antiviral activity against Chikungunya (CHIKV) virus was evaluated using a cytopathic effect inhibition assay. Although a carboxypeptidase Y assay supported a putative mechanism of activation of ProTides built on 5-fluorouridine with such C6-modifications, the Hint docking studies revealed a compromised substrate-activity for the Hint phosphoramidase-type enzyme that is likely responsible for phosphoramidate bioactivation through P-N bond cleavage and free nucleoside 5'-monophosphate delivery. Our observations may support and explain to some extent the poor in vitro biological activity generally demonstrated by the series of 6-substituted-5-fluorouridine phosphoramidates (ProTides) and will be of guidance for the design of novel phosphoramidate prodrugs.

Design of inhibitors of ODCase

ODCase is a highly proficient enzyme responsible for the decarboxylation of orotidine monophosphate to generate uridine monophosphate. ODCase has attracted early attention due to its interesting mechanism of catalysis. In order to exploit therapeutic advantages due to the inhibition of ODCase, one must have selective inhibitors of this enzyme from the pathogen, or a dysregulated molecular mechanism involving ODCase. ODCase inhibitors have potential applications as anticancer agents, antiviral agents, antimalarial agents and potentially act against other parasitic diseases. A variety of C6-substituted uridine monophosphate derivatives have shown excellent inhibition of ODCase. 6-iodouridine is a potent inhibitor of the malaria parasite, and its monophosphate form covalently inhibits ODCase. A variety of inhibitors of ODCase with potential applications as therapeutic agents are discussed in this review.

Structure-activity relationships of orotidine-5'-monophosphate decarboxylase inhibitors as anticancer agents

A series of 6-substituted and 5-fluoro-6-substituted uridine derivatives were synthesized and evaluated for their potential as anticancer agents. The designed molecules were synthesized from either fully protected uridine or the corresponding 5-fluorouridine derivatives. The mononucleotide derivatives were used for enzyme inhibition investigations against ODCase. Anticancer activities of all the synthesized derivatives were evaluated using the nucleoside forms of the inhibitors. 5-Fluoro-UMP was a very weak inhibitor of ODCase. 6-Azido-5-fluoro and 5-fluoro-6-iodo derivatives are covalent inhibitors of ODCase, and the active site Lys145 residue covalently binds to the ligand after the elimination of the 6-substitution. Among the synthesized nucleoside derivatives, 6-azido-5-fluoro, 6-amino-5-fluoro, and 6-carbaldehyde-5-fluoro derivatives showed potent anticancer activities in cell-based assays against various leukemia cell lines. On the basis of the overall profile, 6-azido-5-fluoro and 6-amino-5-fluoro uridine derivatives exhibited potential for further investigations.

Treatment of yellow fever

Yellow fever (YF) is a life-threatening mosquito-borne flaviviral hemorrhagic fever (VHF) characterized by severe hepatitis, renal failure, hemorrhage, and rapid terminal events with shock and multi-organ failure. A live, attenuated vaccine (YF 17D), in wide use for over 60 years, causes a disease identical to wild-type virus at an incidence of 2.5x10(-6). Our current understanding of the pathogenesis and treatment of YF (described in this brief review) is derived from studies of animal models (macaques, hamsters) that reproduce the features of human YF and from descriptive studies of human cases of naturally acquired and vaccine-associated VHF. The least understood, but potentially most important terminal events appear to be due to 'cytokine storm' and represent a potential target for therapeutic interventions. Areas for future study include dissection of cytokine-mediated events in animal models, the pathogenic role of the profound neutrophilia that occurs pre-terminally, the (pathological) role of adaptive immune clearance in pathogenesis, and treatments directed at cytokine storm. Antibody, interferon-alpha, polyICLC and other immune modulators are highly effective when administered before or within a narrow time window after infection, but are ineffective when given after the infection is established. A few antivirals have been evaluated (ribavirin, tiazofurin, carboxamide, pyrazoline compounds). Ribavirin has been used successfully to treat hamsters when the drug is given at high doses up to 2 days after virus infection (shortly before liver infection), but has not shown promise in nonhuman primate models. Future work should focus on evaluating higher doses of ribavirin alone or in combinations with potentially synergistic drugs, including interferons. Also specific inhibitors against other flaviviruses such as dengue virus should be investigated for potential pan-flavivirus activity since recent studies have shown that specific targets such as the flavivirus proteases and helicases are very similar in structure.

Design of inhibitors of orotidine monophosphate decarboxylase using bioisosteric replacement and determination of inhibition kinetics

Inhibitors of orotidine monophosphate decarboxylase (ODCase) have applications in RNA viral, parasitic, and other infectious diseases. ODCase catalyzes the decarboxylation of orotidine monophosphate (OMP), producing uridine monophosphate (UMP). Novel inhibitors 6-amino-UMP and 6-cyano-UMP were designed on the basis of the substructure volumes in the substrate OMP and in an inhibitor of ODCase, barbituric acid monophosphate, BMP. A new enzyme assay method using isothermal titration calorimetry (ITC) was developed to investigate the inhibition kinetics of ODCase. The reaction rates were measured by monitoring the heat generated during the decarboxylation reaction of orotidine monophosphate. Kinetic parameters (k(cat) = 21 s(-1) and KM = 5 microM) and the molar enthalpy (DeltaH(app) = 5 kcal/mol) were determined for the decarboxylation of the substrate by ODCase. Competitive inhibition of the enzyme was observed and the inhibition constants (Ki) were determined to be 12.4 microM and 29 microM for 6-aza-UMP and 6-cyano-UMP, respectively. 6-Amino-UMP was found to be among the potent inhibitors of ODCase, having an inhibition constant of 840 nM. We reveal here the first inhibitors of ODCase designed by the principles of bioisosterism and a novel method of using isothermal calorimetry for enzyme inhibition studies.

6-azapyrimidine-2'-deoxy-4'-thionucleosides: antiviral agents against TK+ and TK- HSV and VZV strains

The synthesis of a series of novel 1-(2-deoxy-4-thio-beta-D-erythro-pentofuranosyl)-(6-azapyrimidine) nucleosides is described. X-ray crystallographic data of the thymidine derivative allowed conformational analysis, which indicated a twist (3T2) sugar conformation. Hydrogen-bonded assemblies for the crystal structure were determined using PLATON software to allow further interpretation of the crystal packing and base interactions. The 6-azapyrimidine nucleosides described were evaluated against a range of viral strains. The thymidine analogue showed pronounced activity against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), varicella-zoster virus (VZV), and vaccinia virus. This compound lost only 5- to 10-fold of its antiviral activity against thymidine kinase (TK)-deficient HSV-1 and VZV strains. These observations suggest that the compounds may not entirely depend on viral TK-catalyzed phosphorylation for antiviral activity and/or use an alternative metabolic activation pathway, and/or display a unique mechanism of antiviral action by the unmetabolized nucleoside analogue.

Minireview: nucleotide prodrugs

Nucleotides have shown interesting biological activities in a wide variety of antiviral, antiproliferative, immunomodulatory and other biological assays, and they present promising drug candidates. Because of their negative charge(s) nucleotides suffer from some disadvantages which can be successfully overcome by the utilization of nucleotide prodrugs. Nucleotide prodrugs were successfully used to increase oral absorption of nucleotides in vivo. By taking advantage of intracellular triggers (reducing potential, enzyme activity, pH), nucleotide prodrugs can be used in vitro for the intracellular delivery of the nucleotide resulting in enhanced potency and in some cases enhanced selectivity. Nucleotide prodrugs have also been utilized for tissue specific delivery of the nucleotides in vivo resulting in altered selectivity and reduced toxicity. For nucleotide prodrugs, their ultimate intended use is (in most cases) in vivo for the treatment of a disease. Thus, it is important to incorporate adequate assays and design criteria into any prodrug effort. In vivo systems are complicated because of metabolism, excretion and tissue distribution of the prodrug and the parent. Thus, results of in vitro assays have to be interpreted cautiously because they may be unsuitable predictors of the in vivo situation.