The importance of the 4'-hydroxyl hydrogen for the anti-trypanosomal and antiviral properties of (+)-5'-noraristeromycin and two 7-deaza analogues

Novel 5'-Norcarbocyclic Pyrimidine Derivatives as Antibacterial Agents

A series of novel 5'-norcarbocyclic derivatives of 5-alkoxymethyl or 5-alkyltriazolyl-methyl uracil were synthesized and the activity of the compounds evaluated against both Gram-positive and Gram-negative bacteria. The growth of Mycobacterium smegmatis was completely inhibited by the most active compounds at a MIC99 of 67 μg/mL (mc²155) and a MIC99 of 6.7⁻67 μg/mL (VKPM Ac 1339). Several compounds also showed the ability to inhibit the growth of attenuated strains of Mycobacterium tuberculosis ATCC 25177 (MIC99 28⁻61 μg/mL) and Mycobacterium bovis ATCC 35737 (MIC99 50⁻60 μg/mL), as well as two virulent strains of M. tuberculosis; a laboratory strain H37Rv (MIC99 20⁻50 μg/mL) and a clinical strain with multiple drug resistance MS-115 (MIC99 20⁻50 μg/mL). Transmission electron microscopy (TEM) evaluation of M. tuberculosis H37Rv bacterial cells treated with one of the compounds demonstrated destruction of the bacterial cell wall, suggesting that the mechanism of action for these compounds may be related to their interactions with bacteria cell walls.

The evolution of nucleoside analogue antivirals: A review for chemists and non-chemists. Part 1: Early structural modifications to the nucleoside scaffold

This is the first of two invited articles reviewing the development of nucleoside-analogue antiviral drugs, written for a target audience of virologists and other non-chemists, as well as chemists who may not be familiar with the field. Rather than providing a simple chronological account, we have examined and attempted to explain the thought processes, advances in synthetic chemistry and lessons learned from antiviral testing that led to a few molecules being moved forward to eventual approval for human therapies, while others were discarded. The present paper focuses on early, relatively simplistic changes made to the nucleoside scaffold, beginning with modifications of the nucleoside sugars of Ara-C and other arabinose-derived nucleoside analogues in the 1960's. A future paper will review more recent developments, focusing especially on more complex modifications, particularly those involving multiple changes to the nucleoside scaffold. We hope that these articles will help virologists and others outside the field of medicinal chemistry to understand why certain drugs were successfully developed, while the majority of candidate compounds encountered barriers due to low-yielding synthetic routes, toxicity or other problems that led to their abandonment.

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This is the first of two invited articles reviewing the development of nucleoside-analogue antiviral drugs.

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It is written for a target audience of virologists and other non-chemists, and for chemists unfamiliar with the field.

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Numerous modifications have been made to the nucleoside scaffold in order to impart therapeutic benefits.

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Nucleoside modifications led to the development of potent antivirals such as acyclovir, entecavir, and tenofovir.

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We examine thought processes, progress in synthetic chemistry and results of antiviral testing that led to approved drugs.

Evaluation of NAD(H) analogues as selective inhibitors for Trypanosoma cruzi S-adenosylhomocysteine hydrolase

S-Adenosylhomocysteine (AdoHcy) hydrolases (SAHHs) from human sources (Hs-SAHHs) bind the cofactor NAD(+) more tightly than several parasitic SAHHs by around 1000-fold. This property suggests the cofactor binding site of this essential enzyme as a potential anti-parasitic drug target, e.g., against SAHH from Trypansoma cruzi (Tc-SAHH). The on-rate and off-rate constants and the equilibrium dissociation constants were determined for NAD(+)/NADH analogues and suggested that NADH analogues were the most promising for selective inhibition of Tc-SAHH. None significantly inhibited Hs-SAHH while S-NADH and H-NADH (see Figure 1) reduced the catalytic activity of Tc-SAHH to < 10% in six minutes of exposure.

Comparative kinetics of cofactor association and dissociation for the human and trypanosomal S-adenosylhomocysteine hydrolases. 2. The role of helix 18 stability

The S-adenosyl- l-homocysteine (AdoHcy) hydrolases (SAHH) from Homo sapiens (Hs-SAHH) and from the parasite Trypanosoma cruzi (Tc-SAHH) are very similar in structure and catalytic properties but differ in the kinetics and thermodynamics of association and dissociation of the cofactor NAD (+). The binding of NAD (+) and NADH in SAHH appears structurally to be mediated by helix 18, formed by seven residues near the C-terminus of the adjacent subunit. Helix-propensity estimates indicate decreasing stability of helix 18 in the order Hs-SAHH > Tc-SAHH > Ld-SAHH (from Leishmania donovani) > Pf-SAHH (from Plasmodium falciparum), which would be consistent with the previous observations. Here we report the properties of Hs-18Pf-SAHH, the human enzyme with plasmodial helix 18, and Tc-18Hs-SAHH, the trypanosomal enzyme with human helix 18. Hs-18Tc-SAHH, the human enzyme with trypanosomal helix 18, was also prepared but differed insignificantly from Hs-SAHH. Association of NAD (+) with Hs-SAHH, Hs-18Pf-SAHH, Tc-18Hs-SAHH, and Tc-SAHH exhibited biphasic kinetics for all enzymes. A thermal maximum in rate, attributed to the onset of local structural alterations in or near the binding site, occurred at 35, 33, 30, and 15 degrees C, respectively. This order is consistent with some reversible changes within helix 18 but does require influence of other properties of the "host enzyme". Dissociation of NAD (+) from the same series of enzymes also exhibited biphasic kinetics with a transition to faster rates (a larger entropy of activation more than compensates for a larger enthalpy of activation) at temperatures of 41, 38, 36, and 29 degrees C, respectively. This order is also consistent with changes in helix 18 but again requiring influence of other properties of the "host enzyme". Global unfolding of all fully reconstituted holoenzymes occurred around 63 degrees C, confirming that the kinetic transition temperatures did not arise from a major disruption of the protein structure.

John Montgomery's legacy: carbocyclic adenosine analogues as SAH hydrolase inhibitors with broad-spectrum antiviral activity

Ever since the S-adenosylhomocysteine (AdoHcy, SAH) hydrolase was recognized as a pharmacological target for antiviral agents (J. A. Montgomery et al., J. Med. Chem. 25:626-629, 1982), an increasing number of adenosine, acyclic adenosine, and carbocyclic adenosine analogues have been described as potent SAH hydrolase inhibitors endowed with broad-spectrum antiviral activity. The antiviral activity spectrum of the SAH hydrolase inhibitors include pox-, rhabdo-, filo-, arena-, paramyxo-, reo-, and retroviruses. Among the most potent SAH hydrolase inhibitors and antiviral agents rank carbocyclic 3-deazaadenosine (C-c3 Ado), neplanocin A, 3-deazaneplanocin A, the 5'-nor derivatives of carbocyclic adenosine (C-Ado, aristeromycin), and the 2-halo (i.e., 2-fluoro) and 6'-R-alkyl (i.e., 6'-R-methyl) derivatives of neplanocin A. These compounds are particularly active against poxviruses (i.e., vaccinia virus), and rhabdoviruses (i.e., vesicular stomatitis virus). The in vivo efficacy of C-c3 Ado and 3-deazaneplanocin A has been established in mouse models for vaccinia virus, vesicular stomatitis virus, and Ebola virus. SAH hydrolase inhibitors such as C-c3Ado and 3-deazaneplanocin A should in thefirst place be considered for therapeutic (or prophylactic) use against poxvirus infections, including smallpox, and hemorrhagic fever virus infections such as Ebola.

Trypanosoma brucei: expression of multiple purine transporters prevents the development of allopurinol resistance

Allopurinol is a hypoxanthine analogue used to treat Leishmania infections that also displays activity against the related parasite Trypanosoma brucei. We have investigated the ease by which resistance to this drug is established in Trypanosoma brucei brucei and correlated this to the mechanisms by which it is accumulated by the parasite. Long-term exposure of procyclic T. b. brucei to 3mM allopurinol did not induce resistance. This appears to be related to the fact that allopurinol was taken up through two distinct nucleobase transporters, H1 and H4, both with high affinity for the drug. The apparent Km for [3H]allopurinol transport by H4 (2.1+/-0.4 microM) was determined by expressing the encoding gene in Saccharomyces cerevisiae. Long-term allopurinol exposure did not change Km (hypoxanthine), Ki (allopurinol), or Vmax values of either H1 or H4 transporters and the cells retained their ability to proliferate with hypoxanthine as sole purine source. This study shows that transport-related resistance to purine antimetabolites is not easily induced in Trypanosoma spp. as long as uptake is mediated by multiple transporters.

Overexpression, purification, and characterization of S-adenosylhomocysteine hydrolase from Leishmania donovani

The gene encoding S-adenosylhomocysteine (AdoHcy) hydrolase in Leishmania donovani was subcloned into an expression vector (pPROK-1) and expressed in Escherichia coli. Recombinant L. donovani AdoHcy hydrolase was then purified from cell-free extracts of E. coli using three chromatographic steps (DEAE-cellulose chromatofocusing, Sephacryl S-300 gel filtration, and Q-Sepharose ion exchange). The purified recombinant L. donovani enzyme exists as a tetramer with a molecular weight of approximately 48 kDa for each subunit. Unlike recombinant human AdoHcy hydrolase, the catalytic activity of the recombinant L. donovani enzyme was shown to be dependent on the concentration of NAD+ in the incubation medium. The dissociation constant (Kd) for NAD+ with the L. donovani enzyme was estimated to be 2.1 +/- 0.2 microM. The Km values for the natural substrates of the enzyme, AdoHcy, Ado, and Hcy, were determined to be 21 +/- 3, 8 +/- 2, and 82 +/- 5 microM, respectively. Several nucleosides and carbocyclic nucleosides were tested for their inhibitory effects on this parasitic enzyme, and the results suggested that L. donovani AdoHcy hydrolase has structural requirements for binding inhibitors different than those of the human enzyme. Thus, it may be possible to eventually exploit these differences to design specific inhibitors of this parasitic enzyme as potential antiparasitic agents.

The latest progress in the synthesis of carbocyclic nucleosides

This review presents the latest developments in the field of carba-nucleosides (1994-1998). Special attention is paid to the synthesis of key precursors to those carba-nucleosides that possess significant biological activities or have novel structures.