Prodrug Strategies for the Development of β-l-5-((E)-2-Bromovinyl)-1-((2S,4S)-2-(hydroxymethyl)-1,3-(dioxolane-4-yl))uracil (l-BHDU) against Varicella Zoster Virus (VZV)

Varicella zoster virus (VZV) establishes lifelong infection after primary disease and can reactivate. Several drugs are approved to treat VZV diseases, but new antivirals with greater potency are needed. Previously, we identified β-l-5-((E)-2-bromovinyl)-1-((2S,4S)-2-(hydroxymethyl)-1,3-(dioxolane-4-yl))uracil (l-BHDU, 1), which had significant anti-VZV activity. In this communication, we report the synthesis and evaluation of numerous l-BHDU prodrugs: amino acid esters (14-26), phosphoramidates (33-34), long-chain lipids (ODE-l-BHDU-MP, 38, and HDP-l-BHDU-MP, 39), and phosphate ester prodrugs (POM-l-BHDU-MP, 41, and POC-l-BHDU-MP, 47). The amino acid ester l-BHDU prodrugs (l-phenylalanine, 16, and l-valine, 17) had a potent antiviral activity with EC₅₀ values of 0.028 and 0.030 μM, respectively. The phosphate ester prodrugs POM-l-BHDU-MP and POC-l-BHDU-MP had a significant anti-VZV activity with EC₅₀ values of 0.035 and 0.034 μM, respectively, and no cellular toxicity (CC₅₀ > 100 μM) was detected. Out of these prodrugs, ODE-l-BHDU-MP (38) and POM-l-BHDU-MP (41) were selected for further evaluation in future studies.

Synthesis of 2'-deoxy-2'-fluoro-2'-C-methyl spiro cyclopentyl carbocyclic uridine analog as potential inhibitors of HCV NS5B polymerase

Synthesis of 1-((4 R,5S,6R,7R)-5,6-dihydroxy-7-(hydroxymethyl)spiro[2.4]heptan-4-yl)pyrimidine-2,4(1H,3H)-dione (12) and its phosphoramidate prodrug 18 is reported. The synthesis of the targeted compound 12 was initiated from triol 1. By the introduction of a substituent methylene group at 6-position of 4, followed by Simmons-Smith cyclopropanation and amination, key intermediate 10 was synthesized. The intermediate amine 10 was utilized to synthesize the nucleoside 12. Furthermore, the nucleoside 12 was derivatized to 2'-α-hydroxy-2'-β-methyl (23) and 2'-α-fluoro-2'-β-methyl (27) analogs. All synthesized derivatives of spiro-cyclopropyl carbocyclic uridine analogs 12, 18, 23 and 27 were evaluated for anti-HCV activity, but none of the compounds, reported in this article show any anti-HCV activity.

Synthesis of an Anti-hepatitis B Agent, 2'-Fluoro-6'-methylene-carbocyclic Adenosine (FMCA) and Its Phosphoramidate (FMCAP)

2'-Fluoro-6'-methylene-carbocyclic adenosine (FMCA, 12) and its phosphoramidate prodrug (FMCAP, 14) have been proven as a potential anti-HBV agent against both adefovir-resistant as well as lamivudine-resistant double (rtL180M/rtM204V) mutants. Furthermore, in vitro, these agents have demonstrated significant activity against lamivudine/entecavir triple mutants (L180M + S202G + M204V). These preliminary results encourage us for further biological evaluation of FMCA and FMCAP to develop as a potential clinical candidate as an anti-HBV agent, which may overcome the problem of drug resistance in HBV therapy. To support the preclinical exploration, a scalable synthesis of this molecule was needed. In this communication, a practical and scalable synthesis of FMCA, and its prodrug, is reported via ketone 1. The selective opening of the isopropylidene group of 2 led to compound 3. Protection of the allylic hydroxyl group of 3, followed by fluorination and deprotection, afforded the key intermediate 10, which was condensed with a Boc-protected adenine, followed by deprotection, furnished the target nucleoside FMCA (12) in high yield. Further coupling of phosphorochloridate of L-alanine isopropyl ester (13) with FMCA gave its phosphoramidate prodrug FMCAP (14) in good yield.

Mechanism of Adefovir, Tenofovir and Entecavir Resistance: Molecular Modeling Studies of How A Novel Anti-HBV Agent (FMCA) Can Overcome the Drug Resistance

Regardless of significant improvement in the area of anti-HBV therapy, resistance and cross-resistance against available therapeutic agents are the major consideration in drug discovery of new agents. The present study is to obtain the insight of the molecular basis of drug resistance conferred by the B and C domain mutations of HBV-polymerase on the binding affinity of four anti-HBV agents [Adefovir (ADV), Tenofovir (TNF), Entecavir (ETV) & 2'-Fluoro-6'-methylene-carbocyclic adenosine (FMCA)]. In this regard, homology modeled structure of HBV polymerase was used for minimization, conformational search and Glide XP docking followed by binding energy calculation on wild-type as well as on mutant HBV-polymerases (N236T, L180M+M204V+S202G & A194T). Our studies suggest a significant correlation between the fold resistances and the binding affinity of anti-HBV nucleosides. The domain B residue, L180 is indirectly associated with other active-site hydrophobic residues such as A87, F88 and M204, whereas the domain C residue, M204 is closely associated with sugar/pseudosugar ring positioning in the active site. These hydrophobic residues can directly influence the interaction of the incoming nucleoside triphosphates and change the binding efficacy. The carbohydrate ring part of natural substrate dATP, dGTP, FMCA and ETV, are occupied in similar passion in the grooves of HBV polymerase active site. The exocyclic double bond of Entecavir and FMCA occupies in the backside hydrophobic pocket (made by residues A87, F88, L180and M204), which enhances the overall binding affinity. Additional hydrogen bonding interaction of 2'-fluorine of FMCA with R41 residue of polymerase promotes a positive binding in wild-type as well as in ADVr, ETVr and TNFr with respect to that of entecavir.

Lymphatic Targeting of anti-HIV Nucleosides: Distribution of 3′-azido-3′-deoxythymidine (AZT) and 3′-azido-2′,3′-dideoxyuridine (AZdU) after Administration of Dipalmitoylphosphatidyl Prodrugs to Mice

Human immunodeficiency virus appears to be proliferating within the lymphatic system throughout the period of clinical latency. Targeting of anti-HIV compounds to the lymphatic tissue may therefore provide therapeutic benefits. The purpose of this investigation was to determine the distribution of 3′-azido-3′-deoxythymidine (AZT) and 3′-azido-2′,3′-dideoxyuridine (AZdU) in lymph nodes in a mouse model after administration of the lipophilic prodrugs dipalmitoylphosphatidyl-azidodeoxythymidine (DPP-AZT) and dipalmitoylphosphatidyl-azidodideoxyuridine (DPP-AZdU). Mice received 50 mg kg−1 of parent nucleoside and 164 mg kg−1 of DPP-AZT (equivalent to 50 mg kg−1 AZT) intravenously or orally and 180mg kg−1 DPP-AZdU (equivalent to 50 mg kg−1 AZdU) orally. Serum, neck, axillary and mesenteric lymph nodes were collected at selected times and AZT and AZdU concentrations were determined by HPLC. The disposition of AZT and AZdU in serum and lymph nodes was significantly altered after intravenous and oral administration of DPP-AZT and oral administration of DPP-AZdU when compared to that after administration of parent nucleoside. Lower peak concentrations of AZT and AZdU were observed in serum and lymph nodes after administration of the phospholipid prodrugs. However, DPP-AZT and DPP-AZdU produced consistently higher concentrations of AZT and AZdU, respectively, 2-3 h after prodrug administration. Half-life values for both nucleosides in serum and lymph nodes were significantly greater after prodrug administration. Greater AUC values for nucleosides were noted in neck (AZT and AZdU) and mesenteric (AZT) lymph nodes after administration of prodrugs compared with values obtained for parent drugs. Furthermore, relative lymph node exposure to AZT and AZdU in the lymph nodes was greater after administration of prodrug than after administration of parent compound. Thus, DPP-AZT and DPP-AZdU show potential as useful prodrugs for the delivery of AZT and AZdU to the lymphatic system.

Mechanism of Antiviral Activities of 3′-Substituted L-Nucleosides against 3Tc-Resistant HBV Polymerase: A Molecular Modelling Approach

Comparison of the active sites of the human HIV-1 reverse transcriptase (RT) and the homology-modelled hepatitis B virus (HBV) polymerase shows that the active sites of both enzymes are open to L-nucleosides, but the position where the 3′-substituent of the L-ribose projects in HBV polymerase is wider and deeper than HIV-1 RT, which enables the HBV polymerase to accommodate various 3′-substituted L-nucleosides. However, the space is not sufficient to accommodate a bulky 3′-substituent such as the 3′-azido group of L-3′-azido-3′-deoxythymidine. Analysis of the minimized structure of rtM204V HBV polymerase/ 3TCTP complex shows that, instead of the steric stress produced by rtV204, a loss of the van der Waals contact around the oxathiolane sugar moiety of 3TCTP caused by the mutation results in the disruption of the active site. Therefore, nucleosides, which are stabilized by additional specific interaction with the enzyme residues, can have more opportunities to circumvent the destabilization by the loss of hydrophobic interaction conferred by mutation. Specifically, the substitution at the 3′-position would be beneficial as the HBV polymerase has wide open space composed of the highly conserved motif (YMDD) where the 3′-substituents of the L-nucleosides project. As an example, our study shows that the 3′-fluorine atom contributes to the antiviral activity of L-3′-Fd4CTP against rtM204V HBV polymerase by readily compensating for the loss of the van der Waals interaction around the 2′,3′-double bond through a formation of a hydrogen bond to the amide backbone of rtD205.

Pharmacokinetics of (-)-β-D-2,6-Diaminopurine Dioxolane and its Metabolite, Dioxolane Guanosine, in Woodchucks (Marmota Monax)

The woodchuck ( Marmota monax) is a useful animal model for evaluating the in-vivo efficacy of antiviral agents against hepatitis B viral infection (HBV). The pharmacokinetics of a newly synthesized antiviral agent (-)-β-D-2,6-diaminopurine dioxolane (DAPD) in woodchucks is reported. DAPD is a nucleoside analogue, having potent and selective activity against human immunodeficiency virus and HBV in vitro. DAPD is susceptible to deamination in vivo by the ubiquitously present enzyme adenosine deaminase yielding the active metabolite dioxolane guanosine (DXG). The pharmacokinetics of DAPD and DXG were characterized following intravenous (i.v.) and oral (p.o.) administration of 20 mg kg−1 of DAPD to woodchucks. Plasma and urine samples were collected, and nucleoside concentrations were determined by HPLC. Following intravenous administration, the half-life of DAPD averaged 6.7 ± 4.3 h, and that of DXG averaged 17.6 ± 14.5 h. The mean total clearance and steady state volume of distribution of DAPD were 0.33 ± 0.14 L h kg−1 and 1.76 ± 0.65 L kg−1, respectively. The oral bioavailability of DAPD ranged from 3.7-8.2%; however, the apparent availability of DXG following oral administration of DAPD was 10.5-53%.

Synthesis of β-Enantiomers of N4-Hydroxy-3′-Deoxy-Pyrimidine Nucleosides and Their Evaluation against Bovine Viral Diarrhoea Virus and Hepatitis C Virus in Cell Culture

N4-Hydroxycytidine (NHC) was recently reported to have anti-pestivirus and anti-hepacivirus activity. It is thought that this nucleoside acts as a weak alternative substrate for the hepatitis C virus (HCV) polymerase. In addition to NHC, 3′-deoxyuridine (3′-dU) was found to inhibit bovine diarrhoea virus (BVDV) production by 1 log10 at 37.2 μM. These initial findings prompted the synthesis of β-D and β-L analogues of (i) base-modified 3′-deoxy-NHC; (ii) 3′-deoxyuridine; and 3′-deoxycytidine. The antiviral activity of these 42 nucleosides was evaluated against BVDV and HCV bicistronic replicon in cell culture. Among the NHC analogues, the antiviral activity observed for the β-L-3′-deoxy-5-fluoro-derivative 1-(3-deoxy-β-L- erythro-pentofuranosyl)-5-fluoro-4-hydrox-yaminopyrimidin-2( 1H)-one and the β-D-3′-deoxy-5-iodo-derivative 1-(3-deoxy-β-D-erythro-pentofuranosyl)-5-iodocytosine in the replicon system (1 log10 reduction at 100 μM) was due to the concomitant toxicity towards intracellular ribosomal RNA levels (CC90 equal or lower than the EC90). In conclusion, none of the newly synthesized derivatives exhibited enhanced antiviral activity compared to the parent nucleoside NHC.

Lymphatic Targeting of Anti-Human Immunodeficiency Virus Nucleosides: Pharmacokinetics of G′-Deoxy-2′,3′-Didehydrothymidine after Intravenous and Oral Administration of Dipalmitoylphosphatidyl Prodrug to Mice

The lymphatic system is a primary target for early anti-human immunodeficiency virus drug therapy. Strategies are currently being sought to enhance the delivery of nucleoside analogues such as 3′-deoxy-2′,3′-didehydrothymidine (stavudine; d4T) toward the lymph and lymph nodes. The purpose of this study was to synthesize dipalmitoylphosphatidyl-d4T (DPP-d4T) as a lipophilic prodrug of d4T and to evaluate the lymphatic distribution of d4T following administration of d4T and DPP-d4T to mice. The pharmacokinetics of d4T were characterized following administration of a single intravenous or oral dose of 50 mg kg−1 d4T and an equimolar dose (214 mg kg−1) of DPP-d4T. Concentrations of d4T in serum and lymph nodes were determined by HPLC. Following administration of d4T, the distribution of d4T into lymph nodes was rapid with maximum concentrations observed within 5 min after dosing. The AUC and half-life values of d4T in three groups of lymph nodes were similar to those in serum. Administration of DPP-d4T resulted in significantly lower concentrations of d4T in serum and lymph nodes. Approximately 67% of the intravenously administered DPP-d4T was biotransformed to parent compound. The apparent oral bioavailability of DPP-d4T was low. While the phospholipid prodrug did not increase d4T concentrations in the lymph nodes, it did provide an extended release of the parent nucleoside, resulting in sustained concentrations of d4T.

PAMAM Dendrimers and Branched Polyethyleneglycol (Nanoparticles) Prodrugs of (-)-β-D-(2R, 4R)-dioxolane-thymine (DOT) and Their Anti-HIV Activity

The synthesis, characterization, anti-HIV activity and cytotoxicity of dendrimers of (–)-β-D-(2 R,4 R)-dioxolane-thymine (DOT) and polyethylene glycol (PEG)–DOT conjugates are described. Dendrimers in this study were polyamidoamine (PAMAM) generation 2.0, 3.0, 5.0 and 6.0, along with 8.0-branched PEG with a molecular weight of 40 kDa. DOT was attached to PAMAM dendrimers or branched PEG via ester or phosphate groups. Size exclusion chromatography was used to purify the dendrimers and PEG conjugates, which were characterized by NMR and MALDI—TOF mass spectrometry. The synthesized PAMAM dendrimers and PEG conjugates were evaluated for anti-HIV activity against HIV-1LAI in primary human peripheral blood mononuclear cells (PBMCs) and cytotoxicity in PBMCs, CEM and Vero cells. PAMAM dendrimers of DOT with ester linkages and particularly phosphate linkers showed an increase in anti-HIV potency in comparison with DOT alone (140- and 56-fold, respectively). Unfortunately, the PAMAM dendrimers also exhibited increased cytotoxicity. Anti-HIV activity of PEG—DOT conjugates was found to be lower than that of DOT.

Synthesis And Antiviral Activity Of 2'-Deoxy-2'-Fluoro-L-Arabinofuranosyl 1,2,3-Triazole Derivatives

The title compounds were prepared by building up the triazole ring at the anomeric position via the glycosyl azides 5 a,b. The anomeric configurations of these nucleosides were assigned by using 1H, 13C and NOESY NMR spectroscopy. The synthesized nucleosides were evaluated against HIV-1 and HBV

Pharmacological reversal of histone methylation presensitizes pancreatic cancer cells to nucleoside drugs: in vitro optimization and novel nanoparticle delivery studies

We evaluated the potential of an investigational histone methylation reversal agent, 3-deazaneplanocin A (DZNep), in improving the chemosensitivity of pancreatic cancer to nucleoside analogs (i.e., gemcitabine). DZNep brought delayed but selective cytotoxicity to pancreatic cancer cells without affecting normal human pancreatic ductal epithelial (HPDE) cells. Co-exposure of DZNep and gemcitabine induced cytotoxic additivity or synergism in both well- and poorly-differentiated pancreatic cell lines by increased apoptosis. In contrast, DZNep exerted antagonism with gemcitabine against HPDE cells with significant reduction in cytotoxicity compared with the gemcitabine-alone regimen. DZNep marginally depended on purine nucleoside transporters for its cytotoxicity, but the transport dependence was circumvented by acyl derivatization. Drug exposure studies revealed that a short priming with DZNep followed by gemcitabine treatment rather than co-treatment of both agents to produce a maximal chemosensitization response in both gemcitabine-sensitive and gemcitabine-resistant pancreatic cancer cells. DZNep rapidly and reversibly decreased trimethylation of histone H3 lysine 27 but increased trimethylation of lysine 9 in an EZH2- and JMJD1A/2C-dependent manner, respectively. However, DZNep potentiation of nucleoside analog chemosensitization was found to be temporally coupled to trimethylation changes in lysine 27 and not lysine 9. Polymeric nanoparticles engineered to chronologically release DZNep followed by gemcitabine produced pronounced chemosensitization and dose-lowering effects. Together, our results identify that an optimized DZNep exposure can presensitize pancreatic cancer cells to anticancer nucleoside analogs through the reversal of histone methylation, emphasizing the promising clinical utilities of epigenetic reversal agents in future pancreatic cancer combination therapies.

Abstract 1025: Optimized DZNep exposure presensitizes pancreatic cancer cells to anticancer nucleoside analogues: potential clinical implications

We evaluated the potential of a histone methylation reversal agent 3-deazaneplanocin A (DZNep) in improving the chemosensitivity of pancreatic cancer to nucleoside analogs (i.e., gemcitabine). DZNep brought delayed but selective cytotoxicity to pancreatic cancer cells without affecting normal human pancreatic ductal epithelial (HPDE) cells. Co-exposure of DZNep and gemcitabine induced cytotoxic additivity or synergism in both well- and poorly-differentiated pancreatic cell lines. In contrast, DZNep exerted antagonism with gemcitabine against HPDE cells with significant reduction in cytotoxicity compared with the gemcitabine-alone regimen. DZNep marginally depended on purine nucleoside transporters for its cytotoxicity, but the transport dependence was circumvented by acyl derivatization. Drug exposure studies revealed that a short priming with DZNep followed by gemcitabine treatment rather than co-treatment of both agents produced a maximal chemosensitization response in both gemcitabine-sensitive and gemcitabine-resistant pancreatic cancer cells. DZNep rapidly and reversibly decreased trimethylation of histone H3 lysine 27 but increased trimethylation of lysine 9 in an EZH2- and JMJD1A/2C-dependent manner, respectively. However, DZNep potentiation of nucleoside analog chemosensitization was found to be temporally coupled to trimethylation changes in lysine 27 and not lysine 9. Polymeric nanoparticles engineered to chronologically release DZNep followed by gemcitabine produced pronounced chemosensitization and dose-lowering effects. Together, our results identify that an optimized DZNep exposure can presensitize pancreatic cancer cells to anticancer nucleoside analogs and emphasize the promising clinical utilities of histone methylation reversal agents in future pancreatic cancer combination therapies.

Citation Format: Sau Wai Hung, Hardik Mody, Sean Marrache, Yangzom D. Bhutia, Franklin Davis, Jong Hyun Cho, Shanta Dhar, Chung K. Chu, Rajgopal Govindarajan. Optimized DZNep exposure presensitizes pancreatic cancer cells to anticancer nucleoside analogues: potential clinical implications. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1025. doi:10.1158/1538-7445.AM2013-1025

Carbocyclic 6-benzylthioinosine analogues as subversive substrates of Toxoplasma gondii adenosine kinase: biological activities and selective toxicities

Toxoplasma gondii adenosine kinase (EC 2.7.1.20) is the major route of adenosine metabolism in this parasite. The enzyme is significantly more active than any other enzyme of the purine salvage in T. gondii and has been established as a potential chemotherapeutic target for the treatment of toxoplasmosis. Several 6-benzylthioinosines have already been identified as subversive substrates of the T. gondii but not human adenosine kinase. Therefore, these compounds are preferentially metabolized to their respective nucleotides and become selectively toxic against the parasites but not its host. In the present study, we report the testing of the metabolism of several carbocyclic 6-benzylthioinosines, as well as their efficacy as anti-toxoplasmic agents in cell culture. All the carbocyclic 6-benzylthioinosine analogues were metabolized to their 5'-monophosphate derivatives, albeit to different degrees. These results indicate that these compounds are not only ligands but also substrates of T. gondii adenosine kinase. All the carbocyclic 6-benzylthioinosine analogues showed a selective anti-toxoplasmic effect against wild type parasites, but not mutants lacking adenosine kinase. These results indicate that the oxygen atom of the sugar is not critical for substrate binding. The efficacy of these compounds varied with the position and nature of the substitution on their phenyl ring. Moreover, none of these analogues exhibited host toxicity. The best compounds were carbocyclic 6-(p-methylbenzylthio)inosine (IC(50)=11.9 microM), carbocyclic 6-(p-methoxybenzylthio)inosine (IC(50)=12.1 microM), and carbocyclic 6-(p-methoxycarbonylbenzylthio)inosine (IC(50)=12.8 microM). These compounds have about a 1.5-fold better efficacy relative to their corresponding 6-benzylthioinosine analogues (Rais et al., Biochem Pharmacol 2005;69:1409-19 [29]). The results further confirm that T. gondii adenosine kinase is an excellent target for chemotherapy and that carbocyclic 6-benzylthioinosines are potential anti-toxoplasmic agents.

A convergent approach for the synthesis of ara-neplanocin a analogues under subzero microwave assisted conditions

A convergent strategy for the synthesis of ara-neplanocin A analogues has been developed. Microwave assisted Mitsunobu reaction proved to be an essential tool both for the 2'-beta-hydroxy inversion and for the coupling reaction with the heterocyclic bases. The exploitation of the present approach allowed generating a family of ara-neplanocins which biological potential is still unexplored.

Efficacy of 2'-C-methylcytidine against yellow fever virus in cell culture and in a hamster model

Yellow fever virus (YFV) continues to cause outbreaks of disease in endemic areas where vaccine is underutilized. Due to the effectiveness of the vaccine, antiviral development solely for the treatment of YFV is not feasible, but antivirals that are effective in the treatment of related viral diseases may be characterized for potential use against YFV as a secondary indication disease. 2'-C-methylcytidine (2'-C-MeC), a compound active against hepatitis C virus, was found to have activity against the 17D vaccine strain of YFV in cell culture (EC(90)=0.32 microg/ml, SI=141). This compound was effective when added as late as 16 h after virus challenge of Vero cells. When administered to YFV-infected hamsters 4 h prior to virus challenge at a dose as low as 80 mg/kg/d, 2'-C-MeC was effective in significantly improving survival and other disease parameters (weight change, serum ALT, and liver virus titers). Disease was improved when compound was administered beginning as late as 3 d post-virus infection. Broadly active antiviral compounds, such as 2'-C-MeC, represent potential for the development of compounds active against related viruses for the treatment of YFV.

(-)-Carbodine: enantiomeric synthesis and in vitro antiviral activity against various strains of influenza virus including H5N1 (avian influenza) and novel 2009 H1N1 (swine flu)

Enantiomerically pure cyclopentyl cytosine [(-)-carbodine 1] was synthesized from d-ribose and evaluated for its anti-influenza activity in vitro in comparison to the (+)-carbodine, (+/-)-carbodine and ribavirin. (-)-Carbodine 1 exhibited potent antiviral activity against various strains of influenza A and B viruses.

Synthesis and anti-hepatitis B virus and anti-hepatitis C virus activities of 7-deazaneplanocin A analogues in vitro

A series of 7-deazaneplanocin A (7-DNPA, 2) analogues were synthesized and evaluated for in vitro antiviral activity against HBV and HCV. The syntheses of target carbocyclic nucleosides were accomplished via a convergent procedure. 7-Substitutions were introduced by using 7-substituted-7-deaza heterocyclic base precursors (F, Cl, Br, and I) or via substitution reactions after the synthesis of the carbocyclic nucleosides. Among the synthesized compounds, 2, 13-15, 24, and 27 exhibited significant anti-HCV activity (EC(50) ranged from 1.8 to 20.1 microM) and compounds 2, 15, 22, and 24 demonstrated moderate to potent anti-HBV activity (EC(50) = 0.3-3.3 microM). In addition, compound 24 also showed activity against lamivudine- and adefovir-associated HBV mutants.

5'-O-Aliphatic and amino acid ester prodrugs of (-)-beta-D-(2R,4R)-dioxolane-thymine (DOT): synthesis, anti-HIV activity, cytotoxicity and stability studies

A series of (-)-beta-D-(2R,4R)-dioxolane-thymine-5'-O-aliphatic acid esters as well as amino acid esters were synthesized as prodrugs of (-)-beta-D-(2R,4R)-dioxolane-thymine (DOT). The compounds were evaluated for anti-HIV activity against HIV-1(LAI) in human peripheral blood mononuclear (PBM) cells as well as for their cytotoxicity in PBM, CEM and Vero cells. Improved anti-HIV potency in vitro was observed for the compound 2-4 (5'-O-aliphatic acid esters) without increase in cytotoxicity in comparison to the parent drug. Chemical and enzymatic hydrolysis of the prodrugs was also studied, in which the prodrugs exhibited good chemical stability with the half-lives from 3 h to 54 h at pH 2.0 and 7.4 phosphate buffer. However, the prodrugs were relatively labile to porcine esterase with the half-lives from 12.3 to 48.0 min.

Understanding the molecular basis of HBV drug resistance by molecular modeling

Despite the significant successes in the area of anti-HBV agents, resistance and cross-resistance against available therapeutics are the major hurdles in drug discovery. The present investigation is to understand the molecular basis of drug resistance conferred by the B and C domain mutations of HBV-polymerase on the binding affinity of five anti-HBV agents [lamivudine (3TC, 1), adefovir (ADV, 2), entecavir (ETV, 3), telbivudine (LdT, 4) and clevudine (l-FMAU, 5)]. In this regard, homology modeled structure of HBV-polymerase was used for minimization, conformational search and induced fit docking followed by binding energy calculation on wild-type as well as on mutant HBV-polymerases (L180M, M204V, M204I, L180M+M204V, L180M-M204I). Our studies suggest a significant correlation between the fold resistances and the binding affinity of anti-HBV nucleosides. The binding mode studies reveals that the domain C residue M204 is closely associated with sugar/pseudosugar ring positioning in the active site. The positioning of oxathiolane ring of 3TC (1) is plausible due the induced fit orientation of the M204 residue in wild-type, and further mutation of M204 to V204 or I204 reduces the final binding affinity which leads to the drug resistance. The domain B residue L180 is not directly close ( approximately 6A) to the nucleoside/nucleoside analogs, but indirectly associated with other active-site hydrophobic residues such as A87, F88, P177 and M204. These five hydrophobic residues can directly affect on the incoming nucleoside analogs in terms of its association and interaction that can alter the final binding affinity. There was no sugar ring shifting observed in the case of adefovir (2) and entecavir (3), and the position of sugar ring of 2 and 3 is found similar to the sugar position of natural substrate dATP and dGTP, respectively. The exocyclic double bond of entecavir (3) occupied in the backside hydrophobic pocket (made by residues A87, F88, P177, L180 and M204), which enhances the overall binding affinity. The active site binding of LdT (4) and l-FMAU (5) showed backward shifting along with upward movement without enforcing M204 residue and this significant different binding mode makes these molecules as polymerase inhibitors, without being incorporated into the growing HBV-DNA chain. Structural results conferred by these l- and d-nucleosides, explored the molecular basis of drug resistance which can be utilized for future anti-HBV drug discovery.

Treatment of Venezuelan equine encephalitis virus infection with (-)-carbodine

Venezuelan equine encephalitis virus (VEEV) may cause encephalitis in humans, for which no FDA-approved antiviral treatment is available. Carbocyclic cytosine (carbodine) has broad-spectrum activity but toxicity has limited its utility. It was anticipated that one of the enantiomers of carbodine would show enhanced activity and reduced toxicity. The activity of the d-(-) enantiomer of carbodine [(-)-carbodine] was evaluated by infectious cell culture assay and was found to have a 50% effective concentration (EC50) of 0.2 microg/ml against the TC-83 vaccine strain of VEEV in Vero cells, while the l-(+) enantiomer had no activity. Virus titer inhibition correlated with intracellular cytidine triphosphate reduction after treatment with (-)-carbodine, as determined by HPLC analysis. Pre-treatment with 200 mg/(kgd) resulted in significant improvement in survival, virus load in the brain, weight change, and mean day-to-death in a mouse model of TC-83 VEEV disease. A single dose of (-)-carbodine resulted in a slight extension of mean time to death in mice infected with wild-type VEEV. Post-virus exposure treatment with (-)-carbodine was effective in significantly improving disease parameters in mice infected with TC-83 VEEV when treatment was initiated as late as 4 days post-virus installation (dpi). It is remarkable that (-)-carbodine is effective when initiated after the establishment of brain infection.

7-Deaza-6-benzylthioinosine analogues as subversive substrate of Toxoplasma gondii adenosine kinase: activities and selective toxicities

Toxoplasma gondii adenosine kinase (EC.2.7.1.20) is the major route of adenosine metabolism in this parasite. The enzyme is significantly more active than any other enzyme of the purine salvage in T. gondii and has been established as a potential chemotherapeutic target for the treatment of toxoplasmosis. Certain 6-benzylthioinosines act as subversive substrates of T. gondii, but not human, adenosine kinase. Therefore, these compounds are preferentially metabolized to their respective nucleotides and become selectively toxic against the parasites but not their host. Moreover, 7-deazaadenosine (tubercidin) was shown to be an excellent ligand of T. gondii adenosine kinase. Therefore, we synthesized 7-deaza-6-benzylthioinosine, and analogues with various substitutions at their phenyl ring, to increase the binding affinity of the 6-benzylthioinosines to T. gondii adenosine kinase. Indeed, the 7-deaza-6-benzylthioinosine analogues were better ligands of T. gondii adenosine kinase than the parent compounds, 6-benzylthioinosine and 7-deazainosine. Herein, we report the testing of the metabolism of these newly synthesized 7-deaza-6-benzylthioinosines, as well as their efficacy as anti-toxoplasmic agents in cell culture. All the 7-deaza-6-benzylthioinosine analogues were metabolized to their 5'-monophosphate derivatives, albeit to different degrees. These results indicate that these compounds are not only ligands but also substrates of T. gondii adenosine kinase. All the 7-deaza-6-benzylthioinosine analogues showed a selective antitoxoplasmic effect against wild type parasites, but not mutants lacking adenosine kinase. The efficacy of these compounds varied with the position and nature of the substitution on their phenyl ring. Moreover, none of these analogues exhibited host toxicity. The best compounds were 7-deaza-6-(p-methoxybenzylthio)inosine (IC(50)=4.6 microM), 7-deaza-6-(p-methoxycarbonylbenzylthio)inosine (IC(50)=5.0 microM), and 7-deaza-6-(p-cyanobenzylthio)inosine (IC(50)=5.3 microM). These results further confirm that T. gondii adenosine kinase is an excellent target for chemotherapy and that 7-deaza-6-benzylthioinosines are potential antitoxoplasmic agents.

Troxacitabine prodrugs for pancreatic cancer

Troxacitabine is a cytotoxic deoxycytidine analogue with an unnatural L-configuration, which is activated by deoxycytidine kinase (dCK). The configuration is responsible for differences in the uptake and metabolism of troxacitabine compared to other deoxynucleoside analogues. The main drawback in the use of most nucleoside anticancer agents originates from their hydrophilic nature, which property requires a high and frequent dosage for an intravenous administration. To overcome this problem several troxacitabine prodrugs modified in the aminogroup with a linear aliphatic chain with a higher lipophilicity were developed. To determine whether these prodrugs have an advantage over Troxacitabine pancreatic cancer cell lines were exposed to Troxacitabine and the lipophilic prodrugs. The addition of linear aliphatic chains to troxacitabine increased sensitivity of pancreatic cancer cell lines to the drug > 100-fold, possibly due to a better uptake and retention of the drug.

Antiviral effect of orally administered (-)-beta-D-2-aminopurine dioxolane in woodchucks with chronic woodchuck hepatitis virus infection

(-)-beta-D-2-Aminopurine dioxolane (APD) is a nucleoside prodrug that is efficiently converted to 9-(beta-D-1,3-dioxolan-4-yl)guanine (DXG). DXG has antiviral activity in vitro against hepatitis B virus (HBV) but limited aqueous solubility, making it difficult to administer orally to HBV-infected individuals. APD is more water soluble than DXG and represents a promising prodrug for the delivery of DXG. A placebo-controlled, dose-ranging efficacy and pharmacokinetic study was conducted with woodchucks that were chronically infected with woodchuck hepatitis virus (WHV). APD was efficiently converted to DXG after oral and intravenous administrations of APD, with serum concentrations of DXG being higher following oral administration than following intravenous administration, suggestive of a considerable first-pass intestinal and/or hepatic metabolism. APD administered orally at 1, 3, 10, and 30 mg/kg of body weight per day for 4 weeks produced a dose-dependent antiviral response. Doses of 3 and 10 mg/kg/day reduced serum WHV viremia by 0.4 and 0.7 log(10) copies/ml, respectively. The 30-mg/kg/day dose resulted in a more pronounced, statistically significant decline in serum WHV viremia of 1.9 log(10) copies/ml and was associated with a 1.5-fold reduction in hepatic WHV DNA. Individual woodchucks within the highest APD dose group that had declines in serum WHV surface antigen levels, WHV viremia, and hepatic WHV DNA also had reductions in hepatic WHV RNA. There was a prompt recrudescence of WHV viremia following drug withdrawal. Therefore, oral administration of APD for 4 weeks was safe in the woodchuck model of chronic HBV infection, and the effect on serum WHV viremia was dose dependent.

Pharmacokinetics of the anti-human immunodeficiency virus agent 1-(beta-D-dioxolane)thymine in rhesus monkeys

Beta-D-dioxolane-thymine (D-DOT) has potent and selective in vitro activity against several clinically important resistant human immunodeficiency virus (HIV) mutants and is in advanced preclinical development. Therefore, the single-dose intravenous and oral pharmacokinetics of D-DOT were studied with three rhesus monkeys. The pharmacokinetic profiles of D-DOT in serum and urine were adequately described by a two-compartment open pharmacokinetic model. D-DOT was rapidly and almost completely absorbed (absorption rate constant = 2.7 h(-1); fraction of oral dose absorbed = 0.82 to 1.06). The average serum beta half-life was 2.16 h. The average central and steady-state volumes of distributions were 0.52 and 1.02 liter/kg of body weight, respectively, and the average systemic and renal clearance values were 0.36 liter/h/kg and 0.18 liter/h/kg. Four or eight percent of administered D-DOT was eliminated in the urine as glucuronide within 8 h after intravenous or oral administration, respectively. D-DOT reached levels in the cerebrospinal fluid in excess of 10 to 20 times the median effective concentration for wild-type HIV and resistant mutants. The potent antiretroviral activity of D-DOT against a lamivudine- and zidovudine-resistant HIV-1 mutant, together with an excellent pharmacokinetic profile for rhesus monkeys, suggest that further development is warranted.

Synthesis, biological evaluation and molecular modeling studies of N6-benzyladenosine analogues as potential anti-toxoplasma agents

Toxoplasma gondii is an opportunistic pathogen responsible for toxoplasmosis. T. gondii is a purine auxotroph incapable of de novo purine biosynthesis and depends on salvage pathways for its purine requirements. Adenosine kinase (EC.2.7.1.20) is the major enzyme in the salvage of purines in these parasites. 6-Benzylthioinosine and analogues were established as "subversive substrates" for the T. gondii, but not for the human adenosine kinase. Therefore, these compounds act as selective anti-toxoplasma agents. In the present study, a series of N(6)-benzyladenosine analogues were synthesized from 6-chloropurine riboside with substituted benzylamines via solution phase parallel synthesis. These N(6)-benzyladenosine analogues were evaluated for their binding affinity to purified T. gondii adenosine kinase. Furthermore, the anti-toxoplasma efficacy and host toxicity of these compounds were tested in cell culture. Certain substituents on the aromatic ring improved binding affinity to T. gondii adenosine kinase when compared to the unsubstituted N(6)-benzyladenosine. Similarly, varying the type and position of the substituents on the aromatic ring led to different degrees of potency and selectivity as anti-toxoplasma agents. Among the synthesized analogues, N(6)-(2,4-dimethoxybenzyl)adenosine exhibited the most favorable anti-toxoplasma activity without host toxicity. The binding mode of the synthesized N(6)-benzyladenosine analogues were characterized to illustrate the role of additional hydrophobic effect and van der Waals interaction within an active site of T. gondii adenosine kinase by induced fit molecular modeling.

In Vitro Optimization of Non-Small Cell Lung Cancer Activity with Troxacitabine,l-1,3-Dioxolane-cytidine, Prodrugs

l-1,3-Dioxolane-cytidine, a potent anticancer agent against leukemia, has limited efficacy against solid tumors, perhaps due to its hydrophilicity. Herein, a library of prodrugs were synthesized to optimize in vitro antitumor activity against non-small cell lung cancer. N4-Substituted fatty acid amide prodrugs of 10-16 carbon chain length demonstrated significantly improved antitumor activity over l-1,3-dioxolane-cytidine. These in vitro results suggest that the in vivo therapeutic efficacy of l-1,3-dioxolane-cytidine against solid tumors may be improved with prodrug strategies.

Interaction of 2'-deoxyguanosine triphosphate analogue inhibitors of HIV reverse transcriptase with human mitochondrial DNA polymerase gamma

Mitochondrial toxicity is a limiting factor in the use of some nucleoside reverse transcriptase inhibitors of HIV. To further understand the impact of structural features on the incorporation and exonuclease removal of nucleoside monophosphate (MP) analogues by human mitochondrial DNA polymerase (pol gamma), transient kinetic studies were done with analogues of 2'-deoxyguanosine triphosphate. The kinetic parameters for the incorporation and removal of carbovir (CBV)-MP, dioxolane guanosine (DXG)-MP and 2',3'-dideoxy-2',3'-didehydroguanosine (d4G)-MP were studied with pol gamma holoenzyme. The importance of the ribose oxygen in incorporation by pol gamma was illustrated by an approximate 3,000-fold decrease in the incorporation efficiency of an analogue lacking the ribose oxygen (CBV-TP) relative to those containing a ribose oxygen (DXG-TP and d4G-TP). As a result, a comparison with previous data for the incorporation by HIV reverse transcriptase showed CBV-TP to be approximately 800-8,000-fold more selective for its antiviral target over pol gamma relative to the other guanosine analogues. However, DXG-TP and d4G-TP were found to be much more selective than previously reported values for mitochondrial toxic nucleoside analogues. Structural modelling based on sequence homology with other polymerase A family members suggests that an interaction between the ribose oxygen and arginine 853 in pol gamma may play a critical role in causing this differential incorporation. Exonuclease removal of a chain-terminating CBV-MP was also found to be more efficient by pol gamma. These results help to further elucidate the structure activity relationships for pol gamma and should aid in the design of more selective antiviral agents.

Biochemical studies on the mechanism of human immunodeficiency virus type 1 reverse transcriptase resistance to 1-(beta-D-dioxolane)thymine triphosphate

A large panel of drug-resistant mutants of human immunodeficiency virus type 1 reverse transcriptase (RT) was used to study the mechanisms of resistance to 1-(beta-d-dioxolane)thymine triphosphate (DOT-TP) and other nucleotide analogs. RT containing thymidine analog-associated mutations (TAM) or RT with a T69S-SG insertion in combination with TAM removed 3'-azido-3'-deoxythymidine-5'-monophosphate or tenofovir more efficiently than DOT-monophosphate from chain-terminated DNA primer/template through ATP-mediated pyrophosphorolysis. For non-ATP-dependent discrimination toward DOT-TP, high levels of resistance were found for RT bearing the Q151M mutation with family mutations, while RT bearing only the M184V or the Y115F mutation conferred no resistance to DOT-TP. A lower degree of resistance to DOT-TP than to tenofovir diphosphate or carbovir-TP was found for RT containing the K65R mutation. In the present studies, 1-(beta-d-dioxolane)guanine triphosphate, another nucleotide with a dioxolane sugar moiety, showed a resistance profile similar to that of DOT-TP. The results suggest that DOT, compared with other approved nucleoside analogs, is overall more resilient to mutations such as TAM, M184V, and K65R, which are commonly found in viruses derived from subjects failing multinucleoside therapy.

Synthesis and anti-HIV activity of (-)-beta-D-(2R,4R)-1,3-dioxolane-2,6-diamino purine (DAPD) (amdoxovir) and (-)-beta-D-(2R,4R)-1,3-dioxolane guanosine (DXG) prodrugs

Prodrugs of (-)-beta-D-(2R,4R)-1,3-dioxolane-2,6-diamino purine (DAPD), organic salts of DAPD, 5'-L-valyl DAPD and N-1 substituted (-)-beta-D-(2R,4R)-1,3-dioxolane guanosine (DXG) have been synthesized with the objective of finding molecules which might be superior to DAPD and DXG in solubility as well as pharmacologic profiles. Synthesized prodrugs were evaluated for anti-HIV activity against HIV-1(LAI) in primary human lymphocytes (PBM cells) as well as their cytotoxicity in PBM, CEM and Vero cells. DAPD prodrugs, modified at the C6 position of the purine ring, demonstrated several folds of enhanced anti-HIV activity in comparison to the parent compound DAPD without increasing the toxicity. The presence of alkyl amino groups at the C6 position of the purine ring increased the antiviral potency several folds, and the most potent compound (-)-beta-D-(2R,4R)-1,3-dioxolane-2-amino-6-aminoethyl purine (8) was 17 times more potent than that of DAPD. 5'-L-Valyl DAPD 20 and organic acid salts 21-24 also exhibited enhanced anti-HIV activity in comparison to DAPD, while DXG prodrugs 16 and 17 exhibited lower potency than that of DXG or DAPD.

D- and L-2',3'-didehydro-2',3'-dideoxy-3'-fluoro-carbocyclic nucleosides: synthesis, anti-HIV activity and mechanism of resistance

Introducing 2'-fluoro substitution on the 2',3'-double bond in carbocyclic nucleosides has provided biologically interesting compounds with potent anti-HIV activity. As an extension of our previous works in the discovery of anti-HIV agents, D- and L-2',3'-unsaturated 3'-fluoro carbocyclic nucleosides were synthesized and evaluated against HIV-1 in human peripheral blood mononuclear (PBM) cells. Among the synthesized L-series nucleosides, compounds 18, 19, 26 and 28 exhibited moderate antiviral activity (EC50 7.1 microM, 6.4 microM, 10.3 microM, and 20.7 microM, respectively), while among the D-series, the guanosine analogue (35, D-3'-F-C-d4G) exhibited the most potent anti-HIV activity (EC50 0.4 microM, EC90 2.8 microM). However, the guanosine analogue 35 was cross-resistant to the lamivudine-resistant variants (HIV-1M184V). Molecular modeling studies suggest that hydrophobic interaction as well as hydrogen-bonding stabilize the binding of compound 35 in the active site of wild type HIV reverse transcriptase (HIV-RT). In the case of L-nucleosides, these two effects are opposite which results in a loss of binding affinity. According to the molecular modeling studies, cross-resistance of D-3'-F-C-d4G (35) to M184V mutant may be caused by the realignment of the primer and template in the HIV-RTM184V interaction, which destabilizes the RT-inhibitor triphosphate complex, resulting in a significant reduction in anti-HIV activity of the D-guanine derivative 35.

Antiviral activity of nucleoside analogues against SARS-coronavirus (SARS-coV)

The recent outbreak of severe acute respiratory syndrome (SARS), which is an acute respiratory illness, is caused by newly discovered SARS coronavirus (SARS-CoV). Herein we describe the antiviral activity of several classes of nucleoside analogues evaluated against SARS-CoV in Vero 76 cells, some of which exhibited moderate activity.

Enantioselective synthesis and antiviral activity of purine and pyrimidine cyclopentenyl C-nucleosides

The enantiomerically pure carbocyclic purine and pyrimidine C-nucleosides 1–4 were synthesized via the key intermediate, 2,3-(isopropylidenedioxy)-4-(trityloxymethyl)-4-cyclopenten-1-ol (5), which was prepared from d-ribose in eight steps. Synthesized compounds were evaluated as potential antiviral agents against HIV, SARSCoV, Punta Toro, West Nile, and Cowpox viruses. However, only 9-deazaneplanocin A (1) exhibited moderate anti-HIV activity.

Synthesis of neplanocin F analogues as potential antiviral agents

Neplanocin F is a natural carbocyclic nucleoside. Herein, we describe the synthesis and antiviral activity of (+/-)-5'-deoxy-neplanocin F analogues. The key intermediate 4, synthesized from the commercially available (+/-)-2-azabicyclo[2.2.1]-hept-5-en-3-one (ABH), was utilized to prepare the target nucleosides. Among the target compounds, 5'-deoxyneplanocin F adenine exhibited moderate anti-HIV activity in human lymphocytes without any marked cytotoxicity.

Cell line dependency for antiviral activity and in vivo efficacy of N-methanocarbathymidine against orthopoxvirus infections in mice

A novel carbocyclic thymidine analog, N-methanocarbathymidine [(N)-MCT], was evaluated for inhibition of orthopoxvirus infections. Efficacy in vitro was assessed by plaque reduction assays against wild-type and cidofovir-resistant strains of cowpox and vaccinia viruses in nine different cell lines. Minimal differences were seen in antiviral activity against wild-type and cidofovir-resistant viruses. (N)-MCT's efficacy was affected by the cell line used for assay, with 50% poxvirus-inhibitory concentrations in cells as follows: mouse=0.6-2.2 microM, rabbit=52-90 microM, monkey=87 to >1000 microM, and human=39-220 microM. Limited studies performed with carbocyclic thymidine indicated a similar cell line dependency for antiviral activity. (N)-MCT did not inhibit actively dividing uninfected cells at 1000 microM. The potency of (N)-MCT against an S-variant thymidine kinase-deficient vaccinia virus was similar to that seen against S-variant and wild-type viruses in mouse, monkey, and human cells, implicating a cellular enzyme in the phosphorylation of the compound. Mice were intranasally infected with cowpox and vaccinia viruses followed 24h later by intraperitoneal treatment with (N)-MCT (twice a day for 7 days) or cidofovir (once a day for 2 days). (N)-MCT treatment at 100 and 30 mg/kg/day resulted in 90 and 20% survival from cowpox virus infection, respectively, compared to 0% survival in the placebo group. Statistically significant reductions in lung virus titers on day 5 occurred in 10, 30, and 100mg/kg/day treated mice. These same doses were also active against a lethal vaccinia virus (WR strain) challenge, and protection was seen down to 10mg/kg/day against a lethal vaccinia virus (IHD strain) infection. Cidofovir (100mg/kg/day) protected animals from death in all three infections.

Synthesis and anti-HIV activity of D- and L-thietanose nucleosides

Various D- and L-thietanose nucleosides were synthesized from D- and L-xylose. The four-membered thietane ring was efficiently synthesized by the cyclization of 1-thioacetyl-3-mesylate (4/38) under basic conditions. Condensation with various heterocyclic bases was conducted via Pummerer-type rearrangement to afford various nucleoside derivatives. Among the synthesized nucleosides, D-uridine (23), D-cytidine (24), D-5-fluorocytidine (25), and L-cytidine (52) analogues showed moderate anti-HIV activity, with EC50 = 6.9, 1.3, 5.8, and 14.1 microM, respectively. However, these four nucleoside analogues are cytotoxic in peripheral blood mononuclear and CEM cells. The other nucleosides are neither active nor cytotoxic. Interestingly, the oxetanocin A analogue 33 was not active. Comparison of the minimized reverse transcriptases (RTs) complexed with the corresponding triphosphates of the cytidine analogue 24 and the adenosine analogue 33 by molecular modeling studies showed that there is no difference in the binding mode of the triphosphate of the cytidine analogue 24 to the active site of HIV-1 RT from that of the triphosphate of the adenosine analogue 33. Modeling studies on the initial monophosphorylation step by deoxycytidine kinase showed that the catalytic efficiency of phosphorylation through a nucleophilic attack of the 4'-hydroxyl group of thietanose on the gamma-phosphate of ATP is diminished in the case of L-cytidine analogue (52) due to the increased distance between the 4'-hydroxyl group and the gamma-phosphate.

Enantiomeric synthesis of carbocyclic analogs ofD- andL-6-azapyrimidine ribonucleosides

An effective and practical synthesis of carbocyclic D- and L-6-azapyrimidine nucleosides (3–8) was described. Starting from D-ribose, a new efficient methodology for the synthesis of L-2,3-O-cyclohexylidene-4-cyclopentenone (23) was developed via a ring-closing metathesis, which was applied for the synthesis of L-cyclopentyl-6-azapyrimidine nucleosides (6–8). The regiospecific introduction of 6-azauracil on the carbocyclic moiety (9 and 25) was also achieved by masking its N3position with a 4-methylthio group.Key words: carbocyclic nucleosides, 6-azapyrimidine nucleosides, enantiomeric synthesis.