Synthesis of novel 3'-C-methyl-apionucleosides: an asymmetric construction of a quaternary carbon by Claisen rearrangement

The synthesis of 2,3-dideoxy-3-C-(hydroxymethyl)-3-C-methyl-D-glycero-tetrofuranosyl++ + nucleosides was accomplished in high enatiomeric purity (98.5% ee) via [3,3]-sigmatropic Claisen rearrangement of (E)(S)-5-benzyloxy-1-tert-butyldimethylsilanyloxy-4-methyl-pent-3- en-2-ol prepared from 2,3-O-isopropylidene-D-glyceraldehyde. The synthesized nucleosides were assayed against human immunodeficiency virus (HIV) and hepatitis B virus in human peripheral blood mononuclear (PBM) and 2.2.15 cells, respectively. 6-Amino-9-[2,3-dideoxy-3-C-(hydroxymethyl)-3-C-methyl-beta-D-glycero- tetrofuranosyl]-2-fluoropurine shows moderate antiviral activity (EC50 = 2.55 microM) against HIV-1 strains and 6-amino-9-[3-deoxy-3-C-(hydroxymethyl)-3-methyl-alpha-D-glycero-tetro furanosyl]-2-fluoropurine exhibits potent anti-HIV activity (EC50 = 0.073 microM) with significant cytotoxicity (IC50 = 1.0 microM).

Synthesis of 2',3'-dideoxy-3'-fluoro-L-ribonucleosides as potential antiviral agents from D-sorbitol

2',3'-Dideoxy-3'-fluoro-L-ribonucleosides were synthesized as potential antiviral agents. The key intermediate, methyl 5-O-benzoyl-2,3-dideoxy-3-fluoro-L-ribofuranoside, which was prepared from D-sorbitol, was condensed with pyrimidine and purine bases to obtain the respective nucleosides. Among them, the cytosine analogue 2',3'-dideoxy-3'-fluoro-alpha-L-cytidine showed a moderate anti-HBV activity.

Beta-L-1,3-dioxolane-cytidine: a novel nucleoside that inhibits proliferation and induces differentiation of keratinocytes in vitro

beta-L-1,3-Dioxolane-cytidine (L-(-)-OddC) is a novel L-nucleoside, and its antitumor activity is under investigation in clinical trials. To evaluate the potential of L-(-)-OddC for treating hyperproliferative diseases of the skin, we examined its activity in human keratinocytes in vitro. The dose of L-(-)-OddC that inhibited the rate of proliferation of keratinocytes by 50% was 50 nM. L-(-)-OddC was about as cytotoxic as 9-beta-D-arabinofuranosylcytosine but was about 1,000 time more potent than 3'-azidothymidine. L-(-)-OddC caused irreversible growth arrest and induced differentiation of keratinocytes. L-(-)-OddC altered morphology, increased the cell size of keratinocytes and increased the expression of involucrin. These data suggest that L-(-)-OddC may have potential as a therapeutic agent against hyperproliferative skin diseases.

Structure-activity relationships of (E)-5-(2-bromovinyl)uracil and related pyrimidine nucleosides as antiviral agents for herpes viruses

A series of (E)-5-(2-bromovinyl)uracil analogues and related nucleosides was synthesized, and their antiviral activities were evaluated. (E)-5-(2-Bromovinyl)-2'-deoxy-L-uridine (L-BVDU, 2), 1-(beta-L-arabinofuranosyl)-(E)-5-(2-bromovinyl)uracil (L-BVAU, 4), (E)-5-(2-bromovinyl)-1-(2-deoxy-2-fluoro-beta-L-ribofuranosyl)uracil (L-FBVRU, 8) and (E)-5-(2-bromovinyl)-1-(2-deoxy-2-fluoro-beta-L-arabinofuranosyl)urac il (L-FBVAU, 10) were synthesized via appropriate 5-iodouracil analogues from L-arabinose. D- and L-Oxathiolane and -dioxolane derivatives 13, 16, 20, 21, and 29-34 were prepared by glycosylation reaction of the oxathiolane and dioxolane intermediates with silylated uracil analogues using TMSI as the coupling agent. The synthesized compounds were evaluated in cell cultures infected with the following viruses: varicella zoster virus (VZV), Epstein Barr virus (EBV), and herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). Among the tested compounds, beta-L-CV-OddU (29), beta-L-BV-OddU (31), and beta-L-IV-OddU (33) exhibited potent in vitro antiviral activity against VZV with EC(50) values of 0.15, 0. 07, and 0.035 microM, respectively, and against EBV with EC(50) values of 0.49, 0.59, and 3.91 microM, respectively.

Synthesis and enantioselectivity of the antiviral effects of (R,Z)-,(S,Z)-methylenecyclopropane analogues of purine nucleosides and phosphoralaninate prodrugs: influence of heterocyclic base, type of virus and host cells

A series of R and S enantiomers of 2-aminopurine methylenecyclopropane analogues of nucleosides was synthesized. Two diastereoisomeric lipophilic phosphate prodrugs derived from R and S enantiomers of 2,6-diaminopurine analogue were also prepared. Enantioselectivity (diastereoselectivity in case of prodrugs) of in vitro antiviral effects was investigated with human and murine cytomegalovirus (HCMV and MCMV, respectively), herpes simplex virus types 1 and 2 (HSV-1 and HSV-2, respectively), human immunodeficiency virus type 1 (HIV-1), hepatitis B virus (HBV), Epstein-Barr virus (EBV) and varicella zoster virus (VZV). Strong differences in enantioselectivity were found between the R and S enantiomers of adenine analogue and enantiomeric 2-aminopurine analogues. Thus, the enantiomers of adenine analogue were equipotent against HCMV but not MCMV, where the S enantiomer is strongly preferred. The same S preference was found throughout the 2-aminopurine series for both HCMV and MCMV. In contrast, R-synadenol in HIV-1 assays was the best agent, whereas the S enantiomers of moderately effective 2-amino-6-cyclopropylamino and 2-amino-6-methoxypurine analogues were preferred. Little enantiomeric preference was found for R and S enantiomers of synadenol and the corresponding enantiomers of 2,6-diaminopurine analogue against HBV. A mixed pattern of enantioselectivity was observed for EBV depending on the type of host cells and assay. Against VZV, the R and S enantiomers of adenine analogue were equipotent or almost equipotent, but throughout the series of 2-aminopurine analogues a distinct preference for the S enantiomers was found. The stereoselectivity pattern of both diastereoisomeric prodrugs mostly followed enantioselectivity of the parent analogues. The varying enantioselectivities in the series of purine methylenecyclopropane analogues are probably a consequence of differences in the mechanisms of action in different virus/host cell systems.

Excision of beta-D- and beta-L-nucleotide analogs from DNA by the human cytosolic 3'-to-5' exonuclease

The cytosolic 3'-to-5' exonuclease from chronic lymphocytic leukemia cells was highly purified, and its ability to remove beta-D- and beta-L-nucleotide analogs from the 3'-end of DNA was determined. The relative rate of excision of beta-D-ddCMP, beta-L-ddCMP, beta-L-FddCMP, beta-L-SddCMP, beta-L-Fd4CMP, and beta-L-OddCMP from the 3'-end of a single-stranded oligonucleotide primer or a primer annealed with complementary DNA and/or RNA templates was assessed. The rate of excision of beta-D-nucleotides from the 3'-end of DNA was higher than that of beta-L-nucleotides, which could be partly attributable to the affinity of the enzyme to beta-D-nucleotide-terminated DNA being 5-fold higher compared with that of beta-L-nucleotide-terminated DNA. The rate of removal of beta-L-Fd4CMP and beta-L-OddCMP from the 3'-end of DNA was at least 8 to 10 times lower compared with that of beta-L-SddCMP. HIV reverse transcriptase could elongate DNA primers after the removal of chain terminators by the cytosolic exonuclease. Concentrations of nucleoside 5'-monophosphate analogs that inhibit the cytosolic exonuclease by 50% were estimated. Among the nucleoside 5'-monophosphate analogs examined, beta-L-Fd4CMP appeared to be the most effective inhibitor of the cytosolic exonuclease, with an ID(50) value of 38 microM.

Preclinical investigation of L-FMAU as an anti-hepatitis B virus agent

Preclinical aspects of a potent anti-hepatitis B virus (HBV) L-nucleoside, 1-(2-fluoro-5-methyl-beta-L-arabino-furanosyl)uracil (L-FMAU) are described. L-FMAU was prepared from L-ribose derivatives via either L-xylose or L-arabinose. L-FMAU shows potent antiviral activity against hepatitis B virus (EC50 5.0 microM in H1 cells) with high selectivity in vitro. L-FMAU is not incorporated into mitochondrial DNA and no significant lactic acid production was observed in vitro. L-FMAU is phosphorylated by thymidine kinase as well as deoxycytidine kinase, ultimately to the triphosphate, which inhibits HBV DNA polymerase as the mechanism of antiviral action. Preliminary in vivo toxological studies suggest no apparent toxicity for 30 days at 50 mg/kg/day in mice and for 3 months in woodchucks (10 mg/kg/day). L-FMAU also has respectable bioavailability in rats. L-FMAU shows potent anti-HBV activity in vivo against woodchuck hepatitis virus in chronically infected woodchucks and there is no significant virus rebound after cessation of the drug treatment.

Spiropentane mimics of nucleosides: analogues of 2'-deoxyadenosine and 2'-deoxyguanosine. Synthesis of all stereoisomers, isomeric assignment, and biological activity

Synthesis of spirocyclic analogues of 2'-deoxyadenosine and 2'-deoxyguanosine (12a-15a and 12b-15b) is described. Rhodium-catalyzed reaction of ethyl diazoacetate with methylenecyclopropane 19, obtained from 2-bromo-2-bromomethylcyclopropane 17 via debromination (16), reduction (18), and acetylation (19), gave a mixture of all four isomeric spiropentanes 20a-20d. Hydrolysis afforded hydroxy carboxylic acids 21a-21d. Acetylation of separated proximal + medial-syn isomers 21a + 21b and medial anti + distal isomers 21c + 21d furnished acetates 22a + 22b and 22c + 22d. Curtius rearrangement effected by diphenylphosphoryl azide in tert-butyl alcohol performed separately with mixtures 22a + 22b and 22c + 22d led to BOC-amino spiropentanes 23a + 23b and 23c + 23d. After deacetylation all isomers 24a-24d were separated and deprotected to give aminospiropentane hydrochlorides 25a-25d. Free bases were of limited stability. The heterocyclic moieties were introduced into individual isomers 25a-25d via 6-chloropurine derivatives 26a-26d or 30a-30d. Ammonolysis of 26a-26d furnished the adenine isomeric series 12a-15a, whereas guanine derivatives 12b-15b were obtained by hydrolysis of 30a-30d with formic acid. The isomeric assignments followed from IR spectra of BOC-aminospiropentanes 24a-24d and NMR spectra of 12a-15a including NOE and (H,H) COSY. The proximal and medial-syn isomers 12a and 12b were modest inhibitors of human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) in culture, whereas the medial-anti isomer 12c was a substrate for adenosine deaminase. The distal isomer 15b was an anti-EBV agent. The medial-syn phosphoralaninate 34 was an effective inhibitor of HCMV replication in vitro. It was also active against herpes simplex virus type 1 (HSV-1), varicella zoster virus (VZV), human immunodeficiency virus (HIV-1), hepatitis B virus (HBV), and EBV with a varying degree of cytotoxicity.

Unique biochemical, cytotoxic, and antitumor activity of camptothecin and 4beta-amino-4'-O-demethylepipodophyllotoxin conjugates

Two compounds having a camptothecin (CPT) analog conjugated to the 4beta-amino-4'-O-demethylepipodophyllotoxin analog were evaluated for their biochemical and biological activities. W1[camptothecin-(para)-4beta-amino-4'-O-demethylepipodophyllotoxin] had no activity against topoisomerase II (TOP II), but inhibited topoisomerase I (TOP I) with an IC(50) value 2-fold higher than CPT. W2 [camptothecin-(ortho)-4beta-amino-4'-O-demethylepipodophyllotoxin] had inhibitory activity against TOP I and TOP II with IC(50) values 1.5-fold higher than either CPT or etoposide (VP-16). Both conjugates had similar cytotoxicity against the KB cell line, although the protein-linked DNA breaks (PLDBs) generated by W2 in KB cells were about 4-fold more than those of W1. No cross-resistance with the two conjugates was seen in a VP-16-resistant KB subline, which showed down-regulation of TOP II and overexpression of the multiple drug resistance-associated protein, or in a vincristine-resistant KB subline with overexpression of gp-170/mdr-1. The CPT-resistant KB variant (KB CPT 100), which has a reduction in TOP I content and another mechanism that occurs post-PLDB formation, was partially resistant to both compounds. W1 was not affected by this post-PLDB resistance mechanism. Cell cycle analysis demonstrated that W1 and W1 had similar cell cycle effects on KB and KB CPT 100 cells, which accumulated in S-phase upon drug treatment. These results suggested that W1 and W2 exerted their cytotoxicity through TOP I. In CPT-resistant cells, however, an unidentified target also may be involved in the cytotoxic action of W1 and TOP II may still be a target for W1. In vivo, W1 was more effective against the growth of human prostate cancer cells in nude mice than VP-16, CPT, or W2. Given its antitumor activity and unique biochemical mechanism of action, W1 warrants exploration as an antitumor compound.

Design of anti-HIV compounds: from nucleoside to nucleoside 5'-triphosphate analogs. Problems and perspectives

To date, human immunodeficiency virus infection remains incurable although a variety of antiviral agents have been identified and characterized. Even though nucleoside analogs have been the most successful prodrugs, there remains the need to develop new compounds that exhibit a more favorable toxicity profile, less susceptible to cross-resistance, and greater efficacy. As prodrugs, the nucleoside analogs should be sequentially phosphorylated by cellular kinases to yield triphosphate form before they can inhibit HIV replication at the reverse transcriptase level. The efficiency of phosphorylation of nucleoside analogs is a key factor in their antiviral activity and strongly depends on nucleoside structure and cell type. In recent years, several attempts have been made to improve therapeutic potential of nucleoside analogs by the use of nucleotide prodrugs (pronucleotides), that can avoid the first step of phosphorylation. This review focuses on problems of intracellular phosphorylation of nucleoside analogs and perspectives of developing of a new class of nucleotide analogs modified at phosphate group as a form for the delivery of nucleotide analogs into the cell.

Synthesis and biological evaluation of 1,3-oxathiolane 5-azapyrimidine, 6-azapyrimidine, and fluorosubstituted 3-deazapyrimidine nucleosides

(2R,5S)-5-Amino-2-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]- 1,2,4-triazine-3(2H)-one (8) and (2R,5R)-5-amino-2-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-1,2,4-tr iazine-3(2H)-one (9) have been synthesized via a multi-step procedure from 6-azauridine. (2R,5S)-4-Amino-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-1,3, 5-triazine-2(1H)-one (11) and (2R,5R)-4-amino-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]- 1,3,5-triazine-2(1H)-one (12), and the fluorosubstituted 3-deazanucleosides (19-24) have been synthesized by the transglycosylation of (2R,5S)-1-[2-[[(tert-butyldiphenylsilyl) oxy]methyl]-1,3-oxathiolan-5-yl] cytosine (2) with silylated 5-azacytosine and the corresponding silylated fluorosubstituted 3-deazacytosines, respectively, in the presence of trimethylsilyl trifluoromethanesulfonate as the catalyst in anhydrous dichloroethane, followed by deprotection of the blocking groups. These compounds were tested in vitro for cytotoxicity against L1210, B16F10, and CCRF-CEM tumor cell lines and for antiviral activity against HIV-1 and HBV.

Excision of beta-L- and beta-D-nucleotide analogs from DNA by p53 protein

The tumor suppressor p53 protein plays a critical role in the cell-cycle progression. The role of the 3'-to-5' exonuclease activity of p53 protein in the DNA repair process remains elusive. Using an in vitro exonuclease assay and defined oligonucleotides terminated with beta-D- and beta-L-nucleoside analogs at the 3'-terminus, we studied the ability of p53 protein to excise beta-L- and beta-D-nucleoside analogs which have anticancer or antiviral potential. p53 protein removes beta-D-nucleoside analogs more efficiently compared to that of beta-L-nucleoside analogs. The affinity of p53 protein for an beta-L-nucleotide terminated primer was 5 fold lower compared to non-modified primer. The hypothesis on an important role of the 3'-to-5' exonuclease activity of p53 protein in the action of nucleoside analogs was proposed.

Synthesis and antiviral activities of enantiomeric 1-[2-(hydroxymethyl) cyclopropyl] methyl nucleosides

Cyclopropyl carbocyclic nucleosides have been synthesized from the key intermediate 2 which was converted to the mesylated cyclopropyl methyl alcohol 3. Condensation of compound 3 with various purine and pyrimidine bases gave the desired nucleosides. All synthesized nucleosides were evaluated for antiviral activity and cellular toxicity. Among them adenine 22 and guanine 23 derivatives showed moderate antiviral activity against HIV-1 and HBV. None of the other compounds showed any significant antiviral activities against HIV-1, HBV, HSV-1 and HSV-2 in vitro up to 100 microM.

Characterization of the antiviral effect of 2',3'-dideoxy-2', 3'-didehydro-beta-L-5-fluorocytidine in the duck hepatitis B virus infection model

A novel L-nucleoside analog of deoxycytidine, 2',3'-dideoxy-2', 3'-didehydro-beta-L-5-fluorocytidine (beta-L-Fd4C), was recently shown to strongly inhibit hepatitis B virus (HBV) replication in the 2.2.15 cell line. Therefore, its antiviral activity was evaluated in the duck HBV (DHBV) infection model. Using a cell-free system for the expression of the DHBV polymerase, beta-L-Fd4C-TP exhibited a concentration-dependent inhibition of dCTP incorporation into viral minus-strand DNA with a 50% inhibitory concentration of 0.2 microM which was lower than that of other tested deoxycytidine analogs, i.e. , lamivudine-TP, ddC-TP, and beta-L-FddC-TP. Further analysis showed that beta-L-Fd4C-TP is likely to be a competitive inhibitor of dCTP incorporation and to cause premature DNA chain termination. In primary duck hepatocyte cultures infected in vitro, beta-L-Fd4C administration exhibited a long-lasting inhibitory effect on viral DNA synthesis but could not clear viral covalently closed circular DNA (CCC DNA). Results of short-term antiviral treatment in experimentally infected ducklings showed that beta-L-Fd4C exhibited the most potent antiviral effect, followed by beta-L-FddC, lamivudine, and ddC. Longer administration of beta-L-Fd4C induced a sustained suppression of viremia (>95% of controls) and of viral DNA synthesis within the liver. However, the persistence of trace amounts of viral CCC DNA detected only by PCR was associated with a recurrence of viral replication after drug withdrawal. In parallel, beta-L-Fd4C treatment suppressed viral antigen expression within the liver and decreased intrahepatic inflammation and was not associated with any sign of toxicity. Our data, therefore, demonstrate that in the duck model of HBV infection, beta-L-Fd4C is a potent inhibitor of DHBV reverse transcriptase activity in vitro and suppresses viral replication in the liver in vivo.

Synthesis of 2',3'-dideoxy-2'-fluoro-L-threo-pentofuranosyl nucleosides as potential antiviral agents

A series of 2',3'-dideoxy-2'-fluoro-L-threo-pentofuranosyl nucleosides has been synthesized as potential antiviral agents. The synthesized compounds were evaluated against HIV-1, HBV, HSV-1, and HSV-2. Among the synthesized analogues, only the cytosine derivative showed moderate antiviral activity against HIV and HBV.

Asymmetric synthesis and antiviral activities of L-carbocyclic 2', 3'-didehydro-2',3'-dideoxy and 2',3'-dideoxy nucleosides

Asymmetric syntheses of L-carbocyclic 2',3'-didehydro-2',3'-dideoxy- and 2',3'-dideoxypyrimidine and purine nucleoside analogues were accomplished, and their anti-HIV and anti-HBV activities were evaluated. The key intermediate, (1S, 4R)-1-benzoyloxy-4-(tert-butoxymethyl)cyclopent-2-ene (7), was prepared by benzoylation of the alcohol 2, selective deprotection of the isopropylidene group of 3, followed by thermal elimination via cyclic ortho ester or deoxygenation via cyclic thionocarbonate. The target compounds were also synthesized by thermal elimination via cyclic ortho esters from protected nucleosides. It was found that L-carbocyclic 2',3'-didehydro-2',3'-dideoxyadenosine (34) exhibited potent anti-HBV activity (EC(50) = 0.9 microM) and moderate anti-HIV activity (EC(50) = 2.4 microM) in vitro without cytotoxicity up to 100 microM.

Synthesis and antiviral activity of phosphoralaninate derivatives of methylenecyclopropane analogues of nucleosides

Phenylmethylphosphoro-L-alaninate prodrugs of antiviral Z-methylenecyclopropane nucleoside analogues and their inactive E-isomers were synthesized and evaluated for their antiviral activity against HCMV, HSV-1, HSV-2, HHV-6, EBV, VZV, HIV-1 and HBV. The adenine Z-analogue was a potent inhibitor of all these viruses but it displayed cellular toxicity. The guanine Z-derivative was active against HCMV, HBV, EBV and VZV and it was not cytotoxic. The 2,6-diaminopurine analogue was the most potent against HIV-1 and HBV and somewhat less against HHV-6, HCMV, EBV and VZV in a non-cytotoxic concentration range. The 2-amino-6-cyclopropylamino and 2-amino-6-methoxypurine prodrugs were also more active than parent analogues against several viruses but with a less favorable cytotoxicity profile. In the E-series of analogues, adenine derivative was active against HIV-1, HBV and EBV, and it was non-cytotoxic. The guanine analogue exhibited a significant effect only against HBV. The 2,6-diaminopurine E-analogue was inactive with the exception of a single EBV assay. The 2-amino-6-methoxypurine Z-methylenecyclopropane nucleoside analogue was an effective inhibitor of HCMV, MCMV and EBV. The 2,6-diaminopurine Z-prodrug seems to be the best candidate for further development.

Antitumor agents. 194. Synthesis and biological evaluations of 4-beta-mono-, -di-, and -trisubstituted aniline-4'-O-demethyl-podophyllotoxin and related compounds with improved pharmacological profiles

As a continuation of our structure-activity relationship studies, several new 4-beta-substituted 4'-O-demethyl-4-desoxypodophyllotoxins bearing mono-, di-, or trisubstituted anilines have been synthesized and evaluated as inhibitors of DNA topoisomerase II and tumor cell growth in tissue culture. Selected compounds were further evaluated as cytotoxic agents using a clonogenic survival assay. The target compounds include 4'-O-demethyl-4beta-[(4' '-(benzimidazol-2' '-yl)anilino]-4-desoxypodophyllotoxin (21), 4'-O-demethyl-4beta-(-)-(4' '-camphanamido-anilino)-4-desoxypodophyllotoxin (25), 4-beta-disubstituted-anilino-4'-demethyl-4-desoxypodophyllotoxins (18-20, 26), 4-alpha-disubstituted-anilino-4'-demethyl-4-desoxypodophyllotoxin (27), 4-beta-trisubstituted-anilino-4'-demethyl-desoxypodophyllotoxin (22, 23), and 4'-O-demethyl-4beta-[4' '-(benzimidazol-2' '-yl)amino]-4-desoxypodophyllotoxin (24). Among the target series, 19, 21, and 24 displayed significant growth inhibitory action against a panel of tumor cell lines including human epidermoid carcinoma of the nasopharynx (KB) and its etoposide-resistant (KB7B) and vincristine-resistant (vin20c KB) subclones, lung carcinoma (A549), human ileocecal carcinoma (HCT-8), human kidney carcinoma (CAKI-1), breast adenocarcinoma (MCF-7), and human malignant melanoma (SK-MEL-2) cells. Compounds 19, 21, 24, and 25 were "cleavable-complex"-forming DNA topoisomerase II inhibitors with either improved or similar activity compared with the prototype drug etoposide (VP-16). Compound 21 was the most active analogue, being 10-fold more potent than etoposide in both cell killing and topoisomerase II inhibition in vitro assays. Using mouse models of antitumor activity, 21 was effective against (P388/0) leukemia but not against the growth of a (MCF7) mammary tumor.

Structure-activity relationships of L-dioxolane uracil nucleosides as anti-Epstein Barr virus agents

A series of 1,3-dioxolanyluracil analogues was prepared from the dioxolane intermediates 2, and their anti-Epstein Barr virus (anti-EBV) activities were determined. The potency of L-dioxolane uracil nucleosides against EBV replication is dependent on the substituents at the 5-position in the following decreasing order: I > Br > Cl > CH3 > CF3 > F. The most active and selective analogue was the iodo derivative (L-I-OddU) with an EC50 value of 0.03 microM and an EC90 value of 0.16 microM. There was no cytotoxicity or depletion of mitochondrial DNA in cells after exposure to L-I-OddU at 50 microM. The action against EBV replication in H1 cells is time-dependent, and EBV DNA in cells treated with L-I-OddU could rebound to pretreatment levels once the drug was removed. In view of the potent antiviral activity plus favorable toxicity profiles, L-I-OddU may be potentially useful for the treatment of EBV-related infectious diseases as well as for delaying the onset or decreasing the incidence of EBV-associated cancers.

Sensitivity of L-(-)2,3-dideoxythiacytidine resistant hepatitis B virus to other antiviral nucleoside analogues

L-(-)2',3'-Dideoxythiacytidine (L(-)SddC, Lamivudine) resistant hepatitis B virus (HBV) develops in patients after prolonged treatment. Point mutations detected in the viral genome from these patients have been shown to be responsible for L(-)SddC resistance. Therefore, new drugs active against L(-)SddC resistant HBV are needed. Using a transient transfection system, we studied the sensitivity of L(-)SddC resistant HBV to other anti-HBV nucleoside analogues. It was found that the L526M mutation alone caused greater resistance to penciclovir (PCV) than did the V553I mutation alone. Both mutations also caused the virus to be less sensitive to L(-)SddC and 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU), although the degree of resistance was much less than that to PCV. The A546V mutation had no impact on the sensitivity to L(-)SddC, L-FMAU, and PCV. When these single mutations were coupled with the M550V/I mutation, all the double mutants were resistant to those drugs. Although 2',3'-dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine (L(-)Fd4C) was also less active, the IC50 of L(-)Fd4C against the L(-)SddC resistant mutant was at least fifty times lower than that against cell growth in culture. DNA polymerase associated with L(-)SddC resistant virions was also found to be less sensitive than that with wild-type HBV to those L-nucleoside triphosphates. All the L(-)SddC resistant mutants were still sensitive to 9-(2-phosphonylmethoxyethyl)-adenine (PMEA). These results suggest that different mutations in the HBV genome have a different impact on its sensitivity to those compounds, and L(-)SddC resistant HBV may also be resistant to PCV, L-FMAU, and L(-)Fd4C. A nucleoside analogue less toxic than PMEA could be developed against L(-)SddC resistant HBV.

Synthesis and anti-HIV and anti-HBV activities of 2'-fluoro-2', 3'-unsaturated L-nucleosides

The synthesis of L-nucleoside analogues containing 2'-vinylic fluoride was accomplished by direct condensation method, and their anti-HIV and anti-HBV activities were evaluated in vitro. The key intermediate 8, the sugar moiety of our target compounds, was prepared from 1,2-O-isopropylidene-L-glyceraldehyde via (R)-2-fluorobutenolide intermediate 5 in five steps. Coupling of the acetate 8 with the appropriate heterocycles (silylated uracil, thymine, N4-benzoylcytosine, N4-benzoyl-5-fluorocytosine, 6-chloropurine, and 6-chloro-2-fluoropurine) in the presence of Lewis acid afforded a series of 2'-fluorinated L-nucleoside analogues (15-18, 23-26, 36-45). The newly synthesized compounds were evaluated for their antiviral activities against HIV-1 in human peripheral blood mononuclear (PBM) cells and HBV in 2.2.15 cells. Cytosine 23, 5-fluorocytosine 25, and adenine 36 derivatives exhibited moderate to potent anti-HIV (EC50 0.51, 0.17, and 1.5 microM, respectively) and anti-HBV (EC50 0.18, 0.225, and 1.7 microM, respectively) activities without significant cytotoxicity up to 100 microM in human PBM, Vero, CEM, and HepG2 cells.

Methylenecyclopropane analogues of nucleosides: synthesis, absolute configuration, and enantioselectivity of antiviral effect of (R)-(-)- and (S)-(+)-synadenol

Synthesis, absolute configuration and antiviral activity of enantiomeric antiviral agents (R)-(-)- and (S)-(+)-synadenol (2 and 3a) are described.

Degradation of topoisomerase I induced by topoisomerase I inhibitors is dependent on inhibitor structure but independent of cell death

DNA topoisomerase I (top I) is the target of the antitumor drug camptothecin (CPT) and its analogs. CPT induces dose- and time-dependent degradation of top I. Degradation of top I also occurs in a CPT-resistant cell line and, therefore, is not a consequence of cell death. Top I degradation is preceded by the appearance of a high molecular weight ladder of top I immunoreactivity and can be blocked by specific inhibitors of the proteasome. We compared the effects of five top I poisons [CPT, topotecan, 6-N-formylamino-12,13-dihydro-1, 11-dihydroxy-13-(beta-D-glucopyranosyl)-5H-indolo[2,3-a]pyrrolo[3, 4-c]carbazole-5,7(6H)-dione (NB506), camptothecin-(para)-4beta-amino-4'-O-demethyl Epipodophyllotoxin (W1), and camptothecin-(ortho)-4beta-amino-4'-O-demethyl Epipodophyllotoxin (W2)] on cleavable complex formation and top I degradation. Although all five drugs induced cleavable complex formation, two of the drugs, NB506 and W1 did not induce top I degradation.

Characterization of a dCTP transport activity reconstituted from human mitochondria

A protein fraction of mitochondria from human acute lymphocytic leukemia cells, which could be reconstituted into proteoliposomes to have dCTP transport activity, has been partially purified by hydroxyapatite and blue Sepharose chromatography. The dCTP transport activity in proteoliposomes was time-dependent and could be activated by Ca2+ and to a lesser extent by Mg2+. None of the other divalent cations tested could activate the transport activity. The Km value of dCTP in the presence of Ca2+ was shown to be 3 microM. dCDP but not dCMP or dCyd could inhibit the transport activity. Other deoxynucleoside triphosphates could also inhibit the uptake of dCTP with the potency dGTP = dATP > TTP. Although ATP could competitively inhibit dCTP uptake with a Ki value of 8 microM, the reconstituted dCTP uptake activity was not sensitive to the ATP/ADP carrier inhibitor atractyloside or the sulfhydryl reagent N-ethylmaleimide. This suggests that the dCTP transport system studied is not the same as the ATP/ADP carrier. In conclusion, these studies describe the first functionally reconstituted mitochondrial carrier that displays an efficient transport activity for dCTP.

Synthesis of 2'-methylene-substituted 5-azapyrimidine, 6-azapyrimidine, and 3-deazaguanine nucleoside analogues as potential antitumor/antiviral agents

2'-Deoxy-2'-methylene-6-azauridine (5) and 2'-deoxy-2'-methylene-6-azacytidine (8) have been synthesized via a multi-step procedure from 6-azauridine. 2'-Deoxy-2'-methylene-5-azacytidine (14a) and 2'-deoxy-2'-methylene-3-deazaguanosine (19a) and their corresponding alpha-anomers (14b and 19b) have been synthesized by the transglycosylation of 3',5'-O-(1,1,3,3- tetraisopropyldisiloxane-1,3-diyl)-2'-deoxy-2'-methyleneu ridine (12) with silylated 5-azacytosine and silylated N2-palmitoyl-3-deazaguanine, respectively, in the presence of trimethylsilyl trifluoromethanesulfonate as the catalyst in anhydrous dichloroethane, followed by separation of the isomers and deprotection of the blocking groups. These compounds were tested for cytotoxicity against B16F10, L1210, and CCRF-CEM tumor cell lines and for antiviral activity against HIV-1, HSV-1, and HSV-2.

(R)-(-)- and (S)-(+)-Synadenol: synthesis, absolute configuration, and enantioselectivity of antiviral effect

Synthesis of (R)-(-)- and (S)-(+)-synadenol (1a and 2a, 95-96% ee) is described. Racemic synadenol (1a + 2a) was deaminated with adenosine deaminase to give (R)-(-)-synadenol (1a) and (S)-(+)-hypoxanthine derivative 5. Acetylation of the latter compound gave acetate 6. Reaction with N, N-dimethylchloromethyleneammonium chloride led to 6-chloropurine derivative 7. Ammonolysis furnished (S)-(+)-synadenol (2a). Absolute configuration of 1a was established by two methods: (i) synthesis from (R)-methylenecyclopropanecarboxylic acid (8) and (ii) X-ray diffraction of a single crystal of (-)-synadenol hydrochloride. Racemic methylenecyclopropanecarboxylic acid (10) was resolved by a modification of the described procedure. The R-enantiomer 8 was converted to ethyl ester 13 which was brominated to give vicinal dibromides 14. Reduction with diisobutylaluminum hydride then furnished alcohol 15 which was acetylated to the corresponding acetate 16. Alkylation-elimination procedure of adenine with 16 yielded acetates 17 and 18. Deprotection with ammonia afforded a mixture of Z- and E-isomers 1a and 19 of the R-configuration. Comparison with products 1a and 2a by chiral HPLC established the R-configuration of (-)-synadenol (1a). These results were confirmed by X-ray diffraction of a single crystal of (-)-synadenol hydrochloride. The latter forms a pseudosymmetric dimer with adenine-adenine base pairing in the lattice with the nucleobase in an anti-like conformation. Enantiomers 1a and 2a exhibit varied enantioselectivity toward different viruses. Both enantiomers are equipotent against human cytomegalovirus (HCMV) and varicella zoster virus (VZV). The S-enantiomer 2a is somewhat more effective than R-enantiomer 1a in herpes simplex virus 1 and 2 (HSV-1 and HSV-2) assays. By contrast, enantioselectivity of antiviral effect is reversed in Epstein-Barr virus (EBV) and human immunodeficiency virus type 1 (HIV-1) assays where the R-enantiomer 1a is preferred. In these assays, the S-enantiomer 2a is less effective (EBV) or devoid of activity (HIV-1).

Synthesis and comparative evaluation of two antiviral agents: beta-L-Fd4C and beta-D-Fd4C

The synthesis of beta-D-Fd4C was achieved in a stereoselective fashion from D-xylose. The antiviral activity and cytotoxicity of beta-D-Fd4C was compared with that of beta-L-Fd4C and 3TC (Lamivudine). Of the three agents compared, beta-L-Fd4C was found to be the most potent antiviral agent.

Preoperative localization procedures for initial surgery in primary hyperparathyroidism

The experience of the surgeon and precise localization of abnormal parathyroid glands determine the success of surgery for primary hyperparathyroidism (HPT). In HPT patients undergoing repeat surgery, the use of localization studies improved the ability to identify the remaining abnormal parathyroid tissue. This study investigated the roles of preoperative localization techniques for initial surgery for primary HPT. From 1985 through 1997, two noninvasive localization procedures, ultrasonography (US) and 201thallium chloride-99mtechnetium pertechnetate subtraction scanning (Tl-Tc), were used prior to initial exploration for primary HPT in 76 patients. Their accuracy was determined on the basis of surgical and pathologic results. The surgical success rate was 96% (73/76). The sensitivities of US and Tl-Tc were 71% and 49%, respectively. The sensitivity of Tl-Tc was higher for the lower parathyroid glands. In 21 of 26 patients who underwent fine-needle aspiration (FNA) of the suspected enlarged parathyroid gland, the diagnosis of parathyroid adenoma was confirmed preoperatively. We conclude that the concomitant use of US and FNA is a safe and convenient method for preoperative localization of the parathyroid glands prior to initial surgical exploration in patients with primary HPT. Bilateral neck exploration by an experienced surgeon should be the routine procedure. US and Tl-Tc alone offer limited localization information, and unilateral exploration should be reserved for selected cases in which the results of these two imaging studies are consistent with one another.

Metabolism of 2',3'-dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine and its activity in combination with clinically approved anti-human immunodeficiency virus beta-D(+) nucleoside analogs in vitro

2',3'-Dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine [L(-)Fd4C] has been reported to be a potent inhibitor of the human immunodeficiency virus (HIV) in cell culture. In the present study the antiviral activity of this compound in two-drug combinations and its intracellular metabolism are addressed. The two-drug combination of L(-)Fd4C plus 2',3'-didehydro-2'-3'-dideoxythymidine (D4T, or stavudine) or 3'-azido-3'-deoxythymidine (AZT, or zidovudine) synergistically inhibited replication of HIV in vitro. Additive antiviral activity was observed with L(-)Fd4C in combination with 2',3'-dideoxycytidine (ddC, or zalcitabine) or 2',3'-dideoxyinosine (ddI, or didanosine). This beta-L(-) nucleoside analog has no activity against mitochondrial DNA synthesis at concentrations up to 10 microM. As we previously reported for other beta-L(-) nucleoside analogs, L(-)Fd4C could protect against mitochondrial toxicity associated with D4T, ddC, and ddI. Metabolism studies showed that this drug is converted intracellularly to its mono-, di-, and triphosphate metabolites. The enzyme responsible for monophosphate formation was identified as cytoplasmic deoxycytidine kinase, and the K(m) is 100 microM. L(-)Fd4C was not recognized in vitro by human mitochondrial deoxypyrimidine nucleoside kinase. Also, L(-)Fd4C was not a substrate for deoxycytidine deaminase. L(-)Fd4C 5'-triphosphate served as an alternative substrate to dCTP for incorporation into DNA by HIV reverse transcriptase. The favorable anti-HIV activity and protection from mitochondrial toxicity by L(-)Fd4C in two-drug combinations favors the further development of L(-)Fd4C as an anti-HIV agent.

(Z)- and (E)-2-(hydroxymethylcyclopropylidene)-methylpurines and pyrimidines as antiviral agents

Several Z- and E-methylenecyclopropane nucleoside analogues were synthesized and tested for antiviral activity in vitro against human and murine cytomegalovirus (HCMV, MCMV), Epstein-Barr virus (EBV), varicella zoster virus (VZV), hepatitis B virus (HBV), herpes simplex virus types 1 and 2 (HSV-1, HSV-2), human herpesvirus 6 (HHV-6) and human immunodeficiency virus type 1 (HIV-1). The Z-2-amino-6-cyclopropylaminopurine analogue was the most effective agent against HCMV (EC50 or EC90 0.4-2 microM) followed by syncytol and the Z-2,6-diaminopurine analogues (EC50 or EC90 3.4-29 and 11-24 microM, respectively). The latter compound was also a strong inhibitor of MCMV (EC50 0.6 microM). Syncytol was the most potent against EBV (EC50 < 0.41 and 2.5 microM) followed by the Z-2,6-diaminopurine (EC50 1.5 and 6.9 microM) and the Z-2-amino-6-cyclopropyl-aminopurine derivative (EC50 11.8 microM). Syncytol was also most effective against VZV (EC50 3.6 microM). Activity against HSV-1, HSV-2 and HHV-6 was generally lower; synthymol had an EC50 of 2 microM against HSV-1 (ELISA) and 1.3 microM against EBV in Daudi cells but was inactive in other assays. The 2-amino-6-cyclopropylamino analogue displayed EC50 values between 215 and > 74 microM in HSV-1 and HSV-2 assays. 2-Amino-6-cyclopropylaminopurine and 2,6-diaminopurine derivatives were effective against HBV (EC50 2 and 10 microM, respectively), whereas none of the analogues inhibited HIV-1 at a higher virus load. Syncytol and the E isomer were equipotent against EBV in Daudi cells but the E isomer was much less effective in DNA hybridization assays. The E-2,6-diaminopurine analogue and E isomer of synthymol were devoid of antiviral activity.

Anti-hepatitis B virus activity and metabolism of 2',3'-dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine

2',3'-Dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine [L(-)Fd4C] was found to be at least 10 times more potent than beta-L-2',3'-dideoxy-3'-thiacytidine [L(-)SddC; also called 3TC, or lamivudine]against hepatitis B virus (HBV) in culture. Its cytotoxicity against HepG2 growth in culture was also greater than that of L(-)SddC (3TC). There was no activity of this compound against mitochondrial DNA synthesis in cells at concentrations upto 10 microM. The dynamics of recovery of virus from the medium of cells pretreated with equal drug concentrations were slower with L(-)Fd4C than with L(-)SddC (3TC). L(-)Fd4C could be metabolized to mono-, di-, and triphosphate forms. The degree of L(-)Fd4C phosphorylation to the 5'-triphosphate metabolite was higher than the degree of L(-)SddC (3TC) phosphorylation when equal extracellular concentrations of the two drugs were used. The apparent K(m) of L(-)Fd4C phosphorylated metabolites formed intracellularly was higher than that for L(-)SddC (3TC). This may be due in part to a difference in the behavior of L(-)Fd4C and L(-)SddC (3TC) towards cytosolic deoxycytidine kinase. Furthermore, L(-)Fd4C 5'-triphosphate was retained longer within cells than L(-)SddC (3TC) 5-triphosphate. L(-)Fd4C 5'-triphosphate inhibited HBV DNA polymerase in competition with dCTP with a Ki of 0.069 +/- 0.015 microM. Given the antiviral potency and unique pharmacodynamic properties of L(-)Fd4C, this compound should be considered for development as an expanded-spectrum anti-HBV drug.

Biochemical characterization of the HIV-1 integrase 3'-processing activity and its inhibition by phosphorothioate oligonucleotides

To better understand HIV-1 integrase (IN) functions, we determined the kinetic parameters of the 3'-processing reaction. Steady-state kinetic analysis performed using Dixon plots indicated that the concentration of active enzyme was 10-fold lower than that calculated by protein determination. The turnover number was low, suggesting that IN remained bound to DNA after cleavage. The catalytic efficiency increased 10-fold from 30 to 37 degrees C and 2-fold from 37 to 42 degrees C. In enzyme assays carried out at 37 degrees C, both single- and double-stranded phosphorothioate oligos bound to IN with an efficiency comparable to that of the phosphodiester duplex substrate. The competition efficiency of single-stranded oligos was directly related to the sequence length. On the other hand, phosphorothioate duplex U5 LTRs modified in the plus strand were capable of both competing with the substrate and directly inhibiting the 3'-processing activity. These results suggest that, in addition to other modes of action (inhibition of gp120-CD4 interaction and reverse transcriptase), phosphorothioate hetero- and homopolimeric oligos also potently inhibit the IN activity.

Role of additional mutations outside the YMDD motif of hepatitis B virus polymerase in L(-)SddC (3TC) resistance

L(-)SddC (3TC) has been shown to be the most promising nucleoside analogue used for the treatment of hepatitis B virus (HBV) infection. Unfortunately, it has been reported that about 12% of HBV-infected patients experience a recurrence of HBV after a period of treatment with 3TC. Point mutations were detected in the HBV polymerase of those viruses from 3TC-resistant patients. A common mutation occurred at methionine in the YMDD motif. In this report, we present mutants that were generated from the HBV genome (adr subtype) by site-directed mutagenesis based on clinical reports from other investigators. With the transient transfection system, it was found that by changing methionine to valine or isoleucine at the YMDD motif, the viral DNA replication would be more than 100-fold less efficient than that of the wild-type virus. Some additional mutations outside the YMDD motif could enhance the replication of the virus containing a YMDD mutation. Various levels of resistance to 3TC were observed in HBV mutants containing point mutations both inside and outside the YMDD motif. These results suggest that the mutations outside the YMDD motif compensate the YMDD mutation to some extent for the viral replication and may also contribute to clinical viral resistance to 3TC.

Interaction of beta-L-2',3'-dideoxy-2',3'-didehydro-5-fluoro-CTP with human immunodeficiency virus-1 reverse transcriptase and human DNA polymerases: implications for human immunodeficiency virus drug design

The work reported in this article has evaluated the relative molecular activity of the 5'-triphosphate of a novel beta-L-nucleoside with an unsaturated ribose residue, beta-L-2', 3'-dideoxy-2',3'-didehydro-5-fluorocytidine (beta-L-Fd4CTP), with that of beta-L-2',3'-dideoxy-5-fluorocytidine (beta-L-FddCTP) and 2', 3'-dideoxycytidine (ddCTP), on DNA strand elongation by human immunodeficiency virus-1 reverse transcriptase (HIV RT) and human DNA polymerases alpha (pol alpha), beta (pol beta), gamma (pol gamma), and epsilon (pol epsilon). The concentrations of beta-L-Fd4CTP that inhibited the yield of products by 50% were 0.20 micro M, 1.8 micro M, and 4.0 micro M for HIV RT, pol gamma, and pol beta, respectively. The beta-L-Fd4CTP at a concentration as high as 40 micro M had no inhibitory effect on pol epsilon, but could inhibit pol alpha by 10-20% at 20 micro M. The Km and relative Vmax values of beta-L-Fd4CTP, beta-L-FddCTP, and ddCTP for incorporation into the standing start point of 5'-[32P]-oligonucleotide primer annealed with M13mp19 phage DNA by HIV RT and human DNA polymerases were evaluated. The efficiency of incorporation (Vmax/Km) of beta-L-Fd4CTP by HIV RT was about 4-fold and 12-fold higher than that of ddCTP and beta-L-FddCTP, respectively. In contrast, the Vmax/Km ratio of beta-L-Fd4CTP for pol gamma was 7-fold lower than that of ddCTP, but 4-fold higher than that of beta-L-FddCTP. Pol alpha could use beta-L-Fd4CTP as a substrate, but only at a high concentration (>20 micro M). Incorporation of beta-L-Fd4CTP by pol epsilon could not be detected. A hypothesis about the preferable recognition of the 2',3'-dideoxy-2',3'-didehydro- structure of beta-L-Fd4CTP to that of the 2',3'-dideoxy-structure of beta-L-FddCTP by HIV RT is discussed.

Telomerase from human leukemia cells: properties and its interaction with deoxynucleoside analogues

Telomerase is a unique reverse transcriptase involved in the maintenance of genomic integrity. In an attempt to understand the properties of this enzyme and to study the effect of deoxynucleoside analogues, we have isolated and partially purified telomerase from the blast cells of a patient with acute myelogenous leukemia. During the course of purification of telomerase, three characteristic forms of this enzyme activity were separated. Two processive forms and one less processive form were noted. All forms of the enzyme activities could be abolished by RNase A and proteinase K treatments, implying that they are ribonucleoproteins. The major form of telomerase was characterized with respect to divalent ion requirements, effect of salt and nonionic detergents. The Km of deoxynucleoside triphosphates was determined with a modified telomerase repeat array protocol assay. Studies with deoxynucleoside analogues indicated that 3'-azido-3'deoxythymidine triphosphate is much more inhibitory than 2',3'-dideoxy 2',3'didehydrothymidine triphosphate, and the cytidine analogue ddCTP was not inhibitory. ddGTP was the most potent inhibitor among all dideoxynucleosides studied.

Decreased drug accumulation without increased drug efflux in a novel MRP-overexpressing multidrug-resistant cell line

KB/7D cells represent a multidrug-resistant subclone of human nasopharyngeal carcinoma KB cells generated by continuous exposure to the topoisomerase II inhibitor VP-16 (etoposide). KB/7D cells also show cross-resistance to doxorubicin and vincristine. Phenotypic traits of the cell line include a 2-fold decrease in topoisomerase II levels and a decrease in the uptake of VP-16 without an increase in the rate of drug efflux or expression of P-glycoprotein, suggesting a novel mechanism associated with the uptake of anticancer drugs. This study demonstrated that the multidrug-resistance associated protein (MRP) is overexpressed in KB/7D cells, and that the loss of resistance in revertant cells correlates with the loss of MRP. The resistance to VP-16 and doxorubicin could be overcome, partially, and resistance to vincristine could be overcome completely, by the L-enantiomer of verapamil, but not by the D-enantiomer or by BIBW 22 (4-[N-(2-hydroxy-2-methyl-propyl)-ethanolamino]-2,7-bis[cis-2,6-++ +dimethylmorpholino)-6-phenylpteridin), an inhibitor of MDR-1. L-Verapamil was shown to be significantly more potent than D-verapamil in modulating the accumulation defect in KB/7D cells towards doxorubicin, as measured by flow cytometry and confocal microscopy, and towards VP-16, as measured by increases in protein-linked DNA strand breaks. This suggests that KB/7D cells are multidrug resistant due to decreases in topoisomerase II levels and the overexpression of MRP, that MRP leads to a decrease in drug accumulation, and that L-verapamil can modulate the MRP-associated accumulation defect and drug-resistance phenotype. This contrasts with previous studies that suggest that MRP causes multidrug resistance by exporting cytotoxic drugs out of the cell and that did not show modulation of MRP by verapamil.

Interaction of human plasma membrane proteins and oligodeoxynucleotides

Two oligodeoxynucleotide (oligodN) binding proteins of 100-110 kDa on plasma membranes of human cell lines were recently identified by us. These two proteins seemed to play a role in oligodN uptake. In this study, the impact of the chain length and the sequence of the oligodN on the interaction with those two proteins was investigated. Chain length of oligodN was an important determinant, but not the sole determinant for the interaction. Binding affinity of oligodNs was determined predominantly by base composition, where pyrimidine bases but not purine bases were required in the sequence to retain high affinity. The binding kinetics of the homopolymers of deoxycytidine (dC21) and deoxythymidine (dT21) suggests that the proteins may have different binding sites, with one site preferring thymine bases and the other cytosine bases. Moreover, some additional plasma membrane proteins were identified, with an apparent molecular mass ranging from 40 to 58 kDa, which could bind thymine bases but not cytosine bases.

Unique inhibitory effect of 1-(2'-deoxy-2'-fluoro-beta-L-arabinofuranosyl)-5-methyluracil 5'-triphosphate on Epstein-Barr virus and human DNA polymerases

1-(2'-Deoxy-2'-fluoro-beta-L-arabinofuranosyl)-5-methyluracil (L-FMAU) was shown to have potent antiviral activity against Epstein-Barr virus (EBV) without any cellular toxicity at concentrations up to 200 microM (Yao et al., Biochem Pharmacol 51: 941-947, 1996). The 5'-triphosphate of L-FMAU was not a substrate for EBV or cellular DNA polymerases, but could inhibit the elongation reaction, 3'-to-5' exonuclease activity, and nucleotide turnover catalyzed by EBV DNA polymerase. DNA synthesis catalyzed by human DNA polymerases was inhibited to a lesser extent. The inhibition pattern of EBV DNA polymerase by L-FMAU-5'-triphosphate (L-FMAU-TP) was consistent with an uncompetitive mechanism when dNTP or template-primer were used as the variable substrates. The Ki values were 38+/-10 microM for the elongation reaction, and about 50+/-10 microM for both nucleotide exchange and 3'-to-5' exonuclease reactions, values that were 10-20 times less than that for GMP. L-FMAU-TP is the first nucleoside 5'-triphosphate shown to have such unique behavior toward DNA polymerases. EBV DNA polymerase could be one of the targets for the inhibitory effect of L-FMAU-TP on EBV replication.

Unique metabolism of a novel antiviral L-nucleoside analog, 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil: a substrate for both thymidine kinase and deoxycytidine kinase

2'-Fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU) is the first L-nucleoside analog with low cytotoxicity discovered to have potent antiviral activities against both hepatitis B virus and Epstein-Barr virus but not human immunodeficiency virus. This spectrum of activity is different from those of the other L-nucleoside analogs examined. L-FMAU enters cells through equilibrative-sensitive and -insensitive nucleoside transport as well as through nonfacilitated passive diffusion. L-FMAU is phosphorylated stepwise in cells to its mono-, di-, and triphosphate forms. In the present study the enzymes responsible for the first step of L-FMAU phosphorylation were identified. This is the first thymidine analog shown to be a substrate not only for cytosolic thymidine kinase and mitochondrial deoxypyrimidine kinase but also for deoxycytidine kinase. This finding suggests that the antiviral activity of L-FMAU will not be limited by the loss or alteration of any of these deoxynucleoside kinases.

Inhibitory effect of 2'-fluoro-5-methyl-beta-L-arabinofuranosyl-uracil on duck hepatitis B virus replication

The antiviral activity of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU), a novel L-nucleoside analog of thymidine known to be an inhibitor of hepatitis B virus (HBV) replication in hepatoma cells (2.2.1.5 cell line), was evaluated in the duck HBV (DHBV) model. Short-term oral administration (5 days) of L-FMAU (40 mg/kg of body weight/day) to experimentally infected ducklings induced a significant decrease in the level of viremia. This antiviral effect was sustained in animals when therapy was prolonged for 8 days. The histological study showed no evidence of liver toxicity in the L-FMAU-treated group. By contrast, microvesicular steatosis was found in the livers of dideoxycytidine-treated animals. L-FMAU administration in primary duck hepatocyte cultures infected with DHBV induced a dose-dependent inhibition of both virion release in culture supernatants and intracellular viral DNA synthesis, without clearance of viral covalently closed circular DNA. By using a cell-free system for the expression of an enzymatically active DHBV reverse transcriptase, it was shown that L-FMAU triphosphate exhibits an inhibitory effect on the incorporation of dAMP in the viral DNA primer. Thus, our data demonstrate that L-FMAU inhibits DHBV replication in vitro and in vivo. Long-term administration of L-FMAU for the eradication of viral infection in animal models of HBV infection should be evaluated.

(Z)- and (E)-2-((hydroxymethyl)cyclopropylidene)methyladenine and -guanine. New nucleoside analogues with a broad-spectrum antiviral activity

New nucleoside analogues 14-17 based on a methylenecyclopropane structure were synthesized and evaluated for antiviral activity. Reaction of 2,3-dibromopropene (19) with adenine (18) led to bromoalkene 20, which was benzoylated to give N6,N6-dibenzoyl derivative 23. Attempts to convert 20 or 23 to bromocyclopropanes 21 and 22 by reaction with ethyl diazoacetate catalyzed by Rh2(OAc)4 were futile. By contrast, 2,3-dibromopropene (19) afforded smoothly (E)- and (Z)-dibromocyclopropane carboxylic esters 24 + 25. Alkylation of adenine (18) with 24 + 25 gave (E)- and (Z)-bromo derivatives 21 + 22. Base-catalyzed elimination of HBr resulted in the formation of (Z)- and (E)-methylenecyclopropanecarboxylic esters 26 + 27. More convenient one-pot alkylation-elimination of adenine (18) or 2-amino-6-chloropurine (30) with 24 + 25 afforded (Z)- and (E)-methylenecyclopropane derivatives 26 + 27 and 31 + 32. The Z-isomers were always predominant in these mixtures (Z/E approximately 2/1). Reduction of 26 + 27 and 31 + 32 with DIBALH afforded (Z)- and (E)-methylenecyclopropane alcohols 14 + 16 and 33 + 34. The latter were resolved directly by chromatography. Compounds 14 + 16 were converted to N6-(dimethylamino)methylene derivatives 28 and 29 which were separated and deprotected to give 14 and 16. Reaction of 33 and 34 with HCO2H led to guanine analogues 15 and 17. The 1H NMR spectra of the Z-analogues 14 and 15 are consistent with an anti-like conformation of the nucleobases. By contrast, 1H NMR and IR spectra of bromo ester 21 are indicative of syn-conformation of adenine. Several Z-(hydroxymethyl)methylenecyclopropanes exhibited in vitro antiviral activity in micromolar or submicromolar range against human and murine cytomegalovirus (HCMV and MCMV), Epstein-Barr virus (EBV), human herpes virus 6 (HHV-6), varicella zoster virus (VZV), and hepatitis B virus (HBV). Analogues 14, 15, and 33 were the most effective agents against HCMV (IC50 1-2.1, 0.04-2.1, and 0.8-5.6 microM), MCMV (IC50 2.1, 0.3, and 0.3 microM) and EBV in H-1 (IC50 0.2, 0.3, and 0.7 microM) and Daudi cells (IC50 3.2, 5.6, and 1.2 microM). Adenine analogue 14 was active against HBV (IC50 2 microM), VZV (IC50 2.5 microM), and HHV-6 (IC50 14 microM). Synadenol (14) and the E-isomer (16) were substrates of moderate efficiency for adenosine deaminase from calf intestine. The E-isomer 16 was more reactive than Z-isomer 14. The deamination of 14 effectively stopped at 50% conversion. Synadenol (14) was also deaminated by AMP deaminase from aspergillus sp.

Calculated RF electric field and temperature distributions in RF thermal ablation: comparison with gel experiments and liver imaging

The present paper is a study of theoretic modeling and experimental tests of that modeling for the local heating due to radiofrequency thermal ablation. A model is developed for the current and the associated electric field produced by a radiofrequency ablation probe. The temperature distributions resulting from the induced ohmic currents in the surrounding material are considered. A comparison is made between the theoretic temperature profiles and the shapes of thermal ablation 'lesions' produced in a controlled gelatin sample experiment. Comparison with the contours of human thermally induced liver lesions found in MRI studies also provides a validation of the success of the modeling. Successful modeling should lead to methods for optimization of the ablation procedure, especially as it is used in interventional MRI.

Structure-activity relationships of 2'-deoxy-2',2'-difluoro-L-erythro-pentofuranosyl nucleosides

Following the recent discoveries that some L-nucleosides are more or equal potent than their D-counterparts, we synthesized 2'-deoxy-2',2'-difluoro-L-erythro-pentofuranosyl nucleosides as potential antiviral agents. The target compounds were synthesized via the key intermediates 7a or 7b from L-gulono gamma-lactone. Compound 2 was oxidatively cleaved and coupled with ethyl bromodifluoroacetate in the presence of activated zinc under Reformatsky conditions to obtain a diasteomeric mixture of 4(R) and 4(S), in a 4:1 ratio. The major 4(R) isomer was cyclized and treated appropriately to obtain the mesylate 8a or 8b, which was condensed with various silyl-protected pyrimidines. Condensation of the alcohol 7a or 7b with 6-chloropurine under Mitsunobu conditions afforded the 6-chlorpurine analogs 53a or 53b and 54a or 54b. Further treatment of the compounds 53a, 54a and 53b, 54b afforded the inosine and adenine derivatives 57-60, respectively. The condensation of 2-amino-6-chloropurine with compound 8a and subsequent treatment with 2-mercaptoethanol/sodium methoxide afforded the guanine analogs 63 and 64. All of the synthesized nucleosides 31-52, 57-60, 63, and 64 were evaluated for antiviral activity and for cellular toxicity. Adenine derivative 57 showed a moderate activity against HIV-1 in PBM cells (3.4 microM). None of the other compounds showed any significant activities against HIV-1, HBV, HSV-1, HSV-2, and toxicity in Vero, CEM, and PBM cell lines up to 100 microM. The X-ray structure of the 5-iodocytosine analog showed a 2'-exo/3'-endo conformation for the carbohydrate moiety, which is different from those of the biologically active compounds (-)-FTC and L-FMAU.

Acute and Delayed Toxicity Studies on the Antiherpesvirus Agents 5-Methoxymethyl-2′-Deoxycytidine and 5-Methoxymethyl-2′-Deoxyuridine

5-Methoxymethyl-2′-deoxycytidine (MMdCyd) and the corresponding deoxyuridine analogue, 5-methoxymethyl-2′-deoxyuridine (MMdUrd) are selective antiherpesvirus agents. MMdCyd (ED501.5 μM) is a more potent inhibitor of herpes simplex virus replication than MMdUrd (ED5030 μM) when maintained in the deoxycytidine form (deamination prevented). The 5′-triphos-phates, MMdCTP and MMdUTP, were synthesized, and incorporation into DNA by mitochondrial DNA polymerase γ was investigated. MMdCTP and MMdUTP were incorporated into DNA in place of dCTP and dTTP, respectively. The effect of MMdCyd and MMdUrd on cell growth (acute toxicity) and prolonged exposure (delayed cytotoxicity) in CEM cells was investigated. The two analogues did not exhibit acute or delayed toxicity (2 weeks exposure) up to 1000 μM. In contrast, at a concentration as low as 0.125 μM of 2′,3′-dideoxycytidine (ddC; control drug), the doubling time of the cells increased after 10 days. At higher concentrations, a very marked increase in doubling time was observed from 6 days onward with ddC treatment. The data suggest that in uninfected cells neither MMdUrd nor MMdCyd are anabolized to the triphosphate form in significant amounts. As a result, little or no MMdCTP or MMdUTP builds up in the mitochondria and thus delayed toxicity is not observed.

Synthesis and anti-hepatitis B virus activity of 9-(2-deoxy-2-fluoro-beta-L-arabinofuranosyl) purine nucleosides

Since the discovery of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU) as a potent anti-HBV and anti-EBV agent, we have studied the structure-activity relationships of 2'-deoxy-2'-fluoro-beta-L-arabinofuranosylpyrimidine nucleosides as anti-HBV agents. Therefore it is rational to extend this study to the purine nucleosides. Thus, 3,5-di-O-benzoyl-2-deoxy-2-fluoro-beta-L-arabinofuranosyl bromide (1), which was prepared from L-xylose via a multistep procedure, was coupled with several purines by the sodium salt method. From this general synthesis, 10 purine nucleosides containing the 2-deoxy-2-fluoro-beta-L-arabinofuranosyl moiety have been obtained. The anti-HBV activity and toxicity of the synthesized nucleosides were evaluated in HepG2 2.2.15 cells. Among them, the adenine (10) and hypoxanthine (15) derivatives exhibit good in vitro anti-HBV activity (EC50 = 1.5 and 8 microM, respectively) without significant toxicity up to 200 microM.

Antitumor agents--CLXXIII. Synthesis and evaluation of camptothecin-4 beta-amino-4'-O-demethyl epipodophyllotoxin conjugates as inhibitors of mammalian DNA topoisomerases and as cytotoxic agents

Two conjugates composed of a camptothecin and a 4'-O-demethyl epipodophyllotoxin derivative joined by an imine linkage were prepared and evaluated as inhibitors of mammalian DNA topoisomerases I and II. Target compounds stimulated cleavable complex formation with both types of enzyme in vitro although activities were reduced at least twofold relative to the activity of unconjugated constituents. The behavior of the most active conjugate as an inhibitor of cell growth closely resembled both topoisomerase I- and II- inhibitory components in that the compound displayed a combined spectrum of activity against various drug-resistant KB sublines. Cytotoxic activity and selectivity were largely retained through conjugation, the exception being a lower than expected activity against a pleiotrophic multidrug-resistant subline. The induced levels and the properties of cellular protein-associated DNA complexes were consistent with topoisomerase involvement and with the in vitro cleavage assay results. Based on the present findings, conjugation afforded cleavable complex-forming topoisomerase inhibitors which display dual target specificity and a broad spectrum of cytotoxic activity against drug-resistant cells.

Inhibition of replication of hepatitis B virus by cytallene in vitro

The acyclic cytosine nucleoside analog cytallene [1-(4'-hydroxy-1',2'-butadienyl)cytosine], which has both (+)- and (-)-enantiomers, was evaluated for its anti-hepatitis B virus (HBV) activity in 2.2.15 cells and was found to have potent activity against HBV DNA synthesis. The R-(-)-enantiomer was found to be the more active of the cytallene enantiomers, with a 50% inhibition concentration against HBV synthesis (HBIC50) of 0.08 microM. Its antiviral activity could be reversed by deoxycytidine (dC) and less efficiently by cytidine. Upon removal of the R-(-)-enantiomer from culture medium, the synthesis of HBV DNA could reinitiate, which suggested that the antiviral action is reversible. The R-(-)-enantiomer was also found to be more cytotoxic than the S-(+)-enantiomer. The degree of cytotoxicity varied among the cell lines, with a 50% inhibition of cell growth at greater than 10 microM. The R-(-)-enantiomer had no effect on HBV RNA synthesis and mitochondrial DNA synthesis at a concentration of 10 times or more than the HBIC50. The two enantiomers cannot be deaminated by dC deaminase, and they can be phosphorylated by cytoplasmic dC kinase. The R-(-)-enantiomer of cytallene is the first acyclic cytosine analog with potent inhibitory activity against HBV similar to those of other L-(-)-ddC analogs.

Altered formation of topotecan-stabilized topoisomerase I-DNA adducts in human leukemia cells

Topotecan (TPT) is a topoisomerase I (topo I) poison that has shown promising antineoplastic activity in solid tumors and acute leukemia. In the present study, a band depletion assay was used to evaluate the ability of TPT to stabilize topo I-DNA adducts in human leukemia cell lines and in clinical leukemia samples ex vivo. This assay showed that 50% of the cellular topo I in HL-60 human myelomonocytic leukemia cells became covalently bound to DNA at an extracellular TPT concentration of 4 micromol/L. In contrast, in 13 clinical specimens of human leukemia harvested before treatment of patients with TPT, the TPT concentration required to stabilize 50% of the cellular topo I in topo I-DNA complexes ranged from 3 to greater than 100 micromol/L (median, 30 micromol/ L). Flow microfluorimetry showed that cellular TPT accumulation varied over only a twofold range and failed to provide evidence for transport-mediated resistance in the clinical samples. These observations raise the possibility that formation of topo I-DNA adducts is diminished in many specimens of refractory/relapsed acute leukemia by a mechanism that might alter topo I sensitivity to TPT.