Search

There is an urgent need for the development of new and safer drugs for the treatment of HIV (human immunodeficiency virus) infection, active against the currently resistant viral strains or directed to novel targets in the viral replicative cycle that may be useful for multiple drug combination. TSAO derivatives are a peculiar group of highly functionalized nucleosides that belong to the so-called nonnucleoside RT inhibitors (NNRTIs). HIV-1 reverse transcriptase (RT) is a key enzyme that plays an essential and multifunctional role in the life cycle of the virus and thus represents a key target for antiviral chemotherapeutic intervention. The dimeric form of the enzyme is absolutely required for all enzymatic activities. Thus, the process of dimerization and subsequent maturation into the p66/p51 heterodimer is essential for a fully functional RT and constitutes a target for therapeutic intervention, however to date such agents have not been developed. TSAO molecules are a peculiar group of non-nucleoside RT inhibitors that exert a unique selectivity for HIV-1 through a specific interaction with the p51 subunit of HIV-1 RT. They interact at the p66/p51 heterodimer interface of the enzyme. They were the first small non peptidic molecules shown to interfere with the dimerization process of the enzyme. This review covers the recent work within this family of compounds aimed at enhancing their interaction with the dimer interface of HIV-1 RT.

The RNA genome of the lentivirus human immunodeficiency virus type 1 (HIV-1) is significantly richer in adenine nucleotides than the statistically equal distribution of the four different nucleotides that is expected. This compositional bias may be due to the guanine-to-adenine (G-->A) nucleotide hypermutation of the HIV genome, which has been explained by dCTP pool imbalances during reverse transcription. The adenine nucleotide bias together with the poor fidelity of HIV-1 reverse transcriptase markedly enhances the genetic variation of HIV and may be responsible for the rapid emergence of drug-resistant HIV-1 strains. We have now attempted to counteract the normal mutational pattern of HIV-1 in response to anti-HIV-1 drugs by altering the endogenous deoxynucleoside triphosphate pool ratios with antimetabolites in virus-infected cell cultures. We showed that administration of these antimetabolic compounds resulted in an altered drug resistance pattern due to the reversal of the predominant mutational flow of HIV (G-->A) to an adenine-to-guanine (A-->G) nucleotide pattern in the intact HIV-1-infected lymphocyte cultures. Forcing the virus to change its inherent nucleotide bias may lead to better control of viral drug resistance development.

A binding site for TSAO-m(3)T at the interface between the p66 and p51 subunits of HIV-1 reverse transcriptase (RT) and distinct from that of "classical" HIV-1 non-nucleoside inhibitors is proposed. The feasibility of the binding mode was assessed by carrying out nanosecond molecular dynamics simulations for the complexes of TSAO-m(3)T with reduced models of both the wild-type enzyme and a more sensitive R172A mutant. The molecular model is in agreement with a previous proposal, with known structure-activity and mutagenesis data for this unique class of inhibitors, and also with recent biochemical evidence indicating that TSAO analogues can affect enzyme dimerization. The relative importance of residues involved in dimer formation and TSAO-RT complex stabilization was assessed by a combination of surface area accessibility, molecular mechanics, and continuum electrostatics calculations. A structure-based modification introduced into the lead compound yielded a new derivative with improved antiviral activity.

Various analogues of the anti-HIV-1 agent TSAO-T, [1-[2',5'-bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]thymine]-3'-spiro-5"-(4"-amino-1",2"-oxathiole-2",2"-dioxide) have been synthesized in which the 5'-TBDMS group has been replaced by alkyl-, alkenyl- or aromatic ether groups, substituted amines, carbamoyl or (thio)acyl groups. The compounds synthesized were evaluated for their inhibitory effect on HIV-1 and HIV-2 replication in cell culture. Replacement of the 5'-TBDMS group by an acyl, aromatic or a cyclic moiety markedly diminish or even eliminate the anti-HIV activity. However, the presence at that position of an alkyl or alkenyl chain, partially retain antiviral activity. These observations suggest that the 5'-TBDMS group of the TSAO molecule plays a crucial role.

TSAO derivatives represent a class of nonnucleoside reverse transcriptase inhibitors (NNRTIs) that consistently select for the Glu138Lys resistance mutation in HIV-1 reverse transcriptase (RT). Seven RT mutants (i.e., Ala, Asp, Gln, Gly, Lys, Phe, and Tyr) were constructed by site-directed mutagenesis. The mutant Glu138Asp, Glu138Lys, Glu138Gln, Glu138Ala, and Glu138Gly RTs retained marked catalytic activity. In contrast, the Glu138Phe and Glu138Tyr RT mutants showed poor RNA-dependent DNA polymerase activity (30 and 4% of wild-type, respectively). TSAO derivatives lost their inhibitory activity against all mutant enzymes, except against the closely related Glu138Asp RT mutant that remained as sensitive to TSAOs as did wild-type RT. Other NNRTIs, including delavirdine, emivirine, and UC-781, and the NRTI ddGTP retained pronounced inhibitory activity against all mutant enzymes. When the amino acid mutations at position 138 of RT were introduced in recombinant virus clones, the sensitivity/resistance spectrum obtained toward the TSAOs and other NNRTIs was similar to those observed for the isolated recombinant mutant enzymes. The Glu138Lys RT mutant virus had the most marked resistance to TSAOs, followed by the Glu138Gln, Glu138Phe, Glu138Gly, Glu138Tyr, and Glu138Ala virus mutants. The Glu138Asp RT mutant virus kept full sensitivity to the TSAO derivatives. Mixtures of Glu138Lys RT mutant virus with the other virus clones mutated at the 138 position resulted in all cases, except for the Glu138Asp and Glu138Gly RT mutant viruses, in an outgrowth of the Glu138Lys RT mutant virus. Since the Glu138Lys RT proved most resistant to TSAO derivatives, was among the most catalytically efficient enzymes, and resulted in highly replication-competent virus, our data explain why the Glu138Lys RT mutant virus strains but not virus strains containing other amino acids at position 138 invariably emerge in cell cultures under TSAO drug pressure., (Copyright 2001 Academic Press.)

A kinetic analysis was performed for the novel 1-(8-phosphonooctyl)-6-amino-5-bromouracil and 1-(8-phosphonooctyl)-7-deazaxanthine inhibitors of Escherichia coli thymidine (dThd) phosphorylase (TPase). The structure of the compounds was rationally designed based on the available crystal structure coordinates of bacterial TPase. These inhibitors reversibly inhibited TPase. Kinetic analysis revealed that the compounds inhibited TPase in a purely competitive or mixed fashion not only when dThd, but also when inorganic phosphate (Pi), was used as the variable substrate. In contrast, the free bases 6-amino-5-bromouracil and 7-deazaxanthine behaved as non-competitive inhibitors of the enzyme in the presence of variable Pi concentrations while being competitive or mixed with respect to thymine as the natural substrate. Our kinetic data thus revealed that the novel 1-(8-phosphonooctyl)pyrimidine/purine derivatives are able to function as multisubstrate inhibitors of TPase, interfering at two different sites (dThd(Thy)- and phosphate-binding site) of the enzyme. To our knowledge, the described compounds represent the first type of such multisubstrate analogue inhibitors of TPase; they should be considered as lead compounds for the development of mechanistically novel type of TPase inhibitors.

The ribonucleoside analogues (E)-5-(2-bromovinyl)uridine (5-BV-Urd) and 3'-spiro-(4'-amino-1',2'-oxathiole-2',2'-dioxide)-5-methyluridine (3'-AOD-5-MeUrd) emerged as potent and selective competitive inhibitors of mitochondrial thymidine kinase (TK)-2 with respect to thymidine (K(i)/K(m) values of 9.0 and 1.2 respectively). Cytosolic TK-1 did not show measurable affinity for these compounds. [(32)P]Phosphate transfer studies from [gamma-(32)P]ATP to 5-BV-Urd and 3'-AOD-5-MeUrd revealed extremely poor substrate activity but potent inhibitory potential of the compounds. It was concluded that the ribonucleosides 5-BV-Urd and 3'-AOD-5-MeUrd represent two new lead compounds for potent and selective inhibitors of mitochondrial TK-2.

HIV-1 samples from six patients undergoing diverse anti-HIV therapies possessed the E138A mutation in their reverse transcriptase (RT) genome. Patients were receiving the following therapies: TIBO monotherapy (one patient); zidovudine plus didanosine combination therapy (one); zidovudine monotherapy (one); sequential therapy with zidovudine, then stavudine and finally zalcitabine plus didanosine (one); and two were drug naive. E138K, not E138A, is a known TSAO-specific resistance mutation, emerging under selective pressure in vitro. Our phenotypic data on the patient isolates, confirmed by data on an E138A mutant acquired through in vitro mutagenesis, indicated that an alanine substitution for glutamate at codon 138 of the HIV-1 RT renders the virus TSAO resistant, confirming the importance of this amino acid residue in the activity of TSAO derivatives. In addition, we have demonstrated through phenotypic analysis of the E138A and A98S mutants (after in vitro mutagenesis) that the mutation A98S, found in one of these patients, could be partially responsible for the phenotypic reversal of TSAO resistance. This reversal could be explained by the restoration of a hydrogen bond between 98S and the main-chain residue L349, which compensates for the loss of the E138-G99 main-chain hydrogen bond. As TSAO derivatives have not been used in the clinical setting, the presence of the E138A mutation at a frequency of 6.7% in our study of 90 TSAO-inexperienced HIV-seropositive individuals implies that 138A of the RT must be a natural variant and that the mutant virus is replication competent. Our observations suggest that the E138A mutation may likely arise in patients under the selective pressure of TSAO or related compounds that show a decreased antiviral potency toward the E138A variant.

A series of acyclic phosphonate derivatives of thymine has been synthesized and tested as multisubstrate analogue inhibitors of Escherichia coli thymidine phosphorylase. The compounds synthesized include 1-(phosphonoalkyl)thymines with six to nine methylenes (1-4, respectively); 1-[(Z)-4-phosphonomethoxy-2-butenyl]thymine (5) and its butyl and 2,3-cis-dihydroxybutyl derivatives (6 and 7, respectively); 1-[(Z)-(4-(phosphonomethoxy)methoxy)-2-butenyl]thymine (8) and also its butyl and 2,3-cis-dihydroxybutyl analogues (9 and 10); and 1-[((Z)-4-(phosphonomethoxy)-2-butenoxy)methyl]thymine (11). Evaluation of these compounds against E. coli revealed significant enzymatic inhibition by 2, 3, 4, 6, and 8 at a concentration of 1000 microM, 3 and 4 being the most potent. Replacement of the thymine base in 3 by 6-amino-5-bromouracil and 7-deazaxanthine afforded compounds 12 and 13, which showed a pronounced improvement of TPase inhibition, comparable to 7-deazaxanthine. When inorganic phosphate was used as a variable substrate, compounds 12 and 13 displayed competitive kinetics with respect to phosphate, indicating a direct interaction of these compounds with the phosphate binding site. Also compounds 12 and 13 were found to be competitive inhibitors of TPase against thymidine as a variable substrate. These results are consistent with the compounds being multisubstrate analogue inhibitors of E. coli TPase, and they represent the first example of such TPase inhibitors.

With the aim of assessing the role that the thymine base of TSAO-T may play in the interaction of TSAO compounds with HIV-1 reverse transcriptase (RT), we have designed, synthesized, and evaluated for their anti-HIV-1 activity a series of 3-spiro sugar derivatives substituted at the anomeric position with nonaromatic rings or with amine, amide, urea, or thiourea moieties that mimic parts or the whole thymine base of TSAO-T. Also, a dihydrouracil TSAO analogue and O-glycosyl 3-spiro sugar derivatives substituted at the anomeric position with methyloxy or benzyloxy groups have been prepared. Compounds substituted at the anomeric position with an azido, amino, or methoxy group, respectively, were devoid of marked antiviral activity (EC50: 10-200 microM). However, the substituted urea sugar derivatives led to an increase in antiviral potency (EC50: 0.35-4 microM), among them those urea derivatives that mimic most closely the intact TSAO-T molecule retained the highest antiviral activity. Also, the dihydrouracil TSAO derivative retained pronounced anti-HIV-1 activity. None of the compounds showed any anti-HIV-2 activity. The results described herein represent the first examples of sugar derivatives that interact in a specific manner with HIV-1 RT. Molecular modeling studies carried out with a prototype urea derivative indicate that a heteroaromatic ring is not an absolute requirement for a favorable interaction between TSAO-T and HIV-1 RT. Urea derivatives, which can mimic to a large extent both the shape and the electrostatic potential of a thymine ring, can effectively replace this nucleic acid base when incorporated into a TSAO molecular framework with only moderate loss of activity.

7-Deazaxanthine (7DX) was identified as a novel inhibitor of thymidine (dThd) phosphorylase (TPase). It inhibited the TPase reaction in a concentration-dependent manner. At 1 mM, it almost completely prevented the TPase-catalysed hydrolysis of dThd to thymine. The 50% inhibitory concentration (IC50 of 7DX was 40 microM in the presence of 100 microM of the natural substrate dThd. 7DX is also endowed with a marked inhibitory effect on angiogenesis. It significantly prevents neovascularisation in the chicken chorioallantoic membrane during development. 7DX is the first purine derivative shown to be a potent inhibitor of purified TPase and angiogenesis.

Novel analogues of the anti-HIV-1 agent TSAO-T, [1-[2',5'-bis-O-(tert- butyldimethylsilyl)-beta-D-ribofuranosyl]thymine]-3'-spiro-5"-(4"-amino- 1",2"-oxathiole-2",2"-dioxide) and its 3-methyl counterpart TSAO-m3T were obtained by modifications at positions 2' or 5' of the sugar moiety. These compounds were evaluated for their inhibitory effect on HIV-1 and HIV-2 replication in cell culture. Introduction of new groups at the 5'-position (i.e. esters, benzylether and silylethers) resulted in compounds that were either inactive or less active than the parent compounds (TSAO-T and TSAO-m3T). Attempts to introduce small silyl ether groups at this position were not successful since these products decomposed during purification. Similar modifications at the 2'-position had a much less pronounced influence on the anti-HIV-1 activity.

Several 4-, 5-, and 6-substituted pyrimidine analogues of the new anti-HIV-1 lead compound [1-[2',5'-bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]thymine] -3'-spiro-5''-(4''-amino-1'',2''-oxathiole 2'',2''-dioxide) (TSAO-T) have been prepared and evaluated as inhibitors of HIV-1 and HIV-2 replication in cell cultures. Reaction of 1,2-di-O-acetyl-5-O-benzoyl-3-C-cyano-3-O-mesyl-D-ribofuranose with 5-substituted pyrimidine bases, followed by treatment with Cs2CO3, afforded, stereoselectively, beta-D-ribofuranosyl-3'-spironucleosides. 2',5'-O-Deacylation and subsequent treatment with tert-butyldimethylsilyl chloride gave the TSAO-5-substituted pyrimidine derivatives. Reaction of 5-halogen-TSAO derivatives with nucleophiles gave 6-substituted-TSAO analogues. Treatment of TSAO-pyrimidine analogues with POCl3/1,2,4-triazole and methylamine or di-methylamine afforded the 4-substituted pyrimidine compounds. Several substituted TSAO-thymine, TSAO-uracil, and TSAO-cytosine derivatives were found to be superior to their unsubstituted TSAO congeners with regard to their antiviral and/or cytotoxic properties.

Several purine and purine-modified analogues of the new lead anti-HIV-1 agent [[2',5'-bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl] thymine]-3'-spiro-5''-(4''-amino-1'',2''-oxathiole 2'',2''-dioxide) (TSAO-T) have been prepared and evaluated as inhibitors of HIV-1-induced cytopathicity. Reaction of O-mesylcyanohydrins of furanos-3'-ulosyladenine with Cs2CO3 afforded beta-D-xylo- and ribofuranosyladenine 3'-spiro nucleosides. Reaction of 1,2-di-O-acetyl-5-O-benzoyl-3-C-cyano-3-O-mesyl-D-ribofuranose with purine bases, followed by treatment with Cs2CO3, stereoselectively afforded beta-D-ribofuranosyl 3'-spiro nucleosides. 2',5'-O-Deacylation and subsequent treatment with tert-butyldimethylsilyl chloride gave the required TSAO derivatives. The 3'-spiro nucleosides with a xylo configuration did not show any anti-HIV activity. However, the purine ribo 3'-spiro nucleosides were potent and selective inhibitors of HIV-1 with a 50% effective concentration in the range of 0.1-1 microM and a selectivity index ranging from 2 to 3 orders of magnitude. Introduction of an alkyl function at N-1 of the purine moiety markedly decreased cytotoxicity without affecting antiviral activity.

Treatment of HIV-1-infected cells with the HIV-1-specific inhibitors hydroxyethoxymethylphenylthiothymine (HEPT), tetrahydroimidazobenzodiazepinones (TIBO), nevirapine, pyridinone, bis(heteroaryl)piperazines (BHAP), and tert-butyldimethylsilylspiroaminooxathioledioxide (TSAO) at a concentration of 0.1 microgram/ml resulted in a rapid breakthrough of resistant virus within three to four subcultivations. At drug concentrations of 0.5 to 1 microgram/ml, emergence of resistant virus was delayed. The drug-resistant HIV-1 strains that originated under these conditions were genetically and phenotypically characterized and showed differential sensitivities against the different classes of HIV-1-specific inhibitors depending on the amino acid substitutions in their reverse transcriptase. Novel amino acid substitutions were found in the reverse transcriptase of BHAP- and pyridinone-resistant mutant HIV-1 strains that had not been reported so far. At 2.5 to 10 micrograms/ml, that is at a concentration 100- to 250-fold higher than the 50% effective concentration (EC50), HEPT, TIBO, nevirapine, pyridinone, and BHAP prevented virus breakthrough after 15 subcultivations. In contrast, 3'-azido-2',3'-dideoxythymidine (AZT), even when administered at a 1000-fold higher concentration (i.e., 1.3 micrograms/ml) than its EC50 failed to prevent virus breakthrough after the second subcultivation. HIV-1-infected cell cultures could apparently be cleared from virus by the HIV-1-specific inhibitors when used at the "knocking-out" concentrations (2.5-10 micrograms/ml), as evidenced by (i) the lack of viral cytopathicity, (ii) the lack of virus-specific envelope glycoprotein expression, (iii) the lack of viral p24 antigen production, and (iv) the apparent absence of proviral DNA in the cells. Moreover, uninfected CEM cell cultures to which HIV-1-infected CEM cells (including syncytia) had been added were protected from destruction by high-concentration treatment with the HIV-1-specific inhibitors, but not with AZT and DDI (2',3'-dideoxyinosine).

Several lipophilic-2',3'-dideoxynucleotide analogues have been synthesized and tested against Human Immunodeficiency Virus (HIV). Glycosyl-oxycarbonylaminosulfonyl-analogues of 3'-deoxythymidine and 2',3'-dideoxyuridine have been synthesized by reaction of 2,3,4,6-tetra-O-benzoyl-alpha-D-glucopyranose with chlorosulfonyl isocyanate and the corresponding 2',3'-dideoxynucleoside. Another series of 5'-phosphate-like-3'-deoxythymidine nucleosides (5'-O-alkyl-sulfamoyl- and 5'-O-carbamoyl-3'-deoxythymidine) have also been prepared. Both series of compounds can be considered as lipophilic nucleotide mimics.

[1-[2',5'-Bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]-3-N- methyl-thymine]-3'-spiro-5''-(4''-amino-1'',2''-oxathiole-2'',2''- dioxide) (TSAO-m3T) is a potent, selective and specific inhibitor of human immunodeficiency virus type 1 replication in vitro. Uptake of TSAO-m3T by human CEM cells is drug concentration-dependent and increased proportionally with increasing initial extracellular TSAO-m3T concentrations up to 20 micrograms/mL. Within 6 hr of incubation, the cells were almost completely saturated with the test compound; further incubation up to 72 hr did not markedly increase the intracellular concentration of the compound. No intracellular metabolic conversion of TSAO-m3T was observed in CEM, MT-4 or MOLT-4 cells. Upon intravenous bolus administration of TSAO-m3T to mice at 0.75 mg/kg, TSAO-m3T was rapidly cleared from the plasma in a mono-exponential manner (half-life: 22 min; distribution volume: 9.5 L/kg; total body clearance: 17.8 L/hr/kg). TSAO-m3T mainly accumulated in the lungs, followed by the heart, kidney and liver. Significant amounts of different metabolites of TSAO-m3T were detected in most tissues, the liver, kidney and spleen being the organs that showed the most extensive metabolism. The principal metabolites identified were TSAO-m3T derivatives in which the t-butyldimethylsilyl moiety at C-2' and/or C-5' had been split off. The free base N3-methylthymine was not detected.

Serial passage of HIV-1 in CEM or MT-4 cell cultures in the presence of different HIV-1-specific reverse transcriptase (RT) inhibitors yielded mutant viruses which were resistant (i.e., 200- to 1000-fold less sensitive) to the homologous compounds. The RT of these mutant HIV-1 strains showed different amino acid substitutions depending on the class of the HIV-1-specific RT inhibitors. The following amino acid substitutions were found: 138 Glu-->Lys (TSAO-T), 181 Tyr-->Cys (nevirapine), 181 Tyr-->Cys (pyridinone), and 100 Leu-->Ile (TIBO R82150). Four TIBO (R82913)-resistant HIV-1 strains contained different amino acid substitutions: 103 Lys-->Asn (strain 2), 100 Leu-->Ile and 138 Glu-->Lys (strain B02), 100 Leu-->Ile and 181 Tyr-->Cys (strain 1), 100 Leu-->Ile and 188 Tyr-->His (strain B22). The level of cross-resistance (or sensitivity) highly depends on the nature of the amino acid substitutions. As a rule, the TSAO-resistant HIV-1 strains (138 Glu-->Lys) and TIBO (R82150 or R82913)-resistant HIV-1 strains (Leu 100-->Ile or 103 Lys-->Asn) are sensitive to the other HIV-1-specific RT inhibitors, whereas the amino acid change 181 Tyr-->Cys results in a significant reduction of sensitivity to all classes of the HIV-1-specific RT inhibitors.

Several analogues of a new lead for anti-HIV-1 agents [1-[2',5'-bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]-thymine] -3'-spiro-5''-(4''-amino-1'',2''-oxathiole 2'',2''-dioxide) (TSAO) modified at positions N-3, O-4 and C-5 of the thymine moiety, have been prepared and evaluated as inhibitors of HIV-1 replication. A new stereoselective synthetic procedure is described. Reaction of 1,2-di-O-acetyl-5-O-benzoyl-3-C-cyano-3-O-mesyl-D-ribofuranose with pyrimidine bases, followed by treatment with Cs2CO3 afforded stereoselectively, beta-D-ribofuranosyl-3'-spiro nucleosides. 2',5'-O-Deacylation and subsequent treatment with tert-butyldimethylsilyl chloride gave the TSAO derivatives. Only those analogues having a tBDMSi group at both the C-5' and C-2' positions of the ribose moiety showed potent anti-HIV-1 activity. The activity ranged from 0.060 microM to 1.0 microM. Introduction of an alkyl or alkenyl function at N-3 of the thymine ring markedly decreased cytotoxicity without affecting the antiviral activity. While markedly active against HIV-1, the TSAO derivatives had no activity against HIV-2 or SIV. They represent the first example of nucleoside analogues with an intact ribose moiety that discriminate between HIV-1 and other retroviruses.

A series of 3'-spiro nucleosides have been synthesized and evaluated as anti-HIV-1 agents. Reaction of O-mesylcyanohydrins of furanos-3'-ulosyl nucleosides with base afforded [1-[2',5'-bis-O- (tert-butyldimethylsilyl)-beta-D-xylo- and -ribofuranosyl]]-3'-spiro-5"- [4"-amino-1",2"-oxathiole 2",2"-dioxide] derivatives of thymine, uracil and 4-N-acetylcytosine 11 and 12. Desilylation of 11 and 12 gave the full deprotected 3'-spiro xylo- and ribofuranosyl nucleosides 13 and 14 or the partially 5'-O-deprotected-3'-spiro beta-D-xylo- and -ribo-nucleosides 15 and 16, or 2'-O-deprotected-3'-spiro beta-D-ribo-nucleoside 17. 2'-Deoxygenation of 17 afforded 2'-deoxy-3'-spiro beta-D-erythro-pentofuranosyl derivative 18. These 3'-spiro derivatives were evaluated for their anti-HIV-1 activity. All 3'-spiro nucleosides having a xylo configuration did not show any anti-HIV-1 activity. 3'-Spiro ribo-nucleosides with none or only one silyl group at C-2' or C-5' or the 2'-deoxy derivative were also inactive at subtoxic concentrations. However, 3'-spiro ribo-nucleosides having two silyl groups at C-2' and C-5' were potent and selective inhibitors of HIV-1. None of the nucleoside analogues that showed anti-HIV-1 activity proved inhibitory to the replication of HIV-2 or SIV.

[2',5'-Bis-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]-3'-spiro- 5"-(4"-amino-1",2"-oxathiole-2", 2"-dioxide)thymine (TSAO-T) is a representative of a novel class of nucleoside analogues that are endowed with a potent and specific activity against human immunodeficiency virus (HIV) type 1 and are targeted at the HIV-1 reverse transcriptase (RT). Inhibition of HIV-1 RT by TSAO-T was reversible and noncompetitive with respect to dGTP as the substrate and poly(C).oligo(dG) as the template/primer. In contrast with the nonnucleoside derivatives tetrahydroimidazo-[4,5,1-jk][1,4]- benzodiazepin-2(1H)-thione (TIBO) (R-82150), nevirapine (BI-RG-587) and the HEPT derivative I-HEPU-SdM, TSAO-T was not inhibitory to HIV-1 RT in the presence of other homopolymeric template/primers. It did not interfere with the DNA-dependent DNA polymerase function of HIV-1 RT, HIV-2 RT, herpes simplex virus type 1 DNA polymerase, or Taq polymerase. However, TSAO-T proved inhibitory to the HIV-1 RT reaction primed by Escherichia coli 16S/23S rRNA, irrespective of the nature of the radiolabeled 2'-deoxynucleotide 5'-triphosphate (dNTP) used. TSAO-T does not act as a DNA chain terminator. It interacts with HIV-1 RT at a nonsubstrate (dNTP)-binding site.