Kinetics of inhibition of endogenous human immunodeficiency virus type 1 reverse transcription by 2',3'-dideoxynucleoside 5'-triphosphate, tetrahydroimidazo-[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thion e, and 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine derivatives

Critical Contribution of Tyr15 in the HIV-1 Integrase (IN) in Facilitating IN Assembly and Nonenzymatic Function through the IN Precursor Form with Reverse Transcriptase

Nonenzymatic roles for HIV-1 integrase (IN) at steps prior to the enzymatic integration step have been reported. To obtain structural and functional insights into the nonenzymatic roles of IN, we performed genetic analyses of HIV-1 IN, focusing on a highly conserved Tyr15 in the N-terminal domain (NTD), which has previously been shown to regulate an equilibrium state between two NTD dimer conformations. Replacement of Tyr15 with alanine, histidine, or tryptophan prevented HIV-1 infection and caused severe impairment of reverse transcription without apparent defects in reverse transcriptase (RT) or in capsid disassembly kinetics after entry into cells. Cross-link analyses of recombinant IN proteins demonstrated that lethal mutations of Tyr15 severely impaired IN structure for assembly. Notably, replacement of Tyr15 with phenylalanine was tolerated for all IN functions, demonstrating that a benzene ring of the aromatic side chain is a key moiety for IN assembly and functions. Additional mutagenic analyses based on previously proposed tetramer models for IN assembly suggested a key role of Tyr15 in facilitating the hydrophobic interaction among IN subunits, together with other proximal residues within the subunit interface. A rescue experiment with a mutated HIV-1 with RT and IN deleted (ΔRT ΔIN) and IN and RT supplied in trans revealed that the nonenzymatic IN function might be exerted through the IN precursor conjugated with RT (RT-IN). Importantly, the lethal mutations of Tyr15 significantly reduced the RT-IN function and assembly. Taken together, Tyr15 seems to play a key role in facilitating the proper assembly of IN and RT on viral RNA through the RT-IN precursor form.

IMPORTANCE

Inhibitors of the IN enzymatic strand transfer function (INSTI) have been applied in combination antiretroviral therapies to treat HIV-1-infected patients. Recently, allosteric IN inhibitors (ALLINIs) that interact with HIV-1 IN residues, the locations of which are distinct from the catalytic sites targeted by INSTI, have been discovered. Importantly, ALLINIs affect the nonenzymatic role(s) of HIV-1 IN, providing a rationale for the development of next-generation IN inhibitors with a mechanism that is distinct from that of INSTI. Here, we demonstrate that Tyr15 in the HIV-1 IN NTD plays a critical role during IN assembly by facilitating the hydrophobic interaction of the NTD with the other domains of IN. Importantly, we found that the functional assembly of IN through its fusion form with RT is critical for IN to exert its nonenzymatic function. Our results provide a novel mechanistic insight into the nonenzymatic function of HIV-1 IN and its prevention.

Cell-Based and Biochemical Analysis of the anti-HIV Activity of Combinations of 3′-azido-3′-deoxythymidine and Analogues of TIBO

The toxicity of 3′-azido-3′-deoxythymidine (AZT) and the appearance of drug-resistant mutants in patients treated with AZT emphasizes the critical importance of the development of alternative strategies for the therapy of AIDS patients. Combination antiviral chemotherapy provides an attractive therapeutic strategy since the dose of the individual agents may be lowered to reduce toxicity and the use of two potent antiviral agents may limit the development of drug resistance. Two analogues of tetrahydro-imidazo[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thione (TIBO) potently and selectively inhibit the replication of HIV-1 in cell culture. In combination with AZT, either of the two TIBO compounds, R82913 and R86183, was highly synergistic in cell culture against HIV-1. However, in biochemical enzyme inhibition assays, utilizing recombinant HIV-1 reverse transcriptase, synergy was not detected at the enzymatic level. These results suggest that one of these two known inhibitors of HIV-1 reverse transcriptase may have a secondary mechanism of action distinct from inhibition of the reverse transcriptase.

Curious discoveries in antiviral drug development: the role of serendipity

Antiviral drug development has often followed a curious meandrous route, guided by serendipity rather than rationality. This will be illustrated by ten examples. The polyanionic compounds (i) polyethylene alanine (PEA) and (ii) suramin were designed as an antiviral agent (PEA) or known as an antitrypanosomal agent (suramin), before they emerged as, respectively, a depilatory agent, or reverse transcriptase inhibitor. The 2',3'-dideoxynucleosides (ddNs analogues) (iii) have been (and are still) used in the "Sanger" DNA sequencing technique, although they are now commercialized as nucleoside reverse transcriptase inhibitors (NRTIs) in the treatment of HIV infections. (E)-5-(2-Bromovinyl)-2'-deoxyuridine (iv) was discovered as a selective anti-herpes simplex virus compound and is now primarily used for the treatment of varicella-zoster virus infections. The prototype of the acyclic nucleoside phosphonates (ANPs), (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine [(S)-HPMPA], (v) was never commercialized, although it gave rise to several marketed products (cidofovir, adefovir, and tenofovir). 1-[2-(Hydroxyethoxy)methyl]-6-(phenylthio)thymine (vi) and TIBO (tetrahydroimidazo[4,5,1-jk][1,4-benzodiazepin-2(1H)]-one and -thione) (vii) paved the way to a number of compounds (i.e., nevirapine, delavirdine, etravirine, and rilpivirine), which are now collectively called non-NRTIs. The bicyclam AMD3100 (viii) was originally described as an anti-HIV agent before it became later marketed as a stem cell mobilizer. The S-adenosylhomocysteine hydrolase inhibitors (ix), while active against a broad range of (-)RNA viruses and poxviruses may be particularly effective against Ebola virus, and for (x) the O-ANP derivatives, the potential application range encompasses virtually all DNA viruses.

Antiviral drug development--success and failure: a personal perspective with a Japanese connection

At the 25th International Conference on Antiviral Research, I received a special recognition for my contribution to the International Society of Antiviral Research over a period of 25 years (from 1987 until 2012). This review follows the theme of my presentation at that event, which comprised 10 reminiscences, all with a Japanese connection concerning the success, or otherwise, in the clinical development of: double- and single-stranded polynucleotides; suramin, a polysulfonate; dextran sulfate, a polysulfate; brivudin; BVaraU; 2',3'-dideoxynucleoside analogues; HEPT; adefovir and tenofovir; CXCR4 antagonists; and elvitegravir.

Synthesis and HIV-1 RT inhibitory action of novel (4/6-substituted benzo[d]thiazol -2-yl)thiazolidin-4-ones. Divergence from the non-competitive inhibition mechanism

Reverse transcriptase (RT) inhibitors play a major role in the therapy of human immunodeficiency virus type 1 (HIV-1) infection. Although, many compounds are already used as anti-HIV drugs, research on development of novel inhibitors continues, since drug resistant strains appear because of prolonged therapy. In this paper, we present the synthesis and evaluation of HIV-1 RT inhibitory action of eighteen novel (4/6-halogen/MeO/EtO-substituted benzo[d]thiazol-2-yl)thiazolidin-4-ones. The two more active compounds (IC50 : 0.04 µM and 0.25 µM) exhibited better inhibitory action than the reference compound, nevirapine. Docking analysis supports a stable binding of the most active derivative to the allosteric centre of RT. Kinetic analysis of two of the most active compounds indicate an uncompetitive inhibition mode. This is a desired characteristic, since mutations that affect activity of traditional non-competitive NNRTIs may not affect activity of compounds of this series. Interestingly, the less active derivatives (IC50 > 40 µM) exhibit a competitive mode of action.

Effects of detergents on the West Nile virus protease activity

Detergents such as Triton X-100 are often used in drug discovery research to weed out small molecule promiscuous and non-specific inhibitors which act by aggregation in solution and undesirable precipitation in aqueous assay buffers. We evaluated the effects of commonly used detergents, Triton X-100, Tween-20, Nonidet-40 (NP-40), Brij-35, and CHAPS, on the enzymatic activity of West Nile virus (WNV) protease. Unexpectedly, Triton X-100, Tween-20, and NP-40 showed an enhancement of in vitro WNV protease activity from 2 to 2.5-fold depending on the detergent and its concentration. On the other hand, Brij-35, at 0.001% enhanced the protease activity by 1.5-fold and CHAPS had the least enhancing effect. The kinetic analysis showed that the increase in protease activity by Triton X-100 was dose-dependent. Furthermore, at Triton X-100 and Tween-20 concentrations higher than 0.001%, the inhibition of compound B, one of the lead compounds against WNV protease identified in a high throughput screen (IC(50) value of 5.7+/-2.5 microM), was reversed. However, in the presence of CHAPS, compound B still showed good inhibition of WNV protease. Our results, taken together, indicate that nonionic detergents, Triton X-100, Tween, and NP-40 are unsuitable for the purpose of discrimination of true versus promiscuous inhibitors of WNV protease in high throughput assays.

Inhibition of endogenous reverse transcription of human and nonhuman primate lentiviruses: potential for development of lentivirucides

In the current study, we extended our previous works on natural endogenous reverse transcription (NERT) and further examined its potential as a virucide molecular target in sexual transmission of primate lentiviruses. HIV-1 and SIV virions were pretreated with select nucleoside (NRTIs) and nonnucleoside RT inhibitors (NNRTIs), either alone or in combination with NERT-stimulating substances. The effects of these antiretrovirals on virion inactivation were analyzed in human T cell lines and primary cell cultures. Pretreatment of HIV-1 virions with physiologic NERT-stimulants and 3'-azido-3'-deoxythymidine 5'-triphosphate (AZT-TP) or nevirapine potently inactivated cell-free HIV-1 virions and resulted in strong inhibition of the viral infectivity. Pretreatment of chimeric SHIV-RT virions with NERT-stimulating cocktail and select antiretrovirals also resulted in virion inactivation and inhibition of viral infectivity in T cell lines. Our findings demonstrate the potential clinical utility of approaches based on inhibiting NERT in sexual transmission of HIV-1, through the development of effective anti-HIV-1 microbicides, such as NRTIs and NNRTIs.

Kinetic analysis of wild-type and YMDD mutant hepatitis B virus polymerases and effects of deoxyribonucleotide concentrations on polymerase activity

Mutations in the YMDD motif of the hepatitis B virus (HBV) DNA polymerase result in reduced susceptibility of HBV to inhibition by lamivudine, at a cost in replication fitness. The mechanisms underlying the effects of YMDD mutations on replication fitness were investigated using both a cell-based viral replication system and an in vitro enzyme assay to examine wild-type (wt) and YMDD-mutant polymerases. We calculated the affinities of wt and YMDD-mutant polymerases for each natural deoxyribonucleoside triphosphate (dNTP) and determined the intracellular concentrations of each dNTP in HepG2 cells under conditions that support HBV replication. In addition, inhibition constants for lamivudine triphosphate were determined for wt and YMDD-mutant polymerases. Relative to wt HBV polymerase, each of the YMDD-mutant polymerases showed increased apparent K(m) values for the natural dNTP substrates, indicating decreased affinities for these substrates, as well as increased K(i) values for lamivudine triphosphate, indicating decreased affinity for the drug. The effect of the differences in apparent K(m) values between YMDD-mutant polymerase and wt HBV polymerase could be masked by high levels of dNTP substrates (>20 microM). However, assays using dNTP concentrations equivalent to those measured in HepG2 cells under physiological conditions showed decreased enzymatic activity of YMDD-mutant polymerases relative to wt polymerase. Therefore, the decrease in replication fitness of YMDD-mutant HBV strains results from the lower affinities (increased K(m) values) of the YMDD-mutant polymerases for the natural dNTP substrates and physiological intracellular concentrations of dNTPs that are limiting for the replication of YMDD-mutant HBV strains.

Inhibitors of human immunodeficiency virus type 1 reverse transcriptase target distinct phases of early reverse transcription

Early HIV-1 reverse transcription can be separated into initiation and elongation phases. Here we show, using PCR analysis of negative-strand strong-stop DNA [(-)ssDNA] synthesis in intact virus, that different reverse transcriptase (RT) inhibitors affect distinct phases of early natural endogenous reverse transcription (NERT). The effects of nevirapine on NERT were consistent with a mechanism of action including both specific and nonspecific binding events. The nonspecific component of this inhibition targeted the elongation reaction, whereas the specific effect seemed principally to be directed at very early events (initiation or the initiation-elongation switch). In contrast, foscarnet and the nucleoside analog ddATP inhibited both early and late (-)ssDNA synthesis in a similar manner. We also examined compounds that targeted other viral proteins and found that Ro24-7429 (a Tat antagonist) and rosmarinic acid (an integrase inhibitor) also directly inhibited RT. Our results indicate that NERT can be used to identify and evaluate compounds that directly target the reverse transcription complex.

Role of Vif in stability of the human immunodeficiency virus type 1 core

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is important for virion infectivity. Previous studies have shown that vif-defective virions exhibit structural abnormalities in the virus core and are defective in the ability to complete proviral DNA synthesis in acutely infected cells. We developed novel assays to assess the relative stability of the core in HIV-1 virions. Using these assays, we examined the role of Vif in the stability of the HIV-1 core. The integrity of the core was examined following virion permeabilization or removal of the lipid envelope and treatment with various triggers, including S100 cytosol, deoxynucleoside triphosphates, detergents, NaCl, and buffers of different pH to mimic aspects of the uncoating and disassembly process which occurs after virus entry but preceding or during reverse transcription. vif mutant cores were more sensitive to disruption by all triggers tested than wild-type cores, as determined by endogenous reverse transcriptase (RT) assays, biochemical analyses, and electron microscopy. RT and the p7 nucleocapsid protein were released more readily from vif mutant virions than from wild-type virions, suggesting that the internal nucleocapsid is less stably packaged in the absence of Vif. Purified cores could be isolated from wild-type but not vif mutant virions by sedimentation through detergent-treated gradients. These results demonstrate that Vif increases the stability of virion cores. This may permit efficient viral DNA synthesis by preventing premature degradation or disassembly of viral nucleoprotein complexes during early events after virus entry.

Selective interaction of the human immunodeficiency virus type 1 reverse transcriptase nonnucleoside inhibitor efavirenz and its thio-substituted analog with different enzyme-substrate complexes

Accumulating data have brought the nonnucleoside reverse transcriptase (RT) inhibitors (NNRTIs) into the forefront of antiretroviral therapy. Among the emerging compounds in this class, a particularly attractive one is efavirenz (Sustiva), recently approved for clinical use by the U.S. Food and Drug Administration. In the present study, the equilibrium dissociation constants for efavirenz binding to the different catalytic forms of human immunodeficiency virus type 1 RT as well as the association and dissociation rates have been determined using a steady-state kinetic approach. In addition, the same enzymological analysis has been extended to the thio-substituted analog, sefavirenz, which showed comparable activity in vitro against RT. Both compounds have been found to act as purely uncompetitive inhibitors at low drug concentrations (5 to 50 nM) and as mixed noncompetitive inhibitors at higher doses (50 to 500 nM). This behavior can be interpreted in terms of the relative affinities for the different catalytic forms of the enzyme. Both efavirenz and sefavirenz showed increasing affinities for the different forms of RT in the following order: free enzyme < (i.e., bound with lower affinity) binary RT-template-primer (TP) complex < ternary RT-TP-deoxynucleoside triphosphate (dNTP) complex. The rate of binding of the two inhibitors to the different enzyme-substrate complexes was well below the diffusion limit (on the order of 10(4) M(-1) s(-1)); however, both inhibitors, when bound to the ternary RT-TP-dNTP complex, showed very low dissociation rates, on the order of 10(-4) s(-1) for both compounds, typical of tightly binding inhibitors. Thus, efavirenz and its thio-substituted derivative sefavirenz appear to be peculiar in their mechanism of action, being selective tightly binding inhibitors of the ternary RT-TP-dNTP complex. Efavirenz is the first clinically approved NNRTI to show this property.

Actinomycin D inhibits human immunodeficiency virus type 1 minus-strand transfer in in vitro and endogenous reverse transcriptase assays

In this report we demonstrate that human immunodeficiency virus type 1 (HIV-1) minus-strand transfer, assayed in vitro and in endogenous reactions, is greatly inhibited by actinomycin D. Previously we showed that HIV-1 nucleocapsid (NC) protein (a nucleic acid chaperone catalyzing nucleic acid rearrangements which lead to more thermodynamically stable conformations) dramatically stimulates HIV-1 minus-strand transfer by preventing TAR-dependent self-priming from minus-strand strong-stop DNA [(-) SSDNA]. Despite this potent activity, the addition of NC to in vitro reactions with actinomycin D results in only a modest increase in the 50% inhibitory concentration (IC50) for the drug. PCR analysis of HIV-1 endogenous reactions indicates that minus-strand transfer is inhibited by the drug with an IC50 similar to that observed when NC is present in the in vitro system. Taken together, these results demonstrate that NC cannot overcome the inhibitory effect of actinomycin D on minus-strand transfer. Other experiments reveal that at actinomycin D concentrations which severely curtail minus-strand transfer, neither the synthesis of (-) SSDNA nor RNase H degradation of donor RNA is affected; however, the annealing of (-) SSDNA to acceptor RNA is significantly reduced. Thus, inhibition of the annealing reaction is responsible for actinomycin D-mediated inhibition of strand transfer. Since NC (but not reverse transcriptase) is required for efficient annealing, we conclude that actinomycin D inhibits minus-strand transfer by blocking the nucleic acid chaperone activity of NC. Our findings also suggest that actinomycin D, already approved for treatment of certain tumors, might be useful in combination therapy for AIDS.

Role of Vif in human immunodeficiency virus type 1 reverse transcription

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is important for virion infectivity. Previous studies have shown that vif mutant HIV-1 virions are defective in their ability to synthesize proviral DNA in vivo. Here, we examine the role of Vif in viral DNA synthesis in the endogenous reverse transcriptase (RT) reaction, an in vitro assay in which virions synthesize viral DNA by using endogenous viral RNA as a template. vif mutant virions showed a significant reduction in endogenous RT activity despite similar levels of exogenous RT activity. Analysis of the viral DNA products on agarose gels demonstrated that this reflects reduced synthesis of short minus- and plus-strand DNA products in addition to those of full genomic length. Quantitative PCR analysis of endogenous reverse transcription provided further evidence for reduced formation of both initial and completed reverse transcripts. Vif had no effect on genomic RNA dimerization or the stability of the RNA dimer linkage. These results suggest that Vif is important for an early event after virus entry but preceding or during the early stages of viral DNA synthesis. This may be due to an intrinsic effect on reverse transcription or a preceding postentry event(s), such as virion uncoating or disassembly of the virion core. Drugs targeted to Vif function may provide a new therapeutic approach to inhibiting HIV-1 reverse transcription.

A two plasmid co-expression system in Escherichia coli for the production of virion-like reverse transcriptase of the human immunodeficiency virus type 1

Many bacterial expression systems have been developed to study the reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1). This enzyme exists in the virions as a heterodimer of a 66 kDa (p66) subunit and a 51 kDa (p51) subunit, originating through proteolytic maturation of the p66 subunit. Most expression systems rely on the processing of p66 by bacterial proteases, this results in a p51 subunit with a non-authentic carboxy-terminus. In contrast, the expression system described produces an RT with an authentic carboxy-terminus. This was achieved by the co-expression of the two subunits of HIV-1 RT, which were each cloned on a different, compatible plasmid in Escherichia coli, and by the use of protease inhibitors during cell lysis. This approach enabled us not only to obtain virion-like RT, as verified by mass spectrometry, but also to monitor the effect of mutations in one or both subunits on the activity of RT and on its sensitivity towards RT inhibitors. The co-expression system described represents a useful method to produce HIV-1 RT, both authentic and mutated, in quantities that allow large-scale studies on the functional organisation of the RT-subunits and the sensitivity of the enzyme to RT inhibitors.

Antiviral therapy for human immunodeficiency virus infections

Depending on the stage of their intervention with the viral replicative cycle, human immunodeficiency virus inhibitors could be divided into the following groups: (i) adsorption inhibitors (i.e., CD4 constructs, polysulfates, polysulfonates, polycarboxylates, and polyoxometalates), (ii) fusion inhibitors (i.e., plant lectins, succinylated or aconitylated albumins, and betulinic acid derivatives), (iii) uncoating inhibitors (i.e., bicyclams), (iv) reverse transcription inhibitors acting either competitively with the substrate binding site (i.e., dideoxynucleoside analogs and acyclic nucleoside phosphonates) or allosterically with a nonsubstrate binding site (i.e., non-nucleoside reverse transcriptase inhibitors), (v) integration inhibitors, (vi) DNA replication inhibitors, (vii) transcription inhibitors (i.e., antisense oligodeoxynucleotides and Tat antagonists), (viii) translation inhibitors (i.e., antisense oligodeoxynucleotides and ribozymes), (ix) maturation inhibitors (i.e., protease inhibitors, myristoylation inhibitors, and glycosylation inhibitors), and finally, (x) budding (assembly/release) inhibitors. Current knowledge, including the therapeutic potential, of these various inhibitors is discussed. In view of their potential clinical the utility, the problem of virus-drug resistance and possible strategies to circumvent this problem are also addressed.

New developments in the chemotherapy of lentivirus (human immunodeficiency virus) infections: sensitivity/resistance of HIV-1 to non-nucleoside HIV-1-specific inhibitors

Of the different steps of the HIV replicative cycle, the reverse transcription step has received most attention as a target for chemotherapeutic intervention. The reverse transcriptase (RT) can be blocked by both nucleoside (nucleotide) and non-nucleoside type of inhibitors. Whereas the former act as competitive inhibitors with respect to the natural substrates or alternate substrates (chain terminators), the latter act allosterically with a non-substrate binding site of the enzyme. Several non-nucleoside types of RT inhibitors have proved to inhibit HIV-1 replication at nanomolar concentrations that are 10(4)- to 10(5)-fold lower than the cytotoxic concentrations. Although a non-nucleoside HIV-1-specific RT inhibitor may rapidly select for virus-drug resistance in cell culture, the resulting mutant strain may or may not show cross-resistance, and in some instances even hypersensitivity, to other HIV-specific RT inhibitors. When used at the appropriate concentrations, HIV-1-specific RT inhibitors are able to completely shut off ("knock-out") virus replication in vitro, under conditions where dideoxynucleoside analogues such as AZT fail to do so. This apparent "sterilizing effect" achieved by the non-nucleoside type of HIV-1-specific RT inhibitors opens new perspectives for the treatment of HIV infections in patients.

Development of a human immunodeficiency virus-1 in vitro DNA synthesis system to study reverse transcriptase inhibitors

A Human immunodeficiency virus type-1 endogenous reverse transcriptase reaction was developed as an in vitro assay to study the inhibition of reverse transcription by antiviral compounds. Conditions were established for producing genomic length (-) strand DNA in high yields and measuring the inhibition of this transcript as the assay endpoint. In addition to genomic length (-) strand DNA, a novel segmented (-) strand product composed of a 6.0 kb reverse transcript of the 5' 2/3 of the viral RNA genome and a 3.5 kb reverse transcript of the 3' 1/3 was observed. The most prominent (+) strand product was the size expected for plus-strong stop DNA. Additional minor (+) strand species were also observed. The triphosphate form of the nucleoside analog inhibitor 3'-azido-3'-deoxythymidine (RETROVIR, Zidovudine, AZT) and BI-RG-587 (nevirapine), a non nucleoside inhibitor, were used to demonstrate the utility of the endogenous system for the analysis of reverse transcriptase inhibitors. In a standard reaction, synthesis of genomic length DNA was 50% inhibited by 0.1 microM AZTTP and 0.1 microM nevirapine.

Antiviral activity of human immunodeficiency virus type 1 protease inhibitors in a single cycle of infection: evidence for a role of protease in the early phase

The antiviral activities of two substrate-based inhibitors of human immunodeficiency virus type 1 (HIV-1) protease, UK-88,947 and Ro 31-8959, were studied in acute infections. H9 and HeLaCD4-LTR/beta-gal cells were infected either with HIV-1IIIB or a replication-defective virus, HIV-gpt(HXB-2). Both inhibitors were capable of blocking early steps of HIV-1 replication if added to cells prior to infection. Partial inhibition was also obtained by addition of inhibitor at the time of or as late as 15 min after infection. The inhibitors were ineffective if added 30 min postinfection. The inhibitory effects were studied by cDNA analysis with PCR followed by Southern blot hybridization and by infectivity assays allowing quantitation of HIV-1 in a single cycle of replication. When UK-88,947-treated H9 cells were coinfected with HIV-1 and human T-cell leukemia virus type I only the replication of HIV-1 was inhibited, demonstrating viral specificity. Pretreating the infectious virus stocks with the inhibitors also prevented replication, indicating that the inhibitors block the action of the viral protease and not a cellular protease. A panel of primer sets was used to analyze cDNA from cell lysates by PCR amplification at 4 and 18 h postinfection. Four hours after infection, viral specific cDNA was detected with all of the four primer pairs used: R/U5, nef/U3, 5' gag, and long terminal repeat (LTR)/gag. However, after 18 h, only the R/U5 and nef/U3 primer pairs and not the 5' gag or LTR/gag primer pair were able to allow amplification of cDNA. The results suggest a crucial role of HIV-1 protease in the early phase of viral replication. Although it is not clear what early steps are affected by the protease, it is likely that the target is the NC protein, as referred from our previous reports of the in situ cleavage of the nucleocapsid (NC) protein by the viral protease inside lentiviral capsids. The results suggest that it is not the inhibition of initiation and progression of reverse transcription but the stability of full-size unintegrated cDNA which is affected in the presence of protease inhibitors. Alternatively, the cleavage of the NC protein may be required for the proper formation of preintegration complex and/or for its transport to the nucleus.

HIV-1-specific RT inhibitors: highly selective inhibitors of human immunodeficiency virus type 1 that are specifically targeted at the viral reverse transcriptase

The TIBO, HEPT, nevirapine, pyridinone, BHAP, TSAO, and alpha-APA derivatives, although belonging to structurally diverging classes of molecules, share remarkable common features. They are specifically active against the reverse transcriptase of HIV-1 (TIBO and HEPT also, to a certain extent, against the reverse transcriptase of SIVagm strains), but not against the reverse transcriptases of HIV-2 or any other retroviruses. Nor are they active against any of the cellular DNA polymerases. These HIV-1-specific RT inhibitors seem to interact with a specific target site (YQYMDDLY) at positions 181-188, which is distinct from, but functionally and spatially related to, the substrate (dNTP) binding site. The tyrosine residues Y181 and Y188 play a crucial role in the interaction of TIBO and its congeners with their target site. The HIV-1-specific RT inhibitors have proven to inhibit the replication of various HIV-1 strains, including AZT-resistant HIV-1 strains, in different cell culture systems, including peripheral blood lymphocytes and monocyte/macrophages. In vitro they exhibit selectivity indexes of up to 5 orders of magnitude, which means that they are inhibitory to virus replication in cell culture at concentrations that are up to 100,000 times lower than the concentrations at which they are toxic to the host cells. As a rule, the HIV-1-specific RT inhibitors are orally bioavailable, as has been demonstrated with the TIBO and HEPT derivatives, nevirapine, pyridinones, and the alpha-APA derivatives in rats, dogs, monkeys, and humans. They sustain plasma drug levels that are well above the concentration required to inhibit virus replication in cell culture. Clinical studies have been undertaken with TIBO R82913, nevirapine, and pyridinones, and others (i.e., alpha-APA R89439) will soon follow. The problem of virus-drug resistance, which seems to readily emerge in vitro, will have to be addressed in the in vivo studies.