Potent and selective inhibition of human immunodeficiency virus type 1 (HIV-1) by 5-ethyl-6-phenylthiouracil derivatives through their interaction with the HIV-1 reverse transcriptase

A scientific career from the early 1960s till 2023: A tale of the various protagonists

In this era spanning more than 60 years (from the early 1960s till today (2023), a broad variety of actors played a decisive role: Piet De Somer, Tom C. Merigan, Paul A. Janssen, Maurice Hilleman, and Georges Smets. Two protagonists (Antonín Holý and John C. Martin) formed with me a unique triangle (the Holý Trinity). Walter Fiers' group (with the help of Jean Content) contributed to the cloning of human β-interferon, and Piet Herdewijn accomplished the chemical synthesis of an array of anti-HIV 2',3'-dideoxynucleoside analogues. Rudi Pauwels, Masanori Baba, Dominique Schols, Johan Neyts, Lieve Naesens, Anita Van Lierde, Graciela Andrei, Robert Snoeck and Dirk Daelemans, as members of my team, helped me in achieving the intended goal, the development of a selective therapy for virus infections. The collaboration with "Lowie" (Guangdi Li) generated a new dimension for the future.

Selected Milestones in Antiviral Drug Development

This review article will describe the (wide) variety of approaches that I envisaged to develop a specific therapy for viral infections: (i) interferon and its inducers, (ii) HSV, VZV and CMV inhibitors, (iii) NRTIs (nucleoside reverse transcriptase inhibitors), NtRTIs (nucleotide reverse transcriptase inhibitors) and NNRTIs (non-nucleoside reverse transcriptase inhibitors) as HIV inhibitors, (iv) NtRTIs as HBV inhibitors, and finally, (v) the transition of an HIV inhibitor to a stem cell mobilizer, as exemplified by AMD-3100 (Mozobil®).

Protease inhibitors as a potential agent against visceral Leishmaniasis: A review to inspire future study

Leishmaniasis is transmitted by sandfly which carries the intracellular protozoa in their midgut. Among visceral, cutaneous and mucocutaneous leishmaniasis, visceral type that is caused by Leishmania donovani is the most lethal one. Findings of leishmanial structure and species took place in 19^th century and was initiated by Donovan. Leishmaniasis is still a major concern of health issues in many endemic countries in Asia, Africa, the Americas, and the Mediterranean region. Worldwide1.5-2 million new cases of cutaneous leishmaniasis and 500,000 cases of visceral leishmaniasis are reported each year. Leishmaniasis is endemic in nearly 90 countries worldwide and close to 12 million new cases of leishmaniasis are reported worldwide annually. Studies on antileishmanial drug development is of major concern as leishmaniasis are the second largest parasitic killer in the world and the available drugs are either toxic or costly. The major surface GP63 protease, also known as Zinc- metalloproteases present on the surface of leishmanial promastigotes, can be targeted for drug development. Protease inhibitors targeting such surface proteases show promising results. Different protease inhibitors have been isolated from marine actinobacteria against many infectious diseases. Metabolites produced by these actinobacteria may have greater importance for the discovery and development of new antileishmanial drugs. Hence, this review discusses the background, current situation, treatment, and protease inhibitors from marine actinobacteria for drug development against GP63 molecules.

Reflections on the Rega Institute for Medical Research, at the fiftieth anniversary of the Rega Stichting vzw (Rega Instituut vzw, Rega Foundation)

The idea to start the Rega Foundation was conceived in 1971 at an informal meeting organized by Prof. Piet De Somer (where Prof. Alfons Billiau, Prof. André Vlerick and I were also present), before the Foundation was formally created in 1972. From the early years some antiviral compounds, such as BVDU and the aminoacyl esters of acyclovir (from which ultimately valacyclovir evolved) originated. The advent of AIDS in 1981 and the discovery of the etiologic agent (HIV) thereof in 1983 have led to the identification of an avalanche of anti-HIV compounds in which the Rega Institute has played a primordial role. Foremost among these compounds was tenofovir, discovered in collaboration with Antonín Holý from the IOCB (Institute of Organic Chemistry and Biochemistry) in Prague. Tenofovir laid the basis for the treatment of HIV (AIDS) and hepatitis B virus (HBV) infections, and in combination with emtricitabine it was the first chemical ever approved by the US FDA (Food and Drug Administration) for the prophylaxis of HIV infections.

Fifty Years in Search of Selective Antiviral Drugs

Fifty years of research (1968-2018) toward the identification of selective antiviral drugs have been primarily focused on antiviral compounds active against DNA viruses (HSV, VZV, CMV, HBV) and retroviruses (HIV). For the treatment of HSV infections the aminoacyl esters of acyclovir were designed, and valacyclovir became the successor of acyclovir in the treatment of HSV and VZV infections. BVDU (brivudin) still stands out as the most potent among the marketed compounds for the treatment of VZV infections (i.e., herpes zoster). In the treatment of HIV infections 10 tenofovir-based drug combinations have been marketed, and tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide (TAF) have also proved effective in the treatment of HBV infections. As a spin-off of our anti-HIV research, a CXCR4 antagonist AMD-3100 was found to be therapeutically useful as a stem cell mobilizer, and has since 10 years been approved for the treatment of some hematological malignancies.

High Plasma Concentrations of Zidovudine (AZT) Do Not Parallel Intracellular Concentrations of AZT-Triphosphates in Infants During Prevention of Mother-to-Child HIV-1 Transmission

OBJECTIVES

Zidovudine (AZT) is mainly used to prevent mother-to-child HIV-1 transmission (PMTCT). Despite serious concerns on AZT-associated toxicity, there is little information on pharmacokinetics of intracellular AZT metabolites in infants.

METHODS

We conducted a prospective study in 31 HIV-uninfected infants who received AZT for PMTCT. Blood samples were obtained from 14 infants on postdelivery days (PDD) 1, 7, 14, and 28 and from 17 infants at 0 and 4 hours after dosing on PDD-1. Plasma AZT concentrations (pAZT) and intracellular concentrations of AZT-monophosphate (icAZT-MP), diphosphate (icAZT-DP), and triphosphate (icAZT-TP) were determined.

RESULTS

Plasma AZT and icAZT-MP concentrations were 2713 nmol/L and 79 fmol/10 cells in PDD-1, but decreased to 1437 nmol/L and 31 fmol/10 cells by PDD-28 (P = 0.02 and P = 0.07 for all PDDs, respectively), whereas those of icAZT-DP and icAZT-TP remained low throughout the sampling period (P = 0.29 and P = 0.61 for all PDDs, respectively) There were no differences in icAZT-TP between infants of the 2 mg/kg 4 times a day dose and 4 mg/kg twice daily dose (P = 0.25), whereas pAZT and icAZT-MP levels were higher in the latter (P < 0.01 and <0.01, respectively). The pAZT and icAZT-MP significantly increased from 0 to 4 hours after dosing (P < 0.001 and <0.001, respectively), whereas icAZT-DP, icAZT-TP levels were not changed (P = 0.41 and 0.33, respectively).

CONCLUSIONS

The level of icAZT-TP did not change with age, time, or a single dose despite the wide range of pAZT concentration. A safer dosage needs to be determined because high pAZT levels do not parallel those of icAZT-TP.

Discovery of novel anti-HIV agents via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry-based approach

INTRODUCTION

In recent years, a variety of new synthetic methodologies and concepts have been proposed in the search for new pharmaceutical lead structures and optimization. Notably, the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry approach has drawn great attention and has become a powerful tool for the generation of privileged medicinal skeletons in the discovery of anti-HIV agents. This is due to the high degree of reliability, complete specificity (chemoselectivity and regioselectivity), mild conditions, and the biocompatibility of the reactants.

AREAS COVERED

Herein, the authors describe the progress thus far on the discovery of novel anti-HIV agents via the CuAAC click chemistry-based approach.

EXPERT OPINION

CuAAC click chemistry is a proven protocol for synthesizing triazole products which could serve as basic pharmacophores, act as replacements of traditional scaffold or substituent modification, be a linker of dual-target or dual-site inhibitors and more for the discovery of novel anti-HIV agents. What's more, it also provides convenience and feasibility for dynamic combinatorial chemistry and in situ screening. It is envisioned that click chemistry will draw more attention and make more contributions in anti-HIV drug discovery in the future.

Approved Antiviral Drugs over the Past 50 Years

Since the first antiviral drug, idoxuridine, was approved in 1963, 90 antiviral drugs categorized into 13 functional groups have been formally approved for the treatment of the following 9 human infectious diseases: (i) HIV infections (protease inhibitors, integrase inhibitors, entry inhibitors, nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and acyclic nucleoside phosphonate analogues), (ii) hepatitis B virus (HBV) infections (lamivudine, interferons, nucleoside analogues, and acyclic nucleoside phosphonate analogues), (iii) hepatitis C virus (HCV) infections (ribavirin, interferons, NS3/4A protease inhibitors, NS5A inhibitors, and NS5B polymerase inhibitors), (iv) herpesvirus infections (5-substituted 2'-deoxyuridine analogues, entry inhibitors, nucleoside analogues, pyrophosphate analogues, and acyclic guanosine analogues), (v) influenza virus infections (ribavirin, matrix 2 protein inhibitors, RNA polymerase inhibitors, and neuraminidase inhibitors), (vi) human cytomegalovirus infections (acyclic guanosine analogues, acyclic nucleoside phosphonate analogues, pyrophosphate analogues, and oligonucleotides), (vii) varicella-zoster virus infections (acyclic guanosine analogues, nucleoside analogues, 5-substituted 2'-deoxyuridine analogues, and antibodies), (viii) respiratory syncytial virus infections (ribavirin and antibodies), and (ix) external anogenital warts caused by human papillomavirus infections (imiquimod, sinecatechins, and podofilox). Here, we present for the first time a comprehensive overview of antiviral drugs approved over the past 50 years, shedding light on the development of effective antiviral treatments against current and emerging infectious diseases worldwide.

Absolute Configuration of the Antiviral Agent (−)-cis-5-Fluoro-1-[2-Hydroxymethyl)-1,3-Oxathiolan-5-yl]Cytosine

The structure and absolute configuration of (−)- cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine (FTC), has been determined by X-ray crystallographic analysis. The results confirm that the L-isomer of the nucleoside analogue is the most active enantiomer and that the correct absolute configuration of (−)-FTC is 5-fluoro-(2′R,5′S)-(−)-1-[2-hydroxymethyl)oxathiolan-5-yl]-fluorocytosine. The two molecules in the asymmetric unit show conformations that combine conformational features of two other classes of potent antiviral nucleosides. Both oxathiolane rings have the 3′-sulphur atom in nearly perfect S3′- exo envelope conformations, similar to what is observed for 3′-azido-3′-deoxythymidine (AZT) and 2′,3′-dideoxycytidine. One of the two molecules has a glycosylic link conformation in which the base is eclipsed with the C5′-O1′ bond. This mimics the high- anti conformation that has been observed in the structures of several 2′,3′-didehydro-2′,3′-dideoxypyrimidine nucleosides but is inaccessible for saturated pyrimidine nucleosides. However, the observed conformations cannot be superimposed adequately with other active antiviral nucleosides to suggest a common ‘active site’ conformation.

Characterization of HIV Reverse Transcriptases with Tyr181→Cys and Leu100→lle Mutations

Two mutants of human immunodeficiency virus (HIV) reverse transcriptase (RT), Tyr181 to Cys and Leu100 to He, have been prepared and characterized by use of various inhibitors. As compared to wild type RT the mutant RT's had lower Kcat/Km values. The Km values were lower with heteropolymeric than with homopolymeric template-primers. Inhibition by phosphonoformate was of mixed type with both wild-type and mutant RT's and the mutants were less sensitive to phosphonoformate than the wild type RT. The non-nucleoside RT inhibitors 9-CI-TIBO and L-697,661 gave a non-competitive inhibition with respect to substrate of the wild type RT. The mutant RT's were inhibited at higher concentrations, showing a mixed type of inhibition with respect to substrate. ddGTP caused a competitive inhibition of wild type and mutant RT's with respect to substrate. RT preparations with different mutations are useful in rapidly characterizing the interaction between various inhibitors and HIV RT and thus facilitate the development of new inhibitors.

3,4-Dihydro-2-Alkoxy-6-Benzyl-4-Oxopyrimidines (DABOs): A New Class of Specific Inhibitors of Human Immunodeficiency Virus Type 1

A series of novel 3,4-dihydro-6-benzyl-4-oxopyrimidines substituted at both the C-5 and the C-2 positions were synthesized as potential anti-HIV agents. Preparation of the title compounds was achieved by condensation of O-methylisourea with methyl 2-alkyl-4-phenylacetylacetate and subsequent displacement of the methoxy group by reaction with a series of linear, ramified and cyclic alkoxy groups containing from three to six carbon units. Methyl 2-alkyl-4-phenylacetylacetates were prepared by alkylation of methyl 4-phenylacetylacetate, which was obtained starting from Meldrum's acid and phenacetyl chloride. Acid hydrolysis of 3,4-dihydro-6-benzyl-2-methoxy-4-oxopyrimidines furnished the corresponding 1,2,3,4-tetrahydro-6-benzyl-2,4-dioxopyrimidines. In acutely infected MT-4 cells, compounds 3e, 3o, 3q and 3r showed an anti-HIV-1 activity as potent and/or selective as HEPT and ddl. Unlike HEPT, the replacement of a methyl for an hydrogen atom at position C-5 of 3,4-dihydro-2-alkoxy-6-benzyl-4-oxopyrimidines (DABOs) did not abolish the antiviral activity, as well as the substitution of the C-5 methyl for an ethyl group did not increase the potency. However, similarly to HEPT and its derivatives, DABOs targeted the HIV-1 reverse transcriptase and neither inhibited the multiplication of HIV-2 in acutely infected MT-4 cells, nor that of HIV-1 in chronically infected H9/IIIB cells.

1-(Ethoxymethyl)-6-(Phenylselenenyl)Pyrimidines with Activity against Human Immunodeficiency Virus Types 1 and 2

The reaction of chloromethyl ethyl ether with bis(trimethylsilyl)uracil derivatives yield 1-(ethoxymethyl)pyrimidines 1a–d in good yield. Lithiation of 1a–d with lithium diisopropylamide at −78°C, followed by reaction with diphenyl diselenide as an electrophile, gave 1-(ethoxymethyl)-6-(phenylselenenyl)ura-cils 2a–d in 70–80% yield. The 6-phenylselenenyl acyclic pyrimidines 2b and 2d exhibited selective in vitro activity against HIV-1 and HIV-2 in primary human lymphocytes. The most potent compound was 1-(ethoxymethyl)-6-(phenylselenenyl)-5-ethyluracil 2d with a median effective concentration of 17 nM in primary human lymphocytes and no discernable cytotoxicity in these cells or rapidly dividing CEM and Vero cells. Well characterized AZT-resistant virus was modestly (3 to 12-fold increase) cross-resistant to compounds 2b and 2d.

Isolation and Characterization of Human Immunodeficiency Virus Type-1 Mutants Resistant to the Non-Nucleotide Reverse Transcriptase Inhibitor MKC-442

MKC-442, 6-benzy 1-1-ethoxymethyl-5-isopropyIuraciI (l-EBU), is a potent and selective non-nucleoside inhibitor of human immunodeficiency virus type-1 (HIV-1) reverse transcriptase (RT). Nevirapine, another non-nucleoside RT inhibitor (NNRTI), is associated with rapid emergence of drug-resistant variants during in vitro passages of HIV-1. The emergence of resistant viruses to MKC-442 or nevirapine was examined in vitro. MT-4 cells infected with a clinical isolate (HE) of HIV-1 were cultivated in medium containing excess concentrations of these drugs, and the drug susceptibilities of the breakthrough viruses recovered from the medium were measured. Although nevirapine lost its antiviral activity after six passages, a delay in the emergence of fully resistant viruses was observed for MKC-442. Two resistant clones for each drug were isolated and nucleotide sequences within the RT region were analysed. An amino acid substitution at position 181 (Tyr to Cys) was found, with additional substitutions at positions 103 (Lys to Arg) and 108 (Val to lle) in the MKC-442-resistant viruses. These clones showed various susceptibilities to MKC-442, and cross-resistance to other NNRTIs but not to AZT. These results suggest that the major binding site of MKC-442 on the HIV-1 RT is the tyrosine residue common to these NNRTIs, and that drug resistance to NNRTIs is dependent on both the quality and the quantity of mutations within the HIV-1 RT gene.

Inhibition of HIV-1-Induced Cytopathogenicity, Syncytium Formation, and Virus-Cell Binding by Naphthalenedisulphonic Acids through Interaction with the Viral Envelope gp120 Glycoprotein

Bis-naphthalenedisulphonic acid derivatives with a biphenyl spacer, 4,4′-[4,4′-biphenyldiylbis(sulphonyl-amino)]bis(5-hydroxy-2,7-naphthalenedisulphonic acid) and 3,3′-[4,4′-biphenyldiylbis(sulphonyl-amino)]bis(1,5-naphthalenedisulphonic acid), have previously been reported as potent and selective inhibitors of human immunodeficiency virus type 1 (HIV-1) replication in cell culture. These compounds have also proved inhibitory to syncytium formation in cocultures of MOLT-4 cells with HIV-1-infected HUT-78 cells. They also inhibit the binding of HIV-1 virions to MT-4 cells as determined by a flow cytometric (FACS) method. Further studies on their mechanism of action by the FACS have revealed that the compounds inhibit the binding of anti-gp120 monoclonal antibody to the viral envelope glycoprotein gp120. Binding of OKT4A/Leu3a monoclonal antibody to the cellular CD4 receptor is not affected by the compounds. These results suggest that the anti-HIV-1 activity of the naphthalenedisulphonic acid derivatives can be attributed to inhibition of the gp120-CD4 interaction through binding of the compounds to the viral gp120 antigen.

Ingenol Derivatives are Highly Potent and Selective Inhibitors of HIV Replication in Vitro

Ingenol 3,5,20-triacetate has recently been identified as a highly potent and selective inhibitor of HIV replication in vitro. To evaluate the potential of ingenol derivatives as anti-HIV agents, several ingenol derivatives have been synthesized and investigated for their anti-HIV activities, structure-activity relationships, and possible mechanisms of action. Among the ingenol derivatives, 13-hydroxyingenol-3-(2,3-dimethylbutanoate)-13-dodecanoate (RD4-2138) proved to be a highly potent and selective inhibitor of HIV replication. Its 50% effective concentration for viral replication in MT-4 cells was 0.07-0.5 nM depending on viral strains, including HIV-2. This concentration was approximately 105-fold lower than its cytotoxic threshold. RD4-2138 was also inhibitory to the syncytium formation induced by cocultivation of Molt-4 cells with Molt-4/IIIB cells (Molt-4 cells chronically infected with HIV-1). Some correlation was observed with the ingenol derivatives between their inhibitory effects on HTLV-IIIB replication and surface CD4 expression in MT-4 cells, suggesting that the mechanism of inhibition is in part attributed to the inhibition of virus adsorption through down-regulation of CD4 molecules in the host cells. However, such correlation was not identified between the inhibition of HTLV-IIIB and the activation of protein kinase C. Thus, they might have a potential as effective anti-HIV agents when toxicity in vivo could be elucidated.

Characterization of the Antiviral Activity of Highly Substituted Pyrroles: A Novel Class of Non-Nucleoside HIV-1 Reverse Transcriptase Inhibitor

As a result of mass screening of the Parke-Davis Pharmaceutical compound library for inhibitors of HIV-1 reverse transcriptase (RT) activity, a novel class of inhibitor, the pyrroles, was identified. Subsequently, a series of analogues was screened for inhibitory activity against HIV-1 and some structure-activity relationships were identified. Further characterization of the most potent pyrrole involved comparing its effects in peripheral blood lymphocytes (PBLs) with its effects in transformed cell lines; the pyrrole had the same efficacy (EC50 = approximately 2 μM) but was less toxic in PBLs (IC50 = 175 μM) than in the cell lines CEM-SS and MT-2 (IC50 = 60-70 μM). The pyrrole was active against a strain of HIV-1 resistant to AZT (strain G9106) but lost activity against both HIV-2 (strain ROD) and a strain of HIV-1 resistant to a non-nucleoside reverse transcriptase inhibitor (the pyridinone-resistant strain A17). Moreover, in direct enzymatic testing against HIV-1 RT purified from virus particles and against RT expressed recombinantly, the pyrrole showed potent inhibitory activity. We conclude that the pyrroles present a novel class of HIV-1 non-nucleoside reverse transcriptase inhibitor.

Synthesis and Antiviral Activity of 1,3-Disubstituted Uracils against HIV-1 and HCMV

The development of new non-nucleoside reverse transcriptase inhibitors (NNRTIs) is an efficient strategy for finding new therapeutic agents against human immunodeficiency virus (HIV). A large number of 6-substituted uracil derivatives have been prepared in order to explore new NNRTIs. However, there are few approaches to anti-HIV agents from 1,3-disubstituted uracil derivatives. Therefore, we tried to prepare several 1,3-disubstituted uracils, which were easily obtainable from uracil by preparation under alkali and Mitsunobu conditions, and examined their antiviral activity against HIV-1 and human cytomegalovirus (HCMV). We found that 1-benzyl-3-(3,5-dimethylbenzyl)uracil and 1-cyanomethyl-3-(3,5-dimethylbenzyl)-4-thiouracil showed powerful inhibition against HCMV and HIV-1, respectively.

1,6-Bis[(benzyloxy)methyl]uracil derivatives-Novel antivirals with activity against HIV-1 and influenza H1N1 virus

A series of 1,6-bis[(benzyloxy)methyl]uracil derivatives combining structural features of both diphenyl ether and pyridone types of NNRTIs were synthesized. Target compounds were found to inhibit HIV-1 reverse transcriptase at micro- and submicromolar levels of concentrations and exhibited anti-HIV-1 activity in MT-4 cell culture, demonstrating resistance profile similar to first generation NNRTIs. The synthesized compounds also showed profound activity against influenza virus (H1N1) in MDCK cell culture without detectable cytotoxicity. The lead compound of this assay appeared to exceed rimantadine, amantadine, ribavirin and oseltamivir carboxylate in activity. The mechanism of action of 1,6-bis[(benzyloxy)methyl]uracils against influenza virus is currently under investigation.

Design, synthesis, and anti-HIV-1 activity of 1-aromatic methyl-substituted 3-(3,5-dimethylbenzyl)uracil and N-3,5-dimethylbenzyl-substituted urea derivatives

BACKGROUND

A new series of 1-aromatic methyl-substituted 3-(3,5-dimethylbenzyl)uracil and N-3,5-dimethylbenzyl-substituted urea derivatives were synthesized and evaluated as non-nucleoside HIV-1 reverse transcriptase inhibitors.

METHODS

A series of new 6-azido and 6-amino derivatives of 1-substituted-3-(3,5-dimethylbenzyl)uracils were synthesized using our previously reported method, and three acyclic derivatives were synthesized from urea. The anti-HIV-1 activities of these compounds were determined based on the inhibition of virus-induced cytopathogenicity in MT-4 cells. The cytotoxicities of the compounds were evaluated using the viability of mock-infected cells.

RESULTS

Some of these compounds showed good-to-moderate activities against HIV-1 with half maximal effective concentration (EC50) values in the submicromolar or subnanomolar range. Compared with emivirine, compound 6-amino-3-(3,5-dimethylbenzyl)-1-(4-aminobenzyl)uracil showed significant anti-HIV-1 activity with an EC50 value of 10 nM and a high selectivity index of 1923. Preliminary structure-activity relationship studies and molecular modeling analyses were carried out to explore the major interactions between HIV-1 reverse transcriptase and the potent inhibitor 6-amino-3-(3,5-dimethylbenzyl)-1-(4-aminobenzyl)uracil; these results may be important for further development of this class of compounds as anti-HIV-1 agents.

CONCLUSION

The excellent activity of 6-amino-3-(3,5-dimethylbenzyl)-1-(4-aminobenzyl)uracil (EC50: 0.010 ± 0.006 µM, SI: >1923) may serve as the basis for conducting further investigations on the behavior of this class of compounds against drug-resistant mutants.

Design, synthesis, and anti-HIV-1 activity of 1-substituted 3-(3,5-dimethylbenzyl)triazine derivatives

BACKGROUND

The reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) is an attractive target for the development of drugs used in the treatment of HIV-1 infection and acquired immune deficiency syndrome (AIDS). We have continued the search for novel anti-HIV-1 agents using the structure-activity relationships of the successful 1,3-disubstituted and 1,3,6-trisubstituted uracil-type HIV-1 RT inhibitors.

METHODS

A series of new triazine analogs were synthesized using an established method. The anti-HIV-1 activities of these compounds were determined based on the inhibition of virus-induced cytopathogenicity in MT-4 cells. The cytotoxicity of the compounds was evaluated by assessing the viability of mock-infected cells.

RESULTS

Some of the compounds showed good-to-moderate activities against HIV-1, with half-maximal effective concentrations (EC50) in the submicromolar range. In particular, a dihydro-1-(4-aminobenzyl)triazine analog showed satisfactory anti-HIV-1 activity with an EC50 of 0.110 µM and a selectivity index (SI) of 909. Furthermore, molecular modeling analyses were performed to explore the major interactions between HIV-1 RT and potent inhibitors. These results may be important for further development of this class of compounds as anti-HIV-1 agents.

CONCLUSION

The satisfactory anti-HIV-1 activity of triazine analogs may serve as the basis for further investigations of the behavior of this class of compounds against drug-resistant mutants.

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.

Dancing with chemical formulae of antivirals: a personal account

A chemical structure is a joy forever, and this is how I perceived the chemical structures of a number of antiviral compounds with which I have been personally acquainted over the past 3 decades: (1) amino acid esters of acyclovir (i.e. valaciclovir); (2) 5-substituted 2'-deoxyuridines (i.e. brivudin); (3) 2',3'-dideoxynucleoside analogues (i.e. stavudine); (4) acyclic nucleoside phosphonates (ANPs) (i.e. cidofovir, adefovir); (5) tenofovir disoproxil fumarate (TDF) and drug combinations therewith; (6) tenofovir alafenamide (TAF, GS-7340), a new phosphonoamidate prodrug of tenofovir; (7) pro-prodrugs of PMEG (i.e. GS-9191 and GS-9219); (8) new ANPs: O-DAPy and 5-aza-C phosphonates; (9) non-nucleoside reverse transcriptase inhibitors (NNRTIs): HEPT and TIBO derivatives; and (10) bicyclam derivatives (i.e. AMD3100).

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.

The nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and protease inhibitors in the treatment of HIV infections (AIDS)

The majority of the drugs currently used for the treatment of HIV infections (AIDS) belong to either of the following three classes: nucleoside reverse transcriptase inhibitors (NRTIs), nonnucleoside reverse transcriptase inhibitors (NNRTIs), and protease inhibitors (PIs). At present, there are 7 NRTIs, 5 NNRTIs, and 10 PIs approved for clinical use. They are discussed from the following viewpoints: (i) chemical formulae; (ii) mechanism of action; (iii) drug combinations; (iv) clinical aspects; (v) preexposure prophylaxis; (vi) prevention of mother-to-child transmission; (vii) their use in children; (viii) toxicity; (ix) adherence (compliance); (x) resistance; (xi) new NRTIs, NNRTIs, or PIs in (pre)clinical development; and (xii) the prospects for a "cure" of the disease.

Anti-human immunodeficiency virus type 1 activity of novel 6-substituted 1-benzyl-3-(3,5-dimethylbenzyl)uracil derivatives

Nonnucleoside reverse transcriptase (RT) inhibitors (NNRTIs) are important components of current combination therapies for human immunodeficiency virus type 1 (HIV-1) infection. In screening of chemical libraries, we found 6-azido-1-benzyl-3-(3,5-dimethylbenzyl)uracil (AzBBU) and 6-amino-1-benzyl-3-(3,5-dimethylbenzyl)uracil (AmBBU) to be highly active and selective inhibitors of HIV-1 replication in vitro. To determine the resistance profiles of these compounds, we conducted a long-term culture of HIV-1-infected MT-4 cells with escalating concentrations of each compound. After serial passages of the infected cells, escape viruses were obtained, and they were more than 500-fold resistant to the uracil derivatives compared to the wild type. Sequence analysis was conducted for RT of the escape viruses at passages 12 and 24. The amino acid mutation Y181C in the polymerase domain of RT was detected for all escape viruses. Docking studies using the crystal structure of RT showed that AmBBU requires the amino acid residues Leu100, Val106, Tyr181, and Trp229 for exerting its inhibitory effect on HIV-1. Four additional amino acid changes (K451R, R461K, T468P, and D471N) were identified in the RNase H domain of RT; however, their precise role in the acquisition of resistance is still unclear. In conclusion, the initial mutation Y181C seems sufficient for the acquisition of resistance to the uracil derivatives AzBBU and AmBBU. Further studies are required to determine the precise role of each mutation in the acquisition of HIV-1 resistance.

Design, synthesis, and biological evaluation of 1-[(2-benzyloxyl/alkoxyl)methyl]-5-halo-6-aryluracils as potent HIV-1 non-nucleoside reverse transcriptase inhibitors with an improved drug resistance profile

Because the emergence of drug-resistant mutants has limited the efficacy of non-nucleoside reverse transcriptase inhibitors (NNRTIs), it is essential to develop new antivirals with better drug resistance and pharmacokinetic profiles. Here we designed and synthesized a series of 1-[(2-benzyloxyl/alkoxyl)methyl]-5-halo-6-aryluracils, the HEPT analogues, and evaluated their biological activity using nevirapine and 18 (TNK-651) as reference compounds. Most of these compounds, especially 6b, 7b, 9b, 11b, and 7c, exhibited highly potent anti-HIV-1 activity against both wild-type and NNRTI-resistant HIV-1 strains. Compound 7b, which had the highest selectivity index (SI = 38 215), is more potent than nevirapine and 18. These results suggest that the introduction of a halogen at the C-5 position may contribute to the effectiveness of these compounds against RTI-resistant variants. In addition, meta substituents on the C-6 aromatic moiety could significantly enhance activity against NNRTI-resistant HIV-1 strains. These compounds can be further developed as next-generation NNRTIs with an improved antiviral efficacy and drug-resistance profile.

Synthesis of 1-benzyl-3-(3,5-dimethylbenzyl)Uracil Derivatives with Potential Anti-HIV Activity

Background:

Nine novel uracil analogues were synthesized and evaluated as inhibitors of HIV-1.

Methods:

Key structural modifications included replacement of the 6-chloro group of 1-benzyl-6-chloro-3-(3,5-dimethylbenzyl)uracil by other functional groups or N1-alkylation of 3-(3,5-dimethylbenzyl)-5-fluorouracil.

Results:

These compounds showed only micromolar potency against HIV-1 in MT-4, though two of them; 6-azido-1-benzyl-3-(3,5-dimethylbenzyl) uracil and 6-amino-1-benzyl-3-(3,5-dimethylbenzyl) uracil were highly potent (half maximal effective concentration =0.067 and 0.069 μM) and selective (selectivity index =685 and 661), respectively. Structure–activity relationships among the newly synthesized uracil analogues suggest the importance of the H-bond formed between 6-amino group of 6-amino-1-benzyl-3-(3,5-dimethylbenzyl) uracil and amide group of HIV-1 reverse transcriptase.

Conclusions:

We discovered two 6-substituted 1-benzyl-3-(3,5-dimethylbenzyl) uracils, (6-azido-1-benzyl-3-(3,5-dimethylbenzyl) uracil and 6-amino-1-benzyl-3-(3,5-dimethylbenzyl) uracil) as novel anti-HIV agents. These compounds should be further pursued for their toxicity and pharmacokinetics in vivo as well as antiviral activity against non-nucleoside reverse transcriptase inhibitor-resistant strains.

Synthesis of 4'-Ethynyl-2'-deoxy-4'-thioribonucleosides and Discovery of a Highly Potent and Less Toxic NRTI

The synthesis of 4'-ethynyl-2'-deoxy-4'-thioribonucleosides was carried out utilizing an electrophilic glycosidation in which 4-ethynyl-4-thiofuranoid glycal 16 served as a glycosyl donor. Electrophilic glycosidation between 16 and the silylated nucleobases (N⁴-acetylcytosine, N⁶-benzoyladenine and N²-acetyl-O⁶-diphenylcarbamoylguanine) was carried out in the presence of N-iodosuccinimide (NIS) leading to the exclusive formation of the desired β-anomers 29, 33 and 36. Anti-HIV studies demonstrated that these 4'-thio nucleosides were less cytotoxic to T-lymphocyte (i.e. MT-4 cells) than the corresponding 4'-ethynyl derivatives of 2'-deoxycytidine (44), 2'-deoxyadenosine (45) and 2'-deoxyguanosine (46). Comparison of the selectivity indices (SI) was made between 4'-thionucleosides (32, 41 and 43) and the corresponding 4'-oxygen analogues 44-46 by using the reported CC₅₀ and EC₅₀ values. In the case of cytosine and adenine nucleosides, comparable SI values were obtained: 32 (545) and 45 (458); 41 (>230) and 45 (1,630). In contrast, 4'-ethynyl-2'-deoxy-4'-thioguanosine 43 was found to possess a SI value of >18,200, which is twenty times better than that of 46 (933).

Novel FRET-based assay to detect reverse transcriptase activity using modified dUTP analogues

We have developed a novel continuous assay to measure reverse transcriptase (RT) polymerase activity. The assay uses fluorescence energy transfer measurements to detect the incorporation of complementary pairs of fluorescently labeled deoxyuridine into cDNA product. The fluorescently labeled dUTP substrates were prepared using commercially available reagents with a simple coupling reaction. The fluorescent dye pairs have significant spectral overlap which allows FRET interaction between dyes incorporated into the cDNA. Using a polyA/oligo dT primer/template, the assay can readily detect DNA polymerase activity from any viral reverse transcriptase enzyme. The reaction proceeds linearly over time, and the rate is proportional to the enzyme concentration. We used the assay to compare the thermostability of a number of wild-type and mutant viral RT enzymes. Our results indicate that the wild-type AMV (avian myeloblastosis virus) enzyme is slightly more stable at 43 degrees C than the HIV-1 (human immunodeficiency virus) or MMLV (Moloney murine leukemia virus) enzymes. The thermostability of the RT enzyme was dramatically increased by the presence of primer/template with the enzyme. We also used the assay to study the effects of inhibitors on HIV-1 RT polymerase activity. This assay may be highly useful for the identification and characterization of potent RT inhibitors which could be candidates for development as therapeutic antiviral agents.

Synthesis and anti-human immunodeficiency virus activity of 4'-branched (+/-)-4'-thiostavudines

Motivated by our recent finding that 4'-ethynylstavudine (4) is a promising anti-human immunodeficiency virus type 1 (HIV-1) agent, we synthesized its 4'-thio analogue, as well as other 4'-thiostavudines having a carbon substituent at the 4'-position, as racemates in this study. Methyl 3-oxo-tetrahydrothiophen-2-carboxylate (5) was used as a starting material to construct the requisite 4-thiofuranoid glycal (13). Introduction of a thymine base was carried out by an electrophilic addition reaction to 13 using N-iodosuccinimide (NIS) and bis(trimethylsilyl)thymine. The desired beta-anomer (16beta) obtained as a major product in this reaction underwent ready elimination with activated Zn to give the 4'-carbomethoxy derivative (18). By using 18 as a common intermediate, 4'-carbon-substituted (CH2OH, CO2Me, CONH2, CH=CH2, CN, and C(triple bond)CH) 4'-thiostavudines were prepared. Among these six compounds, 4'-cyano (28) and 4'-ethynyl (29) analogues were found to show inhibitory activity against HIV-1 with ED50 values of 7.6 and 0.74 microM, respectively. The activity of 29 was comparable to that of stavudine, but 29 was not as active as 4. Optical resolution of 29 was briefly examined.

Chemical library screen for novel inhibitors of Kaposi's sarcoma-associated herpesvirus processive DNA synthesis

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi's sarcoma and certain lymphoproliferative disorders. The role of KSHV lytic replication has been implicated in the tumor pathogenesis. A highly specific molecular complex formed by the KSHV DNA polymerase (POL8) and processivity factor (PF8) is indispensable for lytic viral DNA synthesis and may serve as an excellent molecular anti-KSHV target. The majority of conventional nucleoside-based anti-herpetic DNA synthesis inhibitors require intracellular phosphorylation/activation before they can exert inhibitory activity as competitive substrates for viral DNA polymerases. Novel and more potent inhibitors of KSHV DNA synthesis may be discovered through POL8/PF8-targeted high throughput screening (HTS) of small molecule chemical libraries. We developed a microplate-based KSHV POL8/PF8-mediated DNA synthesis inhibition assay suitable for HTS and screened the NCI Diversity Set that comprised 1992 synthetic compounds. Twenty-eight compounds exhibited greater than 50% inhibition. The inhibitory activity was confirmed for 25 of the 26 hit compounds available for further testing, with the 50% inhibitory concentrations ranging from 0.12+/-0.07 microM (mean+/-S.D.) to 10.83+/-4.19 microM. Eighteen of the confirmed active compounds efficiently blocked KSHV processive DNA synthesis in vitro. One of the hit compounds, NSC 373989, a pyrimidoquinoline analog, was shown to dose-dependently reduce the levels of KSHV virion production and KSHV DNA in lytically induced KSHV-infected BCBL-1 cells, suggesting that the compound blocked lytic KSHV DNA synthesis. HTS for KSHV POL8/PF8 inhibitors is feasible and may lead to discovery of novel non-nucleoside KSHV DNA synthesis inhibitors.

Anti-human immunodeficiency virus type 1 activity and resistance profile of 2',3'-didehydro-3'-deoxy-4'-ethynylthymidine in vitro

2',3'-Didehydro-3'-deoxy-4'-ethynylthymidine (4'-Ed4T) has been identified as a novel nucleoside analog with potent and selective anti-human immunodeficiency virus type 1 (HIV-1) activity and weak cytotoxicity in cell cultures. 4'-Ed4T proved to be 5- to 10-fold more active than its structurally related compound, stavudine (d4T). However, the drug resistance profile of 4'-Ed4T was different from those of d4T and other existing HIV-1 nucleoside reverse transcriptase inhibitors (NRTIs). Approximately 6- to 11-fold decreases in susceptibility to 4'-Ed4T were observed for HIV-1 carrying NRTI-associated mutations (D67N, K70R, T215F, and K219Q) or the lamivudine (3TC)-resistant mutation M184V. In contrast, the susceptibility of the virus carrying the K65R mutation or the multidrug-resistant mutation with the Q151M complex (A62V, V75I, F77L, F116Y, and Q151M) was not altered. Furthermore, the activity of 4'-Ed4T appeared to be enhanced in the presence of K103N, a major nonnucleoside reverse transcriptase inhibitor-resistant mutation. Although 4'-Ed4T was 4.5- to 17.5-fold less active against multidrug-resistant clinical isolates than against a reference strain isolated from a treatment-naïve patient, it was still inhibitory to these isolates at low concentrations. Analysis of 4'-Ed4T-resistant HIV-1 obtained through in vitro selection revealed that the virus was also resistant to 3TC and had two amino acid mutations (P119S and T165A) in addition to the M184V mutation. Since 4'-Ed4T has increased anti-HIV-1 activity, decreased cytotoxicity, and a different resistance profile, it should be considered for further development as a new member of NRTIs.

Studies of non-nucleoside HIV-1 reverse transcriptase inhibitors. Part 2: synthesis and structure-activity relationships of 2-cyano and 2-hydroxy thiazolidenebenzenesulfonamide derivatives

In a previous study, we described the structure-activity relationships (SARs) for a series of thiazolidenebenzenesulfonamide derivatives. These compounds were found to be highly potent inhibitors of the wild type (WT) and Y181C mutant reverse transcriptases (RTs) and modest inhibitors of K103N RT. These molecules are thus considered to be a novel class of non-nucleoside HIV-1 RT inhibitors (NNRTIs). In this paper, we have examined the effects of substituents on both the thiazolidene and benzenesulfonamide moieties. Introduction of a 2-cyanophenyl ring into these moieties significantly enhanced anti-HIV-1 activity, whereas a 2-hydroxyphenyl group endowed potent activity against RTs, including K103N and Y181C mutants. Among the series of molecules examined, 10l and 18b (YM-228855), combinations of 2-cyanophenyl and 4-methyl-5-isopropylthiazole moieties, showed extremely potent anti-HIV-1 activity. The EC50 values of 101 and 18b were 0.0017 and 0.0018 microM, respectively. These values were lower than that of efavirenz (3). Compound 11g (YM-215389), a combination of 2-hydroxyphenyl and 4-chloro-5-isopropylthiazole moieties, proved to be the most active against both K103N and Y181C RTs with IC50 values of 0.043 and 0.013 microM, respectively.

Synthesis and antiviral activities of 1'-carbon-substituted 4'-thiothymidines

4-Thiofuranoid glycals substituted at the 1-position with methyl (5), (t-butyldimethylsilyloxy)methyl (7), and acetoxymethyl (8) groups were prepared from the 3,5-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl) (TIPDS)-4-thiofuranoid glycal (3) by way of LDA-lithiation. N-Iodosuccimide-initiated electrophilic glycosidation between silylated thymine and these 1-carbon-substituted 4-thioglycals gave the respective beta-anomers (9, 10, and 13) stereoselectively. Tin radical-mediated removal of the 2'-iodine atom from these products provided the corresponding 1'-branched 4'-thiothymidine derivatives (11, 12, and 14) in good yields. The 1'-hydroxymethyl derivative (15) served as a precursor for the preparation of the formyl (16), cyanoethenyl (17), and cyano (19) derivatives. Among the deprotected 1'-branched 4'-thiothymidines (20-25), the 1'-methyl analogue 20 showed the most potent anti-HSV-1 activity, but it was much less active than the parent compound 4'-thiothymidine.

Antiviral activity of 4-benzyl pyridinone derivatives as HIV-1 reverse transcriptase inhibitors

In this overview, the antiviral properties of the Curie-pyridinone compounds, a new class of non-nucleoside reverse transcriptase inhibitors (NNRTIs) developed as anti-HIV agents, are described. These compounds are hybrids between hydroxyethoxymethyl-phenylthiothymine (HEPT) and Merck pyridinones. Several structure-activity relationships (SAR) studies between HIV-1 reverse transcriptase (RT) and the Curie-pyridinones are described. The Curie-pyridinones are potent inhibitors of both HIV-1 replication in cell culture and of HIV-1 RT activity in vitro. They are specific to HIV-1 and do not inhibit the replication of HIV-2. The mechanism of inhibition is non-competitive with respect to the natural substrate dGTP. For these reasons, the Curie-pyridinones can be considered as non-nucleoside inhibitors of HIV-1 RT. Moreover, they have the unusual ability to reach the reverse transcription complex inside the extracellular virions and may therefore be useful as retrovirucides. This might lead to the design and synthesis of new drugs able to interact with the retroviral enzyme inside the viral core.

Studies of nonnucleoside HIV-1 reverse transcriptase inhibitors. Part 1: Design and synthesis of thiazolidenebenzenesulfonamides

A random high-throughput screening (HTS) program to discover novel nonnucleoside reverse transcriptase inhibitors (NNRTIs) has been carried out with MT-4 cells against a nevirapine-resistant virus, HIV-1(IIIB-R). The primary hit, a thiazolidenebenzenesulfonamide derivative, possessed good activity. A systematic modification program examining various substituents at the 3-, 4-, and 5-positions on the thiazole ring afforded compounds with enhanced anti-HIV-1 and reverse transcriptase (RT) inhibitory activities. These results confirm the important role of the substituents at these positions and the thiazolidenebenzenesulfonamide motif as a valuable lead series for the next generation NNRTIs.

Induction of cellular resistance to nucleoside reverse transcriptase inhibitors by the wild-type breast cancer resistance protein

Breast cancer resistance protein (BCRP/ABCG2) is a novel member of ATP-binding cassette transporters, which induce multidrug resistance in cancer cells. We previously reported that a high level of BCRP expression in CD4(+) T cells conferred cellular resistance to nucleoside reverse transcriptase inhibitors (NRTIs) of human immunodeficiency virus type 1 (HIV-1). However, this BCRP was found to have a mutation of Arg to Met at position 482 (BCRP(R482M)). The present study demonstrated that the wild-type BCRP (BCRP(WT)) also conferred cellular resistance to NRTIs. MT-4 cells (a CD4(+) T-cell line) highly expressing BCRP(WT) (MT-4/BCRP) were generated and the expression of BCRP(WT) was confirmed by genotypic and phenotypic analyses. Compared to the parental MT-4 cells, MT-4/BCRP cells displayed resistance to zidovudine (AZT) in terms of antiviral activity as well as drug cytotoxicity. In addition, other NRTIs were also less inhibitory to HIV-1 replication in MT-4/BCRP cells than in MT-4 cells. Significant reduction of intracellular AZT accumulation was observed in MT-4/BCRP cells. An analysis for intracellular metabolism of AZT suggested that the resistance was attributed to the increased efflux of AZT and its metabolites in MT-4/BCRP cells. Furthermore, the BCRP-specific inhibitor fumitremorgin C completely restored the reduction of AZT in MT-4/BCRP cells. These results indicate that, like BCRP(R482M), BCRP(WT) also plays an important role in cellular resistance to NRTIs.

Nucleoside analogues exerting antiviral activity through a non-nucleoside mechanism

In analogy with maribavir [1-(beta-L-ribofuranosyl)-isopropylamino-5,6-dichlorobenzimidazole], a nucleoside analogue that acts against human cytomegalovirus (HCMV) by a non-nucleoside mechanism, here I present three other examples of classes of nucleoside analogues (i.e. bicyclic furo[2,3-d]pyrimidine as well as HEPT and TSAO derivatives) that act against either HCMV or human immunodeficiency virus (HIV) through a non-nucleoside mode of action.

Breast cancer resistance protein (BCRP/ABCG2) induces cellular resistance to HIV-1 nucleoside reverse transcriptase inhibitors

Breast cancer resistance protein (BCRP/ABCG2) is a novel member of ATP- binding cassette transporters, which induce multidrug resistance in cancer cells. We found that a high level of BCRP expression in CD4+ T cells conferred cellular resistance to human immunodeficiency virus type-1 (HIV-1) nucleoside reverse transcriptase inhibitors. The cell line MT-4/DOX 500 was established through the long-term culture of MT-4 cells in the presence of doxorubicin (DOX) and had reduced sensitivity to not only DOX but also zidovudine (AZT). MT-4/DOX 500 cells showed reduced intracellular accumulation and retention of DOX and increased ATP-dependent rhodamine 123 efflux. The cells were also resistant to several anticancer agents such as mitoxantrone, 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin, and 7-ethyl-10-hydroxycamptothecin. AZT was 7.5-fold less inhibitory to HIV-1 replication in MT-4/DOX 500 cells than in MT-4 cells. Furthermore, the anti-HIV-1 activity of lamivudine was severely impaired in MT-4/DOX 500 cells. In contrast, the antiviral activity of non-nucleoside reverse transcriptase inhibitors and protease inhibitors was not affected in the cells. MT-4/DOX 500 cells expressed glycosylated BCRP but not P-glycoprotein (ABCB1), multidrug resistance protein 1, 2, or 4 (ABCC1, -2, or -4), or lung resistance-related protein. In addition, the BCRP-specific inhibitor fumitremorgin C completely abolished the resistance of MT-4/DOX 500 cells to AZT as well as to DOX. An analysis for intracellular metabolism of AZT suggests that the resistance is attributed to the increase of ATP-dependent efflux of its metabolites, presumably AZT 5'-monophosphate, in MT-4/DOX 500 cells.