Double-Winged 3-Hydroxypyrimidine-2,4-diones: Potent and Selective Inhibition against HIV-1 RNase H with Significant Antiviral Activity

Inhibiting HCMV pUL89-C Endonuclease with Metal-Binding Compounds

Human cytomegalovirus (HCMV) infects individuals of all ages and establishes a lifelong latency. Current antiviral drugs are suboptimal in efficacy and safety and ineffective against resistant/refractory HCMV. Therefore, there is an unmet clinical need for efficacious, safe, and mechanistically novel HCMV drugs. The recent Food and Drug Administration (FDA) approval of letermovir (LTV) validated the HCMV terminase complex as a new target for antiviral development. LTV targets terminase subunit pUL56 but not the main endonuclease enzymatic function housed in the C terminus of subunit pUL89 (pUL89-C). Structurally and mechanistically, pUL89-C is an RNase H-like viral endonuclease entailing two divalent metal ions at the active site. In recent years, numerous studies have extensively explored pUL89-C inhibition using metal-chelating chemotypes, an approach previously used for inhibiting HIV ribonuclease H (RNase H) and integrase strand transfer (INST). Collectively, the work summarized herein validates the use of metal-binding scaffolds for designing potent and specific pUL89-C inhibitors.

Recent advances on dual inhibitors targeting HIV reverse transcriptase associated polymerase and ribonuclease H

Reverse transcriptase (RT) plays an indispensable role in the replication of human immunodeficiency virus (HIV) through its associated polymerase and ribonuclease H (RNase H) activities during the viral RNA genome transformation into proviral DNA. Due to the fact that HIV is a highly mutagenic virus and easily resistant to single-target RT inhibitors, dual inhibitors targeting HIV RT associated polymerase and RNase H have been developed. These dual inhibitors have the advantages of increasing efficacy, reducing drug resistance, drug-drug interactions, and cytotoxicity, as well as improving patient compliance. In this review, we summarize recent advances in polymerase/RNase H dual inhibitors focusing on drug design strategies, and structure-activity relationships and share new insights into developing anti-HIV drugs.

Repurposing N-hydroxy thienopyrimidine-2,4-diones (HtPD) as inhibitors of human cytomegalovirus pUL89 endonuclease: Synthesis and biological characterization

The terminase complex of human cytomegalovirus (HCMV) is required for viral genome packaging and cleavage. Critical to the terminase functions is a metal-dependent endonuclease at the C-terminus of pUL89 (pUL89-C). We have previously reported metal-chelating N-hydroxy thienopyrimidine-2,4-diones (HtPD) as inhibitors of human immunodeficiency virus 1 (HIV-1) RNase H. In the current work, we have synthesized new analogs and resynthesized known analogs of two isomeric HtPD subtypes, anti-HtPD (13), and syn-HtPD (14), and characterized them as inhibitors of pUL89-C. Remarkably, the vast majority of analogs strongly inhibited pUL89-C in the biochemical endonuclease assay, with IC50 values in the nM range. In the cell-based antiviral assay, a few analogs inhibited HCMV in low μM concentrations. Selected analogs were further characterized in a biophysical thermal shift assay (TSA) and in silico molecular docking, and the results support pUL89-C as the protein target of these inhibitors. Collectively, the biochemical, antiviral, biophysical, and in silico data reported herein indicate that the isomeric HtPD chemotypes 13-14 can serve as valuable chemical platforms for designing improved inhibitors of HCMV pUL89-C.

RNase HI Depletion Strongly Potentiates Cell Killing by Rifampicin in Mycobacteria

Multidrug-resistant (MDR) tuberculosis (TB) is defined by the resistance of Mycobacterium tuberculosis, the causative organism, to the first-line antibiotics rifampicin and isoniazid. Mitigating or reversing resistance to these drugs offers a means of preserving and extending their use in TB treatment. R-loops are RNA/DNA hybrids that are formed in the genome during transcription, and they can be lethal to the cell if not resolved. RNase HI is an enzyme that removes R-loops, and this activity is essential in M. tuberculosis: knockouts of rnhC, the gene encoding RNase HI, are nonviable. This essentiality makes it a candidate target for the development of new antibiotics. In the model organism Mycolicibacterium smegmatis, RNase HI activity is provided by two enzymes, RnhA and RnhC. We show that the partial depletion of RNase HI activity in M. smegmatis, by knocking out either of the genes encoding RnhA or RnhC, led to the accumulation of R-loops. The sensitivity of the knockout strains to the antibiotics moxifloxacin, streptomycin, and rifampicin was increased, the latter by a striking near 100-fold. We also show that R-loop accumulation accompanies partial transcriptional inhibition, suggesting a mechanistic basis for the synergy between RNase HI depletion and rifampicin. A model of how transcriptional inhibition can potentiate R-loop accumulation is presented. Finally, we identified four small molecules that inhibit recombinant RnhC activity and that also potentiated rifampicin activity in whole-cell assays against M. tuberculosis, supporting an on-target mode of action and providing the first step in developing a new class of antimycobacterial drug.

Current medicinal chemistry strategies in the discovery of novel HIV-1 ribonuclease H inhibitors

During HIV-1 genome replication, the viral reverse transcriptase-associated ribonuclease H (RT-associated RNase H) activity hydrolyzes the RNA strand of RNA/DNA heteroduplex intermediates. As of today, HIV-1 RNase H inhibitors (RHIs) remain at an investigational level, although none of them reached clinical trials. Therefore, RNase H remains as an attractive target for drug design and development. In this paper, we review the current status of medicinal chemistry strategies aimed at the discovery of novel RHIs, while discussing problems encountered in their characterization and further development, thereby providing an update on recent progress in the field.

8-Hydroxy-1,6-naphthyridine-7-carboxamides as Inhibitors of Human Cytomegalovirus pUL89 Endonuclease

Human cytomegalovirus (HCMV) replication requires a metal-dependent endonuclease at the C-terminus of pUL89 (pUL89-C) for viral genome packaging and cleavage. We have previously shown that pUL89-C can be pharmacologically inhibited with designed metal-chelating compounds. We report herein the synthesis of a few 8-hydroxy-1,6-naphthyridine subtypes, including 5-chloro (subtype 15), 5-aryl (subtype 16), and 5-amino (subtype 17) variants. Analogs were studied for the inhibition of pUL89-C in a biochemical endonuclease assay, a biophysical thermal shift assay (TSA), in silico molecular docking, and for the antiviral potential against HCMV in cell-based assays. These studies identified eight analogs of 8-hydroxy-1,6-naphthyridine-7-carboxamide subtypes for further characterization, most of which inhibited pUL89-C with single-digit μM IC50 values, and conferred antiviral activity in μM range. TSA and molecular modeling of selected analogs corroborate their binding to pUL89-C. Collectively, our biochemical, antiviral, biophysical and in silico data suggest that 8-hydroxy-1,6-naphthyridine-7-carboxamide subtypes can be used for designing inhibitors of HCMV pUL89-C.

4,5-Dihydroxypyrimidine Methyl Carboxylates, Carboxylic Acids, and Carboxamides as Inhibitors of Human Cytomegalovirus pUL89 Endonuclease

Human cytomegalovirus (HCMV) terminase complex entails a metal-dependent endonuclease at the C-terminus of pUL89 (pUL89-C). We report herein the design, synthesis, and characterization of dihydroxypyrimidine (DHP) acid (14), methyl ester (13), and amide (15) subtypes as inhibitors of HCMV pUL89-C. All analogs synthesized were tested in an endonuclease assay and a thermal shift assay (TSA) and subjected to molecular docking to predict binding affinity. Although analogs inhibiting pUL89-C in the sub-μM range were identified from all three subtypes, acids (14) showed better overall potency, substantially larger thermal shift, and considerably better docking scores than esters (13) and amides (15). In the cell-based antiviral assay, six analogs inhibited HCMV with moderate activities (EC₅₀ = 14.4-22.8 μM). The acid subtype (14) showed good in vitro ADME properties, except for poor permeability. Overall, our data support the DHP acid subtype (14) as a valuable scaffold for developing antivirals targeting HCMV pUL89-C.

Retroviral RNase H: Structure, mechanism, and inhibition

All retroviruses encode the enzyme, reverse transcriptase (RT), which is involved in the conversion of the single-stranded viral RNA genome into double-stranded DNA. RT is a multifunctional enzyme and exhibits DNA polymerase and ribonuclease H (RNH) activities, both of which are essential to the reverse-transcription process. Despite the successful development of polymerase-targeting antiviral drugs over the last three decades, no bona fide inhibitor against the RNH activity of HIV-1 RT has progressed to clinical evaluation. In this review article, we describe the retroviral RNH function and inhibition, with primary consideration of the structural aspects of inhibition.

Development of Human Immunodeficiency Virus Type 1 Resistance to 4'-Ethynyl-2-Fluoro-2'-Deoxyadenosine (EFdA) Starting with Wild-Type or Nucleoside Reverse Transcriptase Inhibitor Resistant-Strains

4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA, MK-8591, islatravir) is a nucleoside reverse transcriptase translocation inhibitor (NRTTI) with exceptional potency against WT and drug-resistant HIV-1, in Phase III clinical trials. EFdA resistance is not well characterized. To study EFdA-resistance patterns as it may emerge in naïve or tenofovir- (TFV), emtricitabine/lamivudine- (FTC/3TC), or zidovudine- (AZT) treated patients we performed viral passaging experiments starting with wild-type, K65R, M184V, or D67N/K70R/T215F/K219Q HIV-1. Regardless the starting viral sequence, all selected EFdA-resistant variants included the M184V RT mutation. Using recombinant viruses, we validated the role for M184V as the primary determinant of EFdA resistance; none of the observed connection subdomain (R358K and E399K) or RNase H domain (A502V) mutations significantly contributed to EFdA resistance. A novel EFdA resistance mutational pattern that included A114S was identified in the background of M184V. A114S/M184V exhibited higher EFdA resistance (∼24-fold) than M184V (∼8-fold) or A114S alone (∼2-fold). Remarkably, A114S/M184V and A114S/M184V/A502V resistance mutations were up to 50-fold more sensitive to tenofovir than WT HIV-1. These mutants also had significantly lower specific infectivity than WT. Biochemical experiments confirmed decreases in the enzymatic efficiency (k_cat/K_m) of WT vs. A114S (2.1-fold) and A114S/M184V/A502V (6.5-fold) RTs, with no effect of A502V on enzymatic efficiency or specific infectivity. The rather modest EFdA resistance of M184V or A114S/M184V (8- and 24-fold), their hypersusceptibility to tenofovir, and strong published in vitro and in vivo data, suggest that EFdA is an excellent therapeutic candidate for naïve, AZT-, FTC/3TC, and especially tenofovir-treated patients.

Metal binding 6-arylthio-3-hydroxypyrimidine-2,4-diones inhibited human cytomegalovirus by targeting the pUL89 endonuclease of the terminase complex

The genome packaging of human cytomegalovirus (HCMV) requires a divalent metal-dependent endonuclease activity localized to the C-terminus of pUL89 (pUL89-C), which is reminiscent of RNase H-like enzymes in active site structure and catalytic mechanism. Our previous work has shown that metal-binding small molecules can effectively inhibit pUL89-C while conferring significant antiviral activities. In this report we generated a collection of 43 metal-binding small molecules by repurposing analogs of the 6-arylthio-3-hydroxypyrimidine-2,4-dione chemotype previously synthesized for targeting HIV-1 RNase H, and by chemically synthesizing new N-1 analogs. The analogs were subjected to two parallel screening assays: the pUL89-C biochemical assay and the HCMV antiviral assay. Compounds with significant inhibition from each assay were further tested in a dose-response fashion. Single dose cell viability and PAMPA cell permeability were also conducted and considered in selecting compounds for the dose-response antiviral testing. These assays identified a few analogs displaying low μM inhibition against pUL89-C in the biochemical assay and HCMV replication in the antiviral assay. The target engagement was further evaluated via a thermal shift assay using recombinant pUL89-C and molecular docking. Overall, our current work identified novel inhibitors of pUL89-C with significant antiviral activities and further supports targeting pUL89-C with metal-binding small molecules as an antiviral approach against HCMV.

Enzyme-triggered DNA nanomimosa: A ratiometric nanoprobe for RNase H activity sensing in living cells

Since ribonuclease H (RNase H) exhibits its importance in a variety of cellular processes. It is necessary to establish strategy for RNase H detection. In this work, we are enlightened by mimosa, a natural plant which can fold in response to stimuli, to construct a DNA tetrahedron-based nanoprobe, termed DNA nanomimosa, to sensing RNase H activity based on fluorescent resonance energy transfer (FRET). The DNA nanomimosa was self-assembled from four DNA chains and one RNA chain. One of the four DNA chains contains a FRET-paired fluorophores-labeled hairpin DNA structures which is unfolded by the RNA chain through hybridization. Without RNase H, the RNA chain separate the two FRET-paired fluorophores in hairpin DNA structure, giving a feeble FRET signal. However, the presence of RNase H can selectively digest the RNA strand in RNA/unfolded-hairpin DNA section, resulting in the hairpin DNA configuration changed from "unfolded" state to "folded" state and further turn on the FRET signal. The DNA nanomimosa can be applied to achieve the determination of RNase H activity by recording the emission intensity of donor and acceptor fluorophores. This strategy shows a low detection limit by 0.017 U/mL, good specificity, and distinct advantages like the self-delivery ability, good biocompatibility, and the capacity to minimize the effects of fluctuations. This design provides a potential application in ribonuclease research and could be expanded for other biomedical research and clinical diagnostics.

Avoiding Drug Resistance in HIV Reverse Transcriptase

HIV reverse transcriptase (RT) is an enzyme that plays a major role in the replication cycle of HIV and has been a key target of anti-HIV drug development efforts. Because of the high genetic diversity of the virus, mutations in RT can impart resistance to various RT inhibitors. As the prevalence of drug resistance mutations is on the rise, it is necessary to design strategies that will lead to drugs less susceptible to resistance. Here we provide an in-depth review of HIV reverse transcriptase, current RT inhibitors, novel RT inhibitors, and mechanisms of drug resistance. We also present novel strategies that can be useful to overcome RT's ability to escape therapies through drug resistance. While resistance may not be completely avoidable, designing drugs based on the strategies and principles discussed in this review could decrease the prevalence of drug resistance.

1,2,4-Triazolo[1,5-a]pyrimidines as a Novel Class of Inhibitors of the HIV-1 Reverse Transcriptase-Associated Ribonuclease H Activity

Despite great efforts have been made in the prevention and therapy of human immunodeficiency virus (HIV-1) infection, however the difficulty to eradicate latent viral reservoirs together with the emergence of multi-drug-resistant strains require the search for innovative agents, possibly exploiting novel mechanisms of action. In this context, the HIV-1 reverse transcriptase (RT)-associated ribonuclease H (RNase H), which is one of the few HIV-1 encoded enzymatic function still not targeted by any current drug, can be considered as an appealing target. In this work, we repurposed in-house anti-influenza derivatives based on the 1,2,4-triazolo[1,5-a]-pyrimidine (TZP) scaffold for their ability to inhibit HIV-1 RNase H function. Based on the results, a successive multi-step structural exploration around the TZP core was performed leading to identify catechol derivatives that inhibited RNase H in the low micromolar range without showing RT-associated polymerase inhibitory activity. The antiviral evaluation of the compounds in the MT4 cells showed any activity against HIV-1 (III_B strain). Molecular modelling and mutagenesis analysis suggested key interactions with an unexplored allosteric site providing insights for the future optimization of this class of RNase H inhibitors.

Cutting into the Substrate Dominance: Pharmacophore and Structure-Based Approaches toward Inhibiting Human Immunodeficiency Virus Reverse Transcriptase-Associated Ribonuclease H

Human immunodeficiency virus (HIV) reverse transcriptase (RT) contains two distinct functional domains: a DNA polymerase (pol) domain and a ribonuclease H (RNase H) domain, both of which are required for viral genome replication. Over the last 3 decades, RT has been at the forefront of HIV drug discovery efforts with numerous nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) approved by the FDA. However, all these RT inhibitors target only the pol function, and inhibitors of RT-associated RNase H have yet to enter the development pipeline, which in itself manifests both the opportunity and challenges of targeting RNase H: if developed, RT RNase H inhibitors would represent a mechanistically novel class of HIV drugs that can be particularly valuable in treating HIV strains resistant to current drugs. The challenges include (1) the difficulty in selectively targeting RT RNase H over RT pol due to their close interplay both spatially and temporally and over HIV-1 integrase strand transfer (INST) activity because of their active site similarities; (2) to a larger extent, the inability of active site inhibitors to confer significant antiviral effect, presumably due to a steep substrate barrier by which the pre-existing substrate prevents access of small molecules to the active site. As a result, previously reported RT RNase H inhibitors typically lacked target specificity and significant antiviral potency. Achieving meaningful antiviral activity via active site targeting likely entails selective and ultrapotent RNase H inhibition to allow small molecules to cut into the dominance of substrates. Based on a pharmacophore model informed by prior work, we designed and redesigned a few metal-chelating chemotypes, such as 2-hydroxyisoquinolinedione (HID), hydroxypyridonecarboxylic acid (HPCA), 3-hydroxypyrimidine-2,4-dione (HPD), and N-hydroxythienopyrimidine-2,4-dione (HTPD). Analogues of these chemotypes generally exhibited improved potency and selectivity inhibiting RT RNase H over the best previous compounds and further validated the pharmacophore model. Extended structure-activity relationship (SAR) on the HPD inhibitor type by mainly altering the linkage generated a few subtypes showing exceptional potency (single-digit nanomolar) and excellent selectivity over the inhibition of RT pol and INST. In parallel, a structure-based approach also allowed us to design a unique double-winged HPD subtype to potently and selectively inhibit RT RNase H and effectively compete against the RNA/DNA substrate. Significantly, all potent HPD subtypes consistently inhibited HIV-1 in the cell culture, suggesting that carefully designed active site RNase H inhibitors with ultrapotency could partially overcome the barrier to antiviral phenotype. Overall, in addition to identifying our own inhibitor types, our medicinal chemistry efforts demonstrated the value of pharmacophore and structure-based approaches in designing active side-directed RNase H inhibitors and could provide a viable path to validating RNase H as a novel antiviral target.

Coumarin: An emerging antiviral agent

Viral infections are responsible for many illnesses, and recent outbreaks have raised public health concerns. Despite the availability of many antiviral drugs, they are often unsuccessful due to the generation of viral mutants and less effective against their target virus. Identifying novel antiviral drugs is therefore of critical importance and natural products are an excellent source for such discoveries. Coumarin is one such natural compound that is a potential drug candidate owing to its properties of stability, solubility, and low toxicity. There are numerous evidences showing its inhibitory role against infection of various viruses such as HIV, Influenza, Enterovirus 71 (EV71) and coxsackievirus A16 (CVA16). The mechanisms involve either inhibition of proteins essential for viral entry, replication and infection or regulation of cellular pathways such as Akt-Mtor (mammalian target of rapamycin), NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), and anti-oxidative pathway including NrF-2 (The nuclear factor erythroid 2 (NFE2)-related factor 2). This review summarizes the present state of understanding with a focus on coumarin's antiviral effect and their possible molecular mechanisms against Influenza virus, HIV, Hepatitis virus, Dengue virus and Chikungunya virus.

From cycloheptathiophene-3-carboxamide to oxazinone-based derivatives as allosteric HIV-1 ribonuclease H inhibitors

The paper focussed on a step-by-step structural modification of a cycloheptathiophene-3-carboxamide derivative recently identified by us as reverse transcriptase (RT)-associated ribonuclease H (RNase H) inhibitor. In particular, its conversion to a 2-aryl-cycloheptathienoozaxinone derivative and the successive thorough exploration of both 2-aromatic and cycloheptathieno moieties led to identify oxazinone-based compounds as new anti-RNase H chemotypes. The presence of the catechol moiety at the C-2 position of the scaffold emerged as critical to achieve potent anti-RNase H activity, which also encompassed anti-RNA dependent DNA polymerase (RDDP) activity for the tricyclic derivatives. Benzothienooxazinone derivative 22 resulted the most potent dual inhibitor exhibiting IC50s of 0.53 and 2.90 μM against the RNase H and RDDP functions. Mutagenesis and docking studies suggested that compound 22 binds two allosteric pockets within the RT, one located between the RNase H active site and the primer grip region and the other close to the DNA polymerase catalytic centre.

Chromenone derivatives as a versatile scaffold with dual mode of inhibition of HIV-1 reverse transcriptase-associated Ribonuclease H function and integrase activity

A number of compounds targeting different processes of the Human Immunodeficiency Virus type 1 (HIV-1) life cycle have been developed in the continuing fight against AIDS. Coumarin-based molecules already proved to act as HIV-1 Protease (PR) or Integrase (IN) inhibitors and also to target HIV-1 reverse transcriptase (RT), blocking the DNA-dependent DNA-polymerase activity or the RNA-dependent DNA-polymerase activity working as common NNRTIs. In the present study, with the aim to exploit a coumarin-based scaffold to achieve the inhibition of multiple viral coded enzymatic functions, novel 4-hydroxy-2H, 5H-pyrano (3, 2-c) chromene-2, 5-dione derivatives were synthesized. The modeling studies calculated the theoretical binding affinity of the synthesized compounds on both HIV-1 IN and RT-associated Ribonuclease H (RNase H) active sites, which was confirmed by biological assays. Our results provide a basis for the identification of dual HIV-1 IN and RT RNase H inhibitors compounds.

Novel Deazaflavin Analogues Potently Inhibited Tyrosyl DNA Phosphodiesterase 2 (TDP2) and Strongly Sensitized Cancer Cells toward Treatment with Topoisomerase II (TOP2) Poison Etoposide

Topoisomerase II (TOP2) poisons as anticancer drugs work by trapping TOP2 cleavage complexes (TOP2cc) to generate DNA damage. Repair of such damage by tyrosyl DNA phosphodiesterase 2 (TDP2) could render cancer cells resistant to TOP2 poisons. Inhibiting TDP2, thus, represents an attractive mechanism-based chemosensitization approach. Currently known TDP2 inhibitors lack cellular potency and/or permeability. We report herein two novel subtypes of the deazaflavin TDP2 inhibitor core. By introducing an additional phenyl ring to the N-10 phenyl ring (subtype 11) or to the N-3 site of the deazaflavin scaffold (subtype 12), we have generated novel analogues with considerably improved biochemical potency and/or permeability. Importantly, many analogues of both subtypes, particularly compounds 11a, 11e, 12a, 12b, and 12h, exhibited much stronger cancer cell sensitizing effect than the best previous analogue 4a toward the treatment with etoposide, suggesting that these analogues could serve as effective cellular probes.

Pharmacophore-based design of novel 3-hydroxypyrimidine-2,4-dione subtypes as inhibitors of HIV reverse transcriptase-associated RNase H: Tolerance of a nonflexible linker

The pharmacophore of active site inhibitors of human immunodeficiency virus (HIV) reverse transcriptase (RT)-associated RNase H typically entails a flexible linker connecting the chelating core and the hydrophobic aromatics. We report herein that novel 3-hydroxypyrimidine-2,4-dione (HPD) subtypes with a nonflexible C-6 carbonyl linkage exhibited potent and selective biochemical inhibitory profiles with strong RNase H inhibition at low nM, weak to moderate integrase strand transfer (INST) inhibition at low μM, and no to marginal RT polymerase (pol) inhibition up to 10 μM. A few analogues also demonstrated significant antiviral activity without cytotoxicity. The overall inhibitory profile is comparable to or better than that of previous HPD subtypes with a flexible C-6 linker, suggesting that the nonflexible carbonyl linker can be tolerated in the design of novel HIV RNase H active site inhibitors.

Design, synthesis, and biologic evaluation of novel galloyl derivatives as HIV-1 RNase H inhibitors

Human immunodeficiency virus (HIV) reverse transcriptase (RT)-associated ribonuclease H (RNase H) remains as the only enzyme encoded within the viral genome not targeted by current antiviral drugs. In this work, we report the design, synthesis, and biologic evaluation of a novel series of galloyl derivatives with HIV-1 RNase H inhibitory activity. Most of them showed IC₅₀ s at sub- to low-micromolar concentrations in enzymatic assays. The most potent compound was II-25 that showed an IC₅₀ of 0.72 ± 0.07 μM in RNase H inhibition assays carried out with the HIV-1_{BH 10} RT. II-25 was 2.8 times more potent than β-thujaplicinol in these assays. Interestingly, II-25 and other galloyl derivatives were also found to inhibit the HIV IN strand transfer activity in vitro. Structure-activity relationships (SAR) studies and molecular modeling analysis predict key interactions with RT residues His539 and Arg557, while providing helpful insight for further optimization of selected compounds.

Synthesis and evaluation of steroidal thiazoline conjugates as potential antiviral agents

Aim: Many heterocyclic compounds derived from natural steroids exhibited broad activities, so this work focused on the investigations on a series of steroidal thiazoline conjugates as antiviral agents. Materials & methods: A series of steroid derivatives containing thiazoline heterocycles were designed and synthesized via a convenient condensation procedure. The compounds were screened for their potential antivirus activities against Enterovirus 71 (EV71) and Coxsackie Virus Type B (CVB3). Results and Conclusion: The in vitro bioassay indicated that compounds 5b, 5g and 5i exhibited excellent antiviral effects on EV71, and compounds 5b, 5e, 6c and 6g presented better antiviral activities against CVB3 compared with the controls ribavirin or pirodavir. These results indicate that these steroidal thiazoline conjugates might be feasible therapeutic candidates against EV71 infection, which might also be considered as promising compounds for optimization of potential antivirus agents.

6-Biphenylmethyl-3-hydroxypyrimidine-2,4-diones potently and selectively inhibited HIV reverse transcriptase-associated RNase H

Human immunodeficiency virus (HIV) reverse transcriptase (RT)-associated ribonuclease H (RNase H) remains an unvalidated drug target. Reported HIV RNase H inhibitors generally lack significant antiviral activity. We report herein the design, synthesis, biochemical and antiviral evaluations of a new 6-biphenylmethyl subtype of the 3-hydroxypyrimidine-2,4-dione (HPD) chemotype. In biochemical assays, analogues of this new subtype potently inhibited RT RNase H in low nanomolar range without inhibiting RT polymerase (pol) or integrase strand transfer (INST) at the highest concentrations tested. In cell-based assays, a few analogues inhibited HIV in low micromolar range without cytotoxicity at concentrations up to 100 μM.

6-Arylthio-3-hydroxypyrimidine-2,4-diones potently inhibited HIV reverse transcriptase-associated RNase H with antiviral activity

Human immunodeficiency virus (HIV) reverse transcriptase (RT) associated ribonuclease H (RNase H) remains the only virally encoded enzymatic function not targeted by current drugs. Although a few chemotypes have been reported to inhibit HIV RNase H in biochemical assays, their general lack of significant antiviral activity in cell culture necessitates continued efforts in identifying highly potent RNase H inhibitors to confer antiviral activity. We report herein the design, synthesis, biochemical and antiviral evaluations of a new 6-arylthio subtype of the 3-hydroxypyrimidine-2,4-dione (HPD) chemotype. In biochemical assays these new analogues inhibited RT RNase H in single-digit nanomolar range without inhibiting RT polymerase (pol) at concentrations up to 10 μM, amounting to exceptional biochemical inhibitory selectivity. Many analogues also inhibited integrase strand transfer (INST) activity in low to sub micromolar range. More importantly, most analogues inhibited HIV in low micromolar range without cytotoxicity. In the end, compound 13j (RNase H IC₅₀ = 0.005 μM; RT pol IC₅₀ = 10 μM; INST IC₅₀ = 4.0 μM; antiviral EC₅₀ = 7.7 μM; CC₅₀ > 100 μM) represents the best analogues within this series. These results characterize the new 6-arylthio-HPD subtype as a promising scaffold for HIV RNase H inhibitor discovery.

Developing and Evaluating Inhibitors against the RNase H Active Site of HIV-1 Reverse Transcriptase

We tested three compounds for their ability to inhibit the RNase H (RH) and polymerase activities of HIV-1 reverse transcriptase (RT). A high-resolution crystal structure (2.2 Å) of one of the compounds showed that it chelates the two magnesium ions at the RH active site; this prevents the RH active site from interacting with, and cleaving, the RNA strand of an RNA-DNA heteroduplex. The compounds were tested using a variety of substrates: all three compounds inhibited the polymerase-independent RH activity of HIV-1 RT. Time-of-addition experiments showed that the compounds were more potent if they were bound to RT before the nucleic acid substrate was added. The compounds significantly inhibited the site-specific cleavage required to generate the polypurine tract (PPT) RNA primer that initiates the second strand of viral DNA synthesis. The compounds also reduced the polymerase activity of RT; this ability was a result of the compounds binding to the RH active site. These compounds appear to be relatively specific; they do not inhibit either Escherichia coli RNase HI or human RNase H2. The compounds inhibit the replication of an HIV-1-based vector in a one-round assay, and their potencies were only modestly decreased by mutations that confer resistance to integrase strand transfer inhibitors (INSTIs), nucleoside analogs, or nonnucleoside RT inhibitors (NNRTIs), suggesting that their ability to block HIV replication is related to their ability to block RH cleavage. These compounds appear to be useful leads that can be used to develop more potent and specific compounds.IMPORTANCE Despite advances in HIV-1 treatment, drug resistance is still a problem. Of the four enzymatic activities found in HIV-1 proteins (protease, RT polymerase, RT RNase H, and integrase), only RNase H has no approved therapeutics directed against it. This new target could be used to design and develop new classes of inhibitors that would suppress the replication of the drug-resistant variants that have been selected by the current therapeutics.

Prenylated phloroglucinols from Hypericum scruglii, an endemic species of Sardinia (Italy), as new dual HIV-1 inhibitors effective on HIV-1 replication

In a search for new potential multitarget anti-HIV compounds from natural products, we have identified in Hypericum scruglii, an endemic and exclusive species of Sardinia (Italy), a potent plant lead. The phytochemical study of the hydroalcoholic extract obtained from its leaves led to the isolation of its most abundant secondary metabolites, belonging to different chemical classes. In particular, three phloroglucinols derivatives were identified, confirming their significance as chemotaxonomic markers of the Hypericum genus. Among them, the 3-(13-hydroxygeranyl)-1-(2'-methylbutanoyl)phloroglucinol was reported here for the first time. All six isolated compounds have been evaluated firstly for the inhibition of both Human Immunodeficiency Virus type 1 (HIV-1) Reverse Transcriptase (RT)-associated DNA Polymerase (RDDP) and Ribonuclease H (RNase H) activities, for the inhibition of HIV-1 integrase (IN) in biochemical assays, and also for their effect on viral replication. Among the isolated metabolites, three phloroglucinol derivatives and quercitrin were effective on both RT-associated RDDP and RNase H activities in biochemical assays. The same active compounds affected also HIV-1 IN strand transfer function, suggesting the involvement of the RNase H active site. Furthermore, phloroglucinols compounds, included the newly identified compound, were able to inhibit the HIV-1 replication in cell based assays.

Design, synthesis and biological evaluations of N-Hydroxy thienopyrimidine-2,4-diones as inhibitors of HIV reverse transcriptase-associated RNase H

Human immunodeficiency virus (HIV) reverse transcriptase (RT) associated ribonuclease H (RNase H) is the only HIV enzymatic function not targeted by current antiviral drugs. Although various chemotypes have been reported to inhibit HIV RNase H, few have shown significant antiviral activities. We report herein the design, synthesis and biological evaluation of a novel N-hydroxy thienopyrimidine-2,3-dione chemotype (11) which potently and selectively inhibited RNase H with considerable potency against HIV-1 in cell culture. Current structure-activity-relationship (SAR) identified analogue 11d as a nanomolar inhibitor of RNase H (IC₅₀ = 0.04 μM) with decent antiviral potency (EC₅₀ = 7.4 μM) and no cytotoxicity (CC₅₀ > 100 μM). In extended biochemical assays compound 11d did not inhibit RT polymerase (pol) while inhibiting integrase strand transfer (INST) with 53 fold lower potency (IC₅₀ = 2.1 μM) than RNase H inhibition. Crystallographic and molecular modeling studies confirmed the RNase H active site binding mode.

A 2-Hydroxyisoquinoline-1,3-Dione Active-Site RNase H Inhibitor Binds in Multiple Modes to HIV-1 Reverse Transcriptase

The RNase H (RNH) function of HIV-1 reverse transcriptase (RT) plays an essential part in the viral life cycle. We report the characterization of YLC2-155, a 2-hydroxyisoquinoline-1,3-dione (HID)-based active-site RNH inhibitor. YLC2-155 inhibits both polymerase (50% inhibitory concentration [IC₅₀] = 2.6 μM) and RNH functions (IC₅₀ = 0.65 μM) of RT but is more effective against RNH. X-ray crystallography, nuclear magnetic resonance (NMR) analysis, and molecular modeling were used to show that YLC2-155 binds at the RNH-active site in multiple conformations.

Design, synthesis and antiviral evaluation of novel heteroarylcarbothioamide derivatives as dual inhibitors of HIV-1 reverse transcriptase-associated RNase H and RDDP functions

In the continuous effort to identify new HIV-1 inhibitors endowed with innovative mechanisms, the dual inhibition of different viral functions would provide a significant advantage against drug-resistant variants. The HIV-1 reverse transcriptase (RT)-associated ribonuclease H (RNase H) is the only viral-encoded enzymatic activity that still lacks an efficient inhibitor. We synthesized a library of 3,5-diamino-N-aryl-1H-pyrazole-4-carbothioamide and 4-amino-5-benzoyl-N-phenyl-2-(substituted-amino)-1H-pyrrole-3-carbothioamide derivatives and tested them against RNase H activity. We identified the pyrazolecarbothioamide derivative A15, able to inhibit viral replication and both RNase H and RNA-dependent DNA polymerase (RDDP) RT-associated activities in the low micromolar range. Docking simulations hypothesized its binding to two RT pockets. Site-directed mutagenesis experiments showed that, with respect to wt RT, V108A substitution strongly reduced A15 IC50 values (12.6-fold for RNase H inhibition and 4.7-fold for RDDP), while substitution A502F caused a 9.0-fold increase in its IC50 value for RNase H, not affecting the RDDP inhibition, reinforcing the hypothesis of a dual-site inhibition. Moreover, A15 retained good inhibition potency against three non-nucleoside RT inhibitor (NNRTI)-resistant enzymes, confirming a mode of action unrelated to NNRTIs and suggesting its potential as a lead compound for development of new HIV-1 RT dual inhibitors active against drug-resistant viruses.