Design, synthesis and antiviral evaluation of novel conjugates of the 1,7,7-trimethylbicyclo[2.2.1]heptane scaffold and saturated N-heterocycles via 1,2,3-triazole linker.

A new series of heterocyclic derivatives with a 1,7,7-trimethylbicyclo[2.2.1]heptane fragment was designed, synthesised and biologically evaluated. Synthesis of the target compounds was performed using the Cu(I) catalysed cycloaddition reaction. The key starting substances in the click reaction were an alkyne containing a 1,7,7-trimethylbicyclo[2.2.1]heptane fragment and a series of azides with saturated nitrogen-containing heterocycles. Some of the derivatives were found to exhibit strong antiviral activity against Marburg and Ebola pseudotype viruses. Lysosomal trapping assays revealed the derivatives to possess lysosomotropic properties. The molecular modelling study demonstrated the binding affinity between the compounds investigated and the possible active site to be mainly due to hydrophobic interactions. Thus, combining a natural hydrophobic structural fragment and a lysosome-targetable heterocycle may be an effective strategy for designing antiviral agents.

Discovery of N-Containing (-)-Borneol Esters as Respiratory Syncytial Virus Fusion Inhibitors.

Respiratory syncytial virus (RSV) causes acute respiratory infections, thus, posing a serious threat to the health of infants, children, and elderly people. In this study, we have discovered a series of potent RSV entry inhibitors with the (-)-borneol scaffold. The active compounds 3b, 5a, 5c, 7b, 9c, 10b, 10c, and 14b were found to exhibit activity against RSV A strain A2 in HEp-2 cells. The most active substances, 3b (IC50 = 8.9 µM, SI = 111) and 5a (IC50 = 5.0 µM, SI = 83), displayed more potency than the known antiviral agent Ribavirin (IC50 = 80.0 µM, SI = 50). Time-of-addition assay and temperature shift studies demonstrated that compounds 3b, 5a, and 6b inhibited RSV entry, probably by interacting with the viral F protein that mediated membrane fusion, while they neither bound to G protein nor inhibited RSV attachment to the target cells. Appling procedures of molecular modeling and molecular dynamics, the binding mode of compounds 3b and 5a was proposed. Taken together, the results of this study suggest (-)-borneol esters to be promising lead compounds for developing new anti-RSV agents.

Borneol Ester Derivatives as Entry Inhibitors of a Wide Spectrum of SARS-CoV-2 Viruses.

In the present work we studied the antiviral activity of the home library of monoterpenoid derivatives using the pseudoviral systems of our development, which have glycoproteins of the SARS-CoV-2 virus strains Wuhan and Delta on their surface. We found that borneol derivatives with a tertiary nitrogen atom can exhibit activity at the early stages of viral replication. In order to search for potential binding sites of ligands with glycoprotein, we carried out additional biological tests to study the inhibition of the re-receptor-binding domain of protein S. For the compounds that showed activity on the pseudoviral system, a study using three strains of the infectious SARS-CoV-2 virus was carried out. As a result, two leader compounds were found that showed activity on the Wuhan, Delta, and Omicron strains. Based on the biological results, we searched for the potential binding site of the leader compounds using molecular dynamics and molecular docking methods. We suggested that the compounds can bind in conserved regions of the central helices and/or heptad repeats of glycoprotein S of SARS-CoV-2 viruses.

Design, Synthesis, and Biological Evaluation of (+)-Camphor- and (-)-Fenchone-Based Derivatives as Potent Orthopoxvirus Inhibitors.

In this work, a library of (+)-camphor and (-)-fenchone based N-acylhydrazones, amides, and esters, including para-substituted aromatic/hetaromatic/cyclohexane ring was synthesized, with potent orthopoxvirus inhibitors identified among them. Investigations of the structure-activity relationship revealed the significance of the substituent at the para-position of the aromatic ring. Also, the nature of the linker between a hydrophobic moiety and aromatic ring was clarified. Derivatives with p-Cl, p-Br, p-CF3, and p-NO2 substituted aromatic ring and derivatives with cyclohexane ring showed the highest antiviral activity against vaccinia virus, cowpox, and ectromelia virus. The hydrazone and the amide group were more favourable as a linker for antiviral activity than the ester group. Compounds 3 b and 7 e with high antiviral activity were examined using the time-of-addition assay and molecular docking study. The results revealed the tested compounds to inhibit the late processes of the orthopoxvirus replication cycle and the p37 viral protein to be a possible biological target.

Quaternary ammonium salts based on (-)-borneol as effective inhibitors of influenza virus.

A series of compounds containing a 1,7,7-trimethylbicyclo[2.2.1]heptane fragment were evaluated for their antiviral activity against influenza A virus strain A/Puerto Rico/8/34 (H1N1) in vitro. The most potent antiviral compound proved to be a quaternary ammonium salt based on (-)-borneol, 10a. In in vitro experiments, compound 10a inhibited influenza A viruses (H1, H1pdm09, and H3 subtypes), with an IC50 value of 2.4-16.8 µM (depending on the virus), and demonstrated low toxicity (CC50 = 1311 µM). Mechanism-of-action studies for compound 10a revealed it to be most effective when added at the early stages of the viral life cycle. In direct haemolysis inhibition tests, compound 10a was shown to decrease the membrane-disrupting activity of influenza A virus strain A/Puerto Rico/8/34. According to molecular modelling results, the lead compound 10a can bind to different sites in the stem region of the viral hemagglutinin.

Synthesis and Antiviral Activity of Camphene Derivatives against Different Types of Viruses.

To date, the 'one bug-one drug' approach to antiviral drug development cannot effectively respond to the constant threat posed by an increasing diversity of viruses causing outbreaks of viral infections that turn out to be pathogenic for humans. Evidently, there is an urgent need for new strategies to develop efficient antiviral agents with broad-spectrum activities. In this paper, we identified camphene derivatives that showed broad antiviral activities in vitro against a panel of enveloped pathogenic viruses, including influenza virus A/PR/8/34 (H1N1), Ebola virus (EBOV), and the Hantaan virus. The lead-compound 2a, with pyrrolidine cycle in its structure, displayed antiviral activity against influenza virus (IC50 = 45.3 µM), Ebola pseudotype viruses (IC50 = 0.12 µM), and authentic EBOV (IC50 = 18.3 µM), as well as against pseudoviruses with Hantaan virus Gn-Gc glycoprotein (IC50 = 9.1 µM). The results of antiviral activity studies using pseudotype viruses and molecular modeling suggest that surface proteins of the viruses required for the fusion process between viral and cellular membranes are the likely target of compound 2a. The key structural fragments responsible for efficient binding are the bicyclic natural framework and the nitrogen atom. These data encourage us to conduct further investigations using bicyclic monoterpenoids as a scaffold for the rational design of membrane-fusion targeting inhibitors.

Biostability study, quantitation method and preliminary pharmacokinetics of a new antifilovirus agent based on borneol and 3-(piperidin-1-yl)propanoic acid.

The stability of the new antifiloviral agent AS-358, which is a derivative of borneol and 3-(piperidin-1-yl)propanoic acid, was studied in the blood and blood plasma of rats in vitro. It was found that both in the blood and in the plasma stabilized by EDTA or heparin, the compound is rapidly hydrolyzed at the ester bond. When sodium fluoride was added to the whole blood, the decomposition of the compound was significantly slowed down, which made it possible to develop and validate a method for the quantitative determination of the agent in this matrix. The method was validated in terms of selectivity, calibration dependence, LLOQ, accuracy and precision, stability in an autosampler, recovery, and carry-over. A 8:2 v/v mixture of methanol containing 2-adamantylamine hydrochloride (internal standard, IS) with 0.2 M aqueous zinc sulfate was used for blood sample treatment and protein precipitation. Analysis was performed by HPLC-MS/MS using reversed phase chromatography. MS/MS detection was performed on a triple quadrupole mass spectrometer 6500 QTRAP (SCIEX) in multiple reaction monitoring (MRM) mode. The transitions 294.5→158.2/98.1 and 152.2→107.2/93.1 were monitored for AS-358 and the IS, respectively. The calibration curve was built in the concentration range of 1-500 ng/mL, the intra-day and inter-day accuracy and precision, carry-over and recovery were within the acceptable limits. The developed method was used for a preliminary study of the pharmacokinetics of the agent AS-358 after its oral administration to rats. It was shown that when the substance was administered at a dose of 200 mg/kg, its concentration in the blood of animals reached 550 ng/mL after 1 h, despite its instability in blood.

(+)-Camphor and (-)-borneol derivatives as potential anti-orthopoxvirus agents.

Although the World Health Organisation had announced that smallpox was eradicated over 40 years ago, the disease and other related pathogenic poxviruses such as monkeypox remain potential bioterrorist weapons and could also re-emerge as natural infections. We have previously reported (+)-camphor and (-)-borneol derivatives with an antiviral activity against the vaccinia virus. This virus is similar to the variola virus (VARV), the causative agent of smallpox, but can be studied at BSL-2 facilities. In the present study, we evaluated the antiviral activity of the most potent compounds against VARV, cowpox virus, and ectromelia virus (ECTV). Among the compounds tested, 4-bromo-N'-((1R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene)benzohydrazide 18 is the most effective compound against various orthopoxviruses, including VARV, with an EC50 value of 13.9 µM and a selectivity index of 206. Also, (+)-camphor thiosemicarbazone 9 was found to be active against VARV and ECTV.

Monoterpenoid-based inhibitors of filoviruses targeting the glycoprotein-mediated entry process.

In this study, we screened a large library of (+)-camphor and (-)-borneol derivatives to assess their filovirus entry inhibition activities using pseudotype systems. Structure-activity relationship studies revealed several compounds exhibiting submicromolar IC50 values. These compounds were evaluated for their effect against natural Ebola virus (EBOV) and Marburg virus. Compound 3b (As-358) exhibited the good antiviral potency (IC50 = 3.7 µM, SI = 118) against Marburg virus, while the hydrochloride salt of this compound 3b·HCl had a strong inhibitory effect against Ebola virus (IC50 = 9.1 µM, SI = 31) and good in vivo safety (LD50 > 1000 mg/kg). The results of molecular docking and in vitro mutagenesis analyses suggest that the synthesized compounds bind to the active binding site of EBOV glycoprotein similar to the known inhibitor toremifene.

Synthesis and structure-activity relationships of novel camphecene analogues as anti-influenza agents.

A chemical library was constructed based on the scaffold of camphecene (2-(E)-((1R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene-aminoethanol). The modifications included introduction of mono-and bicyclic heterocyclic moieties in place of the terminal hydroxyl group of camphecene. All compounds were tested for cytotoxicity and anti-viral activity against influenza virus A/Puerto Rico/8/34 (H1N1) in MDCK cells. Among 15 tested compounds 11 demonstrated a selectivity index (SI) higher than 10 and IC50 values in the micromolar range. The antiviral activity and toxicity were shown to strongly depend on the nature of the heterocyclic substituent. Compounds 2 and 14 demonstrated the highest virus-inhibiting activity with SIs of 106 and 183, and bearing pyrrolidine and piperidine moieties, correspondingly. Compound 14 was shown to interfere with viral reproduction at early stages of the viral life cycle (0-2 h post-infection). Taken together, our data suggest potential of camphecene derivatives in particular and camphor-based imine derivatives in general as effective anti-influenza compounds.

Synthesis and antiviral activity of camphor-based 1,3-thiazolidin-4-one and thiazole derivatives as Orthopoxvirus-reproduction inhibitors.

The Orthopoxvirus genus belongs to the Poxviridae family and includes variola virus (smallpox), cowpox virus, monkeypox virus and vaccinia virus (VV). Smallpox is considered one of the great epidemic disease scourges in human history. It has currently been eradicated; however, it remains a considerable threat as a biowarfare or bioterrorist weapon. The poxvirus, VV, serves as a model virus from which new antiviral therapies against Orthopoxviruses can be identified. Here, a series of nitrogen-sulphur containing heterocycles such as 1,3-thiazolidin-4-one and thiazoles containing a 1,7,7-trimethylbicyclo[2.2.1]heptan scaffold were synthesized and screened for their antiviral activity. The bioassay results showed that the 4b, 4c and 4e thiazoles, which contained a substituted benzene ring, were able to inhibit VV reproduction with IC50 values in the 2.4-3.7 micromolar range whilst exhibiting moderate cytotoxicity. Among the thiazolidin-4-one derivatives, compound 8b, which contained a 4-methylbenzylidene group, displayed good inhibitory activity (IC50 = 9.5 µM) and moderate toxicity.

Discovery of a New Class of Inhibitors of Vaccinia Virus Based on (-)-Borneol from Abies sibirica and (+)-Camphor.

A series of the bornyl ester/amide derivatives with N-containing heterocycles were designed and synthesized as vaccinia virus (VV) inhibitors. Bioassay results showed that among the designed compounds, derivatives 6, 13, 14, 34, 36 and 37 showed the best inhibitory activity against VV with the IC50 values of 12.9, 17.9, 3.4, 2.5, 12.5 and 7.5 µm, respectively, and good cytotoxicity. The primary structure-activity relationship (SAR) study suggested that the combination of a saturated N-heterocycle, such as morpholine or 4-methylpiperidine, and a 1,7,7-trimethylbicyclo[2.2.1]heptane scaffold was favorable for antiviral activity.

Aliphatic and alicyclic camphor imines as effective inhibitors of influenza virus H1N1.

A series of camphor derived imines was synthesised and evaluated in vitro for antiviral activity. Theoretical evaluations of ADME properties were also carried out. Most of these compounds exhibited significant activity against the drug-resistant strains of influenza A virus. Especially, compounds 2 (SI = 632) and 3 (SI = 417) presented high inhibition against influenza subtypes A/Puerto Rico/8/34 and A/California/07/09 of H1N1pdm09. Analysis of the structure-activity relationship showed that the activity was strongly dependent on the length of the aliphatic chain: derivatives with a shorter chain possessed higher activity, while the suppressing action of compounds with long aliphatic chains was lower.

Discovery of a new class of antiviral compounds: camphor imine derivatives.

A new class of compounds featuring a camphor moiety has been discovered that exhibits potent inhibitory activity against influenza A(H1N1)pdm09 and A(H5N1) viruses. The synthesized compounds were characterized by spectroscopic analysis; in addition the structures of compound 2 and 14 were elucidated by the X-ray diffraction technique. Structure-activity relationship studies have been conducted to identify the 1,7,7-trimethylbicyclo[2.2.1]heptanes2-ylidene group as the key functional group responsible for the observed antiviral activity. The most potent antiviral compound is imine 2 with therapeutic index more than 500.

Camphor-based symmetric diimines as inhibitors of influenza virus reproduction.

Influenza is a continuing world-wide public health problem that causes significant morbidity and mortality during seasonal epidemics and sporadic pandemics. The purpose of the study was synthesis and investigation of antiviral activity of camphor-based symmetric diimines and diamines. A set of C2-symmetric nitrogen-containing camphor derivatives have been synthesized. The antiviral activity of these compounds was studied against rimantadine- and amantadine-resistant influenza virus A/California/7/09 (H1N1)pdm09 in MDCK cells. The highest efficacy in virus inhibiting was shown for compounds 2a-e with cage moieties bound by aliphatic linkers. The therapeutic index (selectivity index) for 2b exceeded that for reference compounds amantadine, deitiforin and rimantadine almost 10-fold. As shown by structure-activity analysis, the length of the linker has a dramatic effect on the toxicity of compounds. Compound 2e with -C12H24- linker exhibited the lowest toxicity (CTD50=2216µM). Derivatives of camphor, therefore, can be considered as prospective antiinfluenza compounds active against influenza viruses resistant to adamantane-based drugs.

New quaternary ammonium camphor derivatives and their antiviral activity, genotoxic effects and cytotoxicity.

The synthesis and biological evaluation of a novel series of dimeric camphor derivatives are described. The resulting compounds were studied for their antiviral activity, cyto- and genotoxicity. Compounds 3a and 3d in which the quaternary nitrogen atoms are separated by the C5H10 and С9H18 aliphatic chain, exhibited the highest efficiency as an agent inhibiting the reproduction of the influenza virus A(H1N1)pdm09. The cytotoxicity data of compounds 3 and 4 revealed their moderate activity against malignant cell lines; compound 3f had the highest activity for the CEM-13 cells. These results show close agreement with the data of independent studies on toxicity of these compounds, in particular that the toxicity of compounds strongly depends on spacer length.