A. S.
Sokolova
*ab,
O. I.
Yarovaya
ab,
M. D.
Semenova
a,
A. A.
Shtro
c,
I. R.
Orshanskaya
c,
V. V.
Zarubaev
c and
N. F.
Salakhutdinov
ab
aNovosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Lavrentjev Avenue 9, 630090 Novosibirsk, Russia. E-mail: asokolova@nioch.nsc.ru
bNovosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia
cLaboratory of Chemotherapy, Influenza Research Institute, Prof. Popova St. 15/17, 197376 St. Petersburg, Russia
First published on 3rd March 2017
Herein, we present the design and synthesis of a series of novel heterocyclic derivatives of (−)-borneol and (−)-isoborneol as potent inhibitors of the influenza A virus. All compounds were tested for their toxicity against MDCK cells and for virus-inhibiting activity against the influenza virus A/Puerto Rico/8/34 (H1N1). Compounds 7, 16 and 26 containing a morpholine fragment exhibited the highest efficiency as agents inhibiting the replication of the influenza virus A(H1N1) with selectivity indices of 82, 45 and 65, correspondingly. Derivatives 9 (SI = 23) and 18 (SI = 25) containing a 1-methylpiperazine motif showed moderate antiviral activity. Structure–activity analysis of this new series of borneol derivatives revealed that a 1,7,7-trimethylbicyclo[2.2.1]heptan scaffold is required for the antiviral activity.
Borneol, a bicyclic monoterpenoid alcohol, exists as two enantiomers, the D and L forms. Both the borneol forms occur in the essential oils of numerous medicinal plants, such as valerian (Valeriana officinalis), chamomile (Matricaria chamomilla) and lavender (Lavandula officinalis). Generally, there are two isomers of borneol: borneol and isoborneol, which vary with the location of the hydroxyl group. These substances have a broad spectrum of biological activity. Recently, it has been established that isoborneol shows antiviral activity against herpes simplex virus type 1,7 and both enantiomers of borneol were found to have a highly efficacious positive modulating action at mammalian GABA (γ-aminobutyric acid) inhibitory neurotransmission receptors.8 In addition, borneol shows inhibitory effects on several Gr (−) and Gr (+) pathogenic microorganisms.9
It can be assumed that cage compounds like borneol derivatives containing a 1,7,7-trimethylbicyclo[2.2.1]heptan scaffold have high potential in the synthesis of antiviral agents. Our previous works discovered this scaffold as a promising anti-influenza agent.10–12 The target compound was identified as an inhibitor of the H1N1 influenza virus with a selectivity index (SI) value of 500; this camphor derivative was called camphecene (Fig. 1).13 In contrast to deitiforine that targets the M2 proton channel, camphecene was shown to directly inhibit the acid-induced membrane-disrupting activity of the viral hemagglutinin of influenza A viruses.14 Moreover there are other cage compounds with antiviral activity, for example amino camphor derivatives were identified as a new class of M2 inhibitors with moderate activity.15 Also, compounds based on 1- and 2-adamantylamines exhibited high M2 inhibitor activity.16,17 The ability to inhibit the influenza virus H3N2 was found in amino and imine derivatives with a pinene scaffold;18 compounds containing a pinanamine scaffold with secondary amine and imidazole may increase inhibition of A/M2 channel activity.19
In view of the above, in the present work, several series of borneol derivatives containing a cage structure based on the 1,7,7-trimethylbicyclo[2.2.1]heptan scaffold were synthesized and tested against the influenza virus. According to the literature data, the general direction in the synthesis of biologically active borneol derivatives includes preparation of different borneol esters. These esters were shown to be inhibitors of cyclooxygenase enzymes20 and the replication of coxsackievirus B3. Also, bornyl ester derivatives showed cytotoxic activity against tumor cell lines.21,22 Therefore in this work the general approach to the synthesis of the target compounds included synthesis of different borneol esters containing various heterocycles. Nitrogen heterocycles are attractive structural units in medicinal chemistry. There are a lot of highly effective drugs with the active ingredient containing nitrogen heterocyclic fragments, such as piperazine, piperidine and morpholine.23 In our search for antiviral agents, we screened several different nitrogen-containing heterocyclic fragments for their ability to inhibit reproduction. Moreover, we studied the influence of the length of the aliphatic linker between the ester group and the heterocyclic scaffold.
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Scheme 1 Reagents and conditions: (i) 2-chloroacetyl chloride, Et3N, CH2Cl2 (dry), 25 °C; (ii) 3-chloropropanoyl chloride, Et3N, CH2Cl2 (dry), 25 °C; (iii) Et3N, CH2Cl2 (dry), 25 °C. |
It is well recognised that the stereochemistry of molecules is often crucial in deriving a SAR. The next step in borneol-based SAR analysis was using as the starting materials (−)-isoborneol 22 with different stereochemistries of the hydroxyl group. Compounds 7 and 16 with a morpholine fragment showed the highest antiviral activity, so we synthesised analogues 23 and 24 based on (−)-isoborneol (Scheme 2).
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Scheme 2 Reagents and conditions: (i) 2-chloroacetyl chloride, Et3N, CH2Cl2 (dry), 25 °C; (ii) 3-chloropropanoyl chloride, Et3N, CH2Cl2 (dry), 50 °C; (iii) Et3N, CH2Cl2 (dry), 25 °C. |
To establish the key structural requirements for antiviral activity, we synthesized derivatives 27 (ref. 25) and 28 (ref. 26) without the 1,7,7-trimethylbicyclo[2.2.1]heptan fragment (Fig. 2). As the nitrogen-containing nucleophile, we chose morpholine because borneol derivatives 7 and 16 with the morpholine fragment showed considerable antiviral activity.
The structures of the new, synthesized compounds were confirmed by means of the NMR spectra (1H, 13C NMR) and HR-MS and IR spectra. The 1H NMR spectra of compounds 13–21 and 26 revealed upfield signals of the methylene protons of N–CH2 at δ 2.6–2.8 ppm in contrast to the protons of the starting compound 3 at δ 3.74 ppm. Also, in the 1H NMR spectra of compounds 4–12 and 25 an upfield singlet at δ 3.1 ppm was observed in comparison with the singlet of compound 2 that was revealed at δ 4.03 ppm.
Compound | CC50, μM | IC50, μM | SI | log![]() |
---|---|---|---|---|
a log![]() |
||||
1 | >2142.9 | 155.8 ± 18.3 | >14 | 2.71 |
4 | 301.9 ± 21.4 | 34.0 ± 4.2 | 9 | 4.29 |
5 | 137.6 ± 9.9 | 15.1 ± 1.0 | 9 | 4.85 |
6 | 93.9 ± 7.3 | 10.2 ± 1.4 | 9 | 5.34 |
7 | 580.1 ± 41.1 | 7.1 ± 0.8 | 82 | 3.53 |
8 | 102.9 ± 8.2 | 7.1 ± 0.5 | 14 | 3.68 |
9 | 100.0 ± 7.3 | 4.4 ± 0.5 | 23 | 3.26 |
10 | 51.6 ± 4.1 | 3.2 ± 0.4 | 16 | 3.79 |
11 | >852.0 | 683.1 ± 62.8 | 1 | 4.11 |
12 | 69.3 ± 4.9 | >34.1 | >2 | 6.78 |
13 | 124.7 ± 10.1 | 14.3 ± 1.6 | 9 | 4.45 |
14 | 69.6 ± 5.2 | 7.8 ± 0.8 | 9 | 5.02 |
15 | 56.4 ± 3.8 | 15.3 ± 2.0 | 4 | 5.51 |
16 | 552.5 ± 31.1 | 12.2 ± 1.6 | 45 | 3.36 |
17 | 12.6 ± 1.0 | 6.8 ± 0.9 | 2 | 3.51 |
18 | 48.1 ± 3.3 | 1.9 ± 0.4 | 25 | 3.09 |
19 | 104.7 ± 8.7 | 8.4 ± 0.9 | 12 | 3.62 |
20 | 149.5 ± 11.7 | >98.4 | 2 | 3.94 |
21 | 41.8 ± 2.2 | 7.1 ± 0.9 | 6 | 6.94 |
22 | >974 | >974 | 1 | 2.71 |
25 | >533.8 | 73.7 ± 8.5 | 7 | 3.53 |
26 | 508.5 ± 36.1 | 7.8 ± 1.1 | 65 | 3.36 |
27 | >1886.8 | >1886.8 | 1 | 0.11 |
28 | >1734.1 | 982.7 ± 75.3 | >2 | 0.64 |
Rimantadine | 335.2 ± 26.8 | 67.0 ± 4.9 | 5 | |
Amantadine | 284.1 ± 21.4 | 64.2 ± 4.7 | 4 | |
Deitiforine | 1266.2 ± 81.5 | 208.6 ± 15.4 | 6 | |
Ribavirin | >2000.0 | 24.6 ± 3.1 | >81.0 | |
Oseltamivir | 160.3 ± 11.4 | 0.2 ± 0.03 | 781 |
Adamantane- and norbornane-based derivatives were used as reference compounds due to their close similarity to the compounds under investigation in having rigid cage fragments in their structures. To study the structure–activity relationship, we examined changes of the aliphatic linker and heterocyclic fragment in terms of their effect on toxicity and virus-inhibiting activity. In general, compounds with longer linkers (C2) were slightly more toxic compared to their C1 analogues. The piperazine derivatives 9 and 18 exhibited high antiviral activity with IC50 values of 4.4 ± 0.5 and 1.9 ± 0.4 respectively. Together, with potent activities, these compounds showed high toxicity which results in a relatively low therapeutic index. It was found that the morpholine substituent resulted in a strong reduction of toxicity compared to piperazine derivatives. This led to higher SIs, although the IC50 values of morpholine and piperazine derivatives did not differ (compounds 7 and 8, 16 and 17). Acylation of one of the nitrogen atoms of the piperazine moiety led to strongly decreased toxicity of the compound although its antiviral properties did not increase. The derivatives containing 1-methylpiperazine and 1-ethylpiperazine motifs showed moderate antiviral activity.
It was also demonstrated that in the case of a short linker, the stereochemistry of the compound does not affect the cytotoxicity of the compound but is of critical significance for anti-viral activity. Indeed, compounds 7 and 25 are based on two optical isomers of borneol, and their values of IC50 differ tenfold (7.1 and 73.7, correspondingly). This, however, was not important for two other isomers, 16 and 26, with a longer linker and which have similar values of both toxicity and activity and, therefore, similar SIs (45 and 65, respectively). This phenomenon should be further studied, in particular using computer simulation of ligand–target interaction.
Also, the structure–activity analysis of this new series of borneol derivatives revealed that the 1,7,7-trimethylbicyclo[2.2.1]heptan scaffold is required for the antiviral activity, as compounds 27 and 28 lacking this cage fragment did not possess virus-inhibiting activity. Among all the compounds tested, the highest activity was found in substances 7, 16 and 26, combining a morpholine fragment and 1,7,7-trimethylbicyclo[2.2.1]heptan (SI 82, 45, and 65 correspondingly).
Lipophilicity is a physicochemical property of principal importance in drug discovery and development.27 The quantitative descriptor of lipophilicity, the partition coefficient P, is defined as the ratio of the concentrations of a neutral compound in organic and aqueous phases under equilibrium conditions. To correlate the antiviral activity of the present series of compounds with their lipophilicity, logP values were calculated using ACDLabs/ChemSketch 12.01. All the new, examined compounds have the optimum lipophilicity range with a log
P value of 3.09–3.79.
Footnotes |
† The authors declare no competing interests. |
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c6md00657d |
This journal is © The Royal Society of Chemistry 2017 |