Monoterpenes as a renewable source of biologically active compounds

Analgesic (antinociceptive) and anti-inflammatory activities

Pain is the most common symptom for which patients seek medical attention [5]. Pain management continues to be a major challenge for medicine. Opioids, anticonvulsants, and non-steroidal anti-inflammatory drugs are the main agents used to relieve pain [6]. Inflammation is a common cause of pain. Inflammation is defined as a complex biological response of vascular tissues against aggressive agents [7]. The nonsteroidal anti-inflammatory drugs are most widely used to treat inflammatory diseases and as analgesics, but the gastrointestinal, renal and cardiovascular side effects limit their application. Monoterpenes are a valuable source of novel compounds that have the potential to serve as analgesic and anti-inflammatory agents or as lead molecules for the development of such. Indeed, many essential oils, which consist mainly of monoterpenoids, demonstrated analgesic and anti-inflammatory activities [8], [9], [10].

A number of natural monoterpenes, in particular, (+)- and (−)-limonenes [11], [12], γ-terpinene [13], α-phellandrene [14], (+)-α-pinene [15] and (−)-β-pinene [16] were found to exhibit analgesic activity. Myrcene [17] and (+)-α-pinene [18] demonstrated significant anti-inflammatory activity while (−)-α-pinene was less active and (−)-β-pinene was inactive [18] (Fig. 1). para-Cymene is a naturally occurring aromatic compound and is considered to be a dehydrogenated monoterpene. This compound exhibited an antinociceptive effect and demonstrated anti-inflammatory properties [19], [20]. It was found that the complex of para-cymene with β-cyclodextrin led to prolonged analgesic and anti-inflammatory effects (8 h instead of 2 h for individual para-cymene) [21].

Fig. 1:

Structures of monoterpenes.

Oxygen-containing monoterpenoids occur widely in nature and are used as flavors and fragrances. Many of them demonstrated high analgesic activity. Thus, the analgesic and anti-inflammatory properties were demonstrated for (−)-linalool (Fig. 2) in a number of animal models [22], [23]. The (−)-linalool-β-cyclodextrin complex potentiates the analgesic effect [24] and increases its duration [25]. Significant antinociceptive effect was found for others acyclic monoterpenoids, including citronellol [26] and its acetate [27], geraniol [28], citral [29], citronellal [30] and its complex with β-cyclodextrin [31].

Fig. 2:

Structures of monoterpenoids and compound 2.

(−)-Menthol (Fig. 2) is the best-known oxygen-containing monoterpenoid extracted from the essential oil of the genus Mentha of the Lamiaceae family. (−)-Menthol activates the cold- and menthol-sensitive TRPM8 channels, leading to modulation of peripheral nociception [32]. Other monoterpenoids with para-menthane framework also demonstrating analgesic activity: α-terpineol [33], 1,8-cineole [34] and ketoalcohol (−)-1 [35], as well as aromatic compounds carvacrol [36], [37] and thymol [38], should also be mentioned.

Bicyclic monoterpenoid (−)-myrtenol (Fig. 2) was found to demonstrate the anti-inflammatory and antinociceptive effects [39], although it proved to be ineffective in the study of inhibition of the IL-1β-induced inflammatory and catabolic pathways [18]. Another bicyclic monoterpene alcohol, (−)-borneol, reduced the nociceptive behavior and inflammatory response in mice, including chronic inflammatory and neuropathic pain [40], [41], [42]. Promising analgesic activity was found for α-truxillic acid derivative 2 containing two (−)-borneol fragments [43].

Many chiral heterocyclic compounds belonging to various structural types have been synthesized via the interaction of monoterpenoids verbenol epoxide and diol 3 with aromatic aldehydes (Fig. 3) and tested to detect analgesic activity [44], [45], [46], [47]. These transformations were catalyzed by montmorillonite clays [48], [49], [50], Fe-modified beta zeolite [51], or Ce-MCM-41 [52]. Compounds 4–6 were found to be the most promising ones. It was shown for compounds 5a,b, which are characterized by high activity and low acute toxicity, that neither the absolute configuration nor cis- or trans-arrangement of vicinal oxygen atoms plays a significant role in manifestation of analgesic effect by their stereoisomers [53]. Meanwhile, changes in mutual arrangement of the hydroxyl and methoxy groups in the aromatic ring as one proceeds from 5a to 5b has a crucial effect on their mechanism of action: compound 5a employs the cannabinoid and adrenergic systems, whereas compound 5b targets the opioid and dopaminergic systems [54]. Compound 7, produced by single-stage synthesis from monoterpenoid (−)-isopulegol (Fig. 3), exhibits high activity (1 mg/kg when administered orally), low acute toxicity (LD₅₀>4500 mg/kg) and prolonged effect (for at least 24 h) [55] and shows great promise for further research.

Fig. 3:

Synthesis of heterocyclic compounds based on monoterpenoids.

Compound 8 synthesized via the interaction between monoterpenoid (−)-myrtenal and dimethylbispidinone derived from methenamine [56] had a significant analgesic activity. Unlike nonsteroidal anti-inflammatory drugs, compound 8 does not cause damage to the gastric mucosa.

Thus, a number of monoterpenes, their oxygen-containing derivatives, and heterocyclic products based on them show a significant analgesic activity. Some of these compounds are promising to be used to design new efficient low-toxicity analgesic drugs.

Anticonvulsant activity

Anticonvulsant drugs are primarily developed for the treatment of epilepsy, a neurological condition that affects more than 50 million people worldwide [57]. A series of compound exhibiting a pronounced anticonvulsant effect in vivo was found among monoterpenoids. For example, small anticonvulsant activity was shown for geraniol [58] and linalool oxide [59] (Fig. 4); for geraniol, it could be increased by complexation with β-cyclodextrin. Carvone and its derivatives are the best-studied monoterpenoid-based anticonvulsant agents. Thus, (+)-carvone at a dose of 200 mg/kg, i.p., significantly increased the latency of pentylenetetrazol (PTZ)- and picrotoxin-induced convulsions, close to that of diazepam [60]. (−)-Carvone enantiomer showed no anticonvulsant activity. α,β-Epoxy-carvones exhibited activity for the PTZ model when taken at a greater dose (300 or 400 mg/kg) [61]; their relative and absolute configurations had no fundamental effect on anticonvulsant activity [62]. The use of cyano-carvone (Fig. 4) allowed one to reduce the effective dose to 75 mg/kg [63].

Fig. 4:

Structures of monoterpenoids and compounds 9 and 10.

Monoterpene alcohols, (−)-isopulegol (Fig. 3) and α-terpineol (Fig. 2), were active for the model of PTZ-induced convulsions at a dose of 200 mg/kg [64], [65]. Meanwhile, diol 3 (Fig. 3) exhibited a high anticonvulsant effect for the PTZ model when administered at a dose of 10 mg/kg [66] and, therefore, its effective dose is comparable to that of the recently used anticonvulsants. Significant anticonvulsant activity was detected in aromatic monoterpenoid derivatives thymol, carvacrol (Fig. 2) and its acetate at doses varying from 35 to 100 mg/kg depending on a test used [67], [68]. Borneol (Fig. 2) [69], imine 9 (Fig. 4) [70] synthesized from camphor, and amino ester 10 [71] synthesized from α-pinene [72] are the bicyclic monoterpenoids characterized by high anticonvulsant activity (effective doses 10–30 mg/kg).

Hence, although native monoterpenoids typically exhibit moderate anticonvulsant activity, their modification allows one to increase their activity manifold in some cases.

Application of monoterpenoids for treatment of neurodegenerative disorders

Neurodegenerative disorders such as Alzheimer disease (AD) and Parkinson’s disease (PD) are a heterogeneous group of disorders characterized by progressive loss of neurons. There is no cure for these diseases, and the available medicines can only alleviate some of the symptoms.

AD is one of the best-known neurodegenerative diseases and is responsible for 50–60% of patients with dementia. The cholinergic hypothesis assumes that the level of acetylcholine in AD is reduced due to the loss of the cholinergic neurons and decreased synthesis of acetylcholine [73]. Inhibition of acetylcholinesterase (AChE) that is responsible for acetylcholine utilization makes it possible to increase the level of this neuromediator. Investigation of the AChE inhibition property of bicyclic monoterpenoids demonstrated that monoterpenes (+)- and (−)-α-pinenes (Fig. 1) and (+)-3-carene (Fig. 4) are poor AChE inhibitors with IC₅₀=0.2–0.4 mM [74]. Their oxygen-containing derivatives [74] and para-menthane monoterpenoids [75] turned out to be even less active. At the same time, it was found that oral administration of racemic linalool (Fig. 2) in aged mice with a triple transgenic model of AD reversed the histopathological hallmarks of AD and restored cognitive and emotional functions supposedly due to the anti-inflammatory effect of linalool [76].

PD is the second most common neurodegenerative disorder after AD with its prevalence estimated to be 0.3% in general population [77]. PD is caused by a progressive loss of dopaminergic neurons and terminals from the nigrostriatal pathway and, as AD, is age-related. PD is characterized mostly by motor symptoms including resting tremor, muscular rigidity, bradykinesia, etc.

A number of monoterpenoid alcohols demonstrated promising anti-PD activity in in vivo models based on administration of neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, Fig. 5). MPTP produces reproducible lesions of the nigrostriatal dopaminergic pathway and is the only known dopaminergic neurotoxin that is capable of producing a clinical picture in humans indistinguishable from PD [78]. Geraniol (Fig. 4) at a 100 mg/kg dose reduced PD symptoms both in the acute [79] and chronic [80] models. Moreover, the neuroprotective effect was shown for geraniol using the acrylamide model of neurotoxicity in drosophila [81] and rats [82].

Fig. 5:

Structures of monoterpenoid diols and MPTP.

Monoterpenoid 3 (Fig. 5) at a dose of 20 mg/kg demonstrated potent anti-PD activity in MPTP-treated rodents with different experiment duration and designs [83], [84]. All eight stereoisomers of diol 3 were synthesized and studied using the MPTP mouse model [83]. The absolute configuration of this monoterpenoid was found to greatly influence its anti-PD activity. The most active isomer 3 was identified; it can be synthesized by isomerization of verbenol epoxide (Fig. 3) in the presence of montmorillonite clays [85], [86] or zeolites [87], [88]. This monoterpenoid sufficiently improved both the locomotor and exploratory activities of MPTP-treated animals and demonstrated low acute toxicity (LD₅₀ 4250 mg/kg, mice) [84]. Compound 3 was as effective as the comparator agent Levodopa [83], the “gold standard” in anti-PD therapy.

The investigation of anti-PD activity of compounds 11–14 (Fig. 5), each of them lacking one of the functional groups present in monoterpenoid 3, demonstrated that the positive effect on locomotor and exploratory activity is achieved only if all these groups are present [89]. Meanwhile, it was found that the presence of allylic hydroxy group (compound 13) or endocyclic double bond (compound 14) is not necessary for restoration the exploratory activity of animals. A set of new derivatives of compound 3 was obtained throughout bromide 15 [90] (Fig. 5). Compounds with oxygen- (16) or nitrogen (17)-containing substituents appeared to be inactive. However, compounds with butyl (18) or thioaliphatic (19) substituents effectively restored the locomotor and exploratory activities of MPTP-treated mice.

Compound 20 was synthesized via the interaction of (+)-2-carene with vanillin in the presence of clay [91] (Fig. 6). It was found that this compound at a 30 mg/kg dose reduced PD symptoms in the neuroprotective model of PD caused by single administration of neurotoxin MPTP [92].

Fig. 6:

Synthesis of compound 20.

Hence, oxygen-containing monoterpenoids show great promise as potential lead compounds for treating neurodegenerative diseases, mainly Parkinson’s disease.

Antidepressant and anxiolytic-like effects

Depressive disorder is a common mental disorder associated with a significant negative impact on quality of life and cognitive function. Moreover, depressive disorder is associated with increased risk of disability, suicide and heart failure [93]. Meanwhile, there are several problems connected with decreased efficacy of antidepressant drugs [94]. Application of different medicinal plant species to treat depression was shown to be effective [95]. Thus, antidepressant activity was demonstrated for Litsea glaucescens essential oil [96]. It was shown [96] that monoterpene β-pinene (Fig. 1) and monoterpenoid linalool (Fig. 2), which were tested at dose of 100 mg/kg, are responsible for this effect of the oil.

Significant antidepressant activity in in vivo experiments was also revealed in some other monoterpenoids. Thymol (Fig. 2) (15–30 mg/kg) demonstrated the antidepressant-like effect on a chronic unpredictable mild stress model of depression in mice [97]. α-Phenylselenocitronellal at a dose of 100 mg/kg (Fig. 7), but not citronellal (Fig. 2), was shown to possess antidepressant-like activity in the forced swimming test, classical in vivo test for depression [98].

Fig. 7:

Monoterpenoid-based compounds with antidepressant and anxiolytic-like effects.

The most promising monoterpene derivative for treating depression is (S)-mecamylamine (Fig. 7), which is an antagonist of nicotinic acetylcholine receptors and was previously used as an antihypertensive drug. The compound showed positive effects in a number of animal models of depression. Based on the promising results of phase II clinical trials, (S)-mecamylamine was tested in phase III studies as an adjunct therapy with Citalopram [99]. Unfortunately, (S)-mecamylamine proved to have an effect not significantly higher than that of placebo.

Many monoterpenoids demonstrated some anxiolytic-like effect in rodents. A systematic review of the anxiolytic-like effect of essential oils and their components, mainly monoterpenoids, has been published recently [100]. In addition to these data, compound 21 synthesized by interaction of (−)-myrtenal and 2-aminoadamatane with subsequent reduction (Fig. 7) may be mentioned. Anxiolytic activity in mice in the elevated plus maze test was observed for this compound at a dose of 1 mg/kg [101]. The effect on mice was found to depend on the absolute configuration of myrtenal used for the synthesis and sex of the animals [102].

Hence, although essential oils are commonly used to correct depressive and anxiety disorders, monoterpenoids, which are the key components of these oils, have not found an application yet, probably due to their insufficient activity and effectiveness. Meanwhile, it was demonstrated that their activity can be significantly enhanced by chemical modification of monoterpenoids.

Antiviral activity

Essential oils and their components are used in treatment of upper respiratory tract diseases as expectorants and antiviral agents [103], [104], [105]. Many studies have been devoted to investigation of the antiviral activity of total plant extracts, where monoterpenoids are the major components [106], [107]. The effect of potentiation of the antiviral activity of Oseltamivir under synergistic action when used together with Melissa officinalis extract was reported [108]. These studies are important for understanding the activity of essential oils and extractants but do not provide a clear idea of what natural compounds exhibit the target activity.

In general, the data on antiviral activity of native monoterpenoids are rather scarce. Thus, the antiviral activity against herpes simplex virus of isoborneol (the borneol isomer with respect to position of hydroxyl group) (Fig. 2) [109], cineole and eugenol [110] was studied. It was demonstrated that (+)-isoborneol potently inactivated HSV-1. The antiviral activity of (+)-isoborneol involved the interaction of monoterpene hydroxyl groups with a viral capsid and the inhibition of glycosylation of a viral protein, which resulted in loss of viral infectivity. Citral (Fig. 2), an acyclic monoterpene, has been examined for its antiviral activity against herpes simplex virus type 1 (HSV-1) in vitro [111]. An interesting study on the anti-IBV (infectious bronchitis virus) activities of (−)-α-pinene and (−)-β-pinene using the MTT assay and docking and molecular dynamics simulations was also conducted [112]. These monoterpenes inhibited IBV only when used at a high concentration (~1 mM).

Chemical modification of the natural framework with additional pharmacophore groups can significantly increase the antiviral activity. Hence, citronellal-substituted adamantanes 22 and 23 (Fig. 8) were shown to exhibit a more pronounced activity against influenza virus than unsubstituted aminoadamantanes [113]. Oxygen-containing monoterpenoid with a para-menthane framework (−)-3 (Fig. 2) showed a significant activity against the influenza virus A/California/07/09 (H1N1), and its mononicotinate (+)-24 had a selectivity index (SI) as high as 17. The absolute configuration of compound (+)-24 proved to be critical for its antiviral activity because using its optical antipode led to a loss of the antiviral activity [114]. The antiviral activity of 4-hydroxy-hexahydro-2H-chromenes and 4-fluorine-hexahydro-2H-chromenes with an aromatic substituent, synthesized from monoterpene (−)-verbenone, was studied [115]. Compound 25 seems to be a promising one combining high antiviral potency (IC₅₀=5 mkM) and low cytotoxicity and the SI value of 55.

Fig. 8:

Monoterpenoid-based compounds with antiviral activity.

Compound known as pinanamine [(1R,2R,3R,5S)-3-pinanamine] was found to be more active than amantadine with respect to influenza A M2 channel inhibition [116]. It was found later [117] that attachment of the 4-hydroxyphenyl group to the amine yielded a highly potent M2 inhibitor pinanamine-based imine derivative 26. Linking a secondary amine to the methylimidazole group on the pinanamine scaffold (compounds 27, 28 Fig. 9) further increased the inhibition of M2 channel activity [118], [119].

Fig. 9:

Monoterpenoid-based compounds with antiviral activity.

The derivatives of natural bicyclic monoterpenoids carrying the bicycle[2.2.1] moiety in their backbone are promising starting molecules for synthesizing antiviral compounds. Thus, the substituted 1-norbornylamines 29–31 (Fig. 9) synthesized from (±)-camphor exhibited high activity against influenza virus A and moderate activity against the African swine fever virus [120]. It was demonstrated that methyl-substituted camphor ethylamine (+)-32 represents a new series of anti-influenza virus drugs and could be developed as backup scaffolds for a new generation of M2 inhibitors [121]. A novel class of anti-HIV agents are coumarins substituted with camphanic acid (33 and 34) [122]. It was found in the study aimed at revealing the key pharmacophore fragments that are responsible for manifestation of antiviral activity of these compounds that the orientations of the 3′- and 4′-camphanoyl groups are critical to maintain high anti-HIV activity against both wild-type and drug-resistant HIV-1 strains [123].

Imines based on natural (+)-camphor proved to be a promising source of antiviral agents. Hence, series of aliphatic imines 35, 36 (Fig. 10) [124], iminoalcohols 37 and compounds containing aromatic 39 and heteroaromatic fragments [125] were synthesized and tested for activity against influenza A viruses. Dimeric imines 40 [126] and dimeric bis-quaternized compounds 41 [127] were studied. It was demonstrated in all the cases that reduction of the imine group increases toxicity of the compounds and, therefore, reduces the selectivity index. The selectivity index of compounds 35–41 (Fig. 9) varies from 90 to 500. Camphor derivative 1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene-aminoethanol (camphecene) was shown to exhibit a high level and a broad range [A/Cal (H1N1)pdm09, A/PR (H1N1), A/Aichi (H3N2), A/mallard (H5N2), B/Lee] of inhibiting activity against influenza viruses, which is based on inhibition of viral hemagglutinin and is most prominent at early stages of virus replication [128]. The compound is currently undergoing preclinical trials [129], [130], which prove the high potential of searching for novel antiviral agents among monoterpenoid derivatives, especially those having the framework structure. The derivatives of (−)-borneol and (−)-isoborneol have recently been synthesized as potent inhibitors of the influenza A virus [131]. Compounds 42, 43 and 44 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.

Fig. 10:

Antiviral camphor derivatives.

Antimycobacterial activity

The antimicrobial activity of monoterpenoids and their derivatives has been investigated fairly well: a number of reviews focused on the activity of native natural compounds [132], [133], [134], the antibacterial activity of their synthetic derivatives are also of keen interest among researchers [135], [136], [137], [138]. This review will discuss only monoterpenoid derivatives exhibiting pronounced anti-tuberculosis activity. Mycobacterium tuberculosis (MTB) latently infects one-third of the world’s population causing approximately 9 million cases of active disease each year [139]. Among the monoterpenoid-based compounds, SQ-109 agent (Fig. 11), a derivative of adamantine and geranylamine and an ethambutol analogue [140], has the greatest surface area. This agent has a unique multicomponent mechanism of action, which consists in suppression of the transport enzyme mMpl3 that partakes in synthesis of disaccharide trehalose of the cell wall of mycobacteria [141] and is currently undergoing phase II of clinical trials [142]. In-depth studies of the relationship between the structure of SQ-109 analogues and the anti-tuberculosis activity revealed novel agents 45 and 46 whose effective concentration turned out to be even lower. A series of studies focused on the synthesis of anti-tuberculosis agents containing the bicyclo[2.2.1] framework fragment. Hence, 3-exo-aminoisoborneol derivatives 47 synthesized from (+)-camphor [143]; the product of interaction between bornylamine and α-hydroxy acids, compound 48 [144]; and compound 49 based on (−)-fenchone [145] were revealed as the lead compounds.

Fig. 11:

Monoterpenoid-based compounds with antimycobacterial activity.

A conclusion can be drawn that the use of monoterpenoids as starting molecules to synthesize the compounds that would exhibit high anti-tuberculosis activity is one of the most promising directions in modern medicinal chemistry.