Therapeutic potential of aromatic plant extracts in Alzheimer's disease: Comprehensive review of their underlying mechanisms

Abstract Aims The aim of this review is to outline recent advancements in the application and mechanistic studies of aromatic plant extracts in Alzhermer`s disease (AD) to demonstrate their value in the management of this disease. Background AD is a neurodegenerative disease with a complex pathogenesis characterized by severe cognitive impairment. Currently, there are very few drugs available for the treatment of AD, and treatments are primarily focused on symptom relief. Aromatherapy is a traditional complementary alternative therapy that focuses on the prevention and treatment of the disease through the inhalation or transdermal administration of aromatic plant extracts. Over the past few years, studies on the use of aromatic plant extracts for the treatment of AD have been increasing and have demonstrated a definitive therapeutic effect. Methods We systematically summarized in vitro, in vivo, and clinical studies focusing on the potential use of aromatic plant extracts in the treatment of AD in PubMed, ScienceDirect, Google Scholar, and the Chinese National Knowledge Infrastructure from 2000 to 2022. Results Our literature survey indicates that aromatic plant extracts exert anti‐AD effects by modulating pathological changes through anti‐amyloid, anti‐tau phosphorylation, anti‐cholinesterase, anti‐inflammation, and anti‐oxidative stress mechanisms (Figure 1). Conclusion This review provides a future strategy for the research of novel anti‐AD drugs from aromatic plant extracts.


| INTRODUC TI ON
Alzheimer's disease (AD) is a neurodegenerative disease with a progressive and insidious onset characterized clinically by memory impairment, aphasia, dysfluency, impaired recognition and impaired visuospatial skills, performance dysfunction, character and behavior, along with other manifestations of general dementia. With global aging, AD incidence is increasing and is expected to impose heavy economic burdens on society. That the pathogenesis of AD remains unclear and also poses huge challenges in the study of this disease.
In terms of AD treatment, current drug research and development is based mainly on the neurotransmitter hypothesis and the amyloid cascade hypothesis. To date, the US. Food & Drug Administration has authorized six medicines for AD treatment: donepezil, memantine, rivastigmine, galantamine, tacrine, and aducanumab. The firstline therapeutic agent in the primary phase of clinical treatment of AD is often donepezil, which is an acetylcholinesterase (AChE) inhibitor. 1 During the first 3 months of dosing, patients showed improvement in cognitive symptoms. However, its effects gradually diminished as tolerance developed. N-methyl-D-aspartate receptor agonists such as memantine are often applied to treat moderate to severe AD, and although they improve AD symptoms, they do not stop or delay disease progression. 2 Since beta-amyloid (Aβ) has a significant part in the pathological mechanism of AD and is an early trigger for the development of AD, enhancing Aβ clearance through Aβ-targeted immunotherapy to reduce Aβ deposition to the brain and therefore, improve cognitive performance is a theoretically feasible strategy. Aducanumab, the world's first Aβ monoclonal antibody being used for the prevention of AD, can reduce Aβ deposition, delaying disease clinical progression in a drug-dependent and time-dependent way. 3,4 Over the past 5 years, non-Aβ target drugs have also gained increasing attention, including targets for tau proteins, inflammation, synapses, and neuronal protection. 5,6 However, it is controversial whether the these therapeutic agents can slow down clinical cognitive deterioration in people with AD.
The pursuit of innovative and more potent drugs for the prevention of AD has been a hot topic in the field of drug development. The search for potential therapeutic drugs from traditional medical use experience has become a proven avenue for drug discovery. Essential oils are active components of aromatic plants and are abundant in leaves, seeds, flowers, bark, and roots. 7 These complex, volatile, naturally occurring mixtures of compounds are secondary metabolites of the plant. The main chemical constituents of these aromatic products include monoterpene hydrocarbons, sesquiterpenes, oxygenated sesquiterpenes, oxygenated monoterpenes, and esters, etc. Essential oils are absorbed by the body in three main ways, through the respiratory system, the skin, and orally. 8 Regardless of the administration route, aromatic compounds can easily enter the central nervous system (CNS) by inhalation or crossing the blood-brain barrier (BBB). It had already been indicated that aromatic components have a certain opening effect on the BBB in in vivo mice, and that the lipid position of aromatic constituents may be the material basis of their action on brain tissue. 9 Therefore, essential oils have obvious advantages in the treatment of neurological disorders. Aromatherapy through the application of essential oils may constitute a possible treatment option for AD ( Figure 1).
To evaluate the status of aromatic plant extract application during AD progression, we conducted PubMed, ScienceDirect, Google Scholar, and China national knowledge infrastructure searches of studies published between 2000 and 2022, with key phrases that included Alzheimer's disease, aromatic plant, aromatic compounds, essential oils, and neuroprotection. In this review, recent advances in clinical applications and mechanistic studies of aromatic plant extracts in AD will be outlined to illustrate the value of aromatic plant applications in this field.

| B ENEFI CIAL EFFEC TS OF AROMATI C PL ANT E X TR AC TS IN CLINI C AL AD TRE ATMENT
Patients with AD experience age-related cognitive decline, and commonly suffer from disturbed sleep patterns, anxiety, and agitation. These symptoms usually worsen with age and the severity of dementia. Current clinical research supports the applicability of aromatherapy in cognitive dysfunction and agitation.
In a study of 28 older adults, 17 of whom had AD, aromatherapy use consisted of morning rose and lemon essential oil (LEO) and evening lavender and orange for 28 days. After treatment, all patients showed significant enhancement in self-reported cognitive presentation on the Japanese versions of both the Gott fries, Brane, Steen Scale, and the Type Dementia Assessment Scale. 10 Lavender essential oil, popular due to its ease of application and minimal side effects, significantly reduces non-aggressive physical behaviors in residential patients with dementia. 11 A randomized clinical trial of 3 months of Highlights • Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disease for which only a few drugs are currently available as symptomatic treatment.
• Aromatic plant extracts are indeed effective in improving cognitive function, reducing restlessness, and improving sleep quality in AD patients.
• The pharmacological mechanisms of aromatic plant extracts to improve AD are suggested to be the reduction of Aβ and hyper-phosphorylation of tau protein, anticholinesterase, inhibition of the release of inflammatory cytokines, and reduction of damages of oxidative stress.
• Aromatic plant extracts are considered to have neuroprotective effects and have potential medicinal value for the treatment of AD. therapeutic massage using 2% lavender essential oil on selected acupuncture points in 60 AD patients was found to reduce the severity and distress of behavioral and psychological dementia symptoms. 12 In another study assessing the effects of olfactory neurostimulation showed that cedar scent significantly improved Neuropsychiatric Inventory (NPI) and Zarit Caregiver Burden Interview (J-ZBI) scores in patients with AD without olfactory dysfunction at 4 and 8 weeks. 13 In a double-blind placebo-controlled research study in which AD patients with severe dementia, melissa essential oil therapy, or placebo was placed on the patients' faces and arms twice daily for 4 weeks. Results showed a 30% reduction in patients' Cohen-Mansfield Agitation Scale (CMAI) scores and an overall improvement in agitation and quality of life parameters. 14 Table 1 shows the list of beneficial effects of aromatic plant extracts in clinical AD treatment.
The studies cited above provide evidence for clinical use of aromatic plant extracts. Currently, they are mainly applied in F I G U R E 1 Potential mechanisms of aromatic plant extracts for the treatment of Alzheimer's disease. ↓-reducing, Aβ, amyloid β protein; Ach, acetylcholine; AChE, acetylcholinesterase; APP, amyloid precursor protein; IL-1β, interleukin-1β; IL-6, interleukin-6; ROS, reactive oxygen species; TNFα, tumor necrosis factor α.

| Counteracting effects of aromatic plant extracts on Aβ toxicity and tau phosphorylation pathologies
It is well-known that the characteristic pathology of AD is mainly the deposition of Aβ along with increased neuronal fiber tangles in the brain, so reducing Aβ deposition and inhibiting hyper-phosphorylation of tau protein are essential aspects of anti-Alzheimer's drug mechanism research. Animal experiments have shown that oral administration or inhalation of aromatic essential oils can effectively improve cognitive function in dementia animals by reducing Aβ deposition. A combination of lemon and rosemary oils at night and a combination of lavender and orange oils during the day for 2 months of inhalation could improve cognitive performance in senescence-accelerated mouse prone 8 by reducing brain activity of Aβ and abnormally phosphorylated tau in the mouse brain. 15 In another animal study, the authors reported that combined oral administration of total ginsenosides and essential oil of Acorus tatarinowii significantly increased learning memory and decreased Aβ1-42 levels and reduced age spot formation in a D-galactose-and aluminum chloride-induced AD mouse model. 16 SuHeXiang Wan (SHXW) is a conventional Chinese medicinal preparation usually used for the therapy of CNS disease. The formula consists mainly of aromatic herbs. Research has shown that pre-inhalation of SHXW essential oil modified memory impairment caused by Aβ1-42 and JNK, p38, and tau phosphorylation in the mouse hippocampus. 17 In addition, SHXW essential oil has lipophilic fraction reacting with the lipid parts of cell membranes. It is more frequently used in topical applications rather than internal use. It suggests that SHXW essential oil may have future as an inhaled medicine for the prevention and treatment of AD.
In recent years, many constituents of aromatic plants have been tested for their potential to reduce symptoms of AD or for affecting the disease mechanism. Thymol(2-isopropyl-5-methylphenol) and carvacrol ( Alzheimer's disease rat model. 22 Lim confirmed that post-treatment of curcumin on amyloid pathology in APPSw mice, plaque size was reduced by an average of 14%. 23 Furthermore, Bisceglia demonstrated curcumin could inhibit the formation of toxic Aβ oligomers and limit the formation of insoluble fibrils. 24 Moreover, curcumin has a variety of other biological activities related to the treatment of AD, such as anti-inflammatory and antioxidant effects that will be described later. Based on multi-functional properties of curcumin, therefore, could be a therapeutic option for the treatment of AD, provided limitations in its oral bioavailability can be overcome.
Accumulating evidence suggests that aromatic plant extracts can exert therapeutic effects by reducing plaque size, promoting Aβ clearance, and decreasing phosphorylated tau levels in both transgenic and chemically induced animal models of AD. However, further investigations are necessary to confirm its biological targets as well as potential toxicity before any recommendations can be made for its use as an anti-dementia treatment.

| Counteracting effect of aromatic plant extracts on pathologic AChE activity
Acetylcholine (ACh) has an irreplaceable role in the CNS. It was found that cholinergic neurons, located in the basal forebrain, are severely lost in the brains of AD patients, leading to deficits in memory and attention. ACh present in the synaptic cleft is hydrolyzed by AChE, and this pathological change is therefore thought to be associated with AChE activity. Therefore, inhibition of AChE activity has become a part of the therapeutic interventions for the prevention of AD. Competitive inhibitors of AChE are also the most widely used drugs for the prevention of AD. 25 Several studies have been reported on the cholinesterase inhibitory potential of aromatic plant extracts. In vitro AChE activity was often measured following Ellman's assay. Essential oils (EOs) from Salvia leriifolia Benth. exhibited high BChE inhibitory. 26 Volatile oil from Marlierea racemosa Vell. (Myrtaceae) demonstrated concentration-dependent inhibition of AChE, 27 EOs from the leaves and flowers of Polygonum hydropiper L., 28 sandalwood oil and its chief constituent α-santalol were reported the AChE, BChE inhibitory efficacy. 29 Apart from that, in vivo anti-cholinesterase activities of aromatic plants and their extracts were also determined. The extract of Rosmarinus officinalis L. leaf led to improved long-term memory in scopolamine-induced rats, which can be partially explained by its inhibition of AChE activity in rat brain. 30 In a rat model of acute Aβ  toxicity, Pinus halepensis essential oil reversed the performance of spontaneous alternation rate on Y-maze test, increased spatial memory and restoration of false memories in the radial arm maze test, and reduced AChE activity in hippocampus. 31 It was observed in APP/PS1 mice that 4 weeks of Lemon essential oil treatment could significantly decrease hippocampal AChE, and thus increased ACh levels. 32 As mentioned above, the inhibition of AChE to increase the concentration of neurotransmitter is highly recommended. Some aromatic volatile oils and their major components showed significant cholinesterase inhibitory effects and may improve cognitive performance in AD model animals through this mechanism. Most studies have found that terpenoids in aromatic plant extracts are the main anticholinesterase active components, but their structural diversity complicates the study of structure-activity relationships.

| Counteracting effects of aromatic plant extracts on inflammatory pathology
Increasing evidence suggests that damaged neurons and deposits of insoluble Aβ peptides and neurofibrillary tangles are pathological triggers for neuroinflammation in AD. When these substances bind to the pattern recognition receptors of microglia and astrocytes, they induce transcriptional activation of downstream inflammatory response genes, thereby releasing inflammatory cytokines, the overproduction of which eventually leads to chronic neuroinflammation, thus exacerbating the pathogenesis of AD. 33 On the other hand, during peripheral inflammation, cytokines cross the BBB and cause brain microglia and astrocytes to react, further producing pro-inflammatory mediators. 34 The production of inflammatory cytokines and mediators by microglia and astrocytes directly promotes the occurrence and development of AD. 35 Microglia and astrocytes play an important role of inflammatory responses in the CNS. 36  COX-2 is also involved in neurodegenerative diseases. 42 The non-steroidal anti-inflammatory (NSAIDs) may reduce microglial activation. 43 Hu Y et al.reported trans-caryophyllene decreased the Aβ 1-42 activated overexpression of COX-2 and prostaglandin E 2 (PGE 2 ) generation. 41 In another study, linalool-treated 3xTg-AD mice exhibited a significant reduction in COX-2 in hippocampi and amygdalae from 3xTg-AD mice. 38 The involvement of the inflammatory response in the pathological process of AD has been repeatedly demonstrated. However, clinical trials with anti-inflammatory drugs have not been successful. 44 This suggests that the complex neuroinflammatory pathological process cannot be repaired by intervening at a particular target.

| Counteracting effects of aromatic plant extracts on oxidant pathology
The CNS is highly vulnerable to oxidative stress, due to its high metabolic activity, elevated oxygen requirement, and the presence of high levels of redox-active metals and oxidizable lipids. Several compounds like safranal, linalool, and SHXW essential oil have been found to decrease ROS levels induced by Aβ in rats or mouse. Safranal attenuated MDA levels in the hippocampus and increased SOD activity. 39 Linalool administration attenuated the neurotoxicity of Aβ by reducing nitric oxide (NO) levels. 47 Furthermore, the restoration of GSH-Px, SOD, and CAT by curcumin, 48 clove oil, 49 Acorus Gramineus essential oil, 50 lavender essential oils, 16 and Juniper volatile oil 51 signifies an increase in endogenous antioxidant defenses, which in turn prevents protein oxidation.
Curcumin pre-treatment significantly has been found to modify the alterations in RNS levels in quinolinic acid-induced behavioral and neurological changes in rats. 48 Clove oil is a mind stimulant used in the treatment of anxiety, insomnia, and depression, 49  was also comfirmed by using SOD, GSH-Px, and CAT activities, the total content of the reduced glutathione and protein carbonyl levels. 51 Of concern is that the safety of juniper volatile oil need to be properly addressed. It has been reported a little toxicity of the juniper volatile oil extracted from multiple juniper species in animals.
The antioxidant effect of aromatic plant extracts due to direct scavenging of free radicals and increasing antioxidant capacity. This evidence is particularly strong in the aromatic components of polyphenols and terpenes. Polyphenols and essential oil terpenes protect cellular components from oxidative damage, thereby decrease the danger of diseases associated with oxidative stress. Table 2 shows the list of beneficial effects of aromatic plant extracts in preclinical AD.

| CON CLUS I ON S AND FUTURE DIREC TIONS
Aromatherapy is a traditional complementary alternative therapy that focuses on the prevention and treatment of disease through the inhalation or transdermal administration of aromatic plant extracts. Clinical studies have also shown that aromatic plant extracts can improve cognitive function, reduce agitation, and improve sleep quality in AD patients. These volatile substance-based components are thought to enter the bloodstream mainly through the respiratory mucosa, then through the BBB to the brain, and even diffuse directly to the olfactory nerve to reach the brain and exerting neuroprotective effects through multiple targets and pathways.

Treatment
Behavior Test

Major Finding
Ref.   Furthermore, the ability of the hippocampus to retrieve memories is impaired in AD patients, many older people with AD are hypersensitive to sounds, smells, or temperature in their environment, and unfamiliar strong odors may overstimulate AD patients. However, earlier life memories rely less on the hippocampus for recollection, so it is highly likely that some older people with AD would recognize and may be comforted by their familiar fragrance, then have more effective and better tolerated. In addition, a correlation between olfactory function and brain regions involved in cognition and memory has been reported. Pathologically, amyloid beta protein accumulates in the olfactory nerve at the onset of Alzheimer's disease, and thus olfactory impairment is often observed before cognitive decline occurs in AD patients. 45 There is progressive systemic damage to the olfactory nerve, causing olfactory impairment, which further leads to taste disturbance and loss of appetite, affecting the patient's quality of life. Appropriate olfactory stimuli can be transmitted through the olfactory nerve and be transmitted to the limbic system, including the hypothalamus. 54 Possibly, the essential oils contain chemical constituents with bioactive properties that facilitate the recovery of the olfactory sense by suppressing inflammation and enhancing regeneration. 55 Since the limbic system is closely related to cognition, olfactory stimulation may improve cognitive impairment.

AUTH O R CO NTR I B UTI O N S
MY and YHX contributed to the conception, design, and prepara-

CO N FLI C T O F I NTER E S T S TATEM ENT
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data openly available in a public repository that issues datasets with DOIs.