Saponins are a diverse category of natural compounds that contain one or more sugar chains and a triterpene or steroid aglycone in the basic parent nucleus. This particular category of natural components is a broad-scale distribution in Chinese herbs. Saponins are the predominant active ingredient of KXS, and increasing evidence has indicated that they exert therapeutic properties against memory and cognitive impairment, depression, and neurodegenerative diseases (Lu et al., 2022; Qiu et al., 2022). To date, a total of 65 saponins have been reported in KXS, which mostly consist of polygala saponins and ginsenosides. Structurally, ginsenosides are further classified into four major categories based on the parent ring structure, including the protopanaxadiol type (33–37, 44–46, 50–52, 54–57), the protopanaxatriol type (38–40, 42, 43, 47–49, 53), the oleanane group (41) and pseudoginsenosides of the ocotillol type (63, 64).

The name xanthones designates a class of secondary metabolites normally found in a few higher plant families. The polygalaceae family is an important source of natural xanthones which have the basic mother nucleus of 9H-xanthen-9-one or xanthone. Currently, 5 xanthones from KXS have been identified, including sibiricaxanthone B (66), sibiricaxanthone F (67), 7-O-methylmangiferin (68), polygalaxanthone III (69), and polygalaxanthone XXVIII (70).

Oligosaccharide esters are a sizable class of natural metabolites with sucrose and trisaccharide esters as a character in their chemical structures and have a variety of physiological activities. At present, a total of 24 oligosaccharide esters have been reported in KXS.

There are currently 7 reported triterpenoids that have been identified from KXS. Poria is rich sources of triterpenoids.

In addition to the above ingredients, KXS also contains volatile oils (102–106) and flavonoids (107).

Alzheimer’s disease (AD) is the most common neurodegenerative disease, manifesting progressive cognitive impairment, amnesia, and mental behavioral changes (Conti Filho et al., 2023). The curative effects of KXS on amnesia have been verified throughout millennia. It is widely accepted that the gradual enrichment of Amyloid-β (Aβ) peptide as well as abnormal or hyperphosphorylated Tau are strongly correlated with AD pathogenesis (Khayat et al., 2021; Wegmann et al., 2021; Zhang et al., 2021). As shown in Figure 7, KXS has been proven to possess excellent anti-AD effects by inhibiting neuroinflammation, neurological apoptosis, and oxidative stress, and increasing the supply of neurotransmitters.

AD was induced in male SD rats through an intraperitoneal injection of D-gal (50 mg/kg/d) and a stereotaxic injection of 4 μg Aβ_25-35 (1 μg/μL) for 6 weeks. The rats were orally administered KXS water extracts (10 g/kg/d) or huperzine A (30 μg/kg) from the third week. The spatial learning and memory were measured with the escape latency, platform crossings, and swimming distance to find the removal platform in the target quadrant of the Morris water maze. Neuronal density in hippocampal CA1 regions was performed using immunohistochemistry staining. According to the results of all the detection indexes, the water extract observably decreased the escape latency and swimming distance, increased platform crossings, increased fluorescence intensity of neuronal nuclei, ameliorated central neurotransmitter loss, reduced Tau hyperphosphorylation via regulating the expression of PI3K/Akt. Moreover, the extract dramatically decreased the concentrations of inflammatory factors and ROS-induced neuronal apoptosis via the PI3K/Akt/GSK-3β signaling pathway in the rat hippocampus. In vitro, the extracts potently inhibited the percentage of apoptotic cells and the expressions of Bax/bcl-2 and cleaved-caspase-3, meliorated Aβ_25-35-induced ROS accumulation and LDH release, and reduced the expression of Tau signaling pathway in PC12 cells (at the dose of 1 mg/mL). Based on the results of the in vivo and in vitro experiments, KXS extracts indicated its potentially beneficial role in ameliorating AD progression (Guo et al., 2019c). An in vivo study on AD rats after treatment with KXS indicated that 23 constituents were found in AD rats’ plasma, including 1 from PR, 8 from GR, 4 from Poria, 6 from ATR, and 4 that were only discovered in KXS. meanwhile, 10 components were detected in AD rats’ brains, including 4 from GR and 6 that were only existed in the whole prescription. Furthermore, it was found that the peak areas of some constituents in the KXS group were bigger than those in the single herbs group via semiquantitative analysis. KXS might mediate oxidative stress, neuroinflammation, and apoptosis both in vivo and in vitro, especially when the four single herbs were adopted in combination. These results demonstrated the synergistic function and the whole prescription compatibility rationality of KXS to a certain degree (Guo et al., 2019a).

Meta-analysis of animal experiments is believed to provide a high level of research evidence (Chou et al., 2020). Network pharmacology is helpful in elucidating the mechanisms of the TCM formula in the treatment of complex diseases in a comprehensive and systematic manner (Zhou et al., 2022a). To further elucidate the active ingredients and the mechanisms of KXS, Yi et al. (2020) employed meta-analysis, network pharmacology analyses, and molecular docking to investigate the efficacy and potential underlying action mechanisms of KXS against AD from a holistic prospect. The result indicated that aposcopolamine, stigmasterol, and inermin exhibited good affinity for several key genes associated with memory and cognition, such as muscarinic acetylcholine receptors, adrenergic receptors, and acetylcholinesterase (AChE). Scopolamine-induced amnesia is an excellent model of behavioural-cum-cognitive deficits to study dementing-related illnesses such as AD (Tang, 2019). In scopolamine (3 mg/kg/d i. p.)-induced cognitive dysfunction mice, Luo et al. (2020) validated the anti-inflammatory effects of the ethanol extracts of KXS (at doses of 1.4 and 2.8 g/kg) by decreasing proinflammatory cytokine levels such as tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β both in the hippocampus and cortex, which were consistent with the network pharmacology analyses. More importantly, it was found that cholinergic system dysfunction improvement of KXS was significantly correlated with regulation of the nicotinic acetylcholine receptor CHRNB2. Senescence accelerated mouse prone 8 (SAMP8) was reported to develop typical AD pathologies, such as an abnormal expression of anti-aging factors, inflammation, oxidative stress, tau hyperphosphorylation, amyloid-(A) deposits, abnormal autophagy activity, and endoplasmic reticulum stress (Andrea et al., 2022). Therefore, SAMP8 is widely used and considered as an ideal model for AD research, displaying age-related cognitive decline and memory loss (Xie et al., 2020). In a subsequent study, the potential molecular mechanisms of KXS on AD which was predicted by the network pharmacology approach were experimentally validated in SAMP8. The result indicated that the water extract (at doses of 5.4 g/kg) significantly upregulated AKT phosphorylation, inhibited the activation of GSK3β and CDK5, and suppressed the TLR4/MyD88/NF-КB pathway to suppress neuronal apoptosis and attenuate cognitive impairment, thus alleviating the progression of AD (Jiao et al., 2022). Although dozens of in vivo and in vitro experiments of KXS and its components have supported the anti-AD potentials, explicit pharmacokinetic clinical trials should be executed to further explore the principles of formulating prescriptions and elucidate the correlation between bioactive ingredients and pharmacological function.

Depression, a common but devastating psychiatric illness characterized by anhedonia, despair, anxiety, cognitive changes, and sleep disturbance, has become a pressing public health problem that leads to a heavy socioeconomic burden (Cladder-Micus et al., 2018). More than 264 million people worldwide are suffering from this debilitating illness (Monroe and Harkness, 2022). Unfortunately, depression is an extremely complex disease caused by multiple factors, and the actual etiology of this disease is still unclarified (McCarron et al., 2021). Nowadays, substantial basic research is ongoing to shed light on depression. Many pathological hypotheses including brain-derived neurotrophic factor (BDNF), neurotransmitter imbalance, over-stimulation of inflammatory, hypothalamic-pituitary-adrenal (HPA) axis disturbances, and endogenous metabolites, have been put forward (Fox and Lobo, 2019).

In terms of TCM theory, depression is believed to be caused by the stagnant of Qi (vital energy), moisture, phlegm, or heat, which ultimately leads to brain dysfunction (Li et al., 2023). KXS has the function of tonifying Qi, nourishing the heart, eliminating dampness, and resolving phlegm, thereby for the treatment of depression function. An increasing number of studies have suggested that KXS was used to effectively treat depression by regulating autophagy, neurotrophic factors, neurotransmitters, synaptogenesis, and inflammation in vivo and in vitro experiments (Figure 8) (Zhou et al., 2012; Zhu et al., 2012; Zhu et al., 2013; Dong et al., 2017; Fu et al., 2019). Emerging experimental investigations have suggested that chronic stress which causes many impairments in mood, memory, and cognition, has been considered to play a causal role in depression onset (Cruz-Pereira et al., 2020). The chronic unpredictable mild stress (CUMS) model of depressive disorder is currently the most commonly employed, effective, and reliable experimental tool to assess human psychopathology (Antoniuk et al., 2019). In a preclinical study where fluoxetine (10 mg/kg/d) was used as a positive control, intra-gastric administration of KXS water extract at the dosage of 3, 5, or 10 g/kg/day for 2 weeks remarkably ameliorated depressive-like behaviors of eight CUMS rats in sucrose preference test, forced swimming test and open field test, thereby relieving the symptoms of anhedonia, despair, and anxiety. To be more specific, the sucrose preference was markedly increased in the minimum effective dosage (5 g/kg/day) of the treated group compared with the control group (p < 0.05). The improvements in depressive-like behaviors seen with KXS-H (10 g/kg/d) were similar to the results seen with fluoxetine, demonstrating the efficacy of KXS on depression treatment. In parallel, this study confirmed that KXS inhibited inflammation-stimulated depression via resisting NLRP3 inflammasome activation and inducing autophagy in vivo and in vitro, as evidenced by the decrease in tumor necrosis factor (TNF-α), interleukin (IL)-6, IL-1β mRNA expression and IL-1β protein expression, as well as the accumulation in ROS content (Yu et al., 2021).

KXS was beneficial for modulating the metabolite pathway of neurotrophins (Zhu et al., 2017), inducing synaptic protein expressions (Zhu et al., 2016), and regulating pro-inflammatory cytokines in microglia (Qu et al., 2021), which might account for the concrete regulation mechanism. According to a network pharmacology investigation, 4 compounds including Asarone, Pachymic acid, Ginsenoside Rg1, and Polygalaxanthone III were identified as the key active components of KXS for treating depression (Du et al., 2022). These compounds might target 6 target genes that were closely related to four pathways, such as the serotonergic synapse pathway, neuroactive ligand-receptor interaction pathway, MAPK signaling pathway, and PI3K-Akt signaling pathway (Du et al., 2022).

A considerable number of studies have demonstrated that depressive symptoms are the highly prevalent risk factor for cardiovascular morbidity and mortality (Carney and Freedland, 2016; Bucciarelli et al., 2020; Li et al., 2020). It has been reported that comorbid depression often occurs in patients with coronary heart disease (CHD) (Khandaker et al., 2019), and the comorbidity complicates the depression treatment and worsens the cardiovascular prognosis (Carney and Freedland, 2016). TCM theory specifies that “Heart Governs the Spirit Light”. This means that a dysfunctional heart correlates strongly with mental diseases, such as insomnia, depression, and insanity (Ong et al., 2018). According to one study, KXS water extract (370 mg/kg) provided obvious protection against postmyocardial infarction depression (Hu et al., 2020). Compared to the model group, KXS treatment dramatically improved the levels of ejection fraction and fractional shortening, reduced the levels of lactate dehydrogenase (LDH) and creation kinase MB (CK-MB), increased the activities of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX), and decreased malondialdehyde (MDA) content. In addition, KXS diminished the levels of IL-6, TNF-a, and C-reactive protein (CRP) in myocardial tissue (Hu et al., 2020). Nevertheless, these experiments have obvious limitations in that only one or two doses of KXS were applied, and thus the information on the dose-dependent effect is restricted. As a classical prescription for the treatment of depression, KXS regulates prominent components of gut and brain functions (Cao et al., 2020). The anti-depression molecular mechanisms of KXS remain to be elucidated. The regulatory effects of KXS by which the microbiota-gut-brain axis affects depression are worth studying.

Multi-infarct dementia (MID), which is characterized by neuronal degeneration and multiple lesions in the cerebral gray-white matter, is the most common type of vascular dementia (VD), accompanied by obvious brain tissue pathological damage and cognitive dysfunction (Jellinger, 2013). Accumulated excitatory amino acid (EAA) and flawed energy metabolism in the brain are usually considered two major pathological hallmarks in the progression of MID (Popa-Wagner et al., 2015). Compared with the model group, pretreated with oral KXS water extract (2.12, 1.06 g/kg, 0.7 mg/kg hydergine was used as positive control) for 45 days significantly improved cognitive impairment and hippocampal neuron damage of MID rats, rescued mitochondrial functions by upregulating brain energy, ameliorating mitochondrial swelling as well as improving mitochondrial membrane potential under chronic hypoperfusion conditions, suppressed glutamate neurotoxicity through decreasing the concentration of glutamate and the level of p-NMDAR1, and noticeably activated Shh/Ptch1 signaling pathway in the brain tissue of MID rats. Intriguingly, KXS protected PC12 cells against glutamate-induced neurotoxicity via Shh/Ptch1 signaling pathway (Li et al., 2021b). Nevertheless, further in-depth clinical trials should be conducted to investigate the effectiveness of KXS as a treatment for MID.

Cognitive dysfunction (CD) is characterized by abnormalities in reduced memory, learning, calculation, attention, comprehension, executive functioning, and psychomotor speed (Knight and Baune, 2018; Xie et al., 2020), along with abnormal mental behaviors, which interfere with patients’ daily performance and social abilities (Moussa-Pacha et al., 2020). It is well known that the Morris water maze (MWM) and Y-maze tasks are behavioral experiments designed to determine the spatial memory and learning functions (Wang et al., 2021d).

Appropriate animal models could serve as useful tools for experimental studies of drugs (Zhou et al., 2022b). Scopolamine is a pharmacologic agent that causes serious damage to cognition and functional imaging in various kinds of animals, including rats, mice, dogs, and cats (Klinkenberg and Blokland, 2010), similar to what is seen in cognitive dysfunction in humans. A substantial number of preclinical studies have employed scopolamine as a standard drug for inducing cognitive impairment models in healthy animals (Deng et al., 2019; Amirahmadi et al., 2022). Sleep deficiency or loss leads to the decline of emotion, immunity, memory, and learning (Krone et al., 2021). The rodent models of sleep deprivation show anxiety-like behaviors and cognitive impairment similar to humans (Yin et al., 2017). It is well known that cancer-related disruptions in cognitive function and memory are a common adverse effect of adjuvant chemotherapy (Speidell et al., 2018). Clinical and preclinical studies have indicated that KXS exerts a beneficial effect on CD in different animal models. In twelve scopolamine-induced CD mice, oral administration of 0.7, 1.4, and 2.8 g/kg KXS water extract (3 mg/kg/d of donepezil was used as positive control) for 2 weeks ameliorated learning and memory impairments (shortened the escape latency and increased residence time in target quadrant and the number of target crossings in MWM test, increased the percentage of alternations between the labyrinth arms in the Y-maze), decreased neuronal apoptosis and reversed neuronal degeneration (inhibited the formation of apoptotic bodies and expression of pro-apoptotic protein Bax, and upregulated the expression of anti-apoptotic protein Bcl-2), regulated cholinergic neurotransmitters (increased acetylcholine levels and the activity of choline acetyltransferase), and alleviated oxidative stress damage in the cortex and hippocampus (increased the level and activity of superoxide dismutase and glutathione peroxidase, reduced the level and activity of reactive oxygen species and malondialdehyde) (Xu et al., 2019). Treatment with oral KXS water extract (at daily doses of 500, 250, and 125 mg/kg body weight, 30 mg/kg modafinil was used as positive control) for 14 days improved learning and memory function of paradoxical sleep deprivation (PSD)-induced CD rats by decreasing the glutamic acid (GLU) and γ-aminobutyric acid (GABA) levels and modulating of the expression of some genes in the brain tissue, such as BDNF, cyclic AMP response element binding protein (CREB), and phosphorylated-CREB (p-CREB). The minimal active dose was 250 mg/kg, while the memory enhancement effect of 500 mg/kg KXS was equally effective as that of 30 mg/kg modafinil (Hu et al., 2013). Another study (0.1 mg/kg of huperzine A was used as positive control) showed that intragastric administration of 0.3 or 0.6 g/kg of KXS water extract for 2 weeks improved memory deficit induced by simulated weightlessness in ten rats mainly by inhibiting oxidative stress injury (Qiong et al., 2016). Chemotherapy-related cognitive impairment has been observed in 15% and 50% of oncology patients (Rummel et al., 2021). The therapeutic effect of KXS (at the dosage of 1 g/kg daily by gavage over 3 weeks) was studied on doxorubicin (DOX)-related cognitive impairment in 4T1 breast cancer mice. This study suggested that KXS attenuated DOX-induced cognitive impairment by regulating inflammatory responses, decreasing oxidative stress, and reversing neural degeneration. Additionally, it is worthy that KXS is beneficial to enhance quality of life and prolong survival time in DOX-treated tumor-bearing mice (Lyu et al., 2021). In summary, KXS is a promising prescription for the treatment of memory decline. These results gave inspiration for the future development of commercial agents. Nonetheless, studies investigating the clinical pharmacokinetic parameters and safety evaluation of KXS are scarce, which limits further understanding of the medicinal substance of KXS.

Chronic fatigue syndrome (CFS) is a complex disease characterized by extreme fatigue with associated symptoms of anxiety, depression, pain, and various somatic complaints (Zhang et al., 2022b). The forced wheel running model is a popular model that has been used to simulate the chronic fatigue syndrome (Toval et al., 2021). Cao et al. evaluated the effects of KXS on CFS mice induced by forced wheel running. Briefly, 72 male Kunming mice were randomly separated into six groups (n = 12), including a control group, CFS group, Modafinil-treated CFS group (13 mg/kg/day), and three KXS-treated groups (175, 350 and 700 mg/kg/day). The mice in the control group were not forced to run. Correspondingly, the animals in the other five groups were induced by forced wheel running for 4 weeks. All the mice were anesthetized by intraperitoneal injections of ketamine (80 mg/kg) and xylazine (4 mg/kg). Blood was taken using a disposable syringe from the heart. The liver and muscle of mice had been weighed precisely as instructions. Levels of serum urea nitrogen (SUN), serum lactate dehydrogenase (LDH), liver glycogen (LG), muscle lactic acid (MLA), muscle glycogen (MG), and the expressions of IL-2, and IL-4 secreted by splenocytes were detected. The results showed that electric shock time was prominently reduced in the three KXS-treated groups when compared with the CFS group without drug treatment. Interestingly, the reduction in MLA, SUN, LDH and IL-4 levels and an increase in LG, MG, MLA, and IL-2 levels were demonstrated in these three KXS-treated groups than those in the CFS group without drug treatment (Cao et al., 2012). This elucidates that KXS exhibits comprehensive anti-fatigue effects. However, the specific mechanisms remain unclear, and further investigation is required to perform at animal, cellular, and molecular levels.