The Therapeutic Strategies for Uremic Toxins Control in Chronic Kidney Disease

Uremic toxins (UTs) are mainly produced by protein metabolized by the intestinal microbiota and converted in the liver or by mitochondria or other enzymes. The accumulation of UTs can damage the intestinal barrier integrity and cause vascular damage and progressive kidney damage. Together, these factors lead to metabolic imbalances, which in turn increase oxidative stress and inflammation and then produce uremia that affects many organs and causes diseases including renal fibrosis, vascular disease, and renal osteodystrophy. This article is based on the theory of the intestinal–renal axis, from bench to bedside, and it discusses nonextracorporeal therapies for UTs, which are classified into three categories: medication, diet and supplement therapy, and complementary and alternative medicine (CAM) and other therapies. The effects of medications such as AST-120 and meclofenamate are described. Diet and supplement therapies include plant-based diet, very low-protein diet, probiotics, prebiotics, synbiotics, and nutraceuticals. The research status of Chinese herbal medicine is discussed for CAM and other therapies. This review can provide some treatment recommendations for the reduction of UTs in patients with chronic kidney disease.


Introduction
Chronic kidney disease (CKD) is characterized by a gradual decrease in the glomerular filtration rate and proteinuria. CKD is a global health problem, and its incidence has been increasing. The estimated global prevalence of CKD is 8-14% [1,2]. When kidney function deteriorates gradually, many metabolites accumulate in the body. These accumulated substances, termed as uremic toxins (UTs), can result in adverse pathophysiological outcomes [3]. UTs can affect multiple organs and cause renal fibrosis, vascular calcification, anemia, peripheral arterial disease [4], adynamic bone disease [5], adipocyte dysfunction with insulin resistance [6], impaired immune system [7], uremic pruritus [8], and impaired valsartan-induced neovascularization [9].
In 1999, Vanholder and other researchers established the European Uremic Toxin (EU-Tox) Work Group and divided UTs into three categories based on their solubility, molecular weight, and ability to bind to serum proteins. These categories are small water-soluble This review describes the potential nonextracorporeal methods from bench to bedside that can be used to reduce UTs levels and improve renal function based on the aforementioned mechanisms. The PubMed, Embase, Cochrane Library, Chinese National Knowledge Infrastructure, Airiti library, and Wanfang databases were searched by using the term "uremic toxin". To broaden the search, we further reviewed the included articles and citations utilizing the "related articles" facility on PubMed. This paper is organized as follows. The first section focuses on medications including intestinal sorbents, UA modulators, and enzyme inhibitors. The second section focuses on diet control and supplement therapies such as probiotics, prebiotics, synbiotics, and nutraceuticals. The final section describes complementary and alternative medicine (CAM) and other therapies.

Acarbose
Acarbose is a small intestinal alpha-glucosidase inhibitor that can increase the number of undigested carbohydrates reaching the colon, increase the production of short-chain fatty acid butyrate in the colon, and reduce intraluminal power of hydrogen (pH) value, thereby inhibiting bacterial deamination and increasing the use of ammonia. In a clinical trial, nine healthy people were treated with acarbose for 3 weeks, and their serum p-cresol declined significantly through changes in bacterial amino acid metabolism [26].

AST-120
AST-120 can absorb UTs and their precursors in the gastrointestinal tract and then excrete them in feces, reducing the absorption of UTs into the blood. For example, indole is produced through tryptophan metabolism by bacteria in the gastrointestinal tract. AST-120 absorbs indole and reduces its conversion into IS [31]. In an adenine-induced CKD mouse study, Sato et al. demonstrated that the administration of AST-120 led to a decline in accumulated IS and p-cresol sulfate (PCS) in multiple organs, such as the kidneys, heart, brain, and skeletal muscles [37]. The effect of AST-120 on renal disease progression had been reported in some large-scale clinical trials. The Carbonaceous Oral Adsorbent's Effects on Progression of Chronic Kidney Disease (CAP-KD) trial demonstrated administration of AST-120 can significantly suppress the decrease in estimated glomerular filtration rate (eGFR) for a short observation period of one year [40]. However, it failed to suppress the composite endpoint consisting of doubling of serum creatinine (Scr), Scr ≥ 6.0 mg/dL, endstage renal disease, and death. In Evaluating Prevention of Progression in CKD (EPPIC)-1 and EPPIC-2 trails, AST-120 was administered to 2035 adults with moderate to severe CKD. AST-120 administration was unable to suppress the primary endpoint which is the initiation of dialysis, transplantation, and doubling of Scr levels [32]. However, the secondary endpoint, the eGFR decline rate was significantly suppressed. The Kremezin study against renal disease progression in Korea (K-STAR) trial included measurement of plasma IS concentration. The primary endpoint was not suppressed in the AST-120 group [41]. Similarly, the eGFR decline tended to be suppressed. Finally, a meta-analysis including eight studies revealed that AST-120 can effectively lower IS levels but still controversial in terms of slowing disease progression and all-cause mortality [32]. Additional large-scale randomized controlled trials (RCTs) with longer follow-up durations and standardized outcomes are still necessary to clarify the clinical efficacy of AST-120.

L-Carnitine
Patients undergoing hemodialysis (HD) may develop L-carnitine deficiency due to intestinal malabsorption, reduced carnitine synthesis, and reduced carnitine clearance during dialysis. In a double-blind, placebo-controlled, crossover trial of patients undergoing HD, supplementation of L-carnitine, an antioxidant, for 8 weeks reduced the malondialdehyde (MDA) level, increased reduced/oxidized glutathione, and enhanced glutathione peroxidase activity and protein carbonyl concentration without adverse clinical effects [33]. The intravenous injection of carnitine supplementation has been shown to have a nephroprotective effect on contrast-medium nephropathy in an open-label, crossover study, it can reduce the neutrophil gelatinase associated lipocalin and SCr [42]. An animal study suggested that the nephroprotective effect of L-carnitine could reduce blood urea nitrogen (BUN), Scr, and MDA levels in CKD rats following right nephrectomy, possibly through antioxidative properties and free radical scavenging [38].

Cilastatin
Imipenem is a carbapenem antibiotic with nephrotoxicity. However, cilastatin, the renal dehydropeptidase-I (DHP-1) inhibitor, can prevent imipenem-induced nephrotoxicity by inhibiting the OAT-mediated transport of imipenem [43]. Cilastatin or probenecid ameliorated kidney injury and reduced BUN, Scr, and the renal secretion of imipenem in an imipenem-induced nephrotoxicity rabbit model through inhibition of renal OATs [30].

Cyclosporine A
Cyclosporine A (CsA), a calcineurin inhibitor, is used as an immunosuppressant. Although CsA is considered to be a nephrotoxic agent, its single injection can protect the kidneys during ischemia and reperfusion. In an ischemia-reperfusion (I/R) renal injury mice model, Lemoine et al. demonstrated that the administration of CsA improved mitochondrial respiration and reduced BUN and Scr probably through the inhibition of mitochondrial permeability transition [24].

Enalapril
Enalapril, an angiotensin-converting enzyme inhibitor, is used to treat cardiovascular diseases. Marek et al. found that in Wistar rats, enalapril treatment reduced the plasma trimethylamine-N-oxide (TMAO) level by increasing glomerular filtration and urine output; however, it did not reduce the plasma level of IS or change the bacterial composition in the gut [39].

Folate and Methylcobalamin
Folic acid (FA) is the main cofactor of total homocysteine (Hcy) metabolism, and methylcobalamin (Me-Cbl) is the methylated form of cobalamin. Both play key roles in the conversion of Hcy to methionine. The combined administration of oral FA and intravenous Me-Cbl can reduce plasma Hcy in HD patients [44], which may prevent cardiovascular disease and stroke [45]. In a prospective RCT of patients undergoing HD, both oral FA supplementation and intravenous (i.v.) injection of Me-Cbl plus oral FA normalized plasma total Hcy; however, i.v. injection of Me-Cbl showed no change [34]. Another RCT reported that patients undergoing HD who received i.v. injection of Me-Cbl plus oral FA showed a reduction in Hcy and asymmetric dimethylarginine (ADMA) levels; this reduction possibly resulted from an increase in the metabolic degradation of UTs [25].

Ketoacids
Ketoanalogues have been used in chronic kidney disease for many years. It can combine the excess amino acids to synthesize essential amino acids (EAAs) through transamination, which can reduce the formation of nitrogenous waste and urinary toxins, and reduce the degradation of protein, which in turn, can improve the patient's renal function and prognosis. The transamination of ketoanalogues is bidirectional and is related to the concentration of amino acids. When the concentration of amino acids in the body was high, the body will tend to decompose ketonanalogues instead of synthesizing them into new EAAs [46]. In a pilot study published in 2013, researchers analyzed the physiological data of 32 patients with stage 3 CKD. These patients were randomly divided into two groups: very low protein diet (VLPD) with ketoacid (KA) analog supplementation for 1 week, followed by low protein diet (LPD) administration in the second week, and LPD administration for 1 week, followed by VLPD with KA analog supplementation in the second week. The results indicated that restricted protein consumption with KA analog supplementation reduced the production and intestinal absorption of IS [35]. In another clinical trial, 17 patients were included and randomly divided into two groups: LPD and VLPD with EAAs and KA supplementation. After the 24-week trial, forearm protein metabolism was affected in both groups. Overall, skeletal muscle may adapt to restricted protein intake by combining the responses of decreased protein degradation and increased amino acid recycling [29].

Meclofenamate
Meclofenamate is a type of nonsteroidal anti-inflammatory drug (NSAID) that inhibits hepatic IS production. Saigo et al. used an I/R rat kidney model to demonstrate that meclofenamate considerably improved renal function and reduced BUN, Scr, and IS. In addition, they indicated that meclofenamate could inhibit sulfotransferase (SULT) and reverse the downregulation of rat OAT1 and rat OAT3 expression by preventing prostaglandin E2 generation [28].

Reduced Glutathione
Oxidative stress arises when there is an imbalance between free radical production and antioxidant defense. Uremic toxins can be a source of oxidative stress in CKD patients. Retention of these toxins promotes systemic inflammation via priming polymorphonuclear leukocytes and stimulating CD-8+ cells [47]. Additional associative factors of oxidative stress in CKD include low serum selenium concentration, low platelet glutathione peroxidase activity [48], and lower serum levels of glutathione [48]. Although glutathione is critical to fight against oxidative stress, some evidences disclosed that glutathione is poorly absorbed by oral route mainly due to the action of an intestinal enzyme, the γ-glutamyl transpeptidase which degrades glutathione [49]. Wang et al. ever found that the oral supplementation of reduced glutathione in patients undergoing HD significantly reduced their IL-6 and TNF-α levels but not their Scr or BUN levels in their RCT trial [36]. Although the very low oral bioavailability limits the interest of oral glutathione supplementation, Schmitt et al. demonstrated the superiority of a new sublingual form of glutathione over the oral form in terms of glutathione supplementation [50]. A new sublingual form may be a promising antioxidant therapy in CKD patients in the future.

Diet Control and Diet Supplements
UTs are usually produced through the metabolism of amino acids by intestinal microbiota microbiota, and accumulated UTs can change the composition of intestinal microbiota. Increasing evidence shows that intestinal microbiota plays a crucial role in the development of CKD [51]. Therefore, the current methods of reducing UTs through diet control and supplement therapy include reducing protein intake through a low-protein or plantbased diet [52] and supplementing probiotics, prebiotics, and synbiotics to change the composition of intestinal microbiota; some supplements could exert anti-inflammatory and antioxidative effects [53]. These treatment strategies are briefly discussed below ( Table 2).

Lingonberry
Lingonberry (Vaccinium vitis-idaea L.) is an evergreen dwarf shrub rich in anthocyanins, which has anti-inflammatory, anti-diabetic, and renal protection properties [64,65]. In another study, mice were fed with a high-fat diet (HFD) to induce chronic kidney injury for investigating the effects of lingonberry on CKD. They found that the serum levels of inflammatory cytokines such as TNF-α and IL-6 decreased and renal injury improved in the HFD group that received lingonberry supplementation. In addition, lingonberry supplementation attenuated palmitic acid-induced nuclear factor-kappa B (NF-κB) activation and inflammatory cytokine expression in proximal tubule epithelial cells. Lingonberry supplementation may protect the kidney from metabolic abnormality-induced injury by preventing an inflammatory response [61].

Prebiotic Oligofructose-Enriched Inulin
Oligofructose, a type of oligosaccharide fructans, was used as an alternative sweetener. Probiotic bacteria such as Bifidobacteria and Lactobacilli are saccharolytic in nature. They cause oligofructose to produce SCFAs that exert a positive immunomodulatory effect, maintaining the symbiosis of intestinal microbiota [68]. In a nonrandomized, open-label trial of patients undergoing HD, administration of prebiotics composed of prebiotic oligofructoseenriched inulin (OF-IN) for 4 weeks reduced the generation rate and serum concentration of PCS but not that of IS. Another study proposed that prebiotics possessed the ability to modify intestinal uremic metabolites [54].

Probiotics
Probiotics were defined as "live microorganisms" by the World Health Organization (WHO) and Food and Agriculture Organization in 2002. Emerging evidence suggests that an appropriate supplementation of probiotics can improve gastrointestinal health, strengthen the immune system, lower the incidence of allergies, alleviate the symptoms of lactose intolerance, and reduce the risks of cancer [69,70]. The mechanisms by which probiotics provide their effects may involve modifying the pH of the intestine, defeating pathogens by producing antibacterial compounds, competitively excluding pathogens at the binding site, and combining of harmful mutagens and carcinogens [71]. Many studies have suggested that oral supplementation of a probiotic formula of selected microorganisms may exert a nephroprotective effect by extracting UTs in patients with CKD. In an RCT, the administration of Kibow Biotics probiotic formulation (L. acidophilus KB27, B. longum KB31, and S. thermophilus KB19) in patients with CKD reduced BUN, Scr, and UA levels; enhanced patients' quality of life; and resulted in no adverse effects [55]. Another study demonstrated that the administration of a probiotic containing Bifidobacterium longum in gastric-resistant seamless capsules could effectively reduce serum Hcy and IS levels in patients with HD [56].

Short-Chain Fatty Acids
In CKD and end-stage renal disease (ESRD), gut-derived UTs can cause systemic inflammation, oxidative stress, and cause dysbiosis of the gut. When the anaerobic bacteria that can produce SCFA declines, resulting in insufficient production of SCFA [72]. SCFAs was mainly composed of acetate, propionate, and butyrate and exerted a positive immunomodulatory, anti-inflammatory, anti-oxidative, antibacterial, and anti-diabetic effects effect [73][74][75]. In a nonrandomized pilot study, the food additive sodium propionate (SP), an SCFA, was administered in the form of capsules, and this effectively reduced IS, MDA, and PCS in patients undergoing HD. Moreover, SP reduced pro-inflammatory parameters and oxidative stress and improved insulin resistance and iron metabolism [57].

Soluble Fiber and Omega-3 Fatty Acids
Omega-3 fatty acids (O3FAs) are a group of polyunsaturated fatty acids, among which eicosapentaenoic acid (EPA), ALA, and docosahexaenoic acid (DHA) are closely related to human physiology. O3FAs can be gained primarily from vegetable seed oils or aquatic organisms [76]. O3FA plays a crucial role in the eicosanoid biosynthesis. EPA competes with arachidonic acid to switch the synthesis pathway to promote the production of anti-inflammatory eicosanoids [77]. In addition, eicosanoids derived from O3FA have shown clinical benefits for cardiovascular disease, diabetes, and nephropathies due to their anti-inflammatory effects [77,78].
Omega-3 supplementation could improve oxidative stress in patients with CKD [79]. Eating fiber-rich foods can reduce the risk of kidney disease, inflammation, and death [80]. Soluble fiber and omega-3 fatty acids may reduce IS, PCS, and SDMA by modulating the gut microbiota [63].

Synbiotic
Synbiotics, a combination of probiotics and prebiotics, has immune system regulation and anti-inflammatory effects. It can improve the intestinal environment and reduce concentration of nitrogen-containing metabolites [81,82], was applied to treat intestinal chronic diseases [82], allergic rhinitis and asthma [83]. An uncontrolled trial showed that a synbiotic consisting of Lactobacillus casei, Bifidobacterium breve, and galactooligosaccharides reduced the PCS level but did not change IS or phenol levels in in patients undergoing HD [58].

Vegetarian Diet
Plants are the dietary source of fibers, which shifts the gut microbiota to increased generation of anti-inflammatory compounds and reduced production of uremic toxins [84]. Vegetarian diets have a low endogenous acid load, which could reduce metabolic acidosis in CKD patients and potentially slow CKD progression [84,85]. Moreover, in the primary prevention of CKD, our previous study showed that vegetarian diets a strong negative association between vegetarian diets and the prevalence of CKD [86].
Another review supports this observation and revealed that plant-based foods and Mediterranean (MD) diet or Dietary Approaches to Stop Hypertension (DASH) diets could reduce many types of UTs; however, the potential risk of hyperkaliemia should be examined [87]. A cohort study revealed that vegetarian diet patients in hemodiafiltration have not only lower plasma levels of IS and PCS, but also lower serum level of urea [59].

Vitamin D
CKD patients are prone to vitamin D deficiency, which can cause a variety of diseases. Higher 25-hydroxyvitamin D (25[OH]D) levels can significantly improve the survival rate of CKD patients [88].
Case-control studies have shown that vitamin D supplementation could improve endothelial dysfunction in patients with CKD [89]. An RCT reported that vitamin D supplementation for 8 weeks could reduce the IL-6 and UA levels in patients with nondiabetic CKD stage 3-4 and vitamin D deficiency. In addition, vitamin D supplementation could improve vascular function, as determined by significant positive changes in pulse wave velocity [53].

Curcuma Longa and Boswellia Serrata
Dietary turmeric, rich in curcumin, has anti-inflammatory, anti-oxidant, and chemotherapeutic effects [117]. Curcumin has been shown to reduce inflammation in patients with type 2 diabetes [118]. CKD rat models have confirmed the anti-inflammatory effects of curcumin [119], and Boswellia serrata could improve the inflammatory symptoms of colitis and knee osteoarthritis [120,121]; however, their efficacy in patients with CKD has not been examined. An RCT including patients with CKD revealed a significant decrease in plasma IL-6 following treatment with oral C. longa and B. serrata for 8 weeks compared with a placebo group; however, other variables such as TNF-α and CRP exhibited no significant differences. Moreillon et al. indicated that C. longa and B. serrata reduce IL-6 possibly through inhibition of the pathways of NF-κB and mitogen-activated protein kinase [91].

Danhong Injection and Salvianolic Acids
Danhong injection (DHI) contains the extraction of two Chinese medicines, the radix and rhizome of Salvia miltiorrhiza Bunge (Labiatae) and the flower of Carthamus tinctorius L. (Asteraceae) [123]. Salvianolic acid B, rosmarinic acid, and lithospermic acid are powerful protein-binding ligands, and DHI and salvianolic acids are rich in these substances [123]. In a nephrectomized CKD rat model, injection of DHI and its salvianolic acids through the caudal vein increased the dialysis removal of IS and PCS. DHI and its salvianolic acids showed favorable ability as protein-bound competitors [97].

Uremic Clearance Granule
Uremic clearance granule (UCG), also called as NiaoDuQing (NDQ), created based on the TCM theory consisting of 16 herbs, such as Rheum officinale, Glycyrrhiza uralensis, Astragalus membranaceus, Poria cocos, Sophora flavescens, Chrysanthemum morifolium, and so forth, is the first Chinese medicine approved by the China Food and Drug Administration for the treatment of CKD. UCG has been clinically demonstrated to slow CKD progression [124]. Its effects include improving systemic micro-inflammation [125], reducing Scr [95], and calibrating calcium and phosphorus metabolic disorders [126]. However, its mechanism has not been thoroughly studied [95]. A multicenter double-blind, placebo-controlled, and randomized clinical trial recruited 292 patients with stage 3b-4 CKD proved that UCG could delay CKD progression safely and effectively [95]. In an adenine and unilateral ureteral obstruction (UUO)-induced CKD rat model, Huang et al. reported that the administration of UCG reduced BUN, Scr, and UA levels. In addition, UCG exhibited antifibrosis ability through regulation of extracellular matrix (ECM) degradation and associated signaling pathway activity [98].

Catechin Combined with Vitamin C and Vitamin E
Catechin, a type of flavonoid, is found in green tea and other plant foods, which has the benefit of protecting against congestive heart failure and reducing the incidence of myocardial ischemia [128,129]. Catechin, vitamin C, and vitamin E all have the favorable antioxidant capacity [130][131][132]. A study performed 5/6 nephrectomy in aged Wistar rats to explore the mechanism through which catechin protects against renal dysfunction. Catechin with vitamin C and E supplementation could reduce oxidative stress resulting from aging and renal failure and prevent ADMA accumulation [101].

Cyanidin-3-O-Glucoside
Anthocyanins are polyphenol compounds that are abundant in blueberries and purple corn [133], which can reduce the risk of cardiovascular disease-related mortality [134]. Through in vitro and in vivo investigations, Qin et al. found that cyanidin-3-O-glucoside (C3G), the most widespread anthocyanin, acts as an antioxidative agent and regulates glutathione metabolism, resulting in the reduction of BUN, Scr, and urine albumin-creatinine ratio (UACR) and the amelioration of pathological changes in kidney biopsy in db/db mice [102].

Huangkui Capsule
Huangkui capsule, which is prepared using the extract of the flower Abelmoschus manihot (Linn.) Medicus (A. manihot), was approved by China's State Food and Drug Administration in 1999 for treating chronic nephritis. Studies have confirmed the renal protective effect of the Huangkui capsule in vivo, in vitro, and clinically [105,140]. Flavonoids, the main chemical component of the Huangkui capsule, can be converted into glucuronic acid-sulfate conjugates in vivo, which may contribute to the nephroprotective effects [141]. A multicenter RCT showed that Huangkui capsule has antiproteinuric effect for patients with early stage primary glomerulonephritis, and the other multicenter, double-blind, double-dummy RCT also showed that it can effectively reduce proteinuria in patients with IgA nephropathy. Both RCTs showed that Huangkui capsule treatment has no obvious side effects [142,143]. Cai et al. demonstrated that the Huangkui capsule reduced BUN and Scr in adenine-induced chronic renal failure (CRF) rats. It exerted a preventive effect on tubulointerstitial fibrosis, which participates in the mechanism that inhibits the nicotinamide adenine dinucleotide phosphate oxidase (NADPH)/ROS/ERK pathway [105]. Another 5/6 nephrectomy rat model study revealed that the Huangkui capsule inhibited the synthesis of indole by gut bacteria by interfering with the transport of tryptophan, thereby effectively inhibiting the accumulation of IS in CKD rat [106].

Leonurine
Leonurus cardiac has antibacterial, antioxidant, and anti-inflammatory activities. For thousands of years, it has been traditionally used in China for gynecological diseases such as uterotonic action and postpartum blood stasis. Leonurus cardiac has been used in modern times to treat nervous system dysfunction and cardiovascular disease [144]. Leonurine (LEO), an alkaloid isolated from Leonurus cardiac, exerts antioxidative effects [145]. LEO reduced BUN and Scr levels and downregulated TNF-α, IL-1, IL-6, IL-8, and kidney injury molecule-1 (KIM-1) expression by inhibiting the ROS-mediated NF-kB signaling pathway in a mouse model of lipopolysaccharide (LPS)-induced renal injury [107].

Ligustrazine
Ligustrazine (LIG), an alkaloid extracted from Ligusticum wallichii, could reduce I/Rinduced hepatic and endothelial cell damage by scavenging cytotoxic oxygen free radicals [146,147]. LIG reduced MDA and TNF-α levels in I/R-induced renal injury mice by downregulating oxidative stress and apoptosis and reducing neutrophil infiltration [108].

Notoginsenoside R1
Panax notoginseng is a TCM that promotes blood circulation and removes blood stasis. It is widely used in China to treat cardiovascular diseases. A study reported that notoginsenoside R1 (NR1) is the main component of Panax notoginseng that possesses anti-inflammatory and anticoagulant properties [148]. NR1 mitigated I/R-induced kidney dysfunction as determined by the reduced levels of serum Cr and TNF-α, possibly through suppressing the p38/NF-kB pathway in a rat model of I/R-induced renal injury [109].

Resveratrol
Resveratrol (RSV), a natural polyphenol that originates in grapes, berries, and other dietary constituents, exerts antioxidant activity and anti-atherosclerotic effects by modulating the growth of certain gut microbiota, such as Lactobacillus and Bifidobacterium [152,153]. RSV can promote the release of nitric oxide and prostacyclin to maintain endothelial function and adjust vascular tone [154]. RCT confirmed that RSV has cardioprotective effects in patients with stable coronary artery disease [155]. Chen et al. reported that RSV attenuated TMAO in TMAO-induced atherosclerosis mice through the remodeling of gut microbiota [112].

Rhubarb
Rhubarb is a common herbal medicine composed of the dried rhizomes and roots of Rheum palmatum L, Rheum tanguticum Maxim that is broadly used in sepsis and pancreatitis to protect the intestinal mucosal barrier [156,157]. Rhubarb enema could alleviate IS overload to ameliorate renal tubulointerstitial fibrosis in the 5/6 nephrectomy renal injury rats model by reducing kidney oxidative stress and inflammatory damage [114]. Furthermore, Ji et al. reported that rhubarb enema could reduce IL-1β and IL-6 levels in 5/6 nephrectomy renal injury rats by modifying intestinal microbiota [115].

10-(6 -Plastoquinonyl) Decylrhodamine 19
Mitochondria play an important role in the pathogenesis of uremia, and their damage will lead to the accumulation of UTs [14]. The compartment of the mitochondria is negatively charged in the cell, the design of active molecules combined with cationic carriers can accumulate in mitochondria due to their positive charges [158]. 10-(6 -plastoquinonyl) decylrhodamine 19 (SkQR1), a mitochondrial-targeted antioxidant conjugated with plastoquinone and cationic decylrhodamine 19, possesses high antioxidant capacity in animal models and in vitro [159]. SkQR1 exerted renal protection by increasing erythropoietin levels in gentamicin-induced renal injury rats. In addition, SkQR1 could reduce BUN and MDA levels [116].

Tanshinone I
Danshen, also named Salvia miltiorrhiza, is a traditional Chinese medicine often used to treat cardiovascular diseases. Tanshinone I, an active component of Salvia miltiorrhiza Bunge, has the effects of anti-inflammation, cardiovascular protection, anti-tumor, and anti-hepatic fibrosis [160,161]. Tanshinone I exerted a renal protective effect, as indicated by reduced BUN and Scr levels, through the enhancement of AA metabolism by inducing CYP1A in a mouse model of AA-induced renal injury [113].

Acupuncture
Acupuncture is a popular CAM therapeutic method that can effectively treat certain diseases such as pain and insomnia; this method is recognized by WHO [162]. Acupuncture can stimulate the production of endomorphin-1, encephalin, β-endorphin, and serotonin in plasma and brain tissue, thereby achieving the effects of analgesia, sedation, and immune regulation [163]. In an RCT including 53 patients with CKD, once-weekly electroacupuncture at bilateral Hegu (LI4), Zusanli (ST36), and Taixi (KI3) for 12 weeks improved renal function; the reduction in Scr levels induced by reduced eGFR was greater in the acupuncture group than in the sham acupuncture group [92]. Acupuncture might improve renal function by regulating sympathetic nerves and activating biologically active chemicals [164].

Moxibustion
Moxibustion, a CAM therapy that consists of burning dried moxa on particular acupoints on the body, is used to treat a variety of diseases, such as malposition, diarrhea, soft tissue damage, and dysmenorrhea [165]. Moxibustion methods can be divided into direct moxibustion and indirect moxibustion. Direct moxibustion is to place moxa sticks on acupuncture points, while indirect moxibustion is defined as placing medicinal materials (e.g., mugwort, ginger, etc.) between moxa sticks and acupuncture points [166]. Moreover, the moxa sticks are placed on acupuncture needles inserted into the skin to improve the curative effect [167]. The pharmacological effects of moxibustion are the heat and light radiation of moxibustion burning and aromatherapy [168,169]. Moxibustion may improve CKD by exerting anti-inflammatory effects [170]. A meta-analysis including 23 RCTs revealed that in patients with CKD, moxibustion therapy significantly reduced BUN and Scr levels and improved uremic protein excretion and quality of life. In patients with CKD, moxibustion causes expansion of local renal capillaries and reduces renal podocyte damage [93].

Conclusions
Different treatments, such as medicine, diet control, diet supplement, and CAM, employ different mechanisms of action to improve kidney function or reduce different UTs, including the inhibition of inflammation, cell apoptosis, removal of toxins, regulation of intestinal bacteria, and inhibition of oxidative stress. This evidence supports the potential applications of these therapies. Growing evidence has revealed that the intestine is the main source of UTs. Regulating diet and modulating intestinal microbiota can reduce the accumulation of UTs. Recent studies have indicated that reducing the production of UTs in mitochondria can protect mitochondria, thus reducing the pathological consequences of CKD. Some Chinese medicines have shown the ability to protect the kidneys and reduce UTs; however, certain Chinese medicines are harmful to the kidneys and increased attention should be focused on the use of Chinese medicines. The scale of clinical trials on Chinese medicines is small, and many treatments are still in the animal experiment stage; however, additional large-scale trials are needed to examine their effects on reducing UTs or improving residual renal functions.

Data Availability Statement:
The data used to support the findings of this study are included within the article.