In humans, the multi-enzymatic process of de novo synthesis of BAs occurs in one of two main pathways and is regulated via negative feedback by BA-activated FXR (Shulpekova et al., 2022; Fuchs et al., 2025). BA synthesis represents a major route of cholesterol catabolism (Schwarz, 2004; Chiang, 2013; Li and Chiang, 2014; di Gregorio et al., 2021). The “classical” or neutral pathway, responsible for ∼90% of primary BA production, is initiated and rate-limited in the hepatocytes by cholesterol 7α-hydroxylase (CYP7A1) (Faustino et al., 2016; Chiang and Ferrell, 2020; di Gregorio et al., 2021; Collins et al., 2023). Hydroxylation at the C7 position yields the intermediate, 7α-hydroxycholesterol, which is then acted upon by sterol 27-hydroxylase (CYP27A1) to generate chenodeoxycholic acid (CDCA), or alternatively by 12α-hydroxylase (CYP8B1) before the CYP27A1 step to produce cholic acid (CA) (Figure 1) (Chiang, 2009; di Gregorio et al., 2021).

The “alternative” or acidic pathway, which accounts for ∼10% of primary BA synthesis, is initiated by mitochondrial CYP27A1 in both hepatic and extrahepatic tissues (Schwarz, 2004; di Gregorio et al., 2021). This hydroxylates cholesterol at the C27 position to produce the oxysterol intermediate 27-hydroxycholesterol, which is subsequently modified by oxysterol 7α-hydroxylase (CYP7B1) to generate CDCA (Schwarz, 2004; Chiang, 2009; di Gregorio et al., 2021; Shulpekova et al., 2022; Collins et al., 2023). Both pathways yield amphiphilic molecules with distinct hydrophilic and hydrophobic domains (Hofmann, 1999).

The major primary BAs (CA and CDCA) are predominantly conjugated with glycine and taurine as BSs (Chiang, 2013; Faustino et al., 2016). Conjugation is capable of lowering pKa, increasing hydrophilicity, reducing cytotoxicity, and enhancing solubility under physiological conditions (Faustino et al., 2016; Shulpekova et al., 2022). These properties improve lipid emulsification and facilitate mixed micelle formation in the small intestine (di Gregorio et al., 2021; Collins et al., 2023). Conjugated BAs are stored in the gallbladder during fasting and are released into the duodenum in response to meals (Schwarz, 2004; Chiang, 2009; Faustino et al., 2016). In addition to their digestive role, BAs have also been implicated in stimulating postprandial insulin secretion, thereby contributing to glucose metabolism (Li et al., 2012; Chiang and Ferrell, 2020). In the intestine, gut microbiota deconjugate and transform primary BAs into secondary BAs such as deoxycholic acid (DCA), lithocholic acid (LCA), hyodeoxycholic acid (HDCA) and ursodeoxycholic acid (UDCA) (Faustino et al., 2016; Shulpekova et al., 2022). More recently, gut microbiota are reported to deconjugate and transform primary BAs with amino acids besides glycine and taurine, producing microbial amino-acid-conjugated bile acids (MABAs), although their physiological significance remains to be elucidated (Quinn et al., 2020; Lin et al., 2025).

Approximately 95% of BAs are reabsorbed, with the majority of conjugated BA reuptake occurring in the ileum through the ileal bile acid transporter (IBAT), also known as the apical sodium-dependent bile acid transporter (ASBT), located on the brush border membrane of enterocytes (Chiang, 2009; 2013; Shulpekova et al., 2022). A smaller fraction of protonated, unconjugated BAs are largely absorbed passively along the colon with only about 5% escaping reabsorption for subsequent excretion in feces (Chiang, 2009; 2013; Shulpekova et al., 2022). Around 80% of these primary and secondary BAs then return to the liver from portal blood circulation via sodium taurocholate co-transporting polypeptide (NTCP) activity along the basolateral membrane of hepatocytes (Tan et al., 2024). This crucial reuptake maintains BA levels in systemic circulation and allows for the recycling and reuse of BAs in enterohepatic circulation (Hofmann and Hagey, 2008; Tan et al., 2024). From here, BAs are resecreted into the bile canaliculi via bile salt export pumps (BSEP) to reintegrate into bile (Kubitz et al., 2012). Notably, dysfunction in the regulation or expression of the BA transport proteins IBAT/ASBT, NTCP and or BSEP have been linked with numerous pathologies that will be discussed later in this review. BAs lost to excretion are replenished by de novo synthesis to maintain consistent levels within the body (Chiang, 2009, Ching, 2013).

Upon reentry to circulation in humans, both the conjugated and free forms of BA bind to FXR to regulate further production via one of two pathways (di Gregorio, Cautela and Galantini, 2021; Shulpekova et al., 2022). In the first, BA-bound FXR in ileal enterocytes induces the production of the endocrine peptide fibroblast growth factor 19 (FGF19), which then activates hepatic FGF receptor 4 (FGFR4), resulting in the suppression of CYP7A1 expression and inhibition of the classical BA synthesis pathway (Chiang, 2009; Shulpekova et al., 2022). In the second pathway, BA-bound hepatic FXR forms a complex with retinoid X receptor to produce a small heterodimer partner, leading to suppression of CYP7A1 and CYP8B1 (di Gregorio et al., 2021; Shulpekova et al., 2022). In both pathways, FXR acts as a critical inhibitory receptor to regulate the over or underproduction of BAs (Schwarz, 2004; Chiang, 2009; di Gregorio et al., 2021).

Circulating BAs have also recently been implicated as important signaling metabolites and mediators of neurological disease in both the brain and blood brain barrier (BBB) (Ferrell and Chiang, 2021). The crosstalk between the metabolic functions of the liver, microbial modification and conjugation of BAs in the gut and cellular signaling of BAs within the brain constitute what is known as the gut-liver-brain axis (GLBA) (Ferrell and Chiang, 2021; Sun et al., 2024). Communication between these three systems impact numerous physiological processes including host immunity modulation, metabolism and neurodevelopment (Sun et al., 2024). Blood BAs are also able to cross the BBB to act directly or indirectly with receptors in the brain, though disruption to serum BA levels due to hepatointestinal pathology can influence systemic inflammatory responses, BBB permeability, and neuronal synaptic functioning (Yeo et al., 2023; Sun et al., 2024). Notably, increased serum BA levels have also been found to modulate the permeability of both the BBB through the compromise of tight junctions and gut vascular barrier via the FXR receptor (Quinn et al., 2014; Sun et al., 2024). This suggests interesting implications for the involvement of BA in neurodegenerative disease and its potential role as a therapeutic agent. Current knowledge regarding the mechanisms by which BAs affect the BBB are under investigation.

Dysfunction in the synthesis, storage or circulation of BAs can result in a range of physiological disturbances. Disorders arising from these defects in enterohepatic circulation are generally referred to as cholestatic diseases or cholestasis (Hasegawa et al., 2021; Beuers et al., 2023). This is defined as the impairment of BA flow resulting in a toxic accumulation of BAs and BA metabolites in the liver and systemic circulation (Li and Apte, 2015). Such impairments may be driven by genetic, hormonal, hepatobiliary, metabolic, exogenous, autoimmune or alloimmune factors (Li and Apte, 2015; Sarcognato et al., 2021).

Although the underlying pathophysiology varies, cholestasis can lead to inflammation, fibrosis, cirrhosis, hepatocyte dysfunction, hepatocellular carcinoma, cholangiocarcinoma, and ultimately end-stage liver disease (ESLD) requiring transplantation (Li and Apte, 2015; Sarcognato et al., 2021). Mechanical or structural obstructions to BA flow may further complicate disease progression (Cariello et al., 2022).

In response to rising intrahepatic concentrations of hydrophobic and hepatotoxic BAs, hepatocytes upregulate BA sulfation as a major adaptive detoxification pathway (Alnouti, 2009). Sulfotransferase-mediated conjugation increases BA solubility and reduces membrane-disruptive properties, promoting basolateral efflux and renal elimination (Alnouti, 2009). As a result, sulfated BAs function as a promising marker for distinguishing types of liver injuries and identifying BA overload as well as a protective mechanism to prevent further hepatic damage (Alnouti, 2009; Masubuchi et al., 2016). Dysregulated sulfation has been linked to increased disease severity in cholestasis and contributes to altered immune signaling (Alnouti, 2009; Ito et al., 2024).

Clinically, cholestasis often presents with jaundice due to hepatic insufficiency, persistent fatigue, features of sicca complex, and most notably, pruritus which is reported in up to 80% of affected individuals, which remains one of the most burdensome symptoms and a notable adverse effect of therapeutic BA usage (Beuers et al., 2014; Hirschfield et al., 2017; Beuers et al., 2023). Given this dual unique role of BAs in both disease causation and therapy, understanding their toxic potentials is essential to maximizing benefit and minimizing harm.

While all BAs are amphiphilic, their hydrophobic-hydrophilic balance influences digestive efficacy and cytotoxic potential (Perez and Briz, 2009). The same hydrophobicity that enables effective micelle formation for lipid digestion, also facilitates its interaction with nonpolar cell membranes (Hofmann, 1999). Highly hydrophobic BAs, such as LCA, can disrupt lipid bilayers, increasing membrane permeability and promoting apoptosis (Perez and Briz, 2009). In cholestasis, progressive accumulation of hydrophobic BAs within the bile has experimentally been shown to induce hepatocyte injury, necrosis and subsequent liver damage (Hofmann, 1999; Amaral et al., 2009; Perez and Briz, 2009). This process frequently involves mitochondrial dysfunction characterized by membrane disruption, generation of reactive oxygen species (ROS) and mitochondrial permeability transition (MPT) (Perez and Briz, 2009). As a result of this, the liver relies heavily on BA sulfation which markedly increases hydrophilicity, lowers detergent activity, and enhances solubility in biological fluids to reduce BA overload (Alnouti, 2009).

ROS production activates nuclear factor-κB (NF-κB) signaling via IκB kinase (IKK) activation, driven by upstream mediators such as tumor necrosis factor α (TNF-α) (Lingappan, 2018). Activated NF-κB translocates to the nucleus, promoting transcription of pro-inflammatory, pro-oxidant and anti-apoptotic genes (e.g., Bcl-2 and Bcl-xL) and upregulating inflammatory cytokines such as IL-1β, IL-6 and TNF-α (Catz and Johnson, 2001; Liu et al., 2017; Lingappan, 2018). Notably, prolonged or chronic ROS production can eventually inhibit NF-κB activity, while sustained inflammation further enhances oxidative stress, creating a self-reinforcing cycle of injury (Liu et al., 2017; Forrester et al., 2018).

At pathophysiological concentrations, hydrophobic BAs can also activate transmembrane death receptors such as Fas and TNF-related apoptosis-inducing ligand receptor (TRAILR) (Wei et al., 2020). This recruits the Fas associated death domain (FADD), subsequently leading to the activation of caspases, a family of specialized proteases that execute programmed cell death (Amaral et al., 2009). Caspase-8, the main initiator of death receptor signaling, cleaves cytoplasmic Bid, which activates and induces conformational changes in the Bax and Bak proteins to relay the apoptotic signal to the mitochondria (Amaral et al., 2009; McArthur and Kile, 2018). Bax/Bak pore formation in mitochondrial membranes leads to cytochrome c release into the cytosol, apoptosome assembly with Apaf-1, and activation of caspase-9 and the effector caspases-3, -6, and -7, culminating ultimately in cell death (Amaral et al., 2009; Zhang M. et al., 2017; McArthur and Kile, 2018).

BA activation of TGR5 has been linked to pro-proliferative mechanisms that possess implications in both cancerous and regenerative settings (Pols et al., 2011; Guo et al., 2016). Varied findings have observed that TGR5 was overexpressed in cancers such as esophageal adenocarcinoma but suppressed the proliferation of gastric cancer cells, suggesting a potential tumorigenesis involvement (Guo et al., 2016). Elevated serum BA levels have also been found to enhance the proliferation of cholangiocarcinoma cells in animal models through a proposed TGR5-mediated mechanism (Reich et al., 2016).

The nuclear receptor VDR plays a critical role in hydrophobic BA regulation. Although best known as the mediator of the calcemic hormone 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), VDR is also a high-affinity receptor for the secondary BA, LCA (Makishima et al., 2002; Pavek et al., 2010; Cheng et al., 2014). Activation of VDR by 1,25(OH)₂D₃ or LCA induces the expression of in vivo human cytochrome P450 3A4 (CYP3A4), a critical enzyme involved in the biotransformation of 50% of drugs as well as Vitamin D₃ (Pavek et al., 2010; Kasarla et al., 2022). Although the pregnane X receptor (PXR) is the dominant regulator of CYP3A4 induction in the liver, VDR also transactivates CYP3A4 in the intestine and is broadly expressed across many tissues (Cheng et al., 2014). CYP3A4 hydroxylates LCA, reducing its hydrophobicity while also suppressing intestinal BA transporter expression, inhibiting CYP7A1 to limit further BA synthesis and reducing inflammation through the regulation of the NF-κB pathway (Han and Chiang, 2009; Cheng et al., 2014; Dong et al., 2020). In multiple studies, VDR-deficient mice show markedly worsened LCA-induced hepatotoxicity, altered body and colonic morphology, increased hepatic inflammation and heightened susceptibility to inflammatory bowel disease (IBD) compared to controls, underscoring VDR’s importance in safeguarding against cholestatic injury (Cheng et al., 2014; Dong et al., 2020). Notably, high levels of LCA that activate VDR may limit its practical therapeutic capacity. However, recent studies have explored the potential of LCA derivatives such as LCA acetate which display greater potency and improved safety profiles compared to LCA itself (Adachi et al., 2005; Sasaki et al., 2021).

Hydrophobicity is dictated by the number, degree and position of hydroxylation on the cholesterol backbone, orientation of hydroxyl groups, and the nature of glycine or taurine conjugation (Thomas et al., 2008; Perez and Briz, 2009). Hydrophilic BAs, such as UDCA, glycoursodeoxycholic acid (GUDCA) and tauroursodeoxycholic acid (TUDCA), are favored therapeutically for their lower toxicity and broad clinical utility (Paumgartner and Beuers, 2002; Pusl and Beuers, 2006; Perez and Briz, 2009; Khalaf et al., 2022). In contrast, moderately hydrophobic BAs like CDCA and its semi-synthetic derivative obeticholic acid (OCA) are potent FXR agonists and used to modulate BA synthesis in selected cholestatic conditions (Liu et al., 2014; Zhang Y. et al., 2017). With EC₅₀ values of ∼10 μM for CDCA and ∼100 nM for OCA, the latter’s greater potency has expanded its use despite higher toxicity risk at elevated doses though, notably, OCA has been banned for therapeutic usage in Europe (Zhang Y. et al., 2017; Yang et al., 2024). Hydrophobicity hierarchy (most to least) is: LCA > DCA > OCA > CDCA > CA > HDCA > UDCA > GUDCA > TUDCA (Figure 2) (Heuman, 1989; Perez and Briz, 2009).

Notably, usage of more hydrophobic BAs, such as OCA, is predictably reported to cause BA-induced pruritus (Nevens et al., 2016), making it one of the most frequent and dose-limiting adverse effects of BAs that significantly impair patient quality of life. Although its molecular mechanism remains incompletely understood, our group and others have identified Mas-related G protein-coupled receptor X4 (MRGPRX4), a human GPCR specifically expressed in peripheral itch sensing neurons, as a receptor for BA and bilirubin that potentially mediate BA-induced pruritus (Meixiong et al., 2019; Meixiong, 2025; Yu et al., 2019). However, other studies have questioned this model, reporting that BSs and bilirubin activate MRGPRX4 or other known pruriceptors only at supraphysiological concentrations, and that albumin at physiological levels abolishes such activation, suggesting these metabolites may not directly drive cholestatic itch (Wolters et al., 2025).

Given the harmful effects of cholestasis, certain BAs demonstrate their therapeutic benefits via multiple mechanisms to address a variety of conditions. UDCA in particular is proposed to act through three primary therapeutic mechanisms (Paumgartner and Beuers, 2002).

First, it protects cholangiocytes from BA-induced cytotoxicity by lowering the concentration of highly hydrophobic BAs that can accumulate and cause cellular necrosis in cholangiocytes and hepatocytes (Paumgartner and Beuers, 2002; Perez and Briz, 2009). This reduction also limits the activation of both ligand-dependent and ligand-independent apoptotic death receptors by cytotoxic BA buildup and oxidative stress (Paumgartner and Beuers, 2002; Amaral et al., 2009; Perez and Briz, 2009).

Second, immunomodulative properties activate cell survival signaling pathways that both attenuate apoptosis and prevent mitochondrial dysfunction (Amaral et al., 2009). While unbound FXR exhibits anti-apoptotic effects through the inhibition of caspase-8, BA–activated FXR has also been shown to protect against apoptosis induced by BA overload and deoxysphingolipids, while simultaneously modulating the NF-κB-mediated inflammatory response (Fleishman and Kumar, 2024). This occurs through the promotion of Cytochrome P450 Family 4 Subfamily F (CYP4F)-mediated metabolism of the pro-apoptotic deoxysphingolipids to prevent apoptosis (Fleishman and Kumar, 2024). BA overload is inhibited through the suppression of CYP7A1 and CYP8B1 expression, thereby helping to alleviate cholestasis through the reduction of BA synthesis (Fleishman and Kumar, 2024). BA–activated FXR induces the nuclear receptor small heterodimer partner (SHP), which represses the transcription of these key BA synthetic enzymes, thereby reducing BA accumulation and mitigating apoptosis (Fleishman and Kumar, 2024).

Third, enhanced bile secretion in the hepatocytes and bile duct epithelial cells improve enterohepatic circulation to limit cholestasis (Paumgartner and Beuers, 2002; Amaral et al., 2009; Perez and Briz, 2009).

Recent studies have also elucidated the role of BA-activated TGR5 in NF-κB inflammatory suppression in the liver and stomach. Ligand-bound TGR5 induces a structural shift that activates its coupled Gαs protein and promotes the conversion of ATP to cAMP by adenylate cyclase, resulting in elevated intracellular cAMP levels (Zangerolamo et al., 2025). The increase in cAMP activates protein kinase A (PKA), which plays a key role in suppressing NF-κB signaling by disrupting its activation and limiting inflammatory gene expression (Zangerolamo et al., 2025). TGR5 activation has also been seen to suppress the production of cytokines in both Kupffer and THP-1 cells (Guo et al., 2016). TGR5 can be activated by both conjugated and unconjugated BAs. However, the most potent activators are, ironically, the hydrophobic LCA and DCA, with EC₅₀ values of 0.53 and 1.25 μM respectively, in stark contrast to the significantly weaker activation by UDCA, which has an EC₅₀ of 36.4 μM (Zangerolamo et al., 2025). Notably, evidence also suggests that BA-activated TGR5 may enhance caspase-8 activation, leading to increased apoptotic signaling (Yang et al., 2007). More research is needed to clarify the therapeutic role of BA-activated TGR5 (Figure 3).

BAs have been increasingly implicated in the regulation of glycemic control and overall lipid and glucose metabolism (Zarrinpar and Loomba, 2012; Guo et al., 2016; Cadena Sandoval and Haeusler, 2025). BA activation of TGR5 has been shown to enhance the release of gut-derived incretins such as GLP-1, which stimulate insulin release and suppress glucagon production (Guo et al., 2016; Cadena Sandoval and Haeusler, 2025). Some studies suggest that hydrophilic BAs may act directly on pancreatic β-cells, exerting cytoprotective effects and promoting insulin secretion via TGR5-PKA signaling or through FXR-mediated induction of the TRPA1 ion channel or the FOXA2 transcription factor (Lee et al., 2010; Düfer et al., 2012). However, in vivo mouse models indicate that the predominant metabolic benefits of BAs are likely mediated indirectly through gut hormone release rather than direct β-cell stimulation (Kuhre et al., 2018). Further supporting their metabolic relevance, reduced circulating levels of 12α-hydroxylated BAs, including DCA, have been associated with improved insulin sensitivity in humans (Cadena Sandoval and Haeusler, 2025). TUDCA, an established inhibitor of endoplasmic reticular stress, is also currently being investigated for its role in vascular dysfunction, mitochondrial stabilization, and anti-apoptotic activity (Wu et al., 2024b).

These cytoprotective and metabolic regulatory properties have become the focus of recent research in the potential use of BAs in both metabolic and neurodegenerative diseases, which will be discussed later in this review. Lastly, therapeutic BAs have been suggested to stimulate the liver’s bile secretion by activating signaling pathways that help move transporter proteins such as BSEP and MRP2 to the cell membrane, allowing them to pump bile components more effectively (Paumgartner and Beuers, 2002). Indeed, BAs possess numerous mechanisms by which they are able to potentially treat cholestatic and non-cholestatic conditions. Therefore, this review will primarily focus on the potential therapeutic roles of BAs and their metabolites across a range of reported pathologies, examining specific treatment applications in relation to their corresponding conditions (Table 1).

T2DM is the common, insulin-resistant variant of diabetes mellitus that is commonly associated with obesity, fatty liver disease, cardiovascular issues and presents a major crisis for human health (Nauck et al., 2021). As more research is being conducted on the role of BAs in insulin signaling, energy metabolism and glucose tolerance, BAs have become a promising therapeutic target in the treatment of T2DM.

BA sequestrants, such as colesevelam, are FDA-approved for T2DM and have demonstrated clinically meaningful reductions in HbA1c in meta-analyses of randomized controlled trials, with additional benefits for lipid profiles (Sonne et al., 2014; Hansen et al., 2017). The mechanism is thought to involve modulation of BA signaling through FXR and TGR5, which impacts glucose metabolism and GLP-1 secretion (Sonne et al., 2014).

Novel BA derivatives, such as berberine ursodeoxycholate (HTD1801), have shown promise in recent phase II randomized clinical trials (NCT06411275). HTD1801, an ionic salt of berberine and UDCA, led to significant reductions in HbA1c and improvements in glycemic, cardiometabolic, and liver-related parameters over 12 weeks, with a favorable safety profile (Ji et al., 2025). These effects are attributed to AMP kinase activation, NLRP3 inflammasome inhibition, and improved insulin sensitivity (Ji et al., 2025).

Another recent study identified MABAs as promising clinical markers of glycemic control (Lin et al., 2025). Tryptophan-conjugated CA (Trp-CA) was markedly reduced in patients with T2DM compared with healthy controls, and its administration improved glucose tolerance in diabetic mice (Lin et al., 2025). The GPCR MRGPRE was subsequently identified as the receptor mediating these metabolic effects (Lin et al., 2025). These findings suggest that Trp-CA–MRGPRE signaling may represent a novel therapeutic avenue for T2DM.

Ongoing clinical trials are evaluating additional BA-based therapies, including FXR and TGR5 agonists, and agents that modulate BA transport. Multiple trials on the efficacy and safety of UDCA in T2DM (NCT01337440, NCT05416580, NCT02033876) revealed that treatment resulted in a reduction of HbA1c levels, increased GLP-1 secretion, glucose homeostasis and showed beneficial effects on metabolic and oxidative stress parameters (Shima et al., 2018; Calderon et al., 2020; Lakić et al., 2024). Additional studies (NCT05902468) further exploring UDCA’s therapeutic potential in T2DM are currently underway.

Trials on TUDCA (NCT00771901, NCT03331432) revealed improvements to insulin action and vascular function in participants through the modulation of obesity-related insulin resistance and inhibition of endoplasmic reticulum stress (Amen et al., 2019). A phase II study of OCA (NCT00501592) revealed that a 25–50 mg treatment over 6 weeks led to improved insulin sensitivity, minor loss of body weight and enhanced FGF19 levels in the serum in patients with T2DM and MASLD (Mudaliar et al., 2013). These approaches aim to leverage the hormonal and signaling properties of BAs to improve glucose homeostasis, insulin sensitivity, and reduce inflammation.

CTX is a rare, autosomal recessive disorder of BA synthesis and lipid storage caused by pathogenic mutation to the CYP27A1 gene (DeBarber and Duell, 2021; Islam, Hoggard and Hadjivassiliou, 2021). This leads to reduced levels of primary BAs, particularly CDCA, which in turn disrupts the normal negative-feedback regulation of CYP7A1, allowing for a toxic build up of intermediate metabolites, such as cholestanol, to accumulate in various tissues (Mandia et al., 2019; DeBarber and Duell, 2021; Duell et al., 2023). These deposits commonly aggregate in the central nervous system (CNS), namely the cerebellum, as well as tendons and the lenses of the eyes (Islam et al., 2021). Disease progression can manifest into severe neurocognitive impairment, Parkinsonism, peripheral neuropathy, cerebellar ataxia, dementia, behavioral and psychiatric disturbances, seizures, chronic diarrhea, early onset cataracts and tendon xanthomata if left untreated (Islam et al., 2021; Nóbrega et al., 2022; Kisanuki et al., 2025).

The FDA and European Medicines Agency have approved CDCA for treatment of CTX in adults (Bouwhuis et al., 2023; Jalal et al., 2025). Administration of 250 mg PO TID is generally well tolerated and has been reported to improve neurologic manifestations, slow disease progression and decrease all CTX biomarkers (Nóbrega et al., 2022; Kisanuki et al., 2025). A randomized CDCA withdrawal study on patients with CTX revealed significant increases in metabolite biomarkers when treatment with CDCA was not administered, with 61% of placebo participants requiring rescue medication (Kisanuki et al., 2025). Despite this, around 20% of patients who are administered CDCA for CTX continue to show signs of deterioration, in which case, CA may be utilized as an adjunctive or alternative therapy (Mandia et al., 2019; Nóbrega et al., 2022). Efficacy of CA is noted to significantly reduce cholestanol levels in all patients with no adverse effects reported in one study (Mandia et al., 2019). CDCA treatment does not typically reduce tendon xanthomas nor improve cataracts in patients with CTX and is currently contraindicated for use during pregnancy (Nóbrega et al., 2022). However, one case series of 19 pregnancies noted that no complications were reported in newborns delivered by CDCA-treated birth parents postterm (Duell et al., 2023; Zaccai et al., 2024). CDCA is not currently FDA-approved for usage in pediatric individuals although the current recommended dosing is 5–15 mg/kg/day (Table 3) (Nóbrega et al., 2022).

Hydrophilic BAs, notably UDCA and its conjugates GUDCA and TUDCA, are increasingly recognized for their neuroprotective properties in models of neurological, neurodegenerative, and neuropsychiatric diseases (Zangerolamo et al., 2021; Huang et al., 2022a; Khalaf et al., 2022). Conjugated BAs require active transport into the brain although lipophilic, unconjugated BAs, such as CDCA, are capable of crossing the BBB from peripheral circulation through passive diffusion and exert cytoprotective effects through multiple mechanisms (Mulak, 2021; Zangerolamo et al., 2021; Huang et al., 2022a; Khalaf et al., 2022; Xing et al., 2023). Additionally, they serve as chemical chaperones that support proper protein folding and stability, which is crucial in diseases involving protein misfolding and aggregation (Zangerolamo et al., 2021; Khalaf et al., 2022; Xing et al., 2023).

Preclinical studies demonstrate that UDCA reduces neuronal apoptosis, ROS, and pro-inflammatory cytokine production in models of neurodegeneration, while GUDCA and TUDCA exhibit similar neuroprotective effects across neurological and neuropsychiatric models (Zangerolamo et al., 2021; Huang et al., 2022a). Their therapeutic potential appears to be disease-specific. For instance, all three have shown benefit in Huntington’s disease models, while UDCA and TUDCA are more effective in Parkinson’s (PD) and Alzheimer’s disease (AD) models, and GUDCA in bilirubin encephalopathy (Huang et al., 2022a).

Beyond their direct neuroprotective actions, BAs influence the microbiota-gut–brain axis. Recent animal studies demonstrate that disruptions in the gut microbiota can actively contribute to neurodegenerative disease by reshaping and promoting microglial behavior and activation which, in conjunction with neuroinflammatory responses, are central features of many neurodegenerative disorders (Sampson et al., 2016; Burberry et al., 2020; Wilson et al., 2023; Loh et al., 2024). Although microglial cells exhibit phagocytic properties to eliminate protein aggregates in the brain, excessive uptake of such compounds can lead to neuroinflammation, phagocytic impairment and eventual neurodegeneration (Gao et al., 2023). Because microbial metabolites and signaling pathways exert powerful control over glial activity, the microbiota–gut–brain axis has emerged as a compelling therapeutic target for slowing or modifying the course of neurodegeneration (Cook and Prinz, 2022; Loh et al., 2024).

Importantly, gut microbes modify BA composition through biotransformation, while BAs shape the gut microbial community. Disturbances to bacterial BS hydrolases, which generate deconjugated BAs that are less toxic to the gut microbiota, reshape the BA profile in both the serum and brain, ultimately affecting signaling within the CNS (Wu et al., 2024a). These bidirectional interactions regulate metabolic and signaling pathways that impact neuroinflammation and immune responses relevant to neurodegeneration, and CNS health (Mulak, 2021; Wang et al., 2023; Wu et al., 2024b). Alterations in BA profiles and gut microbiota composition have been observed in AD and underscored by the aforementioned CTX, implicating GLBA crosstalk in neurodegeneration pathophysiology (Mulak, 2021; Wang et al., 2023; Wu et al., 2024b).

In AD, amyloid β (Aβ) plaques and Tau protein aggregates within neurons remain the most recognized biomarkers of disease progression (Wu et al., 2024b). Aβ exposure has been found to activate the early stress c-Jun N-terminal kinase (JNK) pathway which eventually induces apoptosis (Viana et al., 2010). Microtubule affinity–regulating kinase (MARK4), whose overexpression in AD induces synaptic and dendritic spine defects, is also associated with increases in Tau phosphorylation, leading to aggregates (Anwar et al., 2020; Wu et al., 2024b). In terms of BA profiles, LCA-CA and DCA-CA ratios are significantly elevated in reporting studies, with microbiota-derived glyco-DCA/LCA and tauro-LCA also showing significant increases and being associated with reduced cognitive outcomes in patients with AD (Pan et al., 2017; MahmoudianDehkordi et al., 2019).

Therapeutically, CA has been shown to inhibit the Tau phosphorylation activity of MARK4, thereby reducing cognitive deficits and neuronal lesions (Anwar et al., 2020; Wu et al., 2024b). Other MARK4 inhibitors such as donepezil and rivastigmine tartrate are currently undergoing research to explore similar therapeutic potential with promising results (Shamsi et al., 2020). TUDCA attenuates Aβ toxicity by dampening JNK-driven stress signaling, stabilizing mitochondrial membranes, and limiting activation of pro-apoptotic cascades (Wu et al., 2024b). Because clinical antibody therapies against Aβ have shown only modest benefit, these BA-mediated mechanisms offer a complementary therapeutic avenue targeting both Aβ and Tau pathology (Asher and Priefer, 2022). A current phase II trial is underway to evaluate the safety and efficacy of a combination therapy of TUDCA and sodium phenylbutyrate in patients with AD (NCT03533257).

PD by contrast, is a neurodegenerative condition characterized by the progressive loss of catecholamines and cholinergic neurons in conjunction with Lewy body formations composed of aggregated α-synuclein deposits (Hurley et al., 2022). Both UDCA and TUDCA have been observed to inhibit neuroinflammation and reduce glial activity in PD mouse models (Loh et al., 2024). Two recent clinical trials (NCT02967250, NCT03840005) that further evaluate the efficacy of UDCA in PD found that high-dose administration (30 mg/kg/day) was well tolerated in early PD (Payne et al., 2023). Treatment with dioscin, a TGR5 agonist, displayed increased GLP-1 activation and improvements to motor function in mouse models (Sun et al., 2021; Mao et al., 2023; Loh et al., 2024). GLP-1 activity enhances insulin secretion, lowering serum levels of glucose which lead to reduced levels of glial activity (Sun et al., 2021; Mao et al., 2023; Nowell et al., 2023). Co-administration of UDCA and dioscin was shown to enhance the neuroprotective properties of either drug by itself.

Several studies report elevated circulating secondary BAs such as DCA, LCA, and HDCA, and variable but notable changes in primary BAs (Yakhine-Diop et al., 2020; Li et al., 2021; Shao et al., 2021; Hurley et al., 2022). Some studies identify increased plasma CA and elevated glycine- or taurine-conjugated primary BAs, whereas others report reduced glycocholic acid and glycochenodeoxycholic acid in the frontal cortex, highlighting tissue-specific differences that may reflect distinct pathogenic mechanisms (Yakhine-Diop et al., 2020; Shao et al., 2021; Hurley et al., 2022; Kalecký and Bottiglieri, 2023). PD patients and prodromal PD mouse models also show reduced levels of the neuroprotective BAs UDCA and TUDCA, suggesting impaired BA-mediated cytoprotection (Graham et al., 2018; Shao et al., 2021). Additional evidence, including the increased PD risk after cholecystectomy and secondary BA elevations following gut microbiota transfer, supports a role for altered BA signaling within the GLBA in early PD pathogenesis (Kim et al., 2021; Xu et al., 2023).

Despite promising preclinical data, further research is needed to elucidate the molecular mechanisms of BA signaling within the CNS, optimize pharmacokinetics and dosing strategies, and establish efficacy and safety through large-scale clinical trials. Harnessing the neuroprotective and immunomodulatory effects of BAs may open new avenues for treating neurodegenerative and neuropsychiatric disorders, particularly when integrated with targeted delivery systems such as bilosomes.

Bilosomes are BA-stabilized vesicular systems composed of non-ionic surfactants (niosomes), phospholipids (liposomes), and amphiphilic BSs, which confer superior stability, deformability, and protection against enzymatic and acidic degradation compared to conventional liposomes and niosomes (Mitrović et al., 2025). The integration of BSs into the bilayer enhances vesicle integrity in the gastrointestinal tract, promotes mucosal penetration, and facilitates absorption, resulting in increased oral bioavailability of peptides, proteins, and other labile molecules (Aburahma, 2016; Nayak et al., 2023; Mitrović et al., 2025).

Bilosome-based delivery systems demonstrate remarkable versatility across multiple non-invasive administration routes. Surface-modification of bilosomes enhance cell surface receptor identification and ligand detection to allow for improved targeting efficiency and drug delivery to a diverse number of pathologies (Nayak et al., 2023; Priya et al., 2023; Mitrović et al., 2025). In oral drug delivery, bilosomes protect encapsulated agents from gastrointestinal deposition and enhance intestinal absorption (Aburahma, 2016; Nayak et al., 2023; Mitrović et al., 2025). For oral vaccines, bilosomes facilitate efficient antigen uptake via Peyer’s patches, leading to robust mucosal and systemic immune responses, with efficacy demonstrated in preclinical and early clinical studies (Wilkhu et al., 2013; Shukla et al., 2016). For transdermal administration, the ultra-deformable properties of bilosomes allow traversal of the stratum corneum, significantly improving skin penetration and drug deposition (Nayak et al., 2023; Mitrović et al., 2025). In ocular applications, bilosomes enhance corneal permeation, prolong retention time, and improve stability, resulting in better therapeutic availability for ophthalmic conditions (Nayak et al., 2023; Rajbhar et al., 2025). Intranasally, bilosomes protect encapsulated agents from enzymatic degradation, increase mucosal adhesion, and facilitate direct nose-to-brain transport, offering a promising strategy for CNS-targeted therapies and vaccines (Mitrović et al., 2025).

In the context of hepatobiliary diseases, bilosome-based formulations have shown promise in improving the delivery and therapeutic efficacy of agents such as daclatasvir, sofosbuvir, silymarin, and pitavastatin, as well as bioactive polysaccharides th anti-hepatocarcinogenic activity (Matloub et al., 2018; Joseph Naguib et al., 2020; El-Nabarawi et al., 2021). Studies demonstrate that bilosome encapsulation can enhance hepatic targeting, increase drug bioavailability in the liver, and improve pharmacodynamic outcomes in models hepatitis C and hepatocellular carcinoma (Matloub et al., 2018; Joseph Naguib et al., 2020; El-Nabarawi et al., 2021).

Recent advancements include probilosomes and surface-modified bilosomes, which offer improved targeting specificity, formulation stability, and delivery efficiency (Nayak et al., 2023; Rajbhar et al., 2025). These platforms enable incorporation of targeting ligands, including antibodies, carbohydrates or peptides to optimize tissue-specific uptake and therapeutic outcomes (Nayak et al., 2023; Priya et al., 2023; Mitrović et al., 2025; Rajbhar et al., 2025). Polymer coatings such as the positively charged chitosan interact with the negatively charged surface of the bilosome to improve mucoadhesion and protect the vesicles in the gastrointestinal environment (Nayak et al., 2023; Priya et al., 2023; Mitrović et al., 2025). Polyethylene glycol integration increases circulation time and reduces immune recognition while also improving the ability to bypass the first-pass effect and avoid rapid metabolism (Figure 5) (Nayak et al., 2023; Mitrović et al., 2025).

While no bilosome-based therapies have yet received regulatory approval for clinical use, ongoing research is addressing formulation optimization, large-scale manufacturing, and long-term safety. These efforts aim to facilitate clinical translation and expand therapeutic options for diverse diseases.