Urolithin A Choline Salt Type A: A Breakthrough in Bioavailability for Next-Generation Healthy Aging

Urolithin A Choline Salt Type A: A Novel Salt Formulation Designed to Overcome the Absorption Bottleneck

Abstract

Urolithin A (UA), the first identified natural mitophagy inducer, has demonstrated substantial translational potential in anti-aging, muscle function improvement, and immune regulation. However, its extremely poor aqueous solubility and low oral bioavailability severely constrain its clinical efficacy. This study developed a novel Urolithin A choline salt, designated Type A, aiming to address the aforementioned bottlenecks through salt formation and crystal form engineering. In vitro dynamic solubility experiments revealed that the solubility of this salt form in water and simulated intestinal fluid was enhanced by several thousand-fold compared to the free form. In vivo pharmacokinetic studies demonstrated that, at an equimolar dose, the area under the plasma concentration-time curve (AUC) increased by 216%, the peak plasma concentration (Cmax) increased approximately 4-fold, and the time to peak concentration (Tmax) was shortened from 4 hours to 1 hour. X-ray powder diffraction (XRPD) confirmed that this salt form dissociates in aqueous media, releasing free-form UA and generating a high concentration gradient to drive absorption. This study provides robust physicochemical and biological evidence for the efficient oral delivery of Urolithin A.

1. Introduction

As global aging accelerates, the search for interventions capable of delaying functional decline and extending healthspan has become a focal point in biomedical research. Urolithin A (UA) is a postbiotic metabolite produced by gut microbiota from ellagitannins and ellagic acid, and is widely found in the metabolic products of pomegranates, berries, and nuts. Since the landmark study published in Nature Medicine in 2016, abundant evidence has shown that Urolithin A can specifically induce mitophagy through both PINK1/Parkin-dependent and independent pathways, clearing dysfunctional mitochondria and restoring cellular energy metabolic homeostasis.

Clinical studies have further confirmed that Urolithin A supplementation significantly improves muscle endurance, grip strength, and mitochondrial biomarkers in middle-aged populations, and has recently demonstrated potential in reversing immune senescence and enhancing T-cell proliferation. Nevertheless, the clinical application of Urolithin A faces a "dual barrier": first, inter-individual differences in gut microbiota mean that only approximately 40% of individuals can effectively obtain sufficient Urolithin A through dietary conversion; second, even with direct supplementation, conventional free-form Urolithin A (Free Form UA) exhibits extremely low solubility, with oral bioavailability often less than 1%, making it difficult for target tissues (such as skeletal muscle and brain tissue) to reach the effective concentration threshold required to induce mitophagy.

To overcome this absorption bottleneck, salt formation—a classic medicinal chemistry strategy—provides a viable pathway. In this study, choline, which exhibits excellent biocompatibility, was selected as the counterion. The Urolithin A choline salt was successfully prepared, and through systematic polymorph screening, the thermodynamically stable "Type A" polymorph with optimal solubility performance was identified. This article systematically reports the in vitro physicochemical properties, solid-state transformation behavior, and in vivo pharmacokinetic characteristics in rats of this novel salt form, aiming to provide a scientific basis for the development of next-generation high-efficiency Urolithin A formulations.

2. The Absorption Challenge and Physicochemical Barriers of Urolithin A

Urolithin A belongs to the benzo-coumarin class of polyphenolic compounds. Although the multiple phenolic hydroxyl groups in its molecular structure confer antioxidant activity, they also lead to strong intermolecular hydrogen bonding, resulting in a tightly packed crystal lattice and extremely low water solubility.

According to literature and preliminary experimental data, the equilibrium solubility of conventional free-form Urolithin A in a simulated human intestinal environment (FaSSIF, pH 6.5) is only 0.28 µg/mL, and in pure water it is even below the limit of quantification (<0.195 µg/mL). This "poorly soluble" characteristic directly leads to "poorly absorbed": after oral administration, the drug cannot fully dissolve within the limited residence time in the gastrointestinal tract, and the majority is excreted as solid particles in feces. Even for the small fraction that dissolves, the lack of a supersaturation driving force results in extremely low transmembrane transport efficiency.

Furthermore, the activation of mitophagy is concentration-dependent. If the drug concentration in blood and tissues remains chronically below the threshold, not only will the intracellular autophagy pathway fail to be triggered, but it may also lead to a lack of perceived clinical efficacy, i.e., consumers "feeling no effect." Therefore, developing a formulation that can rapidly release high concentrations of free UA in the gastrointestinal tract is a prerequisite for realizing its biological effects.

3. Technological Innovation: Design Rationale of the Choline Salt Type A

To address the aforementioned pain points, we adopted a dual strategy of "salt formation crystal form engineering":

Salt Formation Strategy: Choline was selected as the counterion. Choline is not only an important component of cell membrane phospholipids but also a precursor of the neurotransmitter acetylcholine, possessing extremely high safety and biocompatibility. The acidic phenolic hydroxyl groups of Urolithin A bind to the quaternary ammonium base groups of choline through ionic bonds, disrupting the original strong intermolecular hydrogen bond network and significantly reducing lattice energy, thereby enhancing the thermodynamic driving force for dissolution.

Polymorph Screening: The same salt may exist in multiple crystal forms (polymorphism), with significant differences in dissolution rate and stability. Through high-throughput crystallization screening, we obtained a specific polymorph designated "Type A." This polymorph exhibits characteristic diffraction peaks in the XRPD pattern and demonstrates excellent physicochemical stability in accelerated stability tests, making it the optimal solid form for formulation development.

4. In Vitro Physicochemical Properties and Solid-State Transformation Studies

4.1 Dynamic Solubility Evaluation

Solubility is the prerequisite for absorption. We conducted dynamic solubility tests in simulated gastric fluid (FaSSGF, pH 1.6), simulated intestinal fluid (FaSSIF, pH 6.5), and pure water.

Table 1: Comparison of Dynamic Solubility of Free-Form UA and Choline Salt Type A in Different Media

Note: LOQ = 0.195 µg/mL; data derived from Reference [1].

The data indicate that, in simulated intestinal fluid, the solubility of the choline salt Type A is more than 10-fold higher than that of the free form. More remarkably, in pure water, the choline salt Type A reached a concentration of approximately 1.54 mg/mL at 0.5 hours, whereas the free form was virtually insoluble. This order-of-magnitude enhancement in solubility provides a tremendous concentration gradient potential for in vivo absorption.

4.2 Solid-State Transformation and Dissociation Mechanism

A critical scientific question is: after the highly soluble salt form enters an aqueous environment, will it precipitate as poorly soluble free-form UA, thereby reducing absorption? We characterized the residual solids after dissolution equilibrium using X-ray powder diffraction (XRPD).

Figure 1: XRPD comparison of residual solids after 2 h agitation in water for free-form Type A and choline salt Type A

Figure description: This pattern displays three curves. The black line represents the standard pattern of free-form Type A with specific characteristic peaks; the blue line represents the original pattern of choline salt Type A, with peak positions distinctly different from the free form, confirming it as a new polymorph; the pink line represents the pattern of residual solids after choline salt Type A was agitated in water for 2 hours. The results show that the diffraction peak positions of the pink line completely overlap with the black line (free form) and are entirely different from the blue line (choline salt).

This result reveals the "reactive dissolution" mechanism of choline salt Type A: upon contact with aqueous media, the ionic bonds on the crystal surface of the salt form rapidly dissociate, releasing high concentrations of free Urolithin A molecules into solution, while the residual solid transforms into the thermodynamically more stable free-form Type A. This process creates a localized "high-concentration microenvironment" within the formulation, continuously driving drug molecules to diffuse into the intestinal lumen, effectively avoiding absorption stagnation caused by supersaturation precipitation.

5. In Vivo Bioavailability and Pharmacokinetic Evaluation

The advantages of in vitro data must ultimately be validated in vivo. We commissioned Huantai Biology (Zebrafish Biotechnology) to conduct a single-dose oral gavage pharmacokinetic (PK) study in a Sprague-Dawley (SD) rat model.

5.1 Experimental Design

• Animal Model: Male SD rats, 6 weeks of age, n=6/group.
• Dosing Regimen: After 12 hours of fasting, rats were administered Urolithin A (20 mg/kg) and Urolithin A choline salt (29 mg/kg, equimolar equivalent of Urolithin A) via oral gavage, respectively.
• Analytical Method: Plasma total Urolithin A concentration was determined by LC-MS/MS (after enzymatic hydrolysis).

5.2 Pharmacokinetic Results

Figure 2: Mean plasma concentration-time curves after oral administration of Urolithin A and Urolithin A choline salt in rats

Figure description: The graph plots time (0–24 h) on the x-axis and plasma concentration (ng/mL) on the y-axis. The black circle-connected line represents the conventional Urolithin A group, with a flat curve and relatively low peak; the red square-connected line represents the choline salt Type A group, with a steep rise and a peak significantly higher than the black group, and the Tmax is markedly shifted to the left. The substantial difference in the area under the two curves intuitively demonstrates the enhancement in bioavailability.

Key PK parameters are summarized below:

Table 2: Comparison of Oral Pharmacokinetic Parameters in Rats (Mean ± SD, n=6)

Note: Data derived from Reference [2], Tables 1–3.

5.3 Interpretation of Results

Doubled Total Absorption: The relative bioavailability reached 216%, meaning that under the premise of ingesting the same molar amount of active ingredient, the choline salt Type A can deliver more than 3-fold (3.16-fold) active molecules to the organism. This is decisive for Urolithin A, which requires reaching a specific threshold to activate mitophagy.

Surge in Absorption Rate: Tmax was advanced from 4 hours to 1 hour, indicating that the choline salt rapidly dissociates and dissolves in the gastrointestinal tract without requiring a lengthy disintegration and dissolution process. This is particularly critical for scenarios such as post-exercise recovery that demand rapid response.

Breakthrough in Peak Concentration: Cmax increased approximately 4-fold, directly addressing the pain point of "low tissue distribution concentration," ensuring that the drug can effectively penetrate the blood-brain barrier or enter muscle cells to exert neuroprotective and muscle-enhancing effects.

6. Product Applications and Fields

Based on the significant advantages of Urolithin A choline salt Type A in physicochemical properties and in vivo pharmacokinetics, this novel salt form provides robust technical support for translating Urolithin A from the laboratory to the consumer market. Regarding product development and application, the following fields are the primary targets:

6.1 Sports Nutrition and Anti-Fatigue Supplements

Given that the Tmax of this salt form was substantially shortened from 4 hours to 1 hour and Cmax increased approximately 4-fold, it is highly suitable for development as a "pre-workout" or "post-workout immediate recovery" supplement. The rapid peak characteristic means that active molecules can rapidly enter the bloodstream during the exercise window, efficiently activating mitophagy in skeletal muscle cells, accelerating lactate metabolism and muscle damage repair. Capsule or instant powder formulations are recommended to meet the dual demands of sports populations for rapid onset and portability.

6.2 Interventions for Sarcopenia and Anti-Aging in Middle-Aged and Elderly Populations

For individuals with sarcopenia and frailty in older adults, absorption efficiency is the key determinant of clinical intervention success. The 216% relative bioavailability of choline salt Type A means that, to achieve equivalent target tissue effective concentrations, the single-dose administration can be substantially reduced. This not only significantly lowers the long-term cost for consumers but also reduces the risk of gastrointestinal irritation potentially associated with high-dose polyphenols. It is recommended to develop as a once-daily sustained-release capsule or composite nutritional powder, combined with protein intake, to form a synergistic anti-aging regimen of "promoting anabolism clearing senescence."

6.3 Neuroprotection and Cognitive Health Dietary Supplements

Urolithin A holds potential value in penetrating the blood-brain barrier and inducing mitophagy in brain tissue. As a precursor of the neurotransmitter acetylcholine, choline, when combined with Urolithin A and dissociated in vivo, can simultaneously provide dual neurotrophic support. It is recommended to formulate it with brain-health ingredients such as Omega-3 and B-complex vitamins to develop composite brain-health products targeting high-pressure mental workers and individuals in the early stages of cognitive decline.

7. Discussion and Conclusion

In summary, Urolithin A choline salt Type A is a novel solid form combining high stability, high solubility, and high bioavailability. It not only resolves the "bottleneck" problem in Urolithin A development but also lays a solid formulation foundation for subsequent clinical research in sarcopenia, neurodegenerative diseases, and immune senescence. The development of Urolithin A choline salt Type A represents not merely a technical breakthrough for the solubility problem of a single active ingredient, but also an important practice in the formulation science of natural products. As human clinical data continue to be generated and the product portfolio expands, this novel salt form is expected to reshape the market landscape of Urolithin A, truly bringing the anti-aging benefits of "mitophagy" to the general public.

References

[1] Internal Research Report. Solubility and Polymorph Characterization of Urolithin A Choline Salt Type A. Dynamic Solubility Testing in Biorelevant Media and Water; Figure 8: XRPD comparison of residual solids after 2 h agitation in water for free-form Type A and choline salt Type A.

[2] Huantai Biology (Zebrafish Biotechnology). [202605190001] In Vivo Relative Bioavailability Test Report-2 (Urgent). June 30, 2026. Project No.: 202605190001.

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