IngredientsTriacetyluridine (TAU) is formally designated 2′,3′,5′-tri-O-acetyluridine, reflecting the systematic triacetylation of the three hydroxyl groups on the ribose ring of the nucleoside uridine. Its IUPAC name is [(2R,3R,4R,5R)-3,4-diacetyloxy-5-(2,4-dioxopyrimidin-1-yl)oxolan-2-yl]methyl acetate, with the molecular formula C15H18N2O9 and a molecular weight of 370.311 g/mol. The compound carries the CAS registry number 4105-38-8, and is known under the synonyms PN401, RG2133, 2′,3′,5′-Tri-O-acetyluridine, and Uridine Triacetate. In clinical pharmacology, its adopted International Non-proprietary Name (INN) is uridine triacetate, and it was formerly known under the investigational name vistonuridine.
Uridine triacetate is an orally active tri-acetylated prodrug of uridine, approved for two distinct indications: treatment of hereditary orotic aciduria (brand name Xuriden) and emergency treatment following overdose of the chemotherapy drugs 5-fluorouracil (5-FU) or capecitabine (brand name Vistogard). It was developed, manufactured, and distributed by Wellstat Therapeutics.
Triacetyluridine itself does not occur in nature as a free compound; it is a semi-synthetic derivative of uridine, produced by chemical acetylation of all three free hydroxyl groups on the ribose moiety. Uridine—the parent nucleoside and active moiety—is a naturally occurring compound found throughout biology. Uridine is a pyrimidine nucleoside that plays an essential role in the synthesis of RNA and other key physiologic processes, and has been used as a treatment for patients with hereditary orotic aciduria for more than four decades. Uridine is primarily found in sugar beets, sugarcane, tomatoes, yeast (especially the types used to make beer), organ meats, and broccoli. Human breast milk is notably rich in uridine, hinting at its importance in early brain development. Uridine in human plasma exists at concentrations of 3–8 µM, which is substantially greater than other circulating nucleosides, and is predominantly produced by the liver or adipose tissue for utilization across other tissues.
In pharmaceutical and research settings, triacetyluridine is a white to off-white crystalline solid. It is more lipid-soluble than uridine and resistant to degradation by uridine phosphorylase; it is cleaved by plasma esterases in vivo to release uridine. As an approved pharmaceutical product, it is formulated as oral granules. Xuriden (for hereditary orotic aciduria) is available as oral granules in 2-gram single-use packets. Vistogard (for fluorouracil/capecitabine overdose) is also formulated as oral granules. As a dietary supplement, TAU has been commercially available as the product NucleomaxX®, originally derived from sugar cane extract. Product analysis of NucleomaxX® documented that more than 90% of the nucleoside component is in the form of triacetyluridine (TAU).
Triacetyluridine as a discrete chemical entity has no pre-modern ethnobotanical or traditional medicinal history. It is a semi-synthetic 20th-century compound. However, the clinical and therapeutic history of its parent nucleoside, uridine, extends several decades.
Uridine has been used as a treatment for patients with hereditary orotic aciduria for more than four decades. Its therapeutic potential has also been assessed in a diverse group of clinical disorders including cystic fibrosis, liver dysfunction, chemotherapy toxicity, pervasive developmental delay, schizophrenia, epilepsy, and diabetes-induced peripheral neuropathy, and more recently as a treatment for mitochondrial dysfunction associated with treatment with nucleoside reverse transcriptase inhibitors in patients with HIV infection.
The development of triacetyluridine as a prodrug was specifically motivated by the historically poor oral bioavailability of uridine itself. To overcome these bioavailability limitations, triacetyluridine (TAU, PN401) was developed as a prodrug of uridine, exhibiting significantly higher oral bioavailability and representing a novel mechanism for delivering exogenous pyrimidines to the brain and other tissues. Early clinical interest, beginning in the 1990s and accelerating in the 2000s, was driven by its potential to counteract fluoropyrimidine chemotherapy toxicity, treat hereditary metabolic disease, and support neurological function. Testing of PN401 was undertaken in the United States, sponsored by Wellstat Therapeutics, in HIV-negative people with cancers as well as in others with degenerative diseases of the nervous system, such as Alzheimer's and Parkinson's diseases.
Triacetyluridine is not itself biologically active in the primary sense; its pharmacological actions are attributable almost entirely to its rapid hydrolysis to uridine after oral administration. Uridine triacetate is an acetylated prodrug of uridine. Following oral administration, it is deacetylated by nonspecific esterases present throughout the body, yielding uridine in the circulation. The three acetyl groups on the ribose ring serve two critical pharmacokinetic functions: they substantially increase lipophilicity, improving intestinal absorption, and they confer resistance to uridine phosphorylase, the enzyme primarily responsible for the pre-systemic degradation of orally administered free uridine. TAU is more lipid-soluble than uridine and resistant to degradation by uridine phosphorylase.
Once liberated from TAU, uridine itself participates in a rich set of biochemical pathways. It is a pyrimidine nucleoside consisting of the nucleobase uracil linked to a ribose sugar. Uridine plays an essential role in the synthesis of RNA and other key physiologic processes. Within cells, uridine is phosphorylated to form uridine monophosphate (UMP), uridine diphosphate (UDP), and ultimately uridine triphosphate (UTP). The uridine is taken up and phosphorylated to form uridine triphosphate (UTP) inside cells; UTP then dilutes and effectively competes with fluorouridine triphosphate (FUTP) for incorporation into RNA. Uridine triphosphate is also the direct precursor for the synthesis of cytidine triphosphate (CTP), meaning TAU supplementation can elevate the intracellular pool of multiple pyrimidine nucleotides.
The purinergic system uses uridine triphosphate and diphosphate (UTP; UDP) as transmitters; this system is involved in modulating behavioral motivation and reward, as well as neuroregeneration. Uridine receptors are expressed on all primary cell types within the central nervous system, including neurons, astrocytes, microglia, and endothelium. In the mammalian brain, uridine nucleotides are the native agonist at four specific purinergic receptors: P2Y2, P2Y4, P2Y6, and P2Y14.
Uridine also plays a central role in phospholipid biosynthesis. It is required for the CDP-choline (Kennedy) pathway, which produces phosphatidylcholine—a dominant phospholipid component of neuronal cell membranes—and related phospholipid species critical for synaptic membrane integrity.
The principal mechanism by which TAU exerts its effects is as a high-bioavailability vehicle for uridine delivery. Historically, the use of uridine as a therapeutic agent has been limited by poor bioavailability. In a pharmacokinetic study of orally administered uridine, the maximum tolerated dose was 10–12 g/m2 for a single dose and 5 g/m2 for a multiple-dose regimen (every 6 hours for 3 days), resulting in peak serum uridine concentrations of 60–80 µM after a single dose and a steady-state serum uridine level of 50 µM after multiple doses, with diarrhea as the dose-limiting side effect. By contrast, single and repeated dosing with the TAU-rich supplement NucleomaxX® resulted in peak plasma uridine concentrations 1–2 hours later of 150.9 ± 39.3 µM and 161.4 ± 31.5 µM, respectively—levels known to ameliorate mitochondrial toxicity in vitro.
In the context of fluorouracil (5-FU) and capecitabine toxicity, TAU's mechanism is well characterized. 5-FU and capecitabine cytotoxicity is mediated by their conversion to intracellular fluorouridine nucleotides: 5-fluorouridine triphosphate (FUTP) is misincorporated into RNA, and fluorodeoxyuridine monophosphate (FdUMP) irreversibly inhibits thymidylate synthase (TS), disrupting DNA synthesis, particularly in rapidly dividing cells. Uridine triacetate overcomes the poor oral bioavailability of uridine, delivering and achieving steady-state uridine plasma concentrations of at least 70 µM necessary to protect and rescue normal cells and tissues from 5-FU toxicity. The uridine is taken up and phosphorylated to form UTP inside cells; UTP dilutes and effectively competes with FUTP for incorporation into RNA. Timely administration of uridine triacetate thereby counteracts potentially life-threatening fluoropyrimidine toxicity.
In hereditary orotic aciduria, TAU's mechanism is straightforward replacement therapy. Hereditary orotic aciduria is caused by a defect in uridine monophosphate synthase (UMPS), a bifunctional enzyme that catalyzes the final two steps of the de novo pyrimidine biosynthetic pathway in mammalian cells. The defect in UMP synthase has two primary biochemical consequences: the blockade of de novo UMP synthesis results in a systemic deficiency of pyrimidine nucleotides, accounting for most clinical consequences of the disease. Following oral administration, uridine triacetate is deacetylated by nonspecific esterases present throughout the body, yielding uridine in the circulation—providing uridine to patients who cannot synthesize adequate quantities due to a genetic defect in uridine nucleotide synthesis.
A third mechanism relevant to TAU's neurological and psychiatric research applications involves cellular bioenergetics. Increased intracellular pH values were observed in depressed subjects with bipolar disorder who responded to treatment with triacetyluridine compared to non-responders, an observation that suggests triacetyluridine might improve mitochondrial function in bipolar depression by facilitating oxidative phosphorylation. In preclinical models, uridine has been proposed to support the Krebs cycle and to provide substrate for mitochondrial energy production, particularly under conditions of metabolic stress.
This is the area with the strongest and most clinically decisive evidence, culminating in FDA approval.
On December 11, 2015, the FDA approved uridine triacetate (Vistogard; Wellstat Therapeutics Corporation) for the emergency treatment of adult and pediatric patients following a fluorouracil or capecitabine overdose regardless of the presence of symptoms, and of those who exhibit early-onset, severe, or life-threatening toxicity affecting the cardiac or central nervous system, and/or early-onset, unusually severe adverse reactions (e.g., gastrointestinal toxicity and/or neutropenia) within 96 hours following the end of fluorouracil or capecitabine administration.
Study Design and Results: The approval was based on data from two single-arm, open-label, expanded-access trials in 135 patients receiving uridine triacetate (10 g or 6.2 g/m2 orally every 6 hours for 20 doses) for fluorouracil or capecitabine overdose, or who exhibited severe or life-threatening toxicities within 96 hours following the end of fluorouracil or capecitabine administration. Ninety-six percent of patients met the major efficacy outcome measure, which was survival at 30 days or survival until the resumption of chemotherapy, if prior to 30 days. The most common adverse reactions were vomiting, nausea, and diarrhea.
Of those who overdosed, 97% treated with uridine triacetate were still alive after 30 days, and 89% were alive at 30 days when treated for toxicities. These 30-day survival rates compare very favorably with the historical rate of 16% in a cohort of patients who received best supportive care following fluorouracil overdose.
Evidence Strength: The evidence for this indication is strong and regulatory-grade. While the studies were single-arm (no placebo control), the clinical context (life-threatening overdose) made a randomized placebo-controlled design ethically untenable. The comparison with historical controls showing approximately 16% survival without treatment underlines the magnitude of the effect. The FDA granted Vistogard breakthrough therapy designation prior to approval.
Uridine triacetate is not recommended for the non-emergent treatment of adverse reactions associated with fluorouracil or capecitabine because it may diminish the efficacy of these drugs, and the safety and efficacy of uridine triacetate initiated more than 96 hours following the end of administration of these drugs has not been established.
Xuriden (uridine triacetate) is an orally administered pyrimidine analog uridine replacement product for the treatment of patients with hereditary orotic aciduria—a rare autosomal recessive disorder caused by a defective or deficient enzyme resulting in the body being unable to normally synthesize uridine, a necessary component of RNA. The condition is characterized by retarded growth, anemia, and excessive excretion of orotic acid in the urine.
The FDA approved the pyrimidine analog uridine triacetate (Xuriden) for treatment of hereditary orotic aciduria, a rare autosomal recessive disorder (estimated birth prevalence: <1:1,000,000) in infants and children caused by a deficiency in uridine 5′-monophosphate (UMP) synthase. This deficiency prevents synthesis of uridine nucleotides and causes developmental delays, failure to gain weight, hematologic abnormalities, and excessive urinary excretion of orotic acid, which can lead to urinary obstruction. Xuriden was the first drug to be approved in the US for this indication.
Study Design and Results: The FDA approval of Xuriden was based on an open-label study in 4 patients with hereditary orotic aciduria (3 male, 1 female; age range from 3 to 19 years). Clinical trials investigating this medication showed improvement in anemia and disappearance of megaloblastosis, a decrease in orotic acid levels in the urine, and affected individuals also showed improvement in or remained stable in gaining weight or growth.
Evidence Strength: Given that hereditary orotic aciduria has a birth prevalence of fewer than 1 in 1,000,000, fewer than 25 cases of patients with this disorder worldwide have been reported in the medical literature. The clinical evidence base is therefore very small (n=4) by design, not by choice. The FDA granted approval under orphan drug provisions, weighing biological plausibility, the absence of alternative treatments, and the clinical improvements observed. Long-term safety and efficacy data are limited but ongoing.
TAU has been investigated as a potential intervention in bipolar depression, with a theoretical basis in the putative role of mitochondrial dysfunction and pyrimidine nucleotide depletion in the neurobiology of the disorder.
Key Human Study (Jensen et al., 2008): Eleven patients with bipolar depression were given doses of up to 18 g per day of triacetyluridine (TAU) over 6 weeks to test the effect of uridine on symptoms of depression via the Montgomery-Åsberg Depression Rating Scale (MADRS) and on cellular bioenergetics using phosphorus magnetic resonance spectroscopic imaging (31P-MRSI). The percentage changes for MADRS scores were: Week 2, −23.8%; Week 3, −34.9%; Week 4, −42.5%, with statistically significant time effects of TAU on MADRS scores at Weeks 2, 3, and 4. TAU responders (patients who had a 50% or greater reduction in MADRS scores from baseline at any time) demonstrated a significant difference from non-responders in pH changes from baseline.
It was suggested that TAU treatment may decrease symptoms of depression and improve mitochondrial functioning, and that the time effects of TAU on MADRS scores may reflect TAU effects on early symptom improvement.
A related open-label case series examined uridine (not TAU directly) in adolescents. This report was an open-label case series of seven depressed adolescents with bipolar disorder treated with uridine for 6 weeks. Treatment response was measured with the Children's Depression Rating Scale-Revised and the Clinical Global Impressions scale. Uridine was associated with decreased depressive symptoms and was well tolerated by study participants, with the authors concluding that further systematic studies of uridine are warranted.
Evidence Strength: This evidence is preliminary and weak. The human studies are small (n=11 and n=7), open-label, lack placebo controls, and involve a drug (TAU) ultimately converted to the same active moiety (uridine) shared by multiple supplements. No large randomized controlled trials of TAU specifically for bipolar depression have been published. The spectroscopic findings regarding brain pH are biologically interesting but require replication.
A clinically important area of investigation has been the use of uridine supplementation—delivered via TAU-rich products—to address the mitochondrial toxicity of nucleoside reverse transcriptase inhibitors (NRTIs) used in HIV treatment.
Uridine is a therapy for hereditary orotic aciduria and is being investigated in other disorders caused by mitochondrial dysfunction, including toxicities resulting from treatment with nucleoside reverse transcriptase inhibitors in HIV. A randomized, double-blind, placebo-controlled trial of NucleomaxX® (the TAU-rich supplement) for the treatment of HIV-associated lipoatrophy reported a significant increase in limb fat, intra-abdominal fat, and total body fat. A single case report also described a remarkable salutary effect of only four days of treatment with NucleomaxX®, ameliorating the mitochondrial toxicity caused by stavudine and leading to improvements in myalgias, liver and muscle enzymes, lactate levels, and steatosis, despite continuing treatment with stavudine.
However, the optimal therapeutic dose and plasma or intracellular uridine levels for treatment of NRTI-associated toxicity are not known.
Evidence Strength: Limited but promising. The randomized controlled trial of NucleomaxX® in HIV-associated lipoatrophy provides the best evidence, but it studied a mixed supplement whose active component was identified as TAU only upon later analysis. The mechanistic rationale is strong (NRTIs are pyrimidine analogs that deplete mitochondrial pyrimidines, and replenishing uridine is theoretically restorative), but dedicated RCTs of pure TAU in this setting have not been published.
In preclinical research, TAU (6% in the diet) decreases neurodegeneration in the piriform cortex and striatum, reduces the amount of huntingtin-positive aggregates, and increases BDNF protein levels in the piriform cortex in a transgenic mouse model of Huntington's disease. It also improves rotarod performance and increases survival in transgenic mouse models of Huntington's disease.
The orally bioavailable uridine prodrug PN401 demonstrated protective effects in both R6/2 and N171-82Q transgenic mouse models of Huntington's disease. Treatment with PN401 prolonged survival and improved motor performance in both models.
PN401 has also been shown to protect in the mitochondrial complex I inhibitor MPTP mouse model of Parkinson's disease, with decreased loss of striatal dopamine and dopaminergic neurons in the substantia nigra pars compacta projecting to the striatum.
Evidence Strength: Preclinical only (animal and cell studies). No published randomized controlled trials in human patients with Huntington's or Parkinson's disease have been reported for TAU specifically. The preclinical findings are mechanistically coherent and generated initial research interest, but their translation to humans remains undemonstrated.
The core pharmacokinetic superiority of TAU over oral uridine has been demonstrated in a human PK study. NucleomaxX®, containing predominantly TAU, has significantly greater bioavailability than pure uridine in human subjects and may be useful in the management of mitochondrial toxicity. The demonstration that the nucleoside content of NucleomaxX® is predominantly TAU may explain why such remarkable clinical results have been obtained with NucleomaxX® as the source of uridine supplementation.
The therapeutic potential of uridine has been assessed in a diverse group of clinical disorders including cystic fibrosis, liver dysfunction, chemotherapy toxicity, pervasive developmental delay, schizophrenia, epilepsy, and diabetes-induced peripheral neuropathy. However, for most of these conditions, dedicated clinical trial evidence for TAU specifically (as opposed to free uridine or uridine-containing supplements) is sparse or absent in the published literature.
VISTOGARD was administered at 10 grams orally every 6 hours for 20 doses, or at a body surface area–adjusted dosage of 6.2 grams/m2/dose for 20 doses for patients between 1 and 7 years of age. The FDA urges doctors to instruct patients to take uridine triacetate as soon as possible after an overdose—even if the patient does not display any symptoms—or at the early onset of life-threatening toxicity.
The recommended starting dosage of Xuriden is 60 mg/kg once daily. The dose can be increased to 120 mg/kg (maximum 8 grams) once daily if urine orotic acid levels increase or remain above normal, or for worsening disease. The granules are administered with food (applesauce, pudding, or yogurt) or in milk or infant formula.
In the bipolar depression clinical study, eleven patients with bipolar depression were given doses of up to 18 g per day of triacetyluridine (TAU) over 6 weeks.
In the pharmacokinetic study of the TAU-rich supplement NucleomaxX®, uridine serum levels were followed in individuals for 24 hours after a single dose of 36 grams of Mitocnol (a supplement derived from sugar cane used as the source of TAU).
In pharmacokinetic studies of pure oral uridine (the parent compound, not TAU), the maximum tolerated dose was 10–12 g/m2 for a single dose and 5 g/m2 for a multiple-dose regimen. Because TAU is more bioavailable, lower absolute amounts of TAU are required to achieve equivalent or greater plasma uridine concentrations.
Uridine triacetate is well tolerated, with adverse events being generally mild in severity. The most common adverse reactions observed in the fluorouracil overdose trials were vomiting, nausea, and diarrhea. In the Xuriden safety assessment—4 patients with hereditary orotic aciduria ranging in age from 3 to 19 years who received 60 mg/kg of XURIDEN once daily for six weeks and then continued for at least 24 months at dosages of up to 120 mg/kg once daily—no adverse reactions were reported.
The most clinically important safety concern is the potential for TAU to diminish the anti-tumor efficacy of fluorouracil and capecitabine when used outside the emergency overdose context. The FDA does not recommend uridine triacetate for non-emergency adverse reactions associated with fluorouracil or capecitabine because uridine triacetate may lessen the efficacy of these chemotherapy drugs.
In vitro enzyme inhibition data did not reveal meaningful inhibitory effects of uridine triacetate or uridine on CYP3A4, CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1. In vitro enzyme induction data also did not reveal an inducing effect of uridine triacetate or uridine on CYP1A2, CYP2B6, or related enzymes. In vitro data showed that uridine triacetate was a weak substrate for P-glycoprotein. This profile suggests a low potential for pharmacokinetic drug-drug interactions mediated by CYP enzymes or the P-glycoprotein efflux transporter under typical clinical conditions.
There are no available data on Xuriden (uridine triacetate) use in pregnant women to inform a drug-associated risk. When administered orally to pregnant rats during the period of organogenesis, uridine triacetate at doses similar to the maximum recommended human dose of 120 mg/kg per day was not teratogenic and did not produce adverse effects on embryo-fetal development.
Orally administered uridine triacetate did not affect fertility or general reproductive performance in male animals in pre-clinical testing.
A study in healthy adult subjects receiving uridine triacetate granules under fed and fasted conditions showed no difference in the pharmacokinetics of the released uridine, indicating that TAU can be taken with or without food without meaningful impact on systemic uridine exposure. This is consistent with the approved Xuriden labeling instruction to mix granules with food for palatability.
TAU is resistant to degradation by uridine phosphorylase, the enzyme that normally catabolizes free uridine rapidly after oral ingestion and contributes to its poor bioavailability. This property is pharmacologically advantageous, but it also means that the amount of uridine ultimately entering the systemic circulation from a given TAU dose is substantially greater than from an equimolar dose of free uridine—a consideration relevant to dose selection in research and clinical settings.
Circulating levels of uridine are tightly regulated in the range of 3–8 µM, both by the liver, which synthesizes and degrades uridine, and by erythrocytes that store UDP-glucose, which may be catabolized to provide glucose and uridine. Supraphysiologic uridine concentrations achieved after TAU administration are transient, as the liver's metabolic regulation ultimately returns plasma uridine toward baseline. Uridine is a key constituent in human breast milk and is compounded as an ingredient into commercial infant formula products, providing a long-term historical context for the general safety of uridine at physiological-to-supraphysiological concentrations in humans.
Health conditions that Triacetyluridine may help support.
Body systems that Triacetyluridine may help support.
Go back