Fisetin Encyclopedia — Scientific Background

What Is Fisetin?

Fisetin (CAS 528-48-3), also known as 3,3′,4′,7-tetrahydroxyflavone, 5-desoxyquercetin, or 7-hydroxyflavonol, is a naturally occurring flavonol polyphenol belonging to the flavonoid class of plant secondary metabolites. It is chemically characterized by the molecular formula C₁₅H₁₀O₆ and a molecular weight of 286.24 g/mol. Fisetin is a 7-hydroxyflavonol with additional hydroxy groups at positions 3, 3′, and 4′. It is an orally bioavailable naturally occurring polyphenol found in many fruits and vegetables, with potential antioxidant, neuroprotective, anti-inflammatory, antineoplastic, senolytic, and longevity-promoting activities.

Botanical Sources

Fisetin is found in various fruits and vegetables, with strawberries containing the highest concentration. Other dietary sources include apples, persimmons, grapes, onions, and cucumbers. The primary commercial source for high-purity fisetin extraction is the smoke tree (Cotinus coggygria Scop.), a plant traditionally used in Chinese medicine. Fisetin is extracted from the branches and bark of the smoke tree through processes including alcohol extraction and purification.

Mechanisms of Action

Senolytic Activity — Bcl-2 and Bcl-xL Inhibition

Fisetin's most distinctive mechanism of action is its senolytic activity—the ability to selectively target and eliminate senescent cells. The senolytic properties were examined in terms of Bcl-2 and Bcl-xL inhibition. The results indicated that fisetin not only binds effectively to these proteins but also, with appropriate modifications, may exhibit specific selectivity toward either target. By inhibiting these anti-apoptotic proteins, fisetin promotes the death of senescent cells while sparing healthy cells. Preclinical studies demonstrate that fisetin reduces senescence-related gene expression, including lower abundance of Cdkn1a and Ddit4 in skeletal muscle.

Senescence-Associated Secretory Phenotype (SASP) Modulation

Senescent cells release pro-inflammatory cytokines, chemokines, proteases, and other factors collectively known as the senescence-associated secretory phenotype (SASP), which contribute to systemic aging and age-associated disorders. Fisetin supplementation has been shown to reverse age-related endothelial dysfunction, with responses reflected in circulating CXCL12 concentrations—identifying the SASP and CXCL12 as drivers of age-related endothelial dysfunction and establishing mechanisms of senolytic intervention with fisetin supplementation.

Anti-Inflammatory and Antioxidant Activity

Fisetin inhibits pro-inflammatory mediators such as tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and nuclear factor kappa B (NF-κB). As an antioxidant, fisetin scavenges free radicals, protects cells from oxidative stress, and is able to upregulate Glutathione.

Neuroprotective Effects

Fisetin exerts neuroprotective effects through modulation of key cellular pathways, including mTOR, Wnt/β-catenin, NF-κB, and TRAIL-induced apoptosis, making it a multitargeted pharmacological agent. It has demonstrated the ability to protect against oxidative stress and neurodegeneration in preclinical models.

Sirtuin Regulation and Longevity Promotion

Fisetin promotes cellular metabolism, reduces senescence, and regulates sirtuin function, with potential longevity-promoting activities.

The Bioavailability Challenge

Despite its broad pharmacological potential, fisetin presents significant formulation challenges. The compound has poor water solubility (10.45 μg/mL), high lipophilicity (LogP 3.2), and limited oral bioavailability (approximately 44.1%), which significantly restrict its clinical applications. Fisetin also suffers from reduced blood-brain barrier permeability, further limiting its neuroprotective potential.

These limitations have driven the development of advanced delivery systems, including liposomes, nanoemulsions, ethosomes, glycerosomes, polymeric micelles, and polymeric nanoparticles. Among these, liposomal encapsulation has emerged as one of the most effective strategies for enhancing fisetin bioavailability and therapeutic efficacy.

Liposomal Fisetin

Liposomal encapsulation of fisetin provides a fundamentally different delivery mechanism from conventional forms. The phospholipid bilayer surrounds the fisetin molecule, forming a protective shell that enhances solubility, stability, and cellular uptake. This delivery system:

• Enhances fisetin solubility and dispersibility in aqueous systems

• Protects fisetin from degradation during digestive transit

• Improves intestinal absorption through lipid-mediated pathways

• Provides targeted delivery to senescent cells and tissues

• Achieves up to 7.71-fold higher relative oral bioavailability compared to free fisetin

A PEG/R8 co-modified fisetin liposome formulation (PEG/R8-Fis-Lips) was prepared using a post-insertion method with DSPE-mPEG2000-R8. Pharmacokinetic results showed that the Cmax of PEG/R8-Fis-Lips was enhanced to 604.05 ± 166.73 ng/mL, half-life (t1/2) was prolonged to 5.04 ± 0.63 hours, and relative bioavailability was enhanced to 7.71-fold compared to free fisetin. The formulation also alleviated acetaminophen-induced acute liver injury, demonstrating enhanced therapeutic efficacy.

A separate liposomal fisetin formulation using DOPC and DODA-PEG2000 via thin-film hydration achieved a diameter of 173.5 ± 2.4 nm and demonstrated a 47-fold increase in relative bioavailability compared to free fisetin following intraperitoneal administration in mice.

Clinical Evidence

Knee Osteoarthritis

A Phase I/II randomized, double-blind, placebo-controlled clinical trial evaluated the safety and efficacy of fisetin for treating knee osteoarthritis in individuals aged 40–80 with radiographically confirmed knee osteoarthritis (K-L grades II-IV). Subjects were randomized to receive oral fisetin (3 dosing cycles, each consisting of 20 mg fisetin per kg body weight for 2 consecutive days, followed by 28 days off) or matching placebo capsules over a 12-month study period. Comprehensive data collection included safety assessments, changes in cartilage condition via 3T MRI T2 relaxation, and pain scores. Most adverse events were minor and resolved without intervention, including joint pain, nausea, headache, and dry mouth, with no significant differences in adverse event incidence between groups.

Physical Function and Aging

Preclinical studies have demonstrated that intermittent oral treatment with fisetin (1 week on—2 weeks off—1 week on) improves frailty and grip strength in old mice, with improvements comparable to genetic-based clearance of excess p16+ senescent cells and treatment with the synthetic senolytic ABT-263. Fisetin had no effects in young mice. The improvements in frailty and grip strength were accompanied by favorable modulation of the skeletal muscle transcriptome, including lower abundance of cellular senescence-related genes (Cdkn1a and Ddit4).

Chronic Inflammation

A triple-blind, randomized, placebo-controlled trial is evaluating low-dose fisetin supplementation (100 mg once daily for 7 weeks) and its association with chronic inflammation in middle-aged and older adults aged ≥50 years.

Ongoing Clinical Trials

Ongoing clinical trials are evaluating fisetin across multiple therapeutic areas, including:

• The TROFFi study: Phase II randomized placebo-controlled study of fisetin (20 mg/kg/day on days 1–3 of a 14-day cycle for four cycles) to improve physical function in breast cancer survivors.

• The STOP-Sepsis trial: Multicenter randomized adaptive allocation clinical trial evaluating fisetin as a single oral dose of 20 mg/kg or two oral doses of 20 mg/kg each spaced 1 day apart in 220 elderly patients (age >65) with sepsis.

• A clinical trial in adults with GLILD receiving fisetin 20 mg/kg or placebo on days 0–1 and 28–29.

Safety and Regulatory

Fisetin is generally well tolerated at commonly used doses. A Phase I/II clinical trial in knee osteoarthritis patients reported 185 adverse events, most of which were minor and resolved without intervention or long-term consequences, including joint pain, nausea, headache, and dry mouth. There were no significant differences in adverse event incidence between fisetin and placebo groups (all p > 0.5). Self-reported adverse side effects were generally mild and included dizziness, gastrointestinal distress, fatigue, headache, and elevated creatine kinase.

However, caution is necessary when considering the timing of fisetin treatment and choice of target populations, given that cellular senescence may have beneficial roles in certain contexts. In young female mice, fisetin failed to improve metabolism, cognitive function, or fertility, and detrimental effects were observed in some studies, including increased expression of SASP factors and decreased cognitive performance.

The toxicological profile of fisetin includes safety concerns such as genotoxicity at higher concentrations. Fisetin is widely commercialized as a dietary supplement ingredient and is available in the United States, Europe, and other regions. Testing protocols must adhere to international standards including USP, European Pharmacopoeia, and FAO/WHO Codex Alimentarius. Formulators should verify local regulatory requirements prior to market entry.

This product is intended for research, nutraceutical, and functional food applications. Not intended to diagnose, treat, cure, or prevent any disease.

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