Isoquercitrin

The strongest and most distinctive line, and the reason isoquercitrin has a human file at all. Quercetin flavonoids inhibit extracellular protein disulfide isomerase (PDI) — an enzyme released by activated platelets and endothelium that is required for thrombus formation — and isoquercetin was chosen for the human program because it is better absorbed than rutin, the molecule that first flagged the mechanism ^{7Reference 7Zwicker JI et al. · 2019Clinical trialTargeting protein disulfide isomerase with the flavonoid isoquercetin to improve hypercoagulability in advanced cancer — phase-II clinical trialView study →}.

A human proof-of-mechanism study showed PDI inhibition blocks thrombin generation by interfering with platelet factor-V activation ^{6Reference 6Stopa JD et al. · 2017Protein disulfide isomerase inhibition blocks thrombin generation in humans by interfering with platelet factor V activation — human proof-of-mechanismView study →}.

In the phase-II CATIQ trial in advanced-cancer patients, plain isoquercetin at 1,000 mg/day for 56 days cut plasma D-dimer by ~22%, platelet-dependent thrombin generation by ~57% and soluble P-selectin by ~58%, with no VTE events or major bleeds ^{7Reference 7Zwicker JI et al. · 2019Clinical trialTargeting protein disulfide isomerase with the flavonoid isoquercetin to improve hypercoagulability in advanced cancer — phase-II clinical trialView study →}.

An independent phase-II trial in sickle-cell disease (1,000 mg/day for 28 days) likewise reduced whole-blood coagulation, collagen-induced platelet aggregation, monocyte tissue-factor expression and plasma PDI reductase activity, again with no off-target bleeding ^{8Reference 8Lizarralde-Iragorri MA et al. · 2024RCTIsoquercetin for thromboinflammation in sickle cell disease: a randomized double-blind placebo-controlled trial — phase-II clinical trialView study →}.

Gap: both trials are small, biomarker-endpoint phase-II studies — they establish target engagement and safety, not a proven reduction in clinical thrombosis; and the agent is plain isoquercetin, so the read-through to dietary isoquercitrin exposure is limited.

The one other human signal — but read the form carefully. In a randomized, double-blind, placebo-controlled trial in Japanese-cedar pollinosis, enzymatically-modified isoquercitrin (EMIQ) — a semi-synthetic, glucose-chain-extended derivative engineered for better solubility and absorption — at 100 mg/day for 8 weeks significantly reduced total ocular and ocular-itching scores versus placebo ^{9Reference 9Kawai M et al. · 2009RCTEffect of enzymatically modified isoquercitrin, a flavonoid, on symptoms of Japanese cedar pollinosis: a randomized double-blind placebo-controlled trial — clinical trial (EMIQ)View study →}.

The anti-allergic rationale is quercetin-class mast-cell / histamine-release inhibition, delivered by a better-absorbed glycoside ^{1Reference 1Valentová K et al. · 2014ReviewIsoquercitrin: pharmacology, toxicology, and metabolism — reviewView study →3Reference 3Owczarek-Januszkiewicz A et al. · 2022ReviewEnzymatically Modified Isoquercitrin: Production, Metabolism, Bioavailability, Toxicity, Pharmacology, and Related Molecular Mechanisms — reviewView study →}.

Gap: the tested molecule is EMIQ, not plain isoquercitrin — its improved pharmacokinetics are exactly the point, so the result cannot be transferred to native isoquercitrin at face value; and the benefit was on ocular, not the full symptom, endpoints ^{3Reference 3Owczarek-Januszkiewicz A et al. · 2022ReviewEnzymatically Modified Isoquercitrin: Production, Metabolism, Bioavailability, Toxicity, Pharmacology, and Related Molecular Mechanisms — reviewView study →9Reference 9Kawai M et al. · 2009RCTEffect of enzymatically modified isoquercitrin, a flavonoid, on symptoms of Japanese cedar pollinosis: a randomized double-blind placebo-controlled trial — clinical trial (EMIQ)View study →}.

Isolated isoquercitrin is cytoprotective against oxidative injury, with two reasonably resolved mechanisms. In a mouse intestinal ischemia-reperfusion model, isoquercitrin limited injury by remodeling gut flora and inhibiting NLRP3-inflammasome activation (lower caspase-1, IL-1β, IL-6) ^{10Reference 10Xu H et al. · 2025AnimalIsoquercitrin mitigates intestinal ischemia-reperfusion injury by regulating intestinal flora and inhibiting NLRP3 inflammasome activation — mouse in vivo animal modelView study →}.

In human endothelial (EA.hy926) cells, it protected against hydrogen-peroxide-induced apoptosis via the PI3K/Akt/GSK3β survival axis ^{11Reference 11Zhu M et al. · 2016In vitroIsoquercitrin inhibits hydrogen peroxide-induced apoptosis of EA.hy926 cells via the PI3K/Akt/GSK3β signaling pathway — in vitroView study →}.

Reviews add antioxidant activity through direct radical scavenging and Nrf2/HO-1 induction across many models ^{1Reference 1Valentová K et al. · 2014ReviewIsoquercitrin: pharmacology, toxicology, and metabolism — reviewView study →} — the same antioxidant capacity that makes it horsetail’s flavonoid marker ^{13Reference 13Mimica-Dukić N et al. · 2008In vitroPhenolic compounds in field horsetail (Equisetum arvense L.) as natural antioxidants — in vitro (isoquercitrin as marker)View study →}.

Gap: all cell and rodent work; the in-vivo relevance of the concentrations used is limited by the fact that isoquercitrin is hydrolyzed to quercetin before absorption, so systemic exposure is to quercetin metabolites, not the intact glucoside ^{4Reference 4Sesink AL et al. · 2003AnimalIntestinal uptake of quercetin-3-glucoside in rats involves hydrolysis by lactase phlorizin hydrolase — rat in situView study →5Reference 5Day AJ et al. · 2003AnimalAbsorption of quercetin-3-glucoside and quercetin-4′-glucoside in the rat small intestine: the role of lactase phlorizin hydrolase and the sodium-dependent glucose transporter — rat in situView study →}.

Isolated isoquercitrin shows antidiabetic activity across preclinical models — inhibition of carbohydrate-handling and polyol-pathway enzymes (α-glucosidase, aldose reductase), improved glucose handling and antioxidant protection of pancreatic and vascular tissue — collated in the pharmacology reviews ^{1Reference 1Valentová K et al. · 2014ReviewIsoquercitrin: pharmacology, toxicology, and metabolism — reviewView study →2Reference 2Orfali GC et al. · 2016ReviewReview of anticancer mechanisms of isoquercitin — reviewView study →}. Within the database this fits its presence in the antioxidant leaf-flavonoid fraction of stevia and the flavonoid fraction of goldenrod ^{15Reference 15Chodera A et al. · 1991AnimalEffect of flavonoid fractions of Solidago virgaurea L. on diuresis and levels of electrolytes — rat in vivo animal modelView study →}.

Gap: entirely cell and animal data at concentrations the poor oral bioavailability makes hard to reach systemically; there is no isolated-isoquercitrin human metabolic trial, and any effect is confounded with quercetin’s ^{1Reference 1Valentová K et al. · 2014ReviewIsoquercitrin: pharmacology, toxicology, and metabolism — reviewView study →4Reference 4Sesink AL et al. · 2003AnimalIntestinal uptake of quercetin-3-glucoside in rats involves hydrolysis by lactase phlorizin hydrolase — rat in situView study →}.

The broadest but thinnest tier. Isoquercitrin is one of four principal leaf flavonoids from houttuynia shown to attenuate H1N1-influenza-induced pneumonia in mice, with gut-bacterial biotransformation shaping the active species ^{12Reference 12Tang J et al. · 2023AnimalTherapeutic effects on H1N1-induced pneumonia in mice and intestinal bacteria biotransformation of four main flavonoids from Houttuynia cordata Thunb. — mouse in vivo animal modelView study →}, and reviews catalogue antiviral (including flaviviral) and general anti-inflammatory activity for the isolated molecule across cultured systems ^{1Reference 1Valentová K et al. · 2014ReviewIsoquercitrin: pharmacology, toxicology, and metabolism — reviewView study →2Reference 2Orfali GC et al. · 2016ReviewReview of anticancer mechanisms of isoquercitin — reviewView study →}.

Gap: most of this is in-vitro or mixed-flavonoid animal work at high concentrations; the influenza study tested four flavonoids together rather than isoquercitrin alone, and there are no human antiviral or anti-inflammatory data for the isolated molecule ^{1Reference 1Valentová K et al. · 2014ReviewIsoquercitrin: pharmacology, toxicology, and metabolism — reviewView study →12Reference 12Tang J et al. · 2023AnimalTherapeutic effects on H1N1-induced pneumonia in mice and intestinal bacteria biotransformation of four main flavonoids from Houttuynia cordata Thunb. — mouse in vivo animal modelView study →}.

A rare instance of this molecule being tested as an isolate against a named pathogen. Screening methanol, dichloromethane and cyclohexane extracts of four Alchemilla species — A. glabra, A. monticola, A. fissa and A. viridiflora — against ten Helicobacter pylori clinical isolates plus the ATCC 43504 reference strain, the methanol extracts were the most active, with minimum inhibitory concentrations reaching 4 µg/mL. Two constituents were identified and tested separately: ellagic acid, strongly active at 0.125–0.5 µg/mL, and quercetin-3-O-β-glucoside, active at 2–16 µg/mL ^{16Reference 16Krivokuća M et al. · 2015In vitroAnti-Helicobacter pylori activity of four Alchemilla species (Rosaceae) — in vitroView study →}.

Isolate vs. plant: both the extracts and the two isolated constituents were assayed, which is the informative design — and it shows the glucoside is genuinely active, while the ellagitannin-derived phenolic acid beside it is roughly ten-fold more so. The plant material is Alchemilla, not one of the herbs this molecule is a pharmacopoeial marker for.

Gap: in-vitro susceptibility testing only. No animal or human eradication data exist for the isolated compound, and the pharmacokinetics work against it — the 3-O-glucoside is hydrolysed at the small-intestinal brush border, so an orally dosed molecule does not survive as such to reach a gastric pathogen; a topical-to-the-stomach exposure from a crude preparation is a different proposition from a systemic dose ^{4Reference 4Sesink AL et al. · 2003AnimalIntestinal uptake of quercetin-3-glucoside in rats involves hydrolysis by lactase phlorizin hydrolase — rat in situView study →5Reference 5Day AJ et al. · 2003AnimalAbsorption of quercetin-3-glucoside and quercetin-4′-glucoside in the rat small intestine: the role of lactase phlorizin hydrolase and the sodium-dependent glucose transporter — rat in situView study →16Reference 16Krivokuća M et al. · 2015In vitroAnti-Helicobacter pylori activity of four Alchemilla species (Rosaceae) — in vitroView study →}.