IngredientsApocynin (4-hydroxy-3-methoxyacetophenone) is a major active ingredient from the rhizomes of Picrorhiza kurroa, a botanical plant used as an herbal medicine for treatment of a number of inflammatory diseases. Its systematic IUPAC name is 4-hydroxy-3-methoxyacetophenone; it is also known by the trivial name acetovanillone. Apocynin was first described by Schmiedeberg in 1883 and was isolated from the roots of Apocynum cannabinum (Canadian hemp), and extracts of it were used as official remedies for dropsy and heart troubles.
Apocynin is a naturally occurring methoxy-substituted catechol, experimentally used as an inhibitor of NADPH-oxidase. The hydrophobicity index has been calculated as logP = 0.83, and the oxidation potential determined by cyclic voltammetry is Epa = 0.76 V.
Apocynin was discovered during activity-guided isolation of immunomodulatory constituents from Picrorhiza kurroa, an endangered medicinal plant native to the mountains of India, Nepal, Tibet and Pakistan. Picrorhiza kurroa Royle ex Benth is an important medicinal herbaceous plant belonging to the Scrophulariaceae/Plantaginaceae family, native to the Himalayan regions of India, China, Nepal, and Pakistan. It grows in the hilly regions of the North-Western Himalayan region from Kashmir to Kumaun and Garhwal regions of India and Nepal, found at 3,000–5,000 meters of elevation, growing in rocky crevices and moist, sandy soil.
Apocynin has also been characterized as an essentially nontoxic phenolic compound isolated from the medicinal plant Jatropha multifida, demonstrating that the compound occurs across more than one plant genus. The underground parts (stolons and roots) of P. kurroa contain various ethnopharmacologically active substances including picrosides, picroliv, kutkoside, kutkin, cucurbitacins, and apocynin.
Apocynin is a low-molecular-weight phenolic acetophenone. It is commercially available as a purified powder for research purposes. In its traditional botanical context, it occurs within whole-plant rhizome extracts. Extracts from the roots are used in the Ayurvedic medical tradition of India and Sri Lanka for the preparation of ethnic medicines for the treatment of ailments of liver, heart, joints, and lungs; a 95% ethanolic root extract has been used in laboratory preparations subjected to activity-guided purification. In research and early clinical investigation, apocynin has been formulated as an oral preparation (dissolved in drinking water or administered by oral gavage) and as an inhaled nebulized solution. In one clinical study, 6 ml of apocynin at a total dose of 3 mg (0.5 mg/ml dissolved in sterile 0.9% NaCl) was nebulized for 15–20 minutes through a mouthpiece.
Picrorhiza kurroa (the plant source of apocynin) has been used to treat liver diseases, upper respiratory tract disorders, chronic diarrhea, scorpion sting and fever in the Ayurvedic system of medicine. Picrorhiza kurroa is an important herb in the traditional Chinese and Ayurvedic systems of medicine, used to treat liver and upper respiratory conditions. This drug is known to be in medicinal use since approximately 5,000 years ago.
The plant is known in Ayurveda under the Sanskrit name Katuka or Katuki. Katuki is a well-known herb for its hepatoprotective action and its wide range of pharmacological activities; it is known to help in removing excess fire energy from the body, thus acting as a cooling agent. Other common names include hu huang lian (China), titka kul (Ayurvedic), kutki, katuka, kutka (Hindu, Sanskrit), and kadu (Gujarati).
In traditional Chinese medicine, Picrorhiza has been used to treat hyperemia and dysentery, hemorrhoids, epilepsy, and carbuncles.
Roots of Picrorhiza kurroa are used therapeutically in traditional medicine of almost all Asian countries to treat a manifold of conditions of illness including liver, lung, and spleen ailments. It is important to note that these traditional uses apply to the whole-plant extract rather than to isolated apocynin specifically, since the phytochemical isolation of apocynin as a discrete compound is a product of modern analysis.
The plant has several medicinal properties due to the presence of bioactive components including Picroside I and Picroside II, cucurbitacins, and phenolic components. Kutkin is the active principle of P. kurroa and is comprised of kutkoside and the iridoid glycoside picrosides I, II, and III. Other active constituents include drosin, apocynin, and nine cucurbitacin glycosides, whereas catechol-apocynin has been found active as an anti-inflammatory agent and cucurbitacins for antitumor and cytotoxic effects.
Apocynin is a naturally occurring methoxy-substituted catechol isolated from the roots of Picrorhiza kurroa. Structurally, it is a simple derivative of acetophenone carrying a para-hydroxyl group and a meta-methoxy group on the phenyl ring. Apocynin is a weak free radical scavenger as measured using DPPH, peroxyl radical, and nitric oxide assays when compared to protocatechuic acid, used as a reference antioxidant. On the other hand, apocynin was more effective than protocatechuic acid as a scavenger of the non-radical species hypochlorous acid.
Apocynin is the most employed inhibitor of NADPH oxidase (NOX), a multienzymatic complex capable of catalyzing the one-electron reduction of molecular oxygen to the superoxide anion. The structure of NADPH oxidase is quite complex, consisting of two membrane-bounded elements (gp91phox and p22phox), three cytosolic components (p67phox, p47phox, and p40phox), and a low-molecular-weight G protein (either rac2 or rac1); activation of NADPH oxidase is associated with the migration of the cytosolic components to the cell membrane so that the complete oxidase can be assembled.
It is accepted that the intracellular mechanism of NADPH oxidase inhibition by apocynin is related to blocking the assembly of this multicomponent enzyme complex by preventing the translocation of the cytosolic fractions p47-phox and p67-phox to the membrane fraction; increasing evidence has also demonstrated that apocynin inhibits the expression of NOX components such as p47-phox, p67-phox, and gp91-phox.
Apocynin can decrease the production of superoxide (O₂⁻) from activated neutrophils and macrophages while the ability of phagocytosis remains unaffected. Apocynin is a selective inhibitor of NADPH-oxidase activity and concomitant ROS production, with an IC50 value of 10 μM in activated human neutrophils.
Apocynin seems to act as a prodrug, since it must be initially oxidized into its dimeric form, diapocynin, the supposedly active form of apocynin. In vitro studies indicated conversion of apocynin to diapocynin in the presence of peroxidases, e.g., myeloperoxidase, posing the possibility that diapocynin also contributes to the anti-oxidative action of apocynin. Metabolism in vivo converts the phenol into active metabolites that inhibit the enzyme, which may be due to peroxidase catalysis leading to disruption of the p47phox–p22phox interaction, which is required for translocation of the cytosolic enzyme components to the membrane leading to activation of the enzyme complex.
In vitro studies of NADPH oxidase activity identified a trimer hydroxylated quinone (IIIHyQ) that inhibited NADPH oxidase with an IC50 of 31 nM, indicating that oxidative metabolites of apocynin may be far more potent than the parent molecule. Apocynin itself possessed minimal inhibitory activity in some cell-free in vitro systems, underscoring the prodrug concept. However, this interpretation remains debated: it has been concluded that apocynin was not significantly converted to diapocynin in vivo under some experimental circumstances, and apocynin did not convert to its dimeric form diapocynin in gastric, intestinal, or physiological fluids, or in microsomal stability studies.
Apocynin reacted promptly with the non-radical reactive species H₂O₂ only in the presence of peroxidase; this represents a new pathway for depleting H₂O₂ in cellular experimental models, besides the direct inhibition of NADPH oxidase, and could be relevant for its application as an inhibitor of NOX4, since this isoform produces H₂O₂ and not superoxide anion.
A recent additional mechanism of action associated with apocynin involves increased NO expression and activity. Treatment with apocynin normalized overexpression of NOX2 and its subunit p47phox in aortas of spontaneously hypertensive rats (SHR), and the authors suggest apocynin increases NO bioavailability by different mechanisms, restoring the proper function of vascular endothelium.
Apocynin constitutes a potent inhibitor of NAT activity in rat livers both in vivo and in vitro. Following virtual modelling analyses, thiol functionalities in NATs are hypothesized to play an important role for apocynin binding to the enzyme. The biological significance of this interaction in humans remains under investigation.
Apocynin was rapidly absorbed after oral administration at 50 mg/kg in rats, with peak plasma levels achieved within 5 minutes; plasma levels were observed up to 48 hours, and the bioavailability of apocynin was found to be 8.3%. In vitro plasma protein binding was found to be 83.41–86.07% in rat plasma and 71.39–73.34% in human plasma; apocynin was found stable in gastric (pH 1.2), intestinal (pH 6.8), and physiological (pH 7.4) fluids, including microsomal stability studies.
A study on the bioavailability of apocynin showed that apocynin is rapidly metabolized into glucuronic conjugate. The binding parameters calculated by fluorescence quenching showed that apocynin binds to human serum albumin (HSA) with a binding affinity of 2.19 × 10⁴ M⁻¹.
Oral bioavailability has been reported as low by some groups. Compared with a nitrone derivative (AN-1), apocynin yielded a much shorter half-life (6.1 min vs. 179.8 min) and far lower area under the curve after equimolar intravenous dosing; the absolute bioavailability of oral apocynin was only 2.8% in this study, although this figure differs from other reports, reflecting methodological variability across studies.
Human/Clinical Evidence: This is one of the few areas where human data exist. Peters et al. evaluated the therapeutic potential of inhaled apocynin on ozone-induced bronchial hyperresponsiveness to methacholine in asthmatic patients as a model of inflammatory lung disease; the authors excluded scavenging of ozone by apocynin and concluded that the effect was mitigating ROS production by PMNs and eosinophils that had infiltrated the lung upon ozone exposure. In that clinical trial, the ozone-induced change in PC₂₀ after placebo treatment was −1.94 ± 0.39 doubling doses, while after apocynin it was −0.6 ± 0.33 doubling doses; the difference between apocynin and placebo treatment was 1.3 doubling doses (p = 0.02). Ozone-induced maximal FEV₁ fall was 11.9 ± 1.5% during placebo inhalation compared to 3.85 ± 1.8% during apocynin; apocynin reduced this fall by 8.05% compared to placebo, markedly reducing ozone-induced hyperreactivity and maximal airway narrowing, suggesting that apocynin may have a role in preventing ozone-induced exacerbations of asthma.
A randomized, quadruple-masked, crossover Phase 1 clinical trial (NCT00992667) was registered in which ten nonsmoking patients suffering from mild bronchial asthma (mean age 30 ± 9 years, 5 men, 5 women) participated. In a COPD clinical trial (NCT01402297), apocynin caused a decrease of NO₂⁻ concentration at 30, 60, and 120 minutes after apocynin inhalation compared to placebo; no influence on safety parameters and no adverse effects were observed.
Small-scale early-stage clinical trials for apocynin were conducted for COPD in 2011 and asthma in 2012 but they did not progress any further.
Preclinical Evidence: In male BALB/c mice exposed to cigarette smoke from 9 cigarettes per day, 5 days a week for up to 24 weeks, apocynin treatment at 5 mg/kg/day (intraperitoneal injection) reduced airway neutrophil infiltration by 42% and completely preserved endothelial function and eNOS availability against the oxidative insults of cigarette smoke exposure.
Evidence strength: Preliminary. Two small human clinical trials demonstrated short-term benefit of inhaled apocynin on specific respiratory endpoints, but neither progressed to larger confirmatory trials. The preclinical evidence in rodent COPD models is consistent, but extrapolation to clinical practice is not yet warranted.
Parkinson's Disease: NADPH oxidase appears to be especially important in the modulation of redox-sensitive signaling pathways and has been implicated in neuronal dysfunction and degeneration, and neuroinflammation in diseases ranging from stroke, Alzheimer's and Parkinson's diseases to psychiatric disorders.
In a primate model, treatment with apocynin at 100 mg/kg three times daily started one week before PD induction with MPTP; apocynin limited the typical body weight loss associated with the parkinsonian syndrome, motor function in the apocynin-treated monkeys was improved, indicating anti-Parkinson efficacy, and the number of surviving dopamine neurons was increased, indicating neuroprotective efficacy.
Oral administration of diapocynin (an oxidative metabolite of apocynin) attenuated key neuroinflammatory events including microglial and astroglial activation, iNOS upregulation, and oxidative and nitrative damage in an MPTP mouse model of PD; diapocynin treatment also protected against nigral dopaminergic neuronal damage and behavioral deficits in the animal model.
Mice injected with paraquat and maneb displayed impairments of spatial learning and memory associated with reduced tyrosine hydroxylase expression and increased neurodegeneration; apocynin treatment significantly ameliorated learning and memory deficits as well as hippocampal neurodegeneration and α-synuclein pathology.
Alzheimer's Disease: Lull et al. tested apocynin at a daily oral dose of 10 mg/kg via the drinking water in an hAPP(751)SL transgenic mouse model of AD; they observed in apocynin-treated mice a significant reduction of plaque size within cortex and hippocampus and a reduction of microglia numbers in the cortex, but performance in the Morris water maze swim test, which tests spatial memory organized in the hippocampus, was not markedly improved by the treatment.
Human evidence: To the knowledge of reviewers writing in 2014, apocynin had not been tested in human neurodegenerative disease patients. This remains the situation as reported in the peer-reviewed literature.
Evidence strength: Preclinical only (animal and in vitro studies). The evidence from rodent and primate models is promising and mechanistically coherent, but there are no human clinical trials for neurodegeneration. Evidence is insufficient to draw conclusions about clinical effectiveness.
Reactive oxygen species (ROS) are generated by cell metabolism of oxygen and represent signaling molecules playing an active role in vascular biology; in pathological conditions including hypertension, a ROS excess together with reduced endogenous antioxidant defenses determines a state of oxidative stress; NAD(P)H oxidase is a major ROS source within the vasculature, and a large body of literature has demonstrated that hypertension-associated vascular functional and structural changes are attributable to Nox-driven intravascular ROS generation.
Treatment with apocynin significantly reduced the mean arterial pressure in spontaneously hypertensive rats (SHR); in addition, apocynin improved the impaired acetylcholine hypotensive effect on SHR; and although systemic oxidative stress was high in SHR, SHR treated with apocynin and normotensive rats presented similar systemic oxidative stress levels.
In atherosclerosis-prone apoE-deficient mice, apocynin suppressed the progression of atherosclerosis, decreased 4-hydroxynonenal-positive area in atherosclerotic lesions, and mRNA expression of monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6) in aorta.
Treatment of isolated rat hearts with apocynin before ischemia and at reperfusion showed a significant decrease (p < 0.05) in the levels of TNF-α, IL-1β, and IL-6, compared to the respective untreated controls.
Data in humans are scanty and deserve future consideration; the reduced NO availability and the vascular remodeling are involved in the pathogenesis of atherosclerotic disease and cardiovascular events, and for these reasons apocynin represents a promising candidate for cardiovascular drug discovery.
Evidence strength: Predominantly animal and ex vivo studies. There is mechanistically consistent evidence from multiple rodent models of hypertension and atherosclerosis, but human vascular data are sparse. No adequately powered human cardiovascular outcome trial has been reported.
Researchers investigated the protective efficacy of apocynin in IBD using a chemical-induced mouse colitis model, noting that apocynin is an NADPH-oxidase inhibitor with established safety profiles; apocynin at 400 mg/kg or sulfasalazine at 150 mg/kg were administered for 7 days to mice with DSS-induced colitis. Apocynin treatment in rats has been shown to lessen damage in the colon as well as the enzymatic activity of myeloperoxidase, which is associated with inflammation; apocynin also decreased the number of macrophages and polymorphonuclear leukocytes in the colon.
Evidence strength: Animal and in vitro data only. No human clinical trials have been reported for IBD.
Neutrophils are a key component of the pathogenesis of collagen-induced arthritis and in the mechanisms that lead to the start of inflammation of the joints; the action of apocynin reduces the presence of such cells before inflammation has begun, but it is unable to reverse inflammation that is already present.
Evidence strength: Animal models only, with an important caveat regarding the inability to reverse established inflammation. No human clinical trials have been reported.
In a study investigating apocynin's effects on human lung adenocarcinoma cells (A549), cell viability was measured by MTT assay; IMA, AOPP, MDA, and GSH levels were analyzed; it was determined that apocynin significantly reduced cell viability and directed the cells to apoptosis, increased Bax, NF-κB, and caspase-3 expression, and decreased Bcl-2 expression.
The rhizome of P. kurroa shows anticancerous activities because it contains bioactive components including iridoid glycoside (picroside I and II), cucurbitacins, and apocynin.
Evidence strength: In vitro cell culture and animal model data only. All anticancer evidence for apocynin is preliminary and preclinical.
The following are doses as explicitly reported in the cited scientific literature. No clinical dosing guidelines or approved therapeutic doses exist.
Safety data of apocynin are scarce, but those available show low toxicity and high stability; the LD50 after oral dosing in mice has been estimated at 9 g/kg. In rats, about 80% of intraperitoneally injected apocynin at 120 mg/kg was recovered in unchanged form in a urine sample collected 20 hours later; an intravenous dose of 420 mg/kg apocynin in mice caused minimal signs of toxicity.
No adverse side effects of Picrorhiza extract/apocynin have been reported; apocynin has a very good safety profile in animal studies, and several studies used long-term treatment without any signs of ill-health effects.
Inhaled apocynin has been tested in human volunteers and shown to be well tolerated and effective in reducing ozone- and methacholine-induced airway hyperresponsiveness. In the clinical trial of inhaled apocynin in COPD patients, no influence on safety parameters and no adverse effects were observed. Apocynin has a good safety profile in all animal and human experiments reported to date.
Despite controversies about its selectivity, apocynin has been used as one of the most promising drugs in experimental models of inflammatory and neurodegenerative diseases. The compound is widely used as a research tool to probe NADPH oxidase function, but the degree to which its effects in intact cell and in vivo systems reflect genuine selective NOX inhibition versus broader antioxidant effects remains a subject of scientific debate. Recent evidence proposes that apocynin predominantly acts as an antioxidant in vascular cells rather than as a specific NADPH oxidase inhibitor per se.
Apocynin constitutes a potent inhibitor of NAT activity in rat livers both in vivo and in vitro, and thiol functionalities in NATs are hypothesized to play an important role for apocynin binding to the enzyme. Since NAT enzymes participate in drug metabolism and have been linked to cancer risk from arylamine exposure, this interaction may be pharmacologically relevant, though its clinical implications in humans have not yet been established.
Like many species of medicinal plants, Picrorhiza kurroa is threatened to near extinction due to over-harvesting. This has implications for the sustainable sourcing of botanical preparations containing apocynin.
To the knowledge of reviewers, apocynin has not been tested in human neurodegenerative disease patients. The most appropriate dose and route of administration for apocynin remain unknown, and the relevant underlying mechanisms have not yet been fully elucidated; although the conditions for conducting clinical trials are not yet considered appropriate, experimental evidence supporting the effectiveness of apocynin treatment may encourage further development.
Health conditions that Apocynin may help support.
Body systems that Apocynin may help support.
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