Adhumulone

Adhumulone

Adhumulone is a naturally occurring prenylated phloroglucinol derivative and one of three principal alpha-acid analogues found in the resinous lupulin glands of the hop plant (Humulus lupulus L.). Although it is present in smaller amounts than the dominant alpha acid humulone, adhumulone has attracted scientific interest both for its role in beer brewing chemistry and for its range of pharmacological properties shared with the broader alpha-acid family. Research into adhumulone specifically — as distinct from studies of the alpha-acid class collectively — remains comparatively limited, and the majority of mechanistic and clinical evidence to date pertains to the alpha-acid group as a whole, or to humulone as its primary representative, with adhumulone studied most specifically in the context of enzyme inhibition.

Identity and Chemical Characterization

Botanical Source

Hop (Humulus) is a small genus of flowering plants native to the temperate Northern Hemisphere. The female flowers, commonly called hops, are used as flavoring and stabilizers during beer brewing. Hop is part of the family Cannabaceae, which also includes the genus Cannabis. The lupulin glands of hops secrete a yellow powder of prenylated phloroglucinol derivatives, known as alpha acids, which are essential for foam stability, bitterness, and preservation of beer.

Hop (Humulus lupulus L.) is a climbing, perennial, and dioecious species. Its female inflorescences are rich in phenolic compounds and bitter acids, such as humulone and lupulone, which are used to produce, preserve, and flavor beer.

Chemical Names and Identifiers

Adhumulone, with the chemical formula C₂₁H₃₀O₅, is a complex molecule composed of carbon, hydrogen, and oxygen atoms. Its molecular weight is 362.47 g/mol. Adhumulone is one of three major analogues of hop α-acids. Its systematic IUPAC name is 3,5,6-trihydroxy-2-(2-methylbutanoyl)-4,6-bis(3-methylbut-2-en-1-yl)cyclohexa-2,4-dien-1-one, and it carries the ChEBI identifier CHEBI:143242. The compound is registered under CAS number 31769-65-0 (one registry) and has also been listed under CAS 28374-89-2 in certain databases, reflecting the existence of the compound's stereo- or tautomeric forms. Adhumulone is a fully characterized chemical compound used as a reference standard of API Phloroglucinol.

Structural Features

Adhumulone's structure includes a cyclic ring and multiple hydroxyl groups, making it a versatile compound in terms of its interactions and transformations during brewing. Only slight differences in molecular structures differentiate all analogues of the alpha acids. In the case of adhumulone, the side group of the molecule is 2-methylbutyryl. This distinguishes it from humulone, whose acyl side chain is an isovaleryl (3-methylbutyryl) group, and from cohumulone, which carries an isobutyryl group. The compound belongs to the class of acylphloroglucinols, which are characterized by a cyclohexane-dione core with multiple hydroxyl and prenyl (3-methylbut-2-enyl) substituents.

Position within the Alpha-Acid Series

Adhumulone is one of five identified alpha-acid analogues in hop resin, the others being cohumulone, humulone, prehumulone, and posthumulone. Collectively, these alpha-acid analogues serve as the precursors to iso-alpha acids. These alpha acids mainly consist of humulone (35–70% of total alpha acids), cohumulone (20–65%), and adhumulone (10–15%). Adhumulone levels in hops are typically constant at approximately 15% of the total alpha acid content across all varieties, whereas cohumulone and humulone levels vary (20%–50%) depending on variety. Pre- and posthumulone are minor constituents.

Traditional and Historical Use

Overview of Hops in Historical Medicine

Adhumulone exists exclusively within the hop plant and has no independent history of traditional use; its traditional context is therefore inseparable from that of Humulus lupulus as a whole. The medicinal potential of hop (Humulus lupulus L.) is widely cited in ancient literature and is also acknowledged in several official pharmacopoeias for the treatment of a variety of ailments, mainly related to anxiety states. This is due to the plethora of phytoconstituents — including bitter acids, polyphenols, and prenyl flavonoids — present in the female inflorescences, endowed with anti-inflammatory, antioxidant, antimicrobial, and phytoestrogenic activities.

The oldest report on the medicinal uses of hops can be found in a book dating back to the Middle Ages, specifically to the 11th century, in which the Arabian physician Mesue described the anti-inflammatory properties of this perennial herb. In the 13th century, the Arabian botanist Ibn al-Baytar highlighted the soothing properties of hops. Between 1300 and 1600, there was a widespread use of hops as a remedy for fevers, spleen disorders, as a diuretic, and for liver purging.

Indigenous and Ethnobotanical Use

In North America, various indigenous tribes used hops as a remedy for various ailments. The Delaware, for example, used it for earache and toothache; the Cherokee used it against sleeping disorders; the Navajo against coughs and colds; and the Dakota used hop infusions as a cure for intestinal disorders and wound healing. Hops were also used in Ayurvedic medicine, a traditional medicine used in India since ancient times and still widespread in the subcontinent today.

Sedative and Sleep-Related Traditional Use

The use of hops in traditional medicine as a mild sedative arose from the experience of drowsiness and chronic forms of fatigue that pickers and those who worked with or handled hop inflorescences manifested. Hops have long been used as a sedative and sleeping aid in drinks. In addition, soft pillows filled with hops have been popular. Originally, they were heated and lasted for about three days before being replaced.

Brewing and Preservation Use

Hops have been used in brewing for centuries, with their bitterness and preservative qualities being recognized as early as the Middle Ages. The understanding and manipulation of specific alpha acids, including adhumulone, have evolved significantly over time. The female flower of the hop plant is known for its antimicrobial properties and has long been used as an important preservative and flavoring agent of beer. The antimicrobial aspects of hops are primarily attributed to the alpha acids (humulones) and beta acids (lupulones). Early brewers made use of hop cones directly in the brewing kettle; later, standardized hop extracts and pellets were developed to deliver more reproducible levels of alpha acids including adhumulone.

Key Constituents and Chemical Context

The Alpha-Acid Class

The α-acids (humulones) and β-acids (lupulones) represent a great portion of the dry constituents of hop cones and are associated with flavor and bitterness of beer, besides playing an important role in plant properties, including antimicrobial potential. The main α-acids are cohumulone, humulone, and adhumulone, which are isomerized to iso-α-acids under high temperature conditions.

Isomerization to Iso-Alpha Acids

The thermal isomerization of the α-acids — humulone, cohumulone, and adhumulone — to the iso-α-acids — isohumulone, isocohumulone, and isoadhumulone — proceeds via an acyloin-type ring contraction. When hops are boiled during the brewing process, adhumulone undergoes isomerization, a chemical reaction that converts alpha acids into iso-alpha acids. This transformation is crucial because iso-alpha acids are much more soluble in beer, providing the bitterness that is tasted. The extent of isomerization and the resulting bitterness can be influenced by factors such as boiling time, temperature, and hop variety.

Oxidative Degradation

Oxidation of hops leads to cleavage of adhumulone's 2-methylbutyryl side group and the production of 2-methylbutyric acid, which has a distinctive, pungent odor similar to Roquefort cheese. This is why oxidized hops are often described as "cheesy." This degradative pathway is a quality-control concern in the storage and handling of hop materials, as oxidized alpha acids contribute undesirable sensory characteristics and represent a loss of bittering potential.

Co-occurring Bioactive Compounds

Adhumulone co-occurs in hop resin with numerous other biologically active constituents. These include bitter acids, polyphenols, and prenyl flavonoids present in the female inflorescences, endowed with anti-inflammatory, antioxidant, antimicrobial, and phytoestrogenic activities. The prenylated flavonoid xanthohumol and the phytoestrogen 8-prenylnaringenin are among the most extensively studied co-occurring compounds, and studies of hops pharmacology rarely isolate adhumulone from the broader phytochemical context. In addition to alpha and beta acids, hops also contain essential oils, responsible for hop aroma and flavor in finished beer. The major hop oil constituents of significance are myrcene, humulene, and caryophyllene.

Mechanisms of Action

GABA_A Receptor Modulation

Alpha acids contain cohumulone and adhumulone, which are structurally very similar to humulone and display weak modulatory effects on GABA_A receptors. The primary mechanistic evidence for GABA_A modulation within the alpha-acid class comes from studies on humulone. Recent evidence revealed that humulone, a prenylated phloroglucinol derivative comprising 35–70% of hops alpha acids, may act as a positive modulator of GABA_A receptors at low micromolar concentrations. These findings confirmed humulone's positive allosteric modulation of GABA_A receptor function and its sedative and hypnotic behavior. Humulone modulation can be potentially enhanced by ethanol and hops modulators, suggesting a probable enhancement in the intoxicating effects of ethanol in hops-enriched beer. Adhumulone's own contribution to GABA_A modulation is described as weak relative to humulone, and no studies to date have characterized this interaction with adhumulone in isolation.

Aldo-Keto Reductase Inhibition (AKR1B10)

Isomerised bitter acids — isoadhumulone, isocohumulone, and isohumulone — from hops are known to inhibit members of the aldo-keto-reductase superfamily. Aldo-keto-reductase 1B10 (AKR1B10) is upregulated in various types of cancer and has been reported to promote carcinogenesis. Inhibition of AKR1B10 appears to be an attractive means to specifically treat RAS-dependent malignancies.

However, the closely related reductases AKR1A1 and AKR1B1, which fulfil important roles in the detoxification of endogenous and xenobiotic carbonyl compounds, oftentimes crossreact with inhibitors designed to target AKR1B10. Accordingly, there is an ongoing search for selective AKR1B10 inhibitors that do not interact with endogenous AKR1A1 and AKR1B1-driven detoxification systems. In one key study, unisomerised α-acids — adhumulone, cohumulone, and n-humulone — were separated and tested for their inhibitory potential on AKR1A1, AKR1B1, and AKR1B10.

AKR1B10-mediated farnesal reduction was effectively inhibited by α-acid congeners with K_i-values ranging from 16.79 ± 1.33 µM (adhumulone) to 3.94 ± 0.33 µM (n-humulone). Overall, α-acids showed a strong inhibition with selectivity (115–137 fold) for AKR1B10. These results characterise hop-derived α-acids as a promising basis for the development of novel and selective AKR1B10 inhibitors.

NF-κB and Inflammatory Pathway Inhibition

All three studied classes of hop bitter acids blocked the tumor necrosis factor alpha (TNF)-induced production of the cytokine IL-6, and inhibited the transactivation of the pro-inflammatory transcription factors nuclear factor kappa B (NF-κB), activator protein-1 (AP-1), and cAMP-response element-binding protein (CREB). In this respect, the six-membered ring compounds (alpha acids and beta acids) showed equal potency, whereas the five-membered ring compounds (iso-alpha acids) were effective only when used at higher concentrations.

Furthermore, with regard to the mechanism of NF-κB suppression, a possible role for glucocorticoid receptor alpha (GRα), peroxisome proliferators-activated receptor alpha/gamma (PPARα or PPARγ) was excluded — nuclear receptors also known to inhibit inflammation by directly interfering with the activity of pro-inflammatory transcription factors. This indicates the anti-inflammatory action of alpha acids occurs via a distinct, as yet fully elucidated pathway.

COX-2 Selectivity and Prostaglandin Inhibition

Various hop and modified hop extracts were found to be among the most potent PGE₂ inhibitors in LPS-induced (PGE₂ from COX-2) but not non-induced (PGE₂ from COX-1) RAW 264.7 cells, indicating COX-2 selectivity (ranging from 1.5- to 363-fold). In a human gastric mucosal cell (AGS) model where COX-2 is constitutively expressed, a CO₂ hop extract showed strong inhibition of PGE₂; in contrast, no significant PGE₂ inhibition was observed by other hop extracts, indicating a lack of direct COX enzyme inhibition.

Antimicrobial Mechanisms

The molecular mechanism behind iso-α-acids' antimicrobial activity in Lactobacillus brevis involves their ionophore activity and the dependence of the inhibitory potential on manganese binding. A manganese-binding dependent transmembrane redox reaction (oxidative stress) plays a crucial role in inhibition. This ionophore-type mechanism is relevant to the iso-alpha acid series (isoadhumulone included) rather than to unisomerized adhumulone specifically. While humulones show remarkable antibacterial properties against Gram-positive bacteria such as Bacillus subtilis and Staphylococcus aureus, isohumulones have much weaker effects under neutral pH conditions.

Scientific Evidence by Area of Use

Important Caveat on Adhumulone-Specific Evidence

It must be stated clearly at the outset of this section that adhumulone has rarely been studied in complete isolation in human or clinical research. The majority of the scientific literature addresses the alpha-acid class collectively, or focuses on humulone as the principal alpha acid. Evidence attributed to "hop alpha acids" or "hop bitter acids" includes adhumulone as a component, but attribution of effects specifically to adhumulone cannot be made from most such studies. The one area where adhumulone has been studied specifically is enzyme inhibition (AKR1B10), and that research is in vitro only.

1. AKR1B10 Inhibition and Cancer-Relevant Enzyme Targeting

Evidence type: In vitro biochemical (enzyme inhibition assays). No human clinical trials.

AKR1B10 is upregulated in various types of cancer and has been reported to promote carcinogenesis. Inhibition of AKR1B10 appears to be an attractive means to specifically treat RAS-dependent malignancies. AKR1B10 expression is low in other tissues, where the enzyme is upregulated in cancers, as well as in non-alcoholic fatty liver disease and several skin diseases. In addition, the enzyme's expression is elevated in cancer cells resistant to clinical anti-cancer drugs. Thus, growing evidence supports AKR1B10 as a potential target for diagnosing and treating these diseases.

In the key 2018 study (Seliger et al., Molecules, 23(11):3041), unisomerized adhumulone was isolated from Humulus lupulus extract and tested directly. Unisomerised α-acids — including adhumulone — were tested for their inhibitory potential on AKR1A1, AKR1B1, and AKR1B10. AKR1B10-mediated farnesal reduction was effectively inhibited by α-acid congeners with K_i-values ranging from 16.79 ± 1.33 µM (adhumulone) to 3.94 ± 0.33 µM (n-humulone). Overall, α-acids showed a strong inhibition with selectivity (115–137 fold) for AKR1B10. The results characterise hop-derived α-acids as a promising basis for the development of novel and selective AKR1B10 inhibitors.

Evidence strength: Preliminary. All evidence is from in vitro enzyme assays. Adhumulone was weaker than n-humulone in this assay. No cell-line, animal model, or human clinical data have tested adhumulone's AKR1B10 inhibitory activity in a biological context.

2. Antimicrobial Activity

Evidence type: In vitro (cell/bacterial culture). No clinical trials specifically for adhumulone.

Iso-α-acids are considered the main agents of beer bitterness, also exhibiting antibacterial activity, mainly against Gram-positive bacteria. Bitter acids like alpha-acids such as humulone found in hop possess antimicrobial properties against Gram-positive bacteria including Staphylococcus, Clostridium, and Bacillus.

Strong inverse correlations of MIC and MBC values were obtained with xanthohumol, cohumulone, n+adhumulone, colupulone, and n+adlupulone contents, suggesting that identified chemical hop compounds are directly responsible for antimicrobial activity. The study in question used combined fractions (n+adhumulone was not separated from humulone in these antimicrobial assays), so the contribution of adhumulone specifically to the antimicrobial effect cannot be disaggregated from the available data.

Isohumulone seems crucial for the prevention of the major beer spoiler lactic bacteria, since it exhibits 20 times greater antibacterial activity against Lactobacillus brevis than humulone under low pH conditions, as observed in beers. However, neither isohumulones nor humulones substantially inhibit growth of Gram-negative bacteria.

Evidence strength: In vitro only. No clinical antimicrobial trials exist for adhumulone as an isolated compound. Antimicrobial activity data for the alpha-acid class is well established at the in vitro level, but adhumulone's individual contribution has not been studied independently of the class.

3. Anti-Inflammatory Activity

Evidence type: In vitro and animal models; very limited human data (for rho iso-alpha acids as a class).

Alpha acids as a class, including adhumulone, have demonstrated anti-inflammatory activity in laboratory studies. All three studied classes of hop bitter acids blocked TNF-induced production of IL-6 and inhibited the transactivation of NF-κB, AP-1, and CREB. The six-membered ring compounds (alpha acids and beta acids) showed equal potency, whereas iso-alpha acids were effective only at higher concentrations.

Rho iso-alpha acids (RIAA), a chemically modified derivative of the iso-alpha acid class (which itself is derived from alpha acids including isoadhumulone), have been the most extensively studied in inflammatory contexts. RIAA inhibited GSK-3α/β kinase activity and GSK-3β-dependent phosphorylation of β-catenin in RAW 264.7 cells. In addition, RIAA inhibited NF-κB-mediated inflammatory markers in various cell models, including nitric oxide in LPS-stimulated RAW 264.7 cells, RANKL-mediated TRAP activity in transformed osteoclasts, and TNF-α/IL-1β-mediated MMP-13 expression in SW1353 human chondrosarcoma cells. In a mouse model of collagen-induced arthritis, RIAA ameliorated joint damage; at 250 mg/kg body weight, RIAA had efficacy similar to that of 20 mg/kg body weight of celecoxib.

A pilot human trial of NG440, a combination preparation containing rho iso-alpha acids from hops alongside rosemary and oleanolic acid, was referenced in the inflammation literature, but the multi-ingredient formulation precludes attribution to alpha acids — let alone adhumulone — specifically.

Evidence strength: Preclinical data for the alpha-acid class are robust at the cellular level; animal data exist for modified alpha-acid derivatives. Human clinical evidence for alpha acids as an anti-inflammatory intervention remains limited and largely restricted to combination formulations. No human trials have examined adhumulone in isolation.

4. Sedative and Sleep-Promoting Effects

Evidence type: In vitro receptor pharmacology, animal behavioral studies; limited human studies for hop preparations (not adhumulone in isolation).

The therapeutic potential of alpha acids has been investigated for their wide range of bioactivity, including sedative and hypnotic properties. Alpha acids were found to exhibit sedative and hypnotic properties, indicating their major role in hops' sleep-promoting activity previously reported in animal models and humans.

Alpha acids contain cohumulone and adhumulone, which are structurally very similar to humulone and display weak modulatory effects on GABA_A receptors. Humulone has been studied in considerably more depth in this context. In behavioral tests, humulone shortened sleep onset and increased the duration of sleep induced by pentobarbital and decreased spontaneous locomotion in an open field at 20 mg/kg (i.p.). Sleep duration was increased dose-dependently down to 10 mg/kg (i.p.). These are intraperitoneal animal data; oral bioavailability and equivalent human doses have not been established for adhumulone.

In a caffeine-induced insomnia model, administration of a Saaz-Saphir hops mixture increased sleep time, attributed to the increase in NREM sleep time. Oral administration of the Saaz-Saphir mixture for 3 weeks increased GABA content in the brain and increased expression of the GABA_A receptor. GABA antagonists picrotoxin and bicuculline showed a decrease in sleep activity, confirming that the GABA_A receptor was involved.

Evidence strength: Adhumulone's specific contribution to sedative effects is characterized as weak relative to humulone. Human evidence for sleep benefits exists for mixed hop preparations but has not been attributed to adhumulone specifically. Overall evidence for adhumulone's sedative role is preliminary and indirect.

5. Hepatocellular and Oncology-Relevant In Vitro Activity

Evidence type: In vitro cell line studies. No clinical data.

The chemopreventive effect of α- and iso-α-acids on biological systems has been investigated and extensively reviewed. Apart from their antibiotic capacities and antiangiogenic and antidiabetic properties, α-acids have been reported to interfere with carcinogenesis. In human hepatocarcinoma cells, α-acids significantly reduced phosphorylation of NF-κB as well as AP-1 and ERK1/2 activity, thus reducing migration and proliferation.

Bitter acids from the hop plant Humulus lupulus L. exhibit multiple beneficial biological properties with promising effects in cancer therapy and prevention, but information regarding effects on hepatocellular carcinoma (HCC) is incomplete. Studies used two different hop bitter acid extracts enriched for either α-acids or β-acids to investigate biological activity differences between these groups. At 25 µg/ml, only the β-acid-rich extract started to induce aspartate transaminase (AST) release, with a significant increase detected at 50 µg/ml of both extracts. At lower concentrations both extracts led to dose-dependent inhibition of proliferation, and migration was suppressed at a concentration as low as 5 µg/ml in HCC cells.

Evidence strength: In vitro only. These data use mixed alpha-acid extracts; no evidence isolates adhumulone's contribution. No clinical oncology data exist for adhumulone.

6. Isoadhumulone and Metabolic/Immune Effects

The isomerized form of adhumulone — isoadhumulone — shares bioactive properties with the broader iso-alpha acid class. Although the presence of isohumulones in beer has long been noted, their physiological actions have remained obscure. This might be due to the relatively low stability and limited commercial availability of purified isohumulone, isocohumulone, and isoadhumulone.

A 2026 pilot study published in the European Journal of Nutrition explored a related area: Iso-alpha acids (IAA), bitter compounds derived from hops, are known for their anti-inflammatory properties, but their effects on human immune modulation remain unclear. The study explored the effect of a single oral dose of IAA on LTA-induced inflammatory responses in human immune cells and underlying molecular mechanisms. In a pilot study in healthy female volunteers (n = 5), dose- and time-dependent effects (0–90 mg IAA) on LTA-induced immune responses in monocytes were assessed. While this study addresses iso-alpha acids as a class (which includes isoadhumulone), it does not isolate the contribution of isoadhumulone specifically.

Evidence strength: Early-stage human pilot data (small n, single dose, single sex) for iso-alpha acids as a class. No human evidence specifically isolates isoadhumulone or adhumulone.

Body Systems and Health Areas Associated with Adhumulone

Based on the available literature for adhumulone specifically and alpha acids as a class, the following body systems and health domains are associated with this compound family:

• Central nervous system: Alpha acids contain cohumulone and adhumulone, which are structurally very similar to humulone and display weak modulatory effects on GABA_A receptors. This places adhumulone in the domain of neuroactive phytochemicals, relevant to sleep and anxiolytic activity, though its contribution is weaker than humulone's.
• Immune and inflammatory system: All three classes of active hop bitter acids — alpha-acids, beta-acids, and iso-alpha-acids — have been shown to block the production of TNF-alpha and IL-6, and inhibit the transcription of NF-κB and activator protein-1 (AP-1), thus decreasing the expression of genes related to inflammation.
• Hepatic and metabolic system: The AKR1B10 enzyme inhibited by adhumulone and related alpha acids is also upregulated in non-alcoholic fatty liver disease and skin diseases. Growing evidence supports AKR1B10 as a potential target for diagnosing and treating gastrointestinal cancers and inflammatory bowel diseases, as well as non-alcoholic fatty liver disease and several skin diseases.
• Gastrointestinal system: Beer has been used as folk medicine in Europe for a long time due to its diuretic and stomachic effect. Traditional applications of hops included digestive complaints, fever management, and liver purging.
• Antimicrobial defense: The antimicrobial effects of hops have been known for a long time, and their antimicrobial components inhibit lactic acid bacteria that spoil beer.
• Musculoskeletal system: Iso-alpha acid derivatives have shown effects on inflammatory markers relevant to bone and cartilage degradation in preclinical models, though evidence for adhumulone itself in this domain is absent.

Dosage Forms and Preparations

Forms of the Parent Hop Plant

Adhumulone is not commercially available as an isolated supplement for human consumption. It is encountered in supplementary and functional food contexts exclusively as a component of hop preparations. Hop (Humulus lupulus) is best known for its use in beer brewing owing to its bittering flavor and floral aroma. Today, the brewing industry uses as much as 98% of the produced hop crop worldwide.

Common forms of hop preparations in which adhumulone is present include:

• Whole hop cones/strobili: The unprocessed dried female inflorescences, used in traditional herbal preparations including teas and tinctures.
• Hop pellets and granules: Processed forms used primarily in brewing.
• CO₂ hop extracts: Supercritical fluid extracts standardized for alpha-acid content and used in brewing and as research tools.
• Iso-alpha acid preparations: Derived by thermal isomerization; isoadhumulone is present in these preparations.
• Rho iso-alpha acid (RIAA) extracts: Chemically reduced and purified fractions studied for anti-inflammatory properties.
• Hops dietary supplement capsules and tablets: These contain standardized hop strobile extract and are used for sedative/sleep and anxiety applications; adhumulone is a minor but consistent component.

Dosages in Research

No human clinical dosage data exist for adhumulone as an isolated compound. Dosages reported in studies relate to preparations containing alpha acids as a class or to hop preparations more broadly:

• In a pilot study in healthy female volunteers (n = 5), dose- and time-dependent effects of 0–90 mg iso-alpha acids (IAA) on LTA-induced immune responses were assessed.
• In a mouse model of collagen-induced arthritis, RIAA at 250 mg/kg body weight had efficacy similar to celecoxib at 20 mg/kg body weight. (Animal data only.)
• In behavioral (animal) tests, humulone shortened sleep onset and increased sleep duration induced by pentobarbital at 20 mg/kg (i.p.); sleep duration was increased dose-dependently down to 10 mg/kg (i.p.). (Animal data for humulone, not adhumulone; i.p. route.)
• In the AKR1B10 enzyme inhibition study, adhumulone demonstrated a K_i of 16.79 ± 1.33 µM for inhibition of AKR1B10-mediated farnesal reduction. (In vitro biochemical data only.)

As a fully characterized chemical compound, adhumulone is commercially available as a reference standard of API Phloroglucinol. This standard is compliant with regulatory guidelines and is used for analytical method development, method validation, and quality-controlled applications during synthesis and formulation stages of drug development.

Safety Considerations and Interactions

Occupational and Dermatological Exposure

In hops harvesters, dermatitis has been attributed to mechanical abrasion by the rough hairs on the climbing stem. It has also been suggested that lupulin, the yellow powdery secretion of the glandular hairs on the scales of the strobiles, may be responsible for irritation. Lupulin is the fraction containing alpha acids including adhumulone. Farmers (n = 73) who cultivated hops plants and other crops were administered skin prick tests for allergens of Humulus lupulus as well as grain dust, straw dust, hay dust, storage mites, and antigens of microorganisms typical of farm environments. Cases of airborne allergic contact dermatitis have been reported in hop farmers as an occupational concern.

Phototoxicity

There were no constituents of concern associated with phototoxicity, and there were no indications of phototoxicity in farmer workers exposed to high amounts of Humulus lupulus. Accordingly, phototoxicity assays were not deemed to be required in the safety assessment of hop-derived cosmetic ingredients.

Genotoxicity

The Expert Panel for Cosmetic Ingredient Safety noted the limited scope of an in vitro genotoxicity assay in which an aqueous Humulus lupulus extract increased revertants 2–4 times that of controls. This finding was viewed as of limited interpretive value given the assay's limitations; no follow-up genotoxicity data in mammalian systems were available.

Estrogenic Activity Concern

Because final product formulations may contain multiple botanicals, each containing the same constituents of concern, formulators are advised to be aware of these constituents and to avoid reaching levels that may be hazardous. The Expert Panel expressed concern about the presence of 8-prenylnaringenin, β-myrcene, and quercetin in cosmetics. The phytoestrogen 8-prenylnaringenin, a co-occurring compound in hops, is distinct from adhumulone, but their co-occurrence in preparations is relevant to exposure assessment.

Interaction with Ethanol

Humulone modulation can be potentially enhanced by ethanol and hops modulators, suggesting a probable enhancement in the intoxicating effects of ethanol in hops-enriched beer. Because adhumulone shares structural and mechanistic features with humulone at GABA_A receptors — albeit with weaker activity — the same pharmacodynamic interaction with ethanol may apply, though this has not been directly studied for adhumulone.

Pesticide Residues and Botanical Impurities

The Expert Panel expressed concern about pesticide residues, heavy metals, and substances from plants of other species (weeds) that may be present in botanical ingredients. Multiple fungi and bacteria have been detected co-localized with Humulus lupulus plants. To address these concerns, the cosmetics industry should continue to use current Good Manufacturing Practices (cGMPs) to limit impurities. These considerations apply to hop supplements as well.

Availability as a Pure Reference Standard

Adhumulone is a fully characterized chemical compound used as a reference standard of API Phloroglucinol. The standard is compliant with regulatory guidelines. Adhumulone is used for analytical method development, method validation, and quality-controlled applications during synthesis and formulation stages of drug development, and serves as a reference standard for traceability against pharmacopeial standards (USP or EP). Products containing purified adhumulone as a reference standard are intended for analytical purposes only and are not for human use.

Current Research Status and Evidence Gaps

Adhumulone occupies an unusual position in the phytochemistry literature: it is a well-characterized chemical constituent of hops with a fixed and reliable natural abundance, yet it has been subjected to far less independent scientific investigation than its structural relative humulone or the flavonoid xanthohumol. Experimental data on the biological interaction potential of hop bitter acids, such as (iso-)α-acids, are relatively scarce. This is especially true for non-isomerized α-acids.

The most significant and specific finding for adhumulone is its selective inhibition of AKR1B10 at the biochemical level, with a K_i of 16.79 µM, alongside 115–137 fold selectivity for AKR1B10 over closely related reductases. This is a meaningful preliminary finding that positions adhumulone as a structurally interesting lead for drug development in oncology and metabolic disease contexts, but it remains in vitro evidence only. The therapeutic potential of alpha acids has been investigated for their wide range of bioactivity against bacteria, osteoporosis, angiogenesis, inflammation, and cancer. However, essentially all such research involves alpha acids collectively or humulone individually.

The following evidence gaps are notable in the current literature:

• No human clinical trial has examined adhumulone as an isolated pharmacological agent.
• Oral bioavailability and pharmacokinetics of adhumulone in humans have not been reported.
• The relative biological potency of adhumulone compared to humulone or cohumulone in most assay systems has not been systematically mapped.
• No dose-response, safety, or interaction data in humans exist for adhumulone.
• Animal in vivo studies specific to adhumulone (not alpha-acid mixtures) are absent from the published literature.

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