Indole

Indole: A Comprehensive Encyclopedic Reference

1. Identity and Chemical Nature

1.1 The Indole Scaffold

Indole's chemical structure consists of a bicyclic aromatic ring system — a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Its molecular formula is C₈H₇N. This flat, conjugated structure makes indole electron-rich and chemically reactive, especially at the 3-position, where many biological substitutions occur. Indole serves as a key scaffold in many biologically active molecules including tryptophan, serotonin, melatonin, and several indole alkaloid drugs, and is a central motif not only in microbial metabolites but also in neurotransmitters and pharmaceutical compounds.

Indole was first obtained in 1866 by Adolf von Baeyer during the reductive/thermal decomposition of indigo (e.g., destructive distillation with zinc dust). In 1869, Baeyer proposed the structural formula — a benzene ring fused to a pyrrole ring — which cemented indole as a fundamental heteroaromatic scaffold.

In the context of dietary supplementation, the term "indole" most commonly refers to a family of biologically active indole-containing compounds, the most prominent of which are:

• Indole-3-carbinol (I3C) — also known as Indolylmethanol, 3-Indolylcarbinol, 3-(Hydroxymethyl)indole, Indole-3-methanol, with the CAS Registry Number 700-06-1. It has several common names, including Indolylmethanol, 3-Indolylcarbinol, 3-(Hydroxymethyl)indole, Indole-3-methanol, and I3C.
• 3,3′-Diindolylmethane (DIM) — the primary in vivo condensation product of I3C.
• Microbiota-derived indoles — including indole-3-propionic acid (IPA), indole-3-acetic acid (IAA), indole-3-lactic acid (ILA), skatole (3-methylindole), and others, all arising from the bacterial metabolism of tryptophan.

1.2 Natural Sources

Indole-3-carbinol (I3C) is a major dietary component produced in Brassica vegetables from glucosinolates (GLS) upon herbivores' attack. Indole-3-carbinol and diindolylmethane are naturally occurring compounds found in cruciferous vegetables such as bok choy, broccoli, Brussels sprouts, cabbage, cauliflower, kale, kohlrabi, mustard, rutabagas, and turnips.

Cruciferous vegetables contain sulfurous components called glucosinolates, responsible for their special smell and taste. Glucosinolates are inactive biologically in the organism but are hydrolyzed by the enzyme myrosinase released as a result of chewing, leading to the formation of active derivatives such as isothiocyanates and indoles.

A second major category of dietary indoles arises from the gut microbiome. Intestinal bacteria convert tryptophan mainly into indole through the action of the bacterial enzyme tryptophanase; the latter is induced by tryptophan itself. In mammals, indole originates exclusively from bacterial metabolic activity since host cells do not have the metabolic capacity for the production of this compound. Indole is synthesized from various Gram-positive and Gram-negative bacterial species, including Escherichia coli, Proteus vulgaris, Clostridium spp., and Bacteroides spp.

Tryptophan can also be converted directly or indirectly by the intestinal microbiota into several indole-related compounds, including indole-3-pyruvate, indole-3-lactate, indole-3-propionate, indole-3-acetamide, indole-3-acrylate, indole acetaldehyde, indole-3-acetate, indole-3-aldehyde, 3-methyl-indole (skatole), and indole-3-acetaldehyde.

1.3 Common Supplement Forms and Preparations

Indole-3-carbinol is marketed and promoted as a dietary supplement that is available at health food stores, pharmacies, and via the internet. Indole-3-carbinol may be sold as the sole ingredient in products or in combination nutraceuticals that contain a variety of botanicals and/or vitamins. Some commercial products, generally presented in capsule-type dosage form, include Genius Estrogen Balance-DIM, Indonal partner for the woman; Indonal Man; Zazzee Naturals-DIM, Gynmax; Nutricost-DIM, ProstaIN; INDOL-IN, Now Foods-I3C and GRAV-IN.

The products contain, as declared, between 150 and 400 mg of the active substance, though quantification by high-performance liquid chromatography found amounts much lower than what was declared in several tested products. This evidence generates a critical need to regulate this type of supplement that guarantees the daily consumption of the active substance by people who seek a benefit associated with I3C and DIM.

Indole-3-carbinol is an off-white powder with a melting point of 96° to 99°C. It is soluble in benzene, ethanol, and pentane. Indole-3-carbinol is an unstable compound that undergoes rapid oligomerization in acid pH environments, like the stomach. At low pH, a wide variety of condensation products are formed, ranging from linear and cyclic dimers, trimers, and tetramers to extended heterocyclic compounds such as indolocarbazoles.

2. Traditional and Historical Use

Indole itself as an isolated chemical entity has no pre-modern history of use; it was not identified or named until the 19th century. However, the indole-rich cruciferous plants from which dietary indoles are derived have extensive histories of medicinal use across cultures.

In ancient India, references to mustard seed preparations appear in texts dated around the 1st century CE. The Charaka Samhita, one of the classical Ayurvedic treatises, mentions mustard seeds under medicinal spices used for digestive complaints. Subsequent medieval compendia such as the Sushruta Samhita and the Ashtanga Hridayam expand on mustard oil's applications in fomentation therapies to relieve joint and muscle stiffness.

Isatis tinctoria L. (Brassicaceae), commonly known as woad, is a species with an ancient and well-documented history as an indigo dye and medicinal plant. Currently, I. tinctoria is utilized more often as a medicinal remedy and as a cosmetic ingredient. In 2011, I. tinctoria root was accepted in official European phytotherapy by introduction of its monograph in the European Pharmacopoeia. The biological properties of the raw material have been known from Traditional Chinese Medicine (TCM). The plant is a member of the Brassicaceae family and contains significant amounts of glucosinolates from which indole compounds can be generated.

The specific scientific interest in dietary indoles, particularly I3C, as isolated or concentrated supplements is essentially a 20th-century development. Lee W. Wattenberg, who spent his entire career at the University of Minnesota, was a true pioneer in the field of chemoprevention, and his groundbreaking research uncovered the cancer prevention properties of many dietary compounds, including indole-3-carbinol and diindolylmethane. Early work from Wattenberg's group and others in the 1980s established the framework for understanding indoles as "blocking agents" of chemical carcinogenesis, driving subsequent interest in I3C and DIM as supplements.

Cruciferous vegetable-derived indoles including I3C and DIM are described as "blocking agents" in preclinical models, as greatest efficacy is seen when given prior to and/or during carcinogen exposure.

3. Key Constituents and Active Compounds

3.1 The Glucosinolate–Myrosinase System: Generation of Dietary Indoles

Glucosinolates, a set of compounds containing a glucose component, a sulfur-carbon-nitrogen component, and a variable component, occur in varying ratios in varietals, leaves, stocks, stems, flowers, seeds, and roots of the Brassica (cruciferous) plants. In glucobrassicin, a glucosinolate, the variable component is 3-indolylmethyl. When the cell walls of the plant are destroyed through chopping, grinding, or chewing, an enzyme, myrosinase, is released. Myrosinase aids in the hydrolysis of glucobrassicin, resulting in the release of indole-3-carbinol.

Hydrolysis products of indolylmethyl glucosinolate (glucobrassicin) — I3C, 3,3′-diindolylmethane (DIM), and indole-3-acetonitrile — are inducers of detoxifying enzyme activity. These compounds are formed when the vegetable is chewed or otherwise macerated, leading to contact of glucobrassicin with the enzyme myrosinase, which catalyzes its hydrolysis to the three products. I3C had the strongest inducing activity.

3.2 In Vivo Conversion of I3C to DIM and Other Condensation Products

Indole-3-carbinol is a metabolic precursor for diindolylmethane, with two molecules of indole-3-carbinol required to produce one molecule of diindolylmethane. Acid condensation products formed from I3C during passage through the stomach possess the CYP enzyme modulating effects. The acid condensation products, 3,3′-diindolylmethane (DIM) and 2,3-bis[3-indolylmethyl]indole, were detected in gastric contents and stomach tissue an hour after animals received an oral dose of I3C.

I3C is unstable in acidic milieus; under these conditions it undergoes acid-catalyzed dehydration and condensation to generate a complicated series of oligomeric products in vivo, including DIM (3,3′-diindolylmethane), ICZ (indolo[3,2b]-carbazole), LTr1 (a linear trimer), CTr (a cyclic trimer), and CTet (a cyclic tetramer).

It has been shown that the same array of products results from subjecting indole-3-carbinol to stomach acid, or even plain acidified water. Consequently, numerous workers have concluded that the mixture of dietary indoles, not just indole-3-carbinol itself, is responsible for the beneficial effects seen in providing animals and humans with dietary indoles.

3.3 Microbiota-Derived Indoles from Tryptophan

Indoles represent a wide group of gut bacteria-derived compounds produced from tryptophan, an essential amino acid and the precursor of endogenous synthesis of tryptamine, serotonin, and melatonin. It is estimated that 4–6% of dietary tryptophan is metabolized along the indole pyruvate pathway. The most abundant metabolite is indole, followed by indole-3-acetic acid (IAA) and indole-3-propionic acid (IPA).

The majority of indole alkaloids are biologically active, with some exhibiting excellent antitumor, antibacterial, antiviral, antifungal, and antiplasmodial activities. Consequently, the natural products of this class have attracted considerable attention as potential leads for novel therapeutics. However, data on indole alkaloids, their various structures, and bioactivities are complex due to their diverse sources, such as plants, fungi, bacteria, sponges, tunicates, and bryozoans.

4. Mechanisms of Action

4.1 Modulation of the Aryl Hydrocarbon Receptor (AhR)

DIM has multiple mechanisms of action, the most well-characterized being modulation of aryl hydrocarbon receptor (AhR) signaling. I3C or DIM binds to AhR, which is associated with chaperone proteins. The ligand-bound AhR complex then translocates to the nucleus, where AhR forms a heterodimer with the AhR nuclear translocator (Arnt). The AhR/Arnt complex binds to xenobiotic response elements (XREs) in the promoters of target genes, including the cytochrome P450 (CYP) genes, CYP1A1, CYP1B1, and CYP19, resulting in their transcription and subsequent CYP-mediated biotransformation.

4.2 Phase I and Phase II Enzyme Induction

A considerable number of in vitro and in vivo studies have reported that isothiocyanates and indoles elicit chemopreventive potency through multiple mechanisms that include modulation of phases I and II detoxification pathway enzymes, regulation of cell cycle arrest, and control of cell growth, induction of apoptosis, antioxidant activity, anti-angiogenic effects, and epigenetic regulation.

The proposed mechanism is thought to be AhR-dependent induction of both phase 1 (P450s) and phase 2 (glutathione-S-transferases and UDP-glucuronosyltransferases) enzymes.

4.3 Estrogen Metabolism Modulation

Alteration of cytochrome P450-dependent estrogen metabolism is hypothesized to be an important driver of DIM-dependent breast cancer prevention. The oxidative metabolism of estrogens in humans is mediated primarily by cytochrome P450, many isoenzymes of which are inducible by dietary and pharmacologic agents. One major pathway, 2-hydroxylation, is induced by dietary indole-3-carbinol (I3C), which is present in cruciferous vegetables.

AhR activation by I3C/DIM inhibits CYP19-dependent conversion of androgens to estrogens. Another pathway shows that I3C/DIM increases proteolysis of estrogen receptor α (ERα), leading to reduced ERα-mediated transcription of GATA3.

4.4 Anti-Proliferative and Apoptotic Signaling

I3C and its metabolite DIM exhibit anti-proliferative and apoptotic effects by targeting Akt–NF-κB signaling, caspase activation, cyclin-dependent kinase activities, estrogen metabolism, estrogen receptor signaling, endoplasmic reticulum stress, and BRCA gene expression.

I3C and its metabolite DIM induce overlapping and unique responses in multiple cancer cell lines and tumors, including growth inhibition, apoptosis, and antiangiogenic activities. The mechanisms of these responses are complex and dependent on cell context. I3C and/or DIM activate or inactivate multiple nuclear receptors, induce endoplasmic reticulum stress, decrease mitochondrial membrane potential, and modulate multiple signaling pathways including kinases.

4.5 Epigenetic Regulation

Novel anticancer mechanisms of I3C have been identified, including modulation of epigenetic alterations such as CpG methylation, histone modification, and aberrant expression of microRNA.

4.6 Intestinal Barrier and Immune Modulation by Microbiota-Derived Indoles

Indoles contribute to maintaining the biological barrier of the human intestine, exerting anti-inflammatory activities mainly through activating AhR and PXR receptors to affect the immune system's function, significantly improving intestinal health in conditions such as inflammatory bowel disease, hemorrhagic colitis, and colorectal cancer, and further promoting human health in diabetes mellitus, central system inflammation, and vascular regulation.

Indole and its derivatives are metabolites produced by the metabolism of tryptophan catalyzed by intestinal microorganisms. By activating nuclear receptors, regulating intestinal hormones, and affecting the biological effects of bacteria as signaling molecules, indole and its derivatives maintain intestinal homeostasis and impact liver metabolism and the immune response.

5. Scientific Evidence by Area of Use

5.1 Cancer Chemoprevention and Treatment

Cervical Dysplasia (CIN)

I3C doses of 200 mg/day or 400 mg/day improved the regression of cervical intraepithelial neoplasia (CIN) in a preliminary clinical trial. These preclinical findings led to human trials in cervical dysplasia, breast cancer, vulvar intraepithelial neoplasia, and recurrent respiratory papillomatosis.

Data from early phase clinical trials suggested that I3C is effective against precancerous cervical dysplasia and vulvar intraepithelial neoplasia. These trials, however, are of limited size and represent early-phase evidence. The evidence for I3C and DIM in CIN is among the most replicated in human subjects, though larger randomized controlled trials with clear endpoint definitions are still needed to confirm clinical benefit.

Prostate Cancer

In a phase I dose-escalation study, DIM was orally administered in non-metastatic prostate cancer patients, revealing that PSA levels diminished and emotional functioning improved over time. Furthermore, twice daily administration of 225 mg of DIM was recommended for phase II studies.

Following DIM supplementation in men scheduled for prostatectomy, not only were androgen receptor levels in prostate reduced, but there was exclusion of the receptor from the nucleus.

The daily maximum tolerated dose was 300 mg and the recommended phase II dose was 225 mg. The rate of prostate-specific antigen (PSA) rise in patients taking 225 mg daily initially declined but eventually progressed to placebo rates of increase and/or presented with metastatic disease. This indicates promising early signals but insufficient durable benefit in this advanced-disease context.

Breast Cancer

In humans, much of the focus has been on chemoprevention of breast and prostate cancer. In one study, urine samples were collected from subjects before and after oral ingestion of I3C at 6–7 mg/kg per day. In the first study, seven men received I3C for 1 week; in the second study, 10 women received I3C for 2 months. A profile of 13 estrogens was measured in each sample by gas chromatography-mass spectrometry. The findings supported the hypothesis that I3C-induced estrogen 2-hydroxylation results in decreased concentrations of several metabolites known to activate the estrogen receptor. This effect may lower estrogenic stimulation in women, and I3C may have chemopreventive activity against breast cancer in humans, although the long-term effects of higher catechol estrogen levels in women require further investigation.

More recently, an RCT of a diindolylmethane formulation (DIM, an active I3C metabolite) in breast cancer patients on tamoxifen suggested it encourages beneficial changes in estrogen metabolism and circulating sex hormone-binding globulin levels.

It has been reported that DIM was the only product detected in plasma samples after non-smoking women subjects (n=24; age between 23 and 58 years) with an elevated risk of breast cancer (by family history) ingested oral doses of 400, 600, 800, 1000, and 1200 mg indole-3-carbinol. The maximum plasma concentration (Cmax) of DIM in the women was detected at the 1000 mg I3C oral dose.

Evidence strength: Preclinical evidence (in vitro and animal) is extensive. Human clinical evidence for breast cancer chemoprevention is preliminary; no large-scale placebo-controlled RCTs have established definitive reduction in breast cancer incidence. The clinical trials conducted to date examine primarily the cancer therapeutic potential of DIM and not strictly chemoprevention. Double-blind, placebo-controlled studies in disease-free subjects are needed to better determine the potency and efficacy of DIM as a chemopreventive supplement.

Recurrent Respiratory Papillomatosis

I3C in doses up to 400 mg/day has been used to treat recurrent respiratory papillomatosis. This condition, driven by human papillomavirus, has been a specific target of I3C supplementation, and early clinical evidence suggests a potential role; however, evidence from large controlled trials remains limited.

General Evidence Characterization for Cancer

Current clinical trials regarding I3C are mainly focused on breast, prostate, and colon cancers, whereas DIM clinical trials are for cervical dysplasia and breast and prostate cancers. Clinical and preclinical trials have evaluated the pharmacokinetic/pharmacodynamic effectiveness, antioxidant, and cancer-preventing activities of I3C and DIM, showing promising results. However, the exact mechanism by which they exert anti-cancer and apoptosis-inducing properties has yet to be entirely understood.

5.2 Estrogen Metabolism and Hormonal Health

Both indole-3-carbinol and diindolylmethane are believed to be metabolized by the same enzymatic system used in human cells to metabolize estrogenic compounds. The shift in the ratio of 2-hydroxyestrone to 16α-hydroxyestrone — towards the "protective" 2-hydroxyestrone — has been documented in clinical studies and is considered a potential biomarker for reduced estrogen-related cancer risk.

In premenopausal women, a supplement containing I3C and 7-hydroxymatairesinol, a dietary ingredient, increased the urinary 2:16-hydroxyestrone ratio, a known biomarker for the reduction of breast cancer risk.

Evidence strength: Several small human studies consistently show measurable changes in estrogen metabolite ratios. Whether these metabolic shifts translate into reduced cancer incidence requires demonstration in larger prospective trials. Evidence is mechanistically plausible but clinically preliminary.

5.3 Thyroid Health

A pilot clinical study specifically examined DIM's effects on estrogen metabolism in patients with thyroid proliferative disease. Cruciferous vegetables such as Brussels sprouts, broccoli, kale, cabbage, and cauliflower are rich sources of dietary bioactive compounds, namely I3C and its major metabolite DIM, and DIM was studied for its ability to modulate estrogen metabolism in patients with thyroid proliferative disease. This represents a very early stage of evidence, and no firm clinical recommendations can be drawn.

5.4 Gut Health and Intestinal Homeostasis

Lactobacillus metabolizes tryptophan into ILA, thereby augmenting the expression of key bacterial enzymes implicated in tryptophan metabolism, leading to the synthesis of other indole derivatives including IPA and IAA. ILA, IPA, and IAA possess the ability to mitigate intestinal inflammation and modulate the gut microbiota in both DSS-induced and IL-10 knockout spontaneous colitis animal models.

Ample evidence suggests that indoles derived from gut microbiota metabolism exert significant biological effects and may contribute to the etiology of cardiovascular, metabolic, and psychiatric diseases. However, the majority of the research is limited to experimental studies and only a small number of clinical trials.

Evidence strength: Largely preclinical (in vitro and animal models). Human clinical evidence is sparse. The gut–indole axis is a rapidly developing research area but is not yet established in clinical practice.

5.5 Neuroprotection and Neurology

The microbiota-derived metabolite of tryptophan, indole-3-propionic acid (IPA), was found to target a number of molecular processes and to impact brain function. IPA may affect neuronal activity and survival through key mechanisms. Overall, gathered data confirm neuroprotective features of IPA and support its potential use in high-risk populations, in order to delay the onset and ameliorate the course of neurodegenerative disorders and cognitive impairment.

Human data show lower IPA levels in patients with mild cognitive impairment and Alzheimer's disease than in controls.

A randomized, double-blind, placebo-controlled, multicenter clinical trial was conducted in 63 healthy elderly individuals (≥65 years of age). Participants were administered either placebo (N=31) or probiotic capsules (Bifidobacterium bifidum BGN4 and Bifidobacterium longum BORI; N=32) for 12 weeks. The main finding of this study was that probiotic intervention in healthy elderly individuals increases the level of gut microbial-derived IPA, which exerts neuroprotective effects through the regulation of inflammatory signals in microglial BV2 cells.

Microbiota-derived IPA possesses strong antioxidant and neuroprotective properties, and may ameliorate metabolic, inflammatory, and neurodegenerative changes in various experimental paradigms. Clinical trials evaluating IPA as a promising therapeutic add-on, able to slow down the progress of neurodegenerative disorders such as Alzheimer's or Parkinson's disease and to limit the morphological and behavioral consequences of ischemic stroke, are urgently needed.

Evidence strength: Mechanistic and animal evidence is robust. One small human trial (n=63) indirectly shows elevated IPA following probiotic supplementation. Direct clinical trials using IPA as a supplement in neurological disease are lacking as of this writing. Evidence is promising but preliminary.

5.6 Cardiovascular and Metabolic Disease

Ample evidence suggests that indoles derived from gut microbiota metabolism exert significant biological effects and may contribute to the etiology of cardiovascular and metabolic diseases. Indoxyl sulfate, an indole derivative, has demonstrated nephrotoxicity and cardiovascular toxicity, representing a dual and context-dependent role for the indole family in cardiovascular biology.

Emerging data demonstrate that indoles exert a profound impact on metabolic, immune, cardiovascular, and brain function. However, clinical evidence specific to indole supplementation for cardiovascular or metabolic outcomes in humans is currently lacking; the evidence base is preclinical.

5.7 Antiviral Activity

The potential antiviral effects of I3C, a phytochemical found in cruciferous vegetables, have been investigated. Findings indicate I3C's potential effect in preventing coronavirus cell egression processes that inhibit viral production. Although further studies are needed to clarify the molecular mechanisms whereby HECT family members control virus life cycle, this work opens the possibility of therapeutic use of natural compounds that may reduce the clinical severity of future pandemics.

Evidence strength: This area is in its earliest stages — evidence is in vitro (3D cell culture models) only. No human clinical trials of indole compounds for antiviral purposes have been reported in the peer-reviewed literature to this date.

6. Body Systems Associated with Indoles

• Endocrine system: Modulation of estrogen metabolism via CYP enzyme induction; androgen receptor regulation; effects on thyroid-associated proliferative disease (preliminary).
• Gastrointestinal system: Maintenance of intestinal barrier integrity; regulation of gut microbiota composition; anti-inflammatory effects in models of IBD and colitis.
• Immune system: AhR- and PXR-mediated immune modulation; regulation of inflammatory cytokine production.
• Hepatic system: Induction of CYP1A1, CYP1A2, CYP3A, and CYP1B1 enzymes; phase II enzyme induction; estrogen detoxification.
• Central nervous system: Gut–brain axis signaling via IPA and other microbiota-derived indoles; potential neuroprotection in models of Alzheimer's and Parkinson's disease.
• Oncological: Chemopreventive activity in multiple cancer lines in vitro and in animal models; early human clinical evidence in cervical, breast, and prostate tissues.

7. Dosage Forms and Reported Dosages

The following dosages are reported from identified peer-reviewed clinical studies and government reports and should be understood strictly in that context:

• I3C doses of 200 mg/day or 400 mg/day were used to assess regression of cervical intraepithelial neoplasia (CIN) in a preliminary clinical trial.
• I3C in doses up to 400 mg/day has been used to treat recurrent respiratory papillomatosis.
• In a human estrogen metabolism study, subjects received oral I3C at 6–7 mg/kg per day; seven men received I3C for 1 week, and 10 women received I3C for 2 months.
• In a phase I dose-escalation study in non-metastatic prostate cancer patients, twice-daily 225 mg of DIM was recommended for phase II studies.
• Based on clinical trials primarily in humans with existing disease, supplementation with DIM at 200–400 mg/day is not likely to represent a significant risk, with the caveat that co-administration of some drugs could cause an adverse effect.
• In breast cancer risk pharmacokinetics studies, non-smoking women received oral doses of 400, 600, 800, 1000, and 1200 mg I3C, with DIM detected as the only plasma product; maximum plasma DIM concentration was observed at the 1000 mg I3C dose.
• In NTP toxicology studies in rats, groups received 0, 18.75, 37.5, 75, 150, or 300 mg I3C/kg body weight in corn oil by gavage, 5 days per week for 14 weeks.

The few studies comparing glucobrassicin-rich crucifers such as Brussels sprouts with I3C/DIM supplements have shown that the greater impact of the latter is due to dose. Daily ingestion of kilogram quantities of Brussels sprouts would be required to produce in vivo levels of DIM achievable by supplementation. In clinical trials, these supplement doses have elicited few if any adverse effects.

8. Safety Considerations and Drug Interactions

8.1 General Tolerability in Human Studies

I3C is generally well tolerated when taken orally, but it is unclear if I3C supplementation can benefit humans due to its mixed effects in preliminary studies and its ability to induce cytochrome P450 enzymes, which may cause interactions with several medications.

Reported adverse effects in clinical settings include balance problems, diarrhea, nausea, rash, and tremors; I3C is considered possibly safe when used in doses up to 400 mg daily for periods ranging from 3 to 76 months.

8.2 CYP Enzyme Induction and Drug Interactions

I3C administration to rats markedly induces several cytochrome P450s (CYPs), especially CYP1A1 (approximately 25-fold), while at the same time inhibiting the expression of FMO1. The consequence is a marked shift in the metabolic profile of drugs such as nicotine and tamoxifen, which are substrates for both monooxygenases. Such an effect could lead to adverse drug reactions in humans.

Persons who are taking any medication that contains estrogen (including birth control pills) should be aware that I3C might interfere with the action of this type of medication. Other studies found interactions with the antipsychotic clozapine and the selective serotonin reuptake inhibitor duloxetine.

8.3 Preclinical Toxicology: NTP Findings

Administration of indole-3-carbinol caused increased incidences of nonneoplastic lesions in the small intestine, mesenteric lymph node, and liver of male and female rats; the thyroid gland of male rats; the uterus of female rats; and the liver, glandular stomach, and nose of male and female mice.

Indole-3-carbinol exhibited the potential to be a reproductive toxicant in male and female mice based on significantly decreased sperm motility in all dosed groups of males and a significantly increased probability of extended diestrus in females administered 250 mg/kg.

The absolute and relative liver weights of all dosed groups of male and female rats were significantly increased compared to vehicle controls.

Indole-3-carbinol may undergo reactions in the stomach to produce genotoxic compounds or compounds that can enhance the genotoxicity of other compounds through modulation of key metabolic processes. Indole-3-carbinol was nominated by the National Cancer Institute for toxicity and carcinogenicity testing because of its occurrence in natural products and its potential use as a breast cancer chemopreventive agent.

These NTP findings were generated in animal models at doses substantially higher than those used in human trials; their direct applicability to human supplementation at recommended doses is uncertain, but they have informed ongoing regulatory scrutiny of I3C supplements.

8.4 Intestinal Toxicity in Immunocompromised Models

The intestine appeared to be the target of I3C toxicity in immunocompromised rodent models. The number and width of intestinal villi were significantly altered by I3C, which was associated with a dose-dependent reduction in cell proliferation and increase in apoptosis. Other molecular effects observed for I3C include activation of multiple xenobiotic metabolism pathways. This was the first study to report hazardous effects of I3C supplementation specific to the gastrointestinal tract in an immunocompromised model and should serve as a caution for use of I3C as a dietary supplement.

8.5 Dual Role of Microbiota-Derived Indoles

The toxic influences of indole metabolites cannot be ignored. Indoxyl sulfate, an indole derivative, demonstrates nephrotoxicity and cardiovascular toxicity. Indole not only triggers biofilm formation and antibiotic resistance in gut microbes but also contributes to the progression of kidney dysfunction after absorption by the intestine and sulfation in the liver. This dual (beneficial and harmful) role depends heavily on context, concentration, and the metabolic fate of the parent indole molecule.

8.6 Quality and Labeling Concerns

Analysis by HPLC found amounts of active substance in commercial products much lower than what was declared on labels. This evidence generates a critical need to regulate this type of supplement to guarantee the daily consumption of the active substance by people who seek a benefit associated with I3C and DIM.

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