Naphthoquinones

Naphthoquinones: A Comprehensive Reference

1. Identity and Chemical Classification

Naphthoquinones are a group of aromatic compounds derived from naphthalene, characterized by a conjugated cyclic dione structure that can exist in either a 1,4 or 1,2 arrangement. The 1,4-naphthoquinones (1,4-NQs) are redox active compounds structurally related to naphthalene that are comprised of a benzene moiety (ring A) linearly fused with a fully conjugated cyclic diketone (ring B) in which the carbonyl groups are arranged in the para orientation. The 1,2 isomeric form (ortho-naphthoquinone) is also found in nature, though the 1,4 arrangement is by far the most common and the most extensively studied. There are hundreds of 1,4-naphthoquinones, organic compounds derived from naphthalene.

Naturally occurring naphthoquinones and naphthalenones are a large group of secondary metabolites obtained from several sources including higher plants, fungi, and some bacteria, which showed a wide range of biological and pharmacological activities. Naphthoquinones are colored substances derived from phenylpropanoid and isoprenoid precursors.

Key Named Compounds

• Lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone): Lapachol was identified as the active substance in extracts of the roots of Stereospermum suaveolens and showed antitumor activity against the rat Walker 256 carcinoma.
• β-Lapachone (an ortho-naphthoquinone): β-Lapachone is an o-naphthoquinone with potent antitumor activity.
• Plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone): Plumbagin, a naphthoquinone derivative initially isolated from Plumbago zeylanica L., displays diverse biological actions, such as antioxidant, anti-inflammatory, anti-cancer, and anti-fungal properties.
• Shikonin and its enantiomer alkannin: Enantiomeric natural products alkannin (isolated from the roots of Alkanna tinctoria) and shikonin (isolated from the roots of Lithospermum erythrorhizon) have been used in European and Chinese medicine, respectively, for the treatment of several afflictions, from ulcers to dermatitis.
• Juglone (5-hydroxy-1,4-naphthoquinone): Juglone is a yellow pigment found in black walnut (Juglans regia).
• Lawsone (2-hydroxy-1,4-naphthoquinone): Lawsone is found in henna (Lawsonia inermis), which has been used for centuries in Ayurvedic and Middle Eastern medicine not only as a dye but also for its wound-healing and antifungal effects.
• Vitamin K1 (phylloquinone) and Vitamin K2 (menaquinone): One of the most well-known naphthoquinones is vitamin K, which is essential for blood clotting and bone health. Vitamin K is a derivative of 1,4-naphthoquinone and is found in leafy green vegetables such as spinach and kale.

2. Natural Sources and Botanical Origins

The 1,4-naphthoquinones (1,4-NQs) are a diverse group of natural products found in every kingdom of life. Naphthoquinones are the most commonly occurring type of quinones in nature. They are a diverse family of secondary metabolites that occur naturally in plants, lichens and various microorganisms.

Plant Sources

Many members of the Boraginaceae family produce naphthoquinones in their roots. Plants of the borage family are distributed worldwide and the naphthoquinones from many of these plants have been used in diverse cultures as colorants for cosmetics, fabrics, and foods, and for medicinal applications, including antitumor, anti-inflammatory, and antimicrobial agents.

Prominent naphthoquinone-producing plant genera and species include:

• Tabebuia spp. (also classified as Handroanthus spp.) — inner bark yields lapachol and β-lapachone. Lapachol is isolated from Tabebuia avellanedae and extremely used in American folk medicine for the treatment of many diseases, such as cancer, lupus, and infections.
• Plumbago zeylanica — root source of plumbagin. Plumbago zeylanica (Plumbaginaceae) grows throughout tropical Africa and Asia and the root is used in Indian medicine; it contains juglone in addition to pentacyclic triterpenes.
• Lithospermum erythrorhizon — root source of shikonin. Shikonin is present in the root extract of Lithospermum erythrorhizon, named Zicao, and was used in traditional Chinese medicine for its anti-inflammatory effects.
• Alkanna tinctoria — root source of alkannin. The roots of several Echium species are particularly enriched with alkannins and shikonins, naphthoquinone compounds with well-documented wound-healing, collagen-stimulating, and anticancer properties.
• Juglans nigra, Juglans regia (black and English walnut) — hull and green husk source of juglone. Juglone is found in some species of the family Juglandaceae—for example, Juglans regia, Juglans nigra, and Juglans cineraria—and its use is reported against ringworm, fungal, bacterial, and viral infections and as a cure for heat stroke.
• Lawsonia inermis (henna) — leaves and stems source of lawsone.
• Diospyros spp. (Ebenaceae): In the heartwoods of Diospyros spp., naphthoquinones occur as monomers, complex dimers and trimers. In addition to timber usage (ebony), many species of Diospyros are used worldwide in the traditional medicine of countries where they grow.

Fungal and Microbial Sources

A comprehensive review discusses the chemical characteristics and biological/pharmacological activities of fungal naphthoquinones and naphthalenones reported over the period from 2000 to 2021, highlighting 153 naphthoquinones and their derivatives from twenty-six different fungal genera. Structures and physicochemical properties of 100 naphthoquinone metabolites produced by filamentous fungi have been reviewed; the conditions of pigment formation, biogenesis, and the mechanism of biosynthesis by fungi are described, and sixty-three fungi cultures able to produce naphthoquinones have been listed.

Common Preparations and Forms

Naphthoquinones reach consumers and research settings in several forms:

• Bark decoctions and teas (notably Tabebuia / Pau d'Arco inner bark)
• Root extracts (standardized and non-standardized) from Plumbago, Lithospermum, and Alkanna species
• Black walnut hull tinctures and capsules (juglone-containing)
• Topical henna-based preparations (lawsone)
• Isolated, purified compounds in capsule, tablet, or injectable form (used in pharmacological research)
• Available formulations for commercial Pau d'Arco products include capsules, tablets, liquid extracts, powder, and herbal tea; some products are standardized to 3% naphthoquinones.

3. Traditional and Historical Use

Numerous horticultural plants producing 1,4-NQs have also served as sources of traditional medicines for hundreds of years. The long use history of naphthoquinones has witnessed their functional shift from the original purposes as dyes and ornaments toward medicinal benefits.

South American Traditions

Ethnobotanical and experimental evidence supports the use of Tabebuia species for treating infectious diseases; the compounds responsible for their antimicrobial activity have been isolated, and their structures elucidated, emphasizing among them naphthoquinones such as lapachol. Preparations from the inner bark of Pau d'Arco (Tabebuia spp.) have been used for centuries in the indigenous healing traditions of Amazonian and South American peoples, including for fever, infections, and cancer-like conditions.

Ayurvedic and Unani Medicine

Arnebia euchroma (Ratanjot) has widely been used in the traditional systems of the Unani, Ayurvedic and Chinese medicines recipes due to its anti-fungal and anti-microbial properties. For centuries, naphthoquinone-containing plants were used to treat skin infections, wounds, and fungal conditions. In Ayurvedic and Unani medicine, extracts from these plants were applied to promote wound healing, reduce inflammation, and combat microbial infections.

Traditional Chinese Medicine

Shikonin is present in the root extract of Lithospermum erythrorhizon, named Zicao, and was used in traditional Chinese medicine for its anti-inflammatory effects until studies revealed other interesting properties, like anticancer, antimicrobial and wound healing.

Middle Eastern and South Asian Traditions (Henna / Lawsone)

Lawsonia inermis L. (Family Lythraceae), commonly known as henna, has tremendous pharmaceutical and cosmeceutical applications, which are associated with its historical folk uses in various traditions. Henna, rich in the naphthoquinone lawsone, was not only used as a dye but also as a remedy for headaches, fevers, and skin ailments.

European Folk and Ethnomedicinal Use

Herbal preparations derived from black walnut have been used as hair dyes and skin colorants in addition to being applied topically for the treatment of acne, inflammatory diseases, ringworm, and fungal, bacterial, or viral infections. In folk remedies, black walnut hulls—containing juglone, another prominent naphthoquinone—were renowned for their anti-parasitic and anti-fungal actions. They were often included in tinctures and herbal blends to support digestive health and clear intestinal parasites.

Indigenous Traditions (Aotearoa / New Zealand)

Historical indigenous applications of plants for medicinal uses are well known across the globe and in Aotearoa, with many medicinal applications arising from naphthoquinones (NQ), anthraquinones (AQ) and their quinol or quinone glycosides. Their occurrence in Aotearoa and association with Mātauranga Māori (traditional Māori knowledge)—particularly in Dianella (flax lilies), Phormium (harakeke), Coprosma (karamu) and Bulbinella—has been documented.

Thai Traditional Medicine

In Thai traditional medicine, Impatiens balsamina is commonly used as a topical remedy for wound healing, antimicrobial, and antiallergic purposes, and naphthoquinones are the major identified medicinal constituents of I. balsamina.

Ecological Roles Informing Traditional Knowledge

Naphthoquinones can also be allelopathic, such as in the case of walnut/Juglans, stunting and weakening nearby competing plant species. The compounds' properties include producing harmful oxidative chemicals, disrupting metabolism and even damaging DNA, which pre-adapt them to roles in competition between plants and protection against microbial attack.

4. Key Constituents and Mechanisms of Action

Core Redox Chemistry

The molecular mechanisms of naphthoquinones in cells mainly fall into two categories: inducing oxidant stress by ROS (reactive oxygen species) generation and directly interacting with traditional therapeutic targets in a non-oxidant mechanism. The ability of naphthoquinones to undergo one- and two-electron reduction cycles—generating semiquinone radical intermediates and subsequently superoxide and hydrogen peroxide—is the biochemical foundation for much of their observed biological activity.

The cytotoxicity of naphthoquinones such as juglone and plumbagin is due to two different mechanisms: redox cycling and reaction with glutathione (GSH). Redox cycling results in the generation of the corresponding semiquinone radicals, detected by electron paramagnetic resonance. Incubation of keratinocytes with the quinones generated hydrogen peroxide (H₂O₂) and resulted in the oxidation of GSH to GSSG.

Compound-Specific Mechanisms

Plumbagin: Plumbagin exerts anticancer activity via many molecular mechanisms, such as targeting apoptosis, autophagy pathway, cell cycle arrest, antiangiogenesis pathway, anti-invasion, and antimetastasis pathway. Among these signaling pathways, the key regulatory genes regulated by plumbagin are NF-κβ, STAT3, and AKT. PLB also acts as a potent inducer of reactive oxygen species (ROS), suppressor of cellular glutathione, and novel proteasome inhibitor, causing DNA double-strand break by oxidative DNA base damage.

Shikonin: Recent studies show that shikonin suppresses tumor growth in lung, ovarian, and other cancers through diverse mechanisms, including apoptosis induction, cell cycle arrest, inhibition of metastasis, necroptosis, and disruption of tumor metabolism. In particular, shikonin showed PKM2 inhibitory activity and anti-cancer activity, selectively inhibiting pyruvate kinase M2 (PKM2) rather than PKM1, which in turn reduced glycolytic levels.

Lapachol / β-Lapachone: Reliance on aerobic glycolysis is one of the hallmarks of cancer. Pyruvate kinase M2 (PKM2) is a key mediator of glycolysis in cancer cells. Unlike its structural analog shikonin, a known inhibitor of PKM2, lapachol failed to induce non-apoptotic cell death ferroxitosis in hypoxia. However, melanoma cells treated with lapachol showed a dose-dependent inhibition of glycolysis and a corresponding increase in oxygen consumption.

Topoisomerase inhibition (plumbagin and shikonin): Treatment of a reaction mixture containing plumbagin or shikonin and topoisomerase II resulted in DNA cleavage, suggesting that the mechanism of topoisomerase II-mediated DNA cleavage induced by these naphthoquinones is through formation of a cleavable complex, as seen with known antitumor agents.

Ferroptosis induction (plumbagin): Plumbagin showed higher blood–brain barrier penetration ability than that of lapachol and shikonin and elicited significant growth inhibitory effects in vitro and in vivo. Ferroptosis was the main mechanism of plumbagin-induced cell death.

Anti-inflammatory activity (nitric oxide inhibition): Several naphthoquinones—plumbagin, naphthazarin, juglone, menadione, diosquinone, and 1,4-naphthoquinone—were evaluated for their ability to cause a reduction of nitric oxide (NO) production, when RAW 264.7 macrophages were stimulated with lipopolysaccharide (LPS).

Vitamin K activity: The most well-known naphthoquinone in nutrition is vitamin K, particularly vitamin K1 (phylloquinone) and vitamin K2 (menaquinone), which play essential roles in blood coagulation and bone health.

Neuroprotective mechanisms: An overview of naphthoquinones with neuroprotective effects found that shikonin, plumbagin, and vitamin K prevented oxidative stress through multiple mechanisms. Synthetic naphthoquinones with inhibitory activity on the P2X7 receptor led to a neuroprotective effect on Neuro-2a cells. Naphthazarin was found to act as an inhibitor of the MAO-B enzyme. Vitamin K and synthetic naphthoquinone hybrids with tryptophan or dopamine showed inhibition of the aggregation of α-synuclein. Synthetic derivatives of juglone and naphthazarin were able to protect Neuro-2a cells against neurodegenerative effects of neurotoxins.

5. Scientific Evidence by Area of Health Use

5.1 Oncology / Anticancer Activity

Plumbagin — cell/animal evidence: Plumbagin exhibited anticancer activity against a variety of cancer cell lines including breast cancer, hepatoma, leukemia, melanoma, prostate cancer, brain tumor, tongue squamous cell carcinoma, esophageal cancer, oral squamous cell carcinoma, lung cancer, kidney adenocarcinoma, cholangiocarcinoma, gastric cancer, lymphocyte carcinoma, osteosarcoma, and canine cancer. Plumbagin induced cytotoxicity in human breast cancer cells (MDA-MB-231 and MCF-7) along with cell cycle arrest, DNA damage, apoptosis, and suppression of telomere and telomerase activity, and induced cytoplasmic vacuolation and cell cycle G2/M arrest in MDA-MB-231 cells through inhibition of proteasome and disruption of sulfhydryl homeostasis. However, all evidence to date is from preclinical (in vitro and animal) studies, and no human clinical trials with plumbagin as an anticancer agent have been reported in the peer-reviewed literature.

Plumbagin — non-small cell lung cancer (in vitro): Plumbagin exerted anticancer activity on NSCLC cells by modulating the pro-survival and pro-apoptotic signaling that causes induction of apoptosis. This represents in vitro evidence only.

Shikonin — cancer metabolism (preclinical): Recent studies show that shikonin suppresses tumor growth in lung, ovarian, and other cancers through diverse mechanisms, including apoptosis induction, cell cycle arrest, inhibition of metastasis, necroptosis, and disruption of tumor metabolism. Its multi-targeted activity and relatively low systemic toxicity highlight shikonin as a compelling candidate for development as both a preventive agent and an adjunct to conventional cancer therapies. Evidence is predominantly in vitro and animal-based, with translational studies ongoing.

β-Lapachone — clinical trials: β-Lapachone (ARQ501) demonstrated molecular target activity including the induction of apoptosis in transformed cells, activation of caspase-3, inhibition of NF-κB, and subsequent downregulation of bcl-2. It has progressed to Phase II clinical trials in the United States for advanced solid tumors. However, in clinical trials, various effects of ARQ 761 (a β-lapachone prodrug) were observed in patients, including anemia (79%), fatigue (45%), hypoxia (33%), methemoglobinemia (26%), nausea (17%), and vomiting (17%), with a maximum tolerated dose of 390 mg/m². Despite its potential antitumor activity, β-lapachone exhibits high toxicity, which has encouraged the development and synthesis of new derivatives with greater selectivity and fewer adverse effects.

Lapachol — phase I clinical trial: A Phase I clinical trial of lapachol was performed, revealing an anticoagulant side effect. Lapachol is a vitamin K antagonist with antitumor activity. Subsequent studies documented significant toxicity, particularly hepatotoxicity and anticoagulant effects, in animal models and initial human trials, which limited further development as an anti-cancer agent. Clinical evidence for lapachol's efficacy is extremely limited.

β-Lapachone — cytotoxicity in vitro: β-Lapachone exhibited the strongest antiproliferative activity against HeLa, MCF-7, and A549 cancer cell lines, with IC50 values ranging from 2.85 to 6.24 µM, approaching the activity of doxorubicin under the same in vitro conditions. These findings indicate that lapachol derivatives possess promising bioactivity at the cellular level; however, their pharmacological relevance should be interpreted cautiously in the absence of in vivo or clinical validation.

Evidence strength summary (anticancer): The anticancer evidence for naphthoquinones is largely preclinical — comprising in vitro cell-line studies and animal model experiments — and is substantial in volume. Only β-lapachone has advanced to human clinical trials, where dose-limiting toxicity has been a major challenge. No naphthoquinone has achieved regulatory approval as an anticancer drug as of the latest available literature.

5.2 Antimicrobial Activity

Naphthoquinones are a class of aromatic compounds relevant for their chemical characteristics, structural properties, and biological activity. These compounds are found in nature with a wide range of effects, highlighting their antibacterial, antifungal, and antiprotozoal properties. Additionally, naphthoquinones are used as a scaffold to obtain new derivatives with pharmacological potential, mainly compounds against parasitic diseases.

Antibacterial (in vitro): Among twenty-two naphthoquinone compounds isolated or derived synthetically from culture extracts of Fusarium solani and F. oxysporum, fifteen exhibited antibiotic activity against Staphylococcus aureus, and 12 were active against Streptococcus pyogenes, but none were active at the highest rate of 128 µg/mL against Escherichia coli, Klebsiella pneumoniae, Salmonella typhi, Proteus vulgaris, Serratia marcescens, or Pseudomonas aeruginosa. This suggests activity is principally against Gram-positive bacteria.

Rubromycin CA1, a microbial naphthoquinone, exhibited strong antibacterial activity against Staphylococcus aureus with a MIC of 0.2 µg/mL.

Antifungal (in vitro): Among fungi of medical interest, yeasts of the genus Candida are of extreme importance due to their high frequency of colonization and infection in humans, and the development of naphthoquinones as antifungals for the treatment of Candida species has been a focus of multiple reviews. The antifungal activities of 1,4-naphthoquinone derivatives have obtained MICs of 8 µg/mL and 16 µg/mL against cultures of Candida albicans and Candida parapsilosis, respectively. All reported antifungal data are from in vitro studies; controlled human clinical data are absent.

Antiparasitic: Among natural naphthoquinones, lapachol, α-lapachone, β-lapachone, lawsone, juglone, and plumbagin have been evaluated for their potential as antitrypanosomal activities. In vitro studies found the lethal dose (LD₅₀) on promastigote forms of Leishmania amazonensis using lapachol was 75.60, 72.82, and 58.85 µg/mL at 24, 48, and 72 hours, while for β-lapachone it was 0.65, 1.24, and 0.71 µg/mL, respectively. The naphthoquinones significantly inhibited the survival rate of L. amazonensis amastigotes. A review concluded that natural and synthetic naphthoquinones showed antiparasitic activity in most cases with improved results compared to current drugs, in preclinical studies.

Evidence strength summary (antimicrobial): Evidence is principally in vitro, with no human randomized controlled trials available to date. The activity against Gram-positive bacteria, Candida species, and protozoal parasites is well-documented in laboratory settings, making naphthoquinones a promising scaffold for drug development, but clinical translation remains incomplete.

5.3 Anti-inflammatory Activity

Apart from the traditional effects of wound healing, anti-inflammatory, hemostatic, antifertility, insecticidal and antimicrobial activities, the anticancer potential of naphthoquinones is being more and more realized. The anti-inflammatory effects of various naphthoquinones have been demonstrated in cell and animal studies, primarily through inhibition of pro-inflammatory signaling pathways including NF-κB and reduction of nitric oxide production. Plumbagin's potent anti-oxidant and anti-inflammatory properties offer promising avenues for the treatment of neurodegenerative and cardiovascular diseases. All evidence in this domain for individual naphthoquinones remains preclinical.

5.4 Neuroprotection and Neurodegenerative Disease

Parkinson's disease (PD) is a degenerative disease that affects approximately 6.1 million people and is primarily caused by the loss of dopaminergic neurons. Naphthoquinones have several biological activities explored in the literature, including neuroprotective effects.

Targeted screening using the MTT cell viability test with a mini-library of natural and synthetic 1,4-naphthoquinones and their derivatives was performed to increase the survival of Neuro-2a neuroblastoma cells in in vitro paraquat and 6-hydroxydopamine models of Parkinson's disease. As a result, 10 compounds were selected that could protect neuronal cells from the cytotoxic effects of both paraquat and 6-hydroxydopamine. The five most active compounds significantly protected the activity of nonspecific esterase from the inhibitory effects of neurotoxins, defended cell biomembranes from lytic destruction, and normalized the cell cycle.

Cardioprotective, anti-ischemic, hepatoprotective, neuroprotective, and other new properties have been found for 1,4-naphthoquinone compounds; their role in protecting against neurodegenerative diseases has been established in preclinical research.

Evidence strength (neuroprotection): Evidence is entirely preclinical (in vitro cell models, isolated neuronal preparations, and animal studies). No human clinical trials examining naphthoquinones specifically for neurodegenerative diseases have been identified in the available literature.

5.5 Antidiabetic Activity

In terms of antidiabetic potential of Impatiens balsamina (a naphthoquinone-rich plant), there has been one report demonstrating α-amylase inhibitory activity. An ethanol extract (200–400 µg/mL) of I. balsamina seed was tested for in vitro α-amylase inhibitory activity; the extract showed moderate α-amylase inhibitory activity in comparison to the standard drug acarbose. Evidence in this area is limited to in vitro enzyme inhibition assays, with no human clinical trial data available.

5.6 Wound Healing

In vitro and in vivo studies have shown the wound-healing activities of shikonin and alkannin. Alkannins and shikonins are naphthoquinone compounds with well-documented wound-healing and collagen-stimulating properties. Historically, multiple cultures have applied henna (lawsone), shikonin-containing roots, and plumbagin-rich plant extracts topically for wound healing, and this application has partial preclinical support; controlled human trials are lacking.

5.7 Coagulation and Vitamin K Function

Among all naphthoquinone-related health applications, the role of vitamin K (itself a 1,4-naphthoquinone derivative) in coagulation and bone health is backed by the strongest and most established clinical evidence. Scientific studies have validated the critical function of vitamin K1 and K2 (both naphthoquinones) in activating clotting factors, thus supporting coagulation. This literature is extensive, involves large randomized controlled trials and meta-analyses, and has led to regulatory guidance — but it pertains specifically to vitamin K rather than to naphthoquinones in general.

6. Body Systems Associated with Naphthoquinone Activity

• Oncology/Hematology: Cytotoxic, pro-apoptotic, and anti-proliferative activity documented across multiple cancer cell types and animal models.
• Immune system: Modulation of macrophage activity, pro-inflammatory cytokine inhibition, and antimicrobial defense.
• Coagulation system: Vitamin K-related naphthoquinones are essential coagulation cofactors; lapachol acts as a vitamin K antagonist at higher doses.
• Central nervous system: Preclinical neuroprotection in Parkinson's disease models via anti-oxidant, MAO-B inhibitory, and α-synuclein aggregation-inhibitory mechanisms.
• Integumentary system (skin): Wound healing, anti-inflammatory, and antimicrobial effects documented in topical preparations.
• Gastrointestinal tract: Traditional use for intestinal parasites and infections; in vitro antiparasitic evidence supports this application.
• Skeletal system: Via vitamin K2 (menaquinone) influence on osteocalcin carboxylation and bone mineralization, well established in clinical literature.
• Cardiovascular system: Preliminary preclinical data suggest cardioprotective and anti-ischemic effects; plumbagin's potent anti-oxidant and anti-inflammatory properties offer promising avenues for the treatment of cardiovascular diseases.

7. Dosage Forms and Doses Reported in Studies

Dosage data for naphthoquinones are highly compound- and context-specific. The following are doses as reported in identified sources:

• Lapachol (antiparasitic, in vitro): The lethal dose (LD₅₀) on promastigote forms of L. amazonensis using lapachol was 75.60, 72.82, and 58.85 µg/mL at 24, 48, and 72 hours, respectively.
• β-Lapachone (antiparasitic, in vitro): The LD₅₀ for β-lapachone on promastigote forms was 0.65, 1.24, and 0.71 µg/mL at 24, 48, and 72 hours, respectively.
• Lapachol (animal, anti-metastatic): Serial oral administration of low non-toxic doses of lapachol (5–20 mg/kg) weakly but significantly suppressed metastasis in a murine model.
• β-Lapachone (human, ARQ 761, clinical trial): The maximum tolerated dose was found to be 390 mg/m².
• Pau d'Arco (human, safety/tolerability): For dysmenorrhea, 1,050 mg of Pau d'Arco combined with 75 mg of rutin per day for 8 weeks was well tolerated, with no serious adverse effects reported.
• 1,4-naphthoquinone derivatives (antifungal, in vitro): MICs of 8 µg/mL and 16 µg/mL were obtained against Candida albicans and Candida parapsilosis, respectively.
• Juglone and plumbagin (keratinocyte cytotoxicity, in vitro): Exposure to juglone or plumbagin (1–20 µM) resulted in a concentration-dependent decrease in cell viability.
• I. balsamina extract (antidiabetic, in vitro): An ethanol extract (200–400 µg/mL) of I. balsamina seed was tested for in vitro α-amylase inhibitory activity.

No standardized, evidence-validated therapeutic dose exists for any naturally sourced naphthoquinone compound in human clinical practice beyond the well-established dosing of pharmaceutical vitamin K preparations.

8. Safety Considerations and Drug Interactions

Dose-Dependent Cytotoxicity

Selected naphthoquinones — 1,4-naphthoquinone, menadione, juglone, and plumbagin — were studied in human keratinocytes (HaCaT). These compounds were highly cytotoxic, strongly induced reactive oxygen species (ROS) formation, and depleted cellular glutathione. Moreover, they induced oxidative DNA base damage and accumulation of DNA strand breaks. Neither lawsone nor lapachol (up to 100 µM) were active in any of these assays. This highlights important structural variation in toxicity even within the class.

Gastrointestinal and Systemic Toxicity (Lapachol)

High doses of lapachol (>1.5 g/day) can cause significant gastrointestinal toxicities and increase the risk of bleeding, particularly in cancer patients. Studies documented significant toxicity, particularly hepatotoxicity and anticoagulant effects, in animal models and initial human trials, which limited further development as an anti-cancer agent.

Anticoagulant Interaction

Following oral administration of lapachol, protein C level was reduced maximally without elongation of prothrombin time. A high toxic dose of lapachol promotes metastasis by inducing a hypercoagulable state as a result of vitamin K-dependent pathway inhibition. Lapachol could increase the risk of bleeding when combined with anticoagulant or antiplatelet drugs or supplements. In vitro studies show that it reduces platelet aggregation and could interfere with vitamin K, an essential factor in coagulation; these effects are attributed mainly to lapachol, its active compound.

Clinical Toxicity of β-Lapachone

The clinical application of β-lapachone is hindered by poor solubility and toxicity. In clinical trials, various effects of ARQ 761 were observed in patients, including anemia (79%), fatigue (45%), hypoxia (33%), methemoglobinemia (26%), nausea (17%), and vomiting (17%).

Prooxidant vs. Antioxidant Duality

The same redox chemistry that gives naphthoquinones their antimicrobial and anticancer properties also underlies their cytotoxicity in normal tissues. All tested quinones except lawsone and lapachol strongly induced phosphorylation of the epidermal growth factor receptor (EGFR) and the related ErbB2 receptor tyrosine kinase in human keratinocytes, indicating that certain naphthoquinones can trigger stress-signaling cascades in normal human cells.

Structural Variation and Differential Safety

Hydroxylated naphthoquinones bearing a hydroxyl group on the benzene moiety, such as juglone, shikonin, and plumbagin, markedly stimulated lipid peroxidation and were highly toxic for hepatocytes and keratinocytes. By contrast, lawsone and lapachol, which carry hydroxyl groups on the quinone moiety, demonstrate substantially lower cytotoxicity in keratinocyte assays at equivalent concentrations.

Ecological and Phytotoxic Risks

Juglone and plumbagin induce ROS production on tobacco BY-2 cells, inciting programmed cell death, and juglone induced oxidative damage to the apical meristem of lettuce plants. These 1,4-NQs have also been noted to cause oxidative distress and thiol disruption in microorganisms such as bacteria and fungi. Although these effects are in plant systems, they reflect the potent redox activity of these compounds relevant to any biological target.

Pregnancy, Reproductive Toxicity

Among the traditional effects attributed to naphthoquinones is antifertility activity. This historically recognized effect suggests potential reproductive risk. No comprehensive human safety data exist to define a safe exposure window in pregnancy.

Standardization Concerns

Some commercial products are standardized to 3% naphthoquinones, but these indications are not scientifically validated. The absence of validated standardization methods and the wide variability in naphthoquinone content across plant preparations further complicate safety assessments.

9. Current Research Directions and Evidence Gaps

Several 1,4-NQ natural products derived from horticultural plants have emerged as promising scaffolds for developing new drugs. Ongoing reviews aim to provide an overview of the chemical and pharmacological profile of shikonin and its derivatives, along with their anticancer mechanisms in regulating tumor progression, discussing the potential of shikonin and its derivatives in combination therapy and translational studies relevant to future research and clinical applications in cancer treatment.

As important bioactive compounds, naphthoquinones may open new horizons for the treatment of cancer and drug-resistant bacteria. The chemical modification of naphthoquinones improves their pharmacological properties by introducing amines, amino acids, furan, pyran, pyrazole, triazole, indole, and other chemical groups. Synthetic derivatization is an active and expanding field aimed at improving selectivity, solubility, and tolerability.

Despite extensive preclinical investigation, the major gap in the naphthoquinone literature remains the near-total absence of large, well-controlled human clinical trials for applications beyond vitamin K. For most individual naphthoquinones, the evidence base does not yet support definitive conclusions about efficacy or safety in humans.

References

• Widhalm JR, Rhodes D. "Biosynthesis and molecular actions of specialized 1,4-naphthoquinone natural products produced by horticultural plants." PMC / Horticulture Research, 2016.
• Pereira GK et al. "The Relevance and Insights on 1,4-Naphthoquinones as Antimicrobial and Antitumoral Molecules: A Systematic Review." PMC, 2023.
• ScienceDirect Topics: Naphthoquinone — overview entry with multiple referenced chapters.
• ScienceDirect Topics: Naphthoquinone (Medicine and Dentistry) — overview entry.
• Qiu S et al. "Naphthoquinones: A continuing source for discovery of therapeutic antineoplastic agents." Chemical Biology & Drug Design, 2018.
• Yin Z et al. "Anticancer Effects and Mechanisms of Action of Plumbagin: Review of Research Advances." PMC / BioMed Research International, 2020.
• Shikonin as a Dietary Phytochemical with Multi-Target Anti-Cancer Activities: From Molecular Mechanisms to Translational Applications. PMC, 2025.
• Liao Z et al. "Targeting NQO1/GPX4-mediated ferroptosis by plumbagin suppresses in vitro and in vivo glioma growth." British Journal of Cancer, 2022.
• Plumbagin and shikonin induction of topoisomerase II-mediated DNA cleavage. PubMed, 1992.
• Anticancer mechanism of plumbagin on non-small cell lung cancer cells. PubMed, 2008.
• Rajagopalan S et al. "Lapachol inhibits glycolysis in cancer cells by targeting pyruvate kinase M2." PLOS ONE, 2018.
• Natural and Synthetic Naphthoquinones as Potential Anti-Infective Agents. PubMed, 2021.
• The Antifungal Activity of Naphthoquinones: An Integrative Review. PubMed, 2018.
• Antifungal activity of synthetic naphthoquinones against dermatophytes and opportunistic fungi. PMC, 2014.
• In vitro effects of lapachol and β-lapachone against Leishmania amazonensis. PMC, 2023.
• Natural naphthoquinones and their derivatives as potential drug molecules against trypanosome parasites. PubMed, 2022.
• Santos TB et al. "Naphthoquinones as a Promising Class of Compounds for Facing the Challenge of Parkinson's Disease." PMC / Pharmaceuticals, 2023.
• Neuroprotective Effect of 1,4-Naphthoquinones in an In Vitro Model of Paraquat and 6-OHDA-Induced Neurotoxicity. PMC, 2021.
• Pharmacological Features and Therapeutic Implications of Plumbagin in Cancer and Metabolic Disorders: A Narrative Review. PMC, 2024.
• Klaus V et al. "1,4-Naphthoquinones as inducers of oxidative damage and stress signaling in HaCaT human keratinocytes." Archives of Biochemistry and Biophysics, 2010.
• Inbaraj JJ, Chignell CF. "Cytotoxic Action of Juglone and Plumbagin: A Mechanistic Study Using HaCaT Keratinocytes." Chemical Research in Toxicology, 2004.
• Takashima M et al. "Promotion or suppression of experimental metastasis of B16 melanoma cells after oral administration of lapachol." Toxicology and Applied Pharmacology, 2008.
• New Insights into the Anticancer Effects and Toxicogenomic Safety of Two β-Lapachone Derivatives. PMC, 2025.
• Cell-Specific Production and Antimicrobial Activity of Naphthoquinones in Roots of Lithospermum erythrorhizon. PMC, 2000.
• Antidiabetic Naphthoquinones and Their Plant Resources in Thailand. Chemical and Pharmaceutical Bulletin, 2018.
• 1,4-Naphthoquinones: Some Biological Properties and Application. Chemical and Pharmaceutical Bulletin, 2020.
• Naphthoquinone and anthraquinone derivatives: their properties, occurrence and traditional uses. Chemistry in New Zealand, 2022.
• Unveiling the anticancer potential of plumbagin: targeting pyruvate kinase M2 to induce oxidative stress and apoptosis in hepatoma cells. PMC, 2024.
• Chemical diversity, biological activities and biosynthesis of fungal naphthoquinones: A comprehensive update. Journal of Molecular Structure, 2022.
• Is Nitric Oxide Decrease Observed with Naphthoquinones in LPS Stimulated RAW 264.7 Macrophages a Beneficial Property? PMC, 2011.