Absorption, Distribution and Excretion
Terpenes disrupt the lipid arrangement in the intercellular spaces of the stratum corneum (SC), thereby increasing skin permeability. This effect is applied in transdermal drug delivery techniques, and its effectiveness depends on the physicochemical properties of the terpenes and the amount that penetrates into the stratum corneum; however, terpenes do not necessarily penetrate into active skin tissue, and such penetration is not desirable. This study aimed to investigate the relationship between the skin absorption and elimination kinetics of four cyclic terpenes (α-pinene, β-pinene, eucalyptol, and terpinene-4-ol) when applied in pure form and their physicochemical properties. These terpenes were applied in vitro to human skin. After 1–4 hours, the stratum corneum was separated using an adhesive tape peeling method, and the content in the epidermis/dermis was determined by gas chromatography. Similarly, the terpenes content in the skin was analyzed after 4 hours, 1 hour after absorption. Terpinene-4-ol exhibited the fastest penetration rate and the fastest penetration into all skin layers. All terpenes studied were absorbed by the active epidermis/dermis, but their penetration was a time-dependent process, continuously increasing over 4 hours. Similar to the stratum corneum, terpinene-4-ol accumulated in the epidermis/dermis at the highest rates. Terpenes were rapidly eliminated from the stratum corneum, especially in deeper skin layers, and elimination was even faster if the initial accumulation was small. The cyclic terpenes studied exhibited different penetration and elimination characteristics, failing to penetrate into receptor mediators due to their large accumulation in skin tissue. The closer the log P value of a terpene compound was to 3, the greater its penetration into the stratum corneum. This study aimed to evaluate the in vitro skin penetration of five terpenes—linalool, linalyl acetate, terpinene-4-ol, citronellol, and α-pinene—in pure essential oils or skin preparations (water-in-oil emulsions, oily solutions, or hydrogels) containing 0.75% (w/w) essential oils. The results showed that different carrier types and the log P value of the terpenes affected skin absorption. Compared to topical carriers, pure essential oils accumulate terpenoids in the skin at levels several times higher. Terpinene-4-ol showed better skin permeability in oily solutions (approximately 90 μg/cm²) than in emulsions (60 μg/cm²). Linalyl acetate was not observed to penetrate from the topical carrier into the active skin, but when using an oily solution, the amount of this terpenoid penetrating into the upper stratum corneum increased twofold. In contrast, linalool showed similar skin absorption in both carriers (50-60 μg/cm²). No skin penetration was detected for α-pinene when applied as an oily solution. When applied as a hydrogel, only a small amount (approximately 5 μg/cm²) of this terpenoid was detected in the active skin. Citronellol, when applied as a hydrogel, penetrated all skin layers with a total amount of 25 μg/cm², while no penetration into the active skin layer was observed when applied as an oily solution. Only citronellol penetrated the receptor mediator. This study aimed to evaluate the effects of commonly used topical excipients, namely isopropyl myristate (IPM), oleic acid (OA), polyethylene glycol 400 (PEG400), or Transcutol (TR), on the permeability of terpinen-4-ol (T4OL) contained in pure tea tree oil to human skin. The effects of these excipients were determined by assessing the absorption of T4OL by the human epidermis and detecting perturbations of the stratum corneum structure using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Among the excipients tested, oleic acid enhanced the absorption of T4OL by disrupting the lipid barrier of the stratum corneum. The enhancing effects of other excipients were weaker and should be used with caution. This study also aimed to investigate the pharmacokinetics of purai oil extracted from the rhizome of ginger (Zingiber cassumunar Roxb.) in rat skin. The concentration of free terpinen-4-ol in dermal tissue was determined using microdialysis. The skin pharmacokinetic studies of terpinen-4-ol were conducted under non-occlusive conditions. Topical doses of puraigrette oil were 2, 4, and 8 mg/cm², corresponding to 1.0, 1.9, and 3.8 mg/cm² of terpinen-4-ol, respectively. Following topical application, terpinen-4-ol rapidly distributed into the dermis and exhibited linear pharmacokinetic characteristics, with no significant change in the dose-normalized area under the concentration-time curve (AUC) within the studied dose range. At doses of 2, 4, and 8 mg/cm², the mean percentages of free terpinen-4-ol in the dermis were 0.39 ± 0.06%, 0.41 ± 0.08%, and 0.30 ± 0.03%, respectively. These skin pharmacokinetic studies of terpinen-4-ol can provide information for further formulation development and treatment regimen formulation.

---

Metabolism/Metabolites
(R)-terpinen-4-ol was mixed into an artificial diet at a concentration of 1 mg/g and fed to final instar larvae of the beet armyworm (Spodoptera litura). Metabolites were recovered from the excrement and analyzed spectrally. (R)-terpinen-4-ol was primarily converted to (R)-p-menth-1-en-4,7-diol. Similarly, (S)-terpinen-4-ol was also primarily converted to (S)-p-menth-1-en-4,7-diol. The C-7 position (allyl methyl) of both (R)- and (S)-terpinen-4-ols was preferentially oxidized. We investigated the in vitro metabolism of (+)-terpinen-4-ol in human liver microsomes and recombinant enzymes. The biotransformation of (+)-terpinen-4-ol was studied using gas chromatography-mass spectrometry (GC-MS). Studies have found that human liver microsomal P450 enzymes can oxidize (+)-terpinen-4-ol to (+)-(1R,2S,4S)-1,2-epoxy-p-menthane-4-ol, (+)-(1S,2R,4S)-1,2-epoxy-p-menthane-4-ol, and (4S)-p-menthane-1-en-4,8-diol. The identities of the (+)-terpinen-4-ol metabolites were determined by analyzing the relative abundance and retention time of mass spectrometric fragments using gas chromatography-mass spectrometry (GC-MS). Among the 11 recombinant human P450 enzymes tested, CYP1A2, CYP2A6, and CYP3A4 catalyzed the oxidation of (+)-terpinen-4-ol. Based on multiple pieces of evidence, CYP2A6 and CYP3A4 were identified as the main enzymes for the oxidation of (+)-terpinen-4-ol in human liver microsomes. First, among the 11 recombinant human P450 enzymes tested, CYP1A2, CYP2A6, and CYP3A4 all catalyzed the oxidation of (+)-terpinen-4-ol. Second, (+)-menthol and ketoconazole (known as specific inhibitors of these enzymes) inhibited the oxidation of (+)-terpinen-4-ol. Finally, in 10 human liver microsomes, there was a strong correlation between the activities of CYP2A6 and CYP3A4 and the oxidation activity of (+)-terpinen-4-ol.

Toxicity Summary
Identification and Uses: 1-Terpinen-4-ol is a colorless to pale yellow liquid with a pine-like odor. It is found in derivatives of over 200 plant leaves, herbs, and flowers. It is used in the synthesis of geranium and pepper oils, as well as in perfumery to impart herbal and lavender notes. It is also used as an experimental drug and a topical antibacterial agent. Human Exposure and Toxicity: Terpinen-4-ol can induce apoptosis in human leukemia MOLT-4 cells via both endogenous and exogenous pathways. It inhibits the production of superoxide by monocytes but not neutrophils, suggesting that these components may selectively modulate specific cell types during inflammation. Furthermore, the water-soluble components of tea tree oil can inhibit the production of pro-inflammatory mediators by activated human monocytes. Animal Studies: The oral LD50 in rodents ranges from 1.0 to 4.3 g/kg. One rabbit dermal toxicity study reported an LD50 > 3 g/kg.

---

Interactions
This study compared the antibacterial activities of Melaleuca alternifolia (tea tree) oil and certain components, including single and two-component combinations. Minimum inhibitory concentration (MIC) and time-bactericidal curve assays showed that the main active component of tea tree oil, terpinen-4-ol, exhibited higher activity when used alone than when mixed with tea tree oil. The activity of terpinen-4-ol in combination with γ-terpinene or p-cymene was similar to that of tea tree oil. In the presence of γ-terpinene, the activity and solubility of terpinen-4-ol also decreased in a concentration-dependent manner. Non-oxidized terpenes in tea tree oil appeared to reduce the water solubility of terpinen-4-ol, thereby reducing its efficacy. These findings explain why tea tree oil may have lower in vitro activity than terpinen-4-ol alone and further suggest that the presence of a non-aqueous phase in tea tree oil formulations may limit the microbial availability of its active ingredient. Terpinen-4-ol (4TRP) is a monoterpene alcohol extracted from various aromatic plant essential oils. We investigated the psychopharmacological and electrophysiological activities of 4TRP in male Swiss mice and Wistar rats. 4TRP was administered intraperitoneally (ip) at doses ranging from 25 to 200 mg/kg and intraventricularly (icv) at concentrations of 10, 20, and 40 ng/2 μL. In vitro, 4TRP concentrations of 0.1 mM and 1.0 mM were used. Intraperitoneal injection of 4TRP inhibited PTZ-induced seizures, indicating an anticonvulsant effect. Electroencephalographic recordings showed that intraventricular injection of 4TRP protected the body from PTZ-induced seizures, consistent with behavioral results. To determine whether 4TRP exerts its anticonvulsant effect by modulating GABAergic neurotransmission, we examined 3-mercaptopropionic acid (3-MP)-induced seizures. The results showed that the GABAergic system was involved in the anticonvulsant effect of 4TRP, but flumazenil, a selective antagonist of the benzodiazepine binding site of the GABA_A receptor, did not reverse its anticonvulsant effect, indicating that 4TRP does not bind to the benzodiazepine binding site. Furthermore, 4TRP reduced the sodium current of voltage-dependent sodium channels, suggesting that its anticonvulsant effect may be related to changes in neuronal excitability caused by the regulation of these channels. Artemisia phaeolepis is a perennial herb with a strong volatile aroma, growing in the grasslands of the Mediterranean region. Gas chromatography-flame ionization detection and gas chromatography-mass spectrometry were used to analyze the essential oil extracted from Artemisia phaeolepis. A total of 79 components were identified, accounting for 98.19% of the total essential oil content. The main components were eucalyptol (11.30%), camphor (8.21%), terpinen-4-ol (7.32%), germanene D (6.39%), caryophyllene oxide (6.34%), and caryophyllene (5.37%). The essential oil exhibited significant inhibitory activity against all 10 tested bacterial strains. Eucalyptol, camphor, terpinen-4-ol, caryophyllene, germanene D, and caryophyllene oxide were also studied as major components of the essential oil. Camphor showed the strongest antibacterial activity; terpinen-4-ol, eucalyptol, caryophyllene, and germanene D had moderate activity, while caryophyllene oxide showed weak activity. The study indicates that the antibacterial properties of this essential oil can be attributed to the synergistic effect of its various major and minor components. This study evaluated the effects of the combination of terpinen-4-ol, the main component of tea tree oil, and decanoic acid on inhibiting the growth of Candida albicans hyphae and oral candidiasis in mice, both in vitro and in vivo. Candida albicans hyphae growth was assessed using crystal violet staining. The results showed that the combination of these compounds had a significant synergistic inhibitory effect on Candida albicans growth. The therapeutic effect of this combination was also evaluated in a mouse model of oral candidiasis using microbiological methods, demonstrating significant therapeutic activity. Based on these results, the combination of terpinen-4-ol and decanoic acid is considered a potential candidate drug for the treatment of oral candidiasis. This study also investigated the hypotensive response of intravenous injection of turmeric essential oil (EOAZ) and its main component, terpinen-4-ol (Trp-4-ol), in a deoxycorticosterone-acetate (DOCA)-induced hypertensive rat model. In DOCA-induced hypertensive rats and unilaterally nephrectomized normotensive rats, intravenous administration of EOAZ (1–20 mg/kg) or Trp-4-ol (1–10 mg/kg) reduced mean arterial pressure (MAP) in a dose-dependent manner. However, the antihypertensive response induced by Trp-4-ol was significantly stronger than that induced by the same dose of EOAZ (1–10 mg/kg). DOCA-salt treatment significantly enhanced the maximum percentage decrease in MAP induced by EOAZ or Trp-4-ol. Similarly, the maximum percentage and absolute decrease in MAP induced by intravenous administration of the ganglion blocker hexamethylammonium (30 mg/kg) were significantly greater in DOCA-salt hypertensive rats than in control rats. In DOCA-salt hypertensive rats, pretreatment with hexamethylammonium (30 mg/kg, intravenous) did not affect the enhancement of the EOAZ-induced antihypertensive effect. These results indicate that intravenous administration of EOAZ or Trp-4-ol dose-dependently reduced blood pressure in awake DOCA-induced hypertensive rats, with a more significant effect compared to the unilateral nephrectomy control group. This enhancement is likely primarily related to the increased vascular smooth muscle relaxation induced by EOAZ, rather than to increased sympathetic nervous system activity in this hypertension model. These data further support our previous hypothesis that the antihypertensive effect of EOAZ is partly attributable to the action of Trp-4-ol.

---

Non-human toxicity values
Rabbit dermal LD50 >3 g/kg
Rats oral LD50 4.3 g/kg
Rats oral LD50 1300 mg/kg
Mouse oral LD50 1016 mg/kg

[1]. Terpinen-4-ol, the main component of the essential oil of Melaleuca alternifolia (tea tree oil), suppresses inflammatory mediator production by activated human monocytes. Inflamm Res. 2000 Nov;49(11):619-26.

[2]. Terpinen-4-ol: A Novel and Promising Therapeutic Agent for Human Gastrointestinal Cancers. PLoS One. 2016 Jun 8;11(6):e0156540.

4-Terpinenol is a terpineol, a compound of 1-menthol with a hydroxyl substituent at the 4-position. It is a plant metabolite with various activities including antibacterial, antioxidant, anti-inflammatory, antiparasitic, antitumor, apoptosis-inducing, and volatile oil component activity. It is a terpineol and also a tertiary alcohol. Terpinen-4-ol is currently being studied in the clinical trial NCT01647217 (Demodex blepharitis treatment study). 4-Carvate-menthol has been reported to be present in Anthriscus nitida, Tetradenia riparia, and other organisms with relevant data. Terpinen-4-ol is a metabolite of Saccharomyces cerevisiae. See also: Lavender oil (one of the components); Juniper berry oil (one of the components); Australian tea tree (Peumus boldus) leaves (partial).

---

Therapeutic Uses
Exploratory Therapy: To evaluate the potential anti-inflammatory properties of tea tree oil (an essential oil extracted from the Australian native plant Melaleuca alternifolia), we examined its ability to reduce the production of tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-8, IL-10, and prostaglandin E2 (PGE2) by lipopolysaccharide (LPS)-activated human peripheral blood mononuclear cells in vitro. Sonication of tea tree oil in glass tubes into a medium containing 10% fetal bovine serum (FCS) at a concentration of 0.016% (v/v) was toxic to mononuclear cells. However, a concentration equivalent to 0.125% of the water-soluble components of tea tree oil significantly inhibited LPS-induced TNF-α, IL-1β, and IL-10 production (approximately 50% inhibition) and PGE2 production (approximately 30% inhibition) after 40 hours. Gas chromatography/mass spectrometry analysis identified the water-soluble components of tea tree oil as terpinen-4-ol (42%), α-terpinenol (3%), and 1,8-cineole (2% each). Individual analysis of these components revealed that only terpinen-4-ol inhibited the production of TNFα, IL-1β, IL-8, IL-10, and PGE2 by LPS-activated monocytes after 40 hours. The water-soluble components of tea tree oil were able to inhibit the production of pro-inflammatory mediators by activated human monocytes.
Experimental Treatment: This study aimed to evaluate the regulatory effect of Melaleuca alternifolia oil (tea tree oil) on the production of oxygen free radicals by activated human peripheral blood leukocytes in vitro. We examined the ability of tea tree oil to reduce superoxide production by neutrophils and monocytes stimulated with N-formylmethionine-leucylphenylalanine (fMLP), lipopolysaccharide (LPS), or phorbol 12-myristate 13-acetate (PMA). The results showed that the water-soluble components of tea tree oil had no significant effect on superoxide production by agonist-stimulated neutrophils, but significantly and dose-dependently inhibited superoxide production by agonist-stimulated monocytes. This inhibition was not caused by cell death. Chemical analysis identified the water-soluble components as terpinen-4-ol, α-terpinenol, and 1,8-cineole. Individually, terpinen-4-ol significantly inhibited superoxide production stimulated by fMLP and LPS, but had no effect on superoxide production stimulated by PMA; α-terpinenol significantly inhibited superoxide production stimulated by fMLP, LPS, and PMA; 1,8-cineole had no such effect. The fact that tea tree oil components inhibited superoxide production by monocytes but not neutrophils suggests that these components may selectively modulate specific cell types during inflammation.
Experimental Therapeutics: This study aimed to compare the antibacterial activity of terpinen-4-ol and tea tree oil (TTO) against clinically isolated methicillin-resistant Staphylococcus aureus (MRSA) and coagulase-negative staphylococci (CoNS) and their toxicity to human fibroblasts. The antibacterial activity of the two compounds was compared using the broth microdilution method and a quantitative in vitro time-kill assay. The results showed that terpinen-4-ol exhibited significantly higher antibacterial and bactericidal activity against MRSA and CoNS than tea tree oil (TTO), with higher minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (BCCs). Although the difference was not statistically significant, the time-kill assay clearly demonstrated that terpinen-4-ol had superior antibacterial activity compared to tea tree oil. A comparison of the toxicity of terpinen-4-ol and tea tree oil to human fibroblasts showed that neither compound exhibited toxicity at the tested concentrations over a 24-hour testing period. Terpinen-4-ol exhibited stronger antibacterial activity against MRSA and CoNS than tea tree oil, and neither compound showed toxicity to fibroblasts at the tested concentrations. Terpinen-4-ol should be considered as a single component in formulations for the topical treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections.
Exploratory Treatment Study: This study aimed to investigate the in vitro anticancer activity of Melaleuca alternifolia (tea tree) oil (TTO) and its main active terpene component, terpinen-4-ol, against two invasive mouse tumor cell lines (AE17 mesothelioma and B16 melanoma). The effects of TTO and terpinen-4-ol on the viability of the two tumor cell lines and fibroblasts were assessed using the MTT assay. The induction of apoptosis and necrosis was observed using fluorescence microscopy and quantitatively analyzed by flow cytometry. Ultrastructural changes in tumor cells were observed using transmission electron microscopy, and changes in cell cycle distribution were assessed by flow cytometry. Cell morphology changes were monitored using time-lapse video microscopy. Tea tree oil (TTO) and terpinen-4-ol significantly inhibited the growth of two mouse tumor cell lines in a dose- and time-dependent manner. Interestingly, the cytotoxic doses of TTO and terpinen-4-ol significantly reduced the inhibitory effect on non-tumor fibroblasts. Both TTO and terpinen-4-ol induced necrotic cell death in both tumor cell lines, accompanied by low levels of apoptotic cell death. This primary necrosis was confirmed by video time-lapse microscopy and transmission electron microscopy, which also revealed ultrastructural features such as cell and organelle swelling after TTO treatment. Furthermore, both TTO and terpinen-4-ol exerted their inhibitory effect by inducing G1 phase cell cycle arrest. TTO and terpinen-4-ol exhibited significant antiproliferative activity against both tumor cell lines. In addition, the identification of primary necrosis and cell cycle arrest in invasive tumor cells highlights the potential anticancer activity of tea tree oil and terpinen-4-ol. For more complete data on the therapeutic uses of 4-terpineols (a total of 6), please visit the HSDB record page.

C₁₀H₁₈O

154.25

154.135

562-74-3

11230

Colorless to light yellow liquid

0.9±0.1 g/cm3

209.0±0.0 °C at 760 mmHg

79.4±0.0 °C

0.0±0.9 mmHg at 25°C

1.485

2.99

1

1

1

11

170

0

O([H])C1(C([H])([H])C([H])=C(C([H])([H])[H])C([H])([H])C1([H])[H])C([H])(C([H])([H])[H])C([H])([H])[H]

WRYLYDPHFGVWKC-UHFFFAOYSA-N

InChI=1S/C10H18O/c1-8(2)10(11)6-4-9(3)5-7-10/h4,8,11H,5-7H2,1-3H3

4-methyl-1-propan-2-ylcyclohex-3-en-1-ol

Terpinen4ol; Terpinen 4 ol

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: This product requires protection from light (avoid light exposure) during transportation and storage.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ~100 mg/mL (~648.30 mM)
H2O : ≥ 25 mg/mL (~162.07 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (16.21 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

---

Solubility in Formulation 2: ≥ 2.5 mg/mL (16.21 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

---

View More

Solubility in Formulation 3: ≥ 2.5 mg/mL (16.21 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

---

Solubility in Formulation 4: 100 mg/mL (648.30 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication.

---

 (Please use freshly prepared in vivo formulations for optimal results.)