Methoxsalen (15 mg/kg, i.p., single dosage) raises the Cmax and AUC of nicotine while prolonging its half-life (by four times) and decreasing its clearance (by six times) [1].

Animal/Disease Models: Male adult ICR mice [1]
Doses: 15 mg/kg
Route of Administration: intraperitoneal (ip) injection, single dose
Experimental Results: nicotine plasma concentration is still higher than 10ng/ml at 6 hrs (hrs (hours)). The analgesic and cooling effects of induced nicotine lasted for nearly 6 hrs (hrs (hours)) and 24 hrs (hrs (hours)).

Absorption, Distribution and Excretion
Methoxarin is rapidly metabolized in mice and humans. Approximately 95% of the drug is excreted in the urine within 24 hours as a series of metabolites (Pathak et al., 1977). In rats, after oral administration of (3)H-8-methoxypsoralen, the drug is rapidly absorbed, reaching peak plasma concentrations within 10 minutes. Moderate radioactivity is detected in the liver and kidneys 0.5–4 hours later, with lower levels in other tissues. …Within 24 hours, 62.8% of the radioactivity is excreted in the urine and 20.4% in the feces; within 6 days, 65.1% and 21.9% are excreted in the urine and feces, respectively. 30.0% of the radioactivity is also recovered in the bile within 24 hours; the drug enters the body via enterohepatic circulation. Compared to the suspension, the effective bioavailability of methoxarin is improved when administered in solution form to rats and dogs. In both animals, peak concentrations in the solution group occurred earlier and at higher levels.
After oral administration of 0.6 mg/kg methoxsalen, peak serum concentrations occurred between 0.5 and 2 hours. A significant negative correlation was found between the logarithm of serum concentrations and the minimum phototoxic dose. Therefore, the degree of photosensitivity appears to be correlated with serum methoxsalen levels.
Following a single intravenous injection of 5 mg/kg body weight of 14C-labeled methoxsalen in dogs, the drug rapidly disappeared from the plasma, but a small amount of radioactivity remained for up to 5 weeks post-administration. There is evidence that the persistent plasma radioactivity is due to metabolites bound to plasma proteins. The drug is primarily excreted via urine and bile. Within 72 hours of administration, 45% of the dose is excreted in the urine and 40% in the feces.
For more complete data on the absorption, distribution, and excretion of 8-methoxypsoralen (a total of 8 types), please visit the HSDB records page.

---

Metabolism/Metabolites
After oral administration of 8-methoxypsoralen to rats, the urinary metabolites were: 8-hydroxypsoralen, 5-hydroxy-8-methoxypsoralen, 5,8-dioxopsoralen, 5,8-dihydroxypsoralen, 4,6,7-trihydroxy-5-coumaryl-β-acrylic acid, and 4,6-dihydroxy-7-methoxy-5-coumaryl-β-acrylic acid.
Although the exact metabolic pathway of methoxsalen is not fully understood, the drug is rapidly and almost completely metabolized. Methoxsalen is demethylated to 8-hydroxypsoralen (8-HOP), and methoxsalen and 8-HOP are conjugated with glucuronic acid and sulfuric acid, respectively; in addition, other unidentified metabolites were detected. Methoxsalen, 8-hydroxypsoralen, and their conjugates are all excreted in the urine. Following oral administration of methoxsalen, 80-90% of the drug is excreted in the urine within 8 hours as hydroxylated, glucuronide, and sulfate metabolites; less than 0.1% of the dose is excreted unchanged in the urine. Approximately 95% of the drug is excreted in the urine as metabolites within 24 hours. Methoxsalen is extensively metabolized, with less than 2% excreted unchanged in the urine. Four urinary metabolites have been isolated; three of them are generated by ring-opening of the furan ring: 7-hydroxy-8-methoxy-2-oxo-2H-1-benzopyran-6-acetic acid, α,7-dihydroxy-8-methoxy-2-oxo-2H-1-benzopyran-6-acetic acid, and an unknown conjugate of the former at the 7-hydroxy position. The fourth metabolite is formed by ring-opening of the pyranone ring and is an unknown conjugate of (Z)-3-(6-hydroxy-7-methoxybenzofuran-5-yl)-2-acrylic acid.
The known metabolites of methoxsalen include 9-methoxy-5,7,11-trioxatetracyclo[8.4.0.03,8.04,6]tetradecano-1,3(8),9,13-tetraen-12-one.
Elimination pathway: Methoxsalen is rapidly metabolized in both mice and humans. Approximately 95% of the drug is excreted in the urine within 24 hours as a series of metabolites (Pathak et al., 1977).
Half-life: Approximately 2 hours
The elimination half-life of methoxsalen has been reported to be approximately 0.75–2.4 hours.

Toxicity Summary
Identification and Uses: 8-Methoxypsoralen (8-MOP) belongs to the psoralen or furanocoumarin class of compounds. It can be used as a tanning agent and sunburn protectant. 8-MOP is used in photochemotherapy for psoriasis (PUVA, i.e., 8-MOP used in combination with long-wave UVA radiation). It can also be used in combination with long-wave UVA or sunlight to restore pigmentation to the white patches of skin in patients with idiopathic vitiligo. Oral 8-MOP, used in combination with phototherapy, is used to alleviate the cutaneous manifestations of cutaneous T-cell lymphoma. Human Studies: In Indian patients receiving vitiligo treatment, 12% developed keratosis, but no cancer occurred. In 52 patients receiving continuous PUVA treatment, 12 developed small amounts of epidermal amyloid deposition. In 20 healthy volunteers, 3 received escalating doses of 1% 8-MOP three times weekly and were exposed to ultraviolet radiation, resulting in photosensitivity. Other adverse skin reactions to PUVA therapy include: freckles, hypopigmentation, uneven skin tone or excessive tanning, dry skin, vesicle and bullae formation, generalized desquamation, nonspecific rashes, urticaria, milia, folliculitis, acne-like rashes, exacerbation or spread of psoriasis, hyperpigmentation of psoriatic lesions, tenderness, severe skin pain, onycholysis, nail pigmentation, and exacerbation of underlying photosensitive dermatitis. Kaposi's varicella-like rash has been reported after the initiation of PUVA therapy. Phototoxic reactions may occur with methoxsalen and conventional ultraviolet irradiation, including severe edema and erythema, as well as painful vesicles, burning sensations, and desquamation. Furthermore, PUVA therapy may lead to severe burns requiring hospitalization, as well as significant hyperpigmentation and skin aging. The most common adverse reaction to oral 8-methoxsalen is nausea, occurring in approximately 10% of patients. Gastrointestinal disturbances may also occur with PUVA therapy using 8-methoxsalen. Approximately 10% of patients receiving 8-methoxsalen PUVA therapy experience itching. There have been reports of a male patient developing bilateral macular toxicity after receiving 8-methoxsalen treatment for vitiligo. A 59-year-old white woman developed toxic hepatitis during oral 8-methoxsalen PUVA therapy. There are also reports of cancer development in patients receiving 8-methoxsalen treatment. In a cohort study of 1373 psoriasis patients receiving 8-MOP combined with ultraviolet radiation therapy, 30 patients developed 19 cases of basal cell carcinoma and 29 cases of squamous cell carcinoma. PUVA therapy increases the risk of melanoma. Some patients developed melanoma even more than 5 years after discontinuing PUVA therapy. 8-MOP combined with ultraviolet radiation therapy resulted in a significant increase in lymphocyte chromosomal aberrations in 1/8 of patients, mild but not significant increases in chromosomal aberrations in 6 patients, and no increase in chromosomal aberrations in 1 patient, along with sister chromatid exchange. No chromosomal aberrations or sister chromatid exchanges were observed in psoriasis patients receiving combined therapy, but a significant increase in sister chromatid exchanges was found in leukocytes isolated from patients after in vitro ultraviolet irradiation treatment. More point mutations were observed in patients receiving psoralen and ultraviolet irradiation treatment compared to healthy controls, manifested as an increased incidence of 6-thioguanine-resistant lymphocytes. Animal experiments: Severe reactions occurred one hour after intraperitoneal injection of 40 mg 8-MOP into guinea pigs followed by 24-hour exposure to UVB. White guinea pigs exhibited eyelid ulceration, corneal edema, iris vascular congestion, permanent mydriasis, and multiple punctate opacities in the anterior lens cortex. Black guinea pigs showed milder eyelid and iris damage. No ocular damage was observed in guinea pigs given 80–100 mg/kg 8-MOP unless simultaneously exposed to UVB. Local or intraperitoneal injection of 8-MOP has been reported to have potent photocarcinogenic effects in albino and hairless mice. However, oral administration of 8-MOP has an anti-UV carcinogenic effect on albino mice. Mice with 8-MOP added to their diet had a 38% incidence of ear tumors 180 days after the start of UV treatment, compared to 62% in the control group. Hairless mice were given 40 μg of 8-MOP applied to the skin 30–60 minutes before whole-body exposure to UV (300–400 nm) for 10 minutes daily, 5 days a week. Mice receiving 8-MOP combined with UV irradiation had significantly more tumors per mouse than those not exposed to UV irradiation. Most tumors were squamous cell carcinomas; other tumors included fibrosarcomas, lymphosarcomas, sebaceous adenomas, and hemangiomas. In mice, intraperitoneal injection of 4 mg/kg body weight of 8-MOP followed by exposure to long-wave UV (320–400 nm) irradiation resulted in severe toxic reactions, including erythema, burns, and liver damage. In a 2-year gavage study of male rats, administration of 8-MOP without UV exposure resulted in increased incidence of renal tubular cell proliferation, renal adenoma and renal adenocarcinoma, as well as Zymbal adenocarcinoma. Dose-related non-neoplastic lesions in male rats included worsening of nephropathy, renal mineralization, and forestomach lesions. In female rats, administration of 8-MOP at doses of 37.5 or 75 mg/kg/day for two consecutive years did not reveal carcinogenic activity. Administration of 80 to 160 mg/kg/day to rats during organogenesis resulted in significant fetal toxicity, accompanied by significant maternal weight loss, anorexia, and increased relative liver weight. 8-MOP at doses of 80 mg/kg/day and above caused skeletal malformations and increased variation. Under metabolically activated conditions, 8-MOP was mutagenic in the Ames assay. In the absence of metabolic activation and UV exposure, 8-MOP was disintegrative in vitro (causing sister chromatid exchange and chromosomal aberrations in Chinese hamster ovary cells). 8-MOP can also cause DNA damage, interstrand crosslinks, and DNA repair errors. Methoxarin is a cholinesterase, or acetylcholinesterase (AChE) inhibitor. Cholinesterase inhibitors (or "anticholinesterases") inhibit the activity of acetylcholinesterase. Because acetylcholinesterase has a vital function, chemicals that interfere with its activity are potent neurotoxins; even low doses can cause excessive salivation and lacrimation, followed by muscle spasms and ultimately death. Neurotoxins and substances in many pesticides have been shown to work by binding to serine residues at the active site of acetylcholinesterase, thus completely inhibiting the enzyme's activity. Acetylcholinesterase breaks down the neurotransmitter acetylcholine, which is released at the neuromuscular junction, causing muscle or organ relaxation. Inhibition of acetylcholinesterase results in the accumulation and sustained action of acetylcholine, leading to continuous nerve impulse transmission and an inability to stop muscle contractions. The most common acetylcholinesterase inhibitors are phosphorus-containing compounds designed to bind to the enzyme's active site. Its structural requirements are: one phosphorus atom connecting two lipophilic groups, one leaving group (e.g., a halide or thiocyanate), and one terminal oxygen atom. Many furanocoumarins' mechanisms of action are based on their ability to form photoadducts with DNA and other cellular components, such as RNA, proteins, and membrane proteins like phospholipases A2 and C, calcium-dependent and cAMP-dependent protein kinases, and epidermal growth factor. Furanocoumarins can insert between DNA base pairs and form cycloadducts upon UVA irradiation (L579).

---

Effects during pregnancy and lactation
◉ Overview of medication use during lactation
Currently, there is no information regarding the use of methoxsalen during lactation. Expert opinion indicates that due to the photosensitizing effect of methoxsalen, breastfeeding should be suspended for 24 hours after oral administration to allow 95% of the drug to be excreted in the mother's urine. The same precautions may also be taken for patients with cutaneous T-cell lymphoma treated with methoxsalen. Topical application of methoxsalen during breastfeeding is not contraindicated, but direct contact between the treated skin and the infant's skin should be avoided.
◉ Effects on breastfed infants
No published information found as of the revision date.
◉ Effects on lactation and breast milk
No published information found as of the revision date.

---

Interactions
Psoralen can induce interstrand crosslinks (ICLs) in DNA under UVA light irradiation, thereby blocking DNA replication and transcription. Among psoralen derivatives, 8-methoxypsoralen (8-MOP) is commonly used to treat psoriasis, while amotosarin S59 is used to inactivate bacterial and viral pathogens in blood components. In addition to forming interstrand crosslinks (ICLs), psoralen readily forms various monoadducts (MAs) with thymidine residues in DNA under UVA light irradiation, but the biological significance of these monoadducts is unclear. This paper reports a method combining single-enzyme (nuclease P1) digestion and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the simultaneous quantitative analysis of ICLs and MAs produced by human cells under 8-MOP or S59 and UVA light irradiation. Our results show that as the UVA light dose increased from 0.5 J/cm² to 10.0 J/cm², the S59-induced ICL yield increased from 3.9 damage cells/10³ nucleotides to 12.8 damage cells/10³ nucleotides, approximately 100-fold higher than that induced by 8-MOP. Furthermore, three and five products were identified as 8-MOP-MA and S59-MA, respectively, with the MA yield significantly lower than that of ICLs. As the UVA light dose increased from 0.5 J/cm² to 10.0 J/cm², the yields of the three 8-MOP-MAs were 7.6–2.2, 1.9–9.9, and 7.2–51 per 10(6) nucleotides, respectively, and the yields of the five S59-MAs were 215–19, 106–39, 25–21, 32–146, and 22–26 per 10(6) nucleotides, respectively. Although the MA yields induced by 8-MOP and S59 were lower than those of the corresponding ICLs under the same exposure conditions, the formation of a considerable number of MAs may explain the partial mutation induced by psoralen. /Authors/ Recently, it was reported that methoxsalen (a potent hepatochrome P-450 suicide inhibitor) reduces the metabolic activation of acetaminophen and prevents its hepatotoxicity in mice. /Researchers/ The effects of methoxsalen on acetaminophen metabolism in humans were now investigated. In vitro experiments showed that 100 μM methoxsalen reduced the covalent binding of (3)H-acetaminophen metabolites to microsomal proteins by 40% after incubation of (3)H-acetaminophen with human liver microsomes and an NADPH generation system. In vivo, a single oral dose of methoxsalen (30 mg) 3 hours before acetaminophen (1 g) in 9 healthy volunteers reduced the salivary partial clearance of acetaminophen by 38%, which was reflected in the form of glutathione-derived conjugates (the end product of acetaminophen oxidative metabolism). These observations indicate that methoxsalen can reduce the metabolic activation of acetaminophen in the human body. To determine the effect of methoxsalen on the 7-hydroxylation of coumarin in humans, 5 subjects received 45 mg of methoxsalen and 5 mg of coumarin, respectively. Methoxsalen inhibited the metabolism of coumarin in vivo by 47 ± 9.2% (mean ± standard error). The metabolic rate of methoxsalen in human liver microsomes is 50-100 pmol/mg protein/min (approximately 30% of the activity in mouse liver microsomes). Anti-Cyp2a-5 antibody did not inhibit metabolism in human liver microsomes. NIH 3T3 cells stably expressing the catalytically active CYP2A6 enzyme did not metabolize methoxsalen, indicating that CYP2A6 does not use methoxsalen as a substrate. In pyrazole-induced mouse liver microsomes, anti-Cyp2a-5 antibody inhibited methoxsalen metabolism. Cyp2a-5 protein expressed in Saccharomyces cerevisiae metabolized methoxsalen, indicating that methoxsalen is a substrate of Cyp2a-5. Although kinetic studies showed that the inhibition of coumarin 7-hydroxylation by methoxsalen in human liver microsomes was competitive, methoxsalen does not appear to be a substrate of CYP2A6. Strong metabolic interactions between methoxsalen and coumarin may exist in humans. Furanocoumarins can enhance the bioavailability of CYP3A4 substrate drugs. A randomized crossover study evaluated the potential interaction between methoxsalen and cyclosporine. The study included 12 healthy volunteers who received 40 mg methoxsalen, 200 mg cyclosporine, or a combination of both orally. Results showed that compared to cyclosporine alone, methoxsalen increased the area under the plasma concentration-time curve (AUC) and peak plasma concentration (Cmax) of cyclosporine by 29% (range: -20% to 172%; P < 0.05) and 8% (range: -10% to 26%; P < 0.05), respectively. The geometric mean ratio (95% confidence interval) of AUC between cyclosporine combined with methoxsalen and cyclosporine alone was 1.14 (1.02, 1.27), therefore the two treatment regimens were not bioequivalent. Methoxsalen exhibits clinically significant interactions with cyclosporine in some susceptible individuals. The cause of this susceptibility and the clinical significance of long-term cyclosporine use are not yet clear. Caution should be exercised when using this medication in combination, and more frequent monitoring of cyclosporine plasma concentrations and clinical surveillance are recommended. For more complete data on interactions with 8-methoxypsoralen (a total of 25), please visit the HSDB record page. Non-human toxicity values: Mouse subcutaneous LD50 860 mg/kg; Mouse intraperitoneal LD50 310 mg/kg; Mouse oral LD50 423 mg/kg; Rat intraperitoneal LD50 158 mg/kg; Rat oral LD50 791 mg/kg

[1]. Pharmacokinetic and Pharmacodynamics Studies of Nicotine After Oral Administration in Mice: Effects of Methoxsalen, a CYP2A5/6 Inhibitor. Nicotine Tob Res. 2014 Jan;16(1):18-25.

Therapeutic Uses

Cross-linking agent; photosensitizer
Photochemotherapy (methoxsalen combined with long-wave UVA radiation) is indicated for symptom control of severe, refractory, disabling psoriasis that has not responded well to other therapies and is biopsied. Photochemotherapy must be combined with controlled doses of long-wave ultraviolet radiation. /US product label includes/
Methoxsalen is available orally (conventional capsules) or topically in combination with controlled doses of long-wave ultraviolet (UVA) radiation or sunlight exposure for the treatment of vitiligo hyperpigmentation in patients with idiopathic vitiligo. Currently, liquid-filled capsules have not been approved by the US Food and Drug Administration (FDA) for this purpose. The clinical efficacy of methoxsalen is unstable and unpredictable, with only a small percentage of vitiligo patients achieving cosmetically acceptable results. Complete cure after psoralen treatment is uncommon; only about one-third of vitiligo patients experience significant pigment recovery. A study of 20 patients who received topical methoxsalen and ultraviolet radiation showed that only 3 patients achieved complete pigment recovery. Another study used UVA light irradiation combined with oral methoxsalen or oral trioxazoline for 12-14 months, showing that 73% of vitiligo patients experienced pigment recovery, and 23% of patients showed approximately 73% improvement in pigmentation. The degree, onset time, and duration of pigment recovery varied from person to person. Methoxsalen-induced pigment recovery was faster in fleshy areas such as the face, abdomen, and buttocks than in bony areas such as the back of the hands and feet. To maintain the newly formed pigment, regular medication and some form of UVA light treatment are usually required; however, in one study, 8-14 years after oral methoxsalen combined with conventional ultraviolet phototherapy, 85% of patients still retained 90% or more of the newly formed pigment. Oral methoxsalen combined with UVAR phototherapy is used for palliative treatment of cutaneous T-cell lymphomas (CTCL; e.g., mycosis fungoides, Cezari syndrome). Limited evidence suggests that phototherapy (e.g., twice daily for two consecutive days per month) can reduce the size and/or severity of skin lesions without serious toxicity; some patients have achieved sustained efficacy (2 years or longer). For detailed information on oral methoxsalen combined with phototherapy for patients with cutaneous T-cell lymphoma, clinicians should consult the manufacturer's labels for methoxsalen and the UVAR device, as well as other professional references and published protocols. For more complete data on the therapeutic uses of 8-methoxypsoralen (a total of 6 types), please visit the HSDB record page.
Drug Warning
/Black Box Warning/ Methoxsalen combined with ultraviolet radiation should only be used by physicians with specific competence in the diagnosis and treatment of psoriasis and who are specially trained and experienced in photochemotherapy. The use of psoralen and ultraviolet radiation therapy should be conducted under the ongoing supervision of the aforementioned physician. For the treatment of psoriasis, photochemotherapy should be limited to patients with severe, refractory, disabling disease who have not responded well to other therapies, and should only be used in cases of confirmed diagnosis. Due to the risks of eye damage, skin aging, and skin cancer (including melanoma), physicians should fully inform patients of the inherent risks of this treatment. /Warning/ Methoxsarin soft capsules (USP) should not be used interchangeably with regular methoxsarin capsules or hard methoxsarin capsules. This newer formulation of methoxsarin has significantly higher bioavailability than previous formulations and an earlier onset of photosensitivity. Patients should be treated at the recommended dosage for this product. Before using this formulation for photochemotherapy, the minimum phototoxic dose (MPD) and time to peak phototoxicity should be determined. Methoxsarin is contraindicated in patients with a specific response to psoralen compounds; patients with a history of specific photosensitivity disorders should not begin methoxsarin treatment. Diseases associated with photosensitivity include lupus erythematosus, porphyria cutanea tarda, erythropoietic protoporphyria, porphyria versicolor, xeroderma pigmentosum, and albinism; patients with melanoma or a history of melanoma; patients with invasive squamous cell carcinoma; and aphakic patients, who have a significantly increased risk of retinal damage due to the lack of a lens. Methoxsarin combined with conventional ultraviolet (UV) irradiation can cause phototoxic reactions, including severe edema and erythema, as well as painful blisters, burns, and skin peeling. Furthermore, PUVA therapy can lead to severe burns requiring hospitalization, as well as significant hyperpigmentation and skin aging. When peeling or blisters occur, the skin is more sensitive to UV radiation. Methoxsarin-induced phototoxic reactions most commonly occur when the skin is overexposed to UV radiation or at excessive doses. Severe burns may occur if the treated skin is accidentally exposed to additional UV radiation. Some reports suggest that concurrent use of benzophenone-based sunscreens may slightly reduce the incidence of psoralen-induced phototoxicity. For more complete data on drug warnings for 8-methoxypsoralen (27 in total), please visit the HSDB records page.

---

Pharmacodynamics
Methoxam selectively inhibits deoxyribonucleic acid (DNA) synthesis. The levels of guanine and cytosine are correlated with the degree of cross-linking induced by methoxam. At high concentrations, cellular RNA and protein synthesis are also inhibited.

C12H8O4

216.1895

216.042

298-81-7

Methoxsalen-d3;80386-99-8

4114

White to light yellow solid powder

1.4±0.1 g/cm3

414.8±45.0 °C at 760 mmHg

143-148 ºC

204.7±28.7 °C

0.0±1.0 mmHg at 25°C

1.635

1.93

0

4

1

16

325

0

QXKHYNVANLEOEG-UHFFFAOYSA-N

InChI=1S/C12H8O4/c1-14-12-10-8(4-5-15-10)6-7-2-3-9(13)16-11(7)12/h2-6H,1H3

9-methoxyfuro[3,2-g]chromen-7-one

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

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

DMSO : ~50 mg/mL (~231.28 mM)
H2O : ~0.67 mg/mL (~3.10 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (11.56 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 (11.56 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.

---

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