Absorption, Distribution and Excretion
Benzohexol is absorbed via the gastrointestinal tract. The peak plasma concentration (Cmax) of benztrohexol is 7.2 ng/mL, the time to peak concentration (Tmax) is 1.3 hours, and the area under the curve (AUC) is 201 ngh/mL. Data on the elimination pathway of benztrohexol are unclear. However, it is likely primarily excreted in the urine. A sensitive anticholinergic radioreceptor assay was used to determine the concentration of benztrohexol in human serum, and its pharmacokinetics after short-term and long-term use in patients with dystonia were investigated. Serum concentration-time curves in previously untreated patients showed a biphasic semi-logarithmic curve, including an initial rapid distribution phase and a subsequent slower elimination phase. Long-term treated patients exhibited only the slower elimination phase. Elimination followed first-order kinetics and was rapid, with a half-life of 3.7 ± 0.4 (SEM) hours. Half-life was not correlated with peak serum concentration, age, duration of treatment, or the etiology or severity of dystonia. While acute anticholinergic side effects paralleled increases and decreases in serum anticholinergic drug levels, dystonia responses did not. Trihexyphenidyl hydrochloride is rapidly absorbed from the gastrointestinal tract. After oral administration of trihexyphenidyl hydrochloride tablets, the onset of action is within 1 hour, peak effect lasts 2-3 hours, and duration of action is 6-12 hours. The metabolic pathway of trihexyphenidyl is not yet determined; the drug may be excreted unchanged in the urine. This study investigated the subcellular distribution of bisperidone (BP), trihexyphenidyl (TP), and (-)-quinine cyclobenzyl ester (QNB) in rats after intravenous administration of high doses (3.2 mg/kg) of bisperidone (BP), trihexyphenidyl (TP), and (-)-quinine cyclobenzyl ester (QNB) in the brain, heart, and lungs. The subcellular distribution of clinically commonly used BP or TP is consistent with that of the typical potent central muscarinic receptor antagonist QNB. The concentration-time curves of these drugs in subcellular components of brain tissue exhibit two patterns, both showing a slow decline and paralleling plasma concentrations. Subcellular distribution in the brain and heart depends on the protein content of each component. In lung tissue, the postnuclear component (P2) typically accounts for a 3-5 times higher percentage of total concentration than in the heart. Studies have shown that drug distribution in lung tissue differs from that in the brain and heart, exhibiting a higher affinity for lysosomes and independent of the protein content of the P2 component (containing lysosomes). On the other hand, at low doses (650 ng/kg) of 3H-QNB, compared to the high-dose group, the percentage of each component in total concentration in the brain increased in synaptic membranes and synaptic vesicles, while decreasing in the nucleus and cytoplasm. These results suggest that although the tissue concentration-time curves of anticholinergic drugs appear to decline parallel to plasma concentrations, their subcellular distribution exhibits multiple patterns across different tissues. Twenty-four male subjects were randomly assigned to receive two different oral formulations of trihexyphenidyl hydrochloride (α-cyclohexyl-α-phenyl-1-piperidinpropanol hydrochloride). The dosing regimens were: (1) 5 mg immediate-release tablets twice daily, at midnight and 12 hours later, respectively; and (2) 5 mg extended-release capsules twice daily. The incidence of adverse reactions with the extended-release formulation (SR) was approximately 50% that with the immediate-release formulation (IR). The peak concentration (Cmax) after administration of the SR formulation was significantly lower (p<0.05) than that after the first administration of the IR formulation, and the time to reach Cmax (tmax) with the SR formulation was significantly longer (p<0.05). The mean times for the SR formulation to maintain serum drug concentrations above 50%, 60%, and 70% of Cmax were 11.7 hours, 9.4 hours, and 5.9 hours, respectively, while the corresponding times for the IR formulation were 1.8 hours, 1.2 hours, and 0.9 hours, respectively; all differences were statistically significant (p<0.05). The mean elimination half-life (t1/2) of the SR formulation (10.1 hours) and IR formulation (8.7 hours) were similar (p>0.05). The statistical power of this study was 98.1%, detecting a 20% difference in the area under the curve (AUC0-∞) from time zero to infinity between the different formulations. Although the AUC0-∞ value of the sustained-release formulation was statistically smaller than that of the immediate-release tablet (p<0.05), the difference was less than 20%. Therefore, the sustained-release formulation of trihexyphenidyl hydrochloride is bioequivalent to the immediate-release tablet. /Trihexyphenidyl hydrochloride/

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Metabolism/Metabolites
Data on the metabolism of trihexyphenidyl are not yet clear. However, its degree of metabolism is likely low.
Gas chromatography-mass spectrometry has been used to study trihexyphenidyl and its three metabolites excreted in human urine. Three isomeric hydroxylated metabolites were identified, all of which are 1-(hydroxycyclohexyl)-1-phenyl-3-piperidinylprop-1-ol. 3. The levels of benztropine and its identified metabolites were semi-quantitatively determined in two healthy adults following a single oral dose. Approximately 56% of the dose was excreted as hydroxylated metabolites. The excretion level of benztropine could not be determined by the techniques used.
Half-life: 3.3–4.1 hours

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Biological half-life
The mean elimination half-life of benztropine was 3.2 ± 0.3 hours.
24 male subjects were randomly assigned to receive oral administration of two different formulations of benztropine hydrochloride (α-cyclohexyl-α-phenyl-1-piperidinylpropanol hydrochloride). The dosing regimens were: (1) 5 mg immediate-release tablets twice daily, at 0 hours and 12 hours, respectively; (2) two 5 mg extended-release capsules twice daily. …The mean elimination half-life (t1/2) of the extended-release capsules (10.1 hours) and the immediate-release capsules (8.7 hours) were similar (p>0.05). /Trihexyl Hydrochloride/
A sensitive anticholinergic drug radioreceptor assay was used to determine the level of trihexyl hydrochloride in human serum, and its pharmacokinetics after short-term and long-term use in patients with dystonia were investigated. …Elimination was kineticly first-order and rapid, with a half-life of 3.7 ± 0.4 (SEM) hours. The half-life was not correlated with peak serum concentration, age, duration of treatment, or the etiology or severity of dystonia.

Toxicity Summary
Benzohexol is a selective M1 muscarinic acetylcholine receptor antagonist. It can differentiate between M1 (cortical or neuronal) and peripheral muscarinic subtypes (cardiac and glandular). Benzohexol partially blocks cholinergic activity in the central nervous system, which is the root cause of Parkinson's disease symptoms. It is also thought to increase dopamine availability, a brain chemical essential for initiating and controlling voluntary muscle movement. Hepatotoxicity
Benzohexol has not been reported to cause elevated serum transaminases, but no prospective studies have been conducted to assess its effect on serum enzyme levels. Two cases of acute liver injury death due to benzohexol have been mentioned in Japanese literature, but no details were provided, and no other such injuries have been reported in the subsequent 40 years. Therefore, even if benzohexol does cause liver injury, it must be an extremely rare cause. Probability Score: E (Unproven but suspected clinically significant cause of liver injury).

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Effects during pregnancy and lactation
◉ Overview of medication use during lactation
Limited information suggests that maternal administration of up to 4 mg of trihexyphenidyl daily, concurrently with haloperidol or risperidone, has not produced any adverse effects on breastfed infants. Long-term use of trihexyphenidyl may reduce milk production or the milk ejection reflex, but a single dose is unlikely to interfere with breastfeeding. Concurrent use of antipsychotic medications may increase prolactin levels, thus counteracting the prolactin-lowering effect of trihexyphenidyl. With prolonged use, signs of reduced milk production (e.g., infant not feeling full and poor weight gain) should be observed.
◉ Effects on breastfed infants
A woman with schizophrenia took trihexyphenidyl and haloperidol during three pregnancies and postpartum. The dose of trihexyphenidyl was 4 mg daily during all three pregnancies. She breastfed all three children for 6 to 8 months (feeding duration not specified) using the same dose. At the time of assessment, all children were 16 months old and developmentally age-appropriate at age 8.
A woman diagnosed with undifferentiated schizophrenia took 4-5 mg of risperidone and 2 mg of trihexyphenidyl daily during five pregnancies. She breastfed each infant for 20 to 24 months. None of the children experienced adverse developmental consequences. As of the time of this writing, the three oldest children, aged 26, 23, and 22, have all completed their education and entered the workforce; the two youngest children, aged 15 and 19, are academically successful.
◉ Effects on Lactation and Breast Milk
Anticholinergic drugs can inhibit lactation in animals, possibly by suppressing the secretion of growth hormone and oxytocin. Anticholinergic drugs can also lower serum prolactin levels in non-lactating women. Prolactin levels in mothers who have established lactation may not affect their ability to breastfeed.
A woman with schizophrenia took trihexyphenidyl and haloperidol during three pregnancies and postpartum. She was able to breastfeed all three of her children (feeding duration not specified) for 6 to 8 months. A woman diagnosed with undifferentiated schizophrenia took risperidone 4 to 5 mg and trihexyphenidyl 2 mg daily during five pregnancies. She successfully breastfed each infant for 20 to 24 months. Protein Binding: Data on the degree of protein binding of trihexyphenidyl in plasma are unclear. In dialysis bags, trihexyphenidyl bound to albumin under controlled conditions was 36.13% to 41.92%. Drug Interactions: When trihexyphenidyl hydrochloride is used concomitantly with levodopa, a reduction in the usual dose of both may be necessary. Dosage must be carefully adjusted based on side effects and symptom control. Trihexyphenidyl hydrochloride / ...centrally acting anticholinergic drugs that may exacerbate tardive dyskinesia / caused by chlorpromazine / including...trihexyphenidyl.
Trihexyphenidyl, when used alone at doses of 30 and 50 mg/kg (intraperitoneal injection), did not affect the electroconvulsive threshold. However, when used in combination with sodium valproate, it significantly enhanced its anticonvulsant activity against maximal electroconvulsive-induced seizures in mice, reducing the ED50 from 206 mg/kg to 103 mg/kg and 46 mg/kg, respectively. Chimney test and memory retention test in mice showed that intraperitoneal injection of 30 mg/kg trihexyphenidyl, followed by intraperitoneal injection of 130 mg/kg or 103 mg/kg sodium valproate, respectively, resulted in motor dysfunction and long-term memory impairment, with effects similar to those of sodium valproate alone (at the ED50 of the maximal electroconvulsive dose). Trihexyphenidyl did not alter the total plasma concentration of sodium valproate.

[1]. Binding and functional profiles of the selective M1 muscarinic receptor antagonists trihexyphenidyl and dicyclomine. Br J Pharmacol. 1986 Sep;89(1):83-90.

[2]. Trihexyphenidyl. 2022 Feb 27. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan–.

Therapeutic Uses
Anti-movement disorder medication; anti-Parkinson's disease medication; muscarinic receptor antagonist; parasympathetic nerve blocker.
Trihexyphenidyl hydrochloride is used as adjunctive therapy for various types of Parkinson's syndrome, including post-encephalitis, arteriosclerotic, and idiopathic types. Trihexyphenidyl is also used to relieve Parkinson's disease signs and symptoms caused by extrapyramidal reactions induced by antipsychotic drugs (e.g., butyrophenones, phenothiazines, thioxanthates). Preliminary clinical trial results for trihexyphenidyl hydrochloride in the treatment of other movement disorders, Huntington's disease, spasmodic torticollis, and related diseases are unclear.
Trihexyphenidyl may effectively reduce the frequency and duration of oculomotor crises, reduce salivation, alleviate spastic contractions and involuntary movements characteristic of movement disorders, and relieve mental dullness and depression characteristic of all Parkinson's syndrome patients. As with other anti-Parkinson's disease medications, long-term use of trihexyphenidyl may lead to tolerance. The maximum efficacy of trihexyphenidyl is 20-30% symptom improvement in 50-75% of patients. Typically, to achieve optimal therapeutic effects, trihexyphenidyl is empirically combined with other antimuscarinic, antihistamine, or dopaminergic drugs. Some clinicians consider trihexyphenidyl of little value, but most find it an effective adjunct in a multidimensional treatment regimen for Parkinson's syndrome. Trihexyphenidyl can be used as adjunctive therapy for levodopa in patients with Parkinson's syndrome. This study investigated the effects of two anticholinergic drugs (atropine and trihexyphenidyl) on the elimination of lethal and convulsive effects of soman-induced poisoning in rats. The oxime compound HI-6, when used in combination with the centrally acting anticholinergic trihexyphenidyl, appeared to be more effective than when used with atropine in eliminating the acute toxic effects of soman. The results support the hypothesis that the choice of anticholinergic drug is crucial to the effectiveness of the antidote mixture in the treatment of acute soman poisoning. Adverse reactions to trihexyphenidyl are primarily due to its anticholinergic effects. 30-50% of patients taking trihexyphenidyl experience adverse reactions, including dry mouth, dizziness, blurred vision, nausea, and nervousness. Typical adverse reactions to other antimuscarinic drugs include constipation, tachycardia, mydriasis, dysuria or urinary retention, drowsiness, increased intraocular pressure, fatigue, vomiting, and headache. High doses, a history of allergies to other drugs, or arteriosclerosis may cause central nervous system excitation, typically manifesting as restlessness, anxiety, confusion, delirium, hallucinations, or euphoria. Occasional adverse reactions such as rash, colonic dilatation, paralytic ileus, and purulent parotitis secondary to dry mouth have been reported. Angle-closure glaucoma has been reported in patients taking trihexyphenidyl long-term. In rare cases, trihexyphenidyl can cause mental disorders such as delusions, amnesia, depersonalization, unreality, and one case of suspected delusional disorder. The incidence and severity of adverse reactions are usually dose-related, and sometimes adverse reactions can be avoided by reducing the dose. If a severe reaction occurs, the drug should be discontinued for several days and then restarted at a lower dose. Trihexyphenidyl hydrochloride should be used with caution or contraindicated in patients who do not wish to experience anticholinergic effects. When using trihexyphenidyl hydrochloride, the usual precautions and contraindications for antimuscarinic drugs should be followed. Patients taking antipsychotic drugs long-term may develop tardive dyskinesia, which may also occur after discontinuation of the drug. Anti-Parkinson's disease drugs do not relieve the symptoms of tardive dyskinesia and may even worsen the symptoms in some cases. However, Parkinson's disease and tardive dyskinesia often coexist in patients receiving long-term antipsychotic treatment, and anticholinergic treatment with trihexyphenidyl hydrochloride may relieve some Parkinson's disease symptoms. Because the use of trihexyphenidyl hydrochloride may continue indefinitely in some cases and has atropine-like properties, patients should be closely monitored for a long period to avoid allergic reactions and other adverse reactions. Trihexol hydrochloride has a certain parasympathetic blocking effect and should therefore be used with caution in patients with glaucoma, gastrointestinal or genitourinary obstruction, and elderly men who may have benign prostatic hyperplasia. Elderly patients, especially those over 60 years of age, are often more sensitive to the effects of this type of drug and therefore require strict dosage control. Parasympathetic blocking drugs such as trihexol hydrochloride may induce early glaucoma. For more complete data on trihexol (10 total warnings), please visit the HSDB record page.

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Pharmacodynamics
Trihexol is an anticholinergic drug indicated for adjunctive treatment of Parkinson's disease or for the treatment of drug-induced extrapyramidal symptoms. Because it does not require daily administration, its duration of action is relatively long. It has a wide therapeutic window, and acute toxicity is not fatal even at doses up to 300 mg. Patients should undergo iridocorneal angle examination before administration and intraocular pressure should be monitored during treatment. Patients should be informed of the risks of anhidrosis and hyperthermia.

C20H31NO

301.474

301.241

144-11-6

Trihexyphenidyl hydrochloride;52-49-3

5572

Typically exists as solid at room temperature

1.04g/cm3

447.9ºC at 760 mmHg

258.5ºC

211ºC

8.34E-09mmHg at 25°C

1.546

4.268

1

2

5

22

314

0

C1=CC=C(C=C1)C(CCN2CCCCC2)(C3CCCCC3)O

HWHLPVGTWGOCJO-UHFFFAOYSA-N

InChI=1S/C20H31NO/c22-20(18-10-4-1-5-11-18,19-12-6-2-7-13-19)14-17-21-15-8-3-9-16-21/h1,4-5,10-11,19,22H,2-3,6-9,12-17H2

1-cyclohexyl-1-phenyl-3-piperidin-1-ylpropan-1-ol

Apo-TrihexBenzhexol Trihex Benzhexol free base TrihexyphenidylArtane Parkin Pacitane

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)

May dissolve in DMSO (in most cases), if not, try other solvents such as H2O, Ethanol, or DMF with a minute amount of products to avoid loss of samples

Note: Listed below are some common formulations that may be used to formulate products with low water solubility (e.g. < 1 mg/mL), you may test these formulations using a minute amount of products to avoid loss of samples.

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Injection Formulation 1: DMSO : Tween 80： Saline = 10 : 5 : 85 (i.e. 100 μL DMSO stock solution → 50 μL Tween 80 → 850 μL Saline)
*Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH ₂ O to obtain a clear solution.
Injection Formulation 2: DMSO : PEG300 ：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL DMSO → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)
Injection Formulation 3: DMSO : Corn oil = 10 : 90 (i.e. 100 μL DMSO → 900 μL Corn oil)
Example: Take the Injection Formulation 3 (DMSO : Corn oil = 10 : 90) as an example, if 1 mL of 2.5 mg/mL working solution is to be prepared, you can take 100 μL 25 mg/mL DMSO stock solution and add to 900 μL corn oil, mix well to obtain a clear or suspension solution (2.5 mg/mL, ready for use in animals).

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Injection Formulation 4: DMSO : 20% SBE-β-CD in saline = 10 : 90 [i.e. 100 μL DMSO → 900 μL (20% SBE-β-CD in saline)]
*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.
Injection Formulation 5: 2-Hydroxypropyl-β-cyclodextrin : Saline = 50 : 50 (i.e. 500 μL 2-Hydroxypropyl-β-cyclodextrin → 500 μL Saline)
Injection Formulation 6: DMSO : PEG300 : castor oil : Saline = 5 : 10 : 20 : 65 (i.e. 50 μL DMSO → 100 μLPEG300 → 200 μL castor oil → 650 μL Saline)
Injection Formulation 7: Ethanol : Cremophor : Saline = 10： 10 : 80 (i.e. 100 μL Ethanol → 100 μL Cremophor → 800 μL Saline)
Injection Formulation 8: Dissolve in Cremophor/Ethanol (50 : 50), then diluted by Saline
Injection Formulation 9: EtOH : Corn oil = 10 : 90 (i.e. 100 μL EtOH → 900 μL Corn oil)
Injection Formulation 10: EtOH : PEG300：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL EtOH → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)

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Oral Formulation 1: Suspend in 0.5% CMC Na (carboxymethylcellulose sodium)
Oral Formulation 2: Suspend in 0.5% Carboxymethyl cellulose
Example: Take the Oral Formulation 1 (Suspend in 0.5% CMC Na) as an example, if 100 mL of 2.5 mg/mL working solution is to be prepared, you can first prepare 0.5% CMC Na solution by measuring 0.5 g CMC Na and dissolve it in 100 mL ddH2O to obtain a clear solution; then add 250 mg of the product to 100 mL 0.5% CMC Na solution, to make the suspension solution (2.5 mg/mL, ready for use in animals).

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Oral Formulation 3: Dissolved in PEG400
Oral Formulation 4: Suspend in 0.2% Carboxymethyl cellulose
Oral Formulation 5: Dissolve in 0.25% Tween 80 and 0.5% Carboxymethyl cellulose
Oral Formulation 6: Mixing with food powders

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Note: Please be aware that the above formulations are for reference only. InvivoChem strongly recommends customers to read literature methods/protocols carefully before determining which formulation you should use for in vivo studies, as different compounds have different solubility properties and have to be formulated differently.

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