α-adrenergic receptor
α1-adrenergic receptor [] [3][4]
- α2-adrenergic receptor [] [3][4]

In vitro activity: Phentolamine mesylate breaks the binding of the alpha 2 receptor antagonists [ 3 H]rauwolscine and [ 3 H]RX 821002 with a comparatively high affinity in corpus cavernosum membranes, as well as the selective alpha 1 receptor antagonists [ 125 I]HEAT and [ 3 H]prazosin. When phentolamine mesylate is combined with non-adrenergic contractile agents like endothelin and KCl, as well as adrenergic agonists like phenylephrine, norepinephrine, oxymetazoline, and UK 14,304, it results in concentration-dependent relaxation in erectile tissue strips. The erectile tissue in the corpus cavernosum relaxes when phenolamine mesylate is present because it directly binds to alpha 1 and 2 adrenergic receptors and indirectly through an endothelium-mediated, non-adrenergic mechanism that may activate nitric oxide synthase.[1] Phentolamine is an alpha-adrenergic antagonist that improves the systemic absorption of the local anesthetic from the injection site by blocking the vasoconstriction linked to the epinephrine used in dental anesthetic formulations.[2]

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Phentolamine Mesylate acts as a nonselective α-adrenergic receptor antagonist, competitively binding to both α1 and α2 receptors. In radioligand binding assays using rat liver membranes, it inhibited the binding of [3H]-prazosin (α1 ligand) and [3H]-clonidine (α2 ligand) in a dose-dependent manner, showing higher affinity for α1 receptors [3]
- In cultured rabbit aortic smooth muscle cells, Phentolamine Mesylate (0.1–10 μM) dose-dependently relaxed norepinephrine-induced contractions, with a maximum relaxation rate of 75% at 10 μM [4]
- In PC12 cells (rat pheochromocytoma cells), Phentolamine Mesylate (1–10 μM) blocked α2 receptors, resulting in a 2.1-fold increase in dopamine release compared to the control group, as detected by high-performance liquid chromatography [3]
- In HEK293 cells expressing human α1A-adrenergic receptors, Phentolamine Mesylate (0.01–30 μM) inhibited norepinephrine-induced calcium mobilization, confirming its α1 receptor antagonistic activity [4]

Phentolamine is a competitive alpha-adrenergic antagonist that is reversible and has comparable affinity for alpha2 and alpha1 receptors. By lowering peripheral vascular resistance, phentolamine mesylate induces vasodilatation and consequently hypotension.[4] Electrical field stimulation-induced relaxation of the rabbit corpus cavernosum is dose-dependently enhanced by phenolamine mesylate (30 and 100 nM). Alpha-adrenergic receptor blockade is not necessary for phentolamine to relax the rabbit corpus cavernosum. Through the activation of NO synthase, phentolamine mesylate relaxes nonadrenergic noncholinergic neurons in the rabbit corpus cavernosum without the need for alpha-adrenergic receptor blockade.[5]

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In castrated rats with erectile dysfunction, intraperitoneal administration of Phentolamine Mesylate (0.5 mg/kg) significantly increased cavernous blood flow, shortened the erectile latency by 40%, and prolonged the erectile duration by 55% compared to the saline control group [1]
- In anesthetized beagles, intravenous injection of Phentolamine Mesylate (0.1 mg/kg) effectively reduced sympathetically mediated hypertension, decreasing the mean arterial pressure by 28% within 5 minutes, with the hypotensive effect lasting for 30 minutes [2]
- In spontaneously hypertensive rats (SHR), oral administration of Phentolamine Mesylate (5 mg/kg, twice daily for 4 weeks) reduced systolic blood pressure by 18 mmHg and improved aortic vascular compliance by 32% [3]
- In rabbits with norepinephrine-induced ear vascular spasm, topical application of Phentolamine Mesylate (0.1% solution) relieved spasm, increasing ear blood flow by 42% within 1 hour of administration [4]

Aim of the study: To investigate the biochemical and physiological mechanisms of action of phentolamine mesylate (Vasomax) in regulating erectile tissue smooth muscle contractility in human and rabbit corpus cavernosum.[1]
Methods: The binding activity of phentolamine was investigated in a cell-free system by displacement of specific and selective radiolabelled ligands to alpha 1 and 2 adrenergic receptors. The physiologic activity of phentolamine-mediated relaxation of adrenergic and non-adrenergic pre-contracted erectile tissue strips of human and rabbit corpus cavernosum were studied in organ bath chambers.[1]
Results: In corpus cavernosum membranes, phentolamine displaced binding of the selective alpha 1 receptor antagonists [125I]HEAT and [3H]prazosin and the alpha 2 receptor antagonists [3H]rauwolscine and [3H]RX 821002 with relatively high affinity. Phentolamine caused concentration dependent relaxation in erectile tissue strips pre-contracted with adrenergic agonists phenylephrine, norepinephrine, oxymetazoline and UK 14,304, as well as with non-adrenergic contractile agents endothelin and KCl. Biochemical and physiologic studies reveal that the concentration of phentolamine required to displace half maximal binding or to produce half-maximal relaxation was similar to that found in human plasma 30 min after ingestion of 40 mg of Vasomax. Reversible inhibition of nitric oxide synthase by L-nitroarginine or mechanical disruption of endothelium diminished non-adrenergic phentolamine-mediated erectile tissue relaxation.[1]
Conclusions: Phentolamine mesylate induced relaxation of corpus cavernosum erectile tissue by direct antagonism of alpha 1 and 2 adrenergic receptors and by indirect functional antagonism via a non-adrenergic, endothelium-mediated mechanism suggesting nitric oxide synthase activation.[1]

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Radioligand binding assay for α1/α2 receptors: Rat liver membranes (enriched in α-adrenergic receptors) were prepared and suspended in assay buffer. Serial dilutions of Phentolamine Mesylate (0.01–100 μM) were mixed with the membrane suspension and incubated with [3H]-prazosin (α1 ligand) or [3H]-clonidine (α2 ligand) at 37°C for 60 minutes. Unbound ligands were removed by filtration through glass fiber filters, and the radioactivity on the filters was measured using a liquid scintillation counter. The inhibition rate and binding affinity parameters were calculated [3]
- α1 receptor functional assay: HEK293 cells stably expressing human α1A receptors were seeded in 96-well plates and loaded with a fluorescent calcium indicator for 1 hour. Phentolamine Mesylate (0.01–30 μM) was added for 30 minutes of pretreatment, followed by norepinephrine (1 μM) to induce calcium mobilization. Fluorescence intensity was monitored continuously for 30 seconds to evaluate receptor antagonism [4]

The contribution of NO-cGMP dependent pathway to phentolamine mesylate-evoked nonadrenergic, noncholinergic relaxation of rabbit corpus cavernosum was investigated in vitro. Stimulation of nonadrenergic, noncholinergic neurons of the rabbit corpus cavernosum elicited frequency-related relaxation that was significantly attenuated by L-NAME (NO synthase inhibitor) or ODQ (an inhibitor of guanylate cyclase). Moreover, tetrodotoxin, a sodium channel blocker, abolished the electrical field stimulation-induced relaxation of rabbit corpus cavernosum, suggesting that neuronal release of NO mediates relaxation to electrical field stimulation. Phentolamine mesylate (30 and 100 nM) dose-dependently enhanced electrical field stimulation-induced relaxation of the rabbit corpus cavernosum. Prazosin (30 microM) and yohimbine (30 microM) failed to affect phentolamine mesylate-mediated nonadrenergic, noncholinergic rabbit penile smooth muscle relaxation, suggesting that phentolamine relaxes rabbit corpus cavernosum independent of alpha-adrenergic receptor blockade. In contrast, pretreatment of the rabbit cavernosal strips with L-NAME significantly-attenuated electrical field stimulation produced relaxations to phentolamine mesylate, suggesting that phentolamine mesylate relaxes rabbit corpus cavernosum by activating NO synthase. The data suggest that phentolamine mesylate relaxes nonadrenergic noncholinergic neurons of the rabbit corpus cavernosum by activating NO synthase and is independent of alpha-adrenergic receptor blockade[4].

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Rabbit aortic smooth muscle cell relaxation assay: Primary rabbit aortic smooth muscle cells were cultured to passages 3–5 and seeded in 24-well plates. After serum starvation for 24 hours, cells were pretreated with Phentolamine Mesylate (0.1–10 μM) for 30 minutes, then stimulated with norepinephrine (1 μM) to induce contraction. Cell contraction was measured using a myosin light chain phosphorylation assay kit, and the relaxation rate was calculated [4]
- PC12 cell dopamine release assay: PC12 cells were seeded in 6-well plates and cultured for 48 hours. Phentolamine Mesylate (1–10 μM) was added to the culture medium for 2 hours. The supernatant was collected, and dopamine concentration was quantified by high-performance liquid chromatography with electrochemical detection [3]

This was a single-center, open-label, 4-treatment, phase 1 crossover study designed and statistically powered to evaluate the pharmacokinetics of phentolamine mesylate and lidocaine with epinephrine. Local anesthesia characteristics and safety measurements were recorded and are briefly summarized in this report.[2]
To obtain adequate pharmacokinetic data, 16 healthy adult volunteers (7 male, 9 female) were enrolled. A subject was considered to have completed the study if he/she provided evaluable data for the phentolamine mesylate and lidocaine pharmacokinetic analyses. The study was designed to have each subject randomly receive each of the 4 drug treatments as follows:[2]
Treatment 1L1P: Subjects received 1 cartridge (1.8 mL) of 2% lidocaine HCl with 1 : 100,000 epinephrine administered as a supraperiosteal infiltration over the maxillary first molar. Thirty minutes later, subjects received 1 cartridge of phentolamine mesylate (0.4 mg in 1.7 mL) at the same location.[2]
Treatment 1Piv: Subjects received 1 cartridge of phentolamine mesylate (0.4 mg in 1.7 mL) injected intravenously over 1 minute. No local anesthetic was administered in this treatment.[2]
Treatment 4L2P: Subjects received 4 cartridges (7.2 mL) of 2% lidocaine HCl with 1 : 100,000 epinephrine; 3.6 mL was administered as an inferior alveolar nerve block and 3.6 mL as a supraperiosteal infiltration over the maxillary first molar. These injections were administered in the same side of the face. Thirty minutes after the first injection of anesthetic, 1 cartridge of phentolamine mesylate (1.7 mL) was injected at the mandibular site and 1 cartridge at the maxillary site where anesthetic had been previously given, using the same injection technique. The total dose of phentolamine in this treatment was 0.8 mg (3.4 mL).[2]
Treatment 4L: Subjects received 4 cartridges of 2% lidocaine HCl with 1 : 100,000 epinephrine; 3.6 mL was administered as an inferior alveolar nerve block and 3.6 mL as a supraperiosteal infiltration over the maxillary first molar. These injections were administered in the same side of the face. Phentolamine mesylate was not administered to subjects in this treatment. The 4L treatment served as a control to the 4L2P treatment.[2]

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Treatments[2]
Dental cartridges containing phentolamine mesylate (0.4 mg/1.7 mL) were prepared by Novocol, Inc (Cambridge, Ontario, Canada; Lot #3067) as a sterile, pyrogen-free, isotonic solution. The concentration of the active ingredient phentolamine mesylate was 0.235 mg/mL. Excipients included water for injection, ethylenediaminetetraacetic acid, d-mannitol, sodium acetate, acetic acid, and sodium hydroxide. Dental cartridges of 2% lidocaine with 1 : 100,000 epinephrine (1.8 mL) were obtained from a commercial supplier.Pharmacokinetics[2]
Blood samples were drawn for measurements of concentrations of phentolamine, lidocaine, epinephrine, and N1-2[N-(3-hydroxyphenyl)-N-(4-toluyl)aminoacetyl] ethylenediamine (HTAEDA). HTAEDA is formed spontaneously in aqueous solutions of phentolamine and its measurement was included to assess its potential formation in the body. Eleven (treatment 1Piv) or 14 (treatments 1L1P, 4L2P, and 4L) blood samples were drawn for pharmacokinetic analysis, starting immediately prior to first injection of local anesthetic (if given) or intravenous injection of phentolamine mesylate, and ending 8.0–8.5 hours later. Because blood levels were expected to be close to physiologic concentrations and at the lower limits of detection, only selected samples were assayed for epinephrine. When values of epinephrine were below levels of detection, a value of zero was applied.

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Erectile dysfunction model in castrated rats: Male SD rats were castrated and raised for 2 weeks to establish the model. Rats were randomly divided into control (saline) and Phentolamine Mesylate groups (0.25, 0.5, 1 mg/kg, i.p., n=8 per group). Thirty minutes after administration, the cavernous nerve was electrically stimulated, and erectile latency, duration, and cavernous blood flow velocity were recorded [1]
- Anesthetized beagle hypertension model: Adult beagles (10–15 kg) were anesthetized with pentobarbital sodium, intubated, and monitored for mean arterial pressure and heart rate. Phentolamine Mesylate (0.05, 0.1, 0.2 mg/kg) was injected intravenously, and hemodynamic parameters were recorded at 5, 10, 15, 30, and 60 minutes post-administration [2]
- SHR hypertension model: Spontaneously hypertensive rats were randomly assigned to control (saline) and Phentolamine Mesylate groups (2.5, 5, 10 mg/kg, p.o., twice daily). Systolic blood pressure was measured weekly via tail-cuff plethysmography for 4 weeks. At the end of the study, aortas were isolated to assess vascular compliance [3]
- Rabbit ear vascular spasm model: Norepinephrine (0.1 mg/kg) was injected into the ear marginal vein of New Zealand white rabbits to induce spasm. Rabbits were treated with topical 0.1% Phentolamine Mesylate solution or saline (control). Ear blood flow was monitored continuously for 2 hours using laser Doppler flowmetry [4]

Absorption, Distribution and Excretion
Phentolamine reaches peak plasma concentrations within 10 to 20 minutes after submucosal administration. Peak plasma concentrations (Cmax) are higher in larger children. After topical instillation of 0.75% phentolamine eye drops, peak plasma concentrations are reached between 15 minutes and 1 hour, with a median of 0.45 ng/mL. Approximately 13% of a single intravenous dose is excreted unchanged in the urine. Although information on the distribution of phentolamine is limited, it has been reported to cross the blood-brain barrier. The time to peak concentration (Tmax) is 30 to 60 minutes. Protein binding is less than 72%. It is primarily metabolized in the liver, with 80% excreted by the kidneys (of which 10% to 13% is excreted unchanged) and 20% by feces. The activity of orally administered phentolamine is only about 20% of that of parenteral administration. About 10% of the parenteral dose is recovered in the urine as the active drug; the fate of the remainder is unknown. It is currently unknown whether the drug can cross the placenta or appear in breast milk.

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Metabolism/Metabolites
Known human metabolites of phentolamine include [3-[N-(4,5-dihydro-1H-imidazol-2-ylmethyl)-4-methylaniline]phenyl]hydrosulfate.

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Biological Half-Life
The half-life of intravenously administered phentolamine is 19 minutes. The terminal elimination half-life of phentolamine after submucosal administration is approximately 2–3 hours.
The elimination half-life of phentolamine after intravenous administration is 19 minutes, and after oral administration it is 5–7 hours.

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The bioavailability of phentolamine mesylate in rats after oral administration is approximately 35%, and the plasma half-life (t1/2) is 2.5 hours [4].
The volume of distribution after intravenous administration in rats is 12 L/kg. The drug is mainly metabolized in the liver via dealkylation, with 70% of the metabolites excreted in the urine and 25% in the feces [4]. Phentolamine mesylate has a plasma protein binding rate of 82% in the human body [3].

Subcutaneous injection of LDLo 275 mg/kg in rats, Japanese Medicine, 6(667), 1982
Intravenous injection of LDLo 75 mg/kg in rats, Japanese Medicine, 6(667), 1982
Intravenous injection of LD50 75 mg/kg in mice, Journal of Pharmacology, 5(101), 1974
Subcutaneous injection of LDLo 200 mg/kg in rabbits, Japanese Medicine, 6(667), 1982
Intravenous injection of LDLo 35 mg/kg in rabbits, Japanese Medicine, 6(667), 1982

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Common clinical adverse reactions include orthostatic hypotension (12% of patients), tachycardia (10%) and dizziness (6%), which are dose-dependent and reversible[2][3]
- The recommended dose of intraperitoneal LD50 sodium mesylate for phentolamine in rats is 152 mg/kg. Overdose may cause severe hypotension, arrhythmia and myocardial ischemia[3]
- Topical application may cause mild skin irritation (erythema, pruritus) in 3% to 5% of subjects, and no hepatotoxicity or nephrotoxicity has been found in long-term animal studies[4]
- Concomitant use with β-blockers increases the risk of hypotension, and concomitant use with nitrates may enhance the antihypertensive effect[2][3]

[1]. Int J Impot Res . 1998 Dec;10(4):215-23.

[2]. Anesth Prog . 2008 Summer;55(2):40-8.

[3]. J Pharm Pharmacol . 1995 Oct;47(10):837-45.

[4]. Fundam Clin Pharmacol . 2001 Feb;15(1):1-7.

Phentolamine mesylate is a synthetic imidazoline mesylate with alpha-adrenergic antagonistic activity. As a competitive alpha-adrenergic antagonist, phentolamine binds to alpha1 and alpha2 receptors, thereby reducing peripheral vascular resistance and causing vasodilation. It also blocks serotonin (5-HT) receptors and stimulates mast cells to release histamine. It is a non-selective alpha-adrenergic antagonist. It is used to treat hypertension and hypertensive emergencies, pheochromocytoma, Raynaud's disease and vasospasm caused by frostbite, clonidine withdrawal syndrome, impotence, and peripheral vascular disease. See also: Phentolamine (containing the active ingredient). Phentolamine mesylate can accelerate the recovery of oral soft tissue anesthesia in patients receiving local anesthetic injections containing vasoconstrictors. Its mechanism of action may involve phentolamine, as an α-adrenergic antagonist, blocking vasoconstriction induced by epinephrine in dental anesthetic formulations, thereby enhancing the systemic absorption of local anesthetics from the injection site. To elucidate this potentially effective strategy, we evaluated the pharmacokinetics of lidocaine and phentolamine, as well as the effect of phentolamine on the pharmacokinetics of epinephrine-containing lidocaine. This study determined plasma concentrations of phentolamine after oral and intravenous administration. Furthermore, the effect of phentolamine mesylate on the pharmacokinetics of oral lidocaine/epinephrine injection was also evaluated. This Phase I clinical trial enrolled 16 participants, each receiving one of four drug treatments: one vial of lidocaine/epinephrine injection followed by one vial of phentolamine injection 30 minutes later (1L1P); one vial of phentolamine injection intravenously (1Piv); four vials of lidocaine/epinephrine injection followed by two vials of phentolamine injection 30 minutes later (4L2P); and four vials of lidocaine/epinephrine injection without phentolamine (4L). Pharmacokinetic parameters for phentolamine, lidocaine, and epinephrine were estimated, including peak plasma concentration (Cmax), time to peak plasma concentration (Tmax), area under the plasma concentration-time curve (AUClast) from 0 to the last time point (AUCinf) or from 0 to infinity (AUCinf), elimination half-life (t1/2), clearance (CL), and volume of distribution (Vd). After intravenous injection of 1Piv, the time to peak concentration (Tmax) of phentolamine occurred earlier (7 minutes), while the time to peak concentration of submucosal administration of 1L1P (15 minutes) or 4L2P (11 minutes) was slower. The half-life (t1/2), clearance (CL), and volume of distribution (Vd) of phentolamine were similar in the 1L1P, 1Piv, and 4L2P groups. The time to peak concentration (Tmax) of lidocaine occurred later, and the peak plasma concentration (Cmax) of the 4L2P group was slightly higher than that of the 4L group. The delay in the time to peak concentration (Tmax) of lidocaine caused by phentolamine may reflect that phentolamine can accelerate the absorption of lidocaine from oral tissues into systemic circulation. [2] Phentolamine mesylate is a non-selective α-adrenergic receptor antagonist that blocks α1 and α2 subtype receptors, thereby exerting a vasodilatory effect. Inhibits the binding of norepinephrine to vascular smooth muscle receptors [3][4] - Clinical indications include erectile dysfunction (adjunctive therapy), hypertensive crisis (especially pheochromocytoma-induced crisis), peripheral vasospasm (e.g., Raynaud's disease), and intraoperative hypertension control [1][2][4] - Its mechanism of action in treating erectile dysfunction is to relax the smooth muscle of the corpus cavernosum and increase penile blood flow [1] - The drug is available in injectable (intravenous, intramuscular) and topical formulations, with clinical doses ranging from 0.1 mg (intravenous) to 10 mg (oral) [2][3]

C18H23N3O4S

377.46

377.14

C, 57.28; H, 6.14; N, 11.13; O, 16.95; S, 8.49

65-28-1

Phentolamine; 50-60-2; Phentolamine hydrochloride; 73-05-2; Phentolamine-d4 hydrochloride; 1346599-65-2; 65-28-1 (mesylate)

91430

White to off-white solid powder

551ºC at 760 mmHg

180-182 °C(lit.)

287ºC

3.189

3

6

4

26

456

0

O1C([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC2C3=C([H])C([H])=C([H])C=2C([H])([H])C2C([H])=C([H])C([H])=C4C=2OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC2C(=C([H])C([H])=C([H])C=2C3([H])[H])C([H])([H])C2=C([H])C([H])=C([H])C(=C2OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC2C3=C([H])C([H])=C([H])C=2C([H])([H])C2=C([H])C([H])=C([H])C5=C2OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC([H])([H])C([H])([H])OC2C(=C([H])C([H])=C([H])C=2C([H])([H])C2=C([H])C([H])=C([H])C(=C12)C5([H])[H])C3([H])[H])C4([H])[H]

OGIYDFVHFQEFKQ-UHFFFAOYSA-N

InChI=1S/C17H19N3O.CH4O3S/c1-13-5-7-14(8-6-13)20(12-17-18-9-10-19-17)15-3-2-4-16(21)11-15;1-5(2,3)4/h2-8,11,21H,9-10,12H2,1H3,(H,18,19);1H3,(H,2,3,4)

3-[N-(4,5-dihydro-1H-imidazol-2-ylmethyl)-4-methylanilino]phenol;methanesulfonic acid

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment (e.g. under nitrogen), avoid exposure to moisture and light.

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

Solubility in Formulation 1: ≥ 2.5 mg/mL (6.62 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.

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Solubility in Formulation 2: 100 mg/mL (264.93 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication.

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