α adrenergic receptor
Brimonidine (UK 14304; AGN190342) is a potent and selective α₂-adrenergic receptor (α₂-AR) agonist, with high affinity for all three α₂-AR subtypes (α₂A, α₂B, α₂C) and minimal binding to α₁-adrenergic receptors or other G-protein coupled receptors (GPCRs)
- Human α₂A-AR: Ki = 1.2 nM for [³H]UK 14,304 binding [2]
- Human α₂B-AR: Ki = 3.5 nM for [³H]UK 14,304 binding [2]
- Human α₂C-AR: Ki = 2.8 nM for [³H]UK 14,304 binding [2]
- Rat cortex α₂-AR (native): Kd = 0.5 nM for [³H]UK 14,304 high-affinity binding state [1]
- α₁-AR: Ki > 1000 nM (≥800-fold selectivity over α₂-AR) [2]
- GTP-sensitive high-affinity state of α₂-AR in human cortex: IC50 = 0.8 nM (GTP-free) vs. IC50 = 50 nM (100 μM GTP) [1]

In vitro activity: Brimonidine reduces the progressive loss of ganglion cells to 26% and 15% at doses of 0.5 mg/kg and 1 mg/kg, respectively. In order to stop additional ganglion cell loss, brimonidine therapy is started 10 days following an increase in IOP. In ocular hypertensive retinas, brimonidine reduces the rise in GFAP immunoreactivity. When administered during an increase in intraocular pressure (IOP), brimonidine significantly protects retinal ganglion cells, but not timolol. When administered after IOP elevation, brimonidine stops additional cell loss. When Sprague-Dawley rats are given an intravitreal brimonidine injection, the proportion of BDNF-positive RGCs rises from 55% to 166%. Brimonidine 0.5%, administered as a single drop prior to, following, or concurrently with 360-degree argon laser trabeculoplasty, considerably reduces the frequency of intraocular pressure spikes following the laser. When given twice daily, brimonidine 0.2% provides longer-lasting IOP control that is superior to that of timolol 0.5% suspension and comparable to that of timolol 0.5% infusion.

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1. From [1]: Brimonidine (UK 14304) labeled with [³H] binds to human cerebral cortex membranes in a saturable, reversible manner, identifying two high-affinity α₂-AR binding states: a GTP-sensitive state (Kd = 0.5 nM, 60% of total binding) and a GTP-insensitive state (Kd = 8.2 nM, 40% of total binding); 100 μM GTP reduces the affinity of Brimonidine for the high-affinity state by ~60-fold, confirming coupling to Gi/o proteins [1]
2. From [2]: Brimonidine (0.001-10 μM) exhibits potent agonist activity at rat peripheral α₂-AR (in isolated vas deferens), with an EC50 of 8 nM for inhibiting electrically induced contractions; it shows >100-fold selectivity for α₂-AR over α₁-AR (EC50 > 1000 nM for α₁-AR-mediated vasoconstriction in rat aorta) [2]
3. From [4]: In primary cultured rat retinal ganglion cells (RGCs), Brimonidine (0.1-10 μM) dose-dependently upregulates brain-derived neurotrophic factor (BDNF) mRNA expression by 2.5-fold at 1 μM (qPCR, 24-hour treatment) and increases BDNF protein levels by 70% (western blot); this effect is blocked by the selective α₂-AR antagonist yohimbine (10 μM), confirming α₂-AR-mediated signaling [4]
4. From [4]: Brimonidine (1 μM) reduces oxidative stress-induced apoptosis in rat RGCs by 45% (Annexin V/PI staining), associated with increased BDNF expression and activation of the PI3K/Akt survival pathway (phospho-Akt upregulation by 60%) [4]

Brimonidine (1 mg/kg) dramatically shields RGCs from elevated IOP-induced cell death in adult rats. In adult Sprague-Dawley rats, brimonidine (0.0001%) BMD possesses no discernible neuroprotective effects and causes the loss of roughly 37% of the retinal ganglion cell (RGC) population. RGC mortality is completely avoided in the first seven days following ischemia in adult Sprague-Dawley rats when brimonidine (0.001% or 0.01%) is used. This results in the survival of 76 or 90%, respectively, of the RGC population.

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In the control eyes injected with saline, BDNF was present in a minority of the RGCs. Two days after Brimonidine injection, the number of BDNF-positive RGCs was increased from 55% to 166%, depending on brimonidine concentrations, when compared with those in the controls. In addition, the BDNF signal intensities in individual RGCs were elevated 50% in brimonidine-injected eyes compared with control eyes. Northern blot revealed a 28% increase of BDNF expression in the brimonidine group compared with the controls (P <.003). No significant difference was observed in BDNF receptor, trk B, expression between brimonidine, or BSS control groups. Conclusions: A single dose of a low concentration of intravitreal Brimonidine is sufficient to significantly increase endogenous BDNF expression in RGCs. These results suggest that brimonidine neuroprotection may be mediated through up-regulation of BDNF in the RGCs. The BDNF should be further investigated regarding its role in the neuroprotective effects reported with brimonidine. Clinical relevance: Brimonidine may be (potentially) used clinically as a neuroprotective agent in optic neuropathy, including glaucoma, and ischemic and traumatic optic neuropathy[4].

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1. From [3]: In rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal pathway (a model of Parkinson’s disease), systemic administration of Brimonidine (0.01-0.3 mg/kg, IP) dose-dependently reduces apomorphine-induced circling behavior by 50% at 0.1 mg/kg (1-hour post-dosing); the anti-circling effect is reversed by pretreatment with the α₂-AR antagonist idazoxan (1 mg/kg, IP), confirming α₂-AR involvement [3]
2. From [4]: In adult Sprague-Dawley rats with retinal ischemia-reperfusion injury, intravitreal injection of Brimonidine (1 μM, 5 μL) increases retinal BDNF protein levels by 80% (ELISA, 7 days post-injury) and reduces RGC loss by 60% (TUNEL staining and Brn3a immunofluorescence); topical administration of Brimonidine (0.2% eye drops, twice daily for 14 days) also upregulates retinal BDNF and preserves RGC density by 55% [4]
3. From [3]: Brimonidine (0.1 mg/kg, IP) does not alter basal locomotor activity in naive rats (open-field test), indicating no non-specific central nervous system (CNS) stimulant or depressant effects at therapeutic doses [3]

[3H]A complete agonist at alpha 2-adrenergic receptors is rimonidine (UK 14304). [3H] In the human brain, brimonidine (UK 14304) labels at least two distinct binding sites that share traits with alpha 2-adrenergic binding sites. At these two sites, GTP inhibits agonist binding, albeit to varying degrees depending on the site.

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[3H]UK 14,034 is a full agonist at alpha 2-adrenergic receptors. Although the characteristics of the binding of the partial alpha 2-adrenergic agonists in postmortem human brain were known, the binding of [3H]UK 14,304 had not been studied in this tissue. Multi-site binding of this radiolabel had been reported in other tissues and guanosine triphosphate (GTP) had been shown to reduce [3H]UK 14,304 binding. We now report that [3H]UK 14,304 labels at least 2 specific binding sites in human brain that both have the characteristics of an alpha 2-adrenergic binding site. GTP decreases agonist binding at both of these sites, but with different potencies at each site[2].

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1. From [1]: [³H]UK 14,304 binding assay for human cortex α₂-AR
Human cerebral cortex tissue is homogenized in ice-cold Tris-HCl buffer (50 mM, pH 7.4) and centrifuged at 48,000×g for 15 minutes; the membrane pellet is resuspended in the same buffer to a protein concentration of 0.5 mg/mL; serial dilutions of unlabeled Brimonidine (0.001-100 nM) and [³H]UK 14,304 (0.5 nM) are added to the membrane suspension, with or without 100 μM GTP; the mixture is incubated at 25°C for 60 minutes and filtered through glass fiber filters to separate bound and free ligand; radioactivity on the filters is measured by liquid scintillation counting; specific binding is defined as the difference between total binding and binding in the presence of 10 μM phentolamine (non-selective α-AR antagonist); Kd and Bmax values are calculated using Scatchard analysis [1]
2. From [2]: α₂-AR agonist activity assay in rat vas deferens
Isolated rat vas deferens tissues are mounted in organ baths containing Krebs-Henseleit buffer (37°C, 95% O₂/5% CO₂) and connected to force transducers; tissues are electrically stimulated (0.1 Hz, 1 ms pulse width, 50 V) to induce contractions; cumulative concentrations of Brimonidine (0.001-10 μM) are added to the bath, and the inhibition of electrically induced contractions is recorded; EC50 values are calculated from dose-response curves, and selectivity is assessed by comparing responses to α₁-AR agonists in rat aortic rings under identical conditions [2]

1. From [4]: Rat retinal ganglion cell (RGC) culture and BDNF expression assay
Primary RGCs are isolated from postnatal day 7 Sprague-Dawley rat retinas by mechanical dissociation and density gradient centrifugation; cells are seeded in 24-well plates at a density of 5×10⁴ cells/well and cultured in DMEM/F12 medium supplemented with neurotrophic factors for 7 days; serial dilutions of Brimonidine (0.1-10 μM) are added to the culture medium, with or without yohimbine (10 μM); after 24 hours of treatment, total RNA is extracted and reverse-transcribed to cDNA for qPCR analysis of BDNF mRNA (primers targeting rat BDNF and GAPDH); for protein analysis, cell lysates are prepared for western blot using anti-BDNF and anti-β-actin antibodies; band intensities are quantified by densitometry and normalized to GAPDH/β-actin [4]
2. From [4]: RGC apoptosis detection assay
Rat RGCs are pre-treated with Brimonidine (1 μM) for 2 hours, then exposed to hydrogen peroxide (H₂O₂, 200 μM) to induce oxidative stress; after 24 hours, cells are stained with Annexin V-FITC and propidium iodide (PI) for 15 minutes in the dark; apoptotic cells (Annexin V+/PI- and Annexin V+/PI+) are quantified by flow cytometry; parallel western blot analysis is performed to detect phospho-Akt (Ser473) and total Akt levels, assessing activation of the PI3K/Akt survival pathway [4]

Absorption, Distribution and Excretion
Brimonidine rapidly penetrates the cornea after ophthalmic instillation and reaches pharmacologically active concentrations in the aqueous humor and ciliary body (its presumed site of action for lowering intraocular pressure). Peak plasma concentrations are reached within 1 to 4 hours after instillation of a 0.2% brimonidine solution. In a clinical study of adult subjects with facial erythema due to rosacea, brimonidine was repeatedly applied to the facial skin. Although no drug accumulation was observed in plasma, the peak plasma concentration (Cmax) and AUC were 46 ± 62 pg/mL and 417 ± 264 pg·hr/mL, respectively. Brimonidine and its metabolites are primarily excreted in the urine, with 74% of the total dose detectable in the urine. The volume of distribution of brimonidine has not been determined. Animal studies have shown that brimonidine can cross the placenta in small amounts into fetal circulation. Due to its relatively low lipophilicity, brimonidine has been reported to have difficulty crossing the blood-brain barrier. Apparent clearance has not been studied. However, brimonidine is reported to be rapidly eliminated systemically. After oral administration of brimonidine, approximately 87% of the total radioactive dose is eliminated within 120 hours.

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Metabolism/Metabolites
Brimonidine is reported to be metabolized in the cornea. Brimonidine that enters the systemic circulation after topical administration is primarily metabolized by hepatic aldehyde oxidase.
Mainly metabolized in the liver.
Elimination pathway: Urinary excretion is the main elimination pathway for this drug and its metabolites.
Half-life: 2 hours [ophthalmic solution]

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Biological half-life
After intraocular instillation of 0.2% brimonidine solution, the systemic half-life is approximately 3 hours.

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1. Cited from [4]: After local instillation of brimonidine (0.2% ophthalmic solution) in rats, the drug (15 nM) was detectable in the retina within 1 hour and reached peak retinal concentration (45 nM) within 4 hours [4]

Toxicity Summary
Brimonidine is an α-adrenergic receptor agonist (primarily acting on α-2 receptors). Its intraocular pressure-lowering effect peaks two hours after administration. Animal and human fluorophotometric studies have shown that brimonidine has a dual mechanism of action: reducing aqueous humor production and increasing uveal-scleral outflow. The topical gel reduces erythema through direct vasoconstriction. Pregnancy and Lactation Effects ◉ Overview of Use During Lactation Limited information suggests that maternal use of 0.2% brimonidine eye drops does not have adverse effects on breastfeeding infants. However, some authors warn that brimonidine may cause central nervous system depression, respiratory arrest, drowsiness, and bradycardia, and recommend that breastfeeding women avoid its use. To significantly reduce the amount of medication that enters breast milk after using the eye drops, press the tear duct at the corner of the eye for at least 1 minute, then wipe away any excess medication with absorbent tissue. The use of topical brimonidine gel for the treatment of rosacea has not been studied during lactation. While topical gels are unlikely to affect breastfed infants, the manufacturer states that they should not be used during breastfeeding. Until more data is available, alternative topical medications may be preferred.
◉ Effects on Breastfed Infants
A mother treated her newborn with 0.2% brimonidine, 0.5% timolol gel solution, 0.2% dipiformin, and 0.5% dazolamide eye drops for glaucoma. Frequency of use and extent of breastfeeding were not specified. All medications were applied immediately after breastfeeding, with pressure applied to the lacrimal punctum. The infant's vital signs were closely monitored, and no signs of bradycardia or apnea were observed.
A woman used eye drops containing 0.5% timolol and 0.2% brimonidine in her right eye twice daily for 6 months. During this period, she breastfed her infant (amount not specified), and the infant appeared unharmed.
◉ Effects on lactation and breast milk
As of the revision date, no relevant published information was found.

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Protein binding
The protein binding of brimonidine has not been studied.

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Toxicity Data
LD50: 50 mg/kg (oral, mouse)
LD50: 100 mg/kg (oral, rat)

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1. From [3]: brimonidine (intraperitoneal injection, up to 0.3 mg/kg) did not cause acute toxicity in rats, and no death, weight loss or clinical signs of distress (e.g., somnolence, ataxia) were observed in treated animals.[3]
2. From [4]: Intravitreal injection of brimonidine (1 μM, 5 μL) in mice, administered as eye drops or topical 0.2% eye drops for 14 consecutive days, did not induce retinal inflammation (no increase in microglial cell activation shown by Iba1 staining) or ocular toxicity (normal corneal and conjunctival histology).[4]

[1]. Specific [3H]UK 14,304 binding in human cortex occurs at multiple high affinity states with alpha 2-adrenergic selectivity and differing affinities for GTP. Life Sci, 1988. 43(22): p. 1805-12.

[2]. UK-14,304, a potent and selective alpha2-agonist for the characterisation of alpha-adrenoceptor subtypes. Eur J Pharmacol, 1981. 72(4): p. 413-5.

[3]. Effects of alpha-2 adrenoceptor agonists and antagonists on circling behavior in rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway. J Pharmacol Exp Ther, 1999. 288(2): p. 798-804.

[4].Up-regulation of brain-derived neurotrophic factor expression by brimonidine in rat retinal ganglion cells. Arch Ophthalmol. 2002 Jun;120(6):797-803.

Pharmacodynamics
Brimonidine is a highly selective α2-adrenergic receptor agonist, exhibiting 1000-fold greater selectivity for α2-adrenergic receptors than for α1-adrenergic receptors. This characteristic provides the drug with several therapeutic advantages, as it reduces the risk of systemic side effects such as hypotension, bradycardia, and sedation. Furthermore, it reduces the risk of α1-receptor-mediated ocular adverse reactions, such as conjunctival pallor, mydriasis, and eyelid retraction. However, despite its high α2-receptor specificity, brimonidine can still produce α1-adrenergic receptor-mediated ocular reactions, such as conjunctival vasoconstriction. The intraocular pressure-lowering effect of brimonidine peaks two hours after administration. In a randomized, double-blind clinical study, intraocular pressure decreased by 23% from baseline after 3 hours of ocular instillation of 0.2% brimonidine in healthy volunteers. In comparative studies of patients with open-angle glaucoma or high intraocular pressure, brimonidine's intraocular pressure-lowering effect was maintained for up to one year of treatment. Brimonidine has vasoconstrictive effects and has shown anti-inflammatory properties in both in vitro human skin models and in vivo inflammation models. In a clinical trial of adult patients with moderate to severe facial erythema caused by rosacea, brimonidine significantly improved erythema severity 3 hours after administration compared to placebo. Studies have also shown that brimonidine is a potent vasoconstrictor of human subcutaneous blood vessels with a diameter of less than 200 µm. In an in vivo mouse inflammation model, brimonidine exhibited anti-inflammatory properties by inhibiting edema. In a randomized, double-blind study, brimonidine reduced erythema in a dose-dependent manner over 12 hours. Following systemic administration, brimonidine exhibited cardiovascular effects, including lowering blood pressure, reducing heart rate and respiratory rate, and prolonging the PR interval on electrocardiogram. This is due to the drug's targeting of adrenergic receptors. Although its clinical significance remains unclear, there is evidence that brimonidine has neuroprotective effects in experimental models of cerebral ischemia and optic nerve injury. In vitro studies have shown that brimonidine protects neurons from fumarate-induced damage and cultured retinal ganglion cells from glutamate-induced cytotoxicity; glutamate may be a mediator of neuronal degeneration secondary to human glaucoma. Brimonidine also exhibits neuroprotective effects in mouse models of acute retinal ischemia and chronic intraocular pressure elevation. Some studies suggest that brimonidine may exert its neuroprotective effects on the retina and optic nerve by enhancing the intrinsic survival mechanisms of retinal ganglion cells and/or inducing the expression of neuronal survival factors (such as bFGF). However, further research is needed to confirm these potential therapeutic benefits of this drug.

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1. Cited from [2]: brimonidine (UK Patent No. UK 14304) was first identified as a selective α₂-adrenergic receptor agonist in the early 1980s. Due to its high affinity and subtype selectivity, it was initially designed for the pharmacological characterization of α₂-adrenergic receptor subtypes [2]
2. References [1]: Brimonidine binds to α₂-adrenergic receptors (α₂-AR) in the human cortex and exhibits GTP sensitivity. This is a marker of GPCR coupling with heterotrimeric G protein (Gi/o of α₂-AR), which mediates downstream signal transduction (inhibition of adenylate cyclase, activation of K⁺ channels) [1]
3. Reference [4]: Brimonidine exerts a neuroprotective effect in the retina through α₂-AR-mediated upregulation of BDNF and activation of the PI3K/Akt survival pathway, making it a candidate drug for the treatment of glaucoma and other retinal degenerative diseases [4]. 4. According to [3], in a Parkinson's disease model, brimonidine modulates dopaminergic neurotransmission by activating presynaptic α₂-adrenergic receptors (α₂-AR) on nigrostriatal neurons, thereby reducing dopamine release and alleviating hyperkinetic symptoms (e.g., circling behavior) [3]. 5. Brimonidine (trade name: Alphagan) has been approved by the FDA for the treatment of open-angle glaucoma and ocular hypertension because it can reduce intraocular pressure by inhibiting aqueous humor production and increasing uveal-scleral outflow through α₂-AR-mediated inhibition; no FDA warnings have been reported in the literature regarding the ophthalmic indications of this drug [4].

C11H10BRN5

292.14

291.011

C, 45.23; H, 3.45; Br, 27.35; N, 23.97

59803-98-4

Brimonidine tartrate; 70359-46-5; Brimonidine-d4; 1184971-51-4; 59803-98-4

2435

Light yellow to yellow solid powder

1.8±0.1 g/cm3

432.6±55.0 °C at 760 mmHg

207.5 °C

215.4±31.5 °C

0.0±1.0 mmHg at 25°C

1.798

0.96

2

3

2

17

308

0

BrC1C2C(C([H])=C([H])C=1N([H])C1=NC([H])([H])C([H])([H])N1[H])=NC([H])=C([H])N=2

XYLJNLCSTIOKRM-UHFFFAOYSA-N

InChI=1S/C11H10BrN5/c12-9-7(17-11-15-5-6-16-11)1-2-8-10(9)14-4-3-13-8/h1-4H,5-6H2,(H2,15,16,17)

5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)quinoxalin-6-amine

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)

Solubility in Formulation 1: ≥ 2.5 mg/mL (8.56 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.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (8.56 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 saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 3: 2.5 mg/mL (8.56 mM) 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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 (Please use freshly prepared in vivo formulations for optimal results.)