α2-adrenergic receptor

Clonidine (0.01, 0.1 or 1 μM) significantly and dose-dependently increases the expression of CGRP (α and β) mRNA in endothelial cells. Endothelial cells treated with 1 μM clonidine for 24 hours exhibit a significant increase in NO production. Clonidine-induced CGRP production is modulated by the NO pathway[2].

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Neurons' ability to fire is inhibited by clonidine [5]. The dorsal raphe (DR) nucleus is a system of nuclei in the midline of the lower brainstem, which is considered one of the most important nuclei in the modulation of pain in the central nervous system. Central noradrenergic systems play an important role in the control of cardiovascular regulation and pain transmission. Clonidine, an alpha 2-adrenergic agonist is used extensively in anesthesia research. In this study, we evaluated the involvement of clonidine in the activity of DR nucleus and its possible role in pain modulation. Seventy-four neurons within the DR nucleus in the rat brainstem slice preparation were tested using extracellular recording techniques. Application of noradrenaline (NA), 50 mumol/L, induced firing activity in 68 neurons tested (92%). NA produced a regular long-lasting firing activity on the DR neurons. Fifty-six neurons (88%) previously excited by NA were inhibited by clonidine, 20 mumol/L. Clonidine suppressed the firing activity of neurons. The results indicate that the firing of DR neurons was under noradrenergic influence and was inhibited by clonidine, which in turn alters nociception by modifying the central serotonergic system [5].

Clonidine (50 μg/kg, i.p.) causes a three-hour period of significant rat body temperature reduction, peaking one hour after administration. Rats treated intracerebroventricularly with neutral doses of phentolamine 15 minutes prior to clonidine significantly counteract the hypothermia caused by clonidine[1]. PCP-induced dopamine efflux in the prefrontal cortex is potently suppressed by clonidine (0.003-0.05 mg/kg, i.p.). Clonidine cannot suppress PCP-induced dopamine overflow in the prefrontal cortex when the alpha-2A receptor antagonist BRL-44408 is administered beforehand[3]. Clonidine (0.6 μg i.c.) has no effect on blood pressure in SO rats that have been pretreated with DMSO. On the other hand, clonidine significantly (P < 0.05, one-way ANOVA) lowers blood pressure in SO rats following central adenosine A1R blockade (DPCPX). Contrarily, clonidine (0.6 μg i.c.) significantly lowers blood pressure in ABD rats that have received DMSO pretreatment; crucially, central A1R blockade (DPCPX pretreatment) has no effect on the clonidine-evoked drop in blood pressure in ABD rats (P > 0.05, one-way ANOVA). In SO rats pretreated with DPCPX, clonidine significantly (P < 0.05) raises the RVLM pERK1/2 level in comparison to either basal or clonidine treatment in SO rats pretreated with DMSO. This increase coincides with the onset of the hypotensive response. Clonidine significantly (P < 0.05) increases RVLM pERK1/2 in ABD rats pretreated with vehicle (DMSO), and this response is unaffected by DPCPX pretreatment[4].

N-methyl-D-aspartic acid/glutamate receptor antagonists induce psychotomimetic effects in humans and animals, and much research has focused on the neurochemical and network-level effects that mediate those behavioral changes. For example, a reduction in NMDA-dependent glutamatergic transmission triggers increased release of the monoamine transmitters, and some of these changes are implicated in the cognitive, behavioral and neuroanatomical effects of phencyclidine (PCP). Alpha-2 adrenoceptor agonists (e.g., clonidine) are effective at preventing many of the behavioral, neurochemical and anatomical effects of NMDA antagonists. Evidence has indicated that a key mechanism of the clonidine-induced reversal of the effects of NMDA antagonists is an attenuation of enhanced dopamine release. We have pursued these findings by investigating the effects of alpha-2 agonists on PCP-evoked dopamine efflux in the prefrontal cortex of freely moving rats. Clonidine (0.003-0.1 mg/kg, i.p.) dose-dependently attenuated the ability of PCP (2.5 mg/kg, i.p.) to increase cortical dopamine output. The effects of clonidine were prevented by the alpha-2A subtype selective antagonist BRL-44408 (1 mg/kg, i.p.). Guanfacine, which is an alpha-2 agonist with a higher affinity for the 2A, compared with 2B or 2C, subtypes, also blocked the ability of PCP to increase dopamine efflux in the prefrontal cortex. These data indicate that alpha-2A agonists are effective at counteracting the hyperdopaminergic state induced by PCP and may play a role in their neurobehavioral effects in this putative animal model for schizophrenia [4].

The present study was to determine whether clonidine could induce calcitonin gene-related peptide (CGRP) production and the underlying mechanisms. Human umbilical vein endothelial cells were treated with clonidine and the dose-effect or time-effect relationship of clonidine on CGRP production was examined. Yohimbine (a alpha(2)-adrenoceptor blocker) and L-NAME (an antagonist of nitric oxide synthase, NOS) were chosen to explore the role of alpha(2)-adrenoceptor and nitric oxide pathway in the effect of clonidine on endothelial cell-derived CGRP production. The level of CGRP mRNA or protein was detected by Real Time-PCR or radioimmunoassay. Nitric oxide content was measured by nitroreduction assay. The study showed that clonidine was able to induce CGRP mRNA (alpha- and beta-isoforms) expression in a dose-dependent manner in endothelial cells. The effect of clonidine on endothelial cell-derived CGRP synthesis and secretion was attenuated in the presence of yohimbine. L-NAME treatment could also inhibit clonidine-induced CGRP synthesis and secretion concomitantly with the decreased NO content in culture medium. These results suggest that clonidine could stimulate CGRP synthesis and secretion in endothelial cells through the activation of alpha(2)-adrenoceptor, which is related to the NO pathway [3].

On the day of the experiment, two hours before the baseline sample collection starts, the flow rate is increased to 2 μL/min. Following the collection of four baseline samples, animals are pretreated with an intraperitoneal (i.p.) injection of either 0.9% saline (the vehicle), clonidine (0.0033, 0.01, or 0.05 mg/kg), or guanfacine (0.05 or 0.5 mg/kg). Twenty minutes later, the animals receive an injection of PCP (2.5 mg/kg, i.p.). Dialysates are collected every twenty minutes. BRL (1.0 mg/kg) is given 20 minutes before clonidine in a different study. Furthermore, in certain control studies, the animals are given a single injection of saline, clonidine (0.01 or 0.05 mg/kg), guanfacine (0.5 mg/kg), or BRL (1.0 mg/kg).

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Central adenosine A(1) and A(2A) receptors mediate pressor and depressor responses, respectively. The adenosine subtype A(2A) receptor (A(2A)R)-evoked enhancement of phosphorylated extracellular signal-regulated kinase (pERK) 1/2 production in the rostral ventrolateral medulla (RVLM), a major neuroanatomical target for clonidine, contributes to clonidine-evoked hypotension, which is evident in conscious aortic barodenervated (ABD) but not in conscious sham-operated (SO) normotensive rats. We conducted pharmacological and cellular studies to test the hypothesis that the adenosine A(2A)R-mediated (pERK1/2-dependent) hypotensive action of clonidine is not expressed in SO rats because it is counterbalanced by fully functional central adenosine subtype A(1) receptor (A(1)R) signaling. We first demonstrated an inverse relationship between A(1)R expression in RVLM and clonidine-evoked hypotension in ABD and SO rats. The functional (pharmacological) relevance of the reduced expression of RVLM A(1)R in ABD rats was verified by the smaller dose-dependent pressor responses elicited by the selective A(1)R agonist N(6)-cyclopentyladenosine in ABD versus SO rats. It is important that after selective blockade of central A(1)R with 8-cyclopentyl-1,3-dipropylxanthine in conscious SO rats, clonidine lowered blood pressure and significantly increased neuronal pERK1/2 in the RVLM. In contrast, central A(1)R blockade had no influence on the hypotensive response or the increase in RVLM pERK1/2 elicited by clonidine in ABD rats. These findings support the hypothesis that central adenosine A(1)R signaling opposes the adenosine A(2A)R-mediated (pERK1/2-dependent) hypotensive response and yield insight into a cellular mechanism that explains the absence of clonidine-evoked hypotension in conscious normotensive rats.[2]

Absorption, Distribution and Excretion
Clonixin reaches its maximum concentration 60-90 minutes after oral administration. Race and fasting status do not affect the pharmacokinetics of Clonixin. After oral administration of 100 µg Clonixin tablets, the peak plasma concentration (Cmax) is 400.72 pg/mL, the area under the curve (AUC) is 5606.78 h pg/mL, and the bioavailability is 55-87%. Approximately 50% of the Clonixin dose is excreted unchanged in the urine and 20% in the feces. Depending on the source, the reported volume of distribution of Clonixin is 1.7-2.5 L/kg, 2.9 L/kg, or 2.1 ± 0.4 L/kg. The clearance of Clonixin is 1.9-4.3 mL/min/kg. Animal studies have shown that Clonixin is widely distributed throughout the body; tissue concentrations are higher than plasma concentrations. The average volume of distribution of Clonixin is reported to be 2.1 L/kg. After oral administration, the highest drug concentrations are found in the kidneys, liver, spleen, and gastrointestinal tract. High concentrations are also observed in the lacrimal and parotid glands. Clonixin has a high concentration in the choroid of the eye and is distributed in the heart, lungs, testes, adrenal glands, fat, and muscles. The lowest concentrations are found in brain tissue. Clonixin can be distributed in the cerebrospinal fluid. After epidural infusion, Clonixin rapidly and extensively distributes in the cerebrospinal fluid and readily enters the plasma via epidural veins. In vitro studies show that Clonixin binds to approximately 20-40% of plasma proteins (primarily albumin). Clonixin can cross the placental barrier and is distributed in breast milk. A lactating woman took approximately 0.04 mg of Clonixin hydrochloride orally twice daily and 25 mg of dihydrozirconium orally three times daily. One hour after administration, the Clonixin concentration in plasma was 0.33 ng/mL, and in breast milk collected 2.5 hours after administration, the concentration was 0.6 ng/mL. One hour after breastfeeding, the drug was not detected in the infant's plasma. In healthy volunteers… after intravenous infusion of 300 μg of Clonixin, plasma drug concentrations exhibited a double-exponential decline 10 minutes later, with a rapid half-life of 11 minutes and a slow half-life of 8.5 hours. In healthy volunteers, the pharmacokinetic study time for Clonixin was more than three times longer than previously reported. Approximately 62% of the administered dose was excreted unchanged in the urine, regardless of the dose, formulation, or route of administration. Because the pharmacokinetics of the drug are influenced by enterohepatic circulation, they cannot be described using traditional open-cell or two-compartment models. Plasma Clonixin concentrations and their time course of action are asynchronous. A pharmacokinetic study of Clonixin was conducted in 21 patients with essential hypertension. These patients received two intravenous bolus injections (0.78–3.36 μg/kg) and one oral dose (1.7–2.3 μg/kg). Some patients received multiple therapeutic oral doses (1.1 or 1.9 μg/kg, twice daily) during the dosing interval after 6–12 months of Clonixin monotherapy. With increasing intravenous dose, the clearance constant decreased, and plasma clearance decreased by 74% (9.94–2.61 mL/min/kg), indicating dose-dependent pharmacokinetics. Unlike plasma volume, which increases at the highest dose, the volume of distribution did not change with dose. The pharmacokinetics of a single oral dose were consistent with those of the same intravenous dose. Bioavailability was 90%. During multiple oral doses, the elimination rate constant was lower than that of a single dose. Compared to a single dose (4.17 mL/min/kg), plasma clearance increased (7.18 mL/min/kg). This latter change is likely due to a decrease in bioavailability to approximately 65%. Studies have determined that the pharmacodynamic properties of this drug can explain the changes in pharmacokinetics during dose escalation and multiple dosing.
For more complete data on absorption, distribution, and excretion of Clonixin (8 items in total), please visit the HSDB record page.

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Metabolism/Metabolites
The metabolic mechanism of Clonixin is not fully understood. The main metabolic reaction of Clonixin is the 4-hydroxylation of Clonixin catalyzed by CYP2D6, CYP1A2, CYP3A4, CYP1A1, and CYP3A5. Less than 50% of Clonixin is metabolized in the liver, primarily as an inactive metabolite.
Clonixin hydrochloride is metabolized in the liver. Four metabolites have been detected in humans, but only one, the inactive p-hydroxyClonixin, has been identified. ...In addition to mixed human liver microsomes, the in vitro 4-hydroxylation activity of 17 cDNA-expressed P450 enzymes on Clonixin was evaluated. Five of these P450 enzymes—CYP2D6, 1A2, 3A4, 1A1, and 3A5—catalyzed the production of measurable 4-hydroxyClonixin. Selective inhibition studies in human liver microsomes confirmed that these isoenzymes collectively responsible for the in vitro 4-hydroxylation of Clonixin, with CYP2D6 accounting for approximately two-thirds of the activity. The major role of CYP2D6 in Clonixin metabolism may explain the increased non-renal clearance during pregnancy. The degree of Clonixin biotransformation varies across species. The metabolism of 14C-Clonixin in dogs has been reported, and six components have been isolated and identified. Unaltered Clonixin and its hydroxylated derivatives were detected. Furthermore, dichlorophenylguanidine, previously reported as a canine metabolite, was identified. Three previously undescribed metabolites were also isolated from canine urine. The main metabolic pathways of Clonixin are hydroxylation of the benzene ring and cleavage of the imidazolidin ring. Comparative studies show that Clonixin is metabolized similarly in rats, dogs, and humans, but humans excrete the most unchanged drug, while dogs exhibit the most extensive metabolism. Hepatic metabolism is less common. The concentration of the main metabolite, p-hydroxyClonixin, in urine is less than 10% of the original Clonixin. Four metabolites have been detected, but only p-hydroxyClonixin has been identified. Half-life: 6-20 hours; 40-60% is excreted unchanged in urine, and 20% in feces. The excretion of p-hydroxyClonixin is less than 10%. The elimination half-life after epidural administration is 30 minutes, but can vary between 6-23 hours under other conditions. The elimination half-life of this drug is 6 to 24 hours, with an average of approximately 12 hours.
Pharmacokinetics of Clonixin were studied in healthy volunteers for a period more than three times longer than previously reported. The complete bioavailability and elimination half-life (20 to 25.5 hours) of Clonixin remained unchanged after single and multiple doses.
The plasma half-life in patients with normal renal function is 6–20 hours. The half-life in patients with impaired renal function has been reported to be 18–41 hours. The elimination half-life of this drug may be dose-related, increasing with increasing dose.

Toxicity Summary
Clonixin acts as an agonist of presynaptic α(2) receptors in the nucleus tractus solitarius of the medulla oblongata. Stimulation of these receptors inhibits efferent sympathetic pathways, thereby reducing blood pressure and vascular tone in the heart, kidneys, and peripheral blood vessels. Clonixin is also a partial agonist of presynaptic α(2) adrenergic receptors in peripheral nerves of vascular smooth muscle. Toxicity Data
LD50: 150 mg/kg (oral, rat) LD50: 30 mg/kg (oral, dog) Interactions Potential additive effects (e.g., hypotension, bradycardia). Caution should be exercised when using carvedilol in combination with Clonixin, especially during discontinuation; carvedilol should generally be discontinued first, followed by continued use of Clonixin for several days with a gradual dose reduction. Epidural Clonixin injection may prolong the duration of pharmacological effects of epidural local anesthetics, including sensory and motor blockade.
Because beta-adrenergic blockers may exacerbate rebound hypertension following discontinuation of Clonixin, beta-adrenergic blockers should be discontinued several days before the gradual discontinuation of Clonixin in patients taking both beta-adrenergic blockers and Clonixin. If Clonixin treatment is to be replaced by a beta-adrenergic blocker, it should be administered several days after Clonixin treatment is discontinued.
Because Clonixin can cause bradycardia and atrioventricular block, the possibility of additive effects should be considered when used concomitantly with other drugs affecting sinoatrial node function or atrioventricular node conduction (e.g., guanethidine), beta-adrenergic blockers (e.g., propranolol), calcium channel blockers, or cardiac glycosides.
For more complete data on drug interactions of Clonixin (15 in total), please visit the HSDB record page.

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Non-human toxicity values
Oral LD50 in rats: 126 mg/kg / Clonixin hydrochloride /
Intraperitoneal LD50 in rats: 100 mg/kg / Clonixin hydrochloride /
LD50 in rats (intravenous injection): 29 mg/kg / Clonixin hydrochloride /
LD50 in rats (subcutaneous injection): 77 mg/kg / Clonixin hydrochloride /
For more complete non-human toxicity data for Clonixin (9 types in total), please visit the HSDB record page.

[1]. The involvement of central alpha-adrenergic and histamine H2-receptors in the hypothermia induced by clonidine in the rat. Neuropharmacology. 1980 Jan;19(1):9-15.

[2]. Brainstem adenosine A1 receptor signaling masks phosphorylated extracellular signal-regulated kinase 1/2-dependent hypotensive action of clonidine in conscious normotensive rats. J Pharmacol Exp Ther. 2009 Jan;328(1):83-9.

[3]. Clonidine induces calcitonin gene-related peptide expression via nitric oxide pathway in endothelial cells. Peptides. 2009 Sep;30(9):1746-52.

[4]. Clonidine and guanfacine attenuate phencyclidine-induced dopamine overflow in rat prefrontal cortex: mediating influence of the alpha-2A adrenoceptor subtype. Brain Res. 2008 Dec 30;1246:41-6.

[5]. The effect of clonidine on the activity of neurons in the rat dorsal raphe nucleus in vitro. Anesth Analg. 1994 Aug;79(2):257-60.

Therapeutic Uses

Adrenergic alpha receptor agonist; antihypertensive; sympathomimetic; analgesic
Clonixin hydrochloride and transdermal Clonixin can be used alone or in combination with other classes of antihypertensive drugs to treat hypertension. /Included on US product label/
Epileptic infusion of Clonixin hydrochloride can be used in combination with opioids as adjunctive therapy for severe cancer pain that is not relieved by opioid analgesia alone. /Clonixin hydrochloride; Included on US product label/
Oral Clonixin hydrochloride loading dose regimens can effectively and rapidly lower blood pressure in patients with severe hypertension for whom an emergency is required but no emergency treatment is needed. Hypertensive emergencies are situations where blood pressure needs to be lowered within hours. These situations include upper hypertension, hypertension with papilledema, progressive target organ complications, and severe perioperative hypertension. /Clonixin hydrochloride; Not included on US product label/
For more complete data on the therapeutic uses of Clonixin (15 types), please visit the HSDB record page.
Drug Warning
Sudden discontinuation of Clonixin treatment may cause a rapid increase in systolic and diastolic blood pressure, accompanied by symptoms such as nervousness, agitation, confusion, restlessness, anxiety, insomnia, headache, sweating, palpitations, rapid heart rate, tremor, hiccups, stomach pain, nausea, muscle pain, and increased salivation. The exact mechanism of withdrawal syndrome after discontinuing alpha-adrenergic agonists is not clear, but it may involve increased circulating catecholamine concentrations, increased adrenergic receptor sensitivity, enhanced renin-angiotensin system activity, decreased vagal nerve function, impaired cerebral blood flow autoregulation, and/or dysfunction of alpha-2-adrenergic receptor mechanisms regulating sympathetic output and baroreflex function in the central nervous system.

Due to the risk of rebound hypertension, patients taking Clonixin should be informed of the risk of missing a dose or discontinuing the medication without consulting a doctor. When discontinuing Clonixin treatment, a rapid increase in blood pressure can be minimized or prevented by gradually tapering the dose over 2–4 days.
Some clinicians recommend that when discontinuing transdermal Clonixin, especially in elderly patients, the dosage should be gradually reduced, or the oral Clonixin dosage should be gradually reduced. If the patient is taking Clonixin and a beta-blocker and needs to discontinue Clonixin, the beta-blocker should be discontinued several days before Clonixin is discontinued. It is recommended that Clonixin treatment not be interrupted during surgery; transdermal treatment can continue perioperatively, and oral treatment should continue until 4 hours before surgery. Blood pressure should be closely monitored during surgery, and necessary blood pressure control measures should be prepared. If Clonixin treatment must be interrupted due to surgery, parenteral antihypertensive therapy should be given as needed, and Clonixin treatment should be resumed as soon as possible. If transdermal administration is started perioperatively, it must be noted that therapeutic plasma Clonixin concentrations may not be reached for 2-3 days after the first use of the transdermal delivery system. Implantable epidural catheters carry a risk of infection, including meningitis and/or epidural abscess. The incidence of catheter-related infections is approximately 5-20%, depending on various factors, including the patient's clinical condition, type of catheter used, catheter insertion technique, quality of catheter care, and duration of catheter indwelling. Catheter-related infection should be considered in patients receiving epidural Clonixin who develop fever. Post-marketing surveillance data show that the incidence of fever, malaise, pallor, muscle or joint pain, and leg cramps is as high as 0.5% in patients treated with transdermal Clonixin. For more complete data on Clonixin (22 total warnings), please visit the HSDB record page. Pharmacodynamics: Clonixin acts by activating α-2 adrenergic receptors, with effects including lowering blood pressure, sedation, and hyperpolarization. Due to twice-daily dosing, it has a long duration of action, with a therapeutic window of 0.1 mg to 2.4 mg daily.

C9H9CL2N3

230.094

229.017

C, 46.98; H, 3.94; Cl, 30.82; N, 18.26

4205-90-7

Clonidine hydrochloride;4205-91-8;Clonidine-d4 hydrochloride;67151-02-4

2803

White to off-white solid powder

1.5±0.1 g/cm3

319.3±52.0 °C at 760 mmHg

141-142℃

146.9±30.7 °C

0.0±0.7 mmHg at 25°C

1.671

1.41

2

1

2

14

222

0

GJSURZIOUXUGAL-UHFFFAOYSA-N

InChI=1S/C9H9Cl2N3/c10-6-2-1-3-7(11)8(6)14-9-12-4-5-13-9/h1-3H,4-5H2,(H2,12,13,14)

N-(2,6-dichlorophenyl)-4,5-dihydro-1H-imidazol-2-amine

clonidine; 4205-90-7; Clonidin; Chlornidinum; N-(2,6-Dichlorophenyl)-4,5-dihydro-1H-imidazol-2-amine; Catapres-TTS; Adesipress; Catapres;

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

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

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

DMSO : ~100 mg/mL (~434.61 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (10.87 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 (10.87 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (10.87 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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 (Please use freshly prepared in vivo formulations for optimal results.)