Xanthine oxidase (XO) - IC50 = 0.2 to 50 µM (reported as a relatively weak XO inhibitor in vitro assays). [1]

In HFF and HUVEC cells, allopurinol (0, 10, 100, and 1000 µg/ml; 17 hours) can decrease the expression of HIF-1α and HIF-2α proteins [5]. In a 24-hour period, allopurinol (0, 10, 100, or 1000 µg/ml) diminishes the angiogenic characteristics of HUVEC cells [5].

---

Allopurinol and its metabolite oxypurinol exert direct free radical scavenging effects, copper chelation, superoxide scavenging, inhibition of lipid peroxidation, Ca²⁺-sensitizing effects, and affect heat shock factor expression and antioxidant status in cells. [1]

In mice, allopurinol (39 mg/kg; oral; once daily for 21 days) demonstrates antidepressant effects [3]. In mice, allopurinol (10–400 mg/kg; intraperitoneally) elicits antinociceptive action [4].
In rat models of myocardial ischemia/reperfusion (I/R), allopurinol decreased reperfusion-induced arrhythmias and reduced infarct size (Manning et al., 1984); improved postischemic function and higher ATP levels (Lasley et al., 1988); reduced myocardial damage in isoproterenol-induced infarction (Wexler and McMurtry, 1981). In rabbit I/R, allopurinol decreased reperfusion arrhythmias and protected against ultrastructural damage (Godin et al., 1986). In dog models, allopurinol reduced infarct size (Chambers et al., 1985; Werns et al., 1986) and improved left ventricular function (Stewart et al., 1985), though some studies found no reduction in infarct size (Reimer and Jennings, 1985; Kinsman et al., 1988). In human coronary bypass surgery, allopurinol decreased hospital mortality and increased cardiac index (Johnson et al., 1991); reduced arrhythmias, need for inotropes and perioperative myocardial infarction (Rashid and William-Olsson, 1991); improved postoperative recovery and reduced lipid peroxidation (Coghlan et al., 1994); but other studies failed to demonstrate cardioprotection (Taggart et al., 1994; Yamazaki et al., 1995). [1]
In cerebral ischemia, allopurinol pretreatment reduced infarct size in rat permanent middle cerebral artery occlusion (Martz et al., 1989); reduced cerebral edema and perinatal hypoxic-ischemic brain damage in newborn rats and lambs (Palmer et al., 1990,1993; Shadid et al., 1998). In severely asphyxiated infants, allopurinol tended to improve survival and beneficial effects on free radical formation and cerebral blood flow (Van Bel et al., 1998). [1]
In splanchnic ischemia-reperfusion, allopurinol reduced neutrophil infiltration, bacterial translocation, mucosal injury, and mortality in rats (Grisham et al., 1986; Deitsch et al., 1988; Vaughan et al., 1992; Megison et al., 1990). In cats, allopurinol attenuated necrosis and vascular permeability (Parks et al., 1982; Granger et al., 1986). [1]
In liver and kidney I/R, allopurinol improved morphology and function, decreased hepatic enzyme release, lipid peroxidation, and nuclear factor-κB activation (Linas et al., 1990; Hestin and Johns, 1999; Rhoden et al., 2000b; Matsui et al., 2000). Allopurinol is a constituent of University of Wisconsin organ storage solution. [1]
In chronic heart failure (CHF), allopurinol doubled survival and improved contractility in mouse post-ischemic cardiomyopathy (Stull et al., 2004); improved LV hemodynamics and prevented remodeling in rats (Mellin et al., 2005); decreased myocardial oxygen consumption and increased mechanical efficiency in dogs with pacing-induced CHF (Ekelund et al., 1999; Saavedra et al., 2002). In human CHF, allopurinol improved endothelial function (Doehner et al., 2002; Farquharson et al., 2002); high-dose treatment beneficially affected survival (Struthers et al., 2002), but a large multicenter trial with oxypurinol failed to show significant benefits (Cardiome Pharma). [1]
In vascular disease, allopurinol improved endothelium-dependent vasodilation in hypercholesterolemic rabbits (Ohara et al., 1993; White et al., 1996); reduced blood pressure in spontaneously hypertensive rats (Nakazono et al., 1991); prevented hypertension-induced left ventricular and renal hypertrophy in SHRs (Laakso et al., 1998,2004); improved endothelial function in type 2 diabetics with mild hypertension (Butler et al., 2000). [1]
In other conditions, allopurinol reduced oxidative stress in type 1 diabetic patients (Desco et al., 2002); protected against radiation-, ethanol-, cocaine-, and acetaminophen-induced liver injury (though some effects may be antioxidant rather than XO inhibition). [1]

Western Blot Analysis[5]
Cell Types: HFF, HUVEC Cell
Tested Concentrations: 0, 10, 100, 1000 µg/ml
Incubation Duration: 17 hrs (hours)
Experimental Results: HIF-1α and HIF-2α protein expression diminished in a dose-dependent manner.

---

Animal/Disease Models: 20-30 g, male Swiss albino mouse [3]
Doses: 39 mg/kg
Route of Administration: oral; one time/day for 21 days
Experimental Results: diminished immobility time in FST, immobility time was 129.8± 10.5 seconds.

---

Animal/Disease Models: 30-40 g, male adult Swiss albino mouse [4]
Doses: 10, 50, 100, 200, 400 mg/kg
Route of Administration: intraperitoneal (ip) injection
Experimental Results: Dose dependence in tail flick and thermal stimulation Sexual antinociceptive effects plate.

---

Mouse intraperitoneal (i.p.) injection: Allopurinol was dissolved in 10% Tween solution. Mice received an i.p. injection of vehicle (10% Tween) or allopurinol at doses of 10, 50, 100, 200, or 400 mg/kg in a volume of 10 mL/kg. Treatments were given 20-30 min before behavioral testing [4].
Antagonist pre-treatment protocol: Some animals were pre-treated (15 min in advance) with an i.p. injection of caffeine (30 mg/kg), DPCPX (0.1 mg/kg), SCH58261 (0.5 mg/kg), or naloxone (1 mg/kg) before allopurinol administration [4].
Capsaicin-induced nociception test: 30 min after i.p. treatments, 20 μL of capsaicin (1.6 μg per paw) was injected intraplantarly under the plantar skin of the right hind paw. Mice were observed for 5 min and time spent licking the injected paw was recorded [4].
Glutamate-induced nociception test: 30 min after i.p. treatments, 20 μL of glutamate solution (10 μmol per paw) was injected intraplantarly. Mice were observed for 15 min and licking time recorded [4].
Tail-flick test: Baseline tail-flick latency was measured before treatment. 30 min after i.p. treatments, mice were tested with a tail-flick apparatus (light source focused on tail 2.3 cm rostral to tip; cut-off 10 s). Data expressed as % maximum possible effect (MPE) [4].
Hot-plate test: Hot-plate apparatus maintained at 55 ± 0.5°C. Baseline latency measured before treatment. 30 min after i.p. treatments, response latency (hind paw licking or jumping) recorded with 20 s cut-off. Data expressed as % MPE [4].
Hole-board test: 30 min after i.p. treatments, each mouse was placed in the center of a hole-board apparatus (40×40 cm, 16 holes). Number of head-dips, crossings, rearings, groomings, defecations, and latency to start locomotion were recorded for 5 min [4].
Rotarod test: Mice were trained to remain on a rotating bar (18 rpm) for 120 s. 30 min after i.p. treatments, latency to fall was recorded (maximum 60 s) [4].
Spontaneous locomotor activity test: Activity cages equipped with three parallel photocells automatically recorded number of crossings for 15 min, 30 min after i.p. treatments [4].
Pentobarbital sleeping time test: Mice pre-treated with allopurinol (50, 100, or 200 mg/kg i.p.) or vehicle (30 min in advance) received sodium pentobarbital (30 mg/kg i.p.). Sleeping time (time between loss and recovery of righting reflex) was recorded [4].
Rectal temperature measurement: Measured using a flexible probe before and 30 min after allopurinol (50, 100, or 200 mg/kg) or vehicle treatment [4].
Cerebrospinal fluid (CSF) sampling: Mice were treated with allopurinol (200 mg/kg i.p.) or vehicle. After 30 min, mice were anaesthetized with sodium thiopental (60 mg/kg i.p.) and placed in a stereotaxic apparatus. CSF (10-20 μL per mouse) was drawn by direct puncture of the cisterna magna with an insulin syringe. Samples were centrifuged at 10,000×g for 5 min to obtain cell-free supernatants and stored at -70°C until HPLC analysis [4].
HPLC procedure: Purines (ATP, ADP, AMP, adenosine, GTP, GDP, GMP, guanosine, IMP, inosine, hypoxanthine, xanthine, uric acid) were measured on a C18 column with mobile phase flow rate 1.2 mL/min, column temperature 24°C, UV detection at 254 nm [4].

Absorption, Distribution and Excretion
The drug is absorbed approximately 90% of the gastrointestinal tract. Peak plasma concentrations of allopurinol and oxypurinol typically occur 1.5 hours and 4.5 hours after administration, respectively. After oral administration of 300 mg of allopurinol, peak plasma concentrations of allopurinol and oxypurinol were measured to be approximately 3 μg/mL and 6.5 μg/mL, respectively. Approximately 80% of orally ingested allopurinol is excreted in the urine as various metabolites. Approximately 20% of ingested allopurinol is excreted in the feces. Both allopurinol and oxypurinol are substrates of xanthine oxidase, an enzyme found in the cytoplasm of capillary (including sinusoidal) endothelial cells, with the highest activity in the liver and intestinal lining. Concentrations of allopurinol in human tissues have not yet been reported, but it is presumed that allopurinol and its metabolite oxypurinol are most concentrated in these tissues. In animal studies, allopurinol concentrations were highest in blood, liver, intestines, and heart, and lowest in brain and lung tissues. Because allopurinol and its metabolites are primarily excreted through the kidneys, drug accumulation may occur in patients with renal insufficiency or renal failure; therefore, the dosage of allopurinol should be reduced. A daily dose of 200 mg allopurinol is appropriate when creatinine clearance is 10 to 20 mL/min. When creatinine clearance is below 10 mL/min, the daily dose should not exceed 100 mg. In cases of severe renal impairment (creatinine clearance below 3 mL/min), extended dosing intervals may be necessary. After oral administration, approximately 80-90% of the allopurinol dose is absorbed through the gastrointestinal tract. Peak plasma concentrations are usually reached 2-6 hours after administration. Allopurinol is poorly absorbed when administered rectally in suppository form (cocoa butter or polyethylene glycol base). After rectal administration, plasma concentrations of allopurinol or oxypurinol are extremely low or undetectable. In one study, after a single oral administration of 100 or 300 mg allopurinol to healthy adult men, peak plasma allopurinol concentrations reached approximately 0.5 or 1.4 μg/mL within 1–2 hours, respectively, while peak concentrations of oxopurinol (the active metabolite of allopurinol) reached approximately 2.4 and 6.4 μg/mL within 3–4 hours, respectively. In the same study, after a single intravenous infusion of 100 mg or 300 mg allopurinol (in sodium form), peak plasma concentrations were approximately 1.6 μg/mL and 5.1 μg/mL, respectively, reached within approximately 30 minutes; while peak oxopurinol concentrations were approximately 2.2 μg/mL and 6.2 μg/mL, respectively, reached within approximately 4 hours. In six healthy male and female subjects, after intravenous administration of allopurinol, allopurinol was rapidly cleared from systemic circulation primarily through oxidative metabolism to oxopurinol, and no plasma allopurinol concentration was detected 5 hours post-administration. Approximately 12% of intravenously administered allopurinol is excreted unchanged, 76% is excreted as oxypurine, and the remainder is excreted in the urine as nucleoside conjugates. The rate of rapid conversion of allopurinol to oxypurine is not significantly different after repeated administration. …Oxypurine is primarily excreted unchanged in the urine via glomerular filtration and tubular reabsorption, with a net renal clearance of approximately 30 mL/min. For more complete data on the absorption, distribution, and excretion of allopurinol (13 in total), please visit the HSDB record page. Metabolism/Metabolites Allopurinol is rapidly metabolized to its corresponding xanthine analogue, oxypurine (allopurin), which is also an inhibitor of xanthine oxidase. Both allopurinol and oxypurine inhibit the activity of this enzyme. Allopurinol and oxypurine can also be converted to their respective ribonucleotides via the purine salvage pathway. To date, the mechanisms by which these ribonucleotides relate to the uric acid-lowering effect of allopurinol in humans have not been fully elucidated. These metabolites may inhibit the de novo synthesis of purines by inhibiting aminophosphoribotransferase. No incorporation of these ribonucleotides into DNA has been found. Allopurinol and sodium allopurinol are rapidly metabolized by xanthine oxidase to the pharmacologically active oxopurinol. Multiple administrations do not appear to significantly affect the rapid metabolism of allopurinol to oxopurinol. The pharmacokinetic parameters (e.g., AUC, plasma elimination half-life) of oxopurinol appear similar after oral administration of allopurinol and intravenous administration of sodium allopurinol. Both allopurinol and oxopurinol can bind to alkaloids to form their respective ribonucleosides. Allopurinol-1-nucleoside is the major metabolite of allopurinol and is generally believed to be directly synthesized in vivo by purine nucleoside phosphorylase (PNP). Since this enzyme is primarily responsible for nucleoside degradation in vivo, we used high-performance liquid chromatography and conventional chromatography to determine the metabolites of allopurinol in the urine of children with PNP deficiency. In this patient, approximately 40% of the allopurinol metabolites in the urine were allopurinol-1-nucleoside, confirming that allopurinol-1-nucleoside may be indirectly generated in vivo via allopurinol-1-ribopeptide, a process catalyzed by hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and phosphatases. The major active metabolite, oxopurinol, is detectable in the bloodstream within 15 minutes of allopurinol administration. Oxopurinol concentrations are higher than the parent drug, and long-term use can lead to its accumulation. Oxopurinol is excreted by the kidneys, and its elimination half-life is much longer than that of allopurinol. Oxopurinol accumulates in patients with renal insufficiency; therefore, the allopurinol dosage should be adjusted in such patients. For more complete metabolite/metabolite data on allopurinol (7 metabolites), please visit the HSDB record page.

---

Biological Half-Life
Due to rapid renal clearance, allopurinol has a plasma half-life of 1-2 hours.
In patients with normal renal function, the half-lives of allopurinol and oxypurine are approximately 1-3 hours and 18-30 hours, respectively; however, the half-lives are prolonged in patients with impaired renal function. Allopurinol is primarily cleared from the plasma by conversion to allopurinine, with a half-life of 2-3 hours. The serum half-life of allopurinol is 39 minutes.

---

Allopurinol is rapidly absorbed after oral administration, reaching peak plasma concentrations within 30 to 60 minutes. Oxypurinol has lower oral bioavailability than allopurinol. The plasma half-life of allopurinol is 2-3 hours, whereas the half-life of oxypurinol is much longer (14-30 hours) due to renal reabsorption. [1]

Interactions
Allopurinol inhibits the enzymatic inactivation of 6-mercaptopurine by xanthine oxidase. Therefore, when allopurinol is used in combination with mercaptopurine or azathioprine, the dose of the antitumor drug must be reduced to one-quarter to one-third of the usual dose. Many drugs can increase serum uric acid levels, including most diuretics, pyrazinamide, diazoxide, alcohol, and mecaramine. If these drugs are taken during allopurinol treatment, the allopurinol dose may need to be increased. The combination of allopurinol and cyclophosphamide may increase the incidence of myelosuppression compared to cyclophosphamide alone, but the mechanism of this interaction is unclear. However, a rigorously controlled study in lymphoma patients showed that the combination of allopurinol with cyclophosphamide, doxorubicin, bleomycin, procarbazine, and/or nitrogen mustard did not increase the incidence of myelosuppression in these patients. Patients receiving allopurinol treatment have an unusually high incidence of skin rash after taking ampicillin. For more complete data on drug interactions of allopurinol (15 items in total), please visit the HSDB record page.

---

The most common adverse effects are gastrointestinal distress, hypersensitivity reactions, and skin rash (occurring in 2-8% of patients). The "allopurinol hypersensitivity syndrome" can be fatal, especially in elderly patients with renal insufficiency taking thiazide diuretics. Symptoms include fever, rash, vasculitis, eosinophilia, worsening of renal function, and sometimes aseptic meningitis, nephritis, and hepatic dysfunction. Allopurinol may increase the effects of cyclophosphamide and inhibit the metabolism of oral anticoagulants and probenecid. [1]

[1]. Therapeutic effects of xanthine oxidase inhibitors: renaissance half a century after the discovery of allopurinol. Pharmacol Rev. 2006 Mar;58(1):87-114.

[2]. Antileishmanial effect of allopurinol. Antimicrob Agents Chemother. 1974;5(5):469-472.

[3]. Evaluation of effect of allopurinol and febuxostat in behavioral model of depression in mice. Indian J Pharmacol. 2013 May-Jun;45(3):244-7.

[4]. Anti-nociceptive properties of the xanthine oxidase inhibitor allopurinol in mice: role of A1 adenosine receptors. Br J Pharmacol. 2009 Jan;156(1):163-72.

[5]. Dose-dependent effects of allopurinol on human foreskin fibroblast cells and human umbilical vein endothelial cells under hypoxia. PLoS One. 2015 Apr 1;10(4):e0123649.

Therapeutic Uses
Antimetabolites; antitumor drugs; enzyme inhibitors; gout inhibitors. Allopurinol is indicated for the treatment of patients with primary or secondary gout signs and symptoms (acute attacks, tophi, joint destruction, uric acid stones, and/or kidney disease). /Included on US product label/ Allopurinol is indicated for the treatment of patients with leukemia, lymphoma, and malignant tumors who are receiving cancer treatment and have elevated serum and urinary uric acid levels. Allopurinol treatment should be discontinued when there is no longer any possibility of excessive uric acid production. /Included on US product label/ Allopurinol is indicated for the treatment of patients with recurrent calcium oxalate stones whose daily uric acid excretion exceeds 800 mg/day for men and 750 mg/day for women. Treatment of these patients should be carefully evaluated initially and periodically reassessed to determine whether the treatment is beneficial and whether the benefits outweigh the risks. /Included on US product label/ For more complete data on the therapeutic uses of allopurinol (9 types), please visit the HSDB record page.

---

Drug Warning
Because allopurinol and oxypurine are excreted into breast milk, breastfeeding women should use allopurinol with caution.
Early clinical studies and experience have indicated that some adverse reactions caused by allopurinol (e.g., acute gout attacks, rash) occurred in more than 1% of cases, but current experience suggests that the incidence of adverse reactions to this drug is less than 1%. The reduced incidence of adverse reactions observed in recent experience may be partly due to a slower initiation of treatment and adherence to current prescribing precautions and recommendations.
The most common adverse reaction to oral allopurinol is pruritic maculopapular rash. Exfoliative dermatitis, urticarial dermatitis, erythematous dermatitis, eczematous dermatitis, hemorrhagic dermatitis, and purpuric dermatitis have also occurred. Hair loss, fever, and malaise may also occur alone or concurrently with dermatitis. In addition, there have been reports of severe nasal boils, cellulitis, and ichthyosis. The incidence of rash may be increased in patients with renal insufficiency. Skin reactions may be delayed, reportedly occurring up to 2 years after starting allopurinol treatment. In rare cases, severe hypersensitivity reactions, sometimes life-threatening, may occur following the rash. Cataracts have also developed in some patients with severe dermatitis (including one case of toxic cataract), but the exact relationship between allopurinol and cataracts has not been established. Itching, onycholysis, and lichen planus have also been reported in rare cases among patients receiving allopurinol treatment. Facial edema, sweating, and skin edema have also occurred occasionally, but a causal relationship with the drug has not been established. Local injection site reactions have been reported in patients receiving intravenous allopurinol sodium. For more complete data on allopurinol (28 total), please visit the HSDB records page.

---

Pharmacodynamics
Allopurinol reduces uric acid production by inhibiting the biochemical reactions that precede uric acid formation. This process lowers urate levels, thereby relieving gout symptoms, including tophi pain, joint pain, inflammation, redness, limited range of motion, and swelling.

---

Allopurinol was originally synthesized as an antineoplastic agent but found to inhibit xanthine oxidase, reducing serum and urinary uric acid. It is the cornerstone of gout and hyperuricemia therapy, and also used to prevent tumor lysis syndrome. Its active metabolite oxypurinol irreversibly inhibits XO. The compound also has non-XO-related actions including free radical scavenging, copper chelation, and Ca²⁺ sensitization. It is a constituent of University of Wisconsin organ preservation solution. Novel XO inhibitors (febuxostat, Y-700) are in development with higher potency. [1]

C5H4N4O

136.11146

136.038

315-30-0

Allopurinol sodium;17795-21-0;Allopurinol-d2;916979-34-5

135401907

White to off-white solid powder

1.7±0.1 g/cm3

290.8ºC at 760 mmHg

350 ºC

129.7ºC

1.816

-1.46

2

3

0

10

190

0

OFCNXPDARWKPPY-UHFFFAOYSA-N

InChI=1S/C5H4N4O/c10-5-3-1-8-9-4(3)6-2-7-5/h1-2H,(H2,6,7,8,9,10)

1,5-dihydropyrazolo[3,4-d]pyrimidin-4-one

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

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

DMSO : ~14 mg/mL (~102.86 mM)
H2O : ~1 mg/mL (~7.35 mM)

Solubility in Formulation 1: ≥ 3.33 mg/mL (24.47 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 33.3 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.

---

Solubility in Formulation 2: ≥ 3.33 mg/mL (24.47 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 33.3 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

---

View More

Solubility in Formulation 3: ≥ 0.61 mg/mL (4.48 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 6.1 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.

---

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