COX-3

Phenacetin was more potent at inhibiting COX-3 than was acetaminophen. Under substrate conditions of 30 μM, phenacetin inhibited COX-3 at an IC50 value of 102 μM, as opposed to 460 μM for acetaminophen tested under similar conditions. As with acetaminophen, phenacetin preferentially inhibited COX-3[1].

The effects of long-term smoking on pharmacokinetic profiles of phenacetin in rats are presented in Table 2. Mean plasma concentration-time curves of phenacetin in smoking group and control group are presented in Figure 1A. The results showed that after pretreated with long-term smoking, the AUC(0-∞), t1/2, and Cmax of phenacetin in smoking group were decreased significantly by 32%, 64%, and 27% (P<0.05) compared to those of control group, CL of phenacetin in smoking group was increased significantly by 35% (P<0.05), which indicated that CYP1A2 activity was induced by long-term smoking in rats.[2]

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The animal experiment results showed that baicalin (450 mg/kg, i.v.) significantly decreased the Cmax and CL of phenacetin, and increased C(60 min), t1/2, Vd and AUC (P<0.05). There were significant correlations between percentage of control in C(60 min), t1/2, CL, AUC of phenacetin and Cmax of baicalin in 11 rats (P<0.05). Protein binding experiments in vitro showed that baicalin (0-2000 mg/L) increased the unbound phenacetin from 14.5% to 28.3%. In conclusion, baicalin can inhibit the activity of CYP1A2 in HLMs and exhibit large inter-individual variation that has no relationship with gene polymorphism. Baicalin can change the pharmacokinetics of phenacetin in rats[3].

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Effects of baicalin treatment on phenacetin pharmacokinetics Pharmacokinetics of phenacetin: The plasma concentration versus time profile of phenacetin obtained in the pharmacokinetic studies was given in Figure 3A. This clearly illustrated that the concentration of phenacetin was too low to be detectable at 90 min after administration in control, while it was still (0.12±0.02) mg·L−1 in rats treated with baicalin. As shown in Table 4, baicalin (450 mg/kg, i.v.) was found to significantly decrease the Cmax and CL of phenacetin, and increase C60 min, t1/2, Vd and AUC (P<0.05). The AUC and C60 min of phenacetin in control were (140.2±14.7) mg/L·min and (0.27±0.14) mg/L compared with (162.8±21.1) mg/L·min and (0.55±0.17) mg/L in rats treated with baicalin (450 mg/kg), respectively. Co-administration of baicalin increased the mean AUC of phenacetin by 16%, and the mean C60 min of phenacetin by 104%[3].

Drug Inhibition Assays.[1]
Sf9 cells were infected with high titer viral stocks (moi = 3) and cultured for 48 h. Cells were preincubated with drug for 30 min at 25°C, arachidonic acid (100 μl, final concentration 5 or 30 μM) was then added for an additional 10-min incubation at 37°C. Supernatant was assayed for COX activity by RIA for PGE2. Assays were performed multiple times in triplicate. Inhibition curves were constructed and IC50 values were determined using PRISM 3.0

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Measurement of Rat Plasma Protein Binding of Phenacetin in vitro[3]
The effect of baicalin on protein binding of phenacetin in fresh rat plasma (n = 5) was measured in vitro. The final phenacetin concentration was 7 mg/L and baicalin concentrations varied from 0 to 2000 mg·L−1 in plasma samples. The samples were incubated for 30 min at 37°C and were placed into an ultrafiltration tubes. The samples were centrifuged at 4500 rpm for 15 min. Concentration of phenacetin in the filtrate was determined by the method described above.

Measurement of drug concentration in plasma[2]
Chromatography analysis was performed using an Agilent 1200 HPLC system equipped with a quaternary pump, a degasser, an autosampler, a thermostatted column compartment, and an API 4,000 triple quadrupole instrument (AB/MDSSciex, Ontario, Canada). The separation was achieved on a 150 mm × 2.1 mm, 3.5 µm particle, Agilent Zorbax SB-C18 column at 30 °C. The mobile phase consisted of a mixture of 0.1% formic acid in water and acetonitrile (45:55, v: v) (Merck KGaA, Germany) at a flow rate of 0.4 mL/min. A typical injection volume was 10 µL. The quantification was performed by the peak-area method. The determination of target ions were performed in SIM mode (m/z 180 for phenacetin, m/z 271 for tolbutamide, m/z 167 for chlorzoxazone, m/z 327 for midazolam and m/z 237 for IS) and positive ion electrospray ionization interface. Drying gas flow was set to 6 L/min and temperature to 350 °C. Nebulizer pressure and capillary voltage of the system were adjusted to 20 psi and 3,500 V, respectively. The limits of quantification for phenacetin, tolbutamide, chlorzoxazone, and midazolam were 10, 20, 15 and 8 ng/mL.[2]

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Kinetic parameter of CYP1A2 and IC50 of baicalin to CYP1A2 in HLM from individuals were determined. Moreover the Ki value in pooled HLMs was estimate. The CYP1A2 activity was assessed by formation of acetaminophen from phenacetin, a probe substrate. The incubation mixture contained HLMs (0.3 mg/ml), 100 mM phosphate buffer (pH7.4), phenacetin and baicalin at different concentrations with NADPH (1 mM). The mixture was pre-incubated for 5 min at 37°C and the optimal incubation time was 30 min.[3]
For the biotransformations, eight substrate concentrations were examined over the following ranges: 6.25 to 800 µM for phenacetin. Km and Vmax values of each HLM were determined by nonlinear regression analysis. To estimate the Ki value, different concentrations of phenacetin (12.5, 25, 50,100, 200 µM) and baicalin (0, 10, 20, 40, 80 µM) were used in pooled HLMs (n = 9). The 9 individual HLMs were selected according to CYP1A2 genotype and the value of Km from the 28 individual HLMs. The mechanism of inhibition was estimated graphically from Lineweaver–Burk plots. Ki value was calculated via second plot of the slopes from Lineweaver–Burk plots versus inhibitor concentrations. Moreover, the substrate concentration was chosen close to Km and IC50 of baicalin to CYP1A2 in each HLM was determined.[3]
Termination of the enzyme reaction was by addition of ice-cold acetonitrile. The method of determining acetaminophen, the metabolite of phenacetin, was as follows. The incubation tubes were vortexed and centrifuged then 80 µl clear supernatant was injected to the HPLC system. The mobile phase consisted of methanol and 0.05 M ammonium acetate (20∶80, v/v) at a flow rate of 1 ml·min−1. The UV detection wavelength was 257 nm.[3]

Effects of Baicalin on Phenacetin Pharmacokinetics in Rats in vivo[3]
Sprague–Dawley rats were chosen to conduct this experiment and drug dosing was done via the tail vein. The study was based on a randomized, two-period crossover design at intervals of 4 days. Eleven rats were randomly divided into two groups. Group 1 included 6 rats and group 2 included 5 rats. During the phase I, the rats in group 1 were treated with normal saline (control) and the rats in group 2 were treated with baicalin (450 mg/kg, i.v.). After that an i.v. dose (5 mg/kg) of phenacetin was given immediately. Blood samples were collected before and at 0, 5, 15, 30, 60, 90 and 120 min after administration by orbital bleeding via heparinized capillary tubes. The sample at 0 min was collected immediately after i.v. injection of phenacetin. Plasma was separated from the blood by centrifugation at 4000 rpm for 10 min and was stored at −30°C until analyzed. After a washout period of 4 days, the two groups crossed over to receive the alternative drug.

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Determination of Plasma Phenacetin and Baicalin Concentration[3]
Plasma concentration of phenacetin was determined by HPLC-UV. 1 ml acetic ether was added to 0.1 ml of plasma from each sample and vortexed for 2 min. The samples were centrifuged and the organic phase was evaporated to dryness under a stream of nitrogen. The residue was reconstituted in 100 µl of mobile phase and 50 µl was injected to the HPLC system. The mobile phase consisted of methanol and water (51∶49, v/v) at a flow rate of 1 ml·min−1. The UV detection wavelength was 247 nm. The method of determining plasma baicalin concentration had been reported previously [3].

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After complete the modeling, a cocktail solution at a dose of 5 mL/kg, which contained phenacetin (20 mg/kg), tolbutamide (5 mg/kg), chlorzoxazone (20 mg/kg) and midazolam (10 mg/kg) in CMC-Na solution, was administered orally to all rats in each group. Blood samples of each rat were collected as the following times: pre-dose (0 h) and then at 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, 8, 12, 24, 48 h after probe drugs administration through the tail vein and immediately separated by centrifugation at 13,000 rpm for 10 min to obtain plasma. The total volume of blood taken from each animal did not exceed 2.2 mL. A total of 100 µL plasma samples were transferred to a new tube and stored frozen at −80 °C until analyzed. Rats of smoking group and control group (n=4) were killed. Each liver sample was quickly removed and store at −80 °C[2].

Absorption, Distribution and Excretion
... Oral absorption of phenacetin is markedly influenced by particle size in the preparation, & plasma concentration of phenacetin & acetaminophen are correspondingly variable.
Peak concentration of phenacetin in plasma usually occurs in about 1 hr, & that of acetaminophen derived there from in 1-2 hr.
Absorption following oral administration is rapid ... duration of effect is about 4 hr.
Up to 45% of (14)C was recovered in 16 hr urine & 1% in feces of rats given [acetyl-(14)C]phenacetin per oral.
For more Absorption, Distribution and Excretion (Complete) data for PHENACETIN (9 total), please visit the HSDB record page.

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Metabolism / Metabolites
Metabolised in the body to paracetamol.
Acetaminophen & phenacetin are metabolized primarily by hepatic microsomal enzymes. ... In normal individual, 75 to 80% of administered phenacetin is rapidly metabolized to acetaminophen.
... Phenacetin is converted to at least a dozen other metabolites, by n-deacetylation to para-phenetidin & by hydroxylation & further metabolism of phenacetin & para-phenetidin. An unknown metabolite, but an oxidizing agent, is responsible for methemoglobin formation & hemolysis of red blood cells ... .
Phenacetin is metabolized ... to p-acetamidophenol, which is excreted as glucuronide and sulfate conjugate ... .
... N-hydroxyphenacetin has been identified as metabolite in ... man.
For more Metabolism/Metabolites (Complete) data for PHENACETIN (15 total), please visit the HSDB record page.
Phenacetin has known human metabolites that include N-Hydroxyphenacetin and acetaminophen.
Metabolised in the body to paracetamol.

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Biological Half-Life
The elimination half-life (t1/2)beta varied from 37 to 74 minutes.

Toxicity Summary Phenacetin's analgesic effects are due to its actions on the sensory tracts of the spinal cord. In addition, phenacetin has a depressant action on the heart, where it acts as a negative inotrope. It is an antipyretic, acting on the brain to decrease the temperature set point. It is also used to treat rheumatoid arthritis (subacute type) and intercostal neuralgia. Toxicity Data Acute oral toxicity (LD50): 866 mg/kg [Mouse]. Toxin and Toxin Target Database (T3DB) 12.1.6 Interactions Sodium 3-hydroxy-4-iodo-2-naphthoate and sodium 1-hydroxy-4-bromo-naphthoate inhibited phenacetin absorption in the rat intestine. The blood, brain and kidney levels of phenacetin decreased and the liver level increased in rats following simultaneous oral administration of phenacetin and either one. The blood levels of phenacetin in rabbits were decreased by both but were increased by sodium 1-hydroxy-2-naphthoate, sodium tetrahydro-1-hydroxy-2-naphthoate, and sodium tetrahydro-3-hydroxy-2-naphthoate. All derivatives tested decreased the in vitro metabolism of phenacetin by rat or rabbit liver slices. Niwa H et al; Tohoku Yakka Daigaku Kenkyu Nempo 18: 1 (1971) Hazardous Substances Data Bank (HSDB) The transformation of acetophenetidin to n-acetyl-p-aminophenol was increased in the liver from rats treated with caffeine or antipyrine. The lung and intestine were also capable of metabolizing acetophenetidin to form this metabolite, and this pathway was increased by exposure to cigarette smoke. Following a test dose of acetophenetidin to human subjects, the plasma levels of acetophenetidin were lower in people who smoke cigarettes than in nonsmokers. Thus, in addition to the liver, the lung and intestine may have important roles in the metabolism of acetophenetidin.

[1]. COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: cloning, structure, and expression. Proc Natl Acad Sci U S A. 2002 Oct 15;99(21):13926-31.
[2]. Effects of long-term smoking on the activity and mRNA expression of CYP isozymes in rats. J Thorac Dis. 2015 Oct; 7(10): 1725–1731.
[3]. Inhibition of Baicalin on Metabolism of Phenacetin, a Probe of CYP1A2, in Human Liver Microsomes and in Rats. PLoS One. 2014; 9(2): e89752.
[4]. https://en.wikipedia.org/wiki/Phenacetin

Phenacetin can cause cancer according to California Labor Code.
Phenacetin is an odorless fine white crystalline solid with a lightly bitter taste. Used as an analgesic medicine.
Phenacetin is a member of the class of acetamides that is acetamide in which one of the hydrogens attached to the nitrogen is substituted by a 4-ethoxyphenyl group. It has a role as a non-narcotic analgesic, a peripheral nervous system drug and a cyclooxygenase 3 inhibitor. It is a member of acetamides and an aromatic ether. It is functionally related to a N-phenylacetamide, a 4-ethoxyaniline and a paracetamol.
Phenacetin was withdrawn from the Canadian market in June 1973 due to concerns regarding nephropathy (damage to or disease of the kidney).
Phenacetin is a synthetic, white crystalline solid that is slightly soluble in water and benzene, soluble in acetone and very soluble in pyrimidine. It is used in research as the preferred marker for detecting CYP1A2-based inhibition potential in vitro. Human ingestion of phenacetin can result in a bluish discoloration of the skin due to a lack of oxygen in the blood (cyanosis), dizziness and respiratory depression. It is reasonably anticipated to be a human carcinogen. (NCI05)
Phenacetin was withdrawn from the Canadian market in June 1973 due to concerns regarding nephropathy (damage to or disease of the kidney).
A phenylacetamide that was formerly used in ANALGESICS but nephropathy and METHEMOGLOBINEMIA led to its withdrawal from the market. (From Smith and Reynard, Textbook of Pharmacology,1991, p431)
See also: Aspirin; Butalbital; Caffeine (annotation moved to).

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Drug Indication
Used principally as an analgesic.

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Mechanism of Action
The present study was aimed to test the possible cyclooxygenase (COX)-1/COX-2 selectivity of the old analgesic drug phenacetin and its metabolite p-phenetidine, which exhibits high renal toxicity. Paracetamol (acetaminophen), the main metabolite of phenacetin with low renal toxicity, and indomethacin were selected as reference compounds. Collagen-stimulated platelet thromboxane B2 (TxB2) production and phorbol 12-myristate-13-acetate (PMA)-induced neutrophil prostaglandin E2 (PGE2) synthesis were used as indicators for COX-1 and COX-2 activity, respectively. Phenacetin was even less potent than paracetamol to reduce the production of both TxB2 and PGE2, and no clear preference for either of the COX-enzymes was seen. P-phenetidine was a more potent inhibitor, already at nanomolar level, of the synthesis of these prostanoids than indomethacin and showed some preference to COX-2 inhibition. Somewhat higher, micromolar, concentrations of p-phenetidine also reduced COX-2 expression in neutrophils. We suggest that the very potent inhibitory activity of p-phenetidine on PGE2 synthesis combined with the reduction of COX-2 expression could explain the renal papillary necrosis in phenacetin kidney.
Analgesic nephropathy is a unique drug-induced kidney disease characterized pathologically by renal papillary necrosis and chronic interstitial nephritis, and is the result of excessive consumption of combination antipyretic analgesics. The clinical features of the disorder relate mainly to the papillary necrosis, renal colic, and obstructive uropathy and the development of chronic renal failure in a small percentage of patients. There are significant geographic variations in the clinical features that may be related to the differing combinations of analgesics. The pathogenesis of the disease is in part related to the kidneys' ability to concentrate drugs in the papillae. The following sequence of events presents a plausible explanation for the evolution of the disease. If a combination of phenacetin and aspirin is ingested, the following steps occur. Phenacetin is converted in the gut and liver to acetaminophen by first-pass metabolism. Acetaminophen is then taken up by the kidney and excreted. During its excretion, acetaminophen becomes concentrated in the papillae of the kidney during physiologic degrees of antidiuresis, the concentration being up to five times the intracellular concentration of other tissues. Acetaminophen undergoes oxidative metabolism by prostaglandin H synthase to a reactive quinoneimine that is conjugated to glutathione. If acetaminophen is present alone, there is sufficient glutathione generated in the papillae to detoxify the reactive intermediate. If the acetaminophen is ingested with aspirin, the aspirin is converted to salicylate and salicylate becomes highly concentrated in both the cortex and papillae of the kidney. Salicylate is a potent depletor of glutathione. The mechanism is not completely understood; however, the inhibition of the production of NADPH via the pentose shunt is a possible explanation. With the cellular glutathione depleted, the reactive metabolite of acetaminophen then produces lipid peroxides and arylation of tissue proteins, ultimately resulting in necrosis of the papillae.
The mechanism of analgesic action has not been fully determined. Acetaminophen may act predominantly by inhibiting prostaglandin synthesis in the central nervous system (CNS) and, to a lesser extent, through peripheral action by blocking pain impulse generation. The peripheral action may also be due to inhibition of of the synthesis or actions of other substances that sensitive pain receptors to mechanical or chemical stimulation. /Acetaminophen/
Acetaminophen probably produces antipyresis by acting centrally on the hypothalamic heat-regulating center to produce peripheral vasodilation resulting in increased blood flow through the skin, sweating, and heat loss. The central action probably involves inhibition of prostaglandin synthesis in the hypothalamus. /Acetaminophen/

C10H13NO2

179.22

179.094

C, 67.02; H, 7.31; N, 7.82; O, 17.85

62-44-2

Phenacetin;62-44-2

4754

White to off-white solid powder

1.0±0.1 g/cm3

323.6±44.0 °C at 760 mmHg

133-136 °C(lit.)

149.5±28.4 °C

0.0±0.7 mmHg at 25°C

1.506

2.01

1

2

3

13

162

0

CPJSUEIXXCENMM-UHFFFAOYSA-N

InChI=1S/C10H13NO2/c1-3-13-10-6-4-9(5-7-10)11-8(2)12/h4-7H,3H2,1-2H3,(H,11,12)

N-(4-Ethoxyphenyl)acetamide

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)

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

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

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Injection Formulation 4: DMSO : 20% SBE-β-CD in saline = 10 : 90 [i.e. 100 μL DMSO → 900 μL (20% SBE-β-CD in saline)]
*Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.
Injection Formulation 5: 2-Hydroxypropyl-β-cyclodextrin : Saline = 50 : 50 (i.e. 500 μL 2-Hydroxypropyl-β-cyclodextrin → 500 μL Saline)
Injection Formulation 6: DMSO : PEG300 : castor oil : Saline = 5 : 10 : 20 : 65 (i.e. 50 μL DMSO → 100 μLPEG300 → 200 μL castor oil → 650 μL Saline)
Injection Formulation 7: Ethanol : Cremophor : Saline = 10： 10 : 80 (i.e. 100 μL Ethanol → 100 μL Cremophor → 800 μL Saline)
Injection Formulation 8: Dissolve in Cremophor/Ethanol (50 : 50), then diluted by Saline
Injection Formulation 9: EtOH : Corn oil = 10 : 90 (i.e. 100 μL EtOH → 900 μL Corn oil)
Injection Formulation 10: EtOH : PEG300：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL EtOH → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)

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

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

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

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