Acetaminophen

The chemiacal name of Acetaminophen is N-(4-hydroxy phenyl) acetamide and the trade name is paracetamol belonging to acetanilide antipyretic analgesics. It was first synthesized by Morse in 1878 and first used in clinic by VonMering in 1893. It has become an over the counter drug in the USA since 1955 and our country started production at the end of the 1950’s. Acetaminophen is a white crystalline or a crystalline powder in appearance with melting point from 168℃ to 172℃, odorless, slightly bitter taste, freely soluble in hot water or ethanol, dissolved in acetone, practically insoluble in cold water and petroleum ether. It is stable below 45℃ but will be hydrolyzed into p-aminophenol when exposed to humid air, then oxidized further. The color grades gradually from pink to brown then to black, so it should be sealed and stored in a cool and dry place.Acetaminophen has the antipyretic activity by inhibiting the synthesis of hypothalamic thermoregulation prostaglandins and its strength of antipyretic effect is similar to aspirin. On the other hand, Acetaminophen can produce analgesic effect by inhibiting the synthesis of prostaglandins in the central nervous system and blocking impulses of nociceptive nerve endings, but weaker than aspirin. Compared with aspirin, Acetaminophen has minor irritation, few allergic reactions and other advantages. Its antipyretic and analgesic effect is similar to phenacetin, and   the use of Acetaminophen increases due to limiting or banning using phenacetin in many countries.In clinical, it is mainly used for fever and headache caused by cold and relieving mild to moderate pain such as joint pain, muscle pain, neuralgia, migraine, dysmenorrhea, cancer pain, postoperative analgesia and so on. It can be used for patients who are allergic to aspirin, intolerant of aspirin, or unsuited for aspirin, such as patients with varicella, hemophilia and other hemorrhagic disease (patients having anticoagulant therapy included), as well as patients with slight peptic ulcer and gastritis. In addition, it also can be used for the synthesis of benorylate and used as asymmetric synthetic intermediates, photographic chemicals and stabilizer of hydrogen peroxide.

Obtain prism crystallization from ethanol. Melting point 169-171℃, relative density 1.293(21/4℃). Soluble in ethanol, acetone and hot water, difficult to dissolve in water, insoluble in petroleum ether and benzene. Odorless, bitter. The pH value of saturated aqueous solution is 5.5-6.5.

Acetaminophen is used as antipyretic analgesics. It has the antipyretic activity by means of mediated peripheral vasodilation and perspiration caused by inhibiting the cyclooxygenase which selectively inhibiting the synthesis of hypothalamic thermoregulation prostaglandins, and its strength of antipyretic effect is similar to aspirin. As a peripheral analgesic, it can produce analgesic effect by inhibiting the synthesis and release of prostaglandins and increasing pain threshold. However, its action is weaker than aspirin and it is only effective for mild to moderate pain. There is no obvious anti-inflammation effect.

The oral absorption is rapid and complete, and the peak time occurs 0.5～2h later. The plasma protein binding rate is 25%～50%. This product is equally distributed in the body, 90%～95% is metabolized in the liver and mainly excreted from the kidney combining with glucuronic acid  and about 3% exits the body unchanged in the urine within 24h. Its half-life (t1/2) is 1～4h (average 2h). In case of renal insufficiency t1/2 is not affected, but t1/2 of patients with hepatic insufficiency, newborns or elderly patients may increase and t1/2 of children may decrease. It can be secreted by milk.

1. Using nitrobenzene as raw material
In the presence of concentrated sulfuric acid and sixteen alkyl methyl ammonium chloride, nitrobenzene is transformed into p-Aminophenol by catalytic hydrogenation with Pd/C as catalyst. P-acetaminophen is synthesized acetylation by one-step acylation without separation and the yield is 64.3%. The reaction is as followed:

2. Using paranitrophenol as raw material
With paracetamol as raw material and Pd/C as catalyst, paracetamol is synthesized by hydroacylation on one-step method. The optimum solvent is acetic acid of which the dosage is 2 to 5 times of paranitrophenol and the yield of paracetamol is up to 95%. When Pd-La/C is used as catalyst instead, the yield can reach 97%. The reaction is as followed:
 
3. Using p-aminophenol as raw material
Under these conditions of using p-aminophenol and acetic anhydride as raw materials, zinc powder as the antioxidant, activated carbon as the decolorizing agent and dilute acetic acid as the reaction medium, paracetamol is synthesized by microwave irradiation technology and the yield is up to81.2%. The reaction is as followed:

4. Using p-Hydroxyacetophenone as raw material
First oximate p-Hydroxyacetophenone and then rearrange it to obtain paracetamol by means of Beckmann. Under this method, the yield of 4-hydroxyacetophenone oxime obtained by oximating p-Hydroxyacetophenone is 93.5%. Then we use Hβ molecular sieve as catalyst and acetone as the solvent to obtain acetaminophen by rearrangement and the yield is 81.2 %. In the rearrangement reaction, acetone is used as the solvent and Al-MCM-41 molecular sieve is used as the catalyst. The yield is the highest when the content of phosphoric acid in the catalyst is 30%. The reaction is as followed:
 
5. Using  phenol as raw material
Phenol is used as the raw material and synthesizes paracetamol after acetylation, Fries rearrangement, oxime and Beckmann rearrangement. The yields are 82%, 68.6%, 50.5%,
respectively. The reaction is as followed:

Usage
This product is antipyretic and analgesic whose international nonproprietary name is Paracetamol. It is the most common non anti-inflammatory analgesia-antipyretic drugs without anti inflammatory and anti rheumatism action. Its antipyretic effect is similar to aspirin, but analgesic effect is weak. It is the best of breed of acetanilid drugs. The product is especially suitable for patients who cannot use carboxylic acids drugs. It is used for cold and toothache. Acetaminophen is also used as organic synthesis intermediates, stabilizer of hydrogen peroxide, photographic chemicals.
Dosage   
1. Oral (1) Paracetamol tablets or paracetamol capsules: adults take 300～600mg at a time and 3～4 times a day according to the need. The daily dosage should not be greater than 2g. Defervescence treatment is generally less than 3 days and the administration of pain relief lasts less than 10 days. Children take 10～15mg/kg every 4～ 6 hours. The dosage of children under the age of 12 does not exceed 5 times a day, a five-day course at most. This product should not be taken for a long time.
2. Dispersible tablets: When take tablets, disperse them in warm water dispersion. The commonly used amount of children is 10～15mg/kg every 4～ 6 hours. The dosage of children under the age of 12 does not exceed 5 times a day, a five-day course at most. Children under 3 years old cut back on the amount.

In Osteoarthritis：

• Acetaminophen is recommended by the ACR as first-line drug therapy for pain management of OA. The dose is 325 to 650 mg every 4 to 6 hours on a scheduled basis (maximum dose 4 g/day; maximum 2 g/day if chronic alcohol intake or underlying liver disease). Comparable relief of mild to moderate OA pain has been demonstrated for acetaminophen (2.6 to 4 g/ day) compared with aspirin (650 mg four times daily), ibuprofen (1,200 or 2,400 mg daily), and naproxen (750 mg daily). However, some patients respond better to NSAIDs.
• Acetaminophen is usually well tolerated, but potentially fatal hepatotoxicity with overdose is well documented. It should be used with caution in patients with liver disease and those who chronically abuse alcohol. Chronic alcohol users (three or more drinks daily) should be warned about an increased risk of liver damage or GI bleeding with acetaminophen. Other individuals do not appear to be at increased risk for GI bleeding. Renal toxicity occurs less frequently than with NSAIDs.

1. Allergic reactions: This product has less and slight side effects a dose treatment except for occasional rashes, hives and other allergic reactions. Methemoglobinemia may occur in a few cases.
2. Hepatorenal damage: A large number of long-term use,  hepatorenal damages and thrombocytopenia may occur, even jaundice, oliguria, acute severe hepatitis, which could lead to coma, and death. Using at high dosage may cause nausea, vomiting, stomach pain, stomach cramps, diarrhea, anorexia, sweating, etc.
3. For children under the age of 3, the development of liver and kidney function is not mature with poor detoxification and excretory function, so they should try to avoid using this product. In addition, patients with liver and kidney insufficiency and pregnant women should use cautiously. The long-term drug users should regularly check renal function and hemogram.

It is contraindicated in patients allergic to the product and patients with severe liver and kidney function deficiency.

Organic synthesis intermediates, stabilizer of hydrogen peroxide, photographic chemicals, non anti-inflammatory analgesia-antipyretic drugs.

Produced by acetylation of p-aminophenol.
Method 1: add p-aminophenol into dilute acetic acid, then add glacial acetic acid, heat up to 150℃and react for 7h, add acetic anhydride and react for 2h, check the end point and cool to 25℃ after the acceptance, shake it and filter, water until no acetic acid flavor exists, dry to get crude products.
Method 2: distill p-aminophenol, acetic acid and acid industrial containing more than 50% acid together, the speed of distilling dilute acid for is 1/10 of the total distillate in one hour, check the residue of p-aminophenol less than 2.5% aminophenol by sampling inspection when inner temperature rises up to 130℃, add dilute acid (content of more than 50%), cool to get crystallization. After shaking and filter, first use a small amount of dilute acid to wash, and then use a large number of water till filtrate is near colourless to get crude products. The yield of method 1 is 90%, but the yield of method 2 is 90-95%. Refining methods: add the crude product when the water is heated to near boiling. Heat up to the total dissolution, add activated carbon soaked in water, use dilute acetic acid to adjust till pH=4.2-4.6, boil for 10min. Filter press, add a small amount of sodium bisulfite into the filtrate. Cool to below 20℃, separate crystals out. After shaking and filter, wash and dry to get active ingredients, paracetamol finished products.
Other methods of production are as followed:
(1) p-nitrophenol is reduced by zinc in acetic acid, and acetaminophen is obtained by acetylation at the same time;
(2) put the hydrazone generated from p-hydroxyacetophenone in acid solution containing sulfuric acid, and then add sodium nitrite to get acetaminophen by renversement.

Acetaminophen is an analgesic and antipyretic compound. Unlike many NSAIDs, which inhibit both COX-1 and COX-2, early studies suggested that acetaminophen is a poor inhibitor of both isoforms. However, it does inhibit COX-2 by 83% and COX-1 by 56% in human blood ex vivo, albeit at a high 1,000 mg dose, with IC₅₀ values of 25.8 and 113.7 μM, respectively. Acetaminophen is enzymatically and non-enzymatically converted to several reactive metabolites that contribute to adverse or indirect effects, including liver injury. At toxic doses, the acetaminophen metabolite N-acetyl-4-benzoquinone imine (NAPQI; ) depletes glutathione reserves in the liver, leading to an accumulation of NAPQI and subsequent hepatocyte necrosis. Acetaminophen decreases glutathione levels and reduces glutathione peroxidase activity in mice when administered at a dose of 250 mg/kg and induces ferroptotic cell death in primary mouse hepatocytes, an effect that can be blocked by the ferroptosis inhibitor ferrostatin-1 . Acetaminophen has analgesic and antipyretic properties in animal models.

Acetaminophen differs from the nonsteroidal anti-inflammatory agents described in that it is devoid of anti-inflammatory and antirheumatic properties. It was recently shown that acetaminophen, like aspirin, inhibits cyclooxygenase action in the brain and is even stronger than aspirin. On the other hand, the mechanism of analgesic action of acetaminophen is not fully clear, since it acts poorly on peripheral cyclooxygenase.

White Solid

Trigesic ,Squibb ,US ,1950

manufacture of azo dyes, photographic chemicals.

dispersing agent in liquid scintillation counting

Analgesic; antipyretic

antiinfectant

Acetaminophen is widely used as an analgesic and fever-reducing agent. Acetaminophen is designed for moderate analgesia. It is also effective like aspirin and is used in analgesia for headaches (from weak to moderate pain), myalgia, arthralgia, chronic pain, for oncological and post-operational pain, etc.

Acetaminophen (Tylenol) is an effective antipyretic and analgesic that is well tolerated at therapeutic doses. It has only weak antiinflammatory activity; thus, it is not useful in the treatment of rheumatoid arthritis and other inflammatory conditions.

ChEBI: Paracetamol is a member of the class of phenols that is 4-aminophenol in which one of the hydrogens attached to the amino group has been replaced by an acetyl group. It has a role as a cyclooxygenase 2 inhibitor, a cyclooxygenase 1 inhibitor, a non-narcotic analgesic, an antipyretic, a non-steroidal anti-inflammatory drug, a cyclooxygenase 3 inhibitor, a xenobiotic, an environmental contaminant, a human blood serum metabolite, a hepatotoxic agent, a ferroptosis inducer and a geroprotector. It is a member of phenols and a member of acetamides. It is functionally related to a 4-aminophenol.

About 250 ml of a reaction mixture obtained by the electrolytic reduction of nitrobenzene in sulfuric acid solution and containing about 23 grams of paminophenol by assay is neutralized while at a temperature of 60°C to 65°C, to a pH of 4.5 with calcium carbonate. The calcium sulfate precipitate which forms is filtered off, the precipitate washed with hot water at about 65°C and the filtrate and wash water then combined. The solution is then extracted twice with 25 ml portions of benzene and the aqueous phase is treated with 0.5 part by weight, for each part of p-aminophenol present, of activated carbon and the latter filtered off. The activated carbon is regenerated by treatment with hot dilute caustic followed by a hot dilute acid wash, and reused a minimum of three times.
To the filtrate obtained, there are then added about 0.2 gram of sodium hydrosulfite or sodium sulfite and 15.0 grams of anhydrous sodium acetate in about 27 grams of acetic anhydride at 40°C. The reaction mixture formed is cooled to 8°C to 10°C with stirring and held at this temperature for 60 minutes. A crystalline precipitate of about 27 grams of N-acetyl-paminophenol is obtained melting at 169-171°C. This is equivalent to a yield of 85%.
In lieu of utilizing calcium carbonate as the neutralizing agent, calcium hydroxide, barium hydroxide, barium chloride or other alkaline earth metal salt or hydroxide forming an insoluble sulfate may be employed.

Acephen (G & W); Infants’ Feverall (Actavis); Injectapap (Ortho-McNeil); Neopap (Polymedica); Tylenol (McNeil);Anacin;Crocin.

Analgesic, Antipyretic

Paracetamol, a widely used analgesic and antipyretic is known, in case of overdose, to cause liver damage, frequently with fatal outcome. In recommended dosages this risk does not occur. Paracetamol is listed in the WHO Model List of Essential Drugs.

The Journal of Organic Chemistry, 27, p. 1092, 1962 DOI: 10.1021/jo01050a543
Tetrahedron Letters, 22, p. 1257, 1981 DOI: 10.1016/S0040-4039(01)90289-8

Odorless white crystalline solid. Bitter taste. pH (saturated aqueous solution) about 6.

Slightly soluble in water.

Acetaminophen is sensitive to light. Incompatible with strong oxidizers. 

Flash point data for Acetaminophen are not available; however, Acetaminophen is probably combustible.

Non flammable

Cyclooxygenase inhibitor; may be selective for COX-3 (IC 50 values are 460, > 1000 and > 1000 μ M for canine COX-3, and murine COX-1 and COX-2 respectively). Widely used analgesic and antipyretic agent.

The mechanism of action of paracetamol is not well understood, but it may act in a similar fashion to NSAIDs, with inhibition of cyclo-oxygenase enzymes COX-1 and COX-2 to reduce the phenoxyl radical formation required for COX-1 and 2 activity and prostaglandin synthesis. I t has selectivity for inhibition of prostaglandin synthesis with low concentrations of peroxidases and arachidonic acid, but limited effect at higher concentrations and, therefore, has limited anti-inflammatory effects. Unlike opioids, paracetamol has no well-defined endogenous binding sites. I n some circumstances, it may exhibit a preferential effect on COX-2 inhibition. There is growing evidence of a central antinociceptive effect of paracetamol. It has also been found to prevent prostaglandin production at the cellular transcriptional concentration, independent of COX activity.

Paracetamol is absorbed rapidly from the small intestine after oral administration; peak plasma concentrations are reached after 30–60min. It may also be given rectally and intravenously (either as paracetamol or the prodrug propacetamol). It has good oral bioavailability (70%–90%); rectal absorption is more variable (bioavailability ~50%–80%) with a longer time to reach peak plasma concentration. The plasma half-life is approximately 2–3 h.
Paracetamol is metabolised by hepatic microsomal enzymes mainly to the glucuronide, sulphate and cysteine conjugates. None of these metabolites is pharmacologically active. Aminimal amount of the metabolite N-acetyl-pamino- benzoquinone imine is normally produced by cytochrome P450– mediated hydroxylation. This reactive toxic metabolite is rendered harmless by conjugation with liver glutathione, then excreted renally as mercapturic derivatives. With larger doses of paracetamol, the rate of formation of the reactive metabolite exceeds that of glutathione conjugation, and the reactive metabolite combines with hepatocellular macromolecules, resulting in cell death and potentially fatal hepatic failure. The formation of this metabolite is increased by drugs inducing cytochrome P450 enzymes, such as barbiturates or carbamazepine.

Acetaminophen is weakly acidic (pKa = 9.51) and synthesized by the acetylation of p-aminophenol. It is weakly bound to plasma proteins (18–25%). Acetaminophen is indicated for use as an antipyretic/analgetic, particularly in those individuals displaying an allergy or sensitivity to aspirin. It does not possess anti-inflammatory activity, but it will produce analgesia in a wide variety of arthritic and musculoskeletal disorders. It is available in various formulations, including suppositories, tablets, capsules, granules, and solutions. The usual adult dose is 325 to 650 mg every 4 to 6 hours. Doses of greater than 2.6 g/day are not recommended for long-term therapy because of potential hepatotoxicity issues. Acetaminophen, unlike aspirin, is stable in aqueous solution, making liquid formulations readily available, a particular advantage in pediatric cases.

Side effects are rare and may include hematological reactions, leucopenia, agranulocytosis and other hypersensitivity reactions. Paracetamol has a narrowtherapeutic dose range and overdosage induces severe liver and renal damage via accumulation of a toxic metabolite, N-acetylbenzoquinoneimine (NABQI). Acetylcysteine or methionine, which increase glutathione conjugation of the metabolite, are used as antidote.

Acetaminophen, p-acetaminophenol (3.2.80), is synthesized by reacting p-aminophenol with acetic anhydride [76,77].

Although a major part of the ingested dose of acetaminophen is detoxified, a very small proportion is metabolized via the cytochrome P450-mixed function oxidase pathway to a highly reactive n-acetyl-p-benzoquinoneimine (NAPQI). The toxic intermediate NAPQI is normally detoxified by endogenous glutathione to cysteine and mercapturic acid conjugates and excreted in the urine. Recent studies have shown that hepatic P450s, CYP2E1, and to a lesser extent CYP1A2 are responsible for conversion of acetaminophen to NAPQI. In acetaminophen overdose, the amount of NAPQI increases and depletes endogenous glutathione stores. Time course studies have shown that covalent binding of reactive NAPQI and subsequent toxicity occur only after cellular glutathione stores are reduced by 70% or more of normal. Mitochondrial dysfunction and damage can be seen as early as 15 min after a toxic dose in mice, suggesting that this may be a critical to cellular necrosis. The NAPQI is then thought to covalently bind to critical cellular macromolecules in hepatocytes and cause cell death. Recent proteomic studies have identified at least 20 known proteins that are covalently modified by the reactive acetaminophen metabolite. The resulting acetaminophen-cysteine (APAP-CYS) protein adducts can be quantified via a highpressure liquid chromatography coupled with electrochemical detection (HPLC-EC). Hepatic necrosis and inflammation develop as a consequence of hepatocellular death, which results in development of clinical and laboratory findings consistent with liver failure. A similar mechanism is postulated for the renal damage that occurs in some patients following acetaminophen toxicity.

Acetaminophen (APAP) is metabolized by mice, and nine metabolites are identified in the urine. The main metabolites are APAP-glucuronide and 3-cysteinyl- APAP. Hydroquinone metabolites of S-(2,5- dihydroxyphenyl)cysteine and S-(2,5-dihydroxyphenyl)- N-acetylcysteine result from the benzoquinone metabolite of APAP.

acetaminophen is undergoes rapid first-pass metabolism in the GI tract primarily by conjugation reactions, with the O-sulfate conjugate being the primary metabolite in children and the O-glucuronide being the primary metabolite in adults. A minor, but significant, product of both acetaminophen and phenacetin is the N-hydroxyamide produced by a CYP2E1 and CYP3A4.

Store at RT

Recrystallise Paracetamol from water or EtOH. The 3,5-dinitrobenzamide complex gives orange crystals from hot H2O and has m 171.5o. [Beilstein 13 H 460, 13 I 159, 13 II 243, 13 III 1056, 13 IV 1091.]