Aspirin

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Aspirin, or acetylsalicylic acid (IPA: [əˈsɛtlˌsæləˈsɪlɪk ˈæsɪd]), (acetosal) is a salicylate drug often used as an analgesic (to relieve minor aches and pains), antipyretic (to reduce fever), and as an anti-inflammatory. It also has an antiplatelet ("blood-thinning") effect and is used in long-term, low doses to prevent heart attacks and blood clot formation in people at high risk for developing blood clots. ^[1]

High doses of aspirin may also be given immediately after an acute heart attack; these doses may inhibit the synthesis of prothrombin and therefore produce a second and different anticoagulant effect,^{[citation needed]} although this is not well understood.

The main undesirable side effects of aspirin are gastrointestinal distress—including ulcers and stomach bleeding—and tinnitus, especially in higher doses. Another adverse effect is increased bleeding in menstruating women, due to aspirin's anticoagulant properties. Aspirin is no longer used to control flu-like symptoms or the symptoms of chickenpox in children under 12 years of age due to the risk of Reye's syndrome.^[2]

Aspirin was the first-discovered member of the class of drugs known as non-steroidal anti-inflammatory drugs (NSAIDs), not all of which are salicylates, although they all have similar effects and a similar mechanism of action.

[edit] History

[edit] Salicylic acid

The Greek physician Hippocrates wrote in the 5th century BC about a bitter powder extracted from willow bark that could ease aches and pains and reduce fevers. This remedy was also mentioned in texts from ancient Sumer, Lebanon, and Assyria. The Cherokee and other Native Americans used an infusion of the bark for fever and other medicinal purposes for centuries.^[3] The medicinal part of the plant is the inner bark and was used as a pain reliever for a variety of ailments. The Reverend Edward (Edmund) Stone, a vicar from Chipping Norton, Oxfordshire, England, noted in 1763 that the bark of the willow was effective in reducing a fever.^[4]

The active extract of the bark, called salicin, after the Latin name for the white willow (Salix alba), was isolated in crystalline form in 1828 by Henri Leroux, a French pharmacist, and Raffaele Piria, an Italian chemist. Piria was able to convert the substance into a sugar and a second component, which on oxidation becomes salicylic acid.

Salicylic acid was also isolated from the herb meadowsweet (Filipendula ulmaria, formerly classified as Spiraea ulmaria) by German researchers in 1839. While their extract was somewhat effective, it also caused digestive problems such as gastric irritation, bleeding, diarrhea, and even death when consumed in high doses.

[edit] Modern development

A French chemist, Charles Frederic Gerhardt, was first to prepare acetylsalicylic acid (named aspirin in 1899) in 1853. In the course of his work on the synthesis and properties of various acid anhydrides, he mixed acetyl chloride with a sodium salt of salicylic acid (sodium salicylate). A vigorous reaction ensued, and the resulting melt soon solidified.^[5] Since no structural theory existed at that time Gerhardt called the compound he obtained "salicylic-acetic anhydride" (wasserfreie Salicylsäure-Essigsäure). When Gerhardt tried to dissolve the solid in a diluted solution of sodium carbonate it immediately decomposed to sodium salts of salicylic and acetic acids.^[5] This preparation of aspirin ("salicylic-acetic anhydride") was one of the many reactions Gerhardt conducted for his paper on anhydrides, and he did not pursue it further.

Six years later, in 1859, von Gilm obtained analytically pure acetylsalicylic acid (which he called "acetylirte Salicylsäure", acetylated salicylic acid) by a reaction of salicylic acid and acetyl chloride.^[6] In 1869 Schröder, Prinzhorn and Kraut (Prinzhorn is credited in the paper with conducting the experiments) repeated both Gerhardt's (from sodium salicylate) and von Gilm's (from salicylic acid) syntheses and concluded that both reactions gave the same compound—acetylsalicylic acid. They were first to assign to it the correct structure with the acetyl group connected to the phenolic oxygen.^[7]

In 1897, Felix Hoffmann, a chemist at Friedrich Bayer & Co., obtained acetylsalicylic acid^[8] by a reaction of salicylic acid and acetic anhydride;^[9] that is, essentially repeating the Gilm/Kraut procedure but substituting acetic anhydride for acetyl chloride. This synthesis served as the basis for Bayer claims to discovery of aspirin. According to a legend publicized by Bayer, Hoffmann made some of the formula and gave it to his father, who was suffering from the pain of arthritis and could not stand the side-effects of salicylic acid.^[8] Much later, in 1949, another Bayer researcher, Arthur Eichengrün, who was 81, "claimed that he had instructed Hoffmann to synthesise acetylsalicylic acid and that the latter had done so without knowing the purpose of the work".^[10] In 2000, Walter Sneader of University of Strathclyde in Glasgow reexamined the case and concluded that "Arthur Eichengrün was telling the truth when he wrote that acetylsalicylic acid was synthesized under his direction and that the drug would not have been introduced in 1899 without his intervention".^[10] Subsequently, in his debate with Sneader, Axel Helmstaedter, General Secretary of the International Society for the History of Pharmacy, noted that Sneader did not credit several earlier publications which discussed the Hoffmann-Eichengrün controversy in detail, with most historians doubting Eichengrün's account.^[11] Bayer also countered to Sneader in a press release that according to the records, Hoffmann and Eichengrün held equal positions, and Eichengrün was not Hoffmann's supervisor. Hoffmann was named on the US Patent as the inventor, which Sneader did not mention. Eichengrün, who left Bayer in 1908, had multiple opportunities to claim the priority and had never before 1949 done it; he neither claimed nor received any percentage of the profit from aspirin sales.^[12] This argument did much to obscure the real history of aspirin, whose origin is not the pharmaceutical industry but earlier academic research. Despite the fact that pure aspirin was synthesized by von Gilm and by Kraut's group many years before Hoffmann, Bayer continues to insist that "The active ingredient in Aspirin®, acetylsalicylic acid, was synthesized for the first time in a chemically pure and thus stable form in 1897 by a young chemist working for Bayer, Dr. Felix Hoffmann."^[13]

It was not until the 1970s that the mechanism of action of aspirin and other NSAIDs was elucidated.

[edit] Trademark issues

The brand name Aspirin was coined by the Bayer company of Germany. The name "aspirin" is composed of a- (from acetylirte, meaning acetylated) -spir- (from the plant genus Spiraea) and -in (a common, easily pronounceable ending for drug names at the time). On March 6, 1899, Bayer registered the name Aspirin as a trademark.

Bayer began marketing aspirin in July 1899. It was originally sold as a powder (which is still the preferred form in many European countries) and was an instant success; in 1914, Bayer introduced aspirin tablets.^[8]

Although Bayer had registered Aspirin as a trademark in 1899, the German Patent Office refused to grant Bayer a patent for the drug based on the grounds that neither the product nor the process of preparation were novel.^[8] In 1905, Bayer sued Chemische Fabrik von Heyden in Britain for infringing the British patent on aspirin granted to it in 1900. Hayden claimed that existing prior art, in particular Kraut's work, invalidated Bayer's patent. Bayer advanced the argument that they were first to prepare a pure form of aspirin. The judge agreed with Hayden and invalidated the Bayer patent. A similar struggle took place in the U.S. Circuit court in Chicago; however, in the U.S., Bayer's patent was upheld in 1909.^[8] This created a situation where aspirin in the U.S. was three times more expensive than in Canada, and ten times more expensive than in Europe, resulting in rampant smuggling of aspirin, which Bayer unsuccessfully tried to contain.^[8]

After World War II, Bayer lost the right to use the trademark in many countries because the Allies seized and resold its foreign assets. The right to use the aspirin trademark in the United States (along with all other Bayer trademarks) was purchased from the U.S. government by Sterling Drug in 1918. Even before the patent for the drug expired in 1917, Bayer had been unable to stop competitors from copying the formula and using the name elsewhere, so, with a flooded market, the public was unable to recognize aspirin as coming from only one manufacturer, and in 1921, a landmark ruling by Billings Learned Hand established "aspirin" as a genericized trademark.^[14] Sterling was ultimately acquired by Bayer in 1994, but this did not restore the U.S. trademark. Other countries (such as Canada and many countries in Europe) still consider aspirin a protected trademark.

In some countries the name Aspirin is used as a generic name for the drug instead of the manufacturer's trademark. In countries in which Aspirin remains a trademark, the initialism ASA (for acetylsalicylic acid), or another initialism that matches the local-language term, is used as a generic term.

[edit] Synthesis

The synthesis of aspirin is classified as an esterification reaction, where the alcohol group from the salicylic acid reacts with an acid (acetyl anhydride) to form an ester. Aspirin is commercially synthesized using a two-step process. First, phenol (generally extracted from coal tar) is treated with a sodium base which generates sodium phenolate, which is then reacted with carbon dioxide under high temperature and pressure to yield salicylate, which is acidified, yielding salicylic acid. This process is known as the Kolbe-Schmitt reaction.

Salicylic acid is then acetylated using acetic anhydride, yielding aspirin and acetic acid as a byproduct. This generally tends to produce low yields due to the relative difficulty of its extraction from an aqueous state. A method of extracting higher yields is to acidify with phosphoric acid and heat the reagents under reflux with a boiling water bath for between 40 to 60 minutes.

The original synthesis of aspirin from salicylic acid involved acetylation with acetyl chloride. Unfortunately, the byproduct from this is hydrochloric acid, which is corrosive and environmentally hazardous. As described above, it was then later found that acetic anhydride was a better acylating agent, with the byproduct acetic acid formed, which does not have the unwanted properties of hydrochloric acid and can also be recycled. The salicylic acid/acetic anhydride method is commonly employed in undergraduate teaching labs.

Formulations containing high concentrations of aspirin often smell like vinegar. This is because aspirin can undergo autocatalytic degradation to salicylic acid in moist conditions, yielding salicylic acid and acetic acid.

The acid dissociation constant (pK_a) for Acetylsalicylic acid is 3.5 at 25 °C.^[15] ASA, being a weak acid, dissociates as shown by the following reaction equation:

[edit] Therapeutic uses

Aspirin is one of the most frequently used drugs in the treatment of mild to moderate pain, including that of migraines,^[16] and fever. It is often combined with other analgesics, even though this has never been shown to be more effective or less toxic than aspirin alone. Aspirin has, however, been used in addition to other non-steroidal anti-inflammatory drugs and opioid analgesics in the treatment of pain associated with cancer.^[17]

In high doses, aspirin and other salicylates are used in the treatment of rheumatic fever, rheumatic arthritis, and other inflammatory joint conditions. In lower doses, aspirin also has properties as an inhibitor of platelet aggregation, and has been shown to decrease the incidence of transient ischemic attacks and unstable angina in men, and can be used prophylactically. It is also used in the treatment of pericarditis, coronary artery disease, and acute myocardial infarction.^[17][18] Low doses of aspirin are also recommended for the prevention of stroke, and myocardial infarction in patients with either diagnosed coronary artery disease or who have an elevated risk of cardiovascular disease. It is also possible that women may benefit less from aspirin than men.^[19][20]

[edit] Veterinary uses

Aspirin has been used to treat pain and arthritis in veterinary medicine, primarily in cats and dogs, although it is not particularly recommended, as there are better medications available. Also, dogs are particularly susceptible to the gastrointestinal side effects associated with salicylates.^[21] Horses have also been given aspirin for pain relief, although it is not commonly used due to its relatively short-lived analgesic effects. Horses are also fairly sensitive to the gastrointestinal side effects as well. Nevertheless, it has shown promise in its use as an anticoagulant, mostly in cases of laminitis.^[22] Aspirin's use in animals should only be done under the direct supervision of a veterinarian.

[edit] Experimental uses

There have been some studies in the late 20th century indicating that aspirin may reduce cataract formation, although this is disputed.^[23] The role of aspirin in reducing the rates of many forms of cancer has also been widely studied.

Additionally, aspirin may be effective in reduction of risk of other cancers as well, including those of the prostate,^[24][25] colon,^{[26][27][28][29]} pancreas,^[30] upper GI tract,^[31] and lung.^[32][33] Its potency against tumors of the lung may be due to the fact that such tumors have high amounts of COX-2 enzymes expressed in them, especially adenocarcinomas and tumors caused by asbestosis, and aspirin is known to inhibit both the COX-1 and COX-2 enzymes.^[34] Likewise, if there is a specific connection between COX-2 and lung cancer, other COX-2 inhibitors might also have the same effect. It should be pointed out, however, that this research is not complete, and there is currently no well-established medical recommendation on the use of aspirin or other NSAIDs for use in the treatment or prevention of cancer.

[edit] Contraindications

• Aspirin should be avoided by those known to be allergic to ibuprofen or naproxen.
• Caution should be exercised in those with asthma or NSAID-precipitated bronchospasm.
• It is generally recommended that one seek medical help if symptoms do not improve after a few days of therapy.
• Caution should be taken in patients with kidney disease, peptic ulcers, mild diabetes, gout or gastritis; manufacturers recommend talking to one's doctor before using this medicine.
• Taking aspirin with alcohol or warfarin increases the chance of gastrointestinal hemorrhage (stomach bleeding).
• Children under the age of 12 are discouraged from using aspirin in cold or flu symptoms as this has been linked with Reye's syndrome.^[2]
• Patients with hemophilia or other bleeding tendencies should not take salicylates.
• Some sources recommend that patients with hyperthyroidism avoid aspirin because it elevates T4 levels. ^[35]
• Aspirin is known to cause hemolytic anemia in people who have the genetic disease glucose-6-phosphate dehydrogenase deficiency (G6PD), particularly in large doses and depending on the severity of the disease.

[edit] Adverse effects

• Gastrointestinal complaints (stomach upset, dyspepsia, heartburn, small blood loss). To help avoid these problems, it is recommended that aspirin be taken at or after meals, or be taken in an enteric coated formulation. Undetected blood loss may lead to hypochromic anemia.^[36][37]
• Severe gastrointestinal complaints (gross bleeding and/or ulceration), requiring discontinuation and immediate treatment. Patients receiving high doses and/or long-term treatment should receive gastric protection with high-dosed antacids, ranitidine or omeprazole.
• Frequently, central effects (dizziness, tinnitus, hearing loss, vertigo, centrally mediated vision disturbances, and headaches). The higher the daily dose is, the more likely it is that central nervous system side-effects will occur.
• Sweating, seen with high doses, independent from antipyretic action
• Long-term treatment with high doses (arthritis and rheumatic fever): often increased liver enzymes without symptoms, rarely reversible liver damage. The potentially fatal Reye's syndrome may occur, if given to pediatric patients with fever and other signs of infections. The syndrome is due to fatty degeneration of liver cells. Up to 30% of those afflicted will eventually die. Prompt hospital treatment may be life-saving.^[2]
• Chronic nephritis with long-term use, usually if used in combination with certain other painkillers. This condition may lead to chronic renal failure.^[38]
• Prolonged and more severe bleeding after operations and post-traumatic for up to 10 days after the last aspirin dose. If one wishes to counteract the bleeding tendency, fresh thrombocyte concentrate will usually work.
• Skin reactions, angioedema, and bronchospasm have all been seen infrequently.

[edit] Interactions

Aspirin is known to interact with other drugs. For example, acetazolamide and ammonium chloride have been known to enhance the intoxicating effect of salicyclates, and alcohol also enhances the gastrointestinal bleeding associated with these types of drugs as well. Aspirin is known to displace a number of drugs from protein binding sites in the blood, including the anti-diabetic drugs tolbutamide and chlorpropamide, the immunosuppressant methotrexate, phenytoin, probenecid, valproic acid (as well as interfering with beta oxidation, an important part of valproate metabolism) and any nonsteroidal anti-inflammatory drug. Corticosteroids may also reduce the concentration of aspirin. The pharmacological activity of spironolactone may be reduced by taking aspirin, and aspirin is known to compete with Penicillin G for renal tubular secretion.^[39] Aspirin may also inhibit the absorption of vitamin C.^[40][41][42]

[edit] Dosage

For adults doses of 300 to 1000 mg are generally taken four times a day for fever or arthritis, with a maximum dose of 8000 mg (8 grams) a day.^[43] The correct dose of aspirin depends on the disease or condition that is being treated. For instance, for the treatment of rheumatic fever, doses near the maximal daily dose have been used historically.^{[citation needed]} For the prevention of myocardial infarction in someone with documented or suspected coronary artery disease, doses as low as 75 mg daily (or possibly even lower) are sufficient.

For those under 12 years of age, the dose previously varied with the age, but aspirin is no longer routinely used in children due to the association with Reye's syndrome; paracetamol (known in the United States as acetaminophen) or other NSAIDs, such as ibuprofen, are now used instead. Kawasaki disease remains one of the few indications for aspirin use in children, with aspirin initially started at 7.5–12.5 mg per kilogram of body weight, taken four times a day for up to two weeks and then continued at 5 mg/kg once daily for a further six to eight weeks.^[44]

[edit] Overdose

Aspirin overdose can be acute or chronic. In acute poisoning, a single large dose is taken; in chronic poisoning, supratherapeutic doses are taken over a period of time. Acute overdose has a mortality rate of 2%. Chronic overdose is more commonly lethal with a mortality rate of 25%; chronic overdose may be especially severe in children.^[45]

[edit] Symptoms

Aspirin overdose has potentially serious consequences, sometimes leading to significant morbidity and mortality. Patients with mild intoxication frequently have nausea and vomiting, abdominal pain, lethargy, tinnitus, and dizziness. More significant symptoms occur in more severe poisonings and include hyperthermia, tachypnea, respiratory alkalosis, metabolic acidosis, hyperkalemia, hypoglycemia, hallucinations, confusion, seizure, cerebral edema, and coma. The most common cause of death following an aspirin overdose is cardiopulmonary arrest usually due to pulmonary edema.^[46]

[edit] Toxicity

The toxic dose of aspirin is generally considered greater than 150 mg per kg of body mass. Moderate toxicity occurs at doses up to 300 mg/kg, severe toxicity occurs between 300 to 500 mg/kg, and a potentially lethal dose is greater than 500 mg/kg.^[47] This is the equivalent of many dozens of the common 325 mg tablets, depending on body weight. However children cannot tolerate as much aspirin per unit body weight as adults can.

[edit] Treatment

All overdose patients should be conveyed to hospital for assessment immediately. Initial treatment of an acute overdose includes gastric decontamination. This is achieved by administering activated charcoal which adsorbs the aspirin in the gastrointestinal tract. Stomach pumps are no longer routinely used in the treatment of poisonings but are sometimes considered if the patient has ingested a potentially lethal amount less than 1 hour previously.^[48] Repeated doses of charcoal have been proposed to be beneficial in aspirin overdose.^[49] A study performed found that repeat dose charcoal might not be of significant value.^[50] However, most toxicologists will administer additional charcoal if serum salicylate levels are increasing.

Patients are monitored until their peak salicylate blood level has been determined.^[51] Blood levels are usually performed 4 hours after ingestion and then every 2 hours after that to determine the maximum level. Maximum levels can be used as a guide to toxic effects expected.^[52]

There is no antidote to salicylate poisoning. Frequent blood work is performed to check metabolic, salicylate, and blood sugar levels; arterial blood gas assessments are performed to test for respiratory alkalosis and metabolic acidosis. Patients are monitored and often treated according to their individual symptoms, patients may be given intravenous potassium chloride to counteract hypokalemia, glucose to restore blood sugar levels, benzodiazepines for any seizure activity, fluids for dehydration, and importantly sodium bicarbonate to restore the blood's sensitive pH balance. Sodium bicarbonate also has the effect of increasing the pH of urine, which in turn increases the elimination of salicylate. Additionally, hemodialysis can be implemented to enhance the removal of salicylate from the blood. Hemodialysis is usually used in severely poisoned patients; for example, patients with significantly high salicylate blood levels, significant neurotoxicity (agitation, coma, convulsions), renal failure, pulmonary edema, or cardiovascular instability are hemodialyzed.^[51] Hemodialysis also has the advantage of restoring electrolyte and acid-base abnormalities; hemodialysis is often life-saving in severely ill patients.

[edit] Epidemiology

In the later part of the 20th century the number of salicylate poisonings has declined mainly due to the popularity of other over-the-counter analgesics such as paracetamol (acetaminophen). Fifty-two deaths involving single-ingredient aspirin were reported in the United States in the year 2000. However, in all but three cases, the reason for the ingestion of lethal doses was intentional, predominantly suicides.^[53]

[edit] Mechanism of action

In 1971, the British pharmacologist John Robert Vane, then employed by the Royal College of Surgeons in London, showed that aspirin suppresses the production of prostaglandins and thromboxanes.^[54] For this discovery, he was awarded both a Nobel Prize in Physiology or Medicine in 1982 and a knighthood.

Aspirin's ability to suppress the production of prostaglandins and thromboxanes is due to its competitive and irreversible inactivation of the cyclooxygenase (COX) enzyme. Cyclooxygenase is required for prostaglandin and thromboxane synthesis. Aspirin acts as an acetylating agent where an acetyl group is covalently attached to a serine residue in the active site of the COX enzyme. This makes aspirin different from other NSAIDs (such as diclofenac and ibuprofen), which are reversible inhibitors.

Low-dose, long-term aspirin use irreversibly blocks the formation of thromboxane A₂ in platelets, producing an inhibitory effect on platelet aggregation. This anticoagulant property makes aspirin useful for reducing the incidence of heart attacks. 40 mg of aspirin a day is able to inhibit a large proportion of maximum thromboxane A₂ release provoked acutely, with the prostaglandin I2 synthesis being little affected; however, higher doses of aspirin are required to attain further inhibition.^[55]

Prostaglandins are local hormones (paracrine) produced in the body and have diverse effects in the body, including but not limited to transmission of pain information to the brain, modulation of the hypothalamic thermostat, and inflammation. Thromboxanes are responsible for the aggregation of platelets that form blood clots. Heart attacks are primarily caused by blood clots, and their reduction with the introduction of small amounts of aspirin has been seen to be an effective medical intervention. The side-effect of this is that the ability of the blood in general to clot is reduced, and excessive bleeding may result from the use of aspirin.

There are at least two different types of cyclooxygenase: COX-1 and COX-2. Aspirin irreversibly inhibits COX-1 and modifies the enzymatic activity of COX-2. Normally COX-2 produces prostanoids, most of which are pro-inflammatory. Aspirin-modified COX-2 produces lipoxins, most of which are anti-inflammatory. Newer NSAID drugs called COX-2 selective inhibitors have been developed that inhibit only COX-2, with the hope for reduction of gastrointestinal side-effects.

However, several of the new COX-2 selective inhibitors have been recently withdrawn, after evidence emerged that COX-2 inhibitors increase the risk of heart attack. It is proposed that endothelial cells lining the microvasculature in the body express COX-2, and, by selectively inhibiting COX-2, prostaglandins (specifically PGI2; prostacyclin) are downregulated with respect to thromboxane levels, as COX-1 in platelets is unaffected. Thus, the protective anti-coagulative effect of PGI2 is decreased, increasing the risk of thrombus and associated heart attacks and other circulatory problems. Since platelets have no DNA, they are unable to synthesize new COX once aspirin has irreversibly inhibited the enzyme, an important difference with reversible inhibitors.

Furthermore, aspirin has two additional modes of actions, contributing to its strong analgesic, antipyretic and anti-inflammatory properties:

• It uncouples oxidative phosphorylation in cartilaginous (and hepatic) mitochondria, by diffusing from the inner membrane space as a proton carrier back into the mitochondrial matrix, where it ionizes once again to release protons. In short, aspirin buffers and transports the protons. (Note: This effect in high doses of aspirin actually causes fever due to the heat released from the electron transport chain, instead of its normal antipyretic action.)
• It induces the formation of NO-radicals in the body that enable the white blood cells (leukocytes) to fight infections more effectively. This has been found recently by Dr. Derek W. Gilroy, winning Bayer's International Aspirin Award 2005.^[56]

More recent data suggest that salicylic acid and its derivatives will modulate signaling through NF-κB.^[57] NF-κB is a transcription factor complex that plays a central role in many biological processes, including inflammation.