Structural Biochemistry/Aspirin

History

Chemical structure of 2-acetoxybenzoic acid (IUPAC name), or Aspirin.

Aspirin is in the family of non-steroidal anti-inflammatory drugs otherwise known as NSAIDs medicines. Aspirin has been used for thousands of years. It is believed to have been first used in 400 BC by the Greek physician Hippocrates who would prescribe the bark and leaves of the willow tree to his patients to relieve pain and fever. It turns out that the willow tree is rich in a substance called salicin which was later turned into salicylic acid in 1832 AD by a German chemist named Charles Gergardt. Finally in 1897 AD, Felix Hoffman, a German chemist, invented aspirin which lead to the Bayer’s release of aspirin in 1899 AD. Scientists used X-ray crystallography to determine the 3D structure of the enzyme COX (cyclooxygenase), which helped them study the shapes of the enzyme and how drugs like aspirin block it.

COX Inhibitors

There are many drugs that belong to the NSAID (Non-Steroidal Anti-Inflammatory Drug) family of medicine such as Advil, Ibuprofen, and Aleve. Each of these drugs functions in a similar manner in that they all block a family of enzymes called cyclooxygenases (abbreviated COX). These drugs block two similar enzymes known as COX-1 and COX-2. COX-2 triggers an immune response and swelling in response to infection or injury. Blocking COX-2 is desirable because it can reduce swelling, fever, pain, etc. The problem with these drugs, such as aspirin, is that they also block COX-1. COX-1 produces molecules that protect the stomach wall called prostaglandins. Blocking COX-1 therefore results in ulcers due to the gastric acid being able to digest the de-protected stomach wall. This is a significant problem with long term use of aspirin to treat diseases such as arthritis. These side effects of long term use of aspirin result in about 16,500 deaths every year in the U.S.

The Development of COX-2 Only Inhibiters

Isoleucine

Valine

Structural differences between of isoleucine and valine.

Scientists wondered for many years how COX inhibitors such as aspirin worked so they used X-Ray Crystallography and other biophysical techniques to determine the three dimensional structure of the cyclooxygenases. The structure of an enzyme is important in understanding its function. Visualizing the individual enzyme’s structural folding and bends enables scientists to consider possible inhibitors that might bind to the enzyme. As the structures of COX-1 and COX-2 were being solved a new drug was found that blocks COX-2 but not COX-1, called Celebrex. The solved structures of COX-1 and COX-2 showed why only COX-2 was inhibited. In the binding site for Celebrex in COX-1, an isoleucine is present where there is a valine in COX-2. The structure of valine has a pocket-like end that is easily bound to, whereas isoleucine has an extended side chain structure that makes it impossible for the drug to bind. This allowed for the new drug to be made to bind to that specific target without the bad side effects that were caused by the COX-1 enzyme.

COX-3

Scientist have also developed another inhibitor called COX-3 inhibitors. This inhibitor appears to be more similar to Tylenol. Tylenol does not belong to the NSAID family of medicine because it is not an anti-inflammatory medicine. The future of COX-3 drugs may lead to a more significant understanding of Tylenol.

Synthesis

One of common method for Synthesis of Aspirin is classified as esterification reaction as followed:

Reaction Mechanism

Mechanism of Actions

Antiplatelet effect aspirin

Aspirin is an antiplatelet drug that inhibits cyclooxygenase or COX. COX-2 triggers inflammation as an immune response. By blocking COX-2, aspirin acts as a potent pain killer and anti-inflammatory agent. However, COX-1 enzymes are also the key enzymes in thromboxane A2 (TXA2) generation. Thromboxane TXA2 causes platelets to clump together over the vessel tear to facilitate repair when blood vessels are damaged or are diseased. The aggregation of platelets results in a clot which stops bleeding and aids repair of the damaged blood vessel. Aspirin inhibits COX-1, therefore TXA2, thereby reducing the ability of platelets to accumulate. This is why aspirin is known as a blood thinner or anti-platelet agent.

COX-1 and COX-2 are both cyclooxygenase enzymes with similar structure but different in functions. They both generate prostaglandins from the fatty acid arachidonic acid. However, different prostaglandins are produced from COX-1 and COX-2. Prostaglandins from COX-2 are associated with pain, inflammation and fever while prostaglandins from COX-1 support platelet aggregation, gastrointestinal mucosal integrity and renal function.

Aspirin blocks COX enzymes

In the process of blocking COX, the acetyl group on aspirin is hydrolyzed and then bonded to the alcohol group of serine as an ester. This has the effect of blocking the channel in the enzyme and arachidonic acid can not enter the active site of the enzyme and be converted to prostaglandins.

There is 60% homology between the amino acid structures of COX-1 and COX-2. Aspirin blocks COX-1 by binding to Ser 530 in the same way that it binds to Ser 516 of COX-2. However, the active site of COX-2 is slightly larger than the active site of COX-1, so pharmaceutical companies have been able to develop selective COX-2 inhibitors, such as Celecoxib, Rofecoxib and Meloxicam. These drugs can inhibit the biochemical actions of COX-2 which reduces inflammation but have no effect on COX-1 to prevent damages of the stomach mucosa.

Polymorphism

Polymorphism is the ability of a substance to form more than one crystal structure. This process is essential when it comes to the development of pharmaceutical ingredients. For a long period of time, only one crystal structure was discovered. Then since the 1960s, a second crystalline form was suspected. This second polymorph was first discovered by Vishweshwar and his coworkers in the year 2005. A new crystals was basically found after performing the cocrystallization of aspirin and levetiracetam from hot acetonitrile. However, this second crystalline form is only stable at 100°K and reverts to the first crystalline form at ambient temperature. In the first crystalline form, there exist two salicylic molecules form centrosymmetric dimers through the acetyl groups with the (acidic) methyl proton to carbonyl hydrogen bonds. Furthermore, in the second crystalline form that is newly claimed, each salicylic molecule form the same hydrogen bonds with two neighboring molecules instead of only one. In the end, both polymorphs form the same dimmer structures that is with respect to the hydrogen bonds forms by the carboxylic acid groups.

Before taking Aspirin

Children or teenager who has a fever should NOT take aspirin. Children who have chicken pox or flu symptoms should also not take aspirin due to the potential for aspirin to cause a serious condition called Reye's syndrome, which is primarily seen in children. In addition, adults should also avoid taking aspirin if they have any of the conditions listed below:

• recent history of intestinal, stomach, or intracranial bleeding
• bleeding disorder
• allergy to an non-steroidal anti-inflammatory drug such as Advil, Motrin, Aleve, or Orudis

If an individual is taking aspirin to prevent conditions such as stroke or heart attack, it is crucial to avoid taking ibuprofen at the same time, as this can increase the risk of serious bleeding. This medication can also hurt an unborn baby's heart, and may reduce birth weight or have other serious effects. Therefore, it is important to the tell the doctor before taking aspirin if one is pregnant or plans to become pregnant while taking this medication. It is also essential for breast feeding mother to know that taking aspirin can also pass through breast milk and seriously harm a nursing baby. Hence, all breast feeding mothers should tell the doctor in advance before taking this medication.

Medicinal Uses

Aspirin is part of a group of medications known as non-steroidal anti-inflammatory drugs (NSAIDs) which has been used since the times of the Ancient Greeks. Belonging to a class of salicylates, this drug has multiple uses in treating various ailments associated with pain and inflammation. Typically, it acts by reducing the materials that cause inflammation and pain. Often, aspirin is considered inferior to ibuprofen for the alleviation of pain. However, aspirin is usually ineffective for those pains caused by muscle cramps, bloating, or skin irritation. Thus, treating mild to moderate pain and inflammation are some of the common uses, but aspirin has also been used to treat and prevent heart attack, stroke, chest pain, and other cardiovascular conditions. However, Aspirin appears to offer little use to those at lower risk of heart attack or stroke. This kind of idea is known as primary prevention. Normally while taking aspirin, one should avoid taking other NSAIDs as they will actually inhibit the action of the drug. Coming in various forms (enteric-coated tablets, chewable tablets), aspirin is commonly prescribed by physicians for its uses in preventing cardiovascular problems.

In addition, studies show that aspirin have great effect on cancer, especially its effect on a cancer known as colorectal cancer (CRC). Even multiple meta-analyses implies that regular use of aspirin will definitely reduce the long-term risk of colorectal cancer. For over 100 years, other uses of aspirin may be for fever and pains related with common cold. Basically, aspirine can be a source of cure when it comes to achiness, discomfort, headache, and sore throat pain. Furthermore, existing fever and joint pain often cure quickly within three days of high doses of aspirin. People with pericarditis, coronary artery disease, and acute myocardial infarction can be treated with aspirin too.

There are several side effects and risk factors associated with the use of aspirin. Individuals who are allergic to aspirin, have a history of intestinal or stomach bleeding, bleeding disorders (like hemophilia), and allergies to other NSAIDs should avoid using aspirin. However, prescribers can still recommend aspirin after performing tests and adjusting the dosage based on the patient's needs. For example, patients with specific medical issues such as stomach ulcers, liver disease, blood clotting disorders, or high blood pressure may require extra tests and adjusted doses before safely prescribing aspirin. Also, patients on aspirin should avoid drinking alcohol as alcohol consumption can lead to increased bleeding in the stomach. Typical side effects with aspirin include upset stomach, heartburn, drowsiness, and headache. Because aspiring irritates the lining of the stomach and intestinal tracts, a useful way to reduce this incidence is to drink aspirin with a full glass of milk. One should stop using aspirin if side effect such as hives or difficulty breathing occurs.

Finally, certain medications such as warfarin should be notified to physicians before prescribing aspirin. Page text.

Side Effects

Immediately get medical help if you present one or more of the following symptoms:

• hives
• difficulty breathing
• swelling on the lips, face, tongue or throat

Stop using aspirin and call doctor if you see one or more of the following symptoms:

• black, bloody or tarry stools
• coughing up blood or vomit that looks like coffee grounds
• severe nausea, vomiting or stomach pain
• fever lasting over 3 days
• swelling for over 10 days
• hearing problems (ringing in your ears)

Least serious symptoms might include:

• upset stomach
• heartburn
• headache
• drowsiness

References

Flavio, Guzmán. "Antiplatelet agents: mechanisms of action and general overview." Pharmacology Corner. N.p., 24 2009. Web. 7 Dec 2012.

Vane, J.R, and R.M Botting. "Thrombosis Research." Thrombosis Research. 110. (2003): 255-58. Print.

• http://en.wikipedia.org/wiki/Aspirin
• http://www.drugs.com/aspirin.html