Eicosanoids are an extremely complex group of organic molecules that have a number of functions in the body. Derived from fatty acids, they are signaling molecules, with a short half-life, that control a number of different pathways. Families of eicosanoids include prostaglandins, prostacyclins, thromboxanes, and leukotrienes, each with multiple series of compounds. Depending on which molecules are produced, they may produce inflammation or reduce it. Drugs that affect eicosanoid production include non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ibuprofen, along with adrenal steroids.
The production of eicosanoids starts with the reception of an extracellular signal at a specific sensing point known as a receptor. This activates a cellular membrane enzyme called a phospholipase, which cleaves a particular fatty acid. This is the one step that all of the eicosanoids have in common. The fatty acid generated is a 20-carbon polyunsaturated fatty acid, often arachidonic acid. This compound is one of the essential fatty acids that must be obtained externally from foods.
Once arachidonic acid has been generated, it can be transformed by two separate enzymatic pathways that each add a molecule of oxygen to the fatty acid. Cyclooxygenase (COX) is a family of enzymes that produces the prostaglandins, prostacyclins, and thromboxanes — collectively known as prostanoids. Alternatively, the lipoxygenase family can transform arachidonic acid to produce leukotrienes.
Other molecules, besides arachidonic acid, can be oxygenated to produce eicosanoids. Dietary amounts of omega-6 and omega-3 fatty acids affect this process. Both groups of fatty acids are essential fatty acids but can be divided into the two categories. For example, arachidonic acid belongs to the omega-6 group. The products of these different fatty acids vary in their number of double bonds, and have greatly different physiological effects.
The eicosanoids produced from arachidonic acid tend to promote inflammation. In contrast, those from the omega-3 group tend to either cause less inflammation or prevent it, depending on the molecule produced. There is evidence that increased dietary omega-3 fatty acids promotes good cardiovascular health and helps to lower triglyceride levels.
Different eicosanoids are involved in many varying aspects of mammalian physiology. Among other things, they help regulate blood pressure, modulate the immune system, and affect blood clotting. For instance, the reason many people take aspirin daily is to inhibit a particular thromboxane from stimulating blood clotting, thus lowering the chances of a heart attack.
Since inflammation can cause painful symptoms in certain diseases, such as arthritis, inhibitors of eicosanoid synthesis have been used to block the production of certain eicosanoid molecules. The focus of pharmaceutical inhibitors has been on blocking prostaglandin and leukotriene activities. Perhaps the best-known inhibitors of eicosanoid production are those that inhibit COX activity, thus inhibiting prostaglandin production.
There are two different types of COX, however, with the products of COX-1 being vital to functions like protecting the stomach lining. It is the inducible products of COX-2 that are responsible for painful inflammation. NSAIDs, such as aspirin, block the activities of both COX-1 and COX-2, and can cause peptic ulcers.
Specific types of COX-2 inhibitors have been developed, but there is concern about side effects, like an increased tendency towards stroke and heart attack. Alternative anti-inflammatory drugs are adrenal steroids. These block the activity of one of the key phospholipases in the initial stages of eicosanoid biosynthesis, and so inhibit the production of a number of eicosanoids. This is a major reason why treatment with steroids can produce so many dangerous side effects.
Leukotrienes also produce inflammation. Pharmaceutical drugs have been developed to block leukotrienes from binding to their specific signaling sites. Drugs such as zafirlukast and montelukast help in the management of diseases, like rheumatoid arthritis and asthma, that frequently involve inflammation. The United States Food and Drug Administration (FDA) has issued a warning about potential neuropsychiatric problems with the use of these drugs, however.
