ASPIRIN Session Two of TIP Assignment
History Behind Aspirin Development 2 Pain relief is something that has been sought after since the ancient Greeks and Egyptians used bark and dried leaves of the poplar tree to treat back and abdominal pain. As early as 1850 B.C. the Ebers papyrus recorded the use of plant remedies for the treatment of pain and inflammation. A published Report in 1763 identified willow bark as having analgesic properties. The active component of willow bark was later identified as salicin, which is metabolized to salicylate. In 1832, the French chemist Charles Gerhardt experimented with salicin, generating salicylic acid, and in 1860 Kolbe and Lautemann developed a highly efficient method for the synthesis of salicylic acid from phenol, which led to the use of the compound in the general population as an antiseptic and antipyretic. In 1897, Felix Hoffman from the Bayer Company developed a more palatable form of salicylate by synthesizing acetylsalicylic acid, which was called aspirin and distributed by Bayer in tablet and powder form in 1899.
Aspirin as a drug 3 Salicin of willow bark was used medicinally. Active ingredient was found to be salicylic acid. Salicin is hydrolyzed to saligenin and glucose by β-glucosidase. Saligenin is then slowly oxidized to salicylic acid in the liver. More immediate effect achieved by ingestion of salicylic acid or its salts. Na salts of salicylic acid was used as a pain killer in the 19 th Century despite causing stomach irritation. Acetyl salicylic acid was synthesized as a less irritating derivative. (Glaser, 2001)
Functional groups 4 GI effects seems to be caused by carboxyl group. Reducing the acidity of this group (e.g., converting to an amide, salicylamide) maintains the analgetic actions of salicylic acid derivatives but eliminates the anti-inflammatory properties. Substitution on either the carboxyl or phenolic hydroxyl groups may affect potency and toxicity. Benzoic acid itself has only weak anti-inflammatory activity. Placing the phenolic hydroxyl group meta or para to the carboxyl group abolishes this activity. Substitution of halogen atoms on the aromatic ring enhances potency and toxicity Substitution of aromatic rings at the 5-position of salicylic acid increases anti-inflammatory activity.
5 Crystalline structure and hydrogen bonding of aspirin H-Bonding (Glaser, 2001)
Absorption and Metabolism of aspirin 6 Salicylates are weak acids (acetylsalicylic acid pk a = 3.5), absorption generally takes place primarily from the small intestine and, to a lesser extent, from the stomach by the process of passive diffusion of unionized molecules across the epithelial membranes of the GI tract. Thus, gastric ph is an important factor in the rate of absorption of salicylates. Any factor that increases gastric ph (e.g., buffering agents) will slow its rate of absorption, because more of the salicylate will be in the ionized form. Tablet formulations consisting of small particles are absorbed faster than those of larger particle size. Salicylates are highly bound to plasma protein albumin, with binding being concentration dependent. At low therapeutic concentrations of 100 µg/ml, approximately 90% of aspirin is plasma protein bound, whereas at higher concentrations of approximately 400 µg/ml, only 76% binding is observed
Absorption and metabolism of aspirin 7 continued Aspirin is rapidly absorbed largely intact from the stomach and upper small intestine on oral administration but is rapidly hydrolyzed by plasma esterases. Peak plasma levels usually are achieved within 2 hours after administration. Increasing the ph of the stomach by the addition of buffering agents may affect absorption, because the degree of ionization will be increased.
8 Metabolism of salicylic acid derivatives (Lemke, Williams, Roche, & Zito, 2008)
Mechanism of action of aspirin 9 In 1971, Vane discovered that aspirin interferes with the biosynthesis of prostaglandins (Won Nobel Prize in 1982). The biosynthesis of prostaglandins depends on the enzymes cyclooxygenase 1 (COX-1, aka PGH 2 synthase 1) and cyclooxygenase 2 (COX-2, aka PGH 2 synthase 2), Aspirin is an inhibitor of COX-1 and COX-2 Aspirin, however, is 10 to 100 times more potent against COX-1 than against COX-2 COX-1 is employed for normal, physiological prostaglandin synthesis, and COX-2 makes prostaglandins in inflammatory cells
Side effects 10 disturbances of the GI tract Gastric bleeding induced by salicylates generally is painless but can lead to fecal blood loss and may cause a persistent iron deficiency anemia The mechanism by which salicylates cause gastric mucosal cell damage may be caused by a number of factors, including gastric acidity, ability of salicylates to damage the normal mucosal barrier that protects against the back diffusion of hydrogen ions, ability of salicylates to inhibit the formation of prostaglandins (particularly those of the PGE series, which normally inhibit gastric acid secretion), and inhibition of platelet aggregation (leading to an increased tendency toward bleeding). Reye's syndrome is an acute condition that may follow influenza and chickenpox infections in children from infancy to their late teens, with the majority of cases occurring between the ages of 4 and 12 years. the U.S. Food and Drug Administration (FDA) has proposed that aspirin and other salicylates be labeled with a warning against their use in children younger than 16 years with influenza, chickenpox, or other flu-like illness.
Drug Interactions 11 Acetylsalicylic acid is a weak acid that is highly bound to plasma proteins (50 80%), and it will compete for these plasma protein binding sites with other drugs that are highly bound to these sites. The plasma concentration of free anticoagulant increases in the presence of salicylates, necessitating a possible decrease in the dosage of anticoagulant required to produce a beneficial therapeutic effect Salicylates may inhibit the synthesis of prothrombin by antagonizing the actions of vitamin K The incidence and severity of GI ulcerations may be increased if corticosteroids, salicylates, and NSAIDs are administered together.
Aspirin dosage 12 Usual dose Effect 80 160 mg Antiplatelet 325 1000 mg Analgesic, antipyretic 325 mg 6 grams Anti-inflammatory, tinnitus 6 10 grams Respiratory alkalosis 10 20 grams Fever, dehydration, acidosis > 20 grams Shock, coma
Aspirin Summary 13 Analgesic Antiinflammatory Benefits in addition to analgesia Inhibits COX Irreversible inhibitor of COX Nonselective inhibitor of COX (adverse effects) Drug-drug interactions Reye s syndrome
References 14 Much of the text in this PowerPoint has been adapted from the following resources: Glaser, R. (2001). Aspirin. An ab Initio Quantum- Mechanical Study of Conformational Preferences and of Neighboring Group Interactions. Journal of Organic Chemistry, 66 (3), 771 779. Lemke, T. L., Williams, D. A., Roche, V. F., & Zito, S. W. (2008). Foye's Principles of Medicinal Chemistry. Philadelphia: Wolters Kluwer