NSAIDS Notes Full PDF

UNIVERSITY OF NAIROBI COLLEGE OF HEALTH SCIENCES SCHOOL OF PHARMACY In collaboration with CENTRE FOR OPEN AND DISTANCE LEARNING DEPARTMENT OF PHARMACEUTICAL CHEMISTRY UPC 213: PHARMACEUTICAL CHEMISTRY II: NON-STEROIDAL ANTI-INFLAMMATORY DRUGS AUT...

UNIVERSITY OF NAIROBI COLLEGE OF HEALTH SCIENCES SCHOOL OF PHARMACY In collaboration with CENTRE FOR OPEN AND DISTANCE LEARNING DEPARTMENT OF PHARMACEUTICAL CHEMISTRY UPC 213: PHARMACEUTICAL CHEMISTRY II: NON-STEROIDAL ANTI-INFLAMMATORY DRUGS AUTHOR GRACE N. THOITHI  LECTURE ONE GENERAL CLASSIFICATION OF DRUGS USED TO RELIEVE PAIN Lecture Outline 1.1 Introduction 1.2 Objectives 1.3 Definition of Pain 1.4 Classification of Pain 1.5 Definition of Analgesics 1.6 Classification of Analgesics 1.6.1 Mild Analgesics 1.6.1.1 Analgesic – Antipyretic Drugs 1.6.1.2 Anti-inflammatory Drugs 1.6.2 Strong Analgesics 1.6.2.1 Mu Receptor Agonists 1.6.2.2 Mixed Agonist/Antagonist Analgesics 1.6.2.3 Mu Receptor Antagonist 1.6.2.4 Other Analgesics 1.7 Summary 1.8 References 1.1 Introduction This is our first lecture in a series of lectures during which you will learn about some of the drugs that are used to relieve pain. I will tell you about the classification of pain. In order for you to understand the classification of pain, you need to remember the physiology of the nervous system. Thereafter in this lecture you will learn that analgesics can be classified into ‘mild’ and ‘strong’ analgesics. In this lecture I will give you an overview of classification of analgesics, with a few examples, but we will not look at the details of these drugs. Once you have an overview of the classification of all drugs used for relieving pain, we will start the lectures on non-steroidal anti-inflammatory drugs 1 (NSAIDs). Our aim in this module is to learn about drugs that are used for treatment of mild pain (mild analgesics), of which the biggest category is the non-steroidal anti- inflammatory drugs (NSAIDs). In lectures 2-10, we will see that the NSAIDs have analgesic, antipyretic and anti-inflammatory activities. After that we will discuss analgesic-antipyretic drugs in lecture 11. Lastly, we will discuss drugs that are used for treatment of arthritis and gout in lectures 12 and 13, respectively. Steroids are also useful anti-inflammatory drugs and you will learn about them in another module. Let us now look at the objectives of this lecture. 1.2 Objectives At the end of this lecture you should be able to: 1. Explain what pain is. 2. Discuss the classification of pain. 3. Explain what analgesics are. 4. Describe the various classes of drugs that are in clinical use for relieving pain. 1.3 Definition of Pain Intext Question Have you ever experienced pain? What was it like? What are some of the thoughts that have come to your mind after considering the above question? It is hard to define pain. Pain can be considered to be a syndrome of highly unpleasant sensations, usually with two components: the initial sensation or actual perception; and the psychological effect or reaction component. This later component involves mental modifications and therefore accounts for the variation in the reaction of different people or even variation in the reaction by the same individual to identical pain 2 stimuli. Pain may be described as throbbing, gnawing, splitting, stabbing, pinching, crushing or burning. I believe you now understand pain better. 1.4 Classification of Pain Having understood what pain is, how do we classify pain? There are different ways of classifying pain. One of the common ways is by considering the origin of the pain, and hence the classification into visceral and somatic pain. 1. Visceral pain is of non-skeletal origin and examples are gastric pain, intestinal cramps and colic. 2. Somatic pain emanates from muscle and bone and includes headaches, sprains and arthritic pain. Another way of classifying pain is by considering whether the pain is acute or chronic. 1. Acute pain may be produced by disease, injury, noxious chemicals or physical stimulation. 2. Chronic pain is persistent and pathological in form. The etiology, pathophysiology, functions and diagnosis of acute pain differs from that of chronic pain and so do the means of treatment. Let us now consider the drugs used to relieve pain. 1.5 Definition of Analgesics Drugs that relieve pain are commonly called analgesics or analgetics, but most people prefer to use the term analgesics. These two words are derived from the Greek word meaning ‘painlessness’. Analgesics are also referred to as ‘antinociceptives’. Traditionally, they have been classified into two main categories, ‘mild’ or ‘strong’ analgesics. However, it is worth noting that advancement in drug development has defied this strict separation of the two categories. Today, drugs exist that chemically belong to the ‘mild’ category but whose potency exceeds that of some that are chemically in the ‘strong’ analgesics category. In addition, most traditional ‘strong’ analgesics were addicting, but today there are drugs derived from narcotic (strong) analgesics which are not addicting. Having said this, we shall still use this traditional 3  classification of ‘Strong analgesics’ and ‘Mild analgesics’ for ease of reference. A third main class of drugs that relieve pain is the ‘local anaesthetics’, the prototype of which is codeine. Note that the term ‘anaesthesia’ means loss of all sensation. Local anaesthetics interrupt impulse transmission in all nerve fibres when applied locally near the nerve trunk. In this lecture, we will discuss the classification of weak and strong analgesics. Later in this module, we will discuss in detail the mild analgesics. You will learn details about the strong analgesics and the local anaesthetics in later years of your pharmacy training. 1.6 Classification of Analgesics 1.6.1 Mild Analgesics Intext Question Try and think about a time that you had a headache. What medicine did you take, if any? Where did you obtain it from? Concerning your answer to the above question, I believe that if you used a medicine for your headache, it was an analgesic of the ‘mild’ type. Mild analgesics generally fall into either the analgesic-antipyretic class or to the non-steroidal anti-inflammatory class. Let us look at an overview of these two classes. 1.6.1.1 Analgesic -Antipyretic Drugs Drugs in this class have the ability to alleviate mild, and sometimes, severe pain. They are also effective in relieving fever in febrile subjects. The prototype in this class is paracetamol or acetaminophen (Panadol®, Tylenol®). We will learn more about these medicines in lecture 11. 1.6.1.2 Anti-inflammatory Drugs This class of compounds is made up of drugs with variable and interesting chemical features. However, they all have the ability to alleviate pain (analgesic) and relieve fever 4 (antipyretic) in febrile subjects. In addition, they all have anti-inflammatory activity. Some examples are the salicylates, aryl acetic acids, indole acetic acids, fenamates, pyrazolidines and oxicams which we will discuss in lectures 4-9. An emerging sub-class of NSAIDs are the selective cyclo-oxygenase inhibitors, which we will learn about in lecture 10. 1.6.2 Strong analgesics Intext Question Which drugs are considered to be strong analgesics? Strong analgesics are the most effective drugs for alleviation of severe acute or chronic pain. They are often called ‘opioid analgesics’. The term ‘opioid’ refers to opium- or morphine-like in terms of pharmacological actions. They are sometimes referred to as narcotic analgesics. The prototype is morphine. You will learn more about opioid analgesics in other modules in the coming years of study. Take Note The word ‘narcotic’ literally means that they cause sleep or loss of consciousness (narcosis) and it is also associated with addictive properties. The term ‘narcotic analgesics’ is thus misleading because some of these drugs induce analgesia without narcosis and not all opioids are addicting. 1.6.2.1 Mu Receptor Agonists Drugs in this class act on the mu (µ) opioid receptor and they have the ability to alleviate severe pain. They include morphine, codeine, dihydroxymorphine, dihydrocodeine, oxymorphone, oxycodone, levopharnol, meperidine, methadone, fentanyl, sufentanil, among others. 5 1.6.2.2 Mixed Agonist/Antagonist Analgesics In this class we have important drugs like buprenorphine, butorphanol, nalbuphine, pentazocine and dezocine. Their activity at the µ,  and  opioid receptors may vary. 1.6.2.3 Mu Receptor Antagonist Some mu (µ) opioid receptor antagonists have analgesic activity. Naloxone and naltrexone belong to this class. 1.6.2.4 Other Analgesics Tramadol and propoxyphene are examples of strong analgesics in common use. Activity 1 Make a list of all the classes of drugs we have discussed in this lecture. 2 For each class, name one example (prototype). 3 List the pharmacological activities of each class. 4 Give the mechanism of action of each class. 5 Give the major side effect(s) of each class of drugs. 1.7 Summary In this lecture you have learnt that: 1 Pain is a syndrome of highly unpleasant sensations and it is often treated depending on whether it is acute or chronic, and whether it is mild or severe. 2 Analgesics are broadly classified into ‘mild’ or ‘strong’ analgesics. 3 The two main categories of mild analgesics are antipyretic analgesics and anti-inflammatory analgesics, of which examples are paracetamol and aspirin, respectively. 4 The strong analgesics are also called opioid analgesics and are related to morphine in activity. 5 Local anaesthetics also have analgesic activity 6 1.8 References 1. Introduction to Medicinal Chemistry. Alex Gringauz. Wiley-VCH. New York. 1997 2. Foye’s Principles of Medicinal Chemistry, 6th ed. Thomas L. Lemke, David A. Williams, Victoria F. Roche and S. William Zito. Wolters Kluwer Health/Lippincott Williams and Wilkins. Philadelphia. 2008. 3. Textbook of Pharmacology. 2nd ed. 1980. W. C. Bowman and M. J. Rand. Blackwell Scientific Publications. Oxford. 4. Goodman & Gilmans. 2006. The pharmacological basis of therapeutics. 12th Ed. Laurence L. Brunton, Bruce A. Chabner, Bjorn C. Knollmann. McGraw-Hill Comp. Inc. 7  LECTURE TWO CLASSIFICATION AND MECHANISM OF ACTION OF NSAIDS Lecture Outline 2.1 Introduction 2.2 Objectives 2.3 Definition of NSAIDs 2.4 Mechanism of NSAIDs 2.4.1 Anti inflammatory mechanisms 2.4.1.1 Inflammation 2.4.1.2 Anti-inflammatory action 2.4.2 Anti-pyretic mechanism 2.4.2.1 Fever 2.4.2.2 Antipyretic action 2.4.3 Analgesic mechanism 2.4.3.1 Pain 2.4.3.2 Analgesic action 2.4.4 Mechanism for GIT damage 2.4.4.1 Mucoprotection 2.4.4.2 Mucosa damage 2.5 Approaches to reduce gastric damage 2.5.1 Molecular approach 2.5.2 Pharmaceutical approach 2.6 Classification of NSAIDs 2.6.1 Non-selective cyclo-oxygenase inhibitors 2.6.1.1 Carboxylic acid derivatives 2.6.1.2 Enolic acids 2.6.2 Selective cyclooxygenase II inhibitors 2.7 Uses of NSAIDs 2.8 Summary 2.9 References 8 2.1 Introduction In our first lecture, you learnt what pain is and how it is classified. I also explained that drugs that relieve pain are called analgesics and gave the general classification of analgesics. Mild analgesics can either belong to the antipyretic/analgesics class or to the anti-inflammatory analgesics class, of which examples are paracetamol and aspirin, respectively. The strong analgesics are also called opioid analgesics and are related to morphine in activity. Local anaesthetics also have analgesic properties. In this lecture you will learn about the non-steroidal anti-inflammatory drugs (NSAIDs). In addition to being analgesics, the NSAIDS have anti-pyretic and anti-inflammatory activity. We will learn that their activity is due to inhibition of cyclo-oxygenase, an enzyme involved in mediation of pain and inflammation. You have already covered the module on prostaglandins and you will need to remember the synthesis of prostaglandins and the role of cyclo-oxygenase. The activity of NSAIDs is related to the chemical nature of the drugs and therefore we will learn that they are classified depending on their structural features. Lastly, you will learn about the uses of NSAIDs. 2.2 Objectives At the end of this lecture you should be able to: 1 Explain what the non-steroidal anti-inflammatory drugs are. 2 Discuss the mechanism of action of the non-steroidal anti- inflammatory drugs as analgesic, antipyretic and anti-inflammatory agents. 3 Discuss the mechanism of NSAIDs in causing gastrointestinal damage. 4 Discuss molecular and pharmaceutical approaches to drug design that are used to reduce gastric damage. 5 Describe the various classes of non-steroidal anti-inflammatory drugs. 6 Discuss the general uses of NSAIDs. 9 2.3 Definition of NSAIDs The non-steroidal anti-inflammatory drugs are a group of agents that generally in addition to having analgesic activity, also have antipyretic and anti-inflammatory activity. The NSAIDs are largely used for management of inflammation especially in rheumatic disorders. Inflammation, just like allergies, is considered a defence mechanism to noxious stimuli that threaten the host. For this reason, NSAIDs, antiallergic agents and antiulcer agents are considered to be drugs that affect the immune system. Take note 1 NSAIDs are useful in the treatment of rheumatic disorders. 2 A few of the NSAIDs have uricosuric properties and are useful for the treatment of gout. 2.4 Mechanisms of Action of NSAIDs Intext Question What are the mechanisms by which NSAIDs elicit pharmacological actions? The NSAIDs treat rheumatic conditions by alleviating inflammation (anti-inflammatory), alleviating pain (analgesic) and relieving fever (anti-pyretic). Let us now look at the details involved in each of these mechanisms. You can now assess your answer to the intext question with the discussion I have given below. 2.4.1 Anti-inflammatory mechanisms 2.4.1.1 Inflammation Inflammation is a normal and essential response when there is injury to tissue, when microorganisms invade the host or when there are other diseases or injuries. Inflammation has several components which often follow each other: Release of inflammatory mediators e.g. histamine, bradykinins, 5HT (serotonin), prostaglandins and leukotrienes. 10  Vasodilation. Increased vascular permeability and exudation. Leucocyte migration, and chemotaxis and phagocytosis. Proliferation of connective tissue cells. These maintain the inflammation, which manifests itself as redness, swelling, leaky capillaries and fluid moves out of blood to tissues. Figure 2.1 shows inflammation of the skin. Figure 2.1. Inflammation of the skin ( Source: www.howtogetridofstuff.com). Now that you have understood the outward manifestations of inflammation, let us consider what happens at the molecular level when inflammation occurs. The NSAIDS act as anti-inflammatory agents by inhibition of the enzyme cyclooxygenase and so they inhibit prostaglandin synthesis. Other agents, such as antihistamines, a common example being chlorpheniramine, relieve allergic inflammation. Still others, like lipooxygenase inhibitors, inhibit leukotrienes synthesis. Activity Before you proceed, try and refresh your memory with the following activities: 1 Draw a diagrammatic representation of the synthesis of prostaglandins D, E and F from arachidonic acid. 2 Show all the enzymes involved. 11 2.4.1.2 Anti-inflammatory action In order to understand how the NSAIDs work, let us have a look at the biosynthesis of prostaglandins, as shown in Figure 2.2. Most eicosanoids are derived from arachidonic acid. Arachidonic acid can be acted upon by lipooxygenase to produce leukotrienes or by cyclo-oxygenase to produce prostaglandins PGG2 and PGH2, which give PGD2, PGE2 and PGF2. Prostaglandins A, B and C are unsaturated ketones that arise non- enzymatically from PGE. Prostacyclin (PGI2) has an additional tetrahydrofuran ring which is very unstable and very easily opened up and deactivated. Thromboxane A2 is non-enzymaticaly converted to thromboxane B2. Cyclooxygenase: Three isoforms of cyclooxygenase have been identified: cyclooxygenase-1, cyclooxygenase-2 and cyclooxygenase-3 (COX-1, COX-2 and COX- 3). Both COX-1 and COX-2 are very similar in structure and almost identical in length. Therapeutically, the major difference between them is in their physiological function rather than structure. Cyclooxygenase-1 (COX-1): Cyclooxygenase-1 is continually expressed and it seems to be more specific than COX-2 for fatty acid substrates, and it primarily metabolizes arachidonic acid. COX-1 is useful for maintaining normal physiological functions in the GIT and kidneys by producing cytoprotective prostaglandins (PGE2) which: stimulate bicarbonate and mucus secretion reduce acid production maintenance of kidney function Therefore, inhibition of COX-1 by non-selective NSAIDs leads to side effects such as gastric and renal damage. Cyclooxygenase-2 (COX-2): The expression of COX-2 is induced by cytokines (in vascular smooth muscle, fibroblasts and epithelial cells) and growth factors. Cyclooxygenase-2 accepts a wider range of fatty acid substrates than COX-1 and it metabolizes C-18 and C-20 fatty acid substrates. Selective inhibitors of COX-2 do not bind Arginine-120, which is used by the carboxylic acid group of arachidonic acid and 12 the NSAIDs with a carboxylic acid group (both selective and non-selective). COX-2 functions to produce prostaglandins at inflammatory sites. The treatment of inflammatory disorders by selective COX-2 inhibitors is through inhibition of inducible COX-2 but not the continually expressed COX-1. Cyclooxygenase-3 (COX-3):. Cyclooxygenase-3 is the third and most recently discovered cyclooxygenase isoenzyme, but it is not functional in humans. It is selectively inhibited by analgesic-antipyretic drugs and is potently inhibited by some drugs. We will discuss the analgesic-antipyretics in lecture 11. 2.4.2 Anti-pyretic Mechanisms 2.4.2.1 Fever Body temperature is regulated in the hypothalamus. If there is an infection, body injury, inflammation or other disease states, there is enhanced formation of cytokines (such as interleukin-1) by cells. This induces formation of PGE2 in vascular organs in the pre- optic hypothalamic area and this leads to elevated body temperature via cyclic adenosine monophosphate (cAMP). 2.4.2.2 Antipyretic action Intext Question We have seen that the hypothalamus controls body temperature. How do drugs that reduce fever act? Drugs that act as antipyretics do so by inhibition of PGE2 synthesis at cyclooxygenase level. They may act peripherally or centrally. At the periphery they inhibit the release of endogenous leukocytic pyrogens from cells that are activated by exogenous pyrogens and/or inhibit activation of cells by exogenous pyrogens. At the central level they act by direct competitive inhibition of endogenous pyrogens on hypothalamus and/or inhibition of prostaglandins in the central nervous system. 13 2.4.3 Analgesic mechanism 2.4.3.1 Pain Intext Question How did we describe pain in lecture 1? In lecture one, we said that pain is a manifestation of highly unpleasant sensations, which may be described as throbbing, gnawing, splitting, stabbing, pinching, crushing or burning. In fact, there are different levels of pain as follows: Mechanical or thermal stimuli activate high threshold nociceptors and this sends an impulse to the brain through C nerve fibres and the host withdraws from the source of pain. However, if the stimulus is severe or prolonged there is release of pro-inflammatory agents (prostaglandins, cytokines e.g. interleukin 1 and humoral factors). This release is mediated by substance P, histamine and serotonin and it results in sensitization of nociceptors (pain receptors), thus giving primary hyperalgesia or physiological pain. Secondary hyperalgesia: Large quantities of chemical mediators lower pain threshold and increase excitability of neighbouring nerve endings and pain spreads to areas originally not affected by stimulus. So small stimulus now becomes very painful and gives secondary hyperalgesia or pathological pain. Peripherally, prostaglandins sensitize peripheral pain receptors and so NSAIDs may help to restore normal activity to sensitized nociceptors by inhibiting prostaglandins. Centrally, prostaglandins help pain signal to pass through the central nervous system. Pain passes from the periphery into the spinal cord through pain fibres and requires neurotransmitters such as substance P, glutamate aspartate and neurokinins. Prostaglandins increase conductivity of peripheral pain and central pain fibres. 14 2.4.3.2 Analgesic Action Non-steroidal anti-inflammatory drugs inhibit spinal hyperalgesia through inhibition of prostaglandin synthesis. Generally, NSAIDs have peripheral action although some have central action as well. Endogenous opiates, such as endorphins, inhibit transmission of pain in brain. Below are some examples of NSAIDs activity: It has been demonstrated that aspirin reduces conductivity of pain by prostaglandins Diclofenac has been shown to decrease pain if injected in the thalamus. Ketoprofen inhibits prostaglandins both peripherally and centrally. It is thought that cyclooxygenase 1 may be involved in chronic inflammation so some degree of COX-1 inhibition may be desirable. 2.4.4 Mechanism for GIT Damage Activity Before you proceed, stop and refresh your memory: o Using the knowledge you have on prostaglandins, outline the mechanisms by which COX-1 protects the gastrointestinal mucosa. 2.4.4.1 Mucoprotection Prostaglandin synthetase is present in the gastro-intestinal wall and therefore there are PGEs and PGFs in the GIT. The gastric mucosa secretes PGE into gastric juice and these prostaglandins are responsible for maintaining gastric integrity and protection, a property referred to as cyto-protection. This protection possibly extends to the duodenum, colon, bladder and even other organs. Prostaglandins protect gastric mucosa by inhibiting 15 histamine stimulated hydrochloric acid secretion and increasing bicarbonate and mucus secretion/viscosity, increasing secretion of water into mucosa and increasing blood flow. 2.4.4.2 Mucosa Damage Have you ever heard a person who has taken drugs like aspirin (Ascard®) or ibuprofen (Brufen®) complaining of hyperacidity in the stomach? This is a common complaint in patients who take these medicines. Non-steroidal anti-inflammatory agents damage the mucosa by the so called “dual insult” mechanism. Let us see what this means and how it happens. 1. The first ‘insult’ is because many NSAIDS are organic acids with direct acid damage on mucosa. They also inhibit Cox-1 and this results in increased hydrochloric acid secretion. The weak organic acids such as aspirin cause ‘ion trapping’. Ion trapping means that at the pH of stomach they are non-ionised and easily penetrate mucosal cells. In the cell, the pH rises so they are ionized and cannot escape, so protons enter the cells and cause damage. 2. The second way of ‘insult’ is by inhibition of cyclooxygenase, which leads to prostaglandin inhibition. In addition, inhibition of cyclooxygenase may lead to activation of 5-lipoxygenase which results in production of leukotrienes which generate super oxides which damage mucosa. Figure 2.2 illustrates damage to the stomach mucosa. 16  Figure 2.2. Picture of a peptic ulcer (Source: www.emedicinehealth.com) 2.5 Approaches to Reduce Gastric Damage The pharmacist, as the professional who is responsible for management of the effects of drugs in the patient, is concerned that if a patient has to benefit from a drug, the side effects should be controlled. For this reason, during drug development two broad approaches have been used to reduce gastric damage. These approaches are at the molecular and/or the pharmaceutical level. 2.5.1 Molecular Approach The molecular approach involves designing a drug with a chemical structure which will not lead to gastric damage. At the molecular level several approaches have been used as follows: Prodrugs: The drug may be synthesized as a pro-drug which is not acidic, but which releases the active drug in the body. An example is nabumetone which we shall discuss in lecture five. Non-acidic drugs: Drug design may also focus on getting drugs which have no acidic group. Sulindac, which we shall discuss in lecture 6, is an example. Cyclooxygenase 2 selective drugs: This class of compounds, which we shall discuss in lecture 10, is designed in such a way as to inhibit only cyclooxygenase 2, but not cyclooxygenase 1. It therefore does not inhibit the mucoprotective effects of prostaglandins. 2.5.2 Pharmaceutical Approach The pharmaceutical approach involves using pharmaceutical technology to eliminate or minimize mucosal damage. At the technology level the following approaches have been used: Enteric coated drugs: An enteric coating is a barrier applied to oral medication that controls the location in the digestive system where it is absorbed. Enteric refers 17  to the small intestine, therefore enteric coatings prevent release of medication before it reaches the small intestines. Drugs like ibuprofen are ‘enteric coated’ to minimize damage to stomach mucosa. Combination of a NSAID and a synthetic prostaglandin: The synthetic prostaglandin such as misoprostol supplies the muco-protective actions. One may therefore give a drug like ibuprofen together with misoprostol. Combination of NSAID ( insoluble) and a soluble molecule: An example is the combination of piroxicam with β-cyclodextrin to form a complex which thus reduces damage to the gastric mucosa. Combination of a NSAID and an antacid: The antacid neutralizes the acidity in the stomach. An example is compound magnesium trisilicate. Combination of a NSAID and an inhibitor of gastric secretion: The inhibitor of gastric acid secretion minimizes production of acid in the stomach. Such drugs include cimetidine and ranitidine. Community Activity Visit any community pharmacy and request the pharmacist for the following: 1. To show you samples of ibuprofen, misoprostol, compound magnesium trisilicate and cimetidine. 2. To give you literature inserts of these drugs for you to take away and read. 2.6 Classification of NSAIDs Non-steroidal anti-inflammatory agents can broadly be classified depending on how selective or non-selective they are in their interaction with the different isoforms of the enzyme cyclo-oxygenase. 2.6.1 Non-selective Cyclo-oxygenase Inhibitors 18 Non-selective cyclo-oxygenase inhibitors inhibit both cyclo-oxygenase-1 and cyclooxygenase-2. These can further be classified depending on how they were developed on the basis of structural similarities. They are all acidic in nature, either due to the presence of a carboxylic acid group or due to being enolic acids. 2.6.1.1 Carboxylic Acid Derivatives Salicylates Fenamates Aryl aliphatic acids Indole acetic acids 2.6.1.2 Enolic acids Pyrazolidinediones Oxicams 2.2.2 Selective Cyclooxygenase-2 Inhibitors Selective cyclo-oxygenase-2 inhibitors largely inhibit cyclo-oxygenase-2 but not cyclo- oxygenase-1. Structurally, they either belong to the ‘sulide’ or ‘coxib’ group. Sulides Coxibs 2.7 Uses of NSAIDs From our discussion, the NSAIDs are generally used as: Antiinflammatory agents for relief of inflammation Analgesics for alleviation of pain Antipyretic agents for alleviation of fever Antirheumatic agents which are used in rheumatic conditions Antigout agents for relief of gouty conditions 2.8 Summary 19  In this lecture you have learnt that the non-steroidal anti-inflammatory agents: 1. Have anti-inflammatory, antipyretic and analgesic activity. 2. Produce these activities largely due to the inhibition of cyclooxygenase. 3. Can be classified into non-selective cyclooxygenase inhibitors and selective cyclooxygenase-2 inhibitors. 4. May be used for treatment of gout and/or rheumatic conditions. 2.9 References 1. Introduction to Medicinal Chemistry. 1997. Alex Gringauz. Wiley- VCH. New York. 2. Foye’s Principles of Medicinal Chemistry, 6th ed. 2008. Thomas L. Lemke, David A. Williams, Victoria F. Roche and S. William Zito. Wolters Kluwer Health/Lippincott Williams and Wilkins. Philadelphia. 3. Textbook of Pharmacology. 2nd ed. 1980. W. C. Bowman and M. J. Rand. Blackwell Scientific Publications. Oxford. 4. Goodman & Gilmans. 2006. The pharmacological basis of therapeutics. 12th Ed. Laurence L. Brunton, Bruce A. Chabner, Bjorn C. Knollmann. McGraw-Hill Comp. Inc. 20  LECTURE THREE PHYSICO-CHEMICAL PROPERTIES OF NSAIDS Lecture outline 3.1 Introduction 3.2 Objectives 3.3 Physico-chemical properties of NSAIDs 3.3.1 Partition coefficient 3.3.2 Dissociation of drug 3.4 Absorption and distribution of NSAIDs 3.5 Summary 3.6 References 3.1 Introduction In lecture two, you learnt that in addition to being analgesics, the NSAIDS have anti- pyretic and anti-inflammatory activity. You learnt their mechanisms of action and that their activity is due to inhibition of cyclo-oxygenase, an enzyme involved in mediation of pain and inflammation. The activity of NSAIDs is related to the chemical nature of the drugs and therefore classified into non-selective cyclooxygenase inhibitors and selective cyclooxygenase 2 inhibitors. Lastly, you learnt that some NSAIDs can be used for treatment of gout and/or arthritis. Unlike many other drugs, the NSAIDS have to transverse a series of alternating lipophilic and hydrophilic compartments in order to reach their site of action. For this reason, their physical and chemical properties play a critical role in their transportation to these sites, and hence, their activity. In this lecture, we will look at these physicochemical properties and how they affect the distribution of NSAIDs. 3.2 Objectives 21  At the end of this lecture you should be able to: 1. Explain the physico-chemical properties of NSAIDs. 2. Discuss how the physico-chemical properties of NSAIDs affect their distribution in the body. 3.3 Physico-chemical Properties of NSAIDs Physico-chemical properties of NSAIDs play an important role in the ionization of drugs and hence their absorption and transportation to their sites of action. Before we proceed, let us think of the properties which are important. Activity Before you proceed, stop and refresh your memory: 1. Give the definition of the following terminologies: o pH o pKa o partition coefficient 2. How is partition coefficient determined? 3. Write an equation for the ionization of a weak acid and derive the Henderson-Hasselbach equation. 3.3.1 Partition Coefficient Intext Question What is partition coefficient?? 22 I believe that doing the activity given above has helped you to understand what partition coefficient is. Whenever you have a molecule X, which has access into two compartments, it will tend to distribute itself between the organic (lipophilic) and aqueous (hydrophilic) phases with a partition coefficient P, which is defined as the relative solubility of the molecule between the two phases. Partition coefficient is determined by dissolving the molecule in an aqueous solution and equilibrating it, with some shaking , with an organic solvent. P is defined as the partition between 1-octanol and a buffer at pH 7.4 (physiological pH). This is illustrated below: X X Aq org This partition coefficient can be defined as: P= Cxorg ÷Cxaq Where; Cxorg is the concentration of molecule x in the organic phase, Cxaq is the concentration of molecule x in the aqueous phase. Most drugs are not structurally similar to normal cellular molecules and so they will not be transported across membranes by active transport mechanisms. Their transport is a passive process determined by the partition coefficient. A ratio between lipid and aqueous phase of greater than 0.01 indicates good lipid solubility. 3.3.2 Dissociation of Drug At the same time, remember that in the aqueous phase, any molecule that is capable of ionizing will ionize. Substances that ionize completely are considered to be strong electrolytes, while those that do not are non-electrolytes. Most drugs are in between, being weak electrolytes and are only partially ionized. The ionized part has low lipid 23 solubility, while the non-ionized has higher lipid solubility. The fraction of the total drug concentration that is in the molecular and ionic forms can be indicated by the dissociation constant, Ka. Since the NSAIDs are mostly acids, we can represent their ionization as follows: HA H+ + A- Where, HA is a weak acid and represents the NSAID H+ is the cation (proton) from the ionization, If we take the logarithms of the above we get: log10Ka = log10[H+] + log10[A- ] - log10[HA] Therefore, -log10[H+] = -log10Ka + log10[A-] - log10[HA] This is the well known Henderson-Hasselbalch equation: By using pKa values, we are able to express the strength of an acid (its tendency to dissociate) with reference to the pH scale. If Ka, the dissociation constant, is large, then pKa will have a low numerical value. A strong acid is one which is largely, perhaps completely, dissociated, and which therefore has a high Ka value. Lower values of pKa (e.g. 0.7) correspond to stronger acids. A weak acid is one that is only slightly dissociated in solution, and has a low Ka value. Higher values (e.g. 4.7) correspond to weaker acids. Weak acids have higher pKa than strong ones, thus an acid with a pKa of 5 is 100 times weaker than that with a pKa of 3. The converse is true for bases. Therefore, 24 the distribution of the molecule (log P) is determined by the pKa of the compound and the pH of the medium through which it transverses. Take Note The distribution of the NSAIDs is a function of two parameters: The pKa of the drug The pH of the medium through which it transverses 3.4 Absorption and distribution of NSAIDs Let us look at the example of aspirin which is a drug that is orally taken. It is a weak acid with a pKa of 3.5 as shown on table 3.1. The stomach is highly acidic, with a pH of about 1, while the small intestine has a pH of about 6. The question is, is aspirin more likely to be absorbed in the stomach or in the small intestines? In the stomach, it would be almost completely un-ionized and very ionized in the small intestines. Since we know that the molecular form of the drug is more lipid soluble, the unionized form would be readily absorbed and this would happen in the stomach, which has lipoprotein membranes in its lining. The converse is true: weakly basic drugs have poor absorption in the stomach. However, all physicochemical parameters need to be considered when looking at the absorption of a drug. The other factors that affect absorption and bioavailabitlity are dosage formulation, food contents in the stomach, gastric emptying time, particle size and the presence of drugs like antacids. Indeed, for aspirin, the larger surface area of the small intestines allows absorption to take place primarily in the small intestines and not in the stomach as we would expect from our discussion above. For a NSAID, it has to go through a biological system from the GIT, to the GIT wall, the blood, the capillary wall, synovial fluid and finally the site of action. There are alternate aqueous and organic layers through which it has to transverse to get to the site of action. If a compound is too water soluble, it has a low P, and thus it will remain in the first aqueous layer it finds. If it is too lipid soluble, it has a high P, and it is retained in the first lipid layer. In both cases it will not get to the site of action. For this reason, a drug must 25 have an optimum P value to allow it to partition successively and to get to the site of action. P determines the pharmacokinetics and excretion. Most NSAIDs have similar log P value between 0 and 2 and pKa between 3 and 5. Table 3.1. pKa values of some non-steroidal anti-inflammatory drugs Drug pKa Drug pKa Aspirin 3.5 Indomethacin 4.5 Diflunisal 3.0 Mefenamic acid 4.2 Diclofenac 4.5 Phenyl butazone 4.5 Ibuprofen 5.2 Piroxicam 4.6 Take Note 1. Generally, only lipid soluble, undissociated forms of a drug pass through membranes easily. 2. Ionised species usually cannot pass unless a mediated transport system is present in the membrane. It has been found that the pH of inflamed tissues is less than that of blood and so there is a pH gradient between blood and the site of inflammation which favours movement of acidic compounds into inflamed tissues. 3.5 Summary In this lecture you have learnt that: 1. the distribution of the molecule is determined by the pKa of the compound and the pH of the medium through which it transverses. 2. the non-steroidal anti-inflammatory agents (NSAIDs) are weak acids with pKa 3-5. 3. NSAIDs are largely undissociated in gastric juice and they are dissociated in the small intestines. 4. NSAIDs have a good partition coefficient (P) of 0-2, which allows them to transverse membranes. 26 3.7 References 1. Introduction to Medicinal Chemistry. 1997. Alex Gringauz. Wiley- VCH. New York. 2. Foye’s Principles of Medicinal Chemistry, 6th ed. 2008. Thomas L. Lemke, David A. Williams, Victoria F. Roche and S. William Zito. Wolters Kluwer Health/Lippincott Williams and Wilkins. Philadelphia. 27  LECTURE FOUR SALICYLATES Lecture Outline 4.1 Introduction 4.2 Objectives 4.3 Definition of salicylates 4.3.1 History of aspirin 4.3.2 Absorption and bioavailability of aspirin 4.3.3 Side effects of aspirin 4.4 Mechanism of action of salicylates 4.5 Structure-activity relationships of salicylates 4.6 Structural modifications on aspirin 4.7 Commonly used salicylates 4.8 Synthesis of salicylates 4.9 Metabolism of salicylates 4.10 Analysis of salicylates 4.11 Summary 4.12 References 4.1 Introduction In our last lecture, we learnt that the NSAIDs have to transverse a series of alternating lipophilic and hydrophilic compartments in order to reach their site of action. Therefore, their physical and chemical properties play a critical role in their transportation to these sites, and hence these properties affect their activity. The non-steroidal anti-inflammatory agents (NSAIDs) are weak acids with pKa 3-5 and a partition coefficient (P) of 0-2. They are largely undissociated in gastric juice and they are dissociated in the small intestines. In this lecture, we will discuss the salicylates, the first and oldest class of the NSAIDs. We will see how their structure is related to their activity and their mechanisms of action. We will then discuss their synthesis, metabolism in man and how they are analyzed. 28  4.2 Objectives At the end of this lecture you should be able to: 1. Explain what the salicylates are. 2. Describe the relationship between the structure and the activity of salicylates. 3. Discuss the mechanism of action and uses of the salicylates. 4. Describe the synthesis of salicylates. 5. Discuss the metabolism of salicylates. 6. Demonstrate the analysis of salicylates. 4.3 Definition of Salicylates Intext Question What are salicylate non-steroidal anti-inflammatory drugs? The salicylate NSAIDs are derivatives of salicylic acid. O 6 1 5 OH 2 4 3 OH Figure 4.1. Chemical structure of salicylic acid 4.3.1 History of Aspirin The use of salicylates dates back to 19th century when salicylic acid was first obtained in 1838 from salicin, a glycoside found in willow (Salix alba) and poplar bark. By 1875, sodium salicylate was used as antipyretic-antirheumatic drug. In 1886 several phenyl esters were in use. Aspirin (acetyl salicylic acid) was prepared in 1853 and used as a medicine in 1899. Aspirin (acetyl salicylic acid) is the prototype salicylate and it is also 29 the most widely used analgesic, antipyretic and anti-inflammatory agent. It promotes uric acid secretion and is therefore used in treatment of gouty arthritis. Because it inhibits platelet aggregation, it is used in low doses to protect against stroke and heart attacks (Ascard-75®, Cardispirin®). COOH OCOCH3 Figure 4.2. Chemical structure of aspirin Activity Aspirin has been said to be the most widely used drug in the world. Using reference 2, list all the uses of aspirin that we have mentioned so far and give one use of aspirin that we have not yet mentioned. 4.3.2 Absorption and Bioavailability of Aspirin Aspirin is well absorbed, mainly from the small intestines. The rate of absorption depends on the dosage formulation, gastric pH and particle size of the formulation. Salicylates are protein-bound and this is important in drug interactions. 4.3.3 Side-effects of Aspirin The commonest side effect of aspirin is gastro-intestinal disturbances, including dyspepsia, duodenal bleeding, ulcerations and gastritis. This happens by inhibiting the formation of PGE1, which is cytoprotective. Another side effect associated with aspirin is Reye’s syndrome. This is a potentially fatal disease that affects multiple organs particularly the brain and the liver. Though it may affect adults, children of age 4-14 years are more susceptible, especially if they have had a viral illness like chicken pox or influenza. 30  Take Note Aspirin has also been found to be protective against colon cancer. 4.4 Mechanism of Action of Salicylates The major mechanism of action is inhibition of biosynthesis of prostaglandins by non- selective inhibition of the enzyme cyclo-oxygenase. They may also act by: Inhibition of biosynthesis of histamine Antagonizing functions of various kinins involved in inflammatory reactions Inhibiting release of lysosomal enzymes Inhibiting leukocyte migration 4.5 Structure-activity Relationships of Salicylates 1. The salicylate anion is the active moiety of all salicylate NSAIDs. O O OH Figure 4.3. Chemical structure of salicylate anion 2. The carboxylic acid (-COOH) group is essential for anti-inflammatory activity. Reducing acidity of -COOH maintains the analgesic properties but eliminates the anti- inflammatory properties. An example is salicylamide, which has analgesic but no anti- inflammatory activity.. O NH2 OH 31 Figure 4.4. Chemical Structure of Salicylamide 3. An ortho relationship between the -OH and the -COOH groups is essential for activity. Moving them from ortho to either meta or para position abolishes activity. 4. Halogen substituents on the salicylic acid ring enhance both potency and toxicity. An example is 5-chlorosalicylic acid. 5. Aryl groups (aromatic rings) at position 5 increase anti-inflammatory activity e.g. diflunisal. F COOH F OH Figure 4.5. Chemical structure of diflunisal (Dolobid®) 4.6 Structural Modifications on Aspirin Salicylates have severe gastrointestinal side effects and have short half-life, hence are administered frequently. Modifications have been made aimed at: Decreasing gastro-intestinal side effects Increasing duration of action The following drugs have been developed: 1. A prodrug of salicylic acid O O CH3 N O O Figure 4.6. Chemical structure of a prodrug of aspirin In this drug the -COOH and -OH groups are masked leading to a drug free of gastrointestinal toxicity. 32 2. A halogenated salicylic acid Cl COOH OH Figure 4.7. Chemical structure of 5-chlorosalicylic acid This drug is less polar than aspirin and therefore it has a longer duration of action. However it is more toxic; it causes greater gastrointestinal ulcerations 3. Introduction of a methyl group at position 3 protects against microsomal hydroxylation and thus increases duration of action. COOH COOH OH OCOCH3 CH3 CH3 3-methyl salicylic acid 3-methyl acetyl salicylic acid Figure 4.8. Chemical structures of 3-methylsalicylic acid and 3-methylacetylsalicylic acid 4. Introduction of halogenated aryl group increases anti-inflammatory activity as is the case for diflunisal (Figure 4.5). Take Note The highly non-polar aryl group of diflunisal retards metabolism and hence increases half-life. Diflunisal has greater anti-inflammatory activity than aspirin, but little antipyretic activity. It has no appreciable effect on platelet aggregation and therefore has no significant gastric bleeding side effects. It is used mainly in osteoarthritis, rheumatoid arthritis and other musculoskeletal pains. 33 5. Condensed aspirin molecules OCOCH 3 OCOCH 3 C O C O O Figure 4.9. Chemical structure of aspirin anhydride Aspirin anhydride was synthesized by condensing two aspirin molecules in the hope of: Achieving higher blood levels Having less gastrointestinal disturbances because the -COOH group is masked. However, it was found to be less active than aspirin and to have the same GI irritation since it is unstable and produces aspirin in aqueous media. 6. Aluminium-aspirin preparations Aloxiprin was developed by polymeric condensation of aluminium oxide and aspirin. Al2O3 [C6H4(OOCMe)COO-]5 Figure 4.10. Chemical formula of aloxiprin It was found to be more active than aspirin and it has less GI irritations because aluminium oxide neutralizes gastric acid. 7. Salicylamide In salicylamide (Figure 4.4) the carboxylate group responsible for anti-inflammatory activity is masked. It therefore has analgesic and antipyretic activities, but negligible anti- inflammatory activity. 8. Dimer of salicylic acid O COOH OH C O 34 Figure 4.11. Chemical structure of salsalate This is a dimer of salicylic acid and it is claimed to have fewer gastrointestinal side effects. 9. Prodrug of aspirin and paracetamol O O C Me O O N C Me H O Acetaminophen-aspirin prodrug Figure 4.12. Chemical structure of acetaminophen-aspirin prodrug (Benorylate®) Benorylate is an ester of aspirin and paracetamol (acetaminophen) and upon hydrolysis in the body gives the two drugs. It has the following properties: It has a longer duration of action than aspirin and only needs to be taken twice daily. It has fewer side effects than aspirin, including gastric irritation and bleeding. Mainly used to relieve joint pain and inflammation in osteoarthritis. It is sometimes used for juvenile rheumatoid arthritis. 4.7 Commonly used Salicylates The most common salicylates you will find in the market are aspirin, salicylamide, salsalate, diflunisal, sodium salicylate, sodium thiosalicylate, magnesium salicylate and choline salicylate. Activity Using reference 2, draw the structures of sodium salicylate, sodium thiosalicylate, Magnesium salicylate and choline salicylate. 35 4.8 Synthesis of Aspirin Aspirin can be synthesized by the Kolbe synthesis. Activity Using reference 3, discuss the Kolbe synthesis for synthesis of aspirin with the aid of a schematic diagram. 4.9 Metabolism of Salicylates The first step in the metabolism of aspirin and its derivatives is conversion to salicylic acid, which may be excreted in urine as the free acid (about 10%); or which can be conjugated with glycine to give salicyluric acid (about 75%) or with glucuronic acid to give the glucuronide ester or glucuronide ether (up to 15%). In addition, small amounts of salicylic acid undergo microsomal aromatic hydroxylation, of which the major product (of hydroxylation) is gentisic acid. Figure 4.13 illustrates the metabolism of salicylates. O O O Glu COOH COOH O M+ + OH O Glu OCOCH3 OH 15 % Aspirin Salicylate esters/salts Glucuronyl transferase UDP Glucuronic acid COOH O Acetic acid N-acyltransferase N CH2COOH OH H Salicylic acid Glycine OH Salicyluric acid (major metabolite, 75 %) HO COOH COOH HO COOH OH OH OH Gentisic acid, major OH OH Microsomal aromatic hydroxylation May then be conjugated with sulphate or glucuronic acid 36 Figure 4.13. Metabolism of aspirin and salicylate esters and salts 4.10 Analysis of Salicylates Salicylates are carboxylic acid derivatives and therefore they can be analyzed by titration as demonstrated below with the assay of aspirin. Assay of Aspirin: Samples of aspirin may, in addition to aspirin (acetylsalicylic acid, ASA), have the degradation products acetic acid (AA) and salicylic acid (SA). The assay of aspirin is therefore done in two steps to allow quantification of ASA and give the limits for the degradation products. In the first step aspirin is dissolved in dry ethanol and titrated immediately with 0.1 N NaOH. In this step all acids, ASA (i) , SA (ii) and acetic acid (iii), that can possibly be in an aspirin sample are neutralized to their respective sodium salts, acetylsalicylate sodium, sodium salicylate and sodium acetate. COOH COONa i) + NaOH + H2O O C CH3 O C CH3 O O COOH COONa Hydrolysate products present ii) + NaOH + H2O before dissolution OH OH iii) CH3COOH + NaOH CH3COONa + H2O Figure 4.14. Equations for reactions of the first step of analysis of aspirin In the second step of analysis of aspirin sodium hydroxide is added in excess (e.g. 50 ml) and the mixture is refluxed for 15 minutes. In this step, acetylsalicylate sodium is hydrolysed by NaOH to sodium salicylate and sodium acetate. Sodium salicylate and sodium acetate do not participate in this reaction. The excess NaOH is then back-titrated with 0.1 N HCl. 37  COOH COONa COONa 1 2 NaOH Excess NaOH O C CH3 O C CH3 OH Reflux O O + CH3COONa Figure 4.15 Equations for reactions of the second step of analysis of aspirin The first titration gives the total acids (ASA, SA and acetic acid) present. The second step gives the actual ASA present. The pharmacopoeia set the limit of the difference between NaOH used in the 1st and 2 nd steps as 0.4 ml based on titration of 500 mg of aspirin. This also serves as a limit test. It means that the degradation products (AA and SA) should not exceed the limit given in the pharmacopoeia. 4.10 Summary In this lecture you have learnt that: The salicylates group of non-steroidal anti-inflammatory agents have analgesic, antipyretic and anti-inflammatory activity. In addition, aspirin, which is the prototype promotes uric acid excretion, inhibits platelet aggregation and also protects against colon cancer. Salicylates are non-selective inhibitors of the enzyme cyclo- oxygenase. Salicylates are excreted largely unchanged, but can also be oxidized before excretion. They are often analyzed by titration using 0.1 N NaOH, by virtue of being acids. 38 4.11 References 1. Introduction to Medicinal Chemistry. 1997. Alex Gringauz. Wiley-VCH. New York. 2. Foye’s Principles of Medicinal Chemistry, 6th ed. 2008. Thomas L. Lemke, David A. Williams, Victoria F. Roche and S. William Zito. Wolters Kluwer Health/Lippincott Williams and Wilkins. Philadelphia. 3. Goodman & Gilmans. 2006. The pharmacological basis of therapeutics. 12th Ed. Laurence L. Brunton, Bruce A. Chabner, Bjorn C. Knollmann. McGraw-Hill Comp. Inc. 4. Medicinal Chemistry. 5th ed. 2010. Ashutosh Kar. New Age International Publishers. New Delhi. 5. The British Pharmacopoeia. 2010. www.pharmacopoeia.co.uk 39  LECTURE FIVE ARYL ALIPHATIC ACIDS Lecture outline 5.1 Introduction 5.2 Objectives 5.3 Definition of the aryl aliphatic acids 5.4 Classification of aryl aliphatic acids 5.4.1 Phenyl propionic acids 5.4.2 Phenyl acetic acids/aryl acetic acids 5.4.3 Heteroaryl acetic acids 5.5 Structure-activity relationships of aryl aliphatic acids 5.5.1 SAR of aryl aliphatic acids and indole acetic acids 5.5.2 SAR specific to aryl aliphatic acids 5.6 Mechanism of action of aryl aliphatic acids 5.7 Metabolism of aryl aliphatic acids 5.7.1 Metabolism of ibuprofen 5.7.2 Metabolism of diclofenac 5.8 Analysis of aryl aliphatic acids 5.9 Summary 5.10 References 5.1 Introduction In lecture four, I introduced you to the salicylate class of non-steroidal anti- inflammatory drugs. I explained that they have analgesic, antipyretic and anti- inflammatory activity. In addition, aspirin, which is the prototype, promotes uric acid excretion, inhibits platelet aggregation and also protects against colon cancer. Salicylates are non-selective inhibitors of the enzyme cyclo-oxygenase and they are excreted largely unchanged, but can also be oxidized before excretion. They are often analyzed by titration using 0.1 N NaOH, by virtue of being acids. 40 In this lecture, I will introduce you to the aryl aliphatic acids class of NSAIDs. Sometimes they are referred to as aryl alkanoic acids. We will see how their structure is related to their activity and their mechanisms of action. We will then discuss their synthesis, metabolism in man and the methods of analysis. 5.2 Objectives At the end of this lecture you should be able to: 1. Explain what the aryl aliphatic acids NSAIDS are. 2. Discuss the classification of aryl aliphatic acids 3. Describe the relationship between the structure and the activity of aryl aliphatic acids. 4. Discuss the mechanism of action of the aryl aliphatic acids. 5. Discuss the metabolism of aryl aliphatic acids. 6. Demonstrate the analysis of aryl aliphatic acids. 5.3 Definition of the Aryl Aliphatic Acids Intext Question What are aryl aliphatic acids? The aryl aliphatic acids are NSAIDs which are derivatives of aryl aliphatic acids and they have the general structure: R O Ar C C OH H 41 Figure 5.1. General structure of aryl aliphatic acids Where, R can be H, CH3 or alkyl Ar is an aryl group such as a phenyl or heteroaryl ring This is probably the largest group of NSAIDs and they all have anti-inflammatory, antipyretic and analgesic effects. Their activity is related to both the aliphatic acid moiety, as well as the aryl moiety of the drugs. They are non-selective inhibitors of the enzyme cyclo-oxygenase. They are excreted largely unchanged, but can also be oxidized before excretion. By virtue of being acids, they are often analyzed by titration. 5.4 Classification of aryl aliphatic acids Take Note There are many ways of classifying this group of NSAIDS. We shall categorize them into three broad classes, namely: o Phenyl propionic acids e.g. ibuprofen o Phenyl acetic acids e.g. diclofenac o Heteroaryl acetic acids e.g. indomethacin 5.4.1 Phenyl Propionic Acids This class is also referred to as ‘profens’. The prototype drug in this class of NSAIDs is ibuprofen, which is commonly marketed as Brufen® (Boots Company). In ibuprofen, R is a methyl group, while Ar is a substituted phenyl ring. From the structure of ibuprofen, you can see that it is actually a derivative of propionic acid. Other examples of drugs in this class include flurbiprofen, ketoprofen, naproxen, fenprofen and carprofen. 42  Me Me CH COOH CH COOH Me CH CH2 Me F Ibuprofen Flurbiprofen Me O Me CH COOH CH COOH OMe Ketoprofen Naproxen Me H Me O CH COOH N CH COOH Cl Carprofen Fenprofen Figure 5.2. Chemical structures of some phenyl propionic acids 5.4.2 Phenyl Acetic Acids/Aryl Acetic Acids An example of a phenyl acetic acid is diclofenac (Voltaren ®). Diclofenac is unique because it has the structural features of both the aryl aliphatic acids (specifically phenyl acetic acid) and the fenamates. Fenamates are derived from anthranilic acid. We shall discuss the fenamates in lecture seven. Until the development of the selective NSAIDs, diclofenac was the most potent and widely used NSAID. It is marketed as diclofenac sodium and diclofenac potassium. It is an inhibitor of both cyclo-oxygenase (I and II) and lipo-oxygenase enzymes. 43  CH2COOH NH Cl Cl Figure 5.3. Chemical structure of diclofenac 5.4.3 Heteroaryl Acetic Acids This class of drugs has a heteroaryl moiety and acetic acid moiety. Indomethacin can be considered to be both a heteroaryl acetic acid and an indole acetic acid (Figure 5.4). We shall discuss it in detail in lecture six when we discuss the indole acetic acids. Figure 5.4. Chemical structure of indomethacin 44 Me Cl Me CH2COOH CH2COOH N N O O Me Me Tolmetin Zomepirac COOH O N CH2CO2H O N CH2CH3 H C2H3 Ketorolac Etodolac Etodolac O CH2CH2CO2H N Oxaprozin Figure 5.5. Chemical structures of some heteroaryl acetic acids Other heteroaryl acetic acids include tolmetin, zomepirac, ketorolac, etodolac and oxaprozin. Zomepirac is an analogue of tolmetin which is four times more active than tolmetin, but has been withdrawn due to severe anaphylaxis. These drugs are sometimes classified as indole acetic acids because they have a heteroaryl nucleus. 5.5 Structure-activity relationships of Aryl Acetic Acids 5.5.1 SAR of Aryl Aliphatic Acids and Indole Acetic Acids The following structure–activity relationships apply to both the aryl aliphatic acids and the indole acetic acid classes of NSAIDs. We will discuss the indole acetic acids in lecture six. 1. These drugs must have an acidic center of activity to enable interaction with the receptors. 45  This center is often represented by carboxylic function, as in the structure of ibuprofen above. In some rare cases, the center can be enolic, hydroxamic acid, sulfonamide or a tetrazole ring. This center corresponds to the -COOH group of arachidonic acid. Figure 5.6. Chemical structure of arachidonic acid 2. Derivatives such as esters and amides may be active due to the metabolic hydrolysis products. An example is nabumetone, which has no acidic group but its activity is due to bioactivation to an active acidic metabolite. CH2CH2COCH3 O CH2CH2 COOH N CH3 O Oxaprozin Nabumetone Figure 5.7. Chemical structures of oxaprozin and nabumetone 3. The center of acidity must be located one carbon away from a flat surface which is represented by an aromatic or heteroaromatic ring. The aromatic ring system correlates with the double bonds at C5 and C8 of arachidonic acid. This is the case in ibuprofen, fenoprofen and ketoprofen. 46  Increasing this distance to two or three carbon atoms reduces activity, as in the case of oxaprozin and nabumetone. 4. Substitution of a methyl group on the carbon separating the acid centers from the aromatic ring increases activity. This gives α-methyl acetic acids commonly referred to as “profens”. Examples are ibuprofen, fenoprofen, ketoprofen, flurbiprofen and suprofen. F O CH3 CH3 S CHCOOH CH COOH Flurbiprofen Suprofen Figure 5.8. Chemical structure of flurbiprofen and suprofen Substitution with groups which are larger than a methyl group decreases activity. 5. This methyl group introduces a chiral center and the S(+) enantiomer is the active isomer. The R(-) enantiomer is inactive in vitro. Many manufacturers formulate racemates. In the case of ibuprofen, it has been found that the R enantiomer is biologically converted to S(+) isomer. 6. A second group which is lipophilic may be added as an aromatic ring or alkyl group either attached to or fused to the aromatic center. It is non-coplanar with the aromatic ring and it enhances activity. It seems to correspond to double bonds at C11 and C14 of arachidonic acid. 5.5.2 SAR specific to aryl aliphatic acids 47  The following structure–activity relationships apply only to the aryl aliphatic acids class of NSAIDs and not the indole acetic acids. 1. Substitution of α- methyl group on the alkanoic acid enhances anti-inflammatory actions and reduces many side effects. This is the case with all profens. For example, ibuprofen is more active and less toxic than ibufenac. CH2COOH CH2CH2CH3 Figure 5.9. Chemical structure of ibufenac 2. The aryl propionic acids are optically active. Although many are marketed as racemic mixtures, the S (+) isomer has been found to be responsible for most activity. 3. Derivatives of 2-naphthylpropionic acids are more potent than corresponding acetic acid analogs. 4. For the naphthyl derivatives such as naproxen, small lipophilic groups such as -Cl, -CH3 , -OCH F2 and -OCH3 increase anti-inflammatory activity with –OCH3 being most potent. Larger groups reduce activity. 5. Replacing –COOH group with groups that can be metabolized to –COOH leads to retention of activity. 5.6 Mechanism of action of Aryl acetic Acids Aryl aliphatic acids are non-selective inhibitors of the enzyme cyclooxygenase 5.7 Metabolism of Aryl acetic Acids Activity Before you proceed any further, o Find out what is the major difference between the metabolism of ibuprofen and that of diclofenac in man and make brief notes in the 48  form of schematic diagrams. 5.7.1 Metabolism of Ibuprofen Ibuprofen is nearly completely excreted unchanged in the urine. It may also undergo ώ-1 and ώ-2 oxidation of the p-isobutyl side chain. OH H3C C CH2 major CH3 -1 oxidation OH -2 oxidation H3C CH CH2 CH CH H3C CH3 CH3 -1 oxidation H3C CH CH2 H3C CH CH2 Hydrolytic Hydrolytic oxidation oxidation CH2 OH COOH HOOC major Figure 5.10. Metabolism of ibuprofen 49 5.7.2 Metabolism of Diclofenac Diclofenac is extensively metabolized in the liver mainly by aromatic hydroxylation. 6 1 CH2COOH HO CH2COOH 5 2 4 NH NH 3 Cl 6' 1' Cl Cl Cl 2' 5' 3' 5-hydroxy 4' derivative CH2COOH HO CH2COOH CH2COOH NH NH NH Cl Cl Cl Cl Cl Cl OH OH OH 4'-hydroxy 3'-hydroxy 4',5 - dihydroxy derivative, derivative derivative major Figure 5.11. Metabolism of diclofenac 5.8 Analysis of Aryl acetic Acids Activity Before you proceed any further, o Look at the structure of ibuprofen and give possible methods of analysis o Check the recommended method of ibuprofen in the British Pharmacopoeia or the United States Pharmacopoeia. I hope that you got several possible methods for analysis of ibuprofen. The official method of analysis of ibuprofen is titration 0.1N NaOH, with potentiometric detection. 50 5.9 Summary In this lecture you have learnt that: The aryl aliphatic acids have analgesic, antipyretic and anti- inflammatory activity. They are non-selective inhibitors of the enzyme cyclo- oxygenase. They are further classified into: o Phenyl propionic acids e.g. ibuprofen o Phenyl acetic acids e.g. diclofenac o Heteroaryl acetic acids e.g. indomethacin Ibuprofen is nearly completely excreted unchanged in the urine, while diclofenac is extensively metabolized in the liver mainly by aromatic hydroxylation. They are often analyzed by titration, by virtue of being acids. 5.10 References 1. Introduction to Medicinal Chemistry. 1997. Alex Gringauz. Wiley-VCH. New York. 2. Foye’s Principles of Medicinal Chemistry, 6th ed. 2008. Thomas L. Lemke, David A. Williams, Victoria F. Roche and S. William Zito. Wolters Kluwer Health/Lippincott Williams and Wilkins. Philadelphia. 3. Goodman & Gilmans. 2006. The pharmacological basis of therapeutics. 12th Ed. Laurence L. Brunton, Bruce A. Chabner, Bjorn C. Knollmann. McGraw-Hill Comp. Inc. 4. Medicinal Chemistry. 5th ed. 2010. Ashutosh Kar. New Age International Publishers. New Delhi. 5. The British Pharmacopoeia. 2010. www.pharmacopoeia.co.uk 51  LECTURE SIX INDOLE ACETIC ACIDS Lecture outline 6.1 Introduction 6.2 Objectives 6.3 Definition of the indole acetic acids 6.4 Structure-activity relationships 6.4.1 Structure-activity relationships of indoles and indenes 6.4.2 Notable structural features of sulindac 6.5 Mechanism of action of indole acetic acids 6.6 Metabolism of aryl indole acetic acids 6.6.1 Metabolism of indomethacin 6.6.2 Metabolism of sulindac 6.7 Analysis of indole acetic acids 6.8 Summary 6.9 References 6.1 Introduction In the last lecture, we discussed the aryl aliphatic acid class of non-steroidal anti- inflammatory drugs and found that they are non-selective inhibitors of the enzyme cyclo- oxygenase. We discussed that they are further classified into phenyl propionic acids, phenyl acetic acids and heteroaryl acetic acids. Ibuprofen is nearly completely excreted unchanged in the urine, while diclofenac is extensively metabolized in the liver mainly by aromatic hydroxylation. They are often analyzed by titration, by virtue of being acids. In this lecture, I will introduce you to the indole acetic acid class of NSAIDs. I will show you how their structure, which is based on an indole nucleus, is related to their activity. We will discuss indenes, which are molecules that are isosteric to indoles and they exhibit similar properties to indole. Lastly, we will discuss their metabolism in man and how they can be analyzed. 52 6.2 Objectives Objectives At the end of this lecture you should be able to: 1. Explain what the indole acetic acid NSAIDS are. 2. Describe the relationship between the structure and the activity of indole acetic acids. 3. Discuss the mechanism of action of the indole acetic acids. 4. Discuss the metabolism of indole acetic acids. 5. Explain how the analysis of indole acetic acids and indenes can be carried out. 6.3 Definition of the indole acetic acids Intext Question What are indole acetic acids? The indole acetic acids have an indole nucleus (Figure 6.1) and the prototype in this class is indomethacin (Indocid®), whose IUPAC name is 1-(p-chlorobenzoyl)-5-methoxy-2- methylindole-3-acetic acid. 4 5 3 6 2 7 N 1 Figure 6.1. Chemical structure of indole nucleus The development of the indole acetic acids was based on an earlier assumption that serotonin (5-hydroxytryptomine, 5-HT) was responsible for inflammation. 53  4 HO 3 5 CH2CH2NH2 6 2 N 7 1 Figure 6.2. Chemical structure of 5-hydroxytryptomine Indomethacin (Indocid®) was developed after screening of hundreds of indole compounds for anti-inflammatory activity. The other important drug in this class is sulindac (Clinoril®). As shown in Figure 6.3, indomethacin has an indole nucleus and it is the only true indole drug in clinical use, while sulindac has an indene nucleus. CH3 O CH2COOH F CH2COOH N CH3 CH3 O CH Cl S O CH3 Indomethacin Sulindac Figure 6.3. Chemical structure of indomethacin and sulindac The other drugs that are considered to be indole acetic acids are tolmetin, tenidap, etodolac, ketorolac, zomepirac and carprofen. You will remember that tolmetin, etodolac, ketorolac, and carprofen are also classified as aryl aliphatic acid derivatives as we saw in the last chapter. 54  Activity o Draw the chemical structure of tenidap. o List structural similarities with indomethacin o List structural differences from indomethacin Take Note o Zomepirac is very potent but it has been withdrawn due to causing anaphylactic reactions in patients o Etodolac has significant selective cyclooxygenase 2 inhibition activity 6.4 Structure-activity relationships Activity Before you proceed any further, stop and refresh your memory by drawing the indole nucleus. Show the numbering in this moiety. I hope that the structure you have drawn is the same as that we drew in Figure 6.1. We will refer to that figure as we discuss the relationship between structure and activity. 6.4.1 Structure-activity relationships of indoles and indenes 1. N1 position: Indole N is not essential for activity since indenes are active, e.g. sulindac. 2. C2 position: Alkyl-substituents produce more active compounds than aryl-substituents. A methyl group is the most active of these alkyl substituents. Presence of CH3 at C2 and H at C7 causes the indole ring to swing out of plane and this swing is thought 55  to be essential for activity. Removal of the methyl group gives a compound that is almost coplanar and therefore inactive. Increasing the alkyl chain to one bigger than a methyl group decreases activity. This is due to the steric hindrance of a long chain that affects the fitting of the drug into the receptor. 3. C3 position: The -COOH group is essential for anti-inflammatory activity. Increasing acidity of the carboxyl group generally increases anti-inflammatory action. Replacing it or reducing its acidity decreases activity. Amide analogues are inactive. 4. Methyl group at α position of the acetic group side chain: Produces equipotent compounds. Introduces a chiral centre. Only the S (+) isomer displays anti-inflammatory action. 5. C5 position: Substitution with methoxy, fluoro, and dimethylamino, methyl, alkoxy and acetyl groups yields more active compounds than non-substituted ones. O-Demethylation leads to inactivation of the compounds ( OCH3→OH). 6. The benzoyl moiety: Is essential for activity. This is demonstrated by the fact that metabolism of indomethacin involves hydrolysis of amide group to give an inactive compound and this suggests the benzoyl group is essential. Met OMe CH2COOH inactive CH3 O Cl is essential. N Figure 6.4. Inactive and essential moieties of indomethacin 56  Replacement of benzoyl group with aliphatic carboxylic or aryl carboxylic groups results in compounds much less active. Substitution at para position with F, Cl, trifluoromethyl (-CF3), thiomethyl (- SCH3) groups gives the most active compounds. Metabolism of indomethacin involves amide hydrolysis to give inactive metabolites. 7. The para chlorobenzoyl moiety is free to rotate around the amide bond. The preferred conformation is a chlorobenzoyl group that is cis to the methoxy group. This cis conformation puts the benzoyl group away from the C2 methyl and the compound is non-coplanar due to steric hindrance of C2 methyl and C7 hydrogen. CH3O CH2COOH OMe CH2COOH N CH3 CH3 N O O Cl Cl cis indomethacin trans indomethacin Figure 6.5. Chemical structures of cis and trans indomethacin 8. The presence of the indole moiety is not essential for activity. This is shown by the fact that indenes are active. Indoles, such as indomethacin, have the benzoylindole moiety. On the other hand, indenes, such as sulindac, have the benzylidenylindene moiety. The two groups are isosteric, meaning that they have the same size. 57  N O CH Benzoyl indole 1- Benzylidenyl indene Figure 6.6. Chemical structures of benzoyl indole and 1-benzylidenyl indene The structure-activity relationships of the indenes is similar to that of indoles. 6.4.2 Notable structural features of sulindac We have discussed structural features of the indole acetic acids and how they relate to the indene-based molecules. Let us now see features which are notable in sulindac. 1. Isosteric replacement of indole with indene nucleus. 2. Replacement of benzoyl with benzylidene moiety 3. Also exhibits geometric isomerism due to rotation about C=C double bond. As we discussed for the indoles, the cis isomer is more active than the trans isomer. This shows that both indomethacin and sulindac assume similar conformations at the active site of cyclooxygenase. 4. Replacement of 5-methoxy group with 5-fluoro group results in enhsnced analgesic activity. 5. The para-chloro substituent of indomethacin is replaced with a methylsulfoxide group. This causes decreased water solubility of indenes, which may lead to crystalluria. 6. In sulindac, this poor solubility is alleviated by replacing Cl of phenyl group with a sulfinyl group. 58  Take Note Sulindac has the following advantages over indomethacin 1. Sulindac is a prodrug metabolized to the active sulfide metabolite. The sulfide metabolite is responsible for COX inhibition. 2. It has fewer gastrointestinal side effects than indomethacin since it does not affect prostaglandin synthesis in the gastrointestinal tract. 6.5 Mechanism of action of indole acetic acids Indole acetic acids are non-selective inhibitors of the enzyme cyclooxygenase. 6.6 Metabolism of indole acetic acids We will now look at the metabolism of indomethacin and then at that of sulindac. Before we do so, let us refresh our minds with the activity below. Activity o What are phase I and phase II reactions in relation to drug metabolism? o What are the products of these reactions and what is the pharmacological activity relative to that of the original drug? 6.6.1 Metabolism of Indomethacin Indomethacin is metabolized largely by O-demethylation (50%) followed by conjugation with glucuronic acid (10%) as shown in Figure 6.7. 59  CH3O CH2COOH N CH3 MeO O CH2COO Glu CH3O CH2COOH N CH3 O N CH3 H Cl HO CH2COOH Cl N CH3 O Cl HO CH2COOH N CH3 H HO CH2COOGlu N CH3 H Glu: Glucuronide Figure 6.7. Metabolism of indomethacin 6.6.2 Metabolism of Sulindac. Sulindac is a prodrug in which the sulfoxide is reduced to sulfide (active) which is highly protein bound and is not found in urine. In urine you get the sulfone and glucuronide as shown in figure 6.8. 60  F CH2COOH F CH2COOH CH3 CH3 H H O H3C S H3C S active F CH2COOH F CH2COOH CH3 CH2OH H O OH O S H3C S H3C O F F CH2 COO Glu CH2COOGlu CH2OH OH H O O S H3C O H3C S Glu: Glucuronide Figure 6.8. Metabolism of sulindac 6.7 Analysis As with salicylates and aryl aliphatic acids, indomethacin may be analyzed by titration of the –COOH group with NaOH. However, the British Pharmacopoiea recommends ultraviolet spectrophotometry (UV) for both indomethacin and sulindac. The United States Pharmacopoiea recommends liquid chromatographic analysis with UV detection for both. 6.8 Summary 61  In this lecture, you learnt that indole acetic acids and indenes: Have similar activity and structure-activity relationships. Are non-selective inhibitors of the enzyme cyclo-oxygenase. Are excreted largely unchanged, but can also be oxidized before excretion. Can be analyzed by titration, ultraviolet spectroscopy or by liquid chromatography. 6.9 References 1. Introduction to Medicinal Chemistry. 1997. Alex Gringauz. Wiley-VCH. New York. 2. Foye’s Principles of Medicinal Chemistry, 6th ed. 2008. Thomas L. Lemke, David A. Williams, Victoria F. Roche and S. William Zito. Wolters Kluwer Health/Lippincott Williams and Wilkins. Philadelphia. 3. Medicinal Chemistry. 5th ed. 2010. Ashutosh Kar. New Age International Publishers. New Delhi. 4. The British Pharmacopoeia. 2010. www.pharmacopoeia.co.uk 5. The United States Pharmacopeia. www.uspnf.com/ 62  LECTURE SEVEN FENAMATES Lecture outline 7.1 Introduction 7.2 Objectives 7.3 Definition and examples of fenamates 7.4 Structure-activity relationships of fenamates 7.5 Mechanism of action of fenamates 7.6 Metabolism of fenamates 7.6.1 Metabolism of mefenamic acid 7.6.2 Metabolism of meclofenamic acid 7.7 Analysis of fenamates 7.7.1 Assay of mefenamic acid 7.7.2 Assay of etofenamate 7.8 Summary 7.9 References 7.1 Introduction In lecture six, you were introduced to the indole acetic acids, a class of non-steroidal anti- inflammatory drugs. I explained how their activity is related to their structure and how the indenes, which are molecules that are isosteric to indoles, exhibit similar properties to indoles. We also discussed their metabolism in man and observed that they are excreted largely unchanged, but can also be oxidized before excretion. They can be analyzed by titration, ultraviolet spectroscopy or by liquid chromatography In this lecture, you will learn about fenamates, the last class of NSAIDs that have a carboxylic acid group. We will see how their structure, which is based on anthranilic acid, is related to their activity. We will then discuss their metabolism in man and how they can be analyzed. 63  7.2 Objectives At the end of this lecture you should be able to: 1. Descibe the fenamates class of NSAIDS. 2. Explain the mechanism of action of fenamates. 3. Describe the relationship betwe