Ch_15_summary - CHAPTER 15 MEDICINES AND DRUGS(IB OPTION D SUMMARY Introduction The body has its own natural healing processes that usually

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Unformatted text preview: CHAPTER 15 MEDICINES AND DRUGS (IB OPTION D) SUMMARY Introduction The body has its own natural healing processes that usually overcome tissue damage and infections by microbes, though sometimes medicine or drugs may be required. A medicine or drug is any chemical that does one or more of the following: • Alters the physiological state, including consciousness, activity level or coordination • Alters incoming sensory sensations • Alters mood or emotions The placebo effect is the well established fact that administration of inactive substances, instead of active drugs, still leads to a reported improvement in the condition. Drugs can be administered orally, parenteral (that is by injection, which can be intravenous, intramuscular or subcutaneous), inhalation or rectally. The testing of new drugs is a lengthy process. The stages involved usually include: • Tests on microbes or tissue cultures • Animal tests to establish the LD50 (the dose lethal to half the population) • Double-blind clinical trials The therapeutic window is a measure of the relative margin of safety of a drug. It is the ratio of the lethal dose (LD50) to the therapeutic dose of the drug (ED50 - the dose required for the drug to have a noticeable effect on half the population). The stomach naturally contains HCl, giving it a pH~2. Sometimes excess acidity can cause discomfort, which can be relieved by antacids such as Al(OH)3, Mg(OH)2, CaCO3, and NaHCO3, which neutralise the acid. Analgesics counteract pain. Mild analgesics function by intercepting the pain stimulus at the source, for example by interfering with the production of substances (such as prostaglandins) that cause pain, swelling or fever. Strong analgesics work by temporarily bonding to receptor sites in the brain, preventing the transmission of pain impulses without depressing the central nervous system. • Aspirin and Paracetamol are common mild analgesics. In addition to its analgesic effect, aspirin has been found to be useful in preventing the recurrence of heart attacks and strokes, though it can lead to ulceration and stomach bleeding, as well as allergic reactions and Reye’s syndrome (a potentially fatal liver and brain disorder in children). Paracetamol is very safe in the correct dose (though overdoses can lead to serious liver damage, brain damage and even death). • Morphine, codeine and diamorphine (heroin) are strong analgesics. These have similar carbon structures, but differ in the side chain attached to it. Though very potent, highly effective and lead to a sense of euphoria, these drugs have many negative effects: o they can cause constipation o they cause drowsiness o they are addictive o patients develop a tolerance to the drug so it becomes less effective o long term use can result in sterility © IBID Press 2007 1 CHAPTER 15 MEDICINES AND DRUGS (IB OPTION D) SUMMARY The effect of depressants (though are often described as anti-depressants because they are prescribed to relieve depression) is dose dependent: • at low doses they may exert little or no effect • at moderate doses they induce sedation (soothing, reduction of anxiety) • at higher doses it may induce sleep • at extremely high doses they may cause death. Common examples are diazepam (Valium), nitrazepam (Mogadon) and fluoxetine hydrochloride (Prozac). Valium and Mogadon have very similar structures with a benzene ring fused to a seven-membered heterocyclic ring to which a phenyl group is attached Ethanol, a depressant, is one of the most commonly used drugs. Whilst in the short term it provokes positive feelings through reducing stress and lowering inhibitions, there are serious short and long term negative effects of overindulgence: • Alcohol has even more severe negative effects when combined with other drugs, Alcohol and aspirin together leads to an increased risk of stomach bleeding. Cocaine can react with alcohol to produce cocaethylene which is far more toxic than cocaine. Taking alcohol along with sedatives with alcohol is particularly dangerous as it can easily lead to coma and death. • Detection of alcohol levels is important, especially with regard to drink driving prosecution. One of the simplest is the potassium dichromate(VI) breathalyzer, which turns from orange to green (Cr3+). More sophisticated techniques involve the analysis of blood or urine by chromatography, the absorption of infrared radiation or the intoximeter which uses a fuel cell. Stimulants are the opposite of depressants, which have the effect of making people feel more alert through an increase the heart rate, blood pressure, respiration and wakefulness. Because they override the natural controls on the consumption of energy reserves, excessive use can lead to exhaustion. • Amphetamines and epinephrine (adrenaline) are chemically similar in that both derive from the phenylethylamine structure, hence amphetamines mimic the effects of epinephrine and so are known as sympathomimetic drugs. • Nicotine, in the form of tobacco (for example in cigarettes), is a widely used drug. In the short-term there is a stimulating effect coupled with increased heart rate and blood pressure, as well as a reduction in urine output. In the long term it is addictive and there are significant negative long-term effects including increased risk of heart disease, coronary thrombosis and peptic ulcers. • Caffeine is a respiratory stimulant found in drinks such as tea and coffee. It is a weak diuretic, which when consumed in large amounts can cause anxiety, irritability and sleeplessness. Whilst their structures are not closely related, both caffeine and nicotine contain a tertiary amine group. Antibiotics • Penicillins, initially discovered by Fleming, but developed by Florey and Chain to make large scale production possible, were the first widely used antibacterials (antibiotics). • Penicillins all have a similar structure with a four-membered heterocyclic ring fused to a five-membered one. The side chains attached to these can be altered to enhance their resistance to hydrolysis both in the stomach and by enzymes produced by penicillinresistant bacteria. © IBID Press 2007 2 CHAPTER 15 MEDICINES AND DRUGS (IB OPTION D) SUMMARY • Penicillins interfere with the cross-linking of the cell walls of bacteria, so weakening them, causing them to easily burst. Because they do not have a cell wall, penicillins have no effect on animal cells • If antibacterials are widely used in low concentrations (for example in animal feed), or a course of antibacterials is not fully completed, then bacteria that show some resistance to the antibacterial will survive, which results in the development of drugresistant bacteria. This has resulted in the resurgence of a number of diseases such as tuberculosis (TB). Antivirals Bacteria are single cell microorganisms, measuring about 1 μm, that are capable of independent existence whereas viruses, which basically comprise only genetic material in a protective protein coat, are about a tenth of this size and cannot exist separate from other living organisms, because they rely on invading cells to reproduce. Because viruses are so different to bacteria, antibacterial drugs have no effect on viruses. One class of antiviral drugs depends on altering the cell’s ribosomes (protein production organelles) so that the virus cannot use them to produce its own proteins. Some other antiviral drugs block specific enzymes that only the virus produces. AIDS has been found to be closely associated with the HIV virus. The HIV virus can bind to T cells (a type of white blood cell) rendering it ineffective. This greatly weakens the immune response system that protects the body from infections, rendering the individual much more susceptible to many diseases. The HIV virus has the ability to mutate rapidly, making it difficult to develop a drug or vaccine that will remain effective against it. As with other viruses, the fact that their metabolism is linked closely to that of the cell, makes it difficult to destroy viruses without affecting healthy cells. Drugs that have been developed to combat AIDS are expensive, whilst the disease is very prevalent in developing countries where the cost of this treatment is prohibitive. The stigma associated with AIDS also often prevents people infected seeking diagnosis and treatment. © IBID Press 2007 3 CHAPTER 15 MEDICINES AND DRUGS (IB OPTION D) SUMMARY Enzymes are very stereospecific, that is they will usually only act on one stereoisomer and not the other. Therefore in order to be effective drugs need to be similarly stereo specific, that is usually only one isomer will be biochemically active. Transition metal complex ions, like organic molecules containing a double bond or a ring, can exhibit geometrical (cis- / trans-) isomerism. For example the square planar diamminedichloroplatinum(II) can exist in either a cis- or trans- form. The cis-isomer is found to be a highly effective anti-cancer drug (cisplatin), whereas the trans-isomer inactive. The same is also true of optical isomers, which contain a chiral centre. One example is the drug thalidomide. One enantiomer alleviates morning sickness in pregnant women, while the other can leads to deformities in the limbs of the fetus the woman is carrying. The use of a racemic mixture to treat morning sickness therefore led to the birth of many tragically deformed children in the 1950s. The stereochemistry of penicillins is also vital to their action. The four-membered beta-lactam ring structure has a lot of steric strain owing to its small bond angles. This enhances the reactivity of the amide group within it, allowing it to bond to the enzyme transpeptidase, responsible for the construction of the cell wall of bacteria, inhibiting its action. Diamorphine (heroin) is much more active than morphine. This is because in diamorphine the polar hydroxyl groups in morphine are replaced by the far less polar ester group, increasing its solubility in the lipids of the central nervous system. Traditionally compounds are synthesised individually and then tested for pharmacological activity – an expensive, time consuming process. Combinatorial chemistry simultaneously produces a large number of related compounds (a “combinatorial library”), which can then be tested in parallel. In the case of polymeric compounds often one end of the chain is anchored to a solid support, allowing it to be readily removed from the liquid phase reactants. An alternative approach is to link the biological activity of a molecule to its 3D shape and the polarity of specific regions. Other compounds that produce a similar environment can then be investigated by computer modeling. If they appear to be promising in silico then the compound can be synthesized and tested. Some regions of the body are essentially aqueous and others non-aqueous. Often the efficacy of a drug may be affected by altering its solubility though changing the polarity. Introducing polar groups (e.g. -OH), or those that can form ions (-COOH ⇒ -COO- or –NH2 ⇒ –NH3+) will increase the aqueous solubility, whereas hydrocarbon side-chains will reduce it. Soluble aspirin (where the weak acid is converted to its ionic salt) and fluoxetine hydrochloride are examples of water soluble ionic derivatives. Usually if a molecule has a chiral centre on of the optical isomers will be far more physiological activity than the other. Normal synthesis produces a racemic mixture which is very difficult to separate into the separate isomers. Chiral auxiliaries hold the non-chiral precursor in such a way that the reaction producing the chiral centre can occur in only one manner, producing just on of the enantiomers. Taxol, an anti-cancer drug, is produced by such a stereospecific synthesis. There are a number of drugs, such as lysergic acid diethylamide (LSD), mescaline, psilocybin and tetrahydrocannabinol (THC), are described as mind-altering, or hallucinogens because they create an altered state of conscious. in which normally impossible things seem to occur. LSD, mescaline and psilocybin all have quite similar structures, involving an indole ring, or in the case of mescaline a side chain that can take on a similar conformation to such a ring. Tetrahydrocannabinol (THC), found in cannabis, is significantly less potent than the other mind-altering drugs and in some parts of the world its use is legal. Ther are arguments for and against its legalization. (Shaded area indicates AHL material) © IBID Press 2007 4 ...
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This note was uploaded on 09/23/2010 for the course CS 001 taught by Professor Jix during the Spring '10 term at Riverside Community College.

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