Drug Metabolism

Slide 1 Drug metabolism MBChB 221B Dr Stephen Jamieson Dept of Pharmacology and Clinical Pharmacology Auckland Cancer Society Research Centre Slide 2 Learning objectives • Understand why drug metabolism is important • Learn the major drug metabolism reactions • Appreciate the potential role of drug metabolism in drug-drug interactions and toxicity • Learn the major CYP enzymes and at least one clinically relevant substrate for each Slide Most drugs are lipophilic and will not 3 be renally excreted, as the fraction of drug that is filtered in the glomerulus What is drug metabolism will be reabsorbed back into the bloodstream. For elimination from the • Metabolism is the biotransformation of drugs body, these drugs are generally metabolised into more polar – Enzyme-catalysed chemical change to the drug metabolites that can then be excreted molecule; either building molecule up or breaking from the body in the urine (or the bile). down • Biotransformation reactions typically generate more polar metabolites – Most drugs are lipophilic – Enhance excretion through urine or bile – Metabolites less likely to diffuse into cells Slide Drug metabolism directly influences 4 drug concentrations in the body and therefore can influence the effect of Why is drug metabolism important the drug. Rapidly metabolised high extraction drugs will achieve lower • Drug metabolism can directly influence the concentration-time plasma concentrations than slowly profile in the body metabolised low extraction drugs. – Concentration determines effect 300 ) ) ) L L L / / / g g g m m m Slow metabolism ( ( ( 200 n n n o o o i i i t t t a a a r r r t t t n n n 100 e e e c c c n n n o o o C C C Rapid metabolism 0 0 100 200 300 Time (min) Source: Steve Jamieson Slide In most cases, metabolism will reduce 5 the activity of the drug. A major exception is prodrugs, which require Why is drug metabolism important metabolism for activation. • Drug metabolism can directly influence the concentration-time profile in the body – Concentration determines effect • Major route of drug elimination – Generally inactivates drugs • Can increase drug activity – Activation of prodrug (e.g. codeine to morphine) Codeine Morphine Source: Wikimedia commons Slide Some drug metabolites may be toxic, 6 e.g. paracetamol Why is drug metabolism important • Drug metabolism can directly influence the concentration-time profile in the body – Concentration determines effect • Major route of drug elimination – Generally inactivates drugs • Can increase drug activity – Activation of prodrug (e.g. codeine to morphine) • Production of toxic metabolites – e.g. paracetamol Slide Overdose of paracetamol can result in 7 saturation of glutathione conjugation of a reactive metabolite, leading to Paracetamol metabolism buildup of the reactive metabolite, which can cause liver cell death. Ogilvie et al. Can Med Assoc J. 184:1492-1496, 2012 Slide Drug metabolism can also be 8 responsible for drug drug interactions and is a major source of inter-patient Why is drug metabolism important variability. • Drug metabolism can directly influence the concentration-time profile in the body – Concentration determines effect • Major route of drug elimination – Generally inactivates drugs • Can increase drug activity – Activation of prodrug (e.g. codeine to morphine) • Production of toxic metabolites – e.g. paracetamol • Can explain drug-drug interactions • Source of between patient variability Slide The liver is the major site of drug 9 metabolism. All orally administered drugs have to avoid first pass Possible sites of drug metabolism metabolism before they reach the systemic circulation and become • Liver bioavailable. Some oral drugs, e.g. – Major site of drug metabolism simvastatin, can also be metabolised – First pass metabolism of oral drugs (e.g. morphine) be enzymes present in the gut wall. • May necessitate dosing by other routes • Intestinal wall – CYP450 enzymes Van de Waterbeemd and Griffin. Nat Rev Drug Discov. 2: 192–204, 2003 Slide Microflora in the gut may encounter 10 non-absorbed drug and metabolise it, assisting with absorption. Some drugs Possible sites of drug metabolism are administered as ester prodrugs to enhance their absorption. Esterases • Liver in the plasma can then metabolise the – Major site of drug metabolism ester and release the active drug. – First pass metabolism of oral drugs (e.g. morphine) • May necessitate dosing by other routes • Intestinal wall – CYP450 enzymes • GI tract – Gut bacteria and proteases • Plasma – Esterases (prodrug activation) • Lungs – Metabolism of aerosol sprays Slide Historically drug metabolism reactions 11 have been referred to as phase I (e.g. oxidation, reduction, hydrolysis) and Metabolism reactions phase II (conjugation) reactions with phase I reactions involved in • Oxidation inactivating the drug and unmasking a – Add oxygen atom through loss of electrons (H+) molecule so that phase II reactions – Typically catalysed by CYPs, FMO, oxidases and dehydrogenase enzymes can occur and promote the excretion • Reduction of the metabolite. Many textbooks still + – Add H with or without loss of oxygen atoms refer to metabolism reactions in this • Hydrolysis – Water molecule is added to compound, usually resulting in bond cleavage way. However, these terms are outdated. The so-called phase I • Conjugation reactions don’t always inactivate – Addition of endogenous substrate to increase molecular mass, polarity, water drugs, while conjugation reactions solubility don’t always follow oxidation, – Glucuronidation, sulphation, acetylation, methylation, glutathionylation reduction and hydrolysis reactions or necessarily enhance the excretion of • A single drug may be metabolised by multiple routes and by multiple enzymes the drug. – Any enzyme can be a drug metabolising enzyme Slide The most important oxidative 12 enzymes are the CYP450 family. The CYPP450 enzyme system is Cytochrome P450 enzymes responsible for the metabolism of over 50% of drugs used clinically. Often • Superfamily of metabolising enzymes also called the ‘microsomal mixed – 57 human CYP genes across >20 families function oxidase’ family. These are – CYP families 1-3 are predominantly involved in drug metabolism haem proteins that sit within liver cells, mostly associated with endoplasmic reticulum with a few • Most common drug metabolising enzymes associated with mitochondria. CYPs – Catalyse oxidation reactions oxidise a large variety of drugs but – 5 isoforms metabolise approx 90% of drugs also oxidise a number of endogenous • CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 substances, particularly steroid hormones, fatty acids and • Most common reaction is a monooxygenase reaction prostaglandins. – Requires O2, NADPH and cytochrome P450 reductase + + Drug + O2 + H + NADPH → oxidised drug + H2O + NADP Slide The activity of drug metabolising CYP 13 enzymes can be altered (inhibited or induced) by drugs or environmental CYP-mediated drug metabolism factors, while variability in CYP function exists between individuals • Some drugs inhibit CYPs and other drug-metabolising enzymes through differences in gene – Prevent the metabolism of itself or co-administered drugs expression or genetic polymorphisms. • Increased toxicity Any change in CYP activity (through • Reduced activation of prodrugs induction/inhibition or interindividual variability) can result in altered drug • The expression of some CYP enzymes can be upregulated by environmental factors or drugs concentrations and potentially – Enhance the metabolism of other drugs increased toxicity (if drug levels are • Reduced activity increased) or reduced activity (if drug • Increased levels of toxic metabolites levels are decreased). • Function of CYP enzymes differs amongst individuals – Differences in gene expression – Genetic polymorphisms Slide An example of a CYP1A2 drug drug 14 interaction is in patients who smoke being treated with theophylline. Important CYP isoforms Smoking can induce CYP1A2 so greater doses of theophylline, a CYP1A2 substrate, are required to • CYP1A2 have the same effect as in non- – Clinically relevant substrates: theophylline, caffeine smokers. This can be problematic if – the patient stops smoking and the Drug interactions: cimetidine (inhibition); tobacco, dose isn’t adjusted (Kroon, Am J bbq meat, cruciferous veges (induction) Health-Syst Pharm 64:1917-1921, 2007). • CYP2E1 For CYP2E1, high ethanol intake can – Clinically relevant substrates: ethanol, paracetamol induce CYP2E1 activity and promote metabolism of paracetamol to its toxic – Drug interactions: chronic ethanol consumption metabolite. Need very high doses of increases CYP2E1 expression paracetamol before this becomes an issue. Slide Genetic polymorphisms exist in 15 CYP2D6 that have different phenotypes in their ability to Important CYP isoforms metabolise drugs. This leads to interindividual variability in drug concentrations and drug activity. • CYP2D6 – Clinically relevant substrate: debrisoquine, tricyclic antidepressants (amitriptyline), codeine – Drug interaction: fluoxetine (inhibition) – Over 100 allelic variants Slide Individuals with a genetic 16 polymorphism in CYP2D6 that give a phenotype of “poor metabolisers” CYP2D6 genetic polymorphism need a much lower dose of a CYP2D6 substrate to have the same effect as “ultrarapid metabolisers”. Metabolic ratio of debrisoquine to 4- hydroxy-debrisoquine Meyer. Nat Rev Genet. 5:669-676, 2004 Slide Because of difficulties individualising 17 patient dosing it is simplest to avoid drugs that are metabolised

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