L8 Drug Metabolism Consequences For Drug Development PDF

L8: Drug Metabolism: Consequences for Drug Development Faculty of Life Sciences & Medicine Dr Richard B. Parsons 5BBM0216 Drug Development and Discovery Department of Pharmacy Drug Metab...

L8: Drug Metabolism: Consequences for Drug Development Faculty of Life Sciences & Medicine Dr Richard B. Parsons 5BBM0216 Drug Development and Discovery Department of Pharmacy Drug Metabolism: Consequences for Drug Design  Learning outcomes By the end of this lecture, you will be able to: – Describe what makes an “ideal” drug – Explain the process of drug metabolism and where & why it occurs – Describe the various factors which influence drug metabolism – Describe how drug metabolism can aid in the development of drugs – Explain how drug-drug and drug-dietary interactions occur – Explain why some drugs fail to reach or are removed from the market No such thing as an ideal drug – everything is toxic, and the body has many barriers to drugs getting into the body. 2  Topics in this lecture to exploit Drug metabolism comprises leads to responsible for variability in What makes an Modifiers of Drug Phase 1 & 2 “ideal” drug? Metabolism comprises regulated by categorised influences to avoid into Drug metabolising Design of drugs takes into account enzymes interactions of which to minimise lead to Drug interactions What makes an “ideal” drug? Readily absorbed after oral administration Not metabolised Pharmacodynamic target readily accessible and only site of high-affinity binding High therapeutic efficacy Extensive therapeutic window Optimal plasma half-life – Long enough to have desired effect – Short enough to avoid accumulation and toxicity 3  Oral is better because it has better compliance than other administration forms e.g. suppository, injection, nasal spray. You want a drug that is not metabolized. Binding should be as specific as possible to reduce off target effects. Toxicity is linked to exposure. Why might a NCE be subject to drug metabolism? Cells have evolved enzymes to metabolise endogenous compounds – Synthesis of cellular components – Biochemistry of the cell – “life processes” Cells have also evolved enzymes to metabolise exogenous compounds – Present in diet, environment, etc. detoxification – Compounds are no different in aspect compared with endogenous compounds NCE = new chemical entity. Every drug faces barriers to being absorbed – there are enzymes for breaking down drugs. 4  Why might and NCE be subject to drug metabolism? Drugs are no different to endogenous and exogenous compounds Use the same metabolic enzymes involved in endogenous and exogenous metabolism – “Highjacking” of endogenous biochemical machinery – This is why drug-drug and drug-dietary interactions occur Same machinery used for all compounds What is drug metabolism? DRUG METABOLITE CONJUGATE 5  Both steps won’t happen each time. Where does drug metabolism occur in the body? Main site of metabolism is the liver and gut. Also occurs in brain, skin, lungs 6  Where does drug metabolism occur in the cell? Majority of metabolism occurs in the smooth endoplasmic reticulum as it is rich in lipids and most drugs are lipophilic. 7  Phase 1 and Phase 2 metabolism A bit of history Richard Tecwyn Williams (1909- 1979) was the founder of drug metabolism – Detoxication mechanisms: the metabolism of drugs and allied organic compounds (1949) – Summarised the then-known processes of metabolism of xenobiotics, organising the subject accorging to classes of chemicals Coined the terms “Phase 1” and “Phase 2” – Second edition (1959) contained a chapter which introduced the idea of distinguishing between categories of biotransformation – PHASE 1 & 2 8  Phase 1 vs. Phase 2 Phase 1 2 Phase 2 DRUG METABOLITE CONJUGATE Oxidation Sulphation Reduction Glucuronidation Hydrolysis Glutathione Hydration Methylation Dehalogenation N-acetylation Amino acid conjugation Phase 1 – makes drug more polar so it becomes more reactive Phase 2 – add large molecule onto the drug to make it bigger to be excreted and be detoxified. 9  Phase 1 Biotransformation of the compound to make it more polar – Oxidation, reduction, hydrolysis Changes in biological activity Results in the activation of the compound for the addition of Phase II conjugate – Activation also makes the compound more reactive, thus more potentially toxic Normally switches off pharmacological action of the drug Phase 2 Conjugation reaction, usually to make the compound larger and more water-soluble Addition of a large, water-soluble compound – Almost always results in detoxification – Increases chances of excretion – Many cellular transporters are targeted to conjugates 10  E.g. sulphate makes the drug more water soluble Makes it easier to bind to transporters and transport proteins The challenge to the sequential Phase concept Phase 2 does NOT always need Phase 1 Phase 2 does NOT always occur – Josephy PD et al. (2005) Drug Metabolism Reviews 37: 575-580 E.g. if drug already has OH group like paracetamol, it can go straight to conjugation Drug may not need to be conjugated if it is soluble enough 11  New classifications Oxidations – Cytochrome P450, flavin mono-oxygenase, alcohol dehydrogenase etc. Reductions – Small group of processes, mainly nitro- and azo- reductions Conjugations – Addition of electrophilic adenosine-containing cofactors (PAPS, acetyl coenzyme A, UDP-glucuronic acid, S-adenosylmethionine) Nucleophilic trapping processes – Mainly reactions of GSH with electrophilic xenobiotics, also DNA and protein adduct formation Oxidations – 95% of phase 1 Glutathione – protects against free radicals (nucleophilic trapping processes) 12  Drug metabolising enzymes Cytochrome P450 isoforms Superfamily of haem-thiolate proteins (40 – 65 kDa) that exist in multiple isoforms Responsible for the majority (>90%) of all xenobiotic oxidation reactions Expressed in a variety of tissues – Liver>>>GIT>Kidney>lungs=skin Present in the smooth endoplasmic reticulum in close association with cytochrome P450 reductase – Provides electrons for catalytic cycle Most common for oxidation reactions 13  Haem – contains iron. Thiolate – has thiolate in active site that haem group binds to. Found in smooth endoplasmic reticulum which may also be referred to as a microsome Associated with cytochrome p450 reductase which helps to split oxygen molecules by providing electrons for catalytic cycles. Nomenclature CYP Cytochrome P450 CYP1 Family name, > 40% aa identity with any other CYP CYP1A Sub-family, > 55% aa identity within family with other CYPs CYP1A1 Isoenzyme 14  Cytochrome P450 Xenobiotic = substance that is foreign to a living system. Therefore, the xenobiotics class of p450s are what metabolise drugs. 2E1 is associated with binge drinking. 15  P450s involved in drug metabolism CYP2D6 24% CYP3A 51% CYP2C 19% CYP1A2 CYP2E1 5% 1% CYP3A the most common, followed by CYP2D6 then CYP2C Haem-thiolate group 16  Haem group loses electrons for the catalytic cycle. P450 catalytic cycle Key process: o Substrate binds o Oxygen binds o Two electrons used to split oxygen-oxygen double bond and electron passed on. 17  C-oxidation PGE1 Warfarin C-oxidation leading to dealkylation Also for N- and S- dealkylations 18  N- and S-oxidation Imipramine Chloropromazine Also for primary aliphatic amines Electron withdrawing therefore more reactive 19  Non-P450 drug metabolism Oxidations – Flavin mono-oxygenase – Alcohol dehydrogenase Hydrolyses – Epoxide hydrolase Conjugation (Phase 2) – Glucuronidation (high capacity) – Sulphation (low capacity) – Acetylation (variable capacity) Conjugations Glucuronidation (UGTs) Sulphation (SULTs) One pathway can take over the other. 20  Glucuronidation makes it more reactive and bigger Sulphonation makes it more soluble. Modifiers of metabolism Drug production cannot be a one size fits all approach. Example of extreme reaction: toxic epidermal necrolysis Factors that affect drug metabolism Belle & Singh (2008) Am. Fam. Phys. 77: 1553-1560 Intrinsic factors (things you can’t change) that affect drug metabolism – genetic polymorphism of drug metabolising enzymes, age, ethnicity Age – liver and heart don’t function as well as they could. 21  Extrinsic factors (things you can change) – alcohol consumption, body weight, diet, diseases and conditions, supplements and recreational drugs Drug-drug interactions - CYP3A4 Responsible for largest number of drug biotransformations Major site of expression is liver – Some in small intestine 20-fold variation in metabolic activity amongst individuals Wide substrate profile Drugs use the same pathways, so if more than one drug that uses the same pathway is taken, there will be a limit to how much of the two drugs can be metabolised. The greater the concentration of all drugs, the lower the probability that one drug will be metabolism. 22  CYP3A4 Notable substrates: – Cortisol, ciclosporin, erythromycin, testosterone, midazolam, terfenadine Notable inhibitors – Grapefruit juice, gestodene, ketoconazole Inducers – Barbiturates, rifampicin, dexamethasone Bergamottins inhibit CYP3A4 – means that drugs cannot be metabolised as expected. Other drugs e.g. barbiturates can be inducers, so more CYP3A4 is expressed, and drugs are metabolised more quickly. 23  Terfenadine Non-sedative, antihistamine (H1) drug A pro-drug – Prodrug has antiarrhythmic properties Major routes of metabolism: – Oxidative N-dealkylation – Stepwise oxidation of tert-methyl group to primary alcohol followed by carboxylic acid to produce pharmacologically-active compound Terfenadine was lethal if taken with grapefruit juice. Terfenadine is a prodrug, which has antiarrhythmic properties. CYP3A4 is required to metabolise the prodrug into the safe drug. As grapefruit inhibits CYP3A4, the prodrug accumulated in the system and causes torsades de pointes. 24  Terfenadine metabolism and toxicity Terfenadine – a case history Case history: 29-yr old male Terfenadine 2xday for hayfever Consumed 2xglasses grapefruit juice with dose & then mowed his lawn Collapsed and died from heart attack – terfenadine is a prodrug accumulation causes Torsades des Pointes (Long QT syndrome) – bergamottins inhibit CYP3A4 responsible for conversion of prodrug into active form Previous exam question 25  Terfenadine – effect on cardiac function Drug remains in system so harder to treat the heart attack Genetic polymorphisms Upto 95% of patient variability in individual response is due to genetic factors Affects both Phase 1 and Phase 2 enzymes – Polymorphisms in Phase 1 enzymes more clinically relevant 26  Phase 1 – detoxification therefore more clinically relevant SNPs SNPs are individual changes in DNA sequence from the consensus Give rise to allelic variations Define individuality – Many are benign, some are not Often persist over several generations because of a lack of selective advantage – E.g. blood group, hair colour Metabolic phenotypes Zanger et al. (2004) Naunyn Schmiedebergs Arch Pharmacol. 369: 23-37 27  Three types: extensive metabolisers, intermediate metabolisers, poor metabolisers, ultrarapid metabolisers Patient specific drug design/prescribing – design drug or tailor dosage to someone’s metabolic type. Consequences for drug dosage Appropriate doses for different metabolic types 28  Ethnic variation CYP2C8 – *2 predominates in Africans – *3 predominates in Caucasians CYP2C9 – Predominantly Caucasians – *3>*2 in effect upon metabolism CYP2C19 – *2 & *3 Africans>Asians=Caucasians Mutations had selective advantage Ethnic variation in 2D6 Ingelman-Sundberg et al. (2007) Pharmacol. Therap. 116: 496-526 29  Ultra rapid metaboliser phenotype common along the equator Polymorphisms in Phase 2 - UGTs Most common in promoters Crigler-Najjar-1 syndrome – fatal CN-2 syndrome – survivable Gilbert’s syndrome – relatively mild – inability to glucuronidate bilirubin – most common polymorphism – 19% of some ethnic groups UDP glucosyltransferases. Gilbert’s syndrome common in Irish people 30  Polymorphisms in UGTs Bilirubin buildup causes jaundice. UV treatment breaks up bilirubin and encourages synthesis of the enzyme. 31  Age - geriatrics Decline in renal function – decreased excretion Increase in body fat – increase in drug deposition Decrease in liver mass with age – no decrease in P450 function Decrease in liver blood flow with age – 0.5-1.5% per year – reduce O2 availability for P450 activity – at 70 years up to 40% decline in hepatic blood flow compared to 30 year old Reduction in G.I.T blood flow Blood concentration of drugs increases in elderly people. Age - neonates < 4 weeks in age 30-50% CYP activity of adult t1/2 drugs increased 10 fold Premature babies reduced renal clearance CYP developmental patterns – foetal-specific isoforms, e.g. CYP3A7 endobiotic metabolism CYP patterns similar to adults 2-6 months 24 hours following birth UGT1A1 activated by bilirubin – jaundice GSH conjugation better than adults Phase I and Phase II reach maturity at 2 – 3 years 32  Babies can trap by glutathione (glutathione conjugation) better than adults as they cannot use their mitochondria for metabolism. Could drug metabolism be helpful to the drug designer? Pro-drugs Improve permeability to be a prodrug so it is cleaved within the cell. Prodrugs are normally esters – esterase cleaves it into the drug. 33  L-DOPA Anti-parkinsonian drug L-dopa prodrug has an amino acid group – taken up by amino acid transporters in blood brain barrier and catecholamine transferase cleaves it into dopamine. Other options include adding a sugar group as the brain has lots of glucose transporters. 34  Examples Enalapril (ACE inhibitor) – Oral availability Dipivefrin (glaucoma) – Corneal permeation Xeloda (anti-cancer agent) – Reduced toxicity and high tumour selectivity Rautio et al. (2008) Prodrugs: design and clinical applications. Nature Rev Drug Discovery 7: 255-270 Enalapril, dipivefrin and Xeloda are prodrugs that are metabolised in the body to the active form. 35  “Hard” Drugs Compounds that contain structural characteristics required for activity but are not susceptible to metabolism Increased efficacy by avoiding metabolism – No toxic metabolites are formed Unlikely to result in drug-drug interactions – Lower dose can be administered, thus reducing potential for toxicity – However, less readily eliminated due to lack of metabolism – Not many examples of hard drugs Lisinopril (ACE inhibitor) Hard drugs avoid metabolism so they are not metabolised. “Soft” drugs The opposite of pro-drugs, these compounds are designed and synthesized as active compounds which readily undergo metabolic inactivation – Removal of pharmacological effect – Can produce highly-reactive intermediates Many of which are toxic – Variations in drug metabolism amongst people can give rise to variations in drug response and toxicity Example: remifentanil (µ opiod agonist); atracurium (muscle relaxant) 36  E.g. succinylcholine in surgery is a soft drug used short term to paralyse during surgery. Why do drugs fail? 37  Drug failures Implications for drug development What enzymes are involved in key metabolic pathways? Does clearance depend solely upon a single rate- limiting enzyme? Is that enzyme subject to genetic or environmental variation in activity? Are there any significant potentials for drug-drug and drug-diet interactions? If yes, does development continue? If yes, what restrictions may have to be put in place? You want a drug to be metabolised by several pathways. 38  Assume patients won’t read the patient information leaflet. Paracetamol toxicity – a study 1 Acetanilide – Discovered accidently in 1886 – Antipyretic – Excessive toxictiy (methaemoglobinaemia) Phenacetin introduced in 1887 – Extensively used until implicated in analgesic abuse nephropathy Acetaminophen recognised as metabolite in 1889 – Brodie and Axelrod recognised toxicity due to acetanilide and analgesia due to acetaminophen in 1948/9 Paracetamol introduced into US in 1955 39  NAPQI, a metabolite of paracetamol is toxic. Paracetamol toxicity – a study 2 Paracetamol toxicity – a study 3 Paracetamol overdose most common reason for calls to poison control centres 35% of liver failure due to acetaminophen poisoning, resulting in organ transplantation N-acetylcysteine is the antidote – GSH precursor – Antioxidant properties – Early administration (