Metabolic Changes of Drugs Books: 1. Wilson and Gisvold’s Textbook of Organic Medicinal and Pharmaceutical Chemistry 11th ed. Lippincott, Williams & Wilkins ed. 2. Foye’s Principles of Medicinal Chemistry 3- Burger's Medicinal Chemistry &Drug Discovery. Prof. Faris T. Abachi ( PHD Pharmacy) 3rd Year Pharmacy 2017

outlines Introduction Definitions. General pathways of drug metabolism Sites of drug biotransformation Role of Cytochrome P-450 Enzymes Classification.

Introduction Metabolism play a central role in the elimination of drugs and other foreign compounds ( Xenobiotics) from the body. A solid understanding of drug metabolism pathways is an essential tool for pharmacists in their role of selecting and monitoring appropriate drug therapy for their patients.

Definitions Biochemically speaking: Metabolism means Catabolism (breaking down of substances) + Anabolism (building up or synthesis of substances) But when we speak about drug metabolism, it is only catabolism That is drug metabolism is the break down of drug molecules So what is building the drug molecules? We use the word “synthesis”, then  Biotransformation: It is a specific term used for chemical transformation of xenobiotics in the body/living organism.• a series of enzyme-catalyzed processes—that alters the physiochemical properties offoreign chemicals (drug/xenobiotics) from those that favor absorption across biologicalmembranes (lipophilicity) to those favoring elimination in urine or bile (hydrophilicity )

What Roles are Played by Drug Metabolism? Xenobiotics : These are all chemical substances that are not nutrient for body (foreign to body) and which enter the body through ingestion, inhalation or dermal exposure.They include : drugs, industrial chemicals, pesticides, pollutants, plant and animal toxins, etc.animal toxins, One of four pharmacokinetic parameters, i.e., absorption, distribution, metabolism and excretion (ADME) Elimination of Drugs: Metabolism and excretion together are elimination Xenobiotics : These are all chemical substances that are not nutrient for body (foreign to body) and which enter the body through ingestion, inhalation or dermal exposure.They include : drugs, industrial chemicals, pesticides, pollutants, plant and animal toxins, etc Many drugs are metabolically activated (Prodrugs) Sometimes drugs become more toxic and carcinogenic

Sites/Organs of drug metabolism  The major site of drug metabolism is the liver (microsomal enzyme systems of hepatocytes)Secondary organs of biotransformation• kidney (proximal tubule)• lungs (type II cells)• testes (Sertoli cells)• skin (epithelial cells); plasma. nervous tissue (brain); intestines Enzymes are divided into TWO types:

1-Microsomal enzymes: The endoplasmic reticulum (especially smooth endoplasmic reticulum) of liver and other tissues contain a large variety of enzymes, together called microsomal enzymes(microsomes are minute spherical vesicles derived from endoplasmic reticulum after disruption of cells by centrifugation, enzymes present in microsomes are called microsomal enzymes).

2- Non-microsomal enzymes: Enzymes occurring in organelles/sites other than endoplasmic reticulum (microsomes) are called non-microsomal enzymes.These are usually present in the cytoplasm, mitochondria, etc. and occur mainly in the liver, Gl tract, plasma and other tissues.They are usually non-specific enzymes that catalyse few oxidative reactions, a number of reductive and hydrolytic reactions, and all conjugative reactions other than glucuronidation

General pathways of drug metabolism Drug metabolism reactions have been divided into TWO categories PHASE I ; Functionalization PHASE II ; Conjugation

Phase l Oxidation reactions Aromatic, olefins, benzylic, alicyclic, c-c hetero atoms , N – oxidation, S oxidation, .. Reduction reactions Aldehydes, ketones, nitro, azo ,.. Hydrolytic reactions Hydrloysis of esters & amides

Phase II Glucuronic acid conjugation Sulfate conjugation Glycine, glutamine A.A conjugation Glutathione or mercapturic acid conjugation Acetylation Methylation

Compare between phase I &ll Phase ll Phase l Small large Polar Non-polar ( lipophilic ) Excretion to the urine Not active Metabolite to another product active

Metabolite activity Examples and notes Inactive (detoxification) Similar activity to the drug Different activity Toxic metabolites Routes that result in the formation of inactive metabolites are often referred to as detoxification. The metabolite may exhibit either a different potency or duration of action or both to the original drug.

Sites of Drug Metabolism Liver: Major site, well organized with all enzyme systems The first-pass effect Following drugs are metabolized extensively by first-pass effect: Isoproterenol, Lidocaine Meperidine, Morphine, Pentazocine, Propoxyphene, Propranolol, Nitroglycerin, Salicylamide Intestinal Mucosa: The extra-hepatic metabolism, contains CYP3A4 isozyme Isoproterenol exhibit considerable sulphate conjugation in GI tract Levodopa, chlorpromazine and diethylstilbestrol are also reportedly metabolized in GI tract Esterases and lipases present in the intestine may be particularly important carrying out hydrolysis of many ester prodrugs Bacterial flora present in the intestine and colon reduce many azo and nitro drugs (e.g., sulfasalazine) Intestinal b-glucuronidase can hydrolyze glucuronide conjugates excreted in the bile, thereby liberating the free drug or its metabolite for possible reabsorption (enterohepatic circulation or recycling)

Enzymes Involved in Drug Metabolism CYP450, Hepatic microsomal flavin containing monooxygenases (MFMO or FMO) Monoamine Oxidase (MAO) and Hydrolases Cytochrome P450 system: localized in the smooth endoplasmic reticulum. Cytochrome P450 is a Pigment that, with CO bound to the reduced form, absorbs maximally at 450nm Cytochromes are hemoproteins (heme-thiolate) that function to pass electrons by reversibly changing the oxidation state of the Fe in heme between the 2+ and 3+ state and serves as an electron acceptor–donor P450 is not a singular hemoprotein but rather a family of related hemoproteins. Over 1000 have been identified in nature with ~50 functionally active in humans with broad substrate specificity Simplified apoprotein portion Heme portion with activated Oxygen

Cytochrome P450: Naming Before we had a thorough understanding of this enzyme system, the CYP450 enzymes were named based on their catalytic activity toward a specific substrate, e.g., aminopyrine N-demethylase now known as CYP2E1 Currently, all P450’s are named by starting with “CYP” (CYtochrome P450, N1, L, N2 - the first number( Arabic number ) is the family (>40% homology), the letter is the subfamily (> 55% homology), and the second number is the isoform. The majority of drug metabolism is by ~10 isoforms of the CYP1, CYP2 and CYP3 families in humans Major human forms of P450: Quantitatively, in the liver the percentages of total P450 protein are: CYP3A4 – 28%, CYP2Cx – 20%, CYP1A2 – 12%, CYP2E1 – 6%, CYP2A6 – 4%, CYP2D6 – 4% By number of drugs metabolized the percentages are: CYP3A4 – 35%, CYP2D6 – 20%, CYP2C8 and CYP2C9 – 17%, CYP2C18 and CYP2C19 - 8% CYP 1A1 and CYP1A2 -10%, CYP2E1 – 4%, CYP2B6 – 3% How do you determine the styre

Drug Interactions & Metabolism The drug interactions depend upon: the isoform(s) required by the drug in question, the isoforms altered by concomitant therapy, the type of enzyme alteration (induction or inhibition).

General Metabolic Pathways Oxidation Aromatic moieties Olefins Benzylic & allylic C atoms and a-C of C=O and C=N At aliphatic and alicyclic C C-Heteroatom system C-N (N-dealkylation, N-oxide formation, N-hydroxylation) C-O (O-dealkylation) C-S (S-dealkylation, S-oxidation, desulfuration) Oxidation of alcohols and aldehydes Miscellaneous Hydrolytic Reactions Esters and amides Epoxides and arene oxides by epoxide hydrase Phase II - Conjugation Phase I - Functionalization Drug Metabolism Reduction Aldehydes and ketones Nitro and azo Miscellaneous Glucuronic acid conjugation Sulfate Conjugation Glycine and other AA Glutathion or mercapturic acid Acetylation Methylation

Tetrahydrocannabinol (D1-THC) Metabolism The metabolite is polar, ionisable and hydrophilic

Oxidative Reactions

Hydroxylation is the primary reaction mediated by CYP450 Hydroxylation can be followed by non-CYP450 reactions including conjugation or oxidation to ketones or aldehydes, with aldehydes getting further oxidized to acids Hydroxylation of the carbon α to heteroatoms often lead to cleavage of the carbon – heteroatom bond; seen especially with N, O and S, results in N–, S– or O–dealkylation. Must have an available hydrogen on atom that gets hydroxylated, this is important!!!

Aromatic Hydroxylation OH Epoxide Hydrase OH Epoxide Hydrase Mixed function oxidation of arenes to arenols via an epoxide intermediate arene oxide Major route of metabolism for drugs with phenyl ring Occurs primarily at para position Substituents attached to aromatic ring influence the hydroxylation Activated rings (with electron-rich substituents) are more susceptible while deactivated (with electron withdrawing groups, e.g., Cl, N+R3, COOH, SO2NHR) are generally slow or resistant to hydroxylation

HOME WORK Oxidation of Benzene Ring. 1- Phenytoin 2-Warfarin 3-Propanolol 4-phenybutazone 5- Steroid ( 17-α- Ethyinyestradiol) 6- Atrovastain

Amphetamine Phenytoin p-hydroxyphenytoin Warfarin sodium 17-a-Ethinylestradiol Propranolol Phenylbutazone Atorvastatin