Drug Metabolism Lecture 24 PDF

Principles of Drug Action Chemical Basis of Drug Metabolism I Lecture #24  Medicinal Chemistry Drug-Target Drugs Targets...

Principles of Drug Action Chemical Basis of Drug Metabolism I Lecture #24  Medicinal Chemistry Drug-Target Drugs Targets Drug Interactions Discovery Development & Optimiza sicochemical Property of Drug-1 Enzymes Forces in Drug- Functional Group (FG) Receptor Target Acidity and Basicity of FG Optimization of Drug Interactions SAR, Bioisosterism, Rigidification Enzyme and Discovery & Design sicochemical Property of Drug-2 Receptor Peptide/Protein based drug - Salt and Solubility Interactions Combinatorial & Parallel Chemistry - Chirality Use of Computers in Drug Design (Molecular Modelling, QSAR, AI) Physicochemical Properties of Drug Absorption and Membrane Drug Drug Nanomedicin transporters Metabolism e Examples of Drug Classes  Learning Objectives Identify the key concepts governing drug metabolism. Predict the possible metabolic transformations that could occur for a given drug molecule. – Using any given functional group within a drug molecule, identify the possible types of metabolic transformations it could undergo. – For each Phase I metabolic transformation, review key intermediates and/or the final metabolic product. – For each Phase II metabolic transformation, draw and/or identify the activated intermediate, identify the transferring enzyme, identify the functional groups that can be conjugated by this path, and/or identify any deconjugating enzymes. DRUG DISTRIBUTION Notes Once across the gut wall, the drug enters blood vessels Cells lining blood vessels are loose-fitting Rapid distribution from blood vessels to tissues and organs (leaky’ blood vessels) Drug can quickly cross blood vessel walls through pores between cells Drug is distributed evenly throughout blood supply within 1 min of absorption DRUG DISTRIBUTION Notes Uneven distribution around the body due to the uneven blood supply Blood concentration drops rapidly after absorption due to distribution, macromolecular binding, and storage in fat tissue (e.g. barbiturates) or bone The brain barrier hinders polar drugs from entering the brain tight-fitting cells line the capillaries in the brain - capillaries have a coating of fat cells Can increase the polarity of peripherally acting drugs to reduce CNS side effects  1. Drug Metabolism 1. Drug Metabolism Major mechanism for terminating drug activity to enhance the removal of drug molecules from the body by altering or adding functional groups. Determinant of the duration and intensity of the pharmacological response to a drug. Drug metabolites are usually less active or inactive (exception - prodrugs) Conversion of prodrugs to active drugs  2. Drug Metabolism location 2. Drug Metabolism Location Organs: Primarily in the liver but other organs (GI, kidney, lung, skin, CNS, placenta, and fetus). (Every tissue) Organelle: Microsomes, cytosol, mitochondria A percentage of the orally absorbed drug is metabolized in the liver prior to distribution around the body - the first-pass effect Compounds absorbed by other routes avoid the first pass effect and circulate around the body before reaching the liver (a percentage distributes into fat, cells, and tissues)  1. Drug Metabolism Phases 3. Drug Metabolism Phases Two main categories: Phase I metabolism and Phase II metabolism. Phase I metabolism includes oxidation, reduction, and hydrolysis. Add a polar handle to the molecule; enhance water solubility Cytochrome P450 enzymes catalyze phase I oxidations Phase II metabolism involves conjugation: the addition of endogenous compounds to the drug molecule. Carried out on functional groups which have been added by Phase I reactions DOI:10.1016/s0021-5198(19)60574-3  Key Concepts Key Concepts Enzymes involved in metabolism can’t select between xenobiotic and endogenous compounds (depends on physicochemical properties of substrates/enzymes). Purpose of xenobiotic metabolism: To convert lipophilic substances into more polar derivatives that are excreted more rapidly. Minimum number of transformations. Integrated Nature of Metabolism: - Multiple sequential reactions are often required. - May be competing pathways operating.  Key Concepts Drug molecules may require only Phase I or Phase II metabolism Phase I/N-Dealkylation Example 1 Phase I/N-Dealkylation Example 2 Phase II Conjugation Temazepam glucuronide (inactive, readily excreted metabolite) Temazepam  4. Metabolic Pathways Phase I Reactions 4. Metabolic Pathways: Phase I Reactions Phase I: Convert chemicals to more polar metabolite(s) by introducing or unmasking a polar functional group, e.g. -OH, - NH2, -COOH, -SH. Phase I reaction by Enzymes (Biochemical) A. Oxidation: Cytochrome P450, Flavin monooxygenase, Alcohol/ aldehyde DH, Co-oxidation by PGH synthase, Monoamine oxidase. B. Reduction: Cytochrome P450, NADPH-cytochrome P450 reductase, Carbonyl reductase. C. Hydrolysis: Epoxide hydrolase, Carboxylesterase/Amidase Lipophilic Hydrophilic Cytochrome P450 (CYP450) Catalyze Oxidation/Reduction. Contain heme molecule bound to an iron atom. 450 represents their ability to absorb light of wavelength Also known as mixed-function oxidases due to their ability to introduce oxygen into a wide range of functional groups.  CYP450 isoforms All CYP450 isoforms use the CYP prefix followed by a number designating the family A capital letter designating the subfamily At least 12 families in human biochemistry A second number designating the individual gene. CYP2D6 isoform is form 6 of CYP family 2, subfamily D. Individuals differ in types of Cyt. P450 enzymes present Patient variability in drug metabolism complicates dose levels and leads to different susceptibilities to drugs Mechanism of CYP450 enzymatic oxidation  Pharmacist Alert Drug-drug interactions Drugs that affect the activity of Cyt. P450 enzymes may affect the activity of other drugs (drug-drug interactions) - phenobarbitone enhances activity - cimetidine inhibits activity - results in overdose or underdose of the affected drug - Polymorphism and warfarin (hemorrahage) O H O N O Me OH CH3 CN N NH HN CH2 S CH2 CH2 NH C Et N NHMe O O O Phenobarbitone Cimetidine Warfarin Pharmacist Alert Drug-food interactions Certain foods affect the activity of cytochrome P450 enzymes - brussel sprouts & cigarette smoke enhance activity - grapefruit juice inhibits activity OH antihistamines OH N Terfenadine R=CH3 R Fexofenadine R=CO2H Terfenadine (Seldane) - prodrug for Fexofenadine (Allegra) Metabolized by cytochrome P450 enzymes Metabolism slowed by grapefruit juice Build up of terfenadine leads to cardiac toxicity Fexofenadine favoured in therapy over terfenadine  4. Metabolic Pathways Phase I Reactions 4.1. Oxidation reaction by Cyp450 4.1.1. Aromatic Ring (Hydroxylation) 4.1.2. Alkenes (Hydroxylation) 4.1.3. Aliphatic and Alicyclic Carbon (Hydroxylation) 4.1.4.Carbon next to a double bond Allylic, Benzylic Carbon, imine bond, or carbonyl (Hydroxylation) 4.1.5. Amine, Amides, and Aromatic Nitrogen atoms 4.1.5.1. Deamination (removal of amine group) 4.1.5.2. N-Dealkylation (removal of an alkyl group from N) 4.1.5.3. N-Oxidation (N-oxide) 4.1.6. Sulfur Atoms (Sulfone, sulfoxide) 4.1.7. Oxidative dehalogenation (removal of halogen) 4.1.8. Oxidation of FGs with Carbon-Oxygen Bonds  4. Metabolic Pathways Phase I Reactions 4.1.1.Oxidation of Aromatic rings 4.1. Oxidation reaction by Cyp450 4.1.1. Oxidation of Aromatic Rings (i.e., Aromatic Hydroxylation or Aromatic Oxidation) R R OH Unsubstituted phenyl rings are primarily hydroxylated at the para position. In multiple aromatic rings with substituted positions, the aromatic ring that is LESS sterically hindered is the preferred site for oxidation. Electron withdrawing substituents deactivate aromatic rings (no hydroxylation), while electron donating substituents enhance aromatic hydroxylation Examples 4.1.1. Oxidation of Aromatic Rings C H3 C H3 O O R R OH CO2H CO2H HO Fenoprofen More sterically The carbon atom hindered rings O O that is oxidized O O NH O O NH must contain a hydrogen atom. Paroxetine HO (A general requirement for H 3C N O H 3C N O oxidation with N Cl CyP450) Cl N Diazepam HO O C H3 CO2H X No aromatic hydroxylation  4. Metabolic Pathways Phase I Reactions 4.1.2. Oxidation of Alkenes 4.1.2. Oxidation of Alkene/olefins  In order for this metabolic route to occur, there must be at least one hydrogen atom available (one R = H). R R R O R R R R R  Therefore, the alkene bond present within the structure of tamoxifen cannot undergo alkene oxidation. N O no hydrogen ato no hydrogen atom Tamoxifen  4. Metabolic Pathways Phase I Reactions 4.1.5. Oxidation of Aliphatic & Alicyclic 4.1.3. Oxidation of Aliphatic and Alicyclic Carbon Atoms R CH R CH OH 3 2 R R OH The terminal methyl group in the chain (the omega, ω position). The penultimate (i.e., next to last) carbon atom in the chain (the omega-1, ω -1 position) O HO  -1 oxidation O NH O N H O O NH  N OH H O O Pentobarbital  -1 oxidation O NH N H O Dicyclomine  4. Metabolic Pathways Phase I Reactions 4.1.4. Oxidation of carbon next to double bond 4.1.4. Oxidation of Carbon Atoms Adjacent to Double Bond Includes benzylic carbon atoms, allylic carbon atoms, carbon atoms adjacent to an imine bond, or a carbonyl bond (e.g., ketone, amide, ester). OH R R more sterically hindered benzylic carbon OH C CH Cl O O C N O H S N N HO H H OCH3 O Ethynyl estradiol less sterically benzylic hindered carbon Glyburide benzylic carbon OH Cl C CH O O C N O H S N N H H OCH3 HO O HO benzylic O H benzylic oxidation oxidation  4. Metabolic Pathways Phase I Reactions 4.1.4. Oxidation of Allylic carbon 4.1.4. Allylic Oxidation: Aliphatic carbon directly attached to alkene or non-aromatic double bond R R R' R' OH also allylic, but O O lacks hydrogen atom O O O O O S O S C H3 OH C H3 allylic carbon allylic oxidation Spironolactone N O N O OH allylic carbon allylic oxidation Tamoxifen  4. Metabolic Pathways Phase I Reactions 4.1.4. Carbon next to Imine/Carbonyl bond 4.1.4. Imine bond or a carbonyl bond (e.g., ketone, amide, ester). Imine Carbonyl O O R N R R' N R' R' R' R R OH OH Flurazepam Aminoglutethimide  4. Metabolic Pathways Phase I Reactions 4.1.4. Carbon next to Imine/Carbonyl bond  Examples for Review: Find the benzylic and allylic carbon atoms that can be oxidized C H3 C H3 N C H3 OH C H3 Pentazocine O C H3 HO  1 Tetrahydrocannibinol  4. Metabolic Pathways Phase I Reactions 4.1.5. Oxidation of Amine, Amides and Aromatic N 4.1.5. Oxidation of Amines, Amides, and Aromatic Nitrogen Atoms  There are three major oxidative transformations available for amines and amides:  oxidative deamination (removal of amine group)  oxidative N-dealkylation (removal of alkyl group from N)  N-oxidation (N-Oxide).  Primary, secondary, and tertiary amines are often metabolized differently.  The primary route of oxidative metabolism for aromatic nitrogen atoms is N-oxidation since they cannot undergo oxidative deamination or oxidative N-dealkylation.  Quaternary heterocyclic nitrogen atoms can undergo N-dealkylation, but not oxidative deamination or N-oxidation.  4. Metabolic Pathways Phase I Reactions 4.1.5.1. Oxidative Deamination 4.1.5.1. Oxidative Deamination  This primarily occurs with primary amines; however, it can also occur with secondary amines. In order for oxidative deamination to occur, the α-carbon (i.e., the carbon atom directly adjacent to the nitrogen atom) must be attached to at least one H hydrogen atom. H R 2 O R O R 2 2   + HN Drug NH Drug NH Drug R1 H R1 R1 Aldehyde or Ketone primary or Carbinolamine secondary amine O OH OH H 2N H O O + NH3 Cl Cl Baclofen aldehyde ammonia (primary amine) CH3 H + H 2N CH3 O N H O O primary CH3 CH3 amine Atomoxetine  4. Metabolic Pathways Phase I Reactions 4.1.5.2. Oxidative N-Dealkylation 4.1.5.2. Oxidative N-Dealkylation R R N Me N H R R This metabolic transformation can occur with secondary or tertiary amines or amides. The mechanism is identical to that of oxidative deamination. Similar to oxidative deamination, the alkyl group that is removed must contain a hydrogen atom. N-Dealkylation of tertiary amines will produce secondary amines and N-dealkylation of secondary amines will produce primary amines. In general, smaller alkyl groups are more likely to undergo N- dealkylation than larger groups.  R R 4. Metabolic Pathways Phase I Reactions N Me N H 4.1.5.2. Oxidative N-Dealkylation R R R2 R2 R2 Drug N Drug N H2C Drug N + H O CH3 H 2C Aldehyde secondary or tertiary amine O H Primary or secondary Carbinolamine amine H 3C O N O OC2H5 Cl N oxidative C H3 deamination N N C H3 C H3 Amitriptyline Meperidine Diazepam oxidative oxidative deamination deamination C H3 H O N C H3 H 3C N C H3 H OH H3CO O N O O C H3 HN C H3 NO2 O Atenolol Nicardipine OH H N HO O HO Salmeterol  4. Metabolic Pathways Phase I Reactions 4.1.5.3. N-Oxidation 4.1.5.3. N-Oxidation R-NH2  RNHOH  N-oxidation involves a direct oxidation of the nitrogen atom as opposed to an adjacent carbon atom.  While it is not necessary for an adjacent carbon atom to be attached to a hydrogen atom, the products of N-oxidation will vary based on the presence or absence of a hydrogen atom. C H3 C H3 H NH 2 N HN HN OH N H N-oxidation N H hydroxylamine OCH3 OCH3 Primaquine C H3 loss of water N HN OH oxime N C H3 N-oxidation NH OCH3 HN imine N OCH3 C H3 hydrolysis O HN N H aldehyde OCH3  4. Metabolic Pathways Phase I Reactions 4.1.6.Oxidation of Sulfur atom 4.1.6. Oxidation of Sulfur atom Carbon sulfur groups are susceptible to: a) sulfur dealkylation* b) desulfuration* c) sulfur oxidation (Xenobiotics which contain organic sulfur commonly undergo oxidation). * not common R-S-R′  R-SO2-R′ S S S N S C H3 N S C H3 N S C H3 O O O sulfoxide sulfone N N N N N N C H3 C H3 C H3 Thiethylperazine R-S-R′  R-S-S-R′ sulfhydryl O O O group HS N N S S N C H3 C H 3 disulfide C H3 Captopril O OH HO O O OH  4. Metabolic Pathways Phase I Reactions 4.1.7. Oxidative Dehalogenation 4.1.7. Oxidative Dehalogenation  This type of oxidation can remove halogens from aliphatic chains and aliphatic rings, but not from aromatic rings.  4. Metabolic Pathways Phase I Reactions 4.1.8. Oxidation of Carbon-Oxygen Bonds 4.1.8. Oxidation of FGs with Carbon-Oxygen Bonds Either present within the structure of a drug molecule or added as a result of the oxidation of hydrocarbon rings and chains, it can undergo oxidation to produce aldehydes, ketones, and carboxylic acids. O O R OH R H R OH aldehyde primary hydroxyl OH C H3 H OH C H3 H H N C H3 ADH N C H3 HO O C H3 C H3 HO HO Albuterol ALDH carboxylic acid KEY: HO OH C H3 ADH = Alcohol dehydrogenase H ALDH = Aldehyde dehydrogenase N C H3 O C H3 HO  4. Metabolic Pathways Phase I Reactions 4.2. REDUCTION 4.2. REDUCTION  Involve a gain of hydrogen by the reduced functional group.  least common Phase I metabolic pathway due to the fact that there are only a few functional groups that undergo reduction: O OH H R N N R R NH2 + H2N R C C R R R R Azo Aldehyde and ketone Azoreductase Aldo-keto reductase R NO2 R NH2 Nitro Nitroreductase  4. Metabolic Pathways Phase I Reactions 4.2. REDUCTION 4.2. REDUCTION O OH R N N R R NH2 + H2N R H C C R R R R Azo Aldehyde and ketone Azoreductase Aldo-keto reductase HO2C HO NH2 HO2C CO2H H N + CO2H HO N N H O N Balsalazide H 2N O O H O C H3 N O H aldehyde S S Duloxetine reduction oxidation (minor) (major) O O OH OH O primary carboxylic S alcohol S acid  4. Metabolic Pathways Phase I Reactions 4.2. REDUCTION 4.2. REDUCTION R NO2 R NH2 Nitro Nitroreductase O O N N NH N N NH O2N N O O nitro O O O Nitrofurantoin nitroso O O N NH H N N NH N H 2N N O O primary O HO O amine hydroxylamine  4. Metabolic Pathways Phase I Reactions 4.2. REDUCTION In general: a) Carbonyl group reduces to an alcohol. b) Nitro group reduces to amino derivatives. c) Azo group reduces to amino derivatives. d) N-oxides reduce to tertiary amines. e) Sulfoxides reduce to sulfide. Xenobiotics with functional groups: – RCHO – R2C=O – R2S=O – RSSR – Quinone – N-oxide – RCH=CHR – RN=NR – RNO2 Are all candidates for reduction.  4. Metabolic Pathways Phase I Reactions 4.3. HYDROLYSIS 4.3. HYDROLYSIS O O C C + HO R R OR R OH O O C C + HNR2 R NR2 R OH  Taken literally, the term hydrolysis means “water” (hydro) to “break” (lysis) a bond.  Hydrolysis involves the addition of a molecule of water across a CO— X bond and the subsequent cleavage of that bond.  In most cases, X is an oxygen or nitrogen atom, and for some functional groups, the carbon atom is replaced by a sulfur or phosphorous atom.  Hydrolysis readily occurs with esters, amides, and their cyclic analogs, lactones, and lactams.  Hydrolysis is a very common Phase I metabolic transformation due to the fact that esters, amides, lactones, and lactams are present in a significant number of drug molecules.  4. Metabolic Pathways Phase I Reactions O O 4C. HYDROLYSIS C C + HO R R C H 3 OR R OH C H3 O N H C H3 O OH N H C H3  Hydrolytic enzymes OH are ubiquitous in O the human body. O C H3 C H3 C H3 OH + CH3OH O O N H C H3 O N H C H3  They are present in OH the liver but are also OH ester O carboxylic H 3C O O C H3 C H3 acid H 3CO H C H3 widely distributed in + CH3OOH other organs and O N C H3 hydroxyl N C H3 N Esmolol NH2 + HO Esmolol H C H3 C H3 tissues such as the H 3C O C H3 H 3C O C H3 GI tract, plasma, N HO N C H3 NH2 + HO ON C H3 carboxylic skin, lungs, and H O HO kidneys. lactone acid C H3 C H3 OH O OH O O hydroxyl H 3C O H 3C O O H 3C C H3 HO O H 3C C H3 HO C H3 C H3 OH O OH O O H 3C H 3C H 3C O Simvastatin H 3C O H 3C C H3 H 3C C H3 C H3 C H3 O O  H NH2 + O O 4. Metabolic Pathways Phase I Reactions C H3 O CH OC C + HNR2 4C. HYDROLYSIS 3 R NR C2H 3 R OH O OH + CH3OH O HO O HO OH O OH O C H3 O H 3C C H3 O H 3C O H 3C O N C H3 H 3C O N C H3 H 3C CNH 3 N H 2C H 3+ H 3C HO H amide C H3 C H3 amine carboxylic C H3 acid C H3 H 3 C Lidocaine H 3C O HO O HO O O OH O H OH S O H S O N N amine H 3C NH O 2 N H 3C ONH 2 HN C H3 O C H3 H 3C C H3 O H 3C C H 3 carboxylic OH C H3 C H3 COOH acid COOH lactam H 3 CCephalexin H 3C Pharmacist Alert Prodrugs to improve membrane permeability Esters Used to mask polar and ionizable carboxylic acids Hydrolysed in blood by esterases Used when a carboxylic acid is required for target binding Leaving group (alcohol) should ideally be nontoxic Examples Enalapril for enalaprilate (antihypertensive) O O C C + HO R R OR R OH CH3 RO N N H O O CO2H R=Et Enalapril R=H Enalaprilit 48  Pharmacist Alert Prodrugs to mask toxicity and side effects Mask the groups responsible for toxicity/side effects Used when groups are important for activity Example: Aspirin for salicylic acid O OH CO2H H3C O CO2H Salicylic acid Aspirin Analgesic Phenol masked by ester Causes stomach ulcers Hydrolysed by esterases Due to phenol group in bloodstream 49  Pharmacist Alert Prodrugs to lower water solubility Used to reduce solubility of foul-tasting orally active drugs Less soluble on the tongue Less revolting taste Example: Palmitate ester of chloramphenicol (antibiotic) Palmitate ester OH Cl H H N O O Cl Cl H H O N Cl OH Esterase H O OH O2N H Chloramphenicol O2N 50 51 Pharmacist Alert Prodrugs to increase water solubility Often used for i.v. drugs Allows higher concentration and smaller dose volume May decrease pain at the site of injection Example: Succinate ester of chloramphenicol (antibiotic) HO O Succinate ester OH Cl H H N O O Cl Cl H H N O Cl OH Esterase H O OH O2N H O2N Chloramphenicol 52 53  Key Facts Basic concept of the chemical basis of drug metabolism. Phase I reactions add a polar functional group to a drug. CyP450 performs mostly oxidation and reduction reactions. The carbon atom that is oxidized must contain a hydrogen atom. CYP450 inserts oxygen atoms in drug molecules for metabolism. Remember the oxidation reactions of each functional group with P450. Reduction involves a gain of hydrogen by the reduced functional group. (Azo and Nitro group; Aldehyde and Ketone) Hydrolysis is a very common Phase I metabolic transformation due to the fact that esters, amides, lactones, and lactams are present in a significant number of drug molecules.