Module 4 Med Chem Things to Know

Chlorpromazine Metabolism (FMO-catalyzed)

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Chlorpromazine Metabolism (P450-catalyzed)

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Cyproheptidine Metabolism (P450-catalyzed)

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Drugs that induce CYP at clinically significant doses

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Other enzymes induced by drugs

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Other substances that induce CYP at common exposure levels

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Weak inducers or local (intestinal) effects

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Diazepam Metabolism

(See Image) • It is used for different neurological disorders; it is greasy; & has high lipophillicity and needs to be converted to hydrophilic state to be excreted

Drugs containing carbonyl (C=O), azo (N=N), and nitro (N=O) can be...

...reduced by reductases

Biotransformation may convert:

1. Active drug to inactive metabolite (Most common) 2. Active drug to active metabolite 3. Inactive drug to active metabolite (Prodrugs)

Drug Biotransformation is influences by:

1. Molecular structure of the drug 2. Affinity of drug for its metabolizing enzyme(s) 3. Amount of drug-metabolizing enzyme 4. Availability of co-factors and co-substrates 5. Presence of other xenobiotics that affect the amount or activity of the enzyme (like drug-drug interactions)

__________ are ultra metabolizers of CYP2D6?

5.45%

• Metabolism (biotransformation) represents the listed clearance mechanism for _____ of the top 200 drugs

73% (Renal and biliary excretion, often transporter-facilitated, accounts for the remainder)

What is one way to avoid para-hydroxylation?

Adding a F, since C-F resist metabolism since they are stable

Mercapturic acid

An anionic terminal metabolite, is transported out of cells and excreted in bile or urine

What is metabolism interchangeable with?

Biotransformation

What could a SNP in a promoter region of gene do?

Could affect regulation (either increase or decrease) of the amount of enzyme epxressed

What disease is caused by gene deletion of UGT1A1?

Crigler-Najjar syndrome, it is usually fatal and causes severe unconjugated hyperbilirubinemia

What could the effect of a deletion of a gene be?

Decreased activity of the protein or none at all

FMO is able to oxidize carbons. (True/False)

False, it is only able to oxidize N, S and P

What disease is caused by reduced expression or activity of UGT1A1?

Gilbert's Syndrome, where there is too much unconjugated bilirubin, and it often not known unless challenged

What could the effect of an insertion or tandem repeats of a gene be?

Increased expression

Lecture 4.1: Drug Metabolism better described as Drug Biotransformation

Lecture 4.1: Drug Metabolism better described as Drug Biotransformation

Lecture 4.2: Factors Affecting Drug Biotransformation

Lecture 4.2: Factors Affecting Drug Biotransformation

Lecture 4.3

Lecture 4.3

Lecture 4.4

Lecture 4.4

Lecture 4.5

Lecture 4.5

Lecture 4.7

Lecture 4.7

Lecture 4.8

Lecture 4.8

Affinity of drug for enzyme

Of the three cytochromes P450 known to convert acetaminophen to the reactive metabolite, CYP3A4 has the lowest Km (150 uM; affinity), but CYP2E1 is the next lowest (1 mM), and has higher Vmax

First Pass Biotransformation

Oral Drugs • Biotransformation in intestine and liver will reduce amount of parent drug absorbed into the systemic circulation • Ingested drug absorbed across the intestine is taken by the hepatic portal vein to the liver. From the liver, the drug can reach the main systemic circulatio Drugs administered by other routes • Can have pre-systemic biotransformationin skin (dermal), lung (inhalation)Pre-systemic (First-pass)biotransformation5PHA 5439 Module 4 slides

What does the 450 represent in P450?

The heme-iron complex has absorption maximum at 450 nm, thus named P450.

What is the main organ of biotransformation?

The liver

Often the inducing agent will be a substrate for the induced P450. (True/False)

True, "often" is used since it is not always true. If someone is exposed to a drug that induces P450, doses of drugs metabolized by that form of P450 may need adjustment.

What CTP450 enzyme is bilirubin the substrate for?

UGT1A1

Disposition

What happens to a drug after it enters the body (ADME)

What does systemic mean?

When it's in the blood stream after going into the liver

The process of biotransformation alters the ________ structure of the drug molecule, this is carried out by ________.

chemical; enzymes

Azathioprine (Prodrug that uses GSH)

• .The prodrug tries to protect active drug and eventually GSH cleaves it to produce 6-mercaptopurine, the active form • The 6-mercaptopurine, primary metabolite, can undergo further phase 2 metabolism and be S-methylated at the thiol

What are the most well studied and impactful cytochrome P450?

• 3A4 • 2D6 (has almost 1000 fold variance) • 2C9

Enterohepatic Cycling

• A cycle through which absorbed drug is reintroduced to the intestine. After absorption and exposure to the liver, the drug is stored in the gallbladder and then secreted into the bile to reenter the intestine • Beta-glucuronidase in the lower GI tract hydrolyses the glucuronide conjugate and releases parent drug for re-absorption

Isoniazid metabolism

• Acetylation through NAT-2, where it goes from an active form to an inactive form • It is a drug for tuberculosis • Slow acetylators, toxicity is a concern. • Rapid acetylators, if dose is not adjusted, the drug may be ineffective

S-Ibuprofen (Acyl Glucuronides Formation)

• Acyl glucuronides are chemically reactive. Further reactions of these metabolites may lead to the drug being bound to proteins. The drug-bound protein may trigger an immune response.

Cyproheptidine epoxide metabolism

• After cyproheptidine is converted into an epoxide via CYP450, it can be be opened up using epoxide hydrolase

Phase I Reactions

• Also called "non-synthetic" or "functionalization" reactions. • They introduce or uncover a functional group • Ex. Oxidation, Hydrolysis, Reduction, Dehalogenation, Dealkylation

Phase II Reactions

• Also called "synthetic" or "conjugation") reactions. • An endogenous, usually water-soluble, small molecule (cosubstrate such as glucuronide, sulfate, acetyl or certain amino acids) combines with a functional group (typically hydroxy, amino or carboxylic acid) on the drug or its phase I metabolite • Ex. Glucuronidation, Sulfonation, Acetylation, Amino Acid conjugation, Glutathione conjugation, Methylation, Fatty acid conjugation, cholesterol conjugation

Sulfate Conjugation

• Are anions at physiological pH • Drug Targets: Nucleophilic functional groups like → Hydroxyl group that either aliphatic or aromatic → Amines that are either aliphatic or aromatic → DOES NOT INCLUDE COOH (unlike UGT) • Enzyme: PAPS-sulfotransferase enzyme family (SULT) • Co-substrate: 3'-phospho-adenosine-5'-phosphosulfate (PAPS)

Glutathione

• Are anions at physiological pH and are present everywhere in high concentration to prevent the damage caused by electrophiles; sometimes used for prodrug mechanisms • Drug Targets: Electrophillic functional groups like →Alkyl halides → Some Aryl halides → The α,β-unsaturated ketone group → Epoxides • Enzyme: Glutathione transferases (GST) • Co-substrate: γ-glutamyl-cysteinyl-glycine, a tripeptide also known as glutathione, abbreviated GSH • It can be further metabolized in several steps to the final metabolite, an N-acetyl-cysteine conjugate, which is also known as a mercapturic acid.

Glucuronide conjugation

• Are anions at physiological pH and are stable metabolites with an inversion of configuration • Drug Targets: Nucleophilic functional groups like → Hydroxyl group that either aliphatic or aromatic → Amines that are either aliphatic or aromatic → Thiols → Carboxylate groups → Nucleophilic (acidic) carbons • Enzyme: UDP-glucuronosyl-tranferase enzymes (UGT) • Co-substrate: UDP-glucuronic acid (UDPGA)

Acetylation

• Are neutral ion at physiological pH and its co-substrate is found everywhere • Drug Targets: →Amines (makes an amide) → Hydrazine (H2N-NH2) (makes a N-amide) • Enzyme: N-acetyl-transferases (NAT) → NAT-1 → NAT-2 (first to show genetic poymorphism) • Co-substrate: Acetyl-coenzyme A; has very good availbility (acetyl CoA).

Desloratidine N-glucuronidation

• Arrows indicate potential sites of N-glucuronidation • Glucuronidation of pyridine N (shown) gives a quaternary amine glucuronide

Sulfosalazine metabolism (Azo Reduction)

• Azo converts into an amine that breaks the molecule in 2

Codeine Metabolism

• CYP2D6 to active metabolite of morphine via O-dealkylation • Pts without CYP2D6 have no response • Ultrarapid CYP2D6 metabolizers causes codeine intoxication and possible addiction

CYP450 & Chirality

• CYP450 can metabolize a drug differently based on chirality since CYP450 is also a chiral molecule • Ex. Warfarin's stereoisomers become hydroxylated at different position and by different CYP450s

What is the effect of liver disease on drug-metabolizing enzymes?

• Can lead to lowered drug-metabolizing enzymes like in alcoholic cirrhosis or non-alcoholic steatohepatitis (NASH), would need to decrease dose since metabolism is too slow and would lead to toxicity

Phenylbutazone (C-glucuronide)

• Can occur at an acidic carbon, e.g, if a carbon has flanking keto groups • Forms C-C bond so glucuronide is stable

N-glucuronidation

• Can occur at primary, secondary and tertiary amines • Can occur for heterocycles such as pyrrole, pyridine, piperidine • Ring nitrogen glucuronidation can result in a quaternary amine glucuronide (positive charge) - Formation of quaternary amine glucuronides occurs in people more commonly than in lab animals

Melphalan Metabolism (Glutathione conjugation)

• Cancer drug • The drug has a very potent warhead that reacts with DNA and wil kill cancer cells • But cancer cells become resistant by producing lots of glutathione and eliminating it • But normal cells use glutathione for its protective effects

Variability in enzyme properties due to genetic polymorphism

• Chlorpyrifos is a (prodrug) pesticide that needs to be converted to a toxic form to work, but some people have diminished capability of their enzyme to work, so less toxic metabolite is made and there's less toxicity

Esterification with endogenous fatty acids or alcohols

• Classified as phase II (functional group combined with endogenous molecule) • Products are more fat-soluble • Difficult to follow or verify, because metabolites are stored in fatty tissue • May contribute to prolonged duration of action of some drugs

Aspartame Metabolism (Peptidase)

• Cleaves amide into COOH and amine, releasing two amino acids

Acetaminophen Amide Hydrolysis

• Cleaves amide into amine and carbonyl group cleaved off

St. John's wort - herbal remedy for mood disorders (anxiety, depression)

• Component of St. John's wort induces CYP3A4 • Many commonly used drugs metabolized by CYP3A4 (ex. Ethinylestradiol, acetaminophen, etc.) • Dose may no longer be effective

Isoniazid* (Acetylation of a hydrazine)

• Converts from an active drug to an inactive metabolite (N-acetyl-isoniazid) using NAT-2

What is the effect of cytokines on drug-metabolizing enzymes?

• Cytokines produced during the course of infectious diseases can down-regulate drug-metabolizing enzymes. - E.g. Interferon down-regulates CYP2 and 3 family members, would need to decrease dose since metabolism is too slow and would lead to toxicity

Mephenytoin Metabolism

• Depending on the stereoisomer, a certain modification by CYP450 will predominate

Warfarin Metabolism

• Depending on the stereoisomer, it becomes hydroxylated at different positions and by different CYP450s since different P450s prefer different enantiomers

Xenobiotics

• Describes small molecules not used by the body for nutrition or normal physiological purposes ➢ Includes therapeutic drugs, some herbal remedies, some natural products, food additives, preservatives, pesticides, pollutant chemicals, other small organic molecules

Drug Interactions with other xenobiotics -external factors

• Direct drug-drug interactions • Direct drug-food interactions • Note that in these interactions, the AMOUNT of enzyme is not affected • Could cause inhibition (more common) or enhancement of enzyme activity

Amphetamine Metabolism

• Done using MOA

Amino Acid Conjugation

• Drug needs to first be activated into a coenzyme A ester intermediate before reacting with the amino acid co-substrate; the metabolite is an anion at physiological pH • Drug Targets: → Carboxylic acid (COOH) • Enzyme: Two enzymes are involved: 1. First reaction (activation of drug to CoA ester) catalyzed by acyl-coenzyme A ligase 2. Second reaction (formation of phase II metabolite) is catalyzed by acyl-CoA-amino acid N-acyltransferase • Co-substrate: First acetyl-CoA for the first enzyme; then mainly glycine, but can be glutamine or taurine • Is a good substrate for OAT-1, a renal organic anion transporter

Competition between glucuronidation and sulfonation

• Drugs or their metabolites that contain hydroxyl groups may form either glucuronide or sulfate conjugates • Many drugs do form both conjugates • The predominant metabolite depends on several factors, including drug dose → Low dose prefers SULTS → High dose prefers UGT

P450 Cycle

• Electrons are supplied from the cofactor, NADPH, by a flavoprotein. This is the enzyme NADPH-cytochrome P450 reductase

Cytochrome P450 (P450)

• Enzyme superfamily with > 50 members in humans • All of them have a heme prosthetic group that binds iron via porphyrin and catalyzes the oxidation through the same mechanism • They are membrane-bound enzymes and are found in the ER membrane inside of the cell • Most of them are located in the liver

Epoxide hydrolase

• Epoxide hydrolase enzymes exist in two forms, microsomal and cytosolic • Open up an epoxide by adding water

N-acetyltransferase (NAT-2)

• First recognized genetic polymorphism • Since isoniazid is active and N-acetyl-isoniazid is not, the rate and extent of metabolism will affect the efficacy of the drug in treating tuberculosis • Incidence of slow acetylators varies between ethnic groups. • Asian and Native Americans have a low incidence (0-30%) of slow acetylators. • Swedish people have a high incidence (about 70%) of slow acetylators. • General US population, about 50% slow acetylators • Slow acetylators, toxicity is a concern. • Rapid acetylators, if dose is not adjusted, the drug may be ineffective

CYP2C19

• First shown with S-mephenytoin • High incidence (20-30%) of poor metabolizers among Japanese and other Asian people • Lower incidence (3-5%) of poor metabolizers among Caucasians and Africans

Metabolism of Acetaminophen/Paracetamol/Tylenol

• Forms a toxic metabolite (quinone-imine) • Can also make a sulfate primary metabolite • It can undergo a glutathione (GSH) conjugation and makes NAPQI which is very reactive and toxic (which makes Tylenol overdose dangerous) and eventually becomes a non-toxic metabolite that has the glutathione and becomes mercapturic acid (GSH detoxifies)

Alcohol dehydrogenase (ADH) and Aldehyde dehydrogenase (ALDH)

• Found in cytoplasm (unlike P450 & FMO) and does oxidation of alcohols (ADH) and aldehydes (ALDH) • Targets: Alcohols (ADH) and aldehydes (ALDH) • Cofactor: NADPH OR NADH • Oxidizing agent: O2 (molecular oxygen)

Monoamine oxidase

• Found in mitochondrion (unlike P450 & FMO & ADH & ALDH) and does oxidation of nitrogen-containing drugs • Expressed in brain as well as other tissues (plays a role in neurotransmitter metabolism) • Targets: Nitrogen-containing drugs (primary, secondary and tertiary amines) • Cofactor: NADH (unlike FMO & P450) • Oxidizing agent: O2 (molecular oxygen)

Internal (Intrinsic) Individual Differences that Affect Drug Biotransformation

• Genetic make-up → SNPs → Gene deletion or duplication • Age • Gender • Pregnancy

Procaine Ester Hydrolysis

• Has a short half life • Local anesthetic

Cytochrome P450 Properties

• Heme-iron group that catalyzes the monooxygenation of drugs and endogenous substrates such as steroids (more specific) • Co-factor: NADPH • Additional Enzyme: Flavoprotein reductase enzyme (helps with the e- transfer) • Oxidizing Agent: Molecular oxygen • Reactions That It Catalyses: → Aromatic ring hydroxylation → Aliphatic hydroxylation → N-dealkylation → O-dealkylation → N (amine becomes amide become carbamate) and S oxidation (sulfide become sulfoxide, S double bonded to an O) → Epoxidation (from a double bond) → Dehalogenation → Desaturation (introduce a double bond)

Discovery and diagnosis with debrisoquine

• In the late 1970s research found that some individuals could not hydroxylate debrisoquine, an antihypertensive agent • A patient's CYP2D6 phenotype is often clinically determined via the administration of debrisoquine (a selective CYP2D6 substrate) and subsequent plasma concentration assay of the debrisoquine metabolite (4-hydroxydebrisoquine).

Bioflavonoids in various foods

• Inhibit CYP3A4 and the p-glycoprotein transporter • Mechanism -competition for substrate binding • That's why for some medication, it says to take it on an empty stomach, to avoid these food-drug interactions

Role of Glucuronidation in Irinotecan Biotransformation

• Irintecan is a prodrug that converts into SN-38, its active metabolite • The active metabolite, SN-38, undergoes glucuronidation via UGT1A1, so that it can be excreted, but if there is a problem with the enzyme, there will be build up of the toxic active metabolite

Heme-iron Binding Site

• Iron is held by the porphyrins, there are 4 nitrogens and a part is coordinated (conjugated) w/ a cysteine • There is a substrate binding site for the functional group on the drug to be oxidized • There is a reducing interaction where the reducing enzyme on this side will provide electrons • There is also an oxygen binding site where there is usually H2O, but it will be exchanged for a O2 in the catalystic step

Tamoxifen Metabolism

• It is metabolized by CYP2D6, and a fast metabolizer would have high concentrations of the active drug (4-hydroxyTAM and Endoxifen)

Imidazole-containing drugs and CYP450 inhibition

• Lone pair of electrons on N occupy the oxygen-binding site of cytochrome P450 since the Fe+ of the P450 interacts with the nitrogen instead of the oxygen • This can prevent oxygenation of other drugs that are P450 substrates • Ex. Ketoconazole and cimetidine and Clotrimazole • Metabolism of the second drug is slowed, which could lead to prolonged retention of the drug in the body. If further doses of the second drug are given, toxic levels of this drug may be reached • The interaction with cimetidine or ketoconazole can be avoided or minimized by taking the second drug at a different time that allows the imidazole-containing drug to clear from the liver

If you are a poor metabolizer of CYP2D6, what may you need to do to avoid toxicity?

• Lower the dose, since the the parent compound would not be able to be hydroxylated, which will mean it will need not be able to become hydrophillic enough to be excreted and will work in your system longer and may even accumulate (Ex. Debrisoquine) • If an active metabolite is formed by the action CYP2D6, poor metabolizers will not respond to drug as well (ex. Tamoxifen, Codeine)

MPTP metabolism

• MOA metabolizes until it turns into MPP+ which is toxic and will trigger Parkinson's disease

Ester hydrolases or esterase

• Many drugs and prodrugs contain ester linkages • Ester hydrolases are widespread in the body • Ester groups in drugs are usually hydrolyzed readily and are source of instability

Flavin Monooxygenase (FMO)

• Membrane bound enzyme (like P450) and does oxidation with a flavin group • Targets: Oxidizes N, S (and P) (NOT C!) • Cofactor: NADPH • Oxidizing agent: O2 (molecular oxygen)

Exceptions to the rule that phase IIreactions increase water-solubility:

• Methylation • Fatty acid conjugation • Cholesterol ester formation

Morphine O-glucuronidation

• Morphine can either convert to an active metabolite or an inactive one

Amidases

• Multiple amidases in the body • The amide bond is often more resistant to hydrolysis than the ester bond → Instability of an ester can be avoided by converting the ester into an amide in drug design • Cleaves amide into amine and carbonyl group cleaved off

Age & Glucoronidation

• Newborns have very low levels of UGT proteins, thus drugs such as acetaminophen that are cleared by glucuronidation must be used with caution • Some newborns develop "physiological" jaundice because of their inability to clear bilirubin

Nicotine metabolism and main enzymes

• Nicotine can be directly eliminated using phase II rxns •If someone has a higher level of CYP2A6, they will metabolize nicotine faster, and will need to smoke more to keep nicotine levels steady, so they are more prone to becomig addicted

Reduction

• Not as common as oxidation • Occurs in the lower gastrointestinal tract (bacteria and mucosa) where there is a reducing environment • Nitro groups to amino group • Azo (N=N) groups to amino groups • Cofactor: NADPH

External (Extrinsic) Individual Differences that Affect Drug Biotransformation

• Nutritional status • Diet • Drugs, including herbal drugs • Disease state

Grapefruit Juice (Food-Drug Interaction)

• Occurs in the enterocytes (intestinal cells) • Can occur with drugs that are substrates for CYP3A4, and impact drug bioavailability - Discovered with cyclosporine - Many common drugs • Can occur with drugs that are substrates for the effluxer p-glycoprotein (Module 6) - Many common drugs (bulky, may be cationicat physiological pH) • It is a mechanism based inhibitor (binds to the active site of a substrate)

O-glucoronidation

• Occurs with phenols, alcohols and carboxylic acids • Phenols, alcohols give ether-type glucuronides, relatively stable • Carboxylic acids give acyl-glucuronides that are less stable and are prone to further metabolism

Enzymes that catalyze oxidation reactions

• Oxidation rxns are the most common rxn in drug metabolism 1. Cytochrome 450 monooxygenase (CYP450 or P450) 2. Flavin monooxygenase (FMO) 3. Alcohol and Aldehyde Dehydrogenase (ADH or ALDH) 4. Monoamine oxidase (MOA)

Peptidases

• Peptides form a "special case" of the amide bond. • Peptidase enzymes are vital in protein digestion and are often more substrate-selective than other esterase-amidase enzymes • Action of peptidases limits usefulness of peptide drugs, at least for oral administration (reduces bioavailibility)

Methylation

• Phase II that makes the drug more lipophillic • Drug Targets: → Catechol groups (two phenols or two oxygens on a benzene ring); using COMT → Thiols using TPMT • Enzyme: Depending on the type of target: 1. S-adenosine-methyl transferase/Catechol O-methyl transferase (COMT) 2. TPMT (for thiols) • Co-substrate: S-adenosylmethionine (SAM) • Is a good substrate for OAT-1, a renal organic anion transporter

Esterification

• Phase II that makes the drug more lipophillic and may prolong effects of a drug • Drug Targets: → Hydroxl → Amino groups (either amine or amide) • Enzyme: Ester hydroxylase • Co-substrate: Endogenous fatty acids (like oleic acid or choleterol)

Availability of cofactors and co-substrates - poor nutrition

• Poor nutrition can lead to lower GSH concentrations • Protein-calorie malnutrition leads to lowered free amino acids and reduced amino acid conjugation • Ethanol consumption lowers NADPH and UDPGA concentrations leading to reduced drug monooxygenation and glucuronidation

Pregnant Women & Enzyme Levels

• Pregnant women are usually advised to avoid drugs, but may have to take them under some circumstances • Evidence that some cytochrome P450 enzymes are up-regulated or down-regulated in pregnancy → CYP1A2 decreases (caffeine) → CYP2D6 increases (complicated by genetic polymorphism) → CYP3A4 increases (many drug substrates)

C-glucuronidation

• Quite unusual • Can occur at an acidic carbon, e.g, if a carbon has flanking keto groups • Forms C-C bond so glucuronide is stable

Rifampin and steroids in oral contraceptives

• Rifampin (antibiotics for TB) induces CYP3A4 • Ethinylestradiol (EE, birth control) in oral contraceptive is metabolized by CYP3A4 • Possible consequence of induction, unplanned pregnancy

Age (Intrinsic Factor)

• Several drug-metabolizing enzymes are expressed at low levels in the fetus and infant, then rise slowly during childhood. • There is a general slight decline in metabolizing ability in old age

CYP2D6

• Shows the largest phenotypical variability since it has a high genetic polymorphism • It works on N-containing drugs like bufurolol, propanolol, dextromethorphan, tamoxifen • There are 1 - 15% poor metabolizers in our population

Single Nucleotide Polymorphism (SNP)

• Single nucleotide changes in the gene for a drug biotransformation enzyme or transporter protein → If it is within a coding region, it can change the affinity or stability of an enzyme, leading to different metabolizing capabilities between people

Ethanol Metabolism

• Some individuals (asians) inefficient ALDH enzyme, resulting in buildup of acetaldehyde, which causes flushing and headaches

Phase III

• Some scientists call the process of transporter protein-aided removal of drugs and drug-metabolites out of the cell "phase III" pathways. • Lipid-soluble molecules readily cross cell membranes by simple diffusion, however the overall effect of drug biotransformation is to convert the lipid soluble drug into more water-soluble metabolites which do not readily cross cell membranes by diffusion. • Most drug metabolites, and some drugs, are eliminated from the cell with the aid of transporter proteins. These proteins reversibly bind the drug or drug metabolite, and carry it across the cell membrane into the blood or bile or urine or intestinal contents.

Points of Inhibition of P450

• Step 1 - Can compete with RH (the drug) to bind with the enzyme → Ex. Ritonavir, indinavir, and saquinavir are all CYP3A4 substrates • Step 3: Preventing the O2 binding → Ex. Ketoconazole and cimetidine inhibit by occupying the oxygen-binding site of P450 → Happens in imidazole containing drugs • Step 6 or 7: Inhibitory metabolite binds tightly to P450 (doesn't let go of the P450) → Ex. An erythromycin metabolite inhibits CYP3A4 by binding tightly and preventing metabolism of other drugs

UGT1A1*28

• The common variant of UGT1A1 associated with Gilbert's syndrome • It is also associated with irinotecan toxicity for cancer treatment

Metabolism

• The conversion of a drug to a metabolite

CYP-dependent oxidative dehalogenation

• The first step is CYP-dependent, requiring CYP reductase, NADPH and O2 • Second and Third Step are spontaneous and are elimination of HBr ad hydrolysis respectively

Halothane Metabolism

• The first step is CYP-dependent, requiring CYP reductase, NADPH and O2 • Second and Third Step are spontaneous and are elimination of HBr and hydrolysis respectively

Charcoal-broiled food and theophylline

• Theophylline (for asthma) is metabolized by CYP1A2 • Charcoal-broiled foods contain polycyclic aromatic hydrocarbons that induce CYP1A2 • Theophylline dose is no longer effective

Availability of cofactors and co-substrates - well-nourished person

• UDP-glucuronic acid present in cells at higher concentrations than PAPS (physiological donor of sulfate for SULTS)- this can influence the pathway taken, UGT pathways preferred? • GSH normally present in high concentrations • NADPH and NADH normally present in high concentrations • Acetyl-CoA usually not rate-limiting for acetylation

UGT1A1

• UDP-glucuronosyltransferase (glucuronidation) • There are many genetic variants of it and its genetic polymorphisms have been extensively studied (>100 alleles identified) • Has an important physiological substrate of bilirubin → Gene deletion causes Grigler-Najjir syndrome where there is severe unconjugated hyperbilirubinemia - usually fatal → Reduced expression or activity is called Gilbert's syndrome (too much unconjugated bilirubin) = often not known unless challenged

The most widely prescribed drugs are substrates (or their phase I metabolites are substrates) for phase II enzymes are as follows:

• UGTs ~45% • SULTs ~20% • GSTs ~15% • NATs ~5% • Amino acid conjugation ~5%

Vaproic Acid Metabolism

• Undergoes CYP-dependent desaturation

Metronidazole Metabolism (Reduction)

• Undergoes a reduction where its nitro group becomes an amino group via nitroreductase

Triclosan

→ Low dose prefers SULTS → High dose prefers UGTG