Drug Metabolism and Cytochrome P450

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Questions and Answers

What is the role of NADPH in reactions involving Cytochrome P450?

It acts as an electron acceptor.

It acts as an electron donor. (correct)

It donates oxygen atoms to the product.

It is involved in the conjugation reactions.

Which Cytochrome P450 isoenzyme notation indicates the family and subfamily?

CYP1A

CYP2D (correct)

CYP3A4

CYP2D6

Which reaction is characteristic of Phase I metabolism?

Alcohol oxidation. (correct)

Conjugation with glucuronide.

Methylation.

Acetylation.

What is the primary site of drug metabolism in the body?

Liver Signup and view all the answers

Which of the following substrates is primarily involved in glucuronidation reactions?

UDP glucuronosyltransferases. Signup and view all the answers

What is the consequence of Phase II metabolic reactions?

To facilitate the excretion of drugs through conjugation. Signup and view all the answers

Which of the following is NOT a characteristic of cytochrome P450 enzymes?

They only metabolize aromatic compounds. Signup and view all the answers

Which Phase I reaction involves the removal of a methyl group from a substance?

N-oxidation. Signup and view all the answers

What is the primary role of cytochrome P450 enzymes in drug metabolism?

To chemically alter drug structures through oxidation and reduction reactions Signup and view all the answers

Which of the following best describes Phase I reactions in drug metabolism?

Reactions that convert lipophilic drugs into polar metabolites Signup and view all the answers

Which statement about glucuronidation is accurate?

It is a Phase II metabolic reaction that enhances water solubility Signup and view all the answers

What is the primary outcome of Phase II metabolic reactions?

Conjugation that leads to more water soluble and inactive metabolites Signup and view all the answers

The first pass effect refers to which process?

The reduction in drug concentration when passing through the liver after oral administration Signup and view all the answers

Which of the following statements is true about the main site of drug metabolism?

The liver is the main metabolic organ for drug biotransformation Signup and view all the answers

Which of the following metabolites is an example of a prodrug?

Erythromycin-succinate Signup and view all the answers

What occurs during hydrolytic reactions of Phase I metabolism?

Esters and amides are cleaved to unmask functional groups Signup and view all the answers

What is a significant reason for the metabolism of xenobiotics?

To eliminate toxic metabolites that can be carcinogenic Signup and view all the answers

Where are the drug metabolizing enzymes, known as Mixed Function Oxidases (MFOs), primarily located?

In the smooth endoplasmic reticulum Signup and view all the answers

Which of the following is required by Mixed Function Oxidases for their enzymatic activity?

NADPH and molecular oxygen Signup and view all the answers

What happens to the oxygen utilized by the Mixed Function Oxidases during the oxidation process?

One atom appears in the product while the other is released as water Signup and view all the answers

What term describes the micro-vesicles that are formed from fragments of endoplasmic reticulum during cell homogenization?

Microsomes Signup and view all the answers

Which characteristic distinguishes rough microsomes from smooth microsomes?

Rough microsomes are primarily associated with protein synthesis Signup and view all the answers

How does the designation 'P450' relate to the cytochrome P450 family of enzymes?

It indicates the absorption characteristics of the enzymes at 450nm Signup and view all the answers

Which of the following is NOT a site of metabolism for drugs catalyzed by cellular enzymes?

Nucleus Signup and view all the answers

The main role of Mixed Function Oxidases in drug metabolism is primarily associated with which phase?

Phase 1 metabolism Signup and view all the answers

Signup and view all the answers

Study Notes

Pharmacotherapy Drug Metabolism and Pharmacokinetics

Topics of Study: Drug metabolism and pharmacokinetics
Learning Objectives: Comprehend the purpose of drug metabolism, phases of metabolism and key metabolising systems.
Subjects Discussed: Metabolism, Phase 1 Metabolism, Phase 2 Metabolism

Phases of Metabolism

Phase I: Initial modification of drugs
Phase II: Addition of larger groups to metabolites
Phase III: Transport processes

Processes of Drug Metabolism

Absorption: First and zero order processes, Ionisation , Volume of distribution, Plasma protein binding, Bioavailability, First pass effect
Distribution: Movement of drug throughout the body.
Metabolism: Chemical alteration of drugs to convert them into active or inactive forms, Liver is primary organ of metabolism, Cytochrome P450 (CYP450) enzymes, Glucuronidation, Clearance
Excretion: Removal of drugs from the body, mainly through urine and bile.

Why Metabolism?

Metabolism is necessary for food components, environmental substances, and drugs.
Most metabolites are less pharmacologically active, but some are more active, toxic or carcinogenic.

Mechanisms of Drug Metabolism

Drugs undergo various types of metabolism
Important examples include oxidation and reduction, hydrolysis
Different sites include liver, GI tract, lungs, and kidneys
Metabolism is a fundamental aspect for many processes in the body

Metabolic Phases

Phase I reactions: Chemically alter basic drug structure by oxidative and reductive reactions, hydrolytic reactions, cleave esters and amides to unmask functional groups
Phase II reactions: Couplings in which the drug or metabolite is coupled to an endogenous substrate such as glucuronic acid, conjugate is usually pharmacologically inactive, and is more water soluble so eliminated more rapidly

Sites of Metabolism

Liver: Main site, first-pass metabolism
GI Tract: involved in drug metabolism
Lungs: involved in drug metabolism
Skin: involved in drug metabolism.
Kidneys: involved in drug metabolism

Types of Drug Metabolism

Oxidation: Oxygen is incorporated into the drug molecule.
Reduction: Loss of oxygen from the drug molecule
Hydrolysis: Cleavage of chemical bonds in the drug molecule using water

Cytochrome P450 (CYP450) enzymes

Essential for the majority of drugs eliminated by hepatic metabolism
Absorbed at 450nM when carbon monoxide is present

Intracellular Sites

Endoplasmic reticulum: Majority of drug metabolism occurs here, isolated as microsomal fraction
Mitochondrion
Cytosol
Lysosomes

Microsomal Mixed Function Oxidases (MFOs)

These enzymes require both NADPH and molecular oxygen
Involved in biotransformation of drugs and other substances

Reactions Catalysed by MFOs

Aromatic hydroxylation, Aliphatic hydroxylation, Epoxidation, N or O or S-Dealkylation, Oxidative deamination, N-Oxidation, S-Oxidation, Phosphothioinoate oxidation, Dehalogenation, Alcohol oxidation

CYP450 Isoenzymes

More than 30 subtypes identified
Cause differences in individual abilities to metabolise drugs
Nomenclature, Family, and Subfamily based on structural similarity

Paracetamol Metabolism

Metabolised by 3 different pathways: glucuronidation, sulfation, and cytochrome P450 pathway (producing NAPQI)
In normal doses, NAPQI is conjugated with glutathione and excreted.
P450 mediated phase 1 metabolism of paracetamol leads to production of NAPQI, a reactive intermediate which is hepatotoxic.

Drug Metabolism: Phase II

Conjugation with endogenous substrates for increased aqueous solubility.
Glucuronidation (adding glucuronic acid), Sulfate conjugation, Amino acid conjugation

UGT Enzymes

Largest capacity enzyme system, UDP glucuronosyltransferases (UGTs)
Located in the endoplasmic reticulum
Involved in the metabolism of many drugs, removing their pharmacological effect

Phase III Example

Glucuronidation reactions involving morphine, chloramphenicol, and salicylic acid
UGTs (UDP-Glucuronosyltransferases) are responsible for glucuronidation reactions

Paracetamol Toxicity (and Phase 2)

In high doses, NAPQI overwhelms the body's glutathione supply, leading to liver damage
Phase 2 conjugation with hepatic glutathione detoxifies harmful metabolites.

Phase I and II Reactions

Phase I reactions convert lipophilic drugs to more hydrophilic metabolites.
Phase II reactions conjugate the metabolites with endogenous substrates like amino acids.
The end result of metabolism can be the conversion of an active drug to an inactive compound or to another active drug metabolite

Sequence

Phase II sometimes takes place before Phase I reactions

End Result of Drug Metabolism

Drug is converted from pharmacologically active to inactive (mostly).
Conversion of a drug to an active metabolite.
Conversion of a pharmacologically inactive compound to an active drug – prodrug.

Prodrug Definition

An inactive drug precursor that is metabolised into an active form in the body.

Control of Metabolism

Upregulation/downregulation of CYP and UGT isoforms may alter drug metabolism.
CYP and UGT work in concordance with other systems.
Adaptable responses within cells.

Therapeutic Window of Drug

The range of drug concentrations in the plasma needed to cause a therapeutic response without causing adverse effects.

Enzyme Induction: Clinical Significance

Decreased efficacy of drugs (co-administered drugs stimulate metabolism), Osteomalacia (barbiturates and antiepileptics stimulate vitamin D3 metabolism), Increased dose requirement of benzodiazepines and analgesics in smokers

Induction of Drug Metabolism

Increased synthesis of cytochrome P450 (CYP450)
Several classes of drugs, industrial chemicals, and environmental pollutants including barbituates, carbamazepine, and ethyl alcohol

HME- inducers/HME-inhibitors

Lists several inducers and inhibitors of human microsomal enzymes (HMEs).
Indicates the influence of various substances on hepatic blood flow.

Case Study(s)

63 year-old male prescribed simvastatin 10mg daily, increased to 50mg, presented with rhabdomyolysis
self-administered St. John's Wort to treat depression, impacting efficacy and toxicity of simvastatin.

Characteristics of Case Study

Therapeutic levels of simvastatin achieved by multiple dosage increases
Presence of other drugs increased clearance
St. John's Wort induced P450 enzymes
Toxic effects appeared gradually
Increased drug clearance was reversible.

Cause

Enzyme induction, presence of inducing agent causes increased metabolism
Inducing agent brings about increased synthesis of enzyme isoforms.
Patient had stopped taking inducing agents, requiring higher statin doses
Patient's subsequent rhabdomyolysis due to increased metabolism resulting from higher than therapeutic doses.

Inducing Agents

Anticonvulsants, Steroids, Polycyclic aromatic hydrocarbons, Antibiotics, Recreational agents, Herbal remedies (also known as St. John's Wort)

Mechanism of Action of Inducers

Alterations in transcription within the nucleus, resulting in increases in the expression of CYP isoforms
Consequence increased clearance of co-administered drugs metabolised by these isoforms

Induction Overview

Inducing agent combines with cytoplasmic receptor to form complex
Receptor-inducer complex migrates to the nucleus
Interaction of complex with DNA results in mRNA production
mRNA released to cytoplasm, translation produces increased CYP levels, increasing metabolic rate

Clinical Consequences of Enzyme Induction

Therapeutic failure due to induction
Drug levels restored to therapeutic levels by increasing doses or removal of inducing agent
Effects on patients are not serious as effects are readily restored

Inhibition of Drug Metabolism

Drug interaction whereby one drug inhibits another's metabolism
Two main types are: inhibition of cytochrome P450 and other specific metabolic pathways

Calcium Channel Blocker Example

Felodipine, affected by grapefruit juice which inhibits CYP3A4 in enterocytes.
Reduced bioavailability in presence of grapefruit juice

Case History Example

29 year-old healthy male taking terfenadine for allergic rhinitis
Took grapefruit juice before medication, leading to cardiac arrhythmias within an hour and death
Post-mortem report showed raised levels of terfenadine

Analysis of Case History

Post-mortem report revealed elevated terfenadine levels
No history of impaired liver function
Higher drug levels can cause cardiac arrhythmias
Grapefruit juice inhibited the metabolism of terfenadine

Characteristics of the Analysis

Patient stabilised on dosage indicating drug levels lie within therapeutic window
Grapefruit juice prevented terfenadine clearance
Toxicity resulted from intensified drug action due to accumulation

Enzyme Inhibition

Toxic effects appear rapidly
Toxic effects are rapidly reversible once inhibition is prevented
Inhibition can be a result of self-medication

Mechanisms of Enzyme Inhibition

Competitive inhibition, Noncompetitive Inhibition, Uncompetitive Inhibition, Mechanism-based inhibition

Clinical Consequences of Inhibition

Irreversible inhibition has severe clinical consequences
Elevated drug levels can cause toxicity that worsens over time if inhibitor not removed.
Destruction of enzyme is not readily reversible, making it difficult to reverse the toxic effects rapidly.

Enzyme Induction and Inhibition

Induction lowers drug levels, leading to failure of therapy
Inhibition raises drug levels, often leading to adverse reactions
Graph illustrating the difference between narrow vs wide therapeutic windows

Factors Affecting Drug Metabolism

Internal factors include genetics, age, gender, and disease
External factors include diet and environment

Gender

Menstrual steroids and differences in drug handling between sexes due to drug metabolism variability.
Phase I-III metabolism handles female steroid hormones with enzymes more affected in females.

Age and Neonates

CYP450 activity lower in newborns (<4 weeks), requiring smaller doses of certain drugs.
Developmental differences of metabolising ability can be concerning as differences disappear between 2-6 months for certain drugs

Age and Elderly

Drug metabolism declines after 70 years
Polypharmacy or multiple drug use prevalent issues in elderly population
In vitro studies show similar CYP activities between young and elderly populations.
Physiological limitations (oxygen and NADPH) may also be crucial factors in metabolism

First-Pass Metabolism

Hepatic blood flow reduces by 40% in older adults (>60 yrs) accompanied by a related reduction in renal clearance.
High intrinsic clearance drugs (rapid processing by the liver) are affected by reducing bioavailability with standard doses, creating potential ADR risks for elderly patients.
Warfarin requires a lower dose in elderly compared to younger adults due to reduced metabolism and clearance

Diet

Diet significantly impacts drug metabolism
Antioxidants, plant toxins, and preservatives can impact drug metabolsim

Polyphenols

Extend viability of fruit
Toxin to animals consuming plants.
Naringenin and bergamottin in grapefruit juice
Polyphenols and altering metabolism of Phase I and II reactions

Cruciferous Vegetables

Induce CYP1A2 and glutathione transferases, resulting in rapid clearance of certain compounds.

Barbecued Meats

Induces CYP1A1 and CYP1A2
Increased clearance of paracetamol, amitriptyline, clozapine, fluvoxamine, naproxen, and haloperidol

Effect of Alcohol on Metabolism

Acute ethanol exposure – longer drug half-lives due to preventing NADH/NAD+ ratio
Chronic exposure – phase I and II metabolism and increasing CYP2E1
Enzyme inhibition due to structural changes in hepatocytes.

Cirrhosis

Liver tissue replaced by fibrous tissue, reducing viable hepatocytes.
Phase I metabolism is unaffected, but Phase II glucuronidation is impaired, leading to decreased metabolism of some drugs like paracetamol.
Can result from alcoholic liver disease.

Environmental Factors

Tobacco smoke, motor vehicle exhaust fumes, industrial solvents, and industrial pollutants can induce drug metabolism.
Pesticides such as malathion and parathion induce P450, affect synthesis.

Conclusion

Induction and inhibition of drug-metabolizing enzymes cause modifications in pharmacokinetics.
Induction lowers drug levels, possibly leading to therapeutic failure.
Inhibition raises drug levels, often leading to adverse reactions.

References

Rang & Dale's Pharmacology 8th Edition, P.116 Section 1.9.
Rang, H P, and M M. Dale. Rang & Dale's Pharmacology. Edinburgh: Churchill Livingstone, 2016 8th edition.

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Test your knowledge on the role of Cytochrome P450 enzymes in drug metabolism, including Phase I and Phase II reactions. This quiz covers key concepts such as NADPH involvement, glucuronidation, and the first pass effect. Understand the essential functions and characteristics of cytochrome P450 in pharmacology.