Medicinal Chemistry

💊Medicinal Chemistry Unit 9 – Drug Metabolism and Toxicology

Drug metabolism and toxicology are crucial aspects of pharmacology. They explore how the body processes drugs and the potential harmful effects of substances. Understanding these processes is essential for developing safe and effective medications. This unit covers key concepts like pharmacokinetics, metabolic pathways, and enzymes involved in drug processing. It also delves into factors affecting metabolism, toxicology basics, and drug-drug interactions. Clinical implications and case studies highlight the real-world importance of this knowledge.

Key Concepts and Terminology

  • Pharmacokinetics studies the absorption, distribution, metabolism, and excretion (ADME) of drugs in the body
  • Pharmacodynamics examines the biochemical and physiological effects of drugs on the body and their mechanism of action
  • Biotransformation is the process by which the body modifies a drug through chemical reactions, often to make it more water-soluble for easier elimination
  • Phase I reactions involve oxidation, reduction, or hydrolysis of the drug, typically catalyzed by cytochrome P450 enzymes (CYPs)
  • Phase II reactions involve conjugation of the drug or its metabolites with endogenous substances (glucuronic acid, sulfuric acid, acetic acid) to increase water solubility
  • Bioavailability refers to the fraction of an administered dose of a drug that reaches the systemic circulation unchanged
  • Half-life (t1/2)(t_{1/2}) is the time required for the concentration of a drug in the body to decrease by half
    • Determines the dosing frequency and time to reach steady-state concentrations
  • Clearance is the volume of plasma cleared of a drug per unit time, reflecting the body's ability to eliminate the drug

Principles of Drug Metabolism

  • Drug metabolism primarily occurs in the liver, but can also take place in other tissues (intestines, kidneys, lungs)
  • The main goal of drug metabolism is to convert lipophilic compounds into more water-soluble metabolites for easier elimination from the body
  • Metabolism can lead to the activation of prodrugs, which are inactive compounds that become pharmacologically active after biotransformation
    • Example: Codeine is metabolized to morphine by CYP2D6
  • Metabolism can also result in the inactivation of active drugs, reducing their therapeutic effects
  • Some metabolites may be pharmacologically active or toxic, contributing to the overall effects of the drug
  • First-pass metabolism refers to the metabolism of a drug before it reaches the systemic circulation, which can significantly reduce its bioavailability
    • Occurs primarily in the liver and intestines
  • Genetic variations in drug-metabolizing enzymes can lead to interindividual differences in drug response and toxicity

Major Metabolic Pathways

  • Oxidation reactions, catalyzed by CYPs, introduce oxygen into the drug molecule or remove hydrogen atoms
    • Examples: Hydroxylation, dealkylation, deamination, sulfoxidation
  • Reduction reactions, less common than oxidation, involve the addition of hydrogen atoms or removal of oxygen
    • Examples: Ketone reduction, azo reduction, nitro reduction
  • Hydrolysis reactions involve the cleavage of esters, amides, or peptide bonds by the addition of water
    • Examples: Ester hydrolysis by esterases, amide hydrolysis by amidases
  • Conjugation reactions attach endogenous substances to the drug or its metabolites, increasing water solubility
    • Glucuronidation, the most common conjugation reaction, attaches glucuronic acid to the drug
    • Sulfation attaches sulfuric acid to the drug, typically at hydroxyl or amine groups
    • Acetylation attaches an acetyl group to the drug, usually at amine groups
    • Methylation attaches a methyl group to the drug, often at amine or hydroxyl groups
    • Glutathione conjugation attaches the tripeptide glutathione to the drug, important for detoxification of reactive metabolites

Enzymes Involved in Drug Metabolism

  • Cytochrome P450 enzymes (CYPs) are the most important enzymes in Phase I metabolism, responsible for oxidation, reduction, and hydrolysis reactions
    • Located primarily in the liver, but also found in the intestines, kidneys, lungs, and brain
    • Nomenclature: CYP followed by a number for the family, a letter for the subfamily, and another number for the specific enzyme (e.g., CYP3A4)
  • CYP3A4 is the most abundant CYP in the liver and intestines, metabolizing over 50% of clinically used drugs
    • Examples: Statins, calcium channel blockers, benzodiazepines, immunosuppressants
  • CYP2D6 is highly polymorphic, with over 100 known allelic variants, leading to wide interindividual variability in drug metabolism
    • Examples: Antidepressants, antipsychotics, opioids, beta-blockers
  • CYP2C9 metabolizes many nonsteroidal anti-inflammatory drugs (NSAIDs) and oral anticoagulants
    • Examples: Warfarin, ibuprofen, diclofenac
  • Uridine diphosphate-glucuronosyltransferases (UGTs) are the main enzymes involved in glucuronidation reactions
  • Sulfotransferases (SULTs) catalyze sulfation reactions
  • N-acetyltransferases (NATs) are responsible for acetylation reactions
  • Glutathione S-transferases (GSTs) catalyze the conjugation of glutathione to drugs or reactive metabolites

Factors Affecting Drug Metabolism

  • Age: Neonates and elderly individuals generally have reduced drug metabolism due to immature or declining liver function
  • Genetic polymorphisms in drug-metabolizing enzymes can result in poor, intermediate, extensive, or ultrarapid metabolizers
    • Example: CYP2D6 polymorphisms can lead to increased risk of adverse effects or therapeutic failure
  • Liver disease can impair drug metabolism by reducing the activity of CYPs and other enzymes
  • Drug-drug interactions can occur when one drug induces or inhibits the metabolism of another drug
    • Induction increases the activity of drug-metabolizing enzymes, leading to faster clearance and reduced effectiveness of the affected drug
    • Inhibition decreases the activity of drug-metabolizing enzymes, leading to slower clearance and increased risk of adverse effects
  • Diet and nutritional status can influence drug metabolism
    • Example: Grapefruit juice inhibits CYP3A4, increasing the bioavailability of many drugs
  • Smoking induces CYP1A2, leading to faster metabolism of drugs like clozapine and olanzapine
  • Alcohol consumption can induce CYP2E1, but chronic alcohol use can also impair liver function and drug metabolism

Toxicology Basics

  • Toxicology studies the adverse effects of chemicals, including drugs, on living organisms
  • Toxicity can be acute (short-term, high-dose exposure) or chronic (long-term, low-dose exposure)
  • The dose-response relationship is a fundamental concept in toxicology, describing the relationship between the dose of a substance and the observed effect
    • The dose determines the poison: Even essential substances can be toxic at high doses
  • Lethal dose 50 (LD50) is the dose that causes death in 50% of the exposed population, used to compare the toxicity of different substances
  • Therapeutic index (TI) is the ratio of the lethal or toxic dose to the therapeutic dose, indicating the safety margin of a drug
    • A higher TI means a wider safety margin and lower risk of toxicity
  • Reactive metabolites are chemically reactive compounds formed during drug metabolism that can bind to cellular macromolecules (proteins, DNA) and cause toxicity
    • Example: Acetaminophen (paracetamol) overdose leads to the formation of a reactive metabolite (NAPQI) that depletes glutathione and causes liver damage
  • Idiosyncratic drug reactions are rare, unpredictable adverse reactions that are not dose-dependent and may have an immunological or genetic basis

Drug-Drug Interactions

  • Pharmacokinetic interactions occur when one drug alters the absorption, distribution, metabolism, or excretion of another drug
    • Example: Rifampin, a potent CYP3A4 inducer, can decrease the effectiveness of oral contraceptives
  • Pharmacodynamic interactions occur when drugs have additive, synergistic, or antagonistic effects on the same target or physiological process
    • Example: Combining opioids with benzodiazepines can lead to respiratory depression and increased risk of overdose
  • Protein binding interactions can occur when drugs compete for binding sites on plasma proteins (albumin, alpha-1-acid glycoprotein)
    • The displaced drug may have increased free concentration and enhanced effects or toxicity
  • Transporter interactions involve drugs that are substrates, inhibitors, or inducers of drug transporters (P-glycoprotein, organic anion transporting polypeptides)
    • Example: Cyclosporine, a P-glycoprotein inhibitor, can increase the bioavailability of digoxin
  • Herb-drug interactions can occur between herbal supplements and prescription medications, often involving CYP enzymes or drug transporters
    • Example: St. John's wort, a CYP3A4 inducer, can reduce the effectiveness of many drugs

Clinical Implications and Case Studies

  • Personalized medicine aims to tailor drug therapy based on an individual's genetic profile, particularly for drug-metabolizing enzymes and transporters
    • Example: Dosing of warfarin based on CYP2C9 and VKORC1 genotypes
  • Therapeutic drug monitoring (TDM) involves measuring drug concentrations in the blood to optimize dosing and prevent toxicity
    • Commonly used for drugs with narrow therapeutic indices (lithium, digoxin) or high interindividual variability (tacrolimus, vancomycin)
  • Adverse drug reactions (ADRs) are a major cause of morbidity and mortality, often related to drug metabolism and interactions
    • Example: Stevens-Johnson syndrome and toxic epidermal necrolysis are severe skin reactions associated with certain drugs (allopurinol, carbamazepine)
  • Drug metabolism and toxicology knowledge is crucial for drug development and safety assessment
    • In vitro and in vivo studies are conducted to characterize the metabolic profile and potential toxicity of new drug candidates
  • Clinical case studies illustrate the importance of understanding drug metabolism and interactions in patient care
    • Example: A patient taking warfarin experiences bleeding after starting a course of antibiotics (erythromycin, a CYP3A4 inhibitor)
    • Example: A patient on a stable dose of phenytoin, a CYP2C9 substrate, develops toxicity after starting fluconazole, a CYP2C9 inhibitor


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.