💊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.
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) 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
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