Drug Metabolism and Activation

Drug Metabolism & Inactivation/Activation

• Metabolism is a key component of pharmacokinetics
• Metabolism is often associated with drug inactivation, where active drugs are converted to inactive forms.
• Most metabolism reactions convert active drugs into inactive forms

Drug Metabolism & Termination of Drug Action

• Drug metabolism serves as a mechanism for terminating drug action by converting the drug to an inactive form
• Active drug converts to inactive metabolite through metabolism
• Metabolism makes drugs more polar and water-soluble and less lipid-soluble.
• Less lipid solubility results in less renal reabsorption and increased drug excretion
• Drugs are often metabolized into inactive forms prior to elimination

Drug Metabolism & Activation of Prodrugs

• Some drugs are administered as inactive prodrugs, requiring metabolism for activation
• Inactive prodrugs are metabolized into active drugs
• Metabolism is essential for these drugs to achieve their therapeutic effects; without it, they remain ineffective

Drug Metabolism & Formation of Active Metabolites

• Some drugs form active metabolites; both the parent drug and its metabolites are active
• Benzodiazepines (alprazolam, diazepam) are examples of drugs that form active metabolites

Drug Metabolism & Production of Toxic Metabolites

• Metabolism can sometimes lead to the formation of toxic metabolites
• Acetaminophen overuse can result in a toxic metabolite that damages the liver

Drug Metabolism & Elimination Without Metabolism

• Some drugs do not undergo metabolism at all, and are eliminated from the body unchanged
• Certain drugs that are renally excreted bypass metabolic transformation

Types of Drug Metabolism Reactions: Phase One

• Phase one reactions modify drug structures to add or unmask a polar functional group
• A primary goal of Phase one reactions is to increase water solubility for easier excretion of the drug
• Cytochrome P450 (CYP) enzymes are used to modify drug structure
• Active drugs can be converted to inactive forms in Phase one
• Drugs can be prepared for phase two metabolism in Phase one

Types of Drug Metabolism Reactions: Phase Two (Conjugation)

• Phase two reactions attach endogenous substances to drugs to enhance elimination
• Transferase enzymes are responsible for Phase two reactions
• Conjugation reactions make drugs larger and increase their excretion potential
• Examples of Phase two reactions are glucuronidation, sulfation, and acetylation

Phase One vs Phase Two Comparison

• Phase one reactions modify drug structure while phase two reactions attach large molecules for excretion
• Cytochrome P450 enzymes are used in Phase one, Transferases are used in Phase two
• Phase one can activate or inactivate a drug while Phase two prepares it for elimination
• Oxidation, reduction, and hydrolysis are common in Phase one whereas glucuronidation, acetylation, and sulfation are common in Phase two

Drug Metabolism in Neonates

• Neonates lack fully developed phase two metabolism
• Phase two reactions like conjugation are inefficient in neonates
• Drugs that rely on phase two metabolism may accumulate in neonates, increasing the risk of toxicity

Predicting Metabolic Pathways

• Phase one is not always followed by Phase two.
• Some drugs bypass phase one and go straight to phase two.
• Some drugs bypass metabolism entirely

Major Sites of Drug Metabolism: Liver

• The liver is the primary site of drug metabolism due to its high concentration of drug-metabolizing enzymes.
• Abundant Cytochrome P450 enzymes are found in the liver

Secondary Sites of Drug Metabolism

• Other organs that contribute to drug metabolism are the kidneys (second to the liver), GI tract, skin, and lungs

Intracellular Sites of Metabolism

• Phase one metabolism occurs primarily in the smooth endoplasmic reticulum (ER).
• Phase two metabolism takes place in the cytoplasm.
• Mitochondria and cell membranes also contain some enzymes that contribute to drug metabolism

Phase One Metabolism Reactions

• Phase one metabolism involves chemical modifications to drugs, mainly oxidation, reduction, and hydrolysis
• Oxidation is the most common of the three reactions
• Cytochrome P450 enzymes primarily accomplish oxidation

Cytochrome P450 Enzymes (CYP) Definition & Function

• Cytochrome P450 enzymes mediate oxidation reactions in drug metabolism

Cytochrome P450 Enzymes (CYP) Naming

• CYP enzymes are classified as follows: CYP root + number (family) + letter (subfamily) + number (specific form).
• CYP3A = Family 3, Subfamily A, Gene number 4

Most Active CYP Families in Drug Metabolism

• Major CYP families responsible for drug metabolism are CYP2C, CYP2D, and CYP3A
• CYP3A4 and CYP3A5 metabolize about half of all drugs and is the most significant family for drug metabolism
• CYP2D6 and CYP2C families account for 35% of drug metabolism
• All other CYP families contribute to only 15% of drug metabolism

Cellular Location of CYP Enzymes

• Cytochrome P450 enzymes are located on the surface of the endoplasmic reticulum (ER), where CYP complexes mediate drug oxidation reactions
• CYP enzymes contain a heme group (iron protoporphyrin IX), similar to hemoglobin

Metabolic Process of CYP Enzymes

• CYP enzymes use oxygen as a substrate and Hydrogen derived from NADPH
• They produce an oxidized drug substrate and water as a metabolic byproduct

Role of Compound I (ferryl-oxo species) in CYP450 Mediated Oxidation

• Compound I (a ferryl-oxo species, Fe4+=O): a highly reactive oxidizing agent that donates an oxygen atom to the drug substrate to facilitate its oxidation

Detailed Metabolic Process - Step by Step

• Substrate Binding: lipophilic drug enters the active site of the CYP enzyme in the smooth endoplasmic reticulum of liver cells and binds, causing a conformational change, making the iron more reactive and ready to interact with oxygen. Substrate binding shifts the heme iron from a low-spin to high-spin state, which increases its affinity for oxygen.
• Electron transfer: NADPH transfers the first electron reducing the heme iron, which is essential for binding oxygen.
• Oxygen Activation: Oxygen binds to the heme iron and becomes Fe2+–O2 after gaining an electron from CYP reductase. In the following steps, one oxygen atom gets inserted into the substrate, often turning it into an alcohol, epoxide, or other oxidized form, AND the the second oxygen atom gets reduced to water (H₂O). Creates a highly reactive "Compound I" (a ferryl-oxo species, Fe⁴⁺=O), which is the true oxidizing agent.
• Second Electron Transfer: Another electron is transferred, to splits the oxygen via Compound I, the super-reactive species that oxidizes the drug.

Impact of CYP Enzymes on Metabolism

• Phase one metabolism consists of oxidation, reduction, and hydrolysis.
• Oxidation is the most common reaction, primarily mediated by cytochrome P450 enzymes
• CYP3A is the most important CYP family, responsible for metabolizing ~50% of drugs.
• CYP enzymes are found on the endoplasmic reticulum, mediating oxidation-reduction reactions.
• Final metabolic products include oxidized drug substrates and water.

Cytochrome P450 (CYP450) Inducers Definition & Effect

• CYP inducers are drugs that increase the expression of CYP450 enzymes
• More CYP enzymes result in increased metabolism of substrate drugs
• Lower plasma drug levels and reduced drug effectiveness result from induction

Example Reaction of CYP450 Inducers

• A drug is normally metabolized from active form to inactive form. If a CYP inducer is added, metabolism shifts further to the right, increasing activity while decreasing concentrations and effectiveness

Key CYP Inducers

• Carbamazepine (anti-seizure medication).
• Rifampin (antimicrobial agent).
• St. John's Wort (natural product, used for depression).

Mnemonic for CYP Inducers

• "St. John is riding in his car, riffing."
• St. John’s Wort, Carbamazepine (“Car”), and Rifampin (“Riffing”)

Clinical Scenario CYP Induction

• If rifampin is prescribed for a bacterial infection in a patient taking oral contraceptives, the rifampin induces metabolism, enhancing the reaction AND decreasing plasma drug levels which reduces contraceptive effectiveness, and possibly causing an unintended pregnancy

Cytochrome P450 (CYP450) Inhibitors Definition & Effect

• CYP inhibitors decrease CYP enzyme expression, resulting in less CYP activity which reduces metabolism
• Higher plasma drug levels and increased toxicity risk are outcomes of inhibition

Example Reaction of CYP450 Inhibitors

• A CYP inhibitor blocks the reaction of a drug moving from active to inactive
• Active drug remains in circulation, leading to accumulation
• Increased plasma concentration leads to a higher risk of toxicity

Key CYP Inhibitors

• Cimetidine (peptic ulcer & reflux treatment).
• Amiodarone (antiarrhythmic drug).
• Erythromycin (antibiotic).
• Grapefruit juice (dietary CYP450 inhibitor).

Mnemonic for CYP Inhibitors

• "Amy's Coke & Grapefruit Juice." Amy = Amiodarone, Coke = Cimetidine, E=Erythromycin

Clinical Scenario CYP Inhibition

• A patient taking warfarin as an anticoagulant is later prescribed cimetidine for peptic ulcers; The cimetidine inhibits metabolism, reducing drug breakdown AND increasing plasma drug levels leading to a higher risk of toxicity
• Increased bleeding risk is an expected side effect due to excessive anticoagulation

Clinical Relevance

• CYP450 inducers increase enzyme activity, lowering plasma drug levels and reducing effectiveness
• CYP450 inhibitors decrease enzyme activity, raising plasma drug levels and increasing toxicity risk
• Recognizing CYP enzyme modulation is critical for predicting drug interactions

Phase Two Metabolism Reactions

• Phase two metabolism involves conjugation reactions with the goal of attaching hydrophilic molecules to drugs to enhance excretion and all phase two reactions involve transferase enzymes

Glucuronidation (Most Common Phase Two Reaction) Definition

• Glucuronidation transfers a glucuronic acid molecule to the drug with the help of glucuronosyl transferase

Clinical Consideration for Neonates & Glucuronidation

• Neonates have low levels of glucuronosyl transferase, which leads to impaired metabolism for drugs that require glucuronidation
• Chloramphenicol is metabolized via glucuronidation but neonates cannot metabolize it, leading to toxic effects like Gray Baby Syndrome, with an ashen-gray appearance due to toxicity

Other Key Phase Two Metabolism Reactions

• Sulfation: Transfers a sulfate group with the help of sulfotransferase
• Acetylation: Adds an acetyl group to the drug with the help of acetyltransferase
• Glycine Conjugation: Transfers a glycine molecule with the help of glycine transferase
• Glutathione Conjugation: Attaches acetylcysteine to the drug and is important in acetaminophen metabolism
• Methylation: Transfers a methyl group to the drug with the help of methyltransferase

Gentoype Impact of Acetylation

• Slow acetylators accumulate drugs with the risk for toxicity
• Fast acetylators metabolize faster with less risks
• Patients that are slow acetylators accumulating hydralazine are associated with drug-induced lupus (SLE-like syndrome)

Mnemonic for Phase Two Reactions

• "Sam & The Three Gs are breaking it down in Phase Two!"
• Sulfation, Acetylation, Methylation (SAM) and Glycine, Glutathione, Glucuronidation (Three Gs).

Important Stepwise Glucuronidation & Biochemical Pathway

• UDP-glucose combines with UDP, which forms UDP-glucuronic acid
• UDP-glucuronic acid then attaches to the drug
• These steps creates a larger, more easily excreted metabolite

Real Life Stepwise Aspirin Metabolism Example

• Aspirin undergoes hydrolysis and forms salicylic acid in Phase One
• Salicylic acid is then metabolized via: glucuronidation (Phase Two), glycine conjugation (Phase Two), and Oxidation (Phase One)