BASIC PRINCIPAL PART V.

Role of Biotransformation in Drug Disposition

Metabolic biotransformations occur between drug absorption into circulation and renal elimination. These transformations ensure that drugs are adequately modified for excretion, thus preventing toxicity in the body.

Reactions can be categorized into:

  1. Phase I Reactions: These involve modifications of the drug through enzymatic processes such as oxidation, reduction, and hydrolysis. The goal is to introduce functional groups (e.g., -OH, -COOH, -NH2) which can alter the biological properties of the drug, making it more suitable for further processing.

  2. Phase II Reactions: Involves conjugation processes where Phase I metabolites are linked with a second molecule (e.g., glucuronic acid, sulfate) to form more water-soluble compounds that can be easily excreted by the body.

Drug Metabolism Overview

The primary purpose of metabolism is to convert hydrophobic chemicals into hydrophilic derivatives to facilitate their elimination from the body. This process can even begin before drug absorption, as gut bacteria can metabolize certain drugs. Metabolism plays a critical role in reducing the biological activity of drugs, thus alleviating potential side effects.

Phases of Drug Metabolism

Phase I Reactions:

  • Types of enzymes: Involves various enzymatic reactions that primarily take place in the liver.

  • Mechanism: Functional groups are introduced to the chemical structure of the drug, harmonizing it for Phase II reactions. Common reactions include:

    • Oxidation (e.g., conversion of hydroxyl groups)

    • Reduction (e.g., nitro to amine conversion)

    • Hydrolysis (e.g., ester linkages)

Phase II Reactions:

  • Enzymatic Conjugation: Phase I metabolites are conjugated enzymatically increasing their water solubility, which generally enhances excretion.

  • Key enzymes involved include Glutathione-S-transferases (GSTs), UDP-glucuronosyltransferases (UGTs), and Sulfotransferases (SULT).

Drug Metabolizing Enzymes

Phase I Enzymes:

  1. Cytochrome P450s (CYPs): These enzymes play a significant role in drug metabolism, with CYP3A4 metabolizing more than 50% of clinically used drugs.

  2. Flavin-containing monooxygenases (FMOs): These enzymes also participate in oxygenation processes.

  3. Epoxide hydrolases (EHs): Involved in the hydrolysis of epoxides, often acting on toxic biochemical intermediates.

Phase II Enzymes:

  1. Glutathione-S-transferases (GSTs): These enzymes facilitate the detoxification process by conjugating drugs with glutathione.

  2. UDP-glucuronosyltransferases (UGTs): UGTs are critical for glucuronidation, enhancing the solubility and excretion of drugs and metabolic waste products.

  3. Sulfotransferases (SULT): These enzymes play a vital role in sulfation, crucial for the metabolism of a variety of drugs and hormones.

Sites of Drug Metabolism

  • High Concentration in GI Tract: The liver and intestines have the highest levels of drug-metabolizing enzymes.

  • Drug Interaction with Gut Flora: Drugs may be metabolized by gut bacteria, impacting the drug’s absorption and efficacy.

  • Portal Circulation: After ingestion, drugs enter the portal circulation to the liver where major metabolic activities occur including first-pass metabolism.

First-Pass Elimination

  • Following gut absorption, drugs first pass through the liver before entering systemic circulation.

  • This can significantly reduce the bioavailability of drugs as the liver metabolizes them, primarily via CYP3A4.

CYP Families in Metabolism

  • Drug-metabolizing CYPs predominantly belong to families 1, 2, and 3.

    • CYP2D6 and CYP2C: Critical for metabolizing many common medications.

    • CYP3A subfamilies: Includes CYP3A4, recognized as the most active and crucial for drug metabolism within the liver.

Drug-Drug Interactions

  • Metabolism rates can vary due to drug interactions, which may lead to altered drug efficacy or increased toxicity.

  • Some drugs can act as CYP inducers, enhancing metabolism rates, while others may inhibit CYP enzymes, resulting in decreased metabolism of other drugs.

  • Understanding the relevant CYP enzyme involved is fundamental for preventing adverse interactions and managing patient safety effectively.

Phase II Reactions

  • Glucuronidation by UGTs: This process is vital for drug metabolism and the clearance of substances like bilirubin from the body.

  • Sulfation in SULTs: Sulfation facilitates the metabolism of various compounds and is particularly significant in human physiology related to skin function and drug efficacy.

Glutathione Conjugation

  • Role of GSTs: GSTs mediate the transfer and conjugation of glutathione to chemicals, protecting cells from electrophilic attack.

  • Oxidative Stress: Decreased levels of reduced glutathione can lead to cellular oxidative stress, contributing to various diseases and cellular damage.

N-Acetylation and Methylation

  • N-Acetylation: Specifically targets drugs that contain aromatic amine or hydrazine groups, crucial for mitigating drug toxicity.

  • Methylation: Involves O-, N-, and S-methylation processes vital for the metabolism of neurotransmitters and various other biologically significant compounds.

Elimination Processes

  • The primary modes of drug elimination include:

    • Biotransformation: Conversion into inactive metabolites.

    • Excretion: Through kidneys, liver, lungs, and sweat.

Elimination Kinetics

  • Zero-Order Elimination: This is characterized by a constant amount of drug eliminated over time, independent of plasma concentration.

    • Example: A fixed quantity is eliminated regardless of the drug’s concentration in the bloodstream.

  • First-Order Elimination: This type of elimination is proportional to the drug's plasma level, where a higher concentration results in faster elimination.

    • Elimination Half-Life: The time required to eliminate half of the drug dose, critical for understanding dosing protocols.

Clearance and Dosage Calculations

  • Renal Clearance: Encompasses processes such as glomerular filtration, active secretion, and passive reabsorption of drugs in the renal system.

    • Total Body Clearance Formula: CL = CLR + CLNR, indicating the clearance rates from all elimination pathways.

  • Loading and Maintenance Dose Calculations: These are adjusted based on drug bioavailability and individual patient disease states to ensure therapeutic efficacy.

Practice Problems

Engaging with practice problems will help solidify understanding of drug metabolism and elimination principles, ensuring proficiency in these foundational concepts in pharmacology.