MedChem Module 4 Lecture 4.2

Factors Affecting Drug Biotransformation

• The primary focus of this lecture is understanding the factors that affect drug biotransformation, which is influenced by several key elements:
  • Route of Administration: Different routes can lead to varying biotransformation processes.
  • Interaction with Transporter Proteins: This can influence how drugs are absorbed and distributed in the body.
  • Biotransformation: This refers to the chemical alteration of the drug by metabolism.
  • Drug Dose: The amount of drug administered can affect its disposition in the body.

Sites of Biotransformation

• Biotransformation of drugs can occur at various anatomical locations, the primary and most significant being:
  • Liver: The major organ responsible for biotransformation due to its rich enzyme content.
  • Gastrointestinal (GI) Tract: Particularly in the upper intestine, where orally administered drugs undergo initial biotransformation before systemic circulation.
  • Lungs: For drugs administered via inhalation, where biotransformation can also occur.
  • Skin: For drugs applied dermally.
  • Kidneys: Contributes to the elimination of drugs through urine.
  • Brain: Notable for biotransformation of Central Nervous System (CNS) active drugs.

Mechanism of Drug Disposition

• Post-administration, drugs undergo specific pathways:
  • Drugs move through the GI tract for absorption and then enter the hepatic portal vein before reaching the liver for metabolism.
  • After passing through the liver, drugs or their metabolites enter systemic circulation.
  • First-pass Metabolism: The phenomenon where orally administered drugs are extensively metabolized by the liver before reaching general circulation, which can reduce the concentration of the parent drug.

Enterohepatic Cycling

• Definition: Enterohepatic cycling is a process where absorbed drugs can return to the intestine after metabolism in the liver.
  • This occurs when drugs metabolized in the liver are transported to the gallbladder and eventually reabsorbed in the intestine.
  • Example: After oral administration, drug concentration in the blood can show initial and subsequent peaks due to this reabsorption.

Factors Influencing Drug Biotransformation

• Multiple factors can modify the rate and extent of drug biotransformation, including:
  • Molecular Structure of the Drug: The arrangement of atoms and functional groups dictates metabolism capability.
  • Enzyme Availability: The amount and activity of enzymes impact biotransformation.
  • Affinity of Drug for Enzyme: Drugs with higher affinity to specific enzymes may be metabolized differently.
  • Cofactors and Cosubstrates: Essential elements that enzymes need to function effectively.
  • Drug Interactions: The presence of other drugs can influence metabolic pathways and enzyme activity.

Importance of Functional Group Analysis

• When designing drugs, understanding which functional groups exist is crucial because:
  • Different functional groups undergo various metabolic reactions, which can dramatically influence drug efficacy and safety.
  • Common functional groups include:
  • Amine Groups
  • Alkyl Chains (Cyclohexyl or Linear)
  • Aromatic Rings
  • Carbonyl Groups (including Ketones)
  • Alkenes
  • Thio-ketone Groups

Examples of Metabolic Reactions

• Cytochrome P450 enzymes (P450): A family of enzymes involved in the biotransformation of drugs in the liver.
  • Hydroxylation: Amines, cyclohexyl chains, and aromatic rings can be hydroxylated, resulting in modified structures.
  • For example, an amine can be converted to a hydroxylated amine, altering its properties.

Influence of Fluorine in Drug Design

• The incorporation of a fluorine atom in drug structures can prevent unwanted metabolic reactions, as fluorine-carbon bonds are stable and resistant to enzymatic cleavage.
• Example: In a cholesterol absorption inhibitor, replacing a metabolizable group with fluorine improved bioavailability without undergoing metabolism.

Reduction Reactions

• Drugs with carbonyl or azole groups can undergo reductions via reductases, converting ketones to alcohols, modulating their activity.
  • Example: A ketone can be reduced to a hydroxyl compound.

Hydrolysis of Esther and Amides

• Drugs containing ester bonds can be hydrolyzed by esterases to generate inactive metabolites, such as:
  • Example: Aspirin is hydrolyzed to salicylic acid and acetic acid, leading to loss of pharmacological activity.

Glucuronidation

• A key metabolic process that enhances drug solubility for excretion involves the conjugation of glucuronic acid by the enzyme UDP-glucuronsyl transferase.
  • Drugs containing amine groups or carboxylic acids can undergo this transformation, making them more hydrophilic.
  • Example: An amine group can be covalently modified by attaching a glucuronide residue, facilitating renal excretion.

Sulfation and its Applications

• Drugs containing phenols or alcohols may undergo sulfate conjugation, a less common but important route of metabolism.
  • Example: A beta agonist with a phenolic structure can be sulfated, adding polar sulfate groups to improve excretion.

Toxic Metabolites and Reactive Drug Products

• An important example is Acetaminophen (Tylenol), which can form a toxic quinone, NAPQI, through P450-mediated metabolism.
  • NAPQI is highly reactive and can lead to toxicity, particularly in overdose situations. However, detoxifying pathways using glutathione can convert this intermediate into a non-toxic compound.

Conclusion on Biotransformation Principles

• It is essential for pharmaceutical design to consider how structural features of drugs will determine their metabolic pathways. Functional groups, enzyme availability, and potential for interactions can significantly influence drug action and safety.