Pharmacology of Opioids and Opiates

Lecture Overview

  • Introduction to the topic of opioids and opiates.

  • Outline of learning objectives for the lecture.

    • Differentiate between opioids and opiates.

    • Discuss endogenous opioids and receptors.

    • Explore agonists and antagonists.

    • Identify key structure-activity relationships (SARs).

    • Review metabolic pathways for relevant drugs.

    • Discuss recent topics in pharmacology such as fentanyl.

    • Review log P values.

Opiates vs. Opioids

  • Both are classified as narcotics, inducing sleep and numbness.

    • Opiates:

    • Naturally derived from the opium poppy plant.

    • Examples include opium, morphine, codeine, and heroin.

    • Opioids:

    • Includes synthetic compounds and the broader category that encompasses opiates.

    • Examples of synthetic and semi-synthetic compounds will be covered.

Natural Opium Alkaloids

  • Natural opium alkaloids primarily derived from the opium poppy include compounds like morphine.

    • Chemical Structure of Morphine:

    • Pentacyclic structure.

    • Has five rings—the A, B, C, D, and E rings, with the presence of a bridging ring.

    • Chemical features include the nitrogen atom, classified as alkaloids.

    • Alkaloids:

    • Heterocyclic compounds containing one or more nitrogen atoms, typically derived from natural sources.

Endogenous Opioids

  • Endogenous Opioids:

    • Include enkephalins and endorphins, which are small peptides.

    • Structures identified:

    • Enkephalins: Pentapeptides consisting of amino acids like tyrosine, glycine, phenylalanine, methionine, and leucine.

    • Endorphins: Slightly larger peptides that also bind to opioid receptors, primarily the mu receptor.

    • Their effects mimic those of opiates but can be blocked by opioid antagonists.

Agonists vs. Antagonists

  • Definitions:

    • Agonist: A drug that binds to a receptor, stimulating a response similar to that of an endogenous compound.

    • Antagonist: A drug that binds to a receptor but does not trigger a response; instead, it inhibits the receptor’s normal activity.

    • Binding pathways and effects of agonists and antagonists are illustrated via presynaptic and postsynaptic neurons within pharmacological context.

Structure-Activity Relationships (SAR)

  • SAR Importance:

    • Critical for predicting the activity of compounds based on structural modifications.

    • Focus on regions of structural change affecting agonist vs. antagonist conversion.

    • Morphine Structure Illustration:

    • Contextualizing areas of activity (R groups) affecting response to receptors.

Effects of Substituents on Activity

  • Variations in R group functionality on the alkaloid nitrogen can switch activity from agonist to antagonist.

    • Understand how simple changes can yield significant effects on drug activity.

    • Examples:

    • Larger substituents can block the hydrophobic binding pocket affecting receptor interaction.

    • Hydroxyl groups enhance lipophilicity for better blood-brain barrier permeability.

Morphine vs. Heroin

  • Comparison:

    • Morphine is the active compound; heroin acts as a pro-drug, facilitating improved CNS penetration.

    • Metabolism Details:

    • Both heroin and codeine are metabolized in the body to form morphine, influencing their overall effectiveness and potency.

    • Key enzymes involved in metabolism include CYP3A4.

Fentanyls and Opioid Potency

  • Fentanyls: Highly lipophilic compounds that bind strongly to mu opioid receptors.

    • 80-800 times more potent than morphine, detailing both structure and functional characteristics pertinent to opioid binding.

Dual Action Opioids for Chronic Pain

  • Examples of dual-action opioids (e.g., tramadol) that inhibit reuptake of norepinephrine and serotonin while stimulating mu receptors.

    • Insight into pharmacological synergy, especially concerning chronic pain management.

Pain Management Classification

  • Distinction between nociceptive and neuropathic pain management:

    • Nociceptive pain responds to traditional opioids; neuropathic pain requires different approaches, such as antidepressants or anticonvulsants.

  • Overview of triptans for treating migraine conditions, recognizing their unique serotonergic activity.

Log P Values

  • Log P Definition:

    • Measure of drug lipophilicity, indicating the extent to which a compound partitions between octanol and water.

    • Utilized to assess drug absorption and CNS penetration.

    • Key Data Points:

    • General absorption values range from 1.35 to 1.8.

    • CNS crossing typically requires log P values above 2.2.

Concluding Remarks

  • Recap of structural and functional concepts critical to understanding opioid interactions, potency, and pharmacotherapy.

    • Encourage students to further explore these principles ahead of advanced therapeutic discussions in upcoming lectures.