Pharmacodynamics

Partial Agonists

• Have submaximum intrinsic activity, meaning they cannot achieve the maximum possible response.
• When combined with a full agonist, a partial agonist can block the full agonist's action.
• This is because the partial agonist's activity is always less than the full agonist's activity.
• Partial agonists produce an effect but at a submaximum level.
• Full agonists cannot bind when a partial agonist is present, resulting in submaximum efficacy.
• In the presence of a full agonist, a partial agonist acts as a competitive antagonist due to its lower efficacy.
• Example: Buprenorphine
  • An opioid partial agonist.
  • Provides analgesia for pain relief by stimulating opioid receptors.
  • Has a much lower efficacy than full agonists like morphine.
  • In the presence of morphine, buprenorphine will always have maximum efficacy, and it will antagonize the effects of morphine due to its lower efficacy.
• Another example: Pindolol
  • A beta blocker with partial agonistic activity at beta-adrenoceptors.
  • Has partial efficacy at these receptors.
  • In the presence of a full agonist, it acts as a beta-adrenoceptor blocker.
• Partial agonists have:
  • Affinity for receptors.
  • Intrinsic activity.
  • Submaximum efficacy compared to full agonists.

Drug Interaction with Receptors

• Only drugs or endogenous compounds with a specific chemical structure can interact with receptors.
• Receptors are protein molecules with a 3D structure.
• Agonists or antagonists must fit the shape of the receptors to bind and produce an effect.
• Molecules with different structures cannot interact with receptors.
• Lock and Key Theory:
  • A molecule (key) with a specific shape can only unlock a receptor (lock) with a matching shape.
• Types of interactions between drugs and receptors:
  • Electrostatic interaction.
  • Hydrogenic bond.
  • Van der Waals forces.
  • Hydrophobic interaction.
  • Covalent bond (irreversible).

Receptor Categories

• Receptors can be subdivided into four categories:
  • G protein-coupled receptors (metabotropic receptors).
  • Ion channel-linked receptors (ionotropic receptors).
  • Enzyme-linked receptors.
  • Receptors affecting gene transcription (cytosolic, cytoplasmic, or nuclear receptors).

G Protein-Coupled Receptors (Metabotropic Receptors)

• Coupled with specific G proteins.
• Agonist binding causes conformational changes in the receptor, activating the G protein.
• Activation of the G protein leads to intracellular events via second messenger molecules.
• Upon agonist binding:
  • Conformational changes occur in the receptor.
  • The alpha subunit dissociates from the beta-gamma subunits.
  • Each subunit (alpha, beta-gamma) has its own signaling function.
  • Beta-gamma subunits can activate different signaling cascades.
  • Alpha subunits can activate various enzymes.
• The beta-gamma subunit reassociates with the alpha subunit after dissociation, returning the receptor to its initial state.
• E (Enzymes):
  • G proteins activate different types of enzymes.
  • Three main types of G proteins: $G<em>s$ (stimulatory), $G</em>i$ (inhibitory), and $G_q$ (other).
• Two well-characterized second messenger cascades:
  • $G_s$ mediated activation of adenylyl cyclase:
    • Adenylyl cyclase is an intracellular enzyme.
    • Activation increases cyclic AMP (cAMP) levels.
    • Increased cAMP activates protein kinase A.
    • This leads to a decrease in intracellular calcium.
    • In smooth muscle cells, this results in smooth muscle relaxation.
  • $G_i$ activation:
    • Opens potassium channels.
    • Results in membrane hyperpolarization.
    • Leads to an inhibitory effect.
  • $G_q$ proteins:
    • Associated with phospholipase C.
    • Activation leads to the formation of IP3 (inositol trisphosphate) and diacylglycerol (DAG).
    • IP3 causes calcium release and muscle contractions.
    • DAG activates protein kinase C (PKC), leading to specific pharmacological responses.

Key Enzymes in Intracellular Events

• $PIP_2$ (Phosphoinositol biphosphate).
• $IP_3$ (Inositol three phosphate).
• Diacylglycerol (DAG).
• PKC (Protein Kinase C).

Three Steps in the Cascade

1. Ligand Binding and Receptor Activation
   • Drug or agonist binds to the receptor.
2. G Protein Activation
   • Conformational changes in the receptor activate the G protein.
3. Signal Transduction
   • Engagement of intracellular mechanisms leads to specific pharmacological effects.
   • Smooth muscle relaxation is an example of a final pharmacological effect.

Example: Beta-2 Adrenoceptors in Bronchial Smooth Muscle

• Salbutamol (a beta-2 agonist) binds to beta-2 adrenoceptors in the smooth muscle cells of the bronchus.
• This activates adenylyl cyclase, increasing cAMP levels and decreasing intracellular calcium.
• The result is smooth muscle relaxation and bronchodilation.
• Pharmacological effect: Bronchodilation.
• Clinical Response: Relief of breathlessness due to bronchoconstriction.

Example: Muscarinic Receptors

• Acetylcholine (agonist) stimulates muscarinic receptors in cardiac conduction cells.
• This increases potassium ion permeability, leading to hyperpolarization and bradycardia.
• Antimuscarinic drugs (e.g., atropine) block muscarinic receptors, preventing acetylcholine binding.
• Atropine inhibits bradycardia by preventing endogenous acetylcholine from stimulating the receptors.
  • Antagonists do not produce an effect themselves; the observed effect results from blocking the receptors and preventing the endogenous agonist from binding.

Ionotropic Receptors (Ligand-Gated Ion Channels)

• Drug receptors are structurally attached to an ion channel.
• Binding of the drug causes conformational changes, opening the ion channel.
• This leads to an influx of cations or anions, depending on the receptor type.
• Result: Changes in membrane potential and specific pharmacological effects.
• Examples: Nicotinic receptors, GABA receptors, NMDA receptors.
• Nicotinic Receptors:
  • Acetylcholine binds to alpha subunits, opening the channel and increasing sodium influx.
  • This results in membrane depolarization.

Enzyme-Linked Receptors

• Mediate the first step in signal transduction.
• Example: Insulin receptors.
• Composed of an extracellular domain and an intracellular domain.
• The intracellular domain typically contains an enzyme (e.g., tyrosine kinase).
• Agonist binding causes receptor dimerization and enzyme activation.
• Activation of tyrosine kinase leads to activation of transcriptional factors, resulting in specific cellular responses.
• Insulin Receptors:
  • Insulin binding causes dimerization.
  • Tyrosine kinase activation results in a range of pharmacological effects.

Receptors Affecting Gene Transcription (Intracellular Receptors)

• Located inside the cell.
• Associated with stabilizing proteins that keep the receptors inactive.
• Agonist must migrate through the cellular membrane to bind to the intracellular receptor.
• Binding causes detachment of the stabilizing protein.
• The drug-receptor complex migrates into the nucleus and binds to specific DNA responsive elements.
• This regulates gene transcription, either upregulating or downregulating the synthesis of regulatory proteins.
• Result: Specific cellular responses.
• Glucocorticoid Receptors:
  • Prednisolone (a glucocorticoid) migrates through the cellular membrane and binds to the glucocorticoid receptor.
  • The complex migrates into the nucleus and binds to glucocorticoid-responsive elements of DNA.
  • This upregulates or downregulates the synthesis of different regulatory proteins.
  • Results in a range of pharmacological effects: anti-inflammatory, immunosuppressive, anti-proliferative.

Non-Receptor Mechanisms of Drug Action

Enzymes as Targets

• Enzymes are molecules that break down substrates into metabolites.
• Blocking enzymes can lead to specific pharmacological effects.
• Acetylcholinesterase:
  • Breaks down acetylcholine.
  • Blocking acetylcholinesterase (e.g., by neostigmine) increases acetylcholine levels.
  • This results in an indirect cholinergic effect.
  • Used in the treatment of Alzheimer's disease and myasthenia gravis.
• Phosphodiesterases:
  • Responsible for the inactivation of cyclic AMP.
  • Blocking these enzymes (e.g., by theophylline) increases cyclic AMP levels.
  • Theophylline was previously used to treat bronchial asthma.

Transporters as Targets

• Sodium-Potassium ATPase:
  • Pumps sodium out of and potassium into myocardial cells.
  • Digoxin blocks sodium-potassium ATPase.
  • This increases intracellular sodium, leading to increased calcium levels.
  • Results in increased force of myocardial contraction, which is helpful in patients with congestive heart failure.
• Monoamine Reuptake Inhibitors:
  • Reduce the reuptake of monoamine neurotransmitters.
  • Used in the treatment of depression.
  • Most antidepressants selectively inhibit serotonin or norepinephrine reuptake.

Ion Channels as Targets

• Sodium Channels:
  • Many local anesthetics block sodium channels.
  • They penetrate the membrane, block sodium channels from the inside, and disrupt impulse transmission.
  • Results in local anesthesia.

Physical and Chemical Properties of Drugs

• Drugs can produce effects due to physical or chemical properties, not through receptors.
• Physical Action:
  • Osmotic Diuretics:
    • Increase osmolality of the glomerular filtrate inside the kidney tubules.
    • Facilitate water excretion and inhibit sodium and chloride reabsorption.
    • Produce diuresis.
• Chemical Reaction:
  • Antacids:
    • Relieve gastric pain by chemically interacting with hydrochloric acid in the stomach.
    • Examples: Aluminum hydroxide, magnesium oxide, calcium hydroxide.
    • Neutralize hydrochloric acid, relieving symptoms associated with hypersecretion of hydrochloric acid.

Summary of Pharmacodynamics

• Pharmacodynamics: The study of how drugs affect the body, including their effects and mechanisms of action.
• Majority of drugs produce effects by acting on cellular receptors.
• Four types of drug-receptor interactions: agonist, antagonist, partial agonist, and inverse agonist.
  • Agonist: Affinity and efficacy (intrinsic activity).
  • Antagonist: Affinity but no intrinsic activity.
  • Partial agonist: Affinity and efficacy, but efficacy is submaximum.
  • Inverse agonist: Affinity and negative efficacy (opposite of the full agonist).
• Four types of receptors:
  • G protein-coupled receptors (metabotropic): $G<em>s$, $G</em>i$, and $G_q$ subtypes.
  • Ionotropic receptors: Associated with ion channels.
  • Enzyme-linked receptors: Linked with enzymes (e.g., insulin receptors).
  • Cytoplasmic or nuclear receptors (intracellular): (e.g., glucocorticoid receptors).
• Drugs may also exert effects through:
  • Interaction with enzymes.
  • Interaction with transporters.
  • Interaction with ion channels.
  • Physical action.
  • Chemical reactions.