2. Pharmacodynamics and Ligands

• Pharmacodynamics concerns what drugs do to the body, including their therapeutic and side effects, and mechanism of action
• Pharmacokinetics relates to what the body does to drugs, including absorption, distribution, metabolism, and excretion
• A drug's effect is dependent on its concentration at an active site

Pharmacodynamic variations

• Drug responses can vary due to genetics, which causes variations in protein structure and activity
• Ageing can also effect them, as age-dependent alterations to protein activity
• Disease alters protein activity, and can effect drug response
• Tolerance and interactions between drugs can cause effects as well, affecting drug response

Ligands

• A ligand is an agent producing a signal by binding to a site on a target protein, either physically or chemically
• Association of a ligand to a binding site can be reversible
• Signaling molecules are classified as endogenous (hormones, neurotransmitters, cytokines) or exogenous (drugs)

Specificity and Selectivity of Drugs

• Specific ligands = more effective at a target compared to others
• Selective ligands = mild-moderate greater effectiveness at a target vs others
• Non-selective ligands = minimal difference in effectiveness among multiple targets
• Undesirable effects can take place in other parts of the body when trying to get one desired effect from a drug

Drug Targets

• Drugs can affect the body through
  • Biochemical and physiological effects
  • Mechanism of action
  • Concentration and effect relationships
  • Classification, including interactions with receptors, enzymes, carrier molecules, and ion channels
• Some other drug targets include:
  • Pathogens, which includes fungi, parasites, etc
  • Dietary supplements
  • Direct DNA interaction
  • Amino acids

Most Prescribed Drugs (UK - 2018)

• Atorvastatin & simvastatin - High Cholesterol - Enzyme
• Omeprazole & lansoprazole - Gastric acid - Transporter
• Levothyroxine - Hypothyroidism - Receptor
• Amlodipine - Hypertension - Ion channel
• Ramipril - Thromboembolism - Enzyme
• Bisoprolol - Hypertension - Receptor
• Colecalciferol (Vit. D) - Calcium metabolism - Receptor
• Aspirin - Thromboembolism - Enzyme
• Metformin - Diabetes - Enzyme
• Salbutamol - Asthma / COPD - Receptor

Receptors

• Receptors specifically bind signal molecules and can be classified as cell surface or intracellular
• Cell surface receptors
  • Embedded in the plasma membrane
  • Bind to hydrophilic ligands
• Intracellular receptors
  • Present in the cytoplasm or nucleus
  • Bind to small and hydrophobic ligands that can pass through the cell membrane

Receptor Superfamilies

• Nuclear receptors
• Ligand-gated ion channels
• G-protein coupled
• Kinase linked receptors

Intracellular Receptors

• They function through a simplistic pathway, including transcription regulators
• Nuclear receptors superfamily
  • Found in the cytosol or bound to DNA in the nucleus
  • Receptor bound inhibitory protein
  • Ligand binding
  • Dissociation of the complex and association to coactivator protein
  • Gene transcription

Cell-Surface Receptors

• Ligand-gated ion channels
  • Involved in rapid synaptic signaling
  • Ligand-gated Ca2+ channels
• G protein-coupled receptors
  • GTP-binding protein mediates interaction
  • Glucagon receptor
• Tyrosine kinases receptor
  • Functions as enzymes or is associated with enzymes
  • Insulin receptor

Examples of Cell Surface Receptors

• Ion channel linked: Acetylcholine receptor (Acetylcholine (red))
• G-Protein linked: Serotonin receptor (Serotonin (blue))
• Enzyme linked: Insulin receptor (Insulin (red))

Ligand-Gated Ion Channels (LGIC)

• These receptors are transmembrane proteins allowing the passage of Na+, K+, Ca2+, or Cl-
• Composed of an ion pore and extracellular domain
• Mediate fast synaptic transmission, converting chemical to electrical signals
• The signal molecule acts as a gate, receptor-binds
• Specific ions can pass through the channel when binded

Classification of LGIC

• 3 families, including:
  • Cys-loop receptors
  • Extracellular N-terminal domain that is ligand-binding
  • Anionic or Cationic
  • Specific for NT acetylcholine, serotonin, glycine, glutamate, and γ-aminobutyric acid
  • Ionotropic glutamate receptors
    • Binds to Glutamate
    • Extracellular N-terminal domain that is ligand-binding
  • ATP-gated channels
    • Channel open in response to binding the nucleotide ATP
    • C and N terminus on the intracellular side

Common Cys-Loop Receptor Subunits

• Serotonin (5-HT) includes 5-HT3A, 5-HT3B, 5-HT3C, 5-HT3D, 5-HT3E
• Nicotinic acetylcholine (nAChR) includes α and β subunits
• Glycine (GlyR) includes α and β subunits

G Protein-Coupled Receptors (GPCR)

• GPCRs are transmembrane proteins with an extracellular binding site, and has a cytosolic domain associated with binding to the G protein
• All GPCR's share a similar structure, and share same signal molecules
• G Protein
  • Has 3 protein subunits
  • Has a cytosolic domain
  • Is activated by GTP
  • Some regulate the production of cyclic AMP in the cytosol
  • Some directly regulate ion channels

GPCR Classification

• 3 main families
  • Differ in amino-acid sequence
  • Class 1= Rhodopsin-like
  • Class 2= Secretin receptor family
  • Class 3= Metabotropic glutamate
  • Class 4= Pheromone receptors
  • Class 5= Frizzled/Smoothened, including Cyclic AMP Receptors

Activation of GPCR

• A signal molecule binds to the receptor
• The Gα subunit binds to the receptor, releasing a GDP molecule and binding GTP
• The G protein releases from the receptor, and the Gβγ subunits disassociate
• Two main signal transduction pathways are cAMP and phosphatidylinositol signals

Tyrosine Kinases Receptors (RTKs)

• Receptors made of Transmembrane proteins
• Possessing an Extracellular domain that provides a binding site
• Includes a Cytosolic domain with enzyme activity, or associated enzymes
• Activation occurs by dimerization, GTPase Ras mediates signalling

RTKs Classification

• Epidermal growth factor (EGF)
  • Receptor family of EGF receptors that simulate the survival, growth, proliferation, or differentiation of cells
• Insulin, receptor family of Insulin receptor
  • Stimulates carbohydrate utilization and protein synthesis
• Insulin-like growth factor (IGF1)
  • Receptor family of IGF receptor-1
  • Stimulate cell growth and survival, etc
• Nerve growth factor (NGF), Platelet-derived growth factor (PDGF), Macrophage-colony-stimulating factor (MCSF), Fibroblast growth factor (FGF), Vascular endothelial growth factor (VEGF), and Ephrin receptors

Activation of RTKs process

• An inactive receptor becomes a kinase domain as a monomer
• A signal molecule binds to the receptor, leading to a dimerization of TK domains
• TK domains phosphorylate each other, completing domain activation and creating docking sites for intracellular signaling proteins

Ion Channels

• Ion channels are selective for cations (K+, Na+, Ca2+) or anions (Cl-, rare others)
• Cation channels are selective for a specific ion, or permeable to all three
• Includes Ligand-gated, Leak (low clinical usefulness), or Voltage-gated

Voltage-Gated Ion Channels

• Na+ channel close at normal membrane potential (-70mV)
• The channel opens when the membrane depolarisation (-40mV)

Transporters

• Transporters are proteins moving ions/small molecules across membranes
• May use electrochemical gradient or ATP hydrolysis
• Could move multiple ions and molecules to:
  • Symporters (co-transport)
  • Antiporters (exchange)

Enzymes

• Activation, binding, inhibition, allosteric modulation
• All these principles are similar to much drug action

Receptor Regulation

• Homeostatic activity: physiological processes adapt during drug presence
• Tolerance: lessened drug effects due to:
  • Pharmacodynamic: cellular response to a substance is reduced with repeated use
  • Pharmacokinetics: decreased quantity of the substance reaching a site
  • Drug resistance: drug inactivation increased
  • Behaviour: learning how to handle drug (e.g. alcohol)
• Sensitisation: Effect increases after repeated exposure: also known as reverse tolerance
• Withdrawal Symptoms: occurs upon abrupt discontinuation of drug
• Tachyphylaxis/desensitisation: rapid decrease in response to repeated doses over a short time period

Mechanisms of Sensitisation

• Changes in receptors (conformational change or modification like phosphorylation)
• Receptor translocation, including endocytosis of receptors
• Exhaustion of mediators, or altered metabolism/Physiological adaptation

Affinity and Efficacy

• Affinity measures the bindings of a drug to its target and effect
• Efficacy measures the drug's ability to generate an effect

Binding Experiments and Affinity

• Kd defines the equilibrium dissociation constant for a drug-receptor interaction
  • Kd = ligand concentration at which 50% of receptors are bound
  • A lower Kd indicates a tighter drug-receptor interaction

Concentration-Effect

• Binding is due to concentration; it can be measured directly through biological response
• Physiological effect is also due to concentration
• May be maximal before receptors saturate
• Ratio of physiological effect to binding may not be 1:1
• Receptor heterogeneity

Potency

• Emax defines maximal effect
• EC50 measures the concentration at which the effect is half maximal
• Potency measures the concentration of a drug to cause an effect and is measured by EC50

Agonist Types

• Full agonist induce a maximal physiological response with receptor saturation
• Partial agonist unable to induce a maximal physiological response with receptor saturation
• Inverse agonists binds to the orthosteric site and reduce constitutive activation

Drug Types

• Antagonists are "neutral", possessing zero efficacy.
  • They inhibit the action of an agonist, but has no effect in the absence of the agonist
  • If there is constitutive receptor activity, they do not inhibit it
• The are various the types of antagonism which include: Competitive or non-competitive and Reversible / Irreversible
  • Reversible competitive antagonist: Binds at the active site
  • Reversible non-competitive antagonist: Binds at different site
  • Irreversible antagonist: Competes for binding site and also increases antagonist

Informal Antagonism

• Chemical “antagonism”
  • Antagonist that prevents action
• Pharmacokinetic “antagonism”
  • Antagonist reduces concentration
• Physiological “antagonism”
  • Antagonist opposed to physiological effect

Allosteric Modulation

• Involves binding to receptors
• Orthosteric involves binding for agonist and competitive antagonist
  • Allosteric alters agonist
  • Affecting agonist affinity
  • Directly relating to responses themselves

Quantitative Receptor Theory

• Receptor occupancy theory: it applies the law of mass action to Receptor Theory
  • Rate of a reaction depends on molecule concentration k1XA (Ntot-NA) = K-1 NA
• Pharmacodynamics as Hill–Langmuir equation accounts for drug interaction

Receptor Two-State Model

• Based on constant to describe the interaction between ligand and receptor
• Receptors shift between inactive (R) and active (R*) states
  • Rest: equilibrium lies to left
  • Agonist binding- shifts equilibrium right
  • Antagonist has no difference
  • Inverse agonist moves it further left

Drugs in the Therapeutic Context

• Drug effect occurs when at a certain concentration
• Therapy: includes the effects of the sum, from individual - Must be a good, benefit and effect - Too much activity of desired target is bad

Dose vs Concentration

• Concentration is the amount of a solute per unit and dose, is how much of a substance you take
• Larger sizes have higher concentrations

Dose Response

• Includes drug to receptor interactions
• Involves physiological and therapeutic effects
• Adverse Dose: Curve: is the same but it is focused to find the adverse, also the concentration curve to the clinical

Therapeutic Range and Index

• Therapeutic range defines the concentration of a drug within the body
• The point where drugs are most effective with limited negative side effects
• The therapeutic index: includes median, useful and lethal dose
  • Median useful value affects 50% of test subjects
  • Median Lethal: dosage that affect the other half with death
  • LD/ED ration shows not the clinical use
• Low dosage indicates dangerous dosage