Macromolecular Drug Receptors

• Therapeutic and poisonous medication effects result from these agents interaction with molecules in the patient
• Medications primarily associate with specific macromolecules, changing their activities
• Receptor is a cell or organism component that interacts with a medication and starts a chain of events, resulting in the drug's effects

Receptor Function:

• Central to investigating medication effects and mechanisms, aiding in biologic regulation understanding
• Isolating and characterizing medication receptors, allowing for insight into the molecular foundations of medication action
• Practical consequences for medication development and therapeutic decisions
  • Largely determine quantitative relationships between medication dosage/concentration and pharmacologic effects
  • Determine medication action selectivity
  • Mediate actions of pharmacologic agonists and antagonists

Macromolecular Drug Receptors:

• Clinically relevant medications are generally proteins, discovered after medications that bind to them
• Molecular biology and genome sequencing have helped identify receptors by structural homology
• Orphan receptors natural ligands are unknown and potential future medication targets

Regulatory Protein Receptors:

• Mediate endogenous chemical signals like neurotransmitters, autacoids, and hormones
• Target many therapeutic medications

Other Protein classes as Receptors:

• Enzymes: Inhibited or activated via drug binding,
  • Example: Dihydrofolate reductase receptor for methotrexate
• Transport Proteins: Targeted by medications
  • Example: Na+/K+-ATPase receptor for digitalis glycosides
• Structural Proteins: important drug targets
  • Example: Tubulin receptor for colchicine

Drug Receptor Function Aspects:

• Receptors determine quantitative relationships between medication concentration and pharmacologic response
• Act as regulatory proteins and components in chemical signaling mechanisms, serving as medication targets
• Serve as determinants of therapeutic and toxic effects

Drug Concentration/Response Relation:

• Can be complex; in vitro systems simplify relationship
• Hyperbolic curve describes drug concentration : effect
• E = (Emax × C) / (C + EC50)
  • E: Effect observed at concentration C
  • Emax: Maximal response
  • EC50: Concentration for 50% maximal effect

Mass Action Law:

• Drug agonists bind specific biologic molecules
• Radioactive receptor ligands used to validate occupancy assumption in medication-receptor systems
• Drug binding to receptors (B) relates to concentration (C)
  • B = (Bmax × C) / (C + Kd)
  • Bmax: Total receptor sites
  • Kd: Drug concentration for half-maximal binding (equilibrium dissociation constant)

Equilibrium Dissociation Constant (Kd):

• Represents free drug concentration for half-maximal binding
• Characterizes receptor's drug-binding affinity inversely
  • Low Kd: high affinity, and vice versa
• EC50 and Kd may differ
• Dose-response data plotted as drug effect against log dose, turning hyperbolic curve into sigmoid curve

Receptor-Effector Coupling and Spare Receptors:

• Agonist binding causes receptor conformational shifts and drug occupancy is linked to a pharmacologic response through coupling
• Full agonists strongly change conformational equilibrium
• Coupling is influenced through downstream events
• Some receptors like ligand-gated ion channels show direct dependency on drug occupancy and resulting ion current

Spare Receptors:

• Possible to have maximal biologic response when agonist doesn't occupy all available receptors
• Demonstrated using irreversible antagonists: high agonist concentrations elicit unchanged maximal response
• Spare in total downstream signaling mediator number present
• Spareness appears temporal in some cases
  • Example: β-adrenoceptor activation promotes guanosine triphosphate (GTP) binding to trimeric G protein thus signaling.
• Sensitivity to agonist depends on affinity and spareness

Competitive and Irreversible Antagonists:

• Reduce effects of agonists by either competitively/non-competitively inhibiting them at same time
• Competitive antagonists inhibit agonist response: high antagonist concentrations needed
• Sufficiently high agonist concentrations can overcome a given antagonist concentration, same maximal level of efficacy
• Presence of antagonist raises agonist concentration for a given response, shifting concentration-effect curve to right

Competitive Antagonist (dose ratio):

• C’/C = 1 + [I]/Ki
  • C’: concentration of agonist required to produce a given effect in the presence of a fixed concentration
  • C: agonist concentration required to produce the same effect in the absence of the antagonist
  • Ki:dissociation constant

Noncompetitive Antagonists:

• Agonists cannot surmount inhibitory effect, binding in irreversible or near irreversible fashion
• Unoccupied receptors may be unable to cause responses if too few of them occur due to antagonist proportion
• Distinct advantages and disadvantages on therapeutic level
• Not needing unbound form to inhibit agonist responses once irreversible antagonist has occupied receptors means long duration of action, rate of turnover of receptor molecules

Allosteric Modulators:

• Alter receptor activity without blocking agonist binding through binding to receptor on separate protein site, noncompetitively
• Negative allosteric modulators reduce receptor activity through their binding, but their actions often are reversible
• Positive allosteric modulators enhance effects of receptor's orthosteric agonist

Partial Agonists:

• Produces lower response, not from reduced affinity for receptors
• Can competitively inhibit full agonists
• Can have beneficial and deleterious effects in the clinic
  • Example: buprenorphine (safer analgesic than morphine, fentanyl) is a µ-opioid receptor partial agonist
• Can be effectively antianalgesic when administered in combination with more efficacious opioid drugs

Mechanisms of Drug Antagonism:

• Not all involve medication or endogenous ligand interactions at single receptor type, nor receptor protein involvement at all
  • Example: Protamine is positively charged, used to counteract (heparin) a negative charged anticoagulant drug
• Physiologic antagonism between endogenous regulatory pathways mediated by receptors creates catabolic glucocorticoid actions raises blood glucose levels physiologically opposed by insulin

Drug use:

• Using as physiologic antagonists creates less specific and easily controllable effects than receptors specific antagonists

Signaling Mechanisms & Drug Action:

• Requires understanding how drugs act and variations in structural receptor proteins
• Addresses clinically relevant questions

Transmembrane Signaling:

• Accomplished via different molecular mechanisms
• Specific protein families transduce myriad signals includes cell surface receptor and within cell, enzymes, components that generate, amplify, coordinate, and terminate postreceptor signaling

Five Transmembrane Signaling Mechanisms:

• Lipid-Soluble Ligand: crosses cell membrane to intracellular receptor

• Transmembrane Receptor Protein: intracellular enzymatic activity regulated by ligand binding in extracellular domain

• Transmembrane Receptor: Binds/stimulates protein tyrosine kinase

• Ligand Gated Transmembrane Ion Channel: opens/closes with ligand binding

• Transmembrane Receptor Protein: Stimulates GTP-binding signal transducer protein (G protein) modulates intracellular messenger production

• Not all chemical signals passing cell signal pass w/ 5 established mechanisms: but transducer of most important pharmaceutically used signals

Intracellular Receptors for Lipid-Soluble Agents:

• Some ligands(steroids/thyroid hormones) use intracellular receptors
• Stimulate genes near DNA sequences that regulate gene expression -"Gene-active" receptors: belongs to protein family: evolved from precursor
• Glucocorticoid action Example: hormone binding releases hsp90, enabling DNA binding & activating domains leading to target genes

Mechanism used by Hormones:

• Act through gene regulation
• Produce effects after time period for new protein synethsis
• Effects persist for longer even after concentration is reduced to 0, primarily due to slow turnover that has to occur

Ligand Regulated Transmembrane Enzymes Including Receptor Tyrosine Kinases:

• Class of receptor molecules: insulin, EGF (epidermal growth factor), PDGF (platelet-derived growth factor), ANP(atrial natriuretic peptide), TGF-Beta and many other trophic hormones
• Consist of hormone-binding domain/cytoplasmic enzyme domain like tyrosine kinase a serine kinase, or a guanylyl cyclase domains are connected by hydrophobic segment in plasma membrane

Mechanism of Activation:

• Begins with ligand binding, conformational change: activates tyrosine kinase activity, phosphorylating receptors/downstream signaling proteins
• Insulin use increases glucose/amino-acid uptake, regulates glycogen/triglyceride metabolism inhibitors of receptor tyrosine kinases can treat neoplasic disorders

Inhibitors of Receptor Tyrosine Kinases:

• Monoclonal antibodies
  • Bind to the extracellular domain of a particular receptor and interfere with growth factor binding.
• Membrane permeable small molecule chemicals
  • Inhibits enzymatic activity in the cytoplasm by binding to kinase receptor

Receptor Downregulation:

• Process where Receptor tyrosine kinases action intensity and duration are limited
• Can occur when a receptor is going through endocytosis from cell surface and followed by degradation of internalized receptors/ bound ligands
• Down regulation of transmembrane activation is regulated to limit strength/duration while somatic mutations affect gene expression regulating genes controlling cancerous cell survival/proliferation
• Atrial natriuretic peptide (ANP) regulators bind transmembrane while extracellular domain is guanylyl cyclase, generating cGMP

Cytokine Receptors:

• Respond to heterogeneous group of ligands
• Protein tyrosine kinase activity is not intrinsic to the receptor molecule; Janus-kinase(JAK) binds noncovalently to the receptor
• In the case of the EGF receptor, cytokine receptors dimerize as well

Ion Channels:

• Clinically useful medications act by mimicking agonists, blocking natural agonist examples
• Natural Ligands involve acetylcholine, serotonin, GABA, glutamate all are synaptic transmitters for transmembrane action
• nAChR (nicotinic acetylcholine receptor) is best-cell surface receptor: made-up of subunits. Conformational change occurs that results in passing NA+ ions, activating depolarization

Voltage-Gated Ion Channels:

• Controlled through membrane potential rather than binding neurotransmitters
• Different site of receptor for charged amino acids allows for channel pore membrane potential use
• Verapamil binds a calcium channel that inhibits ion conductance, creating antiarrhythmic effects and reduces pressure without impacting any endogenous transmitter

G Proteins & Second Messengers:

• They use transmembrane components to respond to extracellular messages: cell-surface receptor, GTP-binding protein (G protein), and effector component/enzymes
  • Can increase intracellular concentrations of secondary messages, use transmembrane signal system with different component (cAMP, calcium ion or phosphoinositides)
  • Effector enzyme converts intracellular atp to caMP
    • Corresponding G protein stimulates adenylyl cyclase