What are receptors

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Drug-Receptor Interactions

• What are receptors?
• Traditional model was a rigid Lock and Key
• Lock ? Receptor surface
• Key ? Drug or Ligand

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Drug-Receptor Interactions

• Receptors ? fluid, flexible surfaces or pockets
• Can change 3-D structure as ligand docks
• Most receptors are sites for natural ligands
• Small portion or surface of a macromolecule
• Includes Enzymes, components of cell membranes,
  intracellular protein or nucleic acid,
  antibodies, DNA, RNA
• Ligand - Receptor docking ? structure changes
• Changes function

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Drug-Receptor Interactions

• Receptor - soluble molecule
• Isolated and purified when overexpressed
• Gene inserted into microorganism
• Overproduces protein
• Membrane bound receptor molecules much more
  difficult
• Overexpression can result in a large number of
  copies
• Membrane is then broken into pieces (ie
  Ultrasonication)
• Affinity chromatography
• Ligand covalently attached on a solid phase
  (Agarose)
• Membrane fragment with the receptor binds the
  solid phase
• Wash, wash, wash
• Elute receptor off the solid phase with the
  natural ligand

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Affinity Chromatography
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Drug-Receptor Interactions

• Membrane bound receptors
• Receptors of membranes are primarily proteins
• If the gene sequence is known site directed
  mutagenesis
• Alter one or more amino acids in the receptor
• Has this changed the biological responses to a
  particular ligand?
• How has altering the amino acid within the
  receptor altered the intramolecular forces that
  hold the ligand in the receptor?
• Number of internal hydrogen bonds ? amide bonds
• Number of hydrophobic interactions
• van der Waals forces
• Ionic interactions
• Small molecule interaction with a membrane
  receptor
• Can result in a large change in protein
  conformations
• Structural and physical properties of membrane
  changes

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Receptor Conformation
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Drug-Receptor Interactions

• Drugs can have two fates at the receptor site
• Irreversible covalent bonding with the receptor
  active site
• Active-site-directed irreversible inhibition
• Anticancer agents such as the alkylating agents
• Enzyme inhibitors such as MAOI tranylcyproamine
• Antibacterial agents such as the beta-lactamase
  inhibitors

Occasionally called suicide substrates
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Drug-Receptor Interactions

• Reversible drug-receptor complex
• Most desirable
• Drug can eventually be excreted
• Requires rather weak receptor / drug interactions
  
• When added together afford a stable interaction
• Hydrogen bonds 1 to 7 kcals---proteins and DNA
• Ionic bonding 5 to 10 kcals
• Ion-dipoles bonds1 to 7 kcals
• Dipole-dipole bonds 1 to 7 kcal
• Van der walls 0.5 to 1 kcal
• Hydrophobic bonding1 kcal
• If a molecule has each of these interactions 9.5
  to 33 kcal
• Covalent bonds can range from 40-140 kcal

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Drug-Receptor Interactions

• Agonists or stimulants
• Initiate a desired response
• Intrinsic activity
• Antagonist
• Decrease/prevent the response
• Response - function of number occupied receptors

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Drug-Receptor Interactions

• Dose response curves
• Reveal the affinity and effective concentration
  of a series of drug analogs

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Drug-Receptor Theory

• Occupancy Theory
• Drug and receptor interact with each other
• Complex ? effects ? Conformational changes
• Drugs structure ? affinity

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Occupancy Theory
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Rate Theory

• Agonist or stimulant activity is proportional to
  the rate of drug-receptor combination rather than
  the number of occupied receptors
• Agonist activity is the result of a series of
  rapid association and dissociation of the drug
  and the receptor
• An antagonist has a high association rate but a
  low rate of dissociation

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Other Theories

• Induced-fit theory of enzyme-substrate
  interaction
• Substrate or drug binding to the receptor induces
  3 dimensional conformational changes in the
  macromolecule positioning catalytic groups in the
  correct position to conduct productive chemistry
  or altering membrane behavior (e.g. opens
  channels for calcium)
• Macromolecular perturbation theory
• Small molecule binding produces in a
  macromolecule
• Specific conformational perturbations (Agonist)
• Non-specific conformational perturbations
  (Antagonist)
• An equilibrum mixture of specific and
  non-specific changes (partial agonist or
  antagonistic properties)
• Reality is most likely a mixture or blend of all
  these theories

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Agonism

• Relative Potency
• Relative affinity
• Intrinsic efficacy
• Relative Efficacy
• Neither produce maximal response in tissue

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Antagonism
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Antagonism cont
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Drug-Receptor Interactions

• What factors influence binding?
• Molecular structure
• Isomerism
• Functional groups
• Rigidity
• Peptide bond distance 3.61 angstroms
• Drugs - spacial relationship between functional
  groups is typically a multiple of 3.61
• Conformational changes in drugs occur to
  optimize this

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Stereochemical features of drugs

• Isomerism
• A. Cis and trans isomers in double bonds
• Different physical and chemical
  properties----distribution in a biological system
  are different

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Stereochemical features of drugs

• Isomerism continued
• Conformational isomers as a result of the
  rotation around single bonds between two atoms
• Energy barrier exists between these isomers that
  is sufficiently large that they can often be
  observed
• Examples

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Stereochemical features of drugs

• Isomerism continued
• Conformational isomers as a result of the
  rotation around single bonds between two atoms
• Remember that this is an EQUILIBRUM process

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Stereochemical features of drugs

• Isomerism continued
• Conformational isomers as a result of the
  rotation around single bonds between two atoms
• EQUILIBRUM process - results in CONFORMATIONAL
  FLEXIBILITY
• FLEXIBILITY can lead to multiple modes of action
  at different receptor types
• Example Acetylcholine muscarinic and nicotinic
  receptors
• This can often lead to SIDE EFFECTS due to
  activity at an undesirable site of action

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Stereochemical features of drugs

• Optical Isomerism
• A. Enantiomers - mirror image (plane of symmetry)

Only the L isomer is used in protein synthesis
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Stereochemical features of drugs

• Optical Isomerism
• B. Diastereomers - 2 or more chiral centers
• 2n Number of diasteromers (n of chiral
  centers)
• Example Ephedrine and Pseudoephedrine

Use Hypotension Decongestant
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Stereochemical features of drugs

• Most drugs are diastereomers
• Stereoisomers display different responses
• Receptors - variable
• blends of binding
• Active transport carrier systems
• Chiral, asymmetric molecules such as proteins,
  lipids and carbohydrates
• Preferential binding transport of one
  diastereomer
• Different lipid and water solubilities
  different distribution
• Metabolic enzymes are asymmetric
• One diastereomer preferentially metabolized
• Important when the metabolite is the active
  compound
• Excretion of the drug
• Preferential excretion of one diastereomer over
  another

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Isosterism in drug development

• What is an isostere?? These are structural
  components or functional groups of a molecule
  whose steric, electronic and solubility
  characteristics are interchangeable
• Acyclic steric isoteres are most often exploited

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Isosterism in drug development

• Cyclic steric isosteres are most often exploited

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Molecular modeling

• In the beginning - Ball and Stick
• Computers revolutioned drug development
• Quantum mechanics (not used much) and molecular
  mechanics
• Global minimum energy conformation
• Typically hydrogen bonding, ionic bonding,
  hydrophobic bonding will affect receptor binding
• Lowest energy conformer typically NOT the most
  active
• Solvation factors ignored generally
• X-ray crystallography
• Crystal match the receptor binding conformer?
• Solution conformations can be determined using
  high-resolution nuclear magnetic resonance (NMR)

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Molecular modeling

• Works well in rigid molecules (steroids)
• Highly flexible molecules problematic
• Three-Dimensional Databases
• Brookhaven Protein Database (1000s of proteins)
• Other data bases - NIH
• Examples
• HIV-protease and drugs available to treat AIDS
• Invarase - Saquinovir
• Crixivan - Indinavir
• Norvir - Ritonavir
• Angiotensin-converting enzyme
• Monoamine oxidase