13/14 Quantification of Drug-Receptor Interactions

Question 1

structure - activity relationships

Answer 1

relationship b/w chemical structure and biologic activity

Question 2

recognition sites

Answer 2

sites whose general function is recognition of intercellular chemical signals (like neurotransmitters, hormones, cytokines, etc.)

“substances do not act unless bound”

Question 3

receptors

Answer 3

“receptive substances” that carry recognition sites

• 5% of human genome codes them
• Includes enzymes, membrane channels, transporters, nucleic acids, & various macromolecular targets of drug action
• Common types: ligand-gated ion channels (fastest, transduce chemical to electrical signal), GPCRs (fast, heterotrimers), Enzyme-linked receptors (less fast, intracellular catalytic domain like kinase & autophosphorylation), Nuclear receptors (slow, respond to membrane permeant ligands)

Question 4

agonists

Answer 4

neurotransmitters, hormones, and chemically related drugs which interact with receptors to stimulate the natural cellular response

• Act at [low] (micromolar or less)
• Activity greatly influenced by small changes in chemical structure
• Can be antagonized selectively
  
  • Eg. A given [atropine] can markedly block the action of ACh on smooth muscle, while leaving histamine unchanged

Question 5

antagonists

Answer 5

neurotransmitters, hormones, and chemically related drugs which interact with receptors to inhibit the natural cellular response

• Activity very sensitive to changes in cellular structure

Question 6

pharmacodynamics

Answer 6

effects of drugs on the organism

• Mediated by interactions w/macromolec receptors

Question 7

pharmacokinetics

Answer 7

effects of the organism on drugs

• Absorption, Distribution, Metabolism, Excretion (“AD ME”

Question 8

occupancy model

Answer 8

• magnitude of drug response proportional to occupancy of receptors by drug molecules (law of mass action like Michaelis Mented enzyme-substrate interactions)
  
  • A + R <-> AR
  • By law of mass action, rate of association = k+1 [A][R] and the rate of dissociation = k-1[AR]
    
    • k+1 = rate constant of association
    • k-1 = rate constant of dissociation
  • At steady-state (equilibrium): k+1[A][R] = k-1[AR]
• Equilibrium (dissociation) constant: KA = k-1/k+1
  
  • KA (binding constant)units are concentration
  • Inverse relationship to affinity
    
    • High KA = low affinity (need lots of drug to get response)
    • Low KA = high affinity (little drug to get response)
  • KA = [A]50 = drug concentration that binds 50% receptors
• If [R]T is the total number of receptors, then [R]T - [AR] = [R]
  
  • YA = [AR]/[R]T = [A]/([A]+KA)
    
    • Occupancy = YA (proportion) = concentration [AR] of bound receptor
      
      • Both similarly related to [A]
    • [AR]/[R]T = fraction of receptors occupied by drug molecs

Question 9

Concentration-Occupancy Curve

Answer 9

Approaches asymptote = RT (w/high [A] all A will be bound by receptor

• Bind 50% will give estimation of KA
• Hyperbolic curve

Question 10

log-concentration occupancy curve

Answer 10

Approaches same asymptote

• Sigmoid curve
• increase KA = lower affinity and curve shifts to the right (same max occupancy, but takes more A to fill it)

Question 11

Lineweaver-Burk Plot

Answer 11

1/[AR] = y axis

1/[A] = x axis

x-intercept = -1/KA

y-intercept = 1/[R]T

increase KA = less negative x-intercept and [R]T doesn’t change

Question 12

scatchard plot

Answer 12

yaxis: [AR]/[A]
xaxis: [AR]

x-intercept: [R]T

slope = -1/KA

increase Ka = less steep slope, [R]T doesn’t change

Question 13

when can est of equilibrium constant be made from occupancy plots?

Answer 13

If receptor binding is measured directly (eg radiolabeled ligand) then est of equilibrium constant can be made from these plots

OR

competitive antagonism - curve moves the same amount to the right w/set amount of competitive antagonist for BOTH occupancy and response, so you can determine KB (if B is the competitive antagonist)

• no effect on agonist occupancy or maximum agonist response
• [A]50 increased by a factor of 1+[B]/KB

Distance moved to the right by the log dose-response curve = log (1+[B]/KB

• Doesn’t depend on relationship b/w receptor occupancy and physiological response
• Therefore gives an accurate measure of KB whether occupancy or physiological response data are used

CURVES LOOK THE SAME AS FOR DECREASED AFFINITY/INCREASED KA IN TERMS OF HOW THEY CHANGE

Question 14

dose-response curves

Answer 14

plotted physiological response to agonist

Careful - they can look like they conform to the occupancy equation because plots look linear, but drug responses are rarely proportional to receptor occupancy

[A]50 represents ligand concentration that elicits 50% response, not 50% occupancy

Question 15

partial agonist

Answer 15

has both agonist and antagonist properties & an efficacy of intermediate or low numerical value

has no spare receptors (can never get max response)

In the presence of a full agonist, a partial agonist can behave as a weak agonist or a weak antagonist depending on the ambient level of full agonist

Question 16

response = ?

Answer 16

f(efficacy X occupancy)

Question 17

efficacy of full agonist vs partial agonist vs competitive antagonist

Answer 17

large # vs intermediate/low number vs ZERO

Question 18

insurmountable antagonism

Answer 18

• Cardinal feature of competitive antagonism is that it is surmountable (maximum response is undiminished if a large enough excess of agonist is added)
• Insurmountable antagonists can reduce the maximum response
  
  • Multiple mechanisms
• Antagonist may interact irreversibly w/ same recognition site as agonist
  
  1. Modifies site covalently or by binding so tightly that dissoc is undetectable
• Antagonist interacts reversibly or irreversibly with allosteric site
  
  1. A site other than primary agonist recognition site, to inhibit activation of the receptor
• Antagonists preferentially bind and inhibit agonist-activated receptors = uncompetitive
• Insurmountable antagonists often lumped together and labeled as:
  
  • Noncompetitive= block is not overcome by raising [agonist]
  • Or identified by their specific molecular target (“channel blocker”)

Question 19

what happens to maximum agonist occupancy and [A]50 w/irreversible antagonist on occupancy?

Answer 19

maximum occupancy decresaes to 1-block

[A]50 not affected

depends on degree of block and on the function relating occupancy to response.

Question 20

two state model of an ionotropic receptor without constitutive activity?

Answer 20

• at rest, all receptors inactive
• can’t distinguish b/w a ligand that binds preferentially to Ri and that binds with equal affinity to Ri or Ra
• neither of above ligands disturbs resting equilibrium or elicits any response in teh absence of agonist
• both of these ligands behave as competitive antagonists in the presence of agonist
• full inverse agonist, partial, or neutral antagonist all act as competivie antagonists
• partial agonist and full agonist act w/ positive efficacy

Question 21

two state model of an ionotropic receptor with constitutive activity?

Answer 21

• at rest, enough receptors active to cause functional effects
• can distinguish b/w a ligand that binds preferentially to Ri and one that binds w/equal affinity to Ri or Ra
• a ligand that binds preferentially to Ri “turns off” constitutive activity and is called an inverse agonist
• a ligand that binds with equal affinity to Ri or Ra has no effect on constitutive activity but is equally effective in competitively antagonizing and agonist or inverse agonist and is thus called a neutral antagonist
• full inverse active, partial inverse agonist are negative efficacy
• neutral is both negative and positive efficacy
• full agonist and partial agonist are postive efficacy

Question 22

GPCR and ‘biased agonism’

Answer 22

• Single activated GPCR usu couples to multiple signaling pathway
• Each ligand may stabilize a unique conformation of the same receptor and thus induce a unique pattern of downstream intracellular signals
• Each ligand at the same GPCR will have multiple efficacies - different for each signal (or associated response that is measured)
• = biased agonism or (ligand-directed signaling)
• effect of ligand binding to a GPCR is not the same for all signaling pathways coupled to receptor (eg same ligand can be agonist for one pathway and inverse agonist for another) - efficacy is pathway dependent
• different ligands for same receptor may have different sets of efficacies - implies that the GPCR can adopt multiple conformations depending on associating ligand
• eg TRV130 is a G-protein based ligand at mu opoid receptor that is potently analgesic with reduced GI and respiratory dysfunction compared with morphine
• eg 2 beta blockers stimulated EGFRs and cardiomyocyte survival