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Module 2 # 1 Pharmacodynamics

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Module 2 # 1 Pharmacodynamics. K ash Desai 966-2723 HSc A120 [email protected] Drug Receptors and Pharmacodynamics (how drugs work on the body). The action of a drug on the body , including receptor interactions, dose-response phenomena, and mechanisms of therapeutic and toxic action. 2.

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slide3

Drug Receptors and Pharmacodynamics(how drugs work on the body)

The action of a drug on the body, including receptor interactions, dose-response phenomena, and mechanisms of therapeutic and toxic action.

slide4

2

Pharmacodynamics

(how drugs work on the body)

  • many drugs inhibit enzymes
  • Enzymes control a number of metabolic processes
  • A very common mode of action of many drugs
    • in the patient (ACE inhibitors)
    • in microbes (sulfas, penicillins)
    • in cancer cells (5-FU, 6-MP)
  • some drugs bind to:
    • proteins (in patient, or microbes)
    • the genome (cyclophosphamide)
    • microtubules (vincristine)
pharmacodynamics

3

Pharmacodynamics

  • most drugs act (bind) on receptors
    • in or on cells
    • form tight bonds with the ligand
    • exacting requirements (size, shape, stereospecificity)
    • can be agonists (salbutamol), or antagonists (propranolol)
  • receptors have signal transduction methods
drug receptor
Drug Receptor
  • A macromolecular component of a cell with which a drug interacts to produce a response
  • Usually a protein
types of protein receptors
Types of Protein Receptors
  • Regulatory – change the activity of cellular enzymes
  • Enzymes – may be inhibited or activated
  • Transport – e.g. Na+ /K+ ATP’ase
  • Structural – these form cell parts
dose response curves

5

dose response curves

k 1

[D] + [R] [DR] effect

k -1

k1/k-1 = affinity const.

k-1/k1 = dissociation const.(kd)

at equilibrium:

[D] x [R] x k1 = [DR] x k-1

so that: [DR] = k1

[D] [R] k-1

the lower the kd the more potent the drug

drug receptor binding
Drug - Receptor Binding

D + R DR Complex

Affinity – measure of propensity of a drug to bind receptor; the attractiveness of drug and receptor

  • Covalent bonds are stable and essentially irreversible
  • Electrostatic bonds may be strong or weak, but are usually reversible

Affinity

drug receptor interaction
Drug Receptor Interaction

Efficacy (or Intrinsic Activity) – ability of a bound drug to change the receptor in a way that produces an effect; some drugs possess affinity but NOT efficacy

Effect

DR Complex

slide11

Drug-receptor interaction

k1

Drug + Free Receptor

Drug-receptor Complex

D

(100 - DR)

DR

k-1

Where:

D = drug concentration

DR= concentration of drug-receptor complex

100 - DR = free receptor concentration

drug receptor interaction12
Drug-receptor interaction
  • At equilibrium:

[D] x [R] x k1 = [DR] x k-1

so that: [DR] = k1

[D] [R] k-1

k-1/k1 = dissociation constant (kd)

what can we learn

At equilibrium:

[D] x [R] x k1 = [DR] x k-1

so that: [DR] = k1

[D] [R] k-1

k-1/k1 = dissociation constant (kd)

What can we learn?
  • Ke (k1/k-1) is called the affinity constant
  • DR is the response; D is concentration of drug
  • when DR = 50 percent (effect is half maximal), D (or EC50) is equal to kd or the reciprocal of the affinity constant
  • response is a measure of efficacy
  • drugs that have parallel dose-response curves often have the same mechanism of action
dose response curves 2

% occupancy

6

dose response curves-2

effect = [DR] = Emax* [D]/([D]+EC50)

Concept: spare receptors

arithmetic dose scale
Arithmetic Dose Scale
  • Rate of change is rapid at first and becomes progressively smaller as the dose is increased
  • Eventually, increments in dose produce no further change in effect i.e., maximal effect for that drug is obtained
  • Difficult to analyze mathematically
log dose scale
Log Dose Scale
  • transforms hyperbolic curve to a sigmoid (almost a straight line)
  • compresses dose scale
  • proportionate doses occur at equal intervals
  • straightens line
  • easier to analyze mathematically
potency

Relative position of the dose-effect curve along the dose axis

Has little clinical significance for a given therapeutic effect

A more potent of two drugs is not clinically superior

Low potency is a disadvantage only if the dose is so large that it is awkward to administer

Potency
slide19

Relative Potency

hydromorphone

morphine

codeine

Analgesia

aspirin

Dose

slide20

7

Why are there spare receptors?

  • allow maximal response without total receptor occupancy – increase sensitivity of the system
  • spare receptors can bind (and internalize) extra ligand preventing an exaggerated response if too much ligand is present

The receptor theory assumes that all receptors should be occupied to produce a maximal response. In that case at half maximal effect EC50=kd. Sometimes, full effect is seen at a fractional receptor occupation

a gonists and antagonists

10

Agonists and antagonists
  • agonist has affinity plus intrinsic activity
  • antagonist has affinity but no intrinsic activity
  • partial agonist has affinity and less intrinsic activity
  • competitive antagonists can be overcome
agonist drugs
Agonist Drugs
  • drugs that interact with and activate receptors; they possess both affinity and efficacy
  • two types
    • Full – an agonist with maximal efficacy
    • Partial – an agonist with less then maximal efficacy
slide23

Agonist Dose Response Curves

Full agonist

Partial agonist

Response

Dose

antagonist drug
Antagonist Drug
  • Antagonists interact with the receptor but do NOT change the receptor
  • they have affinity but NO efficacy
  • two types
    • Competitive
    • Noncompetitive
competitive antagonist
Competitive Antagonist
  • competes with agonist for receptor
  • surmountable with increasing agonist concentration
  • displaces agonist dose response curve to the right (dextral shift)
  • reduces the apparent affinity of the agonist i.e., increases 1/Ke
noncompetitive antagonist
Noncompetitive Antagonist
  • drug binds to receptor and stays bound
  • irreversible – does not let go of receptor
  • produces slight dextral shift in the agonist DR curve in the low concentration range
  • this looks like competitive antagonist
  • but, as more and more receptors are bound (and essentially destroyed), the agonist drug becomes incapable of eliciting a maximal effect
d esensitization

11

Desensitization
  • agonists tend to desensitize receptors
    • homologous (decreased receptor number)
    • heterologous (decreased signal transduction)
  • antagonists tend to up regulate receptors
dose response curves 3 quantal dose response curves used in populations response is yes no

8

doseresponsecurves-3quantal dose response curves(used in populations, response is yes/no)

Therapeutic index =Toxic Dose50/Effective Dose50 (TD50/ED50)

dr curve whole animal
DR Curve: Whole Animal
  • Graded – response measured on a continuous scale
  • Quantal – response is an either/or event
    • relates dose and frequency of response in a population of individuals
    • often derived from frequency distribution of doses required to produce a specified effect
effectiveness toxicity lethality
Effectiveness, toxicity, lethality
  • ED50 - Median Effective Dose 50; the dose at which 50 percent of the population or sample manifests a given effect; used with quantal dr curves
  • TD50 - Median Toxic Dose 50 - dose at which 50 percent of the population manifests a given toxic effect
  • LD50 - Median Toxic Dose 50 - dose which kills 50 percent of the subjects
quantification of drug safety
Quantification of drug safety

TD50 or LD50

Therapeutic Index =

ED50

slide32

Drug A

100

sleep

death

Percent

Responding

50

0

ED50

LD50

dose

slide33

Drug B

100

sleep

death

Percent

Responding

50

0

ED50

LD50

dose

slide34

9

The therapeutic index

  • The higher theTI the better the drug.
  • TI’s vary from: 1.0 (some cancer drugs)

to: >1000 (penicillin)

  • Drugs acting on the same receptor or enzyme system often have the same TI: (eg 50 mg of hydrochlorothiazide about the same as 2.5 mg of indapamide)
s ignal transduction

enzyme linked

    • (multiple actions)
  • ion channel linked
    • (speedy)
  • G protein linked
    • (amplifier)
  • nuclear (gene) linked
    • (long lasting)

4

Signal transduction
slide36

Structure:

  • Single polypeptide chain threaded back and forth resulting in 7 transmembrane å helices
  • There’s a G protein attached to the cytoplasmic side of the membrane (functions as a switch).

1. G protein-linked receptors

slide38

2. Tyrosine-kinase receptors

  • Structure:
      • Receptors exist as individual polypeptides
      • Each has an extracellular signal-binding site
      • An intracellular tail with a number of tyrosines and a single å helix spanning the membrane
slide40

3. Ion channel receptors

  • Structure:
      • Protein pores in the plasma membrane
slide41

Intracellular receptors

Not all signal receptors are located on the plasma membrane. Some are proteins located in the cytoplasm or nucleus of target cells.

• The signal molecule must be able to pass through plasma membrane.

Examples:

~Nitric oxide (NO)

~Steroid (e.g., estradiol, progesterone, testosterone) and thyroid hormones of animals).

slide42

B. Second Messengers

      • Small, nonprotein, water-soluble molecules or ions
      • Readily spread throughout the cell by diffusion
      • Two most widely used second messengers are:
        • 1. Cycle AMP
        • 2. Calcium ions Ca2+
slide43

2. Calcium Ions (Ca2+) and Inositol Trisphosphate

      • Calcium more widely used than cAMP
        • used in neurotransmitters, growth factors, some hormones
      • Increases in Ca2+ causes many possible responses:
        • Muscle cell contraction
        • Secretion of certain substance
        • Cell division
slide44

Two benefits of a signal-transduction pathway

  • 1. Signal amplification
  • 2. Signal specificity
  • A. Signal amplification
      • Proteins persist in active form long enough to process numerous molecules of substrate
      • Each catalytic step activates more products then in the proceeding steps
summary

12

Summary
  • most drugs act through receptors
  • there are 4 common signal transduction methods
  • the interaction between drug and receptor can be described mathematically and graphically
  • agonists have both affinity (kd) and intrinsic activity ()
  • antagonists have affinity only
  • antagonists can be competitive (change kd) or
  • non-competitive (change ) when mixed with agonists
  • agonists desensitize receptors.
  • antagonists sensitize receptors.
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