sinal receptors

7/31/2019 Sinal Receptors

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Sinal - Pharmacol I 1

Drug-Receptor Interactions

Dr. Christopher Sinal

Room 5E, Tupper Medical Building

[email protected]

Overview:

Receptor proteins

Quantitation of Drug-Receptor Interactions

Agonists and Antagonists

Spare Receptors

Drug Desensitization

Therapeutic Index

Drug Toxicity

Reference:

Basic and Clinical Pharmacology, B. Katzung 10th Ed., Chapter 2

http://pharmacology.medicine.dal.ca/undergraduate/courses.cfm


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Pharmacodynamics

D + R " DRD: Drug or endogenous ligand

R: Receptor

DR: Drug-Receptor Complex

The study of the relationship of drug concentration to drug effects.

Biological Effect [DR]


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What is a Receptor?

The beneficial therapeutic effects and unwanted toxic effects of drugs are elicitedthrough interactions with proteins

-Enzymes (aspirin + cyclooxygenase)

-Transporters/Carriers (Prozac + serotonin reuptake transporter)

-Ion Channels (local anesthetics + Na+ channels)

-Receptor Proteins (cimetidine + histamine receptor)

The term receptor specifically refers to proteins that participate in intracellularcommunication via chemical signals

Upon recognition of an appropriate chemical signaling molecule (ligand), receptorproteins transmit the signal into a biochemical change in the target cell

Ligands include drugs as well as endogenous signaling molecules such ashormones and neurotransmitters


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Characteristics of Specific Ligand/Receptor Interactions

-none

-compounds exist that have

opposite physiological actions

(e.g. caffeine)

-various

-treatment of allergies, gastric

ulcers

Antagonists

(blockers)

-similar pharmacological actions

over a wide range of chemically

related compounds

-small modifications have major

effects on pharmacological activity

Chemical Specificity

-various stimulatory and

depressive effects on most cells

and tissues

-contraction of bronchial smooth

muscle

-relaxation of vascular smooth

muscle

-stimulation of gastric secretion

Biological Specificity

10-2 - 10-1 M10-5 - 10-8 MEffective Concentration

AlcoholsHistamine


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Major Classes of Receptors

Ligand-Gated Ion Channels

Tyrosine Kinase-Linked Receptors

G-Protein Coupled Receptors

Ligand-Activated Transcription Factors


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Ligand-gated

ion channels

ions

Change in

membrane potential

or

ionic concentration

Cellular

effect

nicotinic acetylcholine

receptor

(milliseconds)

Tyrosine kinase-

linked receptors

Protein

phosphorylation

Cellular

effect

insulin

receptor

(seconds-minutes)

nucleus

mRNA

Cellular

effect

protein

estrogen receptor

(hours)

Ligand-activated

transcription factors

-adrenergic

receptor

(seconds-minutes)

Cellular

effect

Intracellular

2 messenger

(e.g. cAMP)

G-protein

coupled

receptors


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Chemistry of Drug-Receptor Interactions

Most drug-receptor interactions

-reversible

-weak chemical bonds

Irreversible drug-receptor interactions-not common

-strong chemical bonds (covalent)

-e.g. aspirin, anti-tumour drugs

-usually undesirable

-reversal of effects/toxicity

-mutagenicity/carcinogenicity


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Relationship of Drug Concentration and Receptor Binding

1

FractionofReceptorsBo

und(B)

5 100

Drug Concentration [D]

0.5

Hyperbolic Concentration-Binding Curve

Bmax

Kd

B =Bmax " [D]

[D] +Kd

B -Fraction of available receptors

bound

Bmax -Maximal binding of receptors

(=1)

[D] -Concentration of drug

Kd -Equilibrium Dissociation

Constant-Drug concentration at which

1/2 of available receptors are

bound

-Measure of affinity of

drug/receptor interaction


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Sigmoidal Receptor Binding Curves

1

FractionofReceptorsBound(B)

5 100Drug Concentration [D]

Bmax

Kd

0.5

1010.1Drug Concentration [D]

Bmax

Kd

1

FractionofR

eceptorsBound(B)

0.5

-Semi-logarithmic transformation

-Common representation of pharmacological data

-Expands concentration scale at low concentration (where binding is changing rapidly)

-Compresses concentration scale at high concentrations (where binding is changing slowly)

-Does not change value of Bmax and Kd


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Graded Dose-Response Curves

Emax -the maximum response achieved by an agonist

-also referred to as drug efficacy

ED50 -the drug concentration (or dose) at which 50% of Emax is achieved

- also referred to as drug potency

1

Dilation(mm)

50Drug Concentration [D]

Emax

ED50

0.5

10 1010.1Drug Concentration [D]

Emax

ED50

1

Dilation(mm)

0.5


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Quantal Dose-Response Curves

Graded Phenomena: - infinite number of intermediate states

- vessel dilation, blood pressure change, heart rate change

Quantal Phenomena: - all-or-none

- death, pregnancy, cure, pain relief, effect of given magnitude

Quantal Dose-Response Curves

Describe population rather than

single individual responses to

drugs

based on plotting cumulative

frequency distribution of

responders against the log drugdose

Drug Dose

100

Individ

ualsresponding(%

)

1.25

50

2.5 5 10 20

% of individuals whorequire this dose to

respond

cumulative response

Emax

ED50

dose-response curve


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Agonist Types: Its All Relative


100

Effect(%

)

0.01

50

0.1 1.0 10 100

A

A: full agonist

maximum potency,

maximum efficacyC

C: full agonist

reduced potency,

maximum efficacy

D

D: partial agonistreduced potency,

reduced efficacy

B: partial agonist

maximum potency,reduced efficacy

B


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Types of Antagonism

Chemical

-interaction of two drugs in solution such that the effect of active drug is lost

e.g. metal chelators plus toxic metals

Physiological

-interaction of two drugs with opposing physiological actions

e.g. histamine: lowers arterial pressure through vasodilation (H1receptor); epinephrine raises arterial pressure throughvasoconstriction (-adrenergic receptors)

Pharmacological

-blockage of the action of a drug-receptor interaction by another compound

e.g. cimetidine blocks interaction of histamine with H2 receptorsresulting in lower gastric acid secretion


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Pharmacological Antagonists

Bind to receptors, but do not activate signal transduction mechanisms

Biological effects derive from preventing agonist (drugs, endogenous) binding andreceptor activation

Competitive Antagonists

-bind to same site on receptor as agonists

-inhibition can be overcome by increasing agonist concentration (reversible)

-primarily affect agonist potency

-clinically useful

Non-Competitive Antagonists

-bind covalently to same site as agonist (irreversible) or to a site distinct from thatof agonist (irreversible or reversible)

-inhibition cannot be overcome by increasing agonist concentration

-primarily affect efficacy

-limited clinical use


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Competitive Antagonists - Effect on Dose Response Curves

100

Effect(%

)

0.01

Agonist Concentration

50

0.1 1.0 10 100

A: -agonist + no antagonist

-agonist has maximum potency,

maximum efficacy

A B

B: -agonist + competitive

antagonist

-agonist has reduced potency,

but maximum efficacy


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Non-Competitive Antagonists - Effect on Dose Response Curves

Effect(%

)

100

0.01

Agonist Concentration

50

0.1 1.0 10 100

A: -agonist + no antagonist


maximum efficacy

A

BB: -agonist + non-competitive

antagonist


but reduced efficacy


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Constitutive Receptor Activation

R

(inactive)

Biological

Effect

R*(active)spontaneous

DR*

(active)+agonist

agonist (e.g. drug, hormone, neurotransmitter) binding induces a conformational

change in the receptor from the inactive (R) to active (R*) state

conformational change to active state can also occur spontaneously in the absence

of agonist (random kinetic fluctuations)

agonist dramatically increases the probability of conformational change and

stabilizes the active state


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Inverse Agonists Reveal Constitutive Receptor Activity


100

%Bio

logicalEffect

50

0

-50

Agonist

Alone

Antagonist

Alone

Inverse Agonist

Alone

constitutive

receptor

activity

Antagonist impacts receptor activity only in

the presence of agonist

Inverse Agonist

has an independent impact upon

receptor activity produces an effect opposite to

agonist

Agonist

has an independent impact upon

receptor activity


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Constitutive Receptor Activation

R

(inactive)

DR*

(active)

R*(active)

+drug

spontaneous

Biological

Effect

agonist (e.g. drug, hormone, neurotransmitter) binding induces a conformational

change in the receptor from the inactive (R) to active (R*) state

conformational change to active state can also occur spontaneously in the absence

of agonist (random kinetic fluctuations)

agonist dramatically increases the probability of conformational change and

stabilizes the active state

antagonist

inverse

agonist


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A Little More Math

D R DR+k+1

k-1

Biological Effect [DR]

Rate of association = k+1[D][R]

Rate of dissociation = k-1[D-R]law of mass action

At equilibrium: k+1[D][R] = k-1[DR]

[D][R]

[DR] =

k" 1

k+ 1 = K (equilibrium constan t)


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Spare Receptors

In some systems, full agonists are capable of eliciting 50% response with less than50% of the receptors bound (receptor occupancy)

Pool of available receptors exceeds the number required for a full response

Common for receptors that bind hormones and neurotransmitters

Recall:

At equilibrium:

Net effect is to increase sensitivity to ligands

if [R] is increased, the same [DR] can be achieved with a smaller [D]

a similar physiological response is achieved with a smaller [D]

[D

][R

][DR]

= K


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Receptor Occupancy versusPhysiological Response

biological effectED50 = 1.0

100

ReceptorOccupancyorEffect(%)


50

0.1 1.0 10

receptor occupancy

Kd = 1.0

100

ReceptorOccupanc

yorEffect(%)


50

0.1 1.0 10

biological effect

ED50 = 0.1

receptor occupancy

Kd = 1.0

System 1 - No Spare Receptors10 total receptors in system

Kd = 1

Occupancy of 5 receptors (50% of total)elicits 50% of maximum response

ED50 = 1

System 2 - Spare Receptors

50 total receptors in system

Kd = 1Occupancy of 5 receptors (10% of total)elicits 50% of maximal response

ED50 = 0.1


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Receptor Occupancy versusPhysiological Response

100

Response(%

)

50 1000Receptor Occupancy (%)

50

With spare receptors:

50% response = 10% occupancy

Biological effect is proportional to

[DR] only at low drugconcentrations

With

spare receptors

Without spare receptors:

50% response = 50% occupancy

Biological effect is proportional to

[DR] at all drug concentrations

Without

spare receptors


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Drug Desensitization

effect of a drug often diminishes when givencontinuously or repeatedly

desensitization, tachyphylaxis, refractoriness,

resistance, tolerance

receptor-mediated and non-receptor-mediated

mechanisms

Receptor Mediated

- loss of receptor function

- reduction of receptor number

Non-Receptor Mediated

- reduction of receptor-coupled signaling components

- reduction of drug concentration

- physiological adaptation

100

BiologicalEffect

2 100Time (h)

50

4 6 8

Response

continuous ligand exposure


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Receptor Mediated Desensitization

100

%ofControl

2 100Time (h)

50

4 6 8

ResponseReceptor #

continuous ligand exposure

1. Loss of Receptor Function

-rapid desensitization due to change in

receptor conformation

-usually due to feedback of cellular

effects of agonist-Example: phosphorylation of specific

amino acids in G-protein coupled

receptors blocks coupling to G-proteins

loss ofreceptor function

2. Reduction of Receptor Number

-slower, long-term desensitization due to change in receptor number

-usually due to feedback of cellular effects of agonist

-Example: phosphorylation of specific amino acids in G-protein

coupled receptors causes removal from cell surface

reduction ofreceptor number


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Non-Receptor Mediated Desensitization

1. Reduction of Receptor-Coupled Signaling Components-depletion of signaling molecules required for biological response

-Example: prolonged stimulation of G-protein coupled receptors can lead to

depletion of intracellular secondary messengers

2. Increased Metabolic Degradation

-increase in the rate of metabolism and/or elimination of drug-lowers plasma drug concentrations

-Example: barbiturates induce the expression of metabolic enzymes (cytochrome

P450s) that degrade this drug

3. Physiological Adaptation

-reduction or amelioration of drug effects due to opposing homeostatic response

-very few well characterized mechanisms

**all of these receptor and non-receptor dependent factors can also contribute to

interindividual differences in drug response**


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Beneficial versusToxic Drug Effects

Paracelsus 1493-1541

(the father of toxicology)

it is not the nature of the drug that

determines toxicity, but rather the

amount

everything, in excess, is potentially

toxic

all things are poison and not

without poison; only the dose

makes a thing not a poison


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Therapeutic Index

A measure of drug safety Considers dose required for a toxic effect versus that

required for the desired beneficial effect

Beneficial Effect

ED50 = 0.1

In general, a larger T.I.

indicates a clinically safer

drug

Effect(%)

100

0.01


50

0.1 1.0 10

Toxic Effect

ED50 = 1.0

Therapeutic Index (T.I.) = Toxic ED50

Beneficial ED50

Therapeutic Index (T.I.) = 1

0.1

Therapeutic Index (T.I.) = 10


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Beneficial versus Toxic Drug Effects

No drug causes only a single, specific effect

Selectivity in clinical actions is limited to a specific dose range(T.I.)

1. Effects mediated by identical receptors in the

same tissue.

2. Effects mediated by identical receptors in

different tissues.

3. Effects mediated by different receptors.


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Effects Mediated by Identical Receptors in the Same Tissue

-clinically, most serious toxicities occur in this manner

-pharmacological extension of therapeutic actions of drug

-reduced primarily through careful control of drug dosage and monitoring of blood

concentration

Example: Warfarin (coumadin)

-Anticoagulant administered to patients at risk for thrombosis (clot formation)

-Excessive blood concentrations can cause gastrointestinal bleeding and inmore severe cases cerebral and other internal hemorrhage

-Requires careful therapeutic drug monitoring (narrow therapeutic range)

-International Normalized Ratio (INR)

-Compares patient blood clotting time to a recognized standard value


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Effects Mediated by Identical Receptors in Different Tissues

Strategies to reduce toxicity:

-administration at lowest effective dose (monitoring)

-use in combination with other drugs that achieve a similar physiological effect but

through a different mechanism (allows lower dose)

-altered route of administration (e.g inhalers for asthma)

Example: Digoxin

-digitalis glycoside

-treatment of heart failure

-increases intracellular Ca+2 of cardiomyocytes through inhibition of Na+/K+

ATPase pump in cardiomyocytes

-increases force of contraction (positive inotropic effect)

-can also lead to impaired renal function and disruption of systemicelectrolyte balance through inhibition of Na+/K+ ATPase pump

-Therapeutic plasma concentration: 0.5 -1.5 ng/mL

-Toxic: >2 ng/mL


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Effects Mediated by Different Receptors

-often, receptors exist as multiple isoforms with distinct ligand selectivity, tissue expression

patterns and physiological function-drugs may exhibit affinity for multiple isoforms

Example: adrenergic receptors

- important clinical drug targets (e.g. heart failure, hypertension, asthma)

- 9 isoforms in humans (6 alphaplus 3 beta)

Relaxation

(asthma)

Increased force

and rate of

contraction

(heart failure)

Beneficial

Action

2

1

Receptor

Respiratory,

uterine and

vascular smoothmuscle

Heart

Tissue

Terbutaline

Dobutamide

Agonists

2 > >1

1 >2

Agonist

Selectivity

Increased heart

rate, chest pains

Hypotension

(low blood

pressure)

Side Effect

Strategies to reduce toxicity:

-administration at lowest effective dose

-drug combinations

-route of administration

sinal receptors

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