integrated pharmacotherapy i drug targets, ligands ... · pdf file1 integrated pharmacotherapy...

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Integrated Pharmacotherapy I

Drug Targets, Ligands, Receptors, and

Mechanisms of Drug Action

Required reading: Chapters 1 and 2, Basic and Clinical

Pharmacology, 10th Ed., Katzung BG, McGraw Hill, 2007.

Edward Fisher, Ph.D., R.Ph. Professor and Associate Dean for Academic Affairs

Professor and Associate Dean for Academic Affairs

University of Hawaii at Hilo College of Pharmacy [email protected]

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Learning Objectives

After attending these lectures, completing the required reading, &

studying these handouts, you should be able to describe the basic

principles of drug receptors & pharmacodynamics including:

The dose-response relationship

Signaling mechanisms and drug action

The relationship between dose and clinical response

Ligands

• Molecules that carry signals for cellular communication

– Endogenous ligands

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Ligands

• Endogenous ligands

– Neurotransmitters

– Hormones - secreted from endocrine glands

• Ductless and located in connective tissue

• Secreted into blood and have effects at distant sites, they are released:

– In steady amounts (e.g. thyroid hormone)

– Based on specific cycle

» Circadian rhythms (e.g. glucocorticoids)

» 4-week cycle – female hormones

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Types of Hormones

• Steroid hormones– derived from cholesterol • Amino acid derivatives

– Catecholamines – Histamine – Serotonin – Melatonin

• Peptides and proteins – Neuropeptides (vasopressin, oxytocin) – Pituitary hormones (corticotropin, gonadotropins) – Gastrointestinal hormones (insulin)

• Eicosanoids – Derived from polyunsaturated fatty acids especially arachidonic acid

• Prostaglandins, prostacyclins, leukotrienes, thromboxanes

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Types of Hormones

• Hydrophobic hormones – Receptors found in the cell – Act by affecting gene expression – Examples

• Sex hormones – Androgens and estrogens

• Adrenal hormones – Mineralocorticoids and glucocorticoids

• Hydrophilic hormones – Receptors on cell surfaces – Examples

• Catecholamines • Peptide hormones • Eicosanoids

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Mechanisms of Hormone Action • Activation of enzymes

– Usually by changing the releative amounts of enzymes that are phosphorylated

• Modulation of gene expression

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Local Hormones = autocoids

• Characteristics – Transport via blood is not necessary – Have local effects – Short duration of action

• Functions – Defense – Repair – Classifications

• Paracrine – act on neighboring cells – Synaptic transmission – special type of paracrine

• Autocrine – also act on the cell that secreted them – Influence a group of cells to act the in the same way

• Juxtacrine – direct contact is necessary – Gap junctions

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Local Hormones = autocoids

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Introduction to Pharmacology

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Pharmacology: the study of substances that interact with living

systems through chemical processes.

Medical pharmacology – therapeutic application

Toxicology – undesirable effects of chemicals on living systems

Pharmacodynamics: the actions of the drug on the body.

Biochemical effects, mechanism of action (MOA)

Drug classification

Pharmacokinetics: the actions of the body on the drug.

Absorption, distribution, metabolism, & excretion

Involved with the time course of the drug in the body

How the drug effects you

How you effect the drug


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Receptor: part of an organism or cell (macromolecule) that

interacts with a ligand (drug, endogenous molecule) causing a

chain of biochemical events leading to an observable response. Active states vs. inactive states

Drug: any substance that brings about a change in biologic

function through its chemical actions.

Endogenous – synthesized in the body (hormones)

Xenobiotics – chemicals not synthesized by the body

Drug: Pharmacy definition: Articles intended for use in the

diagnosis, cure, mitigation, treatment or prevention of disease in

man or other animals

.


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Drug targets – biomolecules that have a role in the disease

process and are considered to be the site of action for drug

therapy (receptors, enzymes, DNA, ion channels, transport

proteins)

Agonist: a drug that binds to and activates a receptor which

brings about an effect.

Albuterol – β2-selective adrenoceptor agonist


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Antagonist: a drug that binds to a

receptor and prevents agonists from

binding; do not activate receptor;

blockers.

No direct effect, the antagonistic effect

results from the prevention of agonist binding

and activation of the receptor

Atropine – antagonist for muscarinic

cholinoceptors

Curare - antagonist for nicotinic

cholinoceptors

Generally they are structurally bulky to

prevent receptors from going back to active

confirmation, and so that they have more sites

for receptor interaction


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Partial Agonist: a drug that binds to a receptor and activates it,

but the effect is not as great as with a full agonist.

Are agonists if no full agonist is present; are antagonists if a full

agonist is present

Pindolol – partial β receptor agonist

Inverse Agonist: a drug that binds to a receptor and stabilizes it

in the inactive conformation.

Constitutively active receptors – active without binding to agonist

Many drugs that act as competitive antagonists

are really partial agonists


15 Adapted from: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), © 2006 McGraw-Hill Companies

Regulation of the activity of a receptor with conformation-

selective drugs

a = active state

i = inactive state

Two-state Model

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A Little Background on Drugs

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A drug is a substance that brings about a change

in biologic function through its chemical actions.

Sources

Endogenous

Exogenous (xenobiotics)

States

Solids

Liquids

Gases

Size

Vast majority of drugs have a MW between 100 – 1000

MW 100 helps achieve selective receptor binding

MW 1000 inhibits diffusion-mediated distribution

= Chemicals foreign to the

biological system in question

Drugs do not create effects they modulate function

A Little Background on Drugs

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Chirality: stereoisomerism, R or S – “handedness”

Over 50% of all useful drugs are chiral

In the majority of cases, one enantiomer will have different

pharmacodynamics, pharmacokinetics than the other enantiomer

Labetalol (2 chiral centers, 4 enantiomers)

(S,R) – α-blocker

(R,R) – -blocker

(S,S) and (R,S) – inactive

Eutomer – isomer with desired activity

Distomer – isomer with undesired activity

Prototype drug: a member of a drug group that typifies the

most important characteristics of the group.

There are several thousand drugs that are currently available

arranged into ~ 70 groups

Drug Receptors & Pharmacodynamics

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Receptors:

1.) Determine the quantitative relations between dose or

concentration of drug and pharmacologic effects

receptor number in various target tissues

2.) Are responsible for selectivity of drug action

Affinity – determined by chemical forces that cause drug

to bind to the receptor

Efficacy – change in confirmation toward the active state

Intrinsic activity – ability to evoke maximal effect after

binding

3.) Mediate the actions of both pharmacologic agonists and

antagonists

receptor classes, subtype, and isoforms

Some drug’s MOA do not involve receptors: antacids, osmotics.

Receptor:

Class α β

Subtype α1 α2

Isoforms α1a α1b

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Molecular Recognition

• Physiochemical complementarity

– Mostly by noncovalent bonding – ionic and hydrophobic type interactions are most important

– Steric complementarity • Chiral recognition

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Models of binding • Lock and key

• Induced fit

– Seen when ligand causes receptor to change conformation from inactive to active

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Types of Receptors

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Regulatory Proteins:

Mediate actions of most drugs and endogenous chemicals

(neurotransmitters, hormones, autocoids)

Best characterized

Enzymes:

Usually through inhibition

Methotrexate inhibits dihydrofolate reductase

Transport Proteins:

Digoxin inhibits Na+, K+ ATPase

Structural Proteins:

Colchicine inhibits tubulin Prevents polymerization of microtubules

Types of Receptors

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Aspects of Drug Receptor Function

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1.) Relationship between drug concentration (dose)

& pharmacologic response

2.) Signaling mechanisms and drug action

3.) Relationship between dose and clinical response

Relationship Between Dose & Response

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Concentration – Effect curves = Dose – Response curves

From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), © 2007 McGraw-Hill Companies

Hyperbolic curve; concentration-effect reflects concentration-

receptor binding

Occupation theory

Magnitude of pharmacological effect is proportional to percentage of

receptors occupied

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EMAX – maximal response that can be produced by drug

EC50 – concentration of drug that produces 50% maximal effect

Kd – concentration of free drug at which there is 50% maximal binding

equilibrium dissociation constant; measure of affinity

low Kd – high affinity – slow dissociation

high Kd – low affinity – rapid dissociation



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Sigmoidal curve

Expands region of

low drug

concentration

Linear mid-portion

Compresses higher

portion


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Receptor-Effector Coupling:

Transduction process between occupancy & drug response

Coupling efficiency based on 1.) extent of conformational change

(full agonist-full response, partial agonist-partial response) and

2.) biochemical events that transduce occupancy into response

Spare Receptors:

When maximal response can be

elicited by an agonist when not all

available receptors are occupied

C in the figure demonstrates the

concentration of antagonist that

causes available receptors to no

longer be “spare”, they are just

sufficient to still get maximum

response


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Spare Receptors (cont.):

Temporal - when response outlasts binding (GTP binding)

Spare in number

Excess of β1 receptors in heart (90% can be blocked and still

get maximal response)

From: Basic & Clinical Pharmacology, B.G. Katzung

(Ed.), © 2001 McGraw-Hill Companies

Sensitivity of a cell or

tissue to a particular [ ] of

agonist depends on

affinity of receptor for

agonist & degree of

spareness. Spare

receptors increase

sensitivity to drug.


32 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.),

© 2007 McGraw-Hill Companies

Competitive Antagonist:

Reversibly competes with agonist for receptor binding

Antagonism can be overcome by concentration of agonist

Effect is influenced by:

Concentration of antagonist

Concentration of agonist that is

competing for binding to receptors

Propranolol vs. Norepi

Effects vary widely in

individuals due to differences

in clearance

Exercise can overcome effect




Irreversible (noncompetitive) Antagonist:

Antagonism cannot be overcome by concentration of agonist

Not dependent on own rate of elimination

Phenoxybenzamine

irreversible α adrenoceptor

antagonist

control hypertension due to

pheochromocytoma

in case of overdose, cannot

competitively block receptor


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Full agonist at

single

concentration

Full agonist at

single

concentration


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Allosteric Antagonist:

Binds to another part of the molecule

Chemical Antagonist:

A drug may bind to and inactivate another drug

Protamine used to counteract heparin

Desferrioxamine chelates iron

Physiological Antagonist:

One type of functional antagonism – agonists that oppose via action

on a different receptor or system

Use of a separate endogenous regulatory pathway

Glucocorticoids vs. insulin in controlling blood glucose

Effects are less specific & more difficult to control


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Mechanisms of Receptor Antagonism

Adapted from: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), © 2006 McGraw-Hill Companies


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Mechanisms of Receptor Antagonism

Adapted from: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), © 2006 McGraw-Hill Companies

Signaling Mechanisms & Drug Action

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1.) Chemical crosses membrane & interacts with intracellular receptor

2.) Ligand-regulated transmembrane enzyme

3.) Receptor tyrosine kinase

4.) Ion channel

5.) Cell surface receptor linked to an effector enzyme by a G protein From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), © 2007 McGraw-Hill Companies


39 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), © 2007

McGraw-Hill Companies

1.) Intracellular receptors:

Corticosteroids, mineralocorticoids,

sex steroids, thyroid hormone

Stimulate transcription by binding to

response elements

Therapeutically…

hormonal effects are produced

after a lag time of 30 min – few

hours

effects can persist for hours –

days after agonist concentration

is 0 (blood levels are not

indicative of activity)

Nitric Oxide (NO; gas) – stimulates

guanylyl cyclase (not shown here)


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2.) Ligand Regulatory Transmembrane Enzymes:

Ligand activates enzymatic activity of cytoplasmic domain

Similar to tyrosine kinases

3.) Receptor Tyrosine Kinases:

Ligand activates enzymatic activity of bound protein tyrosine kinase

Insulin, epidermal growth factor (EGF), platelet-derived growth

factor (PDGF), atrial natriuretic peptide (ANP), transforming

growth factor- (TGF-)

Mechanism:

autophosphorylation

varied substrate phosphorylation

Activity outlasts receptor binding

Down-regulation

agonist continually binding to receptor causes increased

endocytosis of receptor

Site for drug action for growth factors, cancer


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EGF receptor – typical receptor tyrosine kinase


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Cytokine receptor


43 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), © 2007

McGraw-Hill Companies

4.) Ligand-Gated Ion Channels:

Fastest way; crucial for synapse function

Increases conductance of particular ion across membrane

Examples

nicotinic acetylcholine

receptor (Na+; shown)

GABA (Cl-)

excitatory: glutamate,

aspartate

inhibitory: amino acids

glycine and blocked by

strychnine

Non-specific ion channel

Signaling Mechanisms &

Drug Action

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5.) G Proteins and

2nd Messengers:

“Serpentine”

receptors

Separate excitation

of receptor and

activity of effector

Adrenergic amines,

serotonin,

acetylcholine (M)

Adapted From: Human Physiology, L. Sherwood (Ed.) © 2004 Brooks/Cole

First messenger

(Hormone)

G protein

intermediary

Plasma

membrane

ECF

Receptor

(Binding to receptor

activates a G

protein, the a

subunit activates

adenylyl cyclase)

(Converts)

(Activates)

(Phosphorylates)

Second

messenger

= phosphate

ICF Adenylyl

cyclase GPCRs

Effector element




Duration of action is

dependent on how long GTP

is bound to G protein

R = Receptor

E = Effector (enzyme or ion channel)

Serpentine receptor

Not on affinity of ligand for receptor


46 Adapted From: Human Physiology, L. Sherwood (Ed.) © 2001 Brooks/Cole

Signal amplification

Extracellular

chemical messenger

bound to membrane

receptor

Activated

adenylyl cyclase

Cyclic AMP

Activated

protein kinase

Phosphorylated

(activated) protein

(e.g., an enzyme)

Products of

activated enzyme

Amplification

(10)

(100)

Total number

of molecules

1,000

100,000

10,000,000

Amplification

Amplification

Amplification

(100)

(100)

1

10

1,000

Adapted From: Human Physiology, L. Sherwood (Ed.) © 2004 Brooks/Cole


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G proteins and their receptors and effectors


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Endogenous ligands and their second messengers


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Desensitization: responsiveness to agonist, usually reversible; β receptor

β-Adrenoceptor kinase

β -Arrestin


50

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. Downregulation: number of receptors; β receptor

Signaling Mechanisms & Drug Action Summary of Receptor Types & Signal Transducers

51

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cAMP as a second messenger:

From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.),


Exerts most of its effects by

stimulating cAMP-dependent

protein kinases

Specificity is dependent on the

proteins (enzymes) present

Liver – phosphorylase

kinase vs. glycogen

synthase

Adipocytes – lipase

Smooth muscle – myosin

light chain kinase

Inactivated by

phosphodiesterase (PDE)

caffeine, theophylline are

PDE inhibitors


56

Ca2+ and Phosphoinositides:

From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.),


PLC splits PIP2 into DAG & IP3

DAG stays in the membrane &

activates protein kinase C

(there are many types)

IP3 diffuses through cytoplasm

& releases Ca2+ from internal

storage vesicles; this promotes

Ca2+-calmodulin binding which

regulates Ca2+-dependent

protein kinases

Inactivation

IP3 - dephosphorylation

DAG - phosphorylated

or deacylated

Ca2+ - pumps

PLC = phospholipase C

DAG = diacylglycerol


57 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), ©

2007 McGraw-Hill Companies

cGMP:

Only in a few cell types (vascular smooth muscle, intestinal mucosa)

Similar to cAMP, guanylyl cyclase makes cGMP from GTP

NO activates guanylyl

cyclase

cGMP leads to

dephosphorylation of myosin

light chain – relaxation

ANP binds to a

transmembrane receptor that

activates guanylyl cyclase

found in membranes

cGMP dependent kinases


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Phosphorylation:

Reversible

Amplification: records a molecular memory of the pathway that

has been activated, dephosphorylation erases

the memory

Flexible regulation: differing substrate specificities of the

multiple protein kinases regulated by 2nd

messengers provide branch points that

may be independently regulated

Receptor classes & drug development:

Structure activity relationships give clues about drug receptors

Limited number of regulatory molecules

Overall effects are due to receptors

Diversity of receptors allows for selectivity of drugs

Relationship Between Dose & Clinical Response

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Maximal benefit with minimal toxicity

Graded dose-response relationship:

Potency – EC50 or ED50 (dose needed for 50% of drug’s effect)

dependent on affinity (Kd) and

efficiency of coupling response

Maximal efficacy – limit of the dose-response relationship;

important for clinical effectiveness

dependent on ability to reach relevant receptors

route of administration, absorption, site of action


60 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), ©


Drugs A & B are more potent than

drugs C & D

The pharmacologic potency of A is

less than that of B

Drug A has a larger maximal

efficacy than drug B

Drugs A, C, & D have equal

maximum efficacy which is greater

than the maximum efficacy of B

A very steep curve implies:

cooperative actions of

different systems

need great majority of

receptors to be occupied

narrow therapeutic range

Potency = dose needed for 50% of drug’s effect


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Quantal dose-response curves:

All or none

Determine the dose required to produce an effect in a large number

of individual patients and plotting the cumulative frequency

distribution

ED50 - dose at which 50% of individuals respond (different than

ED50 defined above)

TD50 - dose required to produce a toxic effect in 50% of individuals

(LD50 if death is endpoint)

Therapeutic index – TD50 / ED50

Digoxin – narrow

Benzodiazepines and antipsychotics - wide

The clinically acceptable risk of toxicity depends

critically on the severity of the disease being

treated.


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From: Basic & Clinical Pharmacology, B.G.

Katzung (Ed.), © 2007 McGraw-Hill

Companies

Quantal dose-

response curves


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Variation in drug responsiveness:

Clinical response in individual patients

Idiosyncratic – infrequently observed in most patients

genetic differences in metabolism

immunological differences

Hypersensitivity – true allergy (uncommon)

Hyperreactive – intensity of effect is increased vs. that in most

individuals

Hyporeactive – intensity of effect is decreased vs. that in most

individuals

Tolerance – responsiveness decreases as a consequence of

continued drug administration

Tachyphylaxis – decreased responsiveness that occurs rapidly

after administration of a drug


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Four mechanisms contribute to

variation in drug responsiveness

1.) Alteration in concentration of drug that reaches receptor

2.) Variation in concentration of endogenous receptor ligand

3.) Alteration in number or function of receptors

Up-regulation (thyroid hormone increases receptors in heart;

antagonists like -blockers also do this)

stop antagonist – increase in receptor number - response

to endogenous ligand (need to wean)

Down-regulation

stop agonist – may have too few receptors to get

effective stimulation

2.) Variation in concentration of endogenous receptor ligand

• Saralasin – weak partial agonist at angiotensin II

• Angiotensin II is a potent vasoconstrictor – What would be its effect on blood pressure up or down?

• What would be the effect on blood pressure of giving saralasin to a patient with high levels of angiotensin?

• What would be the effect on blood pressure of giving saralasin to a patient with low levels of angiotensin?

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Pharmacogenetics: identification of genetic factors that contribute

to a particular drug response; may allow the

design of the most appropriate individualized

pharmacologic therapy for patients

4.) Change in responsiveness distal to receptor

Largest and most important class of mechanisms that cause

variation in responsiveness to drug therapy

Age

General health

Severity & pathophysiology of disease

Wrong diagnosis

Compensatory mechanisms – (baroreceptor reflex after

administration of an anti-hypertensive agent)


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Clinical selectivity: beneficial vs. toxic effects of drugs:

From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), ©


Same receptor-effector mechanism,

direct pharmacological

extension

Same receptor, different effectors /

different tissues

Different receptor-effector

mechanisms


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Strategies for lowering adverse effects:

Use lowest dose possible

Add an adjunctive drug that acts on a different receptor mechanism

Anatomical selectivity – refine administration to get more drug to site

of action

No drug causes only a single, specific effect

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Drugs are only selective, not specific

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integrated pharmacotherapy i drug targets, ligands ... · pdf file1 integrated pharmacotherapy...

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