MECHANISMS OF DRUG ACTION

Pharmacotherapeutics:

Types of Therapies Acute therapy- urgent use eg atropine for bradycardia

Maintenance therapy designed to help a primary treatment succeed. Eg maintenance chemotherapy to people who have a cancer in remission to prevent a relapse.

Supplemental therapy to supplement lack of certain agents eg iron

Palliative therapy concentrates on reducing the severity of disease symptoms, rather than providing a cure. Eg Phenothiazines to reduce nausea in chemo

Supportive therapy treatment intended to relieve symptoms or help the patient live with them rather than attempt changes in character structure eg education, counsel

Prophylactic therapy prevention eg vaccines

Pharmacodynamics

– The study of molecular interactions between drugs and body constituents

• Formation of a complex between the drug and a cell component – Drug receptor

• The receptor site where the drug acts to initiate a series of biochemical and physiological effects– Site of action

• Molecular events that follow drug-receptor interactions– Mechanisms of action

Pharmacodynamics cont..

– Relate to the biochemical andphysiological actions of drugs

PharmacodynamicsDrug receptors• Cellular macromolecules– Transmembrane ion channels– Transmembrane receptors coupled to intracellular G proteins – Transmembrane receptors with enzymatic cytosolic domains eg

Tyrosine Kinase receptor acts in phosphorylation reaction– Intracellular receptors, including enzymes, DNA andstructural proteins• A single cell may have hundreds of receptor sites– Drugs interact with these receptor sites andproduce a definable pharmacological response

Pharmacodynamics:

Mechanisms of ActionThe ways by which drugs can produce

therapeutic effects: Once the drug is at the site of action, it

can modify the rate (increase or decrease) at which the cells or tissues function.

A drug cannot make a cell or tissue perform a function it was not designed to perform.

Pharmacodynamics:

Mechanisms of Action Receptor interaction eg DNA & chlorambucil

Enzyme interaction eg toposiomerase and DMXAA

Nonspecific interactions (Biophysical e.g alteration of pH in the stomach acid by Antacid Gaviscon)

ReceptorsMost drugs combine (bind) with specific receptorsto produce a particular response. This association or binding takes place by precise physicochemical and steric interactions between specific groups of the drug and the receptor.

1. Proteins a. Carriersb. Receptors

i. G protein-linked ii. Ligand gated channelsiii. Intracellular

c. Enzymes2. DNA

Endogenous compounds act on their Receptors

Neurotransmitter

Neuropeptides

Hormones

Ionshttp://www.morphonix.com/software/education/science/brain/game/specimens/neurotransmitters.gif

–Receptor classification• According to the type of drug that interactswith a specific receptor– Adrenergic» α1, α2, β1, β2, β3– Cholinergic» Muscarinic (M1, M2, M3), nicotinic– Histaminic» H1, H2, H3– Opioid– Serotonergic– Dopaminergic

Receptor

1) PharmacologicalMediator (i.e. Insulin, Norepinephrine, estrogen)

2) BiophysicalSecond messenger system (i,.e. cAMP, PLC, PLA)

3) Molecular or StructuralSubunit composition (i.e. 5HT1A )

4) AnatomicalTissue (i.e muscle vs ganglionic nAChRs)Cellular (i.e. Membrane bound vs Intracellular)

Classification of Receptors

Types of ReceptorsMEMBRANE BOUND RECEPTORS G-Protein-linked receptors

Serotonin, Muscarinic, Dopaminergic, Noradrenergic Enzyme receptors

Tyrosine kinase Ligand-gated ion channel receptors

Nicotinic, GABA, glutamate

INTRACELLULAR AND NUCLEAR RECEPTORS Hormone receptors Autocoid receptors Growth factors receptors Insulin receptors

G Protein–linked Receptors

http://www.sp.uconn.edu/~bi107vc/images/anim/SigtranRA.gif

Enzyme-like Receptors

Ligand-gated Ion-Channel Receptors

Nuclear Receptors

General Principles of Pharmacology

Pharmacodynamics– Drugs-receptorinteractions• Hydrogen andionic binding arethe most common– Require little energy– May be easily broken

Drug-Receptor InteractionsChemical Bonds

Van der Waals Interactions

Hydrophobic Interactions

Drug-Receptor InteractionsDrug-receptor interactions serve as signals to trigger a cascade of events. This cascade or signaling pathway, is a collection of many cellular responses which serve to amplify the signal and produce a final effect.

Effectors are thus the molecules that translate the drug-receptor interaction into changes in cellular activity.

+ EFFECT DRUG DRUG + RECEPTOR DRUG + RECEPTOR EFFECTOR EFFECTOR INTERACTION COMPLEX SYSTEM

STIMULUS BINDING ACTIVATION TRANSDUCTION AMPLIFICATION RESPONSE

SIGNALLING PATHWAY

Receptor Signaling Pathways

Second Messengers:1. Ions (Ca2+, Na+, K+, Cl-)2. cAMP, cGMP, IP3, Diacylglycerol3. DNA binding – Transcriptional regulation.4. Phosphorylated proteins and enzymes via

tyrosine kinase receptors.

Third Messengers:1. Enzymes (PKC, PKA)2. Ions (Ca2+, K+)

Receptor Signaling Pathways

cAMPcGMPDAG and IP3Arachidonic acidNO and CONa+, Ca2+, K+, Cl-

EFFECTORSAdenylate Cyclase (AC)Guadenylyl Cyclase (GC)Phospholipase C (PLC)Phospholipase A (PLA2)Nitric oxide SynthaseIons

SECOND MESSENGER

Receptor Signaling Pathways

R R R R

Receptor Signaling Pathways

Receptor Down-Regulation

Cells can sense the changes in [ligand] Ligand bound to the receptor can cause

desensitization or tolerance of the target cell to the ligand and will then require higher [ligand] to elicit a response

Desensitization is due to changes in properties or location of the receptor known as receptor down-regulation

Desensitization leads to tolerance – loss of effectiveness

3 Methods of Down-Regulation

1) Remove the receptor from the cell surface Receptor-mediated endocytosis Reduced receptor will yield diminished cellular activity

2) Alterations in the receptor that lowers the affinity for the ligand or 3) alterations so cannot initiate changes in cell function Altered by phosphorylation Remain on the surface but cannot respond to ligand

Pharmacodynamics– Drug-receptor affinity• The ability of a drug to interact with areceptor is a function of its chemicalstructure– Expressed by the drug’s dissociation constant (KD)» The concentration of a drug required toachieve 50% occupancy of its receptors– Most drug-receptor interactions are governed bythe law of mass action» To reach equilibrium, drugs move from highconcentration to low concentration

PharmacodynamicsThe response initiated by a drug is a function of its

intrinsic activity• A drug that has a direct stimulatory effect on a receptor is an

agonist– Strong agonist - high intrinsic activity» Produces a significant response when only a smallnumber of receptors are occupied– Weak agonist - intermediate intrinsic activity» Must be bound to many receptors to produce thesame effect as the strong agonist– Partial agonist - low intrinsic activity» Even when all receptors a occupied, it will notproduce the same effect as a strong agonist-- Spare receptor- full response although not all receptors occupied

may be due to involvement of second messengers.

Pharmacodynamics• A drug that inhibits the action of an

agonist is an antagonist– Receptor antagonist - active site binding» Reversible - competitive active site antagonist

– Receptor antagonist - allosteric binding» Irreversible - noncompetitive allosteric antagonist bind

to other than the active site causing distortion in active site

General Principles of Pharmacology– Nonreceptor antagonist» Chemical antagonist - inactivates agonistbefore it has the opportunity to act e.g protamine-heparin» Physiological agonism and antagonism is the mechanism of

substances to induce the same ultimate effects in the body as other substances, as if they were receptor agonists or antagonists, but without binding to the same receptor. Adrenaline induces platelet aggregation and so does hepatocyte growth factor (HGF). Thus, they are physiological agonists to each other.

Pharmacokinetic antagonist- antagonists that limit drug concentration at the active site e.g by decrease absorption or increase metabolism or excretion

Receptor-Mediated EffectsReceptor-Mediated Effects

% % Maximum Maximum

EffectEffect

[Drug][Drug]

Agonist

Antagonist

Partial agonist

100

80

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40

20

0

1 10010 1000

Dose-Effect ParametersDose-Effect Parameters

PPOTENCY:OTENCY:

EEFFICACY:FFICACY:

The sensitivity of an organ or The sensitivity of an organ or tissue to the drugtissue to the drugthe amount of one drug required to produce a desired effect compared with another agent

The maximum effectThe maximum effectthe drug property that allows the receptor-bound drug to produce its pharmacological effect

PharmacodynamicsEfficacy• The magnitude of response obtained from optimal

receptor site occupancy– Related to intrinsic activity» SAR– “Ceiling dose”

Pharmacodynamics– Potency• Relates two ormore drugs bycomparing thedoses requiredto produce agiven effect– Related toaffinity– Important indeterminingdrug dosage

Comparing Dose-Effect Comparing Dose-Effect CurvesCurves

0

20

40

60

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1 10 100 1000

% of % of Maximal Maximal

EffectEffect

[Drug][Drug]

Drug ADrug A

Drug CDrug C

Drug BDrug B

Effect =Effect = Maximal effect • [Drug]Maximal effect • [Drug]KKDD + [Drug] + [Drug]

Drug InteractionsDrug Interactions

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% of % of Maximal Maximal

EffectEffect

[Drug][Drug]

AgonistAgonist

Agonist + competitive Agonist + competitive antagonistantagonist

Agonist + non-competitive Agonist + non-competitive antagonistantagonist

Classification of Drug interactions between 2 or more ligands

Describes the response when 2 or more drugs are combined:

Synergism: 1+2+3 > 3 Additive synergism: 1+2+3=6 Potenciation: a from of synergism but larger

response: 1+2+3 > 6

Pharmacodynamics– Toxicity• The undesirable effects associated with the given

therapeutic use of a drug– Exaggerations of direct effects» Example: progression from drowsiness to sedation to

hypnosis to death at increasing dosage levels of a CNS depressant

– Concurrent “side” effects» Example: an antihistamine, which is intended to

antagonize histamine action at H1-receptors may also bind to H3-receptors in the CNS and cause drowsiness

Pharmacodynamics

– Margin of safety• Median effectiveDose – ED50• Median ToxicDose – TD50• TherapeuticIndex TD50/ED50

General Principles of Pharmacology• Pharmacodynamics

– Margin of safety• 99% dose-response curve for theTherapeutic effectvs. Dose-responsecurve for a toxicor the lethal effect

Drug response Variation Idiosyncratic response: rare, due to genetic variation eg

allergic reaction (hypersensitivity), metabolic enzymes Refractory response: decreased sensitivity towards drug

stimulation due to prolonged use or continuous use. Reversible – desensitization , receptor down regulation

Drug resistance: eg antibiotics, antitumor agents Tolerance: Decrease organ response towards agonists

after prolonged exposure at the same dose, occurs within days/ weeks.

Tachyphylaxis is a medical term describing a rapidly decreasing response to a drug following administration of the initial doses.

Causes of response variation Alteration in concentration of drug that reaches the receptor; due

to pharmacokinetic variation ADME Variation in conc. of endogenous receptor ligand eg propranolol

effect on heart rate of athlete due to endogenous catecholamines level.

Alterations in number or function of receptors: due to increase/decrease in number of receptor sites or alteration of efficiency of receptor functioning (coupling). Eg withdrawal symptoms from agonist or tachyphylaxis/ tolerance.

Changes in components of response distal to receptor: eg physiologic adaptation by compensatory measure such as increase in sympathetic nervous tone and fluid retention by the kidney leads to tolerance to antihypertensive vasodilator drugs.

DOSE-EFFECT DOSE-EFFECT RELATIONSHIPRELATIONSHIP

The intensity and duration of a drug’s The intensity and duration of a drug’s effects are a function of the drug dose and effects are a function of the drug dose and

drug concentration at the effect sitedrug concentration at the effect site

Monitoring Dose-EffectMonitoring Dose-Effect LevelLevel

Molecular (e.g, enzyme inhibition)Molecular (e.g, enzyme inhibition) Cellular (Cellular (in vitroin vitro tissue culture, blood cells) tissue culture, blood cells) Tissue or organ (Tissue or organ (in vitroin vitro or or in vivoin vivo)) OrganismOrganism

Endpoint used to measure effect may be Endpoint used to measure effect may be different at each leveldifferent at each level

Overall effect = sum of multiple drug effects Overall effect = sum of multiple drug effects and physiological response to drug effectsand physiological response to drug effects

Endpoints to Monitor Drug EffectEndpoints to Monitor Drug Effect

LLEVELEVEL EENDPOINTNDPOINT

MolecularMolecular Caspase inhibitionCaspase inhibition

CellularCellular Proliferation rate, apoptosisProliferation rate, apoptosis

TumorTumor Response (change in tumor size)Response (change in tumor size)

OrganismOrganism Survival, quality of lifeSurvival, quality of life

Farnesyltransferase Inhibitors for Cancer Farnesyltransferase Inhibitors for Cancer (Ras protein)(Ras protein)

Drug-Receptor InteractionsTheory and assumptions of drug-receptor interactions. Drug Receptor interaction follows simple mass-action

relationships, i.e. only one drug molecule occupies each receptor and binding is reversible (We know now there are some exceptions).

For a given drug the magnitude of the response is proportional to the fraction of total receptor sites occupied by drug molecules.

Combination or binding to receptor causes some event which leads to a response.

Response to a drug is graded or dose-dependent.

Law of Mass ActionWhen a drug (D) combines with a receptor (R), it does so at a rate which is dependent on the concentration of the drug and the concentration of the receptor.

D = drugR = receptorDR = drug-receptor complexk1 = rate for associationk2 = rate for dissociationKD = Dissociation ConstantKA = Affinity Constant

 

k1

[D] + [R] [DR] k2

k2 = KD = [D][R] k1 [DR]

1 = KA = k1 = [DR] KD k2 [D] [R]

Dose-Effect EndpointsDose-Effect Endpoints

GradedGraded

QuantalQuantal

•• Continuous scale (Continuous scale (dose dose effect)effect)•• Measured in a single biologic unitMeasured in a single biologic unit

•• Relates dose to intensity of effectRelates dose to intensity of effect

•• All-or-none pharmacologic effectAll-or-none pharmacologic effect

•• Population studiesPopulation studies

•• Relates dose to frequency of effectRelates dose to frequency of effect

0

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25

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Erythropoietin and Erythropoietin and AnemiaAnemia

Erythropoietin Dose [units/kg]Erythropoietin Dose [units/kg]

Peak Peak Hematocrit Hematocrit Increment Increment

[%][%]

Eschbach et al. NEJM 316:73-8, 1987Eschbach et al. NEJM 316:73-8, 1987

Drug-Receptor Drug-Receptor InteractionsInteractions

kk11

kk22

DrugDrug

ReceptorReceptorEffectEffect

Drug-Drug-Receptor Receptor ComplexComplex

Effect =Effect = Maximal effect • [Drug]Maximal effect • [Drug]KKDD + [Drug] + [Drug]

(K(KDD = k = k22/k/k11))

Ligand-binding Ligand-binding domain domain

Effector domain Effector domain

Effect =Effect = [Drug][Drug]KKDD + [Drug] + [Drug]Maximal effectMaximal effect [Drug][Drug]KKDD + [Drug] + [Drug]

Dose-Effect RelationshipDose-Effect Relationship

Effect =Effect = Maximal effect • [Drug]Maximal effect • [Drug]KKDD + [Drug] + [Drug]

Effect =Effect = Maximal effectMaximal effect if [Dose] >> Kif [Dose] >> KDD

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20

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Graded Dose-Effect CurveGraded Dose-Effect Curve

% of % of Maximal Maximal

EffectEffect

[Drug][Drug]ECEC5050

Maximal effectMaximal effect

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Log Dose-Effect CurveLog Dose-Effect Curve

% of % of Maximal Maximal

EffectEffect

[Drug][Drug]

ECEC5050

Application the two curves

Gives critical information in making therapeutic decision

Both curves provides info regarding potency and selectivity of drugs

Graded dose response curve indicates maximal efficacy of drug

Quantal dose-effect curve indicates the potential variablity of responsiveness among individuals

Decisions:* Adapted from Pharmaceutical Executive, January 2000, page 80

PHARMACEUTICAL PRODUCT LIFE CYCLE

Pre-clinical D

evelopment

LeadO

ptimizatio

n

LeadIdentificatio

n

Scale up &

Launch

Post M

arketing

Clinical

Developm

ent

RegulatoryR

eview

PatentExpiration

TRADITIONAL CLINICAL PHASES

• PHASE 1 Healthy Subjects*pharmacology, potency, pharmacokinetic parameters, side-effects

• PHASE 2 Subjects w/ illnessTherapeutic? small group, dose determination for phase 3

• PHASE 3 Pivotal studies in patientsCompare btw test and std drug on larger scale at random

• PHASE 4 Market expansion & surveillanceEfficacy study and side-effects after sales

* Patients used for antineoplastic and anti-HIV drugs.

PHASES OF DRUG DEVELOPMENT