PharmacodynamicsPharmacodynamics

Department of Department of PharmacologyPharmacology

NEIGRIHMS, ShillongNEIGRIHMS, Shillong

ContentsContents

PRINCIPLES AND MECHANISM OF PRINCIPLES AND MECHANISM OF DRUG ACTIONDRUG ACTION

TRANSDUCE MECHANISMSTRANSDUCE MECHANISMS

DOSE-RESPONSE RELATIONSHIPDOSE-RESPONSE RELATIONSHIP

COMBINED DRUG EFFECTSCOMBINED DRUG EFFECTS

What is Pharmacodynamics?What is Pharmacodynamics?

What drugs do to the body when they enter?What drugs do to the body when they enter?

Study of Study of action-effect action-effect of drugs and dose-of drugs and dose-effect relationshipeffect relationship

Defn.: It is the study of biochemical and physiological effects Defn.: It is the study of biochemical and physiological effects of drug and their mechanism of action atof drug and their mechanism of action at organorgan level as level as well as well as cellularcellular levellevel

Also Modification of action of one drug by another drugAlso Modification of action of one drug by another drug

PRINCIPLES OF DRUG ACTIONPRINCIPLES OF DRUG ACTION

- Do NOT impart new functions on any Do NOT impart new functions on any system, organ or cellsystem, organ or cell

- Only alter the PACE of ongoing activityOnly alter the PACE of ongoing activity

STIMULATION STIMULATION DEPRESSIONDEPRESSION IRRITATIONIRRITATION REPLACEMENTREPLACEMENT CYTOTOXIC ACTIONCYTOTOXIC ACTION

PRINCIPLE OF PRINCIPLE OF ACTIONACTION

MODEMODE EXAMPLEEXAMPLE

STIMULATIONSTIMULATION Selective Enhancement Selective Enhancement of level of of level of activity of activity of specialised cells specialised cells - Excessive stimulation is often - Excessive stimulation is often followed by depression of that functionfollowed by depression of that function

Adr stimulates HeartAdr stimulates HeartPilocarpine stimulates Pilocarpine stimulates salivary glandssalivary glandsPicrotoxin – CNS stimulant Picrotoxin – CNS stimulant convulsions convulsions coma coma death death

DEPRESSIONDEPRESSION Selective Diminution Selective Diminution of activity of of activity of specialised cells specialised cells Certain drugs – stimulate one cell type Certain drugs – stimulate one cell type and depress othersand depress others

Barbiturates depress CNS Barbiturates depress CNS Quinidine depresses HeartQuinidine depresses HeartAch – stimulates smooth Ach – stimulates smooth muscle but depresses SA muscle but depresses SA nodenode

IRRITATIONIRRITATION Non-selective Non-selective often often noxiousnoxious effect – effect – applied toapplied to less specialised cells less specialised cells (epithelium, connective tissue)(epithelium, connective tissue)-stimulate associated function -stimulate associated function

Bitters – salivary and gastric Bitters – salivary and gastric secretionsecretionCounterirritants increase Counterirritants increase blood flow to a site blood flow to a site

REPLACEMENTREPLACEMENT Use of natural metabolites, hormones Use of natural metabolites, hormones or their congeners in or their congeners in deficiency deficiency statesstates

Levodopa in parkinsonismLevodopa in parkinsonismIron in anaemiaIron in anaemia

CYTOTOXIC CYTOTOXIC ACTION ACTION

Selective Selective cytotoxic cytotoxic action for action for invading invading parasites or cancer cellsparasites or cancer cells – – for attenuating them without affecting for attenuating them without affecting the host cellsthe host cells

Penicillin, chloroquinePenicillin, chloroquine

Drug Action by Physical/Chemical Drug Action by Physical/Chemical propertiesproperties

Color – Tincture Card co.Color – Tincture Card co. Physical mass – IspaghulaPhysical mass – Ispaghula Physical form – Dimethicone (antifoaming)Physical form – Dimethicone (antifoaming) Smell - Volatile OilsSmell - Volatile Oils Taste - BittersTaste - Bitters Osmotic action – Mannitol, MagsulfOsmotic action – Mannitol, Magsulf Adsorption – Activated CharcoalAdsorption – Activated Charcoal Soothing-demulcent – Soothing agents like calamine Soothing-demulcent – Soothing agents like calamine Oxidizing property – Pot. PermanganateOxidizing property – Pot. Permanganate Chelation – EDTA, dimercaprolChelation – EDTA, dimercaprol Radioactivity - Iodine and othersRadioactivity - Iodine and others Radio-opacity – Barium sulfate Radio-opacity – Barium sulfate Chemical properties – Chelating agents (EDTA, dimercaprol)Chemical properties – Chelating agents (EDTA, dimercaprol) Scavenging effect – Mesna (with cyclophosphamide)Scavenging effect – Mesna (with cyclophosphamide)

MECHANISM OF DRUG ACTION

MECHANISM OF DRUG ACTIONMECHANISM OF DRUG ACTION

MAJORITY OF DRUGS INTERACT MAJORITY OF DRUGS INTERACT WITH TARGET BIOMOLECULES:WITH TARGET BIOMOLECULES:

Usually a ProteinUsually a Protein

1.1. ENZYMESENZYMES

2.2. ION CHANNELSION CHANNELS

3.3. TRANSPORTERSTRANSPORTERS

4.4. RECEPTORSRECEPTORS

1. Enzymes – drug targets1. Enzymes – drug targets All Biological reactions are carried out under All Biological reactions are carried out under

catalytic influence of enzymes – major drug catalytic influence of enzymes – major drug targettarget

Drugs – increases/decreases enzyme mediated Drugs – increases/decreases enzyme mediated reactionsreactions

In physiological system enzyme activities are In physiological system enzyme activities are optimally setoptimally set

Enzyme stimulation is less common by drugs – Enzyme stimulation is less common by drugs – common by endogenous substratescommon by endogenous substrates– Pyridoxine (cofactor in decarboxylase activity)Pyridoxine (cofactor in decarboxylase activity)– Adrenaline stimulates Adrenaline stimulates hepatic glycogen hepatic glycogen

phosphorylase phosphorylase (hyperglycaemia)(hyperglycaemia) Enzyme inhibition – common mode of Enzyme inhibition – common mode of DRUG DRUG

actionaction

Effect of Enzyme stimulationEffect of Enzyme stimulation

Vmax (s) _

Vmax _

½ Vmax (s) -

½ Vmax -

kM(s) kMSubstrate conc.

React

ion

velo

city

Enz. Stm

Enz Ind

Normal

Enzymes – contd.Enzymes – contd.

Nonspecific inhibition: Nonspecific inhibition: Denaturation of Denaturation of proteins – strong acids, heavy metals, proteins – strong acids, heavy metals, alkalies, alcohol, phenols etc.alkalies, alcohol, phenols etc.

Specific Inhibition:Specific Inhibition:

Competitive Noncompetitive

• equilibrium• nonequilibrium

What is specific enzyme inhibition?What is specific enzyme inhibition?

A drug may inhibit A drug may inhibit a particular a particular enzyme without enzyme without affecting others affecting others and influence that and influence that particular particular substrate-enzyme substrate-enzyme reaction ultimately reaction ultimately to influence in the to influence in the product formationproduct formation

Normal

Drug + Enzyme

Competitive InhibitionCompetitive Inhibition

Enzyme Inhibition - ExamplesEnzyme Inhibition - Examples Equilibrium:Equilibrium:

– Physostigmine Vs Acetylcholine (cholinesterase)Physostigmine Vs Acetylcholine (cholinesterase)– Sulfonamides Vs PABA (folate synthetase)Sulfonamides Vs PABA (folate synthetase)– Moclobemide Vs Catecholamines (MAO-A)Moclobemide Vs Catecholamines (MAO-A)– Captopril Vs Angiotensin 1 (ACE)Captopril Vs Angiotensin 1 (ACE)

Nonequilibrium:Nonequilibrium:– Orgnophosphorous compounds/Nerve gases Orgnophosphorous compounds/Nerve gases

(cholinesterase)(cholinesterase) Non-competitive:Non-competitive:

– Acetazolamide (carbonic anhydrase), Omeprazole Acetazolamide (carbonic anhydrase), Omeprazole (HKATPase) , Aspirin (cyclooxygenase), Digoxin (Na+ K+ (HKATPase) , Aspirin (cyclooxygenase), Digoxin (Na+ K+ ATPase) ATPase)

Effects of enzyme inhibition:

Normal Competitive (equilibrium)

2. Ion Channnel2. Ion Channnel

Proteins take part in transmembrane signaling and Proteins take part in transmembrane signaling and regulates ionic compositionregulates ionic composition

Drugs also target these channels: mainly on 3 typesDrugs also target these channels: mainly on 3 types– Ligand gated channelsLigand gated channels– G-protein operated channelsG-protein operated channels– Direct action on channelsDirect action on channels

Examples: BZD opens ligand gated GABAExamples: BZD opens ligand gated GABAA A Cl- channel, Cl- channel, Histamine binds GPCR and activates G-protein, local Histamine binds GPCR and activates G-protein, local anesthetics – directly blocks channelanesthetics – directly blocks channel

Many drugs modulate opening and closing of channels: Many drugs modulate opening and closing of channels: Phenytoin, Ethosuximide, Nifedepine, Quinidine and Phenytoin, Ethosuximide, Nifedepine, Quinidine and Nicorandil etc. Nicorandil etc.

+ +

- -

+ +

--

- -

+ + + +

- -

Na+

+ ++ +

- - - -

Resting (Closed**)

Open

(brief)inactivated

Very slow repolarization in presence of LA

LA receptor

LA have highest affinity for the inactivated formRefractory period

LA acting on Na+ receptorsLA acting on Na+ receptors

3. Transporters3. Transporters Substrates are translocated across membrane by binding to Substrates are translocated across membrane by binding to

specific transporters (carriers) – Solute Carrier Proteins specific transporters (carriers) – Solute Carrier Proteins (SLC)(SLC)

Pump the metabolites/ions in the direction of concentration Pump the metabolites/ions in the direction of concentration gradient or against it.gradient or against it.

Drugs can interact with these transport systemDrugs can interact with these transport system Examples: Probenecid (penicillin and uric acid), Furosmide Examples: Probenecid (penicillin and uric acid), Furosmide

(Na+K+2Cl- cotransport), Hemicholinium (choline uptake) (Na+K+2Cl- cotransport), Hemicholinium (choline uptake) and Vesamicol (active transport of Ach to vesicles), and Vesamicol (active transport of Ach to vesicles), Thiazides block Na+Cl- symporter, Aphetamine (blocks Thiazides block Na+Cl- symporter, Aphetamine (blocks Dopamine reuptake), Reserpine (blocks grannular reuptake Dopamine reuptake), Reserpine (blocks grannular reuptake of NA)of NA)

4. Receptors4. Receptors

Drugs usually do not bind directly with enzymes, Drugs usually do not bind directly with enzymes, channels, transporters or structural proteins, but channels, transporters or structural proteins, but act through specific macromolecules – act through specific macromolecules – RECEPTORSRECEPTORS

Definition: Definition: It is defined as a macromolecule or It is defined as a macromolecule or binding site located on cell surface or inside the binding site located on cell surface or inside the effector cell that serves to recognize the signal effector cell that serves to recognize the signal molecule/drug and initiate the response to it, but molecule/drug and initiate the response to it, but

itself has no other function,itself has no other function, e.g. G-protein e.g. G-protein coupled receptorcoupled receptor

Evidences of Drug action via Evidences of Drug action via receptorsreceptors

1.1. Drugs exhibit structural specificity of Drugs exhibit structural specificity of actionaction

2.2. Competitive Competitive AntagonismAntagonism

3.3. Acetylcholine 1/6000Acetylcholine 1/6000thth of cardiac cells – of cardiac cells – maximal effect maximal effect

1.

2. Piperidine side chain

Some Common TermsSome Common Terms Agonist:Agonist: An agent which activates a receptor to produce an effect An agent which activates a receptor to produce an effect

similar to a that of the physiological signal molecule, e.g. similar to a that of the physiological signal molecule, e.g. Muscarine and NicotineMuscarine and Nicotine

Antagonist:Antagonist: an agent which prevents the action of an agonist on an agent which prevents the action of an agonist on a receptor or the subsequent response, but does not have an a receptor or the subsequent response, but does not have an effect of its own, e.g. atropine and muscarineeffect of its own, e.g. atropine and muscarine

Inverse agonist: Inverse agonist: an agent which activates receptors to produce an agent which activates receptors to produce an effect in the opposite direction to that of the agonist, e.g. an effect in the opposite direction to that of the agonist, e.g. DMCM in BDZ receptorsDMCM in BDZ receptors

Partial agonist: Partial agonist: An agent which activates a receptor to produce An agent which activates a receptor to produce submaximal effect but antagonizes the action of a full agonist, e.g. submaximal effect but antagonizes the action of a full agonist, e.g. opioidsopioids

Ligand:Ligand: any molecule which attaches selectively to particular any molecule which attaches selectively to particular receptors or sites (only binding or affinity but no functional receptors or sites (only binding or affinity but no functional change)change)

Drug – Receptor occupation Drug – Receptor occupation theory – Clark`s equationtheory – Clark`s equation

Drugs can alter cellular function by interacting Drugs can alter cellular function by interacting with receptorswith receptors

D + R D + R DR E DR E (direct (direct

function of D + R)function of D + R)

– But, affinity and intrinsic activity (IA) are differentBut, affinity and intrinsic activity (IA) are different– Competitive antagonist – occupy receptor but no IACompetitive antagonist – occupy receptor but no IA

D + R DR D + R DR SS E E

K1

K2

K1

K2

Some Definitions – contd.Some Definitions – contd.

Affinity:Affinity: Ability of a substrate to bind with Ability of a substrate to bind with receptorreceptor

Intrinsic activity (IA): Intrinsic activity (IA): Capacity to induce Capacity to induce functional change in the receptor in a way that functional change in the receptor in a way that produces an effect; some drugs possess affinity produces an effect; some drugs possess affinity but NOT efficacy but NOT efficacy

If explained in terms of affinity and IA:If explained in terms of affinity and IA: Agonist: Affinity + IA (1)Agonist: Affinity + IA (1) Antagonist: Affinity + IA (0)Antagonist: Affinity + IA (0) Partial agonist: Affinity + IA (0-1)Partial agonist: Affinity + IA (0-1) Inverse agonist: Affinity + IA (0 to -1)Inverse agonist: Affinity + IA (0 to -1)

Drug-receptor binding and Drug-receptor binding and agonismagonism

Drug- Receptor:Drug- Receptor: DRi DRa

DRi DRa

DRi DRa

D

DRi DRa

Full agonist

Partial agonist

Neutral

Inverse agonist

Receptors – contd.Receptors – contd.

Two essential functions:Two essential functions:– RecognitionRecognition of specific ligand molecule of specific ligand molecule– TransductionTransduction of signal into response of signal into response

Two Domains:Two Domains:– Ligand binding domainLigand binding domain– Effectors Domain – undergoes functional Effectors Domain – undergoes functional

conformational changeconformational change

Two State Receptor ModelTwo State Receptor Model

Nature of Nature of Receptors – contd.Receptors – contd.

No hypothesis anymoreNo hypothesis anymore Cell surface receptors remain floated in cell membrane lipidsCell surface receptors remain floated in cell membrane lipids

Functions are determined by the interaction of lipophillic or Functions are determined by the interaction of lipophillic or hydrophillic domains of the peptide chain with the drug moleculehydrophillic domains of the peptide chain with the drug molecule

Non-polar hydrophobic portion of the amino acid remain buried in Non-polar hydrophobic portion of the amino acid remain buried in membrane while polar hydrophilic remain on cell surfacemembrane while polar hydrophilic remain on cell surface

Hydrophilic drugs cannot cross the membrane and has to bind Hydrophilic drugs cannot cross the membrane and has to bind with the polar hydrophilic portion of the peptide chainwith the polar hydrophilic portion of the peptide chain

Binding of polar drugs in ligand binding domain induces Binding of polar drugs in ligand binding domain induces conformational changes (alter distribution of charges and conformational changes (alter distribution of charges and transmitted to coupling domain to be transmitted to effector transmitted to coupling domain to be transmitted to effector domaindomain

All four major families have common properties but individual All four major families have common properties but individual receptors have different amino acid sequencing receptors have different amino acid sequencing

Receptors – contd.Receptors – contd.

Drugs act on Physiological receptors Drugs act on Physiological receptors and mediate responses of and mediate responses of transmitters, hormones, autacoids transmitters, hormones, autacoids and others – cholinergic, adrenergic and others – cholinergic, adrenergic or histaminergic etc.or histaminergic etc.

Drugs may act on true drug Drugs may act on true drug receptors - Benzodiazepine receptorsreceptors - Benzodiazepine receptors

Receptor SubtypesReceptor Subtypes

Example Acetylcholine - Muscarinic and Example Acetylcholine - Muscarinic and NicotinicNicotinic– MM11, M, M22, M, M3 3 etc.etc.– NNMM and N and NNN

Criteria of Classification:Criteria of Classification:

– Pharmacological criteria – potencies of selective agonist and Pharmacological criteria – potencies of selective agonist and antagonists – Muscarinic, nicotinic, alpha and beta adrenergic etc.antagonists – Muscarinic, nicotinic, alpha and beta adrenergic etc.

– Tissue distribution – beta 1 and beta 2Tissue distribution – beta 1 and beta 2– Ligand bindingLigand binding– Transducer pathwayTransducer pathway– Molecular cloningMolecular cloning

The Transducer mechanismThe Transducer mechanism Most transmembrane signaling is accomplished

by a small number of different molecular mechanisms (transducer mechanisms)

Large number of receptors share these handful of transducer mechanisms to generate an integrated response

Mainly 4 (four) major categories:1. GPCR2. Receptors with intrinsic ion channel3. Enzyme linked receptors4. Transcription factors (receptors for gene

expression)

G-protein Coupled ReceptorsG-protein Coupled Receptors

Large family of cell membrane receptors Large family of cell membrane receptors linked to the effector linked to the effector enzyme/channel/carrier proteins through enzyme/channel/carrier proteins through one or more GTP activated proteins (G-one or more GTP activated proteins (G-proteins)proteins)

All receptors has common pattern of All receptors has common pattern of structural organizationstructural organization

The molecule has 7 The molecule has 7 αα-helical membrane -helical membrane spanning hydrophobic amino acid spanning hydrophobic amino acid segments – 3 extra and 3 intracellular segments – 3 extra and 3 intracellular loopsloops

GPCRGPCR

GPCR – contd.GPCR – contd.

G-proteins and EffectorsG-proteins and Effectors

Large number can be distinguished Large number can be distinguished by their by their αα-subunits-subunits

G proteinG protein Effectors pathway Effectors pathway SubstratesSubstrates

GsGs Adenylyl cyclase Beta-receptors, Beta-receptors, H2, D1H2, D1

GiGi Adenylyl cyclase Muscarinic M2Muscarinic M2

D2, alpha-2D2, alpha-2

GqGq Phospholipase CPhospholipase C Alph-1, H1, M1, M3Alph-1, H1, M1, M3

GoGo Ca++ channelCa++ channel K+ channel in K+ channel in heart, smheart, sm

GPCR - 3 Major PathwaysGPCR - 3 Major Pathways

1.1. Adenylyl cyclase:cAMP pathwayAdenylyl cyclase:cAMP pathway

2.2. Phospholipase C: IP3-DAG Phospholipase C: IP3-DAG pathwaypathway

3.3. Channel regulationChannel regulation

1. Adenylyl cyclase: cAMP pathway1. Adenylyl cyclase: cAMP pathway

PKa Phospholambin

Increased Interaction with Faster relaxationCa++

Troponin

Cardiac contractility

OtherFunctionalproteins

Adenylyl cyclase: cAMP Adenylyl cyclase: cAMP pathwaypathway

Main Results:Main Results:– Increased contractility of heart/impulse generationIncreased contractility of heart/impulse generation– Relaxation of smooth musclesRelaxation of smooth muscles– LipolysisLipolysis– GlycogenolysisGlycogenolysis– LipolysisLipolysis– Modulation of junctional transmissionModulation of junctional transmission– Hormone synthesisHormone synthesis– Opens specific type of Ca++ channel – Cyclic nucleotide Opens specific type of Ca++ channel – Cyclic nucleotide

gated channel (CNG) - - -heart, brain and kidneygated channel (CNG) - - -heart, brain and kidney– Responses are opposite in case of AC inhibitionResponses are opposite in case of AC inhibition

2. Phospholipase C:IP3-DAG 2. Phospholipase C:IP3-DAG pathwaypathway

PKc

IPIP33-DAG pathway-DAG pathway

Main Results:Main Results:– Mediates /modulates contractionMediates /modulates contraction– Secretion/transmitter releaseSecretion/transmitter release– Neuronal excitabilityNeuronal excitability– Intracellular movementsIntracellular movements– Eicosanoid synthesisEicosanoid synthesis– Cell ProliferationCell Proliferation– Responses are opposite in case of PLc Responses are opposite in case of PLc

inhibitioninhibition

3. Channel regulation3. Channel regulation Activated G-proteins can open or close ion Activated G-proteins can open or close ion

channels – Ca++, Na+ or K+ etc.channels – Ca++, Na+ or K+ etc. These effects may be without intervention These effects may be without intervention

of any of above mentioned 2of any of above mentioned 2ndnd messengers messengers – cAMP or IP/DAG– cAMP or IP/DAG

Bring about depolarization, Bring about depolarization, hyperpolrization or Ca ++ changes etc.hyperpolrization or Ca ++ changes etc.

Gs – Ca++ channels in myocardium and Gs – Ca++ channels in myocardium and skeletal musclesskeletal muscles

Go and Gi – open K+ channel in heart and Go and Gi – open K+ channel in heart and muscle and close Ca+ in neuronesmuscle and close Ca+ in neurones

Intrinsic Ion Channel ReceptorsIntrinsic Ion Channel Receptors

Intrinsic Ion Channel ReceptorsIntrinsic Ion Channel Receptors

Most useful drugs in clinical medicine act Most useful drugs in clinical medicine act by mimicking or blocking the actions of by mimicking or blocking the actions of endogenous ligands that regulate the flow endogenous ligands that regulate the flow of ions through plasma membrane of ions through plasma membrane channelschannels

The natural ligands include acetylcholine, The natural ligands include acetylcholine, serotonin, aminobutyric acid (GABA), and serotonin, aminobutyric acid (GABA), and the excitatory amino acids (eg, glycine, the excitatory amino acids (eg, glycine, aspartate, and glutamate)aspartate, and glutamate)

Enzyme Linked ReceptorsEnzyme Linked Receptors

2 (two) types of receptors:2 (two) types of receptors:

1.1. Intrinsic enzyme linked receptorsIntrinsic enzyme linked receptors Protein kinase or guanyl cyclase domainProtein kinase or guanyl cyclase domain

2.2. JAK-STAT-kinase binding receptorJAK-STAT-kinase binding receptor

A. Enzyme linked A. Enzyme linked receptorsreceptors

Extracellular hormone-binding domain and a Extracellular hormone-binding domain and a cytoplasmic enzyme domain (mainly protein cytoplasmic enzyme domain (mainly protein tyrosine kinase or serine kinase)tyrosine kinase or serine kinase)

Upon binding the receptor converts from its Upon binding the receptor converts from its inactive monomeric state to an active dimeric inactive monomeric state to an active dimeric statestate

Cytoplasmic domains become phosphorylated on Cytoplasmic domains become phosphorylated on specific tyrosine residues specific tyrosine residues

Enzymatic activities are activated, catalyzing Enzymatic activities are activated, catalyzing phosphorylation of substrate proteins phosphorylation of substrate proteins

Enzyme linked receptors – contd.Enzyme linked receptors – contd.

Enzyme linked receptors – contd.Enzyme linked receptors – contd.

Activated receptors catalyze phosphorylation of tyrosine residues on different target signaling proteins, thereby allowing a single type of activated receptor to modulate a number of biochemical processes

Examples:Examples:– Insulin - Insulin - uptake of glucose and amino acids and

regulate metabolism of glycogen and triglycerides

– Trastuzumab, antagonist of a such type receptor – used in breast cancer

B. JAK-STAT-kinase Binding ReceptorB. JAK-STAT-kinase Binding Receptor

Mechanism closely resembles that of Mechanism closely resembles that of receptor tyrosine kinasesreceptor tyrosine kinases

Only difference - protein tyrosine kinase Only difference - protein tyrosine kinase activity is not intrinsic to the receptor activity is not intrinsic to the receptor moleculemolecule

Uses Janus-kinase (JAK) familyUses Janus-kinase (JAK) family Also uses STAT (signal transducers and Also uses STAT (signal transducers and

activators of transcription)activators of transcription) Examples – cytokines, growth hormones, Examples – cytokines, growth hormones,

interferones etc.interferones etc.

JAK-STAT-kinase ReceptorsJAK-STAT-kinase Receptors

Receptors regulating gene Receptors regulating gene expressionexpression

Lipid soluble biological signals cross the Lipid soluble biological signals cross the plasma membrane and act on intracellular plasma membrane and act on intracellular receptors – NO acts by stimulating cGMPreceptors – NO acts by stimulating cGMP

Receptors for corticosteroids, Receptors for corticosteroids, mineralocorticoids, thyroid hormones, sex mineralocorticoids, thyroid hormones, sex hormones and Vit. D etc. stimulate the hormones and Vit. D etc. stimulate the transcription of genes in the nucleus by transcription of genes in the nucleus by binding with specific DNA sequence – binding with specific DNA sequence – called - “Responsive elements”called - “Responsive elements”

Receptors regulating gene Receptors regulating gene expression – Clinical implicationexpression – Clinical implication

Hormones produce their effects after Hormones produce their effects after a characteristic lag period of 30 a characteristic lag period of 30 minutes to several hours—the time minutes to several hours—the time required for the synthesis of new required for the synthesis of new proteins – gene active hormonal proteins – gene active hormonal drugs take time to be active drugs take time to be active (Bronchial asthma)(Bronchial asthma)

Beneficial or toxic effects persists Beneficial or toxic effects persists even after withdrawaleven after withdrawal

Receptors regulating gene Receptors regulating gene expressionexpression

Summary of TransducersSummary of Transducers

Functions of ReceptorsFunctions of Receptors

To Regulate signals from outside the cell To Regulate signals from outside the cell to inside the effector cell – signals not to inside the effector cell – signals not permeable to cell membranepermeable to cell membrane

To amplify the signalTo amplify the signal To integrate various intracellular and To integrate various intracellular and

extracellular signalsextracellular signals To adapt to short term and long term To adapt to short term and long term

changes and maintain homeostasis.changes and maintain homeostasis.

Non-receptor mediated drug Non-receptor mediated drug actionaction

Physical and chemical means - Antacids, Physical and chemical means - Antacids, chelating agents and cholestyraminechelating agents and cholestyramine

Enzymes, Ion channels and transportersEnzymes, Ion channels and transporters Alkylating agents: binding with nucleic Alkylating agents: binding with nucleic

acid and render cytotoxic activity – acid and render cytotoxic activity – Mechlorethamine, cyclophosphamide etc.Mechlorethamine, cyclophosphamide etc.

Antimetabolites: purine and pyrimidine Antimetabolites: purine and pyrimidine analogues – 6 MP and 5 FUanalogues – 6 MP and 5 FU

Receptor RegulationReceptor Regulation

Up regulation of receptors:Up regulation of receptors:– In tonically active systems, prolonged In tonically active systems, prolonged

deprivation of agonist (by denervation deprivation of agonist (by denervation or antagonist) results in supersensitivity or antagonist) results in supersensitivity of the receptor as well as to effector of the receptor as well as to effector system to the agonist. Sudden system to the agonist. Sudden discontinuation of Propranolol, Clonidine discontinuation of Propranolol, Clonidine etc.etc.

– Unmasking of receptors or proliferation Unmasking of receptors or proliferation or accentuation of signal amplificationor accentuation of signal amplification

Receptor RegulationReceptor Regulation

Continued exposure to an agonist or Continued exposure to an agonist or intense receptor stimulation causes intense receptor stimulation causes desensitization or refractoriness: receptor desensitization or refractoriness: receptor become less sensitive to the agonistbecome less sensitive to the agonist

Examples – beta adrenergic agonist and Examples – beta adrenergic agonist and levodopalevodopa

Causes:Causes:1.1. Masking or internalization of the receptorsMasking or internalization of the receptors

2.2. Decreased synthesis or increased destruction Decreased synthesis or increased destruction of the receptors (down regulation)of the receptors (down regulation)

DesensitizationDesensitization

Sometimes response to all agonists which act Sometimes response to all agonists which act through different receptors but produce the same through different receptors but produce the same overt effect is decreased by exposure to anyone overt effect is decreased by exposure to anyone of these agonists – heterologous desensitizationof these agonists – heterologous desensitization

Homologous – when limited to the agonist which Homologous – when limited to the agonist which is repeatedly activatedis repeatedly activated

R+ TransducerHomologous

Ach

Hist

Heterologous

Mechanism of desensitizationMechanism of desensitization

ßARK (beta-adrenergic receptor kinase)Beta-arrestin

Dose-Response RelationshipDose-Response Relationship

Drug administered – 2 components of dose- Drug administered – 2 components of dose- response response – Dose-plasma concentrationDose-plasma concentration– Plasma concentration (dose)-response relationshipPlasma concentration (dose)-response relationship

E = E = Emax X [D]

Kd + [D]

E is observed effect of drug dose [D], Emax = maximum response,KD = dissociation constant of drug receptor complex

E max

Dose-Response CurveDose-Response Curve

dose Log dose

% re

spon

se

% re

spon

se

100% -

50% -

100% -

50% -

E = Emax X [D]

Kd + [D]

Dose-Response CurveDose-Response Curve

Advantages:Advantages:– A wide range of drug doses can easily A wide range of drug doses can easily

be displayed on a graphbe displayed on a graph– Potency and efficacy can be comparedPotency and efficacy can be compared– Comparison of study of agonists and Comparison of study of agonists and

antagonists become easierantagonists become easier

How we get DRC in vitro How we get DRC in vitro Practically??Practically??

Example: Frog rectus muscle and Example: Frog rectus muscle and Acetylcholine response – in Acetylcholine response – in millimetersmillimeters– Can compare with a drug being studied Can compare with a drug being studied

for having skeletal muscle contracting for having skeletal muscle contracting property.property.

PracticallyPractically

log1 = 0log10 = 1Log20 =1.30Log40 = 1.60Log 80 = 1.90Log160 = 2.20

Potency and efficacyPotency and efficacy

Potency:Potency: It is the amount of drug required to It is the amount of drug required to produce a certain responseproduce a certain response

Efficacy:Efficacy: Maximal response that can be elicited by Maximal response that can be elicited by a druga drug

Resp

onse

Drug in log conc.

1 2 3 4

Potency and efficacy - Potency and efficacy - ExamplesExamples

Aspirin is less potent as well as less efficacious Aspirin is less potent as well as less efficacious than Morphinethan Morphine

Pethidine is less potent analgesic than Morphine Pethidine is less potent analgesic than Morphine but eually efficaciousbut eually efficacious

Diazepam is more potent but less efficacious than Diazepam is more potent but less efficacious than phenobarbitonephenobarbitone

Furosemide is less potent but more efficacious Furosemide is less potent but more efficacious than metozolonethan metozolone

Potency and efficacy are indicators only in Potency and efficacy are indicators only in different clinical settings e.g. Diazepam Vs different clinical settings e.g. Diazepam Vs phenobarbitone (overdose) and furosemide vs phenobarbitone (overdose) and furosemide vs thaizide (renal failure)thaizide (renal failure)

Slope of DRCSlope of DRC Slope of DRC is also importantSlope of DRC is also important Steep slope – moderate increase in dose markedly increase Steep slope – moderate increase in dose markedly increase

the response (individualization)the response (individualization) Flat DRC – little increase in response occurs in wide range Flat DRC – little increase in response occurs in wide range

of doses (standard dose can be given to most ptients)of doses (standard dose can be given to most ptients) Example: Hydralazine and Hydrochlorothiazide DRC in Example: Hydralazine and Hydrochlorothiazide DRC in

HypertensionHypertensionHydralazine

ThiazideFall

in

BP

Therapeutic index (TI)Therapeutic index (TI)

Therapeutic Index = Therapeutic Index = Median Lethal Dose (LD50)

Median Effective dose (ED50)

Idea of margin of safety Margin of Safety

Therapeutic index (TI)Therapeutic index (TI)

It is defined as the gap between therapeutic It is defined as the gap between therapeutic effect DRC and adverse effect DRC (also called effect DRC and adverse effect DRC (also called margin of safety)margin of safety)

Combined Effects of DrugsCombined Effects of Drugs Drug Synergism:Drug Synergism:

– Additive effect (1 + 1 = 2)Additive effect (1 + 1 = 2) Aspirin + paracetamol, amlodipine + atenolol, Aspirin + paracetamol, amlodipine + atenolol,

nitrous oxide + halothanenitrous oxide + halothane– Supra-additive effect (1 + 1 = 4)Supra-additive effect (1 + 1 = 4)

Sulfamethoxazole + trimethoprim, levodopa + Sulfamethoxazole + trimethoprim, levodopa + carbidopa, acetylcholine + physostigminecarbidopa, acetylcholine + physostigmine

PABA DHFA THFAPABA DHFA THFA Sulfamethoxazole TrimethoprimSulfamethoxazole Trimethoprim

Folate synthase Dihydrofolate

reductase

Drug AntagonismDrug Antagonism

1.1. Physical: CharcoalPhysical: Charcoal2.2. Chemical: KMnOChemical: KMnO44, Chelating agent, Chelating agent3.3. Physiological antagonism: Physiological antagonism:

Histamine and adrenaline in Histamine and adrenaline in bronchial asthma, Glucagon and bronchial asthma, Glucagon and InsulinInsulin

4.4. Receptor antagonism:Receptor antagonism:a.a. Competitive antagonism (equilibrium)Competitive antagonism (equilibrium)b.b. Non-competitiveNon-competitivec.c. Non-equilibrium (competitive)Non-equilibrium (competitive)

Receptor antagonism - curvesReceptor antagonism - curveso Competitive:Competitive:

o Antagonist is chemically similar to agonist and binds to Antagonist is chemically similar to agonist and binds to same receptor moleculessame receptor molecules

o Affinity (1) but IA (0), Result – no responseAffinity (1) but IA (0), Result – no responseo Log DRC shifts to the rightLog DRC shifts to the righto But, antagonism is reversible – increase in concentration of But, antagonism is reversible – increase in concentration of

agonist overcomes the blockagonist overcomes the blocko Parallel shift of curve to the right sideParallel shift of curve to the right side

o Non-competitive:Non-competitive:o Allosteric site binding altering receptor not to bind with Allosteric site binding altering receptor not to bind with

agonistagonisto No competition between them – no change of effect even No competition between them – no change of effect even

agonist conc. .is increasedagonist conc. .is increasedo Flattening of DRCFlattening of DRC

Receptor antagonism - curvesReceptor antagonism - curves

Non – equilibrium:Non – equilibrium:– Antagonists Binds receptor with strong Antagonists Binds receptor with strong

bondbond– Dissociation is slow and agonists cannot Dissociation is slow and agonists cannot

displace antagonists (receptor displace antagonists (receptor occupancy is unchanged)occupancy is unchanged)

– Irreversible antagonism developesIrreversible antagonism developes– DRC shifts to the right and Maximal DRC shifts to the right and Maximal

response loweredresponse lowered

Drug antagonism DRCDrug antagonism DRC

Drug antagonism DRC – non-Drug antagonism DRC – non-competitive antagonismcompetitive antagonism

Resp

ons

e

Shift to the right and lowered response

Drug in log conc.

Agonist

Agonist+ CA (NE)

Spare ReceptorSpare Receptor

When only a fraction of the total When only a fraction of the total population of receptors in a system, population of receptors in a system, are needed to produce maximal are needed to produce maximal effect, then the particular system is effect, then the particular system is said to have spare receptorssaid to have spare receptors

Example – Adrenaline (90%)Example – Adrenaline (90%)

Competitive Vs NC antagonismCompetitive Vs NC antagonism

CompetitiveCompetitive Binds to same receptorBinds to same receptor Resembles chemicallyResembles chemically Parallel right shift of DRC Parallel right shift of DRC

in increasing dose of in increasing dose of agonistagonist

Intensity depends on the Intensity depends on the conc. Of agonist and conc. Of agonist and antagonistantagonist

Example – Ach and Example – Ach and atropine, Morphine and atropine, Morphine and NaloxonNaloxonee

NoncompetitiveNoncompetitive Binds to other siteBinds to other site No resemblanceNo resemblance Maximal response is Maximal response is

suppressedsuppressed Depends only on Depends only on

concentration of concentration of antagonistantagonist

Diazepam - BicucullineDiazepam - Bicuculline

SummarySummary Basic Principles of PharmacodynamicsBasic Principles of Pharmacodynamics Mechanisms of drug action – Enzymes, Ion channels, Mechanisms of drug action – Enzymes, Ion channels,

Transporters and Receptors with examplesTransporters and Receptors with examples Definitions of affinity, efficacy, agonist and antagonists etc.Definitions of affinity, efficacy, agonist and antagonists etc. Drug transducer mechanismsDrug transducer mechanisms GPCR and different GPCR transducing mechanisms – cAMP, GPCR and different GPCR transducing mechanisms – cAMP,

Protein kinase etc.Protein kinase etc. Up regulation and down regulation of receptors and Up regulation and down regulation of receptors and

desensitizationdesensitization Principles of dose response curves and curves in relation to Principles of dose response curves and curves in relation to

agonist, competitive antagonist etc.agonist, competitive antagonist etc. Therapeutic index, margin of safety and risk-benefit ratio Therapeutic index, margin of safety and risk-benefit ratio

conceptsconcepts Combined effects of drugs – synergism etc. Combined effects of drugs – synergism etc. Dose response curve (DRC) – agonist and antagonistDose response curve (DRC) – agonist and antagonist

Thank youThank you