PHARMACODYNAMICS

RVS Chaitanya KoppalaAssistant ProfessorLovely professional University

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

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

Defn.: It is the study of biochemical and physiological effects of drug and their mechanism of action at organ level as well as cellular level

Also Modification of action of one drug by another drug

PRINCIPLES OF DRUG ACTION

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

- Only alter the PACE of ongoing activity

• STIMULATION • DEPRESSION• IRRITATION• REPLACEMENT• CYTOTOXIC ACTION

MECHANISM OF DRUG ACTION

MECHANISM OF DRUG ACTION

• MAJORITY OF DRUGS INTERACT WITH TARGET BIOMOLECULES:

Usually a Protein 1. ENZYMES2. ION CHANNELS3. TRANSPORTERS4. RECEPTORS

4. Receptors

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

• Definition: It is defined as a macromolecule or binding site located on cell surface or inside the effector cell that serves to recognize the signal molecule/drug and initiate the response to it, but itself has no other function, e.g. Muscarinic (M type) and Nicotinic (N type) receptors of Cholinergic system

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

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

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

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

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

• Ligand: (Latin: ligare – to bind) - any molecule which attaches selectively to particular receptors or sites (refers only binding or affinity but no functional change)

Evidences of Drug action via receptors – Historical

1. Drugs exhibit structural specificity of action: example - Catecholamines

2. Competitive Antagonism: Between agonists and antagonists (Atropine - M type receptors) – by Langley

3. Acetylcholine - 1/6000th of cardiac cell surface – maximal effect – by Clark

Drug – Receptor occupation theory – Clark`s equation (1937)

• Drugs are small molecular ligands (pace of cellular function can be altered)

• Drug (D) and receptor (R) interaction governed by “law of mass action”

• Effect (E) Is the direct function of the Drug-Receptor complex

• But DR complex may not be sufficient to elicit E (response)• D must be able to bring a conformational change in R to get E• Affinity and Intrinsic activity (IA)

D + R DR E (direct function of DR complex)K1

K2

Receptor occupation theory – contd.

• Affinity: Ability to bind with a Receptor• Intrinsic activity (IA): Capacity to induce functional

change in the receptor• Competitive antagonists have Affinity but no IA• Therefore, a theoretical quantity (S) – denoting

strength was interposed

D + R DR S EK1

K2

Definitions redefined

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

Two-state receptor model

Drug-receptor binding and agonism

• Drug- Receptor: DRi DRa

DRi DRa

DRi DRa

D

DRi DRa

Full agonist

Partial agonist

Neutral

Inverse agonist

Nature of Receptors

• Not hypothesis anymore – proteins and nucleic acids• Isolated, purified, cloned and amino acid sequencing done• Cell surface receptors remain floated in cell membrane lipids• Non-polar hydrophobic portion of the amino acid remain buried in

membrane while polar hydrophilic remain on cell surface• Major classes of receptors have same structural motif – pentameric etc.• But, majority of individual receptor molecules are made up of non-

identical subunits – ligand binding brings about changes in structure or alignment of subutits

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

• Many drugs act on Physiological receptors – also true drug receptors

Receptor Subtypes• Evaluation of receptors and subtypes – lead to discovery of

various newer target molecules• Example Acetylcholine - Muscarinic and Nicotinic

– M1, M2, M3 etc.– NM and NN

– α (alpha) and β (beta) ….

• Criteria of Classification:– Pharmacological criteria – potencies of selective agonist and antagonists

– Muscarinic, nicotinic, alpha and beta adrenergic etc.– Tissue distribution – beta 1 and beta 2– Ligand binding– Transducer pathway and Molecular cloning

Action – effects !• Receptors : Two essential functions:

• Recognition of specific ligand molecule• Transduction of signal into response

• Two Domains:• Ligand binding domain (coupling proteins)• Effectors Domain – undergoes functional conformational change

• “Action”: Initial combination of the drug with its receptors resulting in a conformational change (agonist) in the later, or prevention of conformational change (antagonist)

• “Effect”: It is the ultimate change in biological function brought about as a consequence of drug action, through a series of intermediate steps (transducers)

The 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 and amplified response

• Mainly 4 (four) major categories:1. G-protein coupled receptors (GPCR)2. Receptors with intrinsic ion channel3. Enzyme linked receptors4. Transcription factors (receptors for gene expression)

G-protein CoupledReceptors (GPCR)• Large family of cell membrane

receptors linked to the effector enzymes or channel or carrier proteins through one or more GTP activated proteins (G-proteins)

• All receptors has common pattern of structural organization

• The molecule has 7 α-helical membrane spanning hydrophobic amino acid segments – 3 extra and 3 intracellular loops

• Agonist binding - on extracellular face and cytosolic segment binds coupling G-protein

GPCR – contd.• G-proteins float on the

membrane with exposed domain in cytosol

• Heteromeric in composition with alpha, beta and gamma subunits

• Inactive state – GDP is bound to exposed domain

• Activation by receptor GTP displaces GDP

• The α subunit carrying GTP dissociates from the other 2 – activates or inhibits “effectors”

• βɣ subunits are also important

GPCR - 3 Major Pathways

1. Adenylyl cyclase:cAMP pathway2. Phospholipase C: IP3-DAG pathway3. Channel regulation

1. Adenylyl cyclase: cAMP pathway

PKA Phospholambin

Increased Interaction with Faster relaxationCa++

Troponin

Cardiac contractility

OtherFunctionalproteins

Adenylyl cyclase: cAMP pathway• Main Results:

– Increased contractility of heart/impulse generation– Relaxation of smooth muscles– Lipolysis– Glycogenolysis– Inhibition of Secretions– Modulation of junctional transmission– Hormone synthesis– Additionally, opens specific type of Ca++ channel – Cyclic nucleotide

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

2. Phospholipase C:IP3-DAG pathway

PKc

IP3-DAG pathway

• Main Results:– Mediates /modulates contraction– Secretion/transmitter release– Neuronal excitability– Intracellular movements– Eicosanoid synthesis– Cell Proliferation– Responses are opposite in case of PLc inhibition

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

– Ca++, Na+ or K+ etc.• These effects may be without intervention of any of

above mentioned 2nd messengers – cAMP or IP/DAG• Bring about depolarization, hyperpolrization or Ca ++

changes etc.• Gs – Ca++ channels in myocardium and skeletal

muscles• Go and Gi – open K+ channel in heart and muscle and

close Ca+ in neurones

G-proteins and Effectors

• Large number can be distinguished by their α-subunits

G protein Effectors pathway Substrates

Gs Adenylyl cyclase - PKA Beta-receptors, H2, D1

Gi Adenylyl cyclase - PKA Muscarinic M2D2, alpha-2

Gq Phospholipase C - IP3 Alpha-1, H1, M1, M3

Go Ca++ channel - open or close

K+ channel in heart, sm

Intrinsic Ion Channel Receptors

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

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

Transducer 2 ….

Receptors with Intrinsic Ion Channel

Enzyme Linked Receptors

• 2 (two) types of receptors:

1. Intrinsic enzyme linked receptors• Protein kinase or guanyl cyclase domain

2. JAK-STAT-kinase binding receptor

Transducer 3 ….

A. Enzyme linked receptors

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

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

t-Pr-K gets activated – tyrosine residues phosphorylates on each other

Also phosphorylates other SH2-Pr domain substrate proteins Ultimately downstream signaling function Examples – Insulin, EGF -------antagonist of a such type

receptor – used in breast cancer

B. JAK-STAT-Kinase Binding Receptor

Mechanism closely resembles that of receptor tyrosine kinases

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

Uses Janus-kinase (JAK) familyAlso uses STAT (signal transducers and activators of

transcription)Examples – cytokines, growth hormones,

interferones etc.

JAK-STAT-kinase Receptors

Receptors regulating gene expression Intracellular (cytoplasmic or nuclear) receptors Lipid soluble biological signals cross the plasma membrane and

act on intracellular receptors Receptors for corticosteroids, mineralocorticoids, thyroid

hormones, sex hormones and Vit. D etc. stimulate the transcription of genes in the nucleus by binding with specific DNA sequence – called - “Responsive elements” – to synthesize new proteins

Hormones produce their effects after a characteristic lag period of 30 minutes to several hours – gene active hormonal drugs take time to be active (Bronchial asthma)

Beneficial or toxic effects persists even after withdrawal

Transducer 4 ….

Receptors of gene expression - Image

Receptor RegulationUp regulation of receptors:

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

– 3 mechanisms - Unmasking of receptors or proliferation or accentuation of signal amplification

Receptor Regulation – contd.Continued exposure to an agonist or intense receptor

stimulation causes desensitization or refractoriness: receptor become less sensitive to the agonist

Examples – beta adrenergic agonist and levodopaCauses:

1. Masking or internalization of the receptors2. Decreased synthesis or increased destruction of the

receptors (down regulation) - Tyrosine kinase receptors

Functions of Receptors - Summary

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

2. To amplify the signal3. To integrate various intracellular and extracellular

signals4. To adapt to short term and long term changes and

maintain homeostasis.

Non-receptor mediated drug action – clinically relevant examples

Physical and chemical means - Antacids, chelating agents and cholestyramine etc.

Alkylating agents: binding with nucleic acid and render cytotoxic activity – Mechlorethamine, cyclophosphamide etc.

Antimetabolites: purine and pyrimidine analogues – 6 MP and 5 FU – antineoplastic and immunosuppressant activity

Dose-Response Relationship• Drug administered – 2 components of dose- response

– Dose-plasma concentration– Plasma concentration (dose)-response relationship

• E is expressed as

Emax X [D]

Kd + [D]

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

E max

Dose-Response Curve

dose Log dose

% re

spon

se

% re

spon

se

100% -

50% -

100% -

50% -

E = Emax X [D]

Kd + [D]

Dose-Response Curve

• Advantages:– Stimuli can be graded by Fractional change in

stimulus intensity– A wide range of drug doses can easily be displayed

on a graph– Potency and efficacy can be compared– Comparison of study of agonists and antagonists

become easier

Potency and efficacy• Potency: It is the amount of drug required to produce a

certain response• Efficacy: Maximal response that can be elicited by the drug

Res

pons

e

Drug in log conc.

1 2 3 4

Potency and efficacy - Examples• Aspirin is less potent as well as less efficacious than Morphine• Pethidine is less potent analgesic than Morphine but eually

efficacious• Diazepam is more potent but less efficacious than

phenobarbitone• Furosemide is less potent but more efficacious than

metozolone• Potency and efficacy are indicators only in different clinical

settings e.g. Diazepam Vs phenobarbitone (overdose) and furosemide vs thaizide (renal failure)

Slope of DRC• Slope of DRC is also important• Steep slope – moderate increase in dose markedly increase the response

(individualization)• Flat DRC – little increase in response occurs in wide range of doses

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

Hydralazine

ThiazideFall

in B

P

Selectivity

• Drugs produce different effects – not single• DRC of different effects may be different• Example – Isoprenaline – Bronchodilatation

and cardiac stimulation – same DRC• Salbutamol – different (selective

bronchodilatation)

Therapeutic index (TI)

• In experimental animals

• Therapeutic Index = Median Lethal Dose (LD50)

Median Effective dose (ED50)

Idea of margin of safety Margin of Safety

Therapeutic index (TI)• It is defined as the gap between minimal therapeutic effect

DRC and maximal acceptable adverse effect DRC (also called margin of safety)

Combined Effects of Drugs• Drug Synergism:

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

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

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

• PABA DHFA THFA Sulfamethoxazole Trimethoprim

Folate synthase Dihydrofolate

Reductase

Drug Antagonism

1. Physical: Charcoal2. Chemical: KMnO4, Chelating agent3. Physiological antagonism: Histamine and

adrenaline in bronchial asthma, Glucagon and Insulin

4. Receptor antagonism:a. Competitive antagonism (equilibrium)b. Non-competitivec. Non-equilibrium (competitive)

Receptor antagonism - curveso Competitive:

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

Affinity (1) but IA (0), Result – no response Log DRC shifts to the right But, antagonism is reversible – increase in concentration of agonist overcomes

the blocko Parallel shift of curve to the right side

o Non-competitive: Allosteric site binding altering receptor not to bind with agonist No competition between them – no change of effect even agonist conc. .is

increased Flattening of DRC of agonist by increasing the conc. Of antagonist

Receptor antagonism - curves

• Non – equilibrium:– Antagonists Binds receptor with strong bond– Dissociation is slow and agonists cannot displace

antagonists (receptor occupancy is unchanged)– Irreversible antagonism developes– DRC shifts to the right and Maximal response

lowered

Drug antagonism DRC

Drug antagonism DRC – non-competitive antagonism

Res

pons

e

Shift to the right and lowered response

Drug in log conc.

Agonist

Agonist+ CA (NE)

Competitive Vs NC antagonismCompetitive Binds to same receptor Resembles chemically Parallel right shift of DRC in

increasing dose of agonist Intensity depends on the conc. Of

agonist and antagonist Example – Ach and atropine,

Morphine and Naloxone

Noncompetitive Binds to other site No resemblance Maximal response is

suppressed Depends only on

concentration of antagonist Diazepam - Bicuculline

Summary Basic Principles of Pharmacodynamics Mechanisms of drug action – Enzymes, Ion channels, Transporters and

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

etc. Up regulation and down regulation of receptors and desensitization Principles of dose response curves and curves in relation to agonist,

competitive antagonist etc. Therapeutic index, margin of safety and risk-benefit ratio concepts Combined effects of drugs – synergism etc. Dose response curve (DRC) – agonist and antagonist

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