Mech of action of Drugs

PHARMACODYNAMICS…: 

PHARMACODYNAMICS… ( Greek : dynamis – power) ‘What the drug does to the body’ This includes physiological and biochemical effects of drugs and their mechanism of action at macromolecular/subcellular/organ system levels.

Principles of Drug Action: 

Drugs can potentially alter the rate of body function i.e. can only increase or decrease the rate , they do not create or impart new effects. They modulate intrinsic physiological functions . Principles of Drug Action

Principles of Drug Action…: 

Types of drug action Stimulation- -adrenaline (heart) pilocarpine (salivary glands) Depression- -barbiturates (CNS), quinidine (heart) Irritation- -irritant purgatives Replacement- -levodopa (parkinsonism), insulin (DM) Cytotoxic- -chemotherapeutic agents Principles of Drug Action…

Principles of Drug Action…: 

Drugs can produce many effects Primary effect / therapeutic / desired effect Secondary effect / Undesirable / Adverse effects Principles of Drug Action…

MECHANISMS OF DRUG ACTION : 

MECHANISMS OF DRUG ACTION Drug receptor interaction Biophysical interaction e.g. Alcohol, Volatile GA Chemical interaction i.e. Antacids, Heparin & Protamine sulphate, Chelating agents & heavy metals

MECHANISMS OF DRUG ACTION… : 

MECHANISMS OF DRUG ACTION… 4. Alteration of specific processes unique to micro-organisms /cancer cells: Antibacterial i.e. Streptomycin, Penicillins Antiviral i.e. Ribavirin Antifungal i.e. Clotrimazole Antiparasitic i.e. Chloroquine Alkylating agents i.e. Cyclophosphamide Antimetabolites i.e. Methotrexate Spindle poisons i.e. Vincristine Hormone / Hormone antagonists i.e. Prednisolone / Tamoxifen

MECHANISMS OF DRUG ACTION… : 

MECHANISMS OF DRUG ACTION… 5. Physical Action Mass of the drug ---- Bulk laxatives Adsorptive property ---- Charcoal, Kaolin , Osmosis i.e. Diuretic (Mannitol), Purgative (Magnesium sulphate) Radioactivity ----- 131 I Radio opacity ---- contrast media , Urograffin

1.DRUG RECEPTOR INTERACTION: 

1.DRUG RECEPTOR INTERACTION RECEPTOR Definition: Specific cellular macromolecule to which the drug binds to initiate its biological effect by producing a conformational change in the macro molecule OR Any target molecule with which a drug molecule has to combine in order to elicit its specific effects Usually a protein

1.DRUG RECEPTOR INTERACTION…: 

1.DRUG RECEPTOR INTERACTION… Receptor It is a dynamic structure i.e. there may be increase, decrease in synthesis or it may be up-regulated or down regulated Biological effect may be Increased secretion of a gland Contraction of a muscle Decrease in heart rate Change in permeability

Characteristics of receptors: 

Characteristics of receptors Most of the receptors are proteins, a few are other molecules such as DNA Receptor site (Recognition site) : Specific region of the receptor to which drug molecule binds Determine the quantitative relations between the dose or conc. of drug and pharmacologic effects Responsible for selectivity / specificity of drug action Mediate the actions of both pharmacologic agonists, antagonists, partial agonists and inverse agonists

Receptor: 

Receptor A. Most of the receptors are proteins in nature Best characterized drug receptors are Regulatory proteins e.g. receptors for neurotransmitters /autacoids / hormones (Adrenergic drugs, Antihistamines,Thyroxin) Some act as Transport proteins Na+-K+-ATPase (Digoxin) Some are Structural proteins Tubulin (Colchicine)

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B. Drug-enzyme interaction Enzymes (functional proteins) are very important target of drug actions. The drug molecule is a substrate analogue that acts as competitive inhibitor of the enzyme, either reversibly or irreversibly Carbonic anhydrase (inhibited by Acetazolamide) Xanthine oxidase (inhibited by Allopurinol) Cyclooxygenase (inhibited by aspirin irreversibly)

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C. Drug-channel interaction Some ion channels incorporate a receptor and open only when the receptor is occupied by an agonist Verapamil (Calcium channels blocker) block the L-type f calcium channes BDZ facilitate Cl channel opening LAs – Na channel blockers

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D. Interaction with nucleic acid 5 F U (Anticancer drug) DNA & RNA synthesis inhibitor

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RECEPTOR LIGANDS Latin Ligare ---- to bind Ligand is a molecule which attaches selectively to a particular receptor Coupling (transduction process between occupancy of receptors and drug response) Drug Receptor Binding Interaction-Types Reversible (ionic bond) Irreversible (covalent bond)

Parameters of drug action : 

Parameters of drug action Affinity (tendency of a drug to combine with the receptor) Intrinsic activity / Efficacy (capacity to innitiate a series of chain of reactions resulting in its effect) Agonist : Affinity + Intrinsic activity e.g. Adrenaline, Isoprenaline , Acetylecholine Antagonist: Affinity but NO Intrinsic activity e.g. Propranolol , Atropine

Antagonist: 

Antagonist two types Competitive Noncompetitive

Competitive Antagonist: 

Competitive Antagonist competes with agonist for receptor surmountable with increasing agonist concentration displaces agonist dose response curve to the right (dextral shift) reduces the apparent affinity of the agonist

Noncompetitive Antagonist: 

Noncompetitive Antagonist drug binds to receptor and stays bound irreversible – does not let go of receptor produces slight dextral shift in the agonist DR curve in the low concentration range but, as more and more receptors are bound (and essentially destroyed), the agonist drug becomes incapable of eliciting a maximal effect

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Partial agonist (Agonist / Antagonist): Affinity + Some degree of Intrinsic activity Produce a lower response even at full receptor occupancy In the presence of an agonist it acts as an antagonist e.g, Oxpranolol, pentazocine

Agonists and antagonists : 

A gonists and antagonists agonist has affinity plus intrinsic activity antagonist has affinity but no intrinsic activity partial agonist has affinity and less intrinsic activity competitive antagonists can be overcome

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Endogenous ligand (e.g. endorphin on opoid receptor) Agonist (e.g. morphine on opoid receptor) Antagonist (e.g. naloxone on opoid receptor) Classification of drugs

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Inverse agonist Affinity + Intrinsic activity but the effects produced are specifically opposite to that of agonists e.g. Betacarbolines at Benzodiazepines receptor.

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Spare & Orphan receptors Spare: allow maximal response without total receptor occupancy – increase sensitivity of the system can bind (and internalize ) extra ligand preventing an exaggerated response if too much ligand is present **beta 1 receptors on heart Orphan: ligand is not known

Importance of receptors : 

Importance of receptors Receptors determine the quantitative relation between dose & Pharmacologic effects of a drug (the receptor affinity for binding a drug determines the conc of drug required to form a significant no of drug-receptor complexes, and the total no of receptors may limit the maximal effect a drug may produce) Selectivity of action (molecular size, shape, electric charge of a drug determine whether and with what affinity it will bind to a particular receptor among the vast array of chemically different binding sites available in a cell, tissue or patient) Receptors mediate actions of both agonists & antagonists.

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Receptors subserve two essential functions Recognition of the specific ligand molecule. Transduction of the signal into a response. Transducer Mechanisms These are highly complex multi-step processes that provide for amplification & integration of concurrently received extra-and intra-cellular signals at each step.

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Effectors These are molecules that translate the drug-receptor interaction into a change in cellular activity. Examples: Enzyme Adenylyl cyclase. Some receptors are also effectors: Tyrosine kinase is part of Insulin receptor.

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Second messengers These are chemical substances produced under the effect of “effectors” . They amplify & further transmit the signal to produce the response. e.g. Cyclic AMP DAG

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Efficacy Maximal effect an agonist can produce Potency Denotes the amount of drug needed to produce a given effect

Potency: 

Potency Relative position of the dose-effect curve along the dose axis Has little clinical significance for a given therapeutic effect A more potent of two drugs is not clinically superior Low potency is a disadvantage only if the dose is so large that it is awkward to administer

REGULATION OF RECEPTORS: 

REGULATION OF RECEPTORS Down regulation / Desensitization of receptors Decrease in number of receptors on prolonged use of some agonist drugs leading to refractoriness e.g. Adrenergic drugs (salbutamol) may become ineffective as bronchodilators in asthmatic patients on chronic use. Up regulation / Super sensitivity Increase in number of receptors on long term use of some antagonists e.g. Beta blockers (propranolol) sudden with drawl after prolonged use may lead to an attack of angina.

Desensitization: 

D esensitization agonists tend to desensitize receptors antagonists tend to up regulate receptors

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MOLECULAR / SIGNALLING MECHANISMS AND DRUG ACTION

SIGNALLING MECHANISMS: 

SIGNALLING MECHANISMS Five basic mechanisms of transmembrane Signaling a. Lipid soluble ligand that crosses the membrane and acts on intracellular receptors b Transmembrane receptor protein whose intracellular enzymatic activity is allosterically regulated by a ligand that binds to a site on the protein’s extra cellular domain c. Binding to extra cellular domain of trans membrane receptor bound to separate intracellular protein tyrosine kinase molecule

SIGNALLING MECHANISMS: 

d. Binding to and directly regulating the opening and closing of an ion channel e. Binding to a cell surface receptor linked to an effector enzyme by a G protein (GTP binding signal protein) which inturn changes the concentration of intracellular second messenger SIGNALLING MECHANISMS

SIGNALLING MECHANISMS…: 

Trans membrane diffusion and action on intracellular receptors a: Stimulation of cytoplasmic enzyme i.e. Guanylyl cyclase by Nitric oxide. b: Stimulation of gene transcription by: Corticosteroids Thyroid hormone Vitamin D SIGNALLING MECHANISMS…

Trans membrane diffusion and action on intracellular receptors : 

Trans membrane diffusion and action on intracellular receptors Receptors that control gene transcription ( gene active receptors) Receptors are intracellular proteins, so ligands must first enter cells. Receptors consist of a three domains: DNA-binding domain. Ligand-binding domain. Transcription activating domain.

Trans membrane diffusion and action on intracellular receptors : 

Trans membrane diffusion and action on intracellular receptors Without ligand receptor is stabilized in an inactive state by hsp90 When steroid binds to receptor site hsp90 is dissociated receptor is converted the into active configuration. Activated R can initiate transcription of genes by binding to Response elements (specific DNA sequences near the genes)

Trans membrane diffusion and action on intracellular receptors : 

Trans membrane diffusion and action on intracellular receptors Important therapeutic consequences: Effects are slow in onset (30 min /several hrs) as they are due to altered protein synthesis Effects can persists for hours / days after the agonist conc. has been reduced to zero ----- due to slow turn over of enzymes & proteins. One type of nuclear receptor is responsible for the increased expression of drug-metabolizing enzymes induced by many therapeutic agents.

2.Receptor Located on transmembrane Enzymes : 

2.Receptor Located on transmembrane Enzymes Trans-membrane proteins Large extra cellular domain Intracellular domain having tyrosine kinase / guanylate cyclase / serine kinase activity. Examples: Epidermal growth factor, Insulin, Atrial natriuretic peptide, Platelet derived growth factor.

2.Receptor Located on transmembrane Enzymes: 

2 .Receptor Located on transmembrane Enzymes Binding of drug to receptor site on extra cellular domain Conformational change Activates enzymatic activity of effector enzyme- --- Tyrosine kinase. Dimerisation & Auto phosphorylation of tyrosine residues. They also bind to of a variety of intracellular proteins. These proteins modulate a number of biochemical processes i.e Insulin increases glucose & AA uptake Regulation of metabolism of glycogen & triglycerides.

2.Receptor Located on transmembrane Enzymes: 

2 .Receptor Located on transmembrane Enzymes Imp. implications Inhibitors of tyrosine kinase. Intensity & DOA of these ligands is limited by down regulation ----- Accelerated endocytosis of R from cell surface & degradation of ligand

3. Cytokine receptor : 

3. Cytokine receptor Trans membrane receptor having: Extra cellular domain Intracellular domain that binds non-covalently to a separate TK from Janus-kinase family( JAK) EXAMPLES: Growth hormone Erythropoietin Interferon

3. Cytokine receptor: 

3. Cytokine receptor Signaling mech. When the receptor is occupied , dimerization and activation of JAK – which phosphorylates tyrosine residues. Tyrosine phosphates lead to phosphorylation of STAT molecules (Signal transducers and activators of transcription) STAT dimers travel to the nucleus and regulate transcription

4. Ligand-gated ion channel (ionotropic receptors) : 

4. Ligand-gated ion channel (ionotropic receptors) Several structural families, the commonest 4-5 trans-membrane sub units, arranged around a central aqueous channel. The drug binds to receptor site & directly opens the channel within milliseconds. Involved mainly in fast synaptic transmission. ↑ trans-membrane conductance of relevant ion. Alteration of electrical potential across the membrane.

4. Ligand-gated ion channel (ionotropic receptors) : 

4. Ligand-gated ion channel (ionotropic receptors) Examples Synaptic transmitter: Acetylcholine at nicotinic receptor Gammaaminobutyric acid type A (GABA A ) Glycine 5­hydroxytryptamine at 5-HT 3 receptors

4. Ligand-gated ion channel (ionotropic receptors) : 

4. Ligand-gated ion channel (ionotropic receptors) Ligand-grated ion channels can be regulated by multiple mechanisms including phosphorylation & endocytosis. In the CNS these are involved in learning & memory.

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors : 

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors Family of 7 transmembrane or serpentine receptors. One of the intracellular loops is larger than the others and interacts with the G-protein. The G-protein is a membrane protein comprising of: 3 subunits αβγ α -subunit possesses GTPase activity . Several types of G-proteins

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors : 

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors When drug binds to a cell surface receptor binding site. Conformational change. Transmitted to C loops The α -subunit dissociates. Release of GDP from G pr. & entry of GTP. Then it can activate an effector: A membrane enzyme i.e. Adenylyl cyclase Ion channel (In some cases the βγ -subunit may be the activator species.)

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors : 

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors This changes the concentration of intracellular second messenger i.e. Cyclic Adenosine Monophosphate (cAMP) Calcium and Phosphoinositides (IP3 and diacyl glycerol) Cyclic Guanosine Monophosphate (cGMP)

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors : 

5. G protein-coupled receptors (GPCR) Or Metabotropic receptors Activation of the effector is terminated when the bound GTP molecule is hydrolyzed The α ­subunit recombines with βγ . Examples Muscarinic acetylcholine receptor Adrenoceptors Neuropeptide receptors.

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Receptor Types Coupling protein Effector Effector substrate Second messenger response Result M1,M3, Alpha Gq Phospholipase C Membrane lipids IP3  DAG Ca 2+  Protein kinase Beta , D1 Gs Adenyl cyclase ATP cAMP Ca 2+ influx Enz. activity Alpha 2 , M2 Gi Adenyl cyclase ATP cAMP Ca 2+ influx & Enz activity

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G Protein Ligand for receptors Effectors Second Messenger Gs Beta adrenergic amines  Adenylyl cyclase  cAMP G i1, G i2, G i3 α 2 adrenergic amine Acetylcholine Opioids  Adenylyl cyclase  cAMP Open cardiac K+ channels G olf Odorants  Adenylyl cyclase  cAMP G q Acetylcholine, bombesin, serotonin & others  Phospholipase C IP3  DAG & Cytoplasmic Ca++ G t1 G t2 Photons  cGMP -Phosphodiesterase  cGMP

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Effectors activated or inhibited by G-proteins Adenylate cyclase (AC) AC catalyses formation of the intracellular messenger cAMP. cAMP activates various protein kinases. PK control cell function in many different ways by causing phosphorylation of various Enzymes Carriers Other proteins.

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Phospholipase C Catalyses the formation of two intracellular messengers from membrane phospholipids: i) Inositol trisphosphate (IP 3 ) ii) Diacylglycerol (DAG) IP 3 releases Ca 2+ from intracellular compartments So ↑ free cytosolic Ca 2+ . ↑ free Ca 2+ initiates many events: Contraction Secretion Enzyme activation Membrane hyperpolarisation.

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DAG activates protein kinase C Protein kinase C controls many cellular functions by phosphorylating a variety of proteins

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Regulation of receptors: Desensitisation of G-protein-coupled receptors occurs as a result of phosphorylation by specific receptor kinases, causing the receptor to become non-functional and to be internalised. There is a large family of phosphatases that act to reverse the effects of kinases.

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Receptor-linked G-proteins also control: Phospholipase A  Formation of arachidonic acid and eicosanoids Potassium and calcium channels  affecting: Membrane excitability Transmitter release Contractility

Receptor & Disease: 

Receptor & Disease No. of disease states are directly linked to receptor malfunction Myasthenia gravis Grave’s disease Mutation in receptors