Pharmacodynamic Anna Wiktorowska-Owczarek

Pharmacodynamics Mechanisms of drug action and the relationship between drug concentration and its effect

Pharmacodynamics Can be defined as: – the study of the biochemical and physiological effects of drugs and their mechanisms of action –the chemical or physical interactions between drug and target cell

Why do we want to know mechanisms of drug action? Such a complete analysis provides the basis for –the rational therapeutic use of drug –adverse effects –the design of new therapeutic agents

Mechanisms of drug action The effects of most drugs result from their interaction with macromolecular components of the organism. These interactions initiate the biochemical and physiological changes that the characteristic of the response to the drug.

Mechanism of drug action Drug in site of action ↓ Drug interaction with component of the organism ↓ Alteration of function of the component ↓ Initiation of the biochemical and physiological changes

Drug receptors Proteins form the most important class of drug receptors. The term receptor denotes the component of the organism with which the chemical agent was presumed to interact. Membrane receptors contain one or more hydrophobic membrane- spanning α-helical segments, linking the extracellular ligand-binding region of the receptor to the intracellular domain which is involved in signalling.

Interaction with receptor Agonist ↓ Receptor ↓ Generation of second message ↓ Change in cellular activity

Drug receptors Agonist combines with receptor and activate the receptor. Agonists initiate changes in cell function, producing effects of various types Antagonist may combine at the same site without causing activation. Antagonist blocks the binding of the endogenous agonist. Ligand is small molecule that may function either as an agonist or as an antagonist.

Ligands Hormones Neurotransmitters Exogenous substances (drug)

Antagonist Agonist

Drug receptors Drugs acting on receptors may be agonists or antagonists Agonists initiate changes in cell function, producing effects of various types; antagonists bind to receptors without initiating such changes. Agonist potency depends on two parameters: affinity (i.e. tendency to bind to receptors) and efficacy (i.e. ability, once bound, to initiate changes which lead to effects).

Drug receptors For antagonists, efficacy is zero. Full agonists (which can produce maximal effects) have high efficacy; partial agonists (which can produce only submaximal effects) have intermediate efficacy.

Drug receptors Tolerance –this term is used to describe a more gradual decrease in responsiveness to a drug, taking days or weeks to develope. Tolerance to drug effects results in a decrease in response with repeated doses. Tachyphylaxis is a medical term describing a rapidly decreasing response to a drug following administration of the initial doses.

Drug receptors Desensitisation is used to describe both long-term or short-term changes in dose-response relationship arising from a decrease in response of the receptor. Desensitisation can occur by a number of mechanisms: –Decreased receptor numbers (downregulation) –Decreased receptor binding affinity –Decreased G-protein coupling

Drug targets Receptors (for hormones/neurotransmitters) –adrenergic β-receptor blockers Enzymes –angiotensin converting enzyme inhibitors Carrier molecules –serotonin reuptake inhibitors Ion channels –GABA agonists Idiosyncratic targets (metal ions, gastric content) –cholestyramine Nucleic acids –antiviral agents

Types of receptors Receptor-operated channels G-protein-coupled receptors Tyrosine kinase receptors DNA-coupled receptors

Receptor-operated channels Subunits – 4 TMs each Binding of ligand  conformational changes  opening of ion-selective pore  membrane depolarization or hyperpolarization Three states: –open –closed –inactivated Very rapid transduction (ms)

Receptor-operated channels Examples –GABA A receptor benzodiazepines –nicotinic cholinergic receptor –glycine receptor –5-HT 3 serotonin receptor

G-protein-coupled receptors Membrane proteins with 7 transmembrane helical domains –7-TM receptors N-terminal part  extracellular  binds ligands C-terminal part  intracellular  binds G-proteins

G-protein-coupled receptors G-proteins  trimeric proteins –three subunits: α, β, γ –ligand binds to receptor  G-protein separates from the receptor and α separates from βγ dimer –α and βγ stimulate intracellular signalling pathways (depending on subtypes) adenylate cyclase (AC)  (+) or (-)  cAMP  protein kinase A (PKA) phospholipase C-β (PLC)  DAG, IP 3  PKC, Ca 2+ channel

The G-protein system The α-subunit binds GDP/GTP, it also has GTPase activity. The α- subunit/GTP complex is active while GTP is bound to it. The α- subunit/GTP complex is inactivated when the GTP is hydrolysed to GDP. The β-subunits remains associated with the γ-subunit when the receptor is occupied and the combined βγ- subunit may activate cellular enzyme. The γ-subunit

Second messenger systems Cyclic nucleotide system. This system is based on cyclic nucleotides such as cyclic adenosine monophosphate (cAMP), which is synthesised from ATP via enzyme adenylate cyclase. The cAMP is inactivated by hydrolysis by a phosphodiesterase enzyme to give AMP.

Second messenger systems The phosphatidylinositol system. This system is based on inositol 1,4,5- triphosphate (IP 3 ) and diacylglycerol (DAG), which are synthesised from the membrane phospholipid phosphatidylinositol 4,5-bis-phosphate (PIP 2 ), by the enzyme phospholipase Cβ.

Types of G-proteins G s stimulates membrane-bound adenylate cyclase to increase cAMP G i (and G o ) inhibits adenylate cyclase to decrease cAMP G q (and G 12 ) activates phospholipase C

The intracellular consequences of receptor activation and G-protein dissociation. Receptor linked to G-protein Gi Gs + - Adenylate cyclase cAMPProtein kinase A Phospholipase C DAG IP3 Protein kinase C Release of calcium from sarcoplasmic reticulum Intracellular enzymes Ion channels (Ca and K) Contractile proteins Gq +

G-protein-coupled receptors Examples: –adrenergic receptors α-adrenomimetics  vasoconstriction –muscarinic cholinergic receptors –dopamine receptors antipsychotic drugs  antagonists

Tyrosine kinase receptors Three domains –extracellular (ligand binding) domain –transmembrane domain –intracellular (catalytic) domain  tyrosine kinase activity Ligand binding  autophosphorylation  binding and phosphorylation of other target proteins

Tyrosine kinase receptors Examples: –insulin receptor –epidermal growth factor receptor –VEGF -receptor

DNA-coupled receptors Binding to nuclear DNA fragments when activated by ligands –promote or inhibit gene expression –sometimes ligand binding causes dissociation of inhibitory protein (e.g., HSP 90 ) –stay in the cytoplasm  agonist must enter the cell when activated migrate to the nucleus – slow process

DNA-coupled receptors Examples: –Corticosteroids: Glucocorticoid receptor Mineralocorticoid receptor –Thyroid hormone receptor –Vitamin D receptor –Retinoic acid receptor

Other sites of drug action Specific enzymes: –Acetylcholinesterase (AChE) → anticholinesterase drugs –Cyclo-oxygenase → NSAIDs (Non Steroidal Anti-Inflammatory Drugs ) –Angiotensin-converting enzyme → ACE inhibitors –Phosphodiesterase → Phosphodiesterase inhibitors

Other sites of drug action Specific enzymes and nucleic acid. The anticancer drugs inhibit enzymes involved in purine, pyrimidine or DNA synthesis.

Other sites of drug action Specific cell membrane ion pumps. –For example, Na/K-ATPase in the brain is activated by the anticonvulsant phenytoin whereas that in cardiac tissue is inhibited by digoxin; –K/H-ATPase (proton pump) in gastric parietal cells is inhibited by omeprazole.

Other sites of drug action Ion channels –Voltage-gated Na channels → Local anaesthetic –Voltage –gated Ca channels → Dihydropyridines