How Drug Act, Basics of Molecular Pharmacology.pptx

How Drug Act, Basics of Molecular Pharmacology.pptx
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1. How Drug Act?? (Molecular Aspects) Dr. Muhammad Masood Ahmed PhD-Pharmacology R.Ph

2. Ligand - any molecule which attaches selectively to particular receptors. Affinity - The degree to which a substance tends to combine with another. Intrinsic activity – Intrinsic activity is defined as a  proportionately constant ability of the agonist to activate the receptor   Specificity - A drug that interacts with a single type of receptor that is expressed on only a limited number of differentiated cells will exhibit high specificity.

3. Agonists Drugs that bind to physiological receptors and mimic the regulatory effects of the endogenous signaling compounds are termed agonists . ( Drug must have affinity and intrinsic activity to be an agonist) If the drug binds to the same recognition site as the endogenous agonist (the primary or orthosteric site * on the receptor) the drug is said to be a primary agonist . * The orthosteric sites are  the sites for binding of the substrates or competitive inhibitors of enzymes and agonists or competitive antagonists of receptors . Allosteric agonists bind to a different region on the receptor referred to as an allosteric site * *The allosteric site is  a site that allows molecules to either activate or inhibit (or turn off) enzyme/drug activity .

4. Cont’d……. Partial agonists – A drug that binds to receptor blocking access of natural agonist and also capable of low degree of activation (affinity + some intrinsic activity) eg . Pindolol & Oxprenolol at Beta receptors. Inverse agonists – A drug that on binding to receptor produces effects which are specifically opposite to those of agonist, eg . Beta Carbolines at Benzodiazepine receptors

5. Antagonist Drugs that block or reduce the action of an agonist are termed antagonists . ( Drug must have affinity but no intrinsic activity) Antagonism most commonly results from competition with an agonist for the same or overlapping site on the receptor (a syntopic interaction *) ( * An interaction between ligands that bind to the same recognition site) Physical antagonist binds to the drug and prevents its absorption like charcoal binds to alkaloids and prevents their absorption. Chemical antagonist combines with a substance chemically like chelating agents binds with the metals. ( Protamine Sulphate & Heparin) Physiological antagonist produces an action opposite to a substance but by binding to the different receptors e.g. adrenaline is a physiological antagonist of histamine because adrenaline causes bronchodilatation by binding to β 2 receptors, which is opposite to bronchoconstriction caused by histamine through H1 receptors. Pharmacological antagonists produce no effect , shows no intrinsic activity.

6. Types of Target for Drug Action The protein targets for drug action on mammalian cells that are divided into following four (4)classes Receptors Ion channels Enzymes Transporters (Carrier Molecules)

7. Receptors Transduction is the process by which a virus transfers genetic material from one bacterium to another Transcription is the process of copying a segment of DNA into RNA. Transformation In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane. Replication is  the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules

8. Ion Channels Vasodilator drugs such as dihydropyridines which inhibit the opening of L-type Ca++ Channels Benzodiazepines tranquillizers . These drugs bind to the GABA A Receptor Chloride channel complex that is distinct from GABA binding site Sulfonylureas used in treating Diabetes which act on ATP-gated K+ channels of pancreatic beta Cells and enhance Insulin Secretion. Types of Ca Channels 1. L-Type 2. N- type 3. P-type /Q.-type 4. R-type 5. T-type P: purkinjee Cells or Neuron discovered by Czech   anatomist   Jan Evangelista Purkyně , who characterized the cells in 1839. (L,N,P are (HVA)High Voltage Activated) but R & T are intermediate

9. Enzymes Many drug target enzymes, often drug molecule is a substrate analogue that acts as a competitive inhibitor of the enzyme (e.g. captopril -ACE inhibitor) in other cases, the binding is irreversible and non-competitive ( e.g.Aspirin , acting on cyclo-oxygenase (Cox-1 &2) Drugs may also act as false substrates , where the drug molecule undergoes chemical transformation to form an abnormal product that subverts the normal metabolic pathway. An example is the anticancer drug fluorouracil , which replaces Uracil

10. Transporters The movement of ions and small polar organic molecules across cell membranes generally occurs either through channels, or through the agency of a transport protein. Hydrolysis of ATP provides the energy for transport of substances against their electrochemical gradient. Such as distinct ATP binding site, and are termed ABC (ATP-Binding Cassette) transporters. Sodium pump (Na+-K+- ATPase and multidrug resistance (MDR transporters that eject cytotoxic drugs from cancer and microbial cells, conferring resistance to these therapeutic agents

11. PHYSIOLOGICAL RECEPTORS Ligand -gated Ion channels ( Ionotropic Receptors) G-Protein Coupled Receptors (GPCRs) Kinase -Linked receptors a. Transmembrane , enzymes b. Transmembrane , non-enzymes 4. Nuclear receptors e.g Intracellular enzymes   Receptor: In Pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems

12. Types of Receptors

13. Receptor- Effector Linkage

14. Cellular Pathways Activated by Physiological Receptors Signal Transduction Pathways Physiological receptors have at least two major functions, ligand binding and message propagation (i.e., signaling). Two functional domains within the receptor: - ligand -binding domain and - effector domain . The regulatory actions of a receptor may be exerted directly on its cellular target(s), on effector protein(s), or may be conveyed by intermediary cellular signaling molecules called transducers .

15. LIGAND-GATED ION CHANNELS

16. Receptors with intrinsic ion channel Ligand gated ion channels. Ligand : In coordination chemistry, a ligand is  an ion or molecule (functional group) that binds to a central atom to form a coordination complex. No intervention of G-protein or second messenger. Response is fastest (in milliseconds). e.g . GABA A , 5HT 3 , NMDA

17. Voltage-activated Na + channel with the pore in the open and closed state.

18. Structure of the nicotinic acetylcholine receptor (a typical ligand -gated ion channel. The five receptor subunits (α2, β, γ, δ) form a cluster surrounding a central transmembrane pore.

19. Ligand -gated nicotinic acetylcholine receptor expressed in the skeletal muscle neuromuscular junction. The pore is made up of five subunits, each with a large extracellular domain and four transmembrane helices. The helix that lines the pore is shown in blue.

20. Receptor It is defined as a macromolecule or binding site located on the surface or inside the effector cell that serves to ( i ) recognize the signal molecule/drug and (ii) initiate the response to it, but itself has no other function. Macromolecules are made up of basic molecular units. They include the proteins ( polymers of amino acids ), nucleic acids (polymers of nucleotides), carbohydrates (polymers of sugars) and lipids (with a variety of modular constituents).

21. Agonist An agent which activates a receptor to produce an effect similar to that of the physiological signal molecule. An agent which activates a receptor to produce an effect in the opposite direction to that of the agonist. An agent which prevents the action of an agonist on a receptor or the subsequent response, but does not have any effect of its own. Inverse agonist Antagonist

22. Partial agonist An agent which activates a receptor to produce submaximal effect but antagonizes the action of a full agonist. Ligand ( Latin: ligare —to bind ) Any molecule which attaches selectively to particular receptors or sites. The term only indicates affinity. Affinity or ability to bind without regard to functional change: agonists and competitive antagonists are both ligands of the same receptor. Ligand

23. Efficacy Efficacy is the maximal response a drug can produce. Potency is a measure of the dose that is required to produce a response. Potency (strength) refers to   the amount of drug (usually expressed in milligrams) needed to produce an effect . Intrinsic activity is  a measure of the ability of a drug that is bound to the receptor to generate an activating stimulus and produce a change in cellular activity . Potency Intrinsic activity

24. Potent Drug A highly potent drug evokes a given response at low concentrations. (e.g.,  fentanyl , alprazolam , risperidone , bumetanide , bisoprolol ) while a drug of lower potency evokes the same response only at higher concentrations. (e.g., meperidine , diazepam, ziprasidone , furosemide , metoprolol )

25. “Opiates” vs. “ opioids ” Although these terms are often used interchangeably they are different:  Opiates refer to natural opioids such as heroin, morphine and codeine.  Opioids refer to all natural, semisynthetic , and synthetic opioids .

26. The two-state receptor model

27. ACTION-EFFECT SEQUENCE It is the initial combination of the drug with its receptor resulting in a conformational change in the latter ( in case of agonists ), or prevention of conformational change through exclusion of the agonist ( in case of antagonists ). [a  conformational change  is a change in the shape of a  macromolecule , often induced by environmental factors] Drug action:

28. Drug effect It is the ultimate change in biological function brought about as a consequence of drug action, through a series of intermediate steps i -e transducer. Transducer These are a large family of cell membrane receptors which are linked to the effector (enzyme/ channel/carrier protein) through one or more GTPactivated proteins ( Gproteins ) for response effectuation.

29. Receptors Essential functions Receptors subserve two essential functions, viz , recognition of the specific ligand molecule and transduction of the signal into a response . Accordingly, the receptor molecule has a ligand binding domain (spatially and energetically suitable for binding the specific ligand ) and an effector domain

30. G-Protein Coupled Receptor

31. G-Protein Coupled Receptor STRUCTURAL FAMILY FUNCTIONAL FAMILY PHYSIOLOGICAL LIGANDS EFFECTORS AND TRANSDUCERS EXAMPLE DRUGS   GPCR β Adrenergic receptors NE, Epi , DA ( Dihydroxyphenyl-alanine ) G s ; AC   Dobutamine   Muscarinic cholinergic receptors ACh G i and G q ; AC, ion channels, PLC   Atropine   Eicosanoid receptors Prostaglandins, leukotrienes , thromboxanes G s , G i and G q proteins   Misoprostol , montelukast

32. G-Protein Coupled Receptor Transduction

33. G-Proteins G-Proteins a family of heterotrimeric GTP-binding regulatory proteins termed G proteins ( heterotrimer (plural heterotrimers ) (chemistry, biochemistry) A trimer , especially  a biologically active one , derived from two or more different (but similar) monomers). G proteins are signal transducers that convey the information that agonist is bound to the receptor from the receptor to one or more effector proteins. The G protein heterotrimer is composed of a guanine nucleotide-binding α subunit , which confers specific recognition to both receptors and effectors, and an associated dimer of β and ϒ subunits. G–protein-regulated effectors include enzymes such as - adenylyl cyclase , - phospholipase C, -cyclic GMP phosphodiesterase (PDE6), -membrane ion channels selective for Ca 2+ and K + .

34. The subunits fall into four families ( G s , G i , G q , and G 12/13 ). The G s α -subunit uniformly activates adenylyl cyclase ; the G i α -subunit can inhibit certain isoforms of adenylyl cyclase ; the G q α -subunit activates all forms of phospholipase C; and the G 12/13 α -subunits couple to guanine nucleotide exchange factors (GEFs), such as p115RhoGEF for the small GTP-binding proteins Rho* and Rac *. * Rho : A ρ factor (Rho factor) is  a prokaryotic protein involved in the termination of transcription . * Rac :  is a subfamily of the Rho family of GTPases , small (~21 kDa ) signaling G  proteins  (more specifically a GTPase ). When bound to GTP,  Rac  is activated. G Proteins-Types

35. G-Protein Suptypes and Their Functions

36. G Protein–Coupled Receptors ( GCPRs ) GPCRs span the plasma membrane as a bundle of seven alfa -helices. Extracellular binding site for ligand . Cytosolic binding site for transducer G-protein.

37. (RGS: Regulator of G protein signaling)

38. PI3 K: Phosphoinositide 3-kinases GRKs: GPCRs Kinase Arrestins are  adaptor proteins that function to regulate G protein-coupled receptor (GPCR) signaling and trafficking

39. Adenylyl cyclase-cAMP pathway ↑ cAMP (2 nd messenger) PK A phosphorylation Various functions -↑ heart contraction -Smooth muscle relaxation - Glycogenolysis - lipolysis

40. Phospholipase C: IP 3 -DAG pathway PIP 2 IP 3 + DAG Mobilize Ca 2+ PKc activation Activation of MLCK (Myosin Light chain Kinase ), PKc - contraction -neural excitation -cell proliferation

41. Signal Transduction Signal transduction (also known as cell signaling) is  the transmission of molecular signals from a cell's exterior to its interior . Signals received by cells must be transmitted effectively into the cell to ensure an appropriate response. This step is initiated by cell-surface receptors. e.g. Adrenaline via Beta Adrenergic receptors & Ach via Muscarinic receptors

42. RESENSITIZATION AND DOWN-REGULATION OF RECEPTOR Agonist binding to receptors initiates signalling by promoting receptor interaction with G proteins (G s ) located in the cytoplasm. Agonist-activated receptors are phosphorylated by a G protein-coupled receptor kinase (GRK), preventing receptor interaction with G s and promoting binding of a different protein, - β arrestin ( β - Arr ), to the receptor. The receptor- β arrestin complex binds to coated pits, promoting receptor internalization. Dissociation of agonist from internalized receptors reduces - β Arr binding affinity, allowing dephosphorylation of receptors by a phosphatase . Return of receptors to the plasma membrane result in the efficient resensitization of cellular responsiveness. Repeated or prolonged exposure of cells to agonist favors the delivery of internalized receptors to lysosomes , promoting receptor down-regulation rather than resensitization .

43. Kinase - linked receptors

44. Enzyme linked receptors They are of two types With intrinsic enzymatic activity. ( Transmembrane , Enzymes ) Without intrinsic enzymatic activity (but bind a JAK-STAT kinase on activation ( Transmembrane , Non-Enzymes )

45. receptor tyrosine kinase and a cytokine receptor . Activation of the EGF receptor. The extracellular structure of the unliganded receptor (a) contains four domains (I-IV), which rearrange significantly upon binding two EGF molecules. (b). The conformational changes lead to activation of the cytoplasmic tyrosine kinase domains and tyrosine phosphorylation of intracellular regions to form SH2 binding sites. (c). The adapter molecule Grb2 binds to the phosphoryated tyrosine residues and activates the Ras -MAP kinsase cascade

46. JAKs- STATS PATHWAY Binding of the cytokine causes dimerization of the receptor and recruits the Janus Kinases (JAKs) to the cytoplasmic tails of the receptor. JAKs trans- phosphorylate and lead to the phosphorylation of the signal transducers and activators of transcription (STATs) . The phosphorylated STATS translocate to the nucleus and regulate transcription. There are proteins termed suppressors of cytokine signaling (SOCS) that inhibit the JAK-STAT pathway

47. NATRIURETIC PEPTIDE RECEPTORS

48. NO SYNTHASE AND SOLUBLE GUANYLATE CYCLASE The cellular effects of cyclic GMP on the vascular system are mediated by a number of mechanisms, but especially by PKG. For example, in vascular smooth muscle, activation of PKG leads to vasodilation by: Inhibiting IP 3 -mediated Ca 2+ release from intracellular stores. Phosphorylating voltage-gated Ca 2+ channels to inhibit Ca 2+ influx. Phosphorylating phospholamban , a modulator of the sarcoplasmic Ca 2+ pump, leading to a more rapid reuptake of Ca 2+ into intracellular stores. Phosphorylating and opening the Ca 2+ -activated K + channel leading to hyperpolarization of the cell membrane, which closes L-type Ca 2+ channels and reduces the flux of Ca 2+ into the cell.

49. Nuclear Receptors

50. Receptors regulating gene expression Several biologic ligands are sufficiently lipid-soluble to cross the plasma membrane and act on intracellular receptors. e.g. steroids (corticosteroids, mineralocorticoids , sex steroids, vitamin D), and thyroid hormone, whose receptors stimulate the transcription of genes by binding to specific DNA sequences near the gene whose expression is to be regulated. Target DNA sequences called response elements.

51. In the absence of hormone, the receptor is bound to hsp90 , a protein that appears to prevent normal folding of several structural domains of the receptor. Binding of hormone to the ligand -binding domain triggers release of hsp90. This allows the DNA-binding and transcription-activating domains of the receptor to fold into their functionally active conformations, so that the activated receptor can initiate transcription of target genes. Cont’d….

52. Bibliography Essentials of Medical Pharmacology -7 th edition by KD Tripathi Goodman & Gilman's the Pharmacological Basis of Therapeutics  12 th edition by Laurence Brunton  (Editor) Rang & Dale’s Pharmacology, 7 th Edition Lippincott's Illustrated Reviews: Pharmacology  - 6 th edition by Richard A. Harvey Basic and Clinical pharmacology 11 th edition by Bertram G Katzung Rang & Dale's Pharmacology -7 th edition  by Humphrey P. Rang Clinical Pharmacology 11 th edition By Bennett and Brown, Churchill Livingstone Principles of Pharmacology 2 nd edition by HL Sharma and KK Sharma Review of Pharmacology by Gobind Sparsh

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