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Pharmacology A Selective Overview Carl Rosow, M.D., Ph.D.

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Presentation on theme: "Pharmacology A Selective Overview Carl Rosow, M.D., Ph.D."— Presentation transcript:

1 Pharmacology A Selective Overview Carl Rosow, M.D., Ph.D.

2 Disclosures Consultant – Salix Pharmaceuticals

3 Topics for Today Pharmacodynamics –Receptors, potency, efficacy –Agonists, antagonists –The dose response curve Cholinergic Pharmacology –Nicotinic and muscarinic effects –Acetylcholinesterase –Cholinesterase inhibitors: effects and side effects

4 Topics for Friday Pharmacokinetics –Volume of distribution –Clearance –Drug Metabolism Variability in Drug Response –Idiosyncratic variability –Pharmacogenetic variability

5 Case Sydney S. is a 62 year old male with a diagnosis of early-onset Alzheimer’s Dementia. One month ago, his dose of donepezil was increased to improve his increasing memory lapses. His wife says that he now seems a bit more alert and (possibly) less forgetful. Sydney doesn’t like it, because now the medicine gives him stomach cramps and diarrhea.

6 What does Cholinergic Pharmacology have to do with… …Sydney’s memory loss, diarrhea, and dizziness? …effects and side effects of the drugs used to treat this patient?

7 Where do we find cholinergic neurons? 1.Brain 2.Motor nerves in skeletal muscle 3.Autonomic ganglia 4.Parasympathetic post-ganglionic nerves

8 Cholinergic Neurons in the Brain Nucleus Basalis of Meynert – involved in memory, alertness Pedunculopontine projection involved in sleep-wake cycle Cholinergic receptors in the brain are “Nicotinic n ” and “Muscarinic”

9 Alzheimer Disease: Loss of Cholinergic neurons Degeneration of Nucleus Basalis of Meynert Loss of neurons in pedunculopontine projection Drugs for ALZHEIMER’S increase synaptic Ach of remaining neurons.

10 Motor Nerve Synapse “Nicotinic m ” Receptors MYASTHENIA GRAVIS: Nicotinic m receptor autoantibody. Drugs for Myasthenia increase synaptic ACh to compete with the antibody.

11 Sympathetic Div. Parasympathetic Div. Ach Nicotinic n Ach Muscarinic  1,  2,  1,  2) DA, ACh NE Autonomic Nervous System

12 ANS End-Organ Effects  ACh(musc)ACh effect Heart Contractility  ↓-- Heart Rate  Slow HR Blood Vessel Tone  ↓ (NO)-- Salivary Glands  Secretions GI Tone  Peristalsis GI Sphincter  Relax sphinct. Detrusor Contraction  Empty bladder Urine Sphincter  Relax sphinct. Pupil Radial Muscle  -- Pupil Circ. Muscle  Small pupil Ciliary Muscle  Accommodate Atropine!

13 Diphenhydramine (Benadryl) is an antihistamine with powerful muscarinic antagonist properties. These are likely to account for which of the following side effects? 1.Constipation 2.Sedation 3.Urinary retention 4.Dry mouth 5.Tachycardia

14 Returning now to principles of pharmacology and receptors… Pharmacodynamics: The action of drugs. “What the drug does to the body” Pharmacokinetics: The absorption, distribution, metabolism, and excretion of drugs. “What the body does to the drug”

15 Drug Receptors A macromolecular component of a cell that interacts with a drug and initiates an effect Drug-receptor interaction determines –Structure-activity relationship –Selectivity/Stereoselectivity –Potency –Efficacy (Intrinsic Activity) –Agonist or antagonist properties

16 Structural Specificity An Example Ac CH=CH 2 MORPHINE: An opioid analgesic Change hydroxys to acetoxys → HEROIN: more potent analgesic Change methyl to allyl → NALORPHINE: a hallucinogen and opioid antagonist Change to dextro enantiomer → d-MORPHINE: cough suppressant, no analgesic effect [“Stereospecificity”]

17 Potency and Efficacy (Not the same!) More/less potentFull/partial agonist Which of these (potency or efficacy) is more important to a clinician?

18 “Antagonists”

19 Receptor Superfamilies Ligand Gated Ion Channels [Nicotinic-ACh] G Protein Coupled Receptors [Muscarinic-ACh] Enzyme-Linked Cell Surface Receptors [insulin] Intracellular Receptors [cortisone] Voltage Gated Ion Channels [lidocaine] Endocytotic Receptors [LDL-cholesterol] Numerous Others

20 The Ligand-Gated Ion Channel Superfamily Nicotinic Cholinergic[Na +, K + ] GABA A [Cl - ] Serotonin (5-HT 3 )[Na +, K + ] Glycine[Cl - ]

21 Nicotinic Cholinergic Receptor

22 GABA A Receptor

23 Regulation of Fast LGIC Responses (GABA A Receptor) Deactivation Desensitization : A conformational change

24 The GPCR Superfamily Family 1 GPCRs Adenosine receptors Adrenergic receptors Cannabinoid receptors Cholecystokinin receptors Dopamine receptors Galanin receptors Histamine receptors Muscarinic receptors Neuropeptide Y receptors Opioid receptors Purinergic receptors Serotonin receptors Somatostatin receptors Tachykinin receptors Family 2 GPCRs CRF receptors VIP & PACAP receptors Family 3 GPCRs Calcium sensing receptors GABAB receptors Metabotropic glutamate receptors GPCR ligands are: Neurotransmitters Small peptides Nucleotides

25 G-Protein Coupled Receptor Agonist Binding

26 G-Protein Coupled Receptor

27 Activation of Adenylyl Cyclase and Phospholipase C  s or  i qq

28 Drug-Receptor Binding (1) At the molecular level, drug binding is a chemical interaction characterized by a DISSOCIATION CONSTANT K D. Strength of binding is AFFINITY (1/K D ) D + R RD, where D x k on k off

29 Drug-Receptor Binding (2) Can be rearranged to So, an increase in bound receptors [RD] can be increase in [D] or increase in [R] K D is constant Next slide

30 Drug-Receptor Binding (3) Assume total number of receptors R tot = R + RD is constant, then the fraction of bound receptors is: A Langmuir Isotherm

31 Ligand-Receptor Binding Curves Binding curve often covers 4 orders of magnitude A semilogarithmic plot is useful Drugs that bind to the same receptor have parallel curves

32 Assume Binding is Proportional to Response : Concentration- response curve looks like the binding curve. Why do we use the 50% effect?

33 Drug-Receptor Binding vs. Response Why the Difference? (EC 50 ) < K D SPARE RECEPTORS! 50% effect at much less than 50% binding

34 The Dose-Response Curve The Sine Qua Non of drug effect Characterized by –E max – maximal effect (efficacy or intrinsic activity) –EC 50 or ED 50 – dose for ½ maximal effect (potency) –Slope or  (characteristic of particular ligand- receptor interaction – should be monotonic)

35 Which of These Drugs is the Most Potent?

36 “Affinity” vs. “Efficacy” Ligand (D) binds to Receptor (R) causing a measurable effect. D + R RDRD* K off K on  Affinity = PotencyEfficacy (Intrinsic Activity)

37 Agonists (Affinity vs. Efficacy) The molecular de-activation equilibrium constant is a drug-specific factor. When  is a smaller number, it denotes more receptor activation (greater efficacy).

38 Agonist Efficacy  = 0.01  = 0.1  = 1  = 10  = 3 PartialAgonists No Spare Receptors Full Agonists Spare Receptors

39 Induced-Fit Hypothesis 1.An AGONIST binds to a receptor and activates it by inducing a conformational change (induced fit) 2.Bound resting receptors (RD) become active (RD*) 3.Active receptors then become inactive Inactive receptors must recycle to resting in order to work again. D + R RDRD* KDKD 

40 Let’s get back to Sydney S.… He still needs a cholinergic agonist for his dementia. What are the possible drug targets?

41 Hemicholinium (-) Vesamicol (-) Latrotoxin (+) Botulinum Toxin (-) Neostigmine (+) Donepezil (+) Nicotine (+) → (-) Curare (-) Muscarine (+) Atropine (-) (+) = ↑ cholinerg. effect (-) = ↓ cholinerg. effect  -Conotoxin (-)

42 Nicotinic “Agonists” Acetylcholine – Cannot be administered exogenously. Why? Nicotine and congeners are “agonists,” but they cause twitch, then flaccid paralysis. Why? –Not hydrolyzed by Acetylcholinesterase –Nicotinic receptors cannot return to resting state –Membrane stays depolarized, but no Na+ current This is called DEPOLARIZING BLOCK How can we produce a cholinergic AGONIST effect?

43 Neostigmine (+) Donepezil (+)

44 Acetylcholinesterase (ChE) COO - (3) Rate: 600,000/min!!

45 ChE inhibition: Neostigmine COO - (3) Form carbamylated enzyme intermediate resistant to hydrolysis But - does not enter CNS Neostigmine

46 Cholinergic Therapy Alzheimer’s Dementia –Symptomatic treatment: orally active AChEI that cross BBB donepezil, rivastigmine, or galantamine Myasthenia Gravis –Diagnosis: short-acting AChEI like edrophonium –Treatment: longer-acting AChEI like neostigmine or pyridostigmine –All work on muscles in periphery, not CNS What side effects do you expect from an AChEI?

47 Case Sydney S. is a 62 year old male with a diagnosis of early-onset Alzheimer’s Dementia. One month ago, his dose of donepezil was increased to improve his increasing memory lapses. His wife says that he now seems a bit more alert and (possibly) less forgetful. Sydney doesn’t like it, because now the medicine gives him stomach cramps and diarrhea.

48 ANS Effects  ACh(musc)ACh effect Heart Contractility  ↓-- Heart Rate  Slow HR Blood Vessel Tone  ↓ (NO)-- Salivary Glands  Secretions GI Tone  Peristalsis GI Sphincter  Relax sphinct. Detrusor Contraction  Empty bladder Urine Sphincter  Relax sphinct. Pupil Radial Muscle  -- Pupil Circ. Muscle  Small pupil Ciliary Muscle  Accommodate

49

50 Acetylcholinesterase Inhibitors } } Nerve Gases, (Irreversible ChE block) Relaxant Reversal, Myasthenia, Alzheimer’s (Temporary ChE block)

51 Organophosphates Powerful, irreversible ChE antagonists “Nerve Gases”: Soman, VX –Absorbed through skin, eyes, lungs –Synapse flooded with Ach – receptors can’t reset –↓ Nicotinic effect --- Lethal paralysis in seconds –↑↑ Muscarinic effects --- Bradycardia, secretions Treatment –Reactivate ChE to ↑ Nicotinic receptors – PRALIDOXIME (2-PAM) –Block Muscarinic receptors -- ATROPINE

52 Muscarinic ACh receptors are 1.Voltage gated ion channels 2.Ligand gated ion channels 3.G-protein coupled receptors 4.Intracellular receptors 5.Receptor tyrosine kinases

53 Donepezil changes the amount of ACh in cholinergic synapses, because it 1.Increases ACh release 2.Increases ACh storage 3.Increases ACh synthesis 4.Decreases ACh reuptake 5.Decreases ACh breakdown

54 Which of the following are muscarinic cholinergic effects? 1.Bladder contraction 2.Tachycardia 3.Pupil dilation 4.Constipation 5.Salivation

55 What did Dr. Stanley Goodspeed (Nicholas Cage) inject when he was exposed to this stuff? 1.Neostigmine 2.Pralidoxime 3.Curare 4.Atropine 5.Prozac

56 Test Question Why do all of the cholinergic “agonists” (except acetylcholine) produce weakness and paralysis?


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