Topic 10 Drugs of the Nervous System. Ch 19,20 Patrick Part VI- Nervous system -Corey

Transcription

1 Topic 10 Drugs of the ervous System h 19,20 Patrick Part VI- ervous system -orey

2 ontents Part 1: holinergics & anticholinesterases 1. erve Transmission 2. eurotransmitter 3. Transmission 4. holinergic receptors 4.1. icotinic receptor 4.2. Muscarinic receptor - G Protein coupled receptor 5. holinergic agonists 5.1. Acetylcholine as an agonist 5.2. icotine and muscarine as cholinergic agonists 5.3. Requirements for cholinergic agonists 6. SAR for acetylcholine 7. Binding site (muscarinic) 8. Active conformation of acetylcholine 9. Instability of acetylcholine 10. Design of cholinergic agonists 11. Uses of cholinergic agonists

3 LIERGI ERVUS SYSTEM

4 1. erve Transmission Peripheral nervous system S Brain Peripheral nerves Muscle eart Gastrointestinal tract (GIT) Spinal cord

5 1. erve Transmission Peripheral nervous system S (Somatic) Ach () Skeletal muscle S (Autonomic) Sympathetic Synapse Ach () A Parasympathetic Ach () Adrenaline Adrenal medulla Synapse AUTMI Ach () Ach (M) Smooth muscle ardiac muscle

6 1. erve Transmission Synapses erve impulse erve A Receptors erve ew signal Vesicles containing neurotransmitters Release of neurotransmitters Receptor binding and new signal

7 2. eurotransmitter Acetylcholine (Ach) Acetyl holine

8 3. Transmission process Signal in nerve 1 Signal erve erve 2. Acetylcholine Vesicle Acetylcholinesterase enzyme holinergic receptor

9 3. Transmission process Vesicles fuse with membrane and release Ach erve 1 erve 2 Signal

10 3. Transmission process erve 2

11 3. Transmission process Receptor binds Ach Induced fit triggers 2 o message Triggers firing of nerve 2 Ach undergoes no reaction 2 o ssage erve 2

12 3. Transmission process Ach departs receptor Receptor reverts to resting state Ach binds to acetylcholinesterase erve 2

13 3. Transmission process Ach hydrolysed by acetylcholinesterase erve 2 Acetylcholine Acetic acid holine

14 3. Transmission process holine binds to carrier protein holine erve 1 erve 2 arrier protein for choline

15 3. Transmission process holine transported into nerve erve 1 erve 2

16 3. Transmission process Ach resynthesised erve 1 erve 2 E 1 = holine acetyltransferase 3 SoA E holine Acetylcholine

17 3. Transmission process Ach repackaged in vesicles erve 1 erve 2

18 4. holinergic receptors Receptor types ot all cholinergic receptors are identical Two types of cholinergic receptor - nicotinic and muscarinic amed after natural products showing receptor selectivity icotine 2 3 L-(+)-Muscarine Activates cholinergic receptors at nerve synapses and on skeletal muscle Activates cholinergic receptors on smooth muscle and cardiac muscle Acetylcholine is natural messenger for both receptor types

19 Peripheral nervous system S (Somatic) Ach () Skeletal Muscle SMATI S (Autonomic) Sympathetic Synapse Ach () A Parasympathetic Ach () Adrenaline Adrenal medulla Synapse AUTMI Ach () Ach (M) Smooth Muscle ardiac Muscle

20 4.1 icotinic receptor ontrol of ationic Ion hannel: Receptor Binding site ssenger ell membrane Five glycoprotein subunits traversing cell membrane Induced fit Gating (ion channel opens) ell membrane

21 4.1 icotinic receptor The binding sites Binding sites Ion channel β α α γ ell membrane δ α γ β δ α 2xα, β, γ, δ subunits Two ligand binding sites mainly on α-subunits

22 4.2 Muscarinic receptor - G Protein coupled receptor Activation of a signal protein Receptor binds messenger leading to an induced fit pens a binding site for a signal protein (G-protein) messenger induced fit closed open G-protein bound G-protein split

23 4.2 Muscarinic receptor - G Protein coupled receptor Activation of membrane bound enzyme G-Protein is split and subunit activates a membrane bound enzyme Subunit binds to an allosteric binding site on enzyme Induced fit results in opening of an active site Intracellular reaction is catalysed Enzyme Enzyme subunit active site (closed) active site (open) Intracellular reaction

24 5. holinergic agonists 5.1 Acetylcholine as an agonist Advantages atural messenger Easily synthesized 3 Ac Ac 2 Disadvantages Easily hydrolysed in stomach (acid catalysed hydrolysis) Easily hydrolysed in blood (esterases( esterases) o selectivity between receptor types o selectivity between different target organs

25 5. holinergic agonists 5.2 icotine and muscarine as cholinergic agonists Advantages More stable than Ach Selective for main cholinergic receptor types Selective for different organs Disadvantages Activate receptors for other chemical messengers Side effects

26 5. holinergic agonists 5.3 Requirements for cholinergic agonists Stability to stomach acids and esterases Selectivity for cholinergic receptors Selectivity between muscarinic and nicotinic receptors Knowledge of binding site SAR for acetylcholine

27 6. SAR for acetylcholine Ethylene bridge Acetoxy 3 4 o itrogen Quaternary nitrogen is essential Bad for activity

28 6. SAR for acetylcholine Ethylene bridge Acetoxy 3 4 o itrogen Distance from quaternary nitrogen to ester is important Ethylene bridge must be retained Bad for activity

29 6. SAR for acetylcholine Ethylene bridge Acetoxy 3 4 o itrogen Ester is important Bad for activity

30 6. SAR for acetylcholine Ethylene bridge Acetoxy 3 4 o itrogen Minimum of two methyl groups on quaternary nitrogen 3 Et Et Et 3 Et Bad for activity Active

31 6. SAR for acetylcholine Ethylene bridge Acetoxy 3 4 o itrogen thyl group of acetoxy group cannot be extended 3 3 Bad for activity

32 6. SAR for acetylcholine Ethylene bridge Acetoxy 3 4 o itrogen onclusions: Tight fit between Ach and binding site thyl groups fit into small hydrophobic pockets Ester interacting by -bonding Quaternary nitrogen interacting by ionic bonding

33 7. Binding site (muscarinic) hydrophobic pocket 3 Trp-307 Asp Trp-616 Trp hydrophobic pockets hydrophobic pocket Asn-617

34 7. Binding site (muscarinic) vdw 3 -bonds Ionic bond Trp-307 Asp Trp-616 Trp-613 vdw vdw Asn-617

35 7. Binding site (muscarinic) Possible induced dipole dipole interaction between quaternary nitrogen and hydrophobic aromatic rings in binding site + induces dipole in aromatic rings δ + R δ + δ - 3 δ -

36 8. Active conformation of acetylcholine Several freely rotatable single bonds Large number of possible conformations Active conformation does not necessarily equal the most stable conformation

37 8. Active conformation of acetylcholine Rigid Analogues of acetylcholine MUSARIE 2 3 Rotatable bonds locked within ring Restricts number of possible conformations Defines separation of ester and 4.4A 5.9A Muscarinic receptor icotinic receptor

38 9. Instability of acetylcholine 3 3! "! + eighbouring group participation Increases electrophilicity of carbonyl group Increases sensitivity to nucleophiles

39 10. Design of cholinergic agonists Requirements orrect size orrect pharmacophore - ester and quaternary nitrogen Increased stability to acid and esterases Increased selectivity

40 10. Design of cholinergic agonists Use of steric shields Rationale Shields protect ester from nucleophiles and enzymes Shield size is important Must be large enough to hinder hydrolysis Must be small enough to fit binding site

41 10. Design of cholinergic agonists thacholine * 3 hinders binding to esterases and provides a shield to nucleophilic attack Properties asymmetric centre Three times more stable than acetylcholine Increasing the shield size increases stability but decreases activity Selective for muscarinic receptors over nicotinic receptors S-enantiomer is more active than the R-enantiomer Stereochemistry matches muscarine ot used clinically 2 3 MUSARIE (S) (R)

42 10. Design of cholinergic agonists Use of electronic factors Replace ester with urethane Stabilises the carbonyl group! " 2 2 =! + 2

43 10. Design of cholinergic agonists 2 3 arbachol Properties Resistant to hydrolysis Long lasting 2 and 3 are equal sizes. Both fit the hydrophobic pocket 2 = bio-isostere isostere Muscarinic activity = nicotinic activity Used topically for glaucoma

44 10. Design of cholinergic agonists Steric + Electronic factors 2 * 3 Bethanechol Properties Very stable rally active Selective for the muscarinic receptor Used to stimulate GI tract and urinary bladder after surgery

45 10. Design of cholinergic agonists icotinic selective agonist * 3 * asymmetric centre

46 11. Uses of cholinergic agonists icotinic selective agonists Treatment of myasthenia gravis - lack of acetylcholine at skeletal muscle causing weakness Muscarinic selective agonists Treatment of glaucoma Switching on GIT and urinary tract after surgery Treatment of certain heart defects. Decreases heart muscle activity and decreases heart rate

47 Peripheral nervous system S (Somatic) Ach () Skeletal Muscle SMATI S (Autonomic) Sympathetic Synapse Ach () A Parasympathetic Ach () Adrenaline Adrenal medulla Synapse AUTMI Ach () Ach (M) Smooth Muscle ardiac Muscle

48 ontents Part 1: holinergics & anticholinesterases 1. erve Transmission 2. eurotransmitter 3. Transmission process 4. holinergic receptors 4.1. icotinic receptor 4.2. Muscarinic receptor - G Protein coupled receptor 5. holinergic agonists 5.1. Acetylcholine as an agonist 5.2. icotine and muscarine as cholinergic agonists 5.3. Requirements for cholinergic agonists 6. SAR for acetlcholine 7. Binding site (muscarinic) 8. Active conformation of acetylcholine 9. Instability of acetylcholine 10. Design of cholinergic agonists 11. Uses of cholinergic agonists

49 LIERGI ERVUS SYSTEM

50 1. erve Transmission Peripheral nervous system S Brain Peripheral nerves Muscle eart Gastrointestinal tract (GIT) Spinal cord

51 1. erve Transmission Peripheral nervous system S (Somatic) Ach () Skeletal muscle S (Autonomic) Sympathetic Synapse Ach () A Parasympathetic Ach () Adrenaline Adrenal medulla Synapse AUTMI Ach () Ach (M) Smooth muscle ardiac muscle

52 1. erve Transmission Synapses erve impulse erve A Receptors erve ew signal Vesicles containing neurotransmitters Release of neurotransmitters Receptor binding and new signal

53 ontents Part 2: holinergics & anticholinesterases 12. holinergic Antagonists (Muscarinic receptor) Atropine yoscine (scopolamine) omparison of atropine with acetylcholine Analogues of atropine Simplified Analogues SAR for Antagonists Binding Site for Antagonists 13. holinergic Antagonists (icotinic receptor) urare Binding Analogues of tubocurarine

54 12. holinergic Antagonists (Muscarinic receptor) Drugs which bind to cholinergic receptor but do not activate it Prevent acetylcholine from binding pposite clinical effect to agonists - lower activity of acetylcholine Postsynaptic nerve Postsynaptic nerve Ach Ach Ach Antagonist

55 12. holinergic Antagonists (Muscarinic receptor) linical Effects Decrease of saliva and gastric secretions Relaxation of smooth muscle Decrease in motility of GIT and urinary tract Dilation of pupils Uses Shutting down digestion for surgery phthalmic examinations Relief of peptic ulcers Treatment of Parkinson s s Disease Anticholinesterase poisoning Motion sickness

56 12. holinergic Antagonists (Muscarinic receptor) 12.1 Atropine 2 easily racemised * Racemic form of hyoscyamine Source - roots of belladonna (1831) (deadly nightshade) Used as a poison Used as a medicine decreases GIT motility antidote for anticholinesterase poisoning dilation of eye pupils S side effects - hallucinations

57 12. holinergic Antagonists (Muscarinic( receptor) 12.2 yoscine (scopolamine) 2 * Source - thorn apple-datura- Datura-jimsonweed dical use - treatment of motion sickness S effects, hallucinations

58 12. holinergic Antagonists (Muscarinic receptor) 12.3 omparison of atropine with acetylcholine Relative positions of ester and nitrogen similar in both molecules itrogen in atropine is ionised Amine and ester are important binding groups (ionic + -bonds) Aromatic ring of atropine is an extra binding group (vdw( vdw) Atropine binds with a different induced fit - no activation Atropine binds more strongly than acetylcholine

59 12. holinergic Antagonists (Muscarinic receptor) 12.4 Analogues of atropine Br 3 ( 3 ) Ipratropium (bronchodilator & anti-asthmatic) Atropine methonitrate (lowers GIT motility) Analogues are fully ionized Analogues unable to cross the blood brain barrier o S side effects

60 12. holinergic Antagonists (Muscarinic receptor) 12.5 Simplified Analogues Pharmacophore = ester + basic amine + aromatic ring Et Et (Et) 3 Br 2 2 l Amprotropine Tridihexethyl bromide Propantheline chloride

61 12. holinergic Antagonists (Muscarinic receptor) 12.5 Simplified Analogues Tropicamide (opthalmics) yclopentolate (opthalmics) Benztropine (Parkinsons disease) Benzhexol (Parkinsons disease) 2 Pirenzepine (anti-ulcer)

62 12. holinergic Antagonists (Muscarinic receptor) 12.6 SAR for Antagonists R R' R' R' = Aromatic or eteroaromatic Important features Tertiary amine (ionised) or a quaternary nitrogen Aromatic ring Ester -Alkyl groups (R) can be larger than methyl (unlike agonists) Large branched acyl group R = aromatic or heteroaromatic ring Branching of aromatic/heteroaromatic rings is important

63 12. holinergic Antagonists (Muscarinic receptor) 12.6 SAR for Antagonists l 2 2 R 2 Active Inactive

64 12. holinergic Antagonists (Muscarinic receptor) 12.6 SAR for Antagonists vs. Agonists SAR for Antagonists Tertiary amine (ionized) or quaternary nitrogen Aromatic ring Ester -Alkyl groups (R) can be larger than methyl R = aromatic or heteroaromatic Branching of Ar rings important SAR for Agonists Quaternary nitrogen Aromatic ring Ester -Alkyl groups = methyl R =

65 12. holinergic Antagonists (Muscarinic receptor) 12.7 Binding Site for Antagonists van der Waals binding regions for antagonists Acetylcholine binding site REEPTR SURFAE

66 12. holinergic Antagonists (Muscarinic receptor) 12.7 Binding Site for Antagonists 2 2 l 2 2 Asn 2 2 R 2

67 13. holinergic Antagonists (icotinic receptor) 13.1 urare Extract from curare plant- Strychnos toxifera Used for poison arrows auses paralysis (blocks acetylcholine signals to muscles) Active principle = tubocurarine 2 2 Tubocurarine

68 13. holinergic Antagonists (icotinic receptor) Pharmacophore Two quaternary centres at specific separation (1.15nm) Different mechanism of action from atropine based antagonists Different binding interactions linical uses euromuscular blocker for surgical operations Permits lower and safer levels of general anaesthetic Tubocurarine used (previously) as neuromuscular blocker but side effects

69 13. holinergic Antagonists (icotinic receptor) 13.2 Binding protein complex (5 subunits) diameter=8nm S 8nm 9-10nm a) Receptor dimer b) Interaction with tubocurarine Probably not bridging Ach sites Tubocurarine Acetylcholine binding site

70 13. holinergic Antagonists (icotinic receptor) 13.3 Analogues of tubocurarine 3 ( 2 ) Decamethonium Suxamethonium Long lasting Long recovery times Side effects on heart Esters incorporated Shorter lifetime (5 min) Fast onset and short duration Side effects at autonomic ganglia

71 13. holinergic Antagonists (icotinic receptor) 13.3 Analogues of tubocurarine Pancuronium (R=) Vecuronium (R=) Steroid acts as a spacer for the quaternary centres (1.09nm) Acyl groups are added to introduce the Ach skeleton Faster onset then tubocurarine but slower than suxamethonium Longer duration of action than suxamethonium (45 min) o effect on blood pressure and fewer side effects

72 13. holinergic Antagonists (icotinic receptor) 13.3 Analogues of tubocurarine 2 2 ( 2 ) Atracurium Design based on tubocurarine and suxamethonium Lacks cardiac side effects Rapidly broken down in blood both chemically and metabolically Avoids patient variation in metabolic enzymes Lifetime is 30 minutes Administered as an i.v. drip Self destruct system limits lifetime

73 13. holinergic Antagonists (icotinic receptor) 13.3 Analogues of tubocurarine 2 Ph ATIVE R - 2 Ph IATIVE R Atracurium stable at acid p ofmann elimination at blood p (7.4)

74 13. holinergic Antagonists (icotinic receptor) 13.3 Analogues of tubocurarine Mivacurium 3 Faster onset (2 min) Shorter duration (15 min)

75 ontents Part 3: holinergics & anticholinesterases 14. Acetylcholinesterase Role ydrolysis reaction catalysed Effect of inhibition Structure of enzyme complex Active site - binding interactions Active site - chanism of catalysis 15. Anticholinesterases Physostigmine chanism of action Physostigmine analogues rganophosphates Anticholinesterases as Smart Drugs

76 14. Acetylcholinesterase 14.1 Role ydrolysis and deactivation of acetylcholine Prevents acetylcholine reactivating receptor Signal erve erve 2 Acetylcholinesterase enzyme

77 14. Acetylcholinesterase 14.2 ydrolysis reaction catalysed Acetylcholine Acetic acid holine active inactive

78 14. Acetylcholinesterase 14.3 Effect of inhibition Enzyme inhibitor (Anticholinesterase) 2 o ssage Ach Inhibitor blocks acetylcholinesterase Ach is unable to bind Ach returns to receptor and reactivates it Enzyme inhibitor has the same effect as a cholinergic agonist erve 2

79 14. Acetylcholinesterase 14.4 Structure of enzyme complex S S Enzyme S S S S S S Enzyme S S S S S S S S S S Enzyme collagen

80 : 14. Acetylcholinesterase 14.5 Active site - binding interactions Anionic binding region Ester binding region Serine Aspartate istidine hydrophobic pockets vdw vdw :: Ionic 2 2 -bond 3 Tyrosine Anionic binding region similar to cholinergic receptor site Binding and induced fit strains Ach and weakens bonds Molecule positioned for reaction with is and Ser

81 14. Acetylcholinesterase 14.6 Active site - chanism of catalysis 3 :: : 3 : : : R : Serine (ucleophile) istidine (Base) istidine (Base catalyst) 3 : : R : : 3 R : : : istidine Acid catalyst istidine

82 14. Acetylcholinesterase 14.6 Active site - chanism of catalysis 2 3 : : : R : 3 R istidine istidine 3 : : : 3 : _ : : : istidine istidine Basic catalyst

83 : 14. Acetylcholinesterase 14.6 Active site - chanism of catalysis 3 _ : : : : 3 : : : : istidine Basic catalyst istidine (Acid catalyst) : _ : : 3 : 3 : istidine

84 14. Acetylcholinesterase Serine and water are poor nucleophiles chanism is aided by histidine acting as a basic catalyst holine and serine are poor leaving groups Leaving groups are aided by histidine acting as an acid catalyst Very efficient x 10 6 faster than uncatalysed hydrolysis Acetylcholine hydrolysed within 100 µsecs of reaching active site An aspartate residue is also involved in the mechanism

85 14. Acetylcholinesterase The catalytic triad An aspartate residue interacts with the imidazole ring of histidine to orient and activate it : :: : : : : Serine (ucleophile) istidine (Base) Aspartate

86 15. Anticholinesterases Inhibitors of acetylcholinesterase enzyme Block hydrolysis of acetylcholine Acetylcholine is able to reactivate cholinergic receptor Same effect as a cholinergic agonist

87 15. Anticholinesterases 15.1 Physostigmine Urethane or arbamate Pyrrolidine atural product from the African calabar (ordeal) bean- Physostigma arbamate is essential (equivalent to ester of Ach) Aromatic ring is important Pyrrolidine is important (ionized at blood p) Pyrrolidine is equivalent to the quaternary nitrogen of Ach

88 15.2 chanism of action : : : Ar : Ar : : : Physostigmine Ar : : : : : : : : : Ar

89 15.2 chanism of action : : : Ar : -Ar Stable carbamoyl intermediate : ydrolysis very slow : Rate of hydrolysis slower by 40 x 10 6

90 15.2 chanism of action : Stable carbamoyl intermediate : :: arbonyl group 'deactivated'

91 15.3 Physostigmine analogues 2 3 (ionised at blood p) Miotine Simplified analogue Susceptible to hydrolysis rosses BBB as free base S side effects eostigmine Fully ionized annot cross BBB o S side effects More stable to hydrolysis Extra -methyl group increases stability

92 15.4 rganophosphates a) erve gases i Pr i Pr P P i Pr F F Dyflos (Diisopropyl fluorophosphonate) Sarin Agents developed in World War 2 Agents irreversibly inhibit acetylcholinesterase Permanent activation of cholinergic receptors by Ach Results in death

93 15.4 rganophosphates b) chanism of action Serine - Serine i Pr i Pr P F i Pr P i Pr STABLE F - Irreversible phosphorylation P- bond very stable

94 15.4 rganophosphates c) dicinal organophosphate S P Et Et Ecothiopate Used to treat glaucoma Topical application Quaternary is added to improve binding interactions Results in better selectivity and lower, safer doses

95 15.4 rganophosphates d) rganophosphates as insecticides Et P S Et 2 Parathion P S S Malathion 2 Et 2 2 Et Relatively harmless to mammals Agents act as prodrugs in insects tabolised by insects to produce a toxic metabolite

96 15.4 rganophosphates d) rganophosphates as insecticides MAMMALS ISETS Et Et P S PARATI (Inactive Prodrug) 2 Insect xidative desulphurisation Et P Et 2 Active drug Mammalian tabolism Et S Phosphorylates enzyme P Et IATIVE & excreted DEAT

97 15.4 rganophosphates e) Design of rganophosphate Antidotes Strategy Strong nucleophile required to cleave strong P- bond Find suitable nucleophile capable of cleaving phosphate esters Water is too weak as a nucleophile ydoxylamine is a stronger nucleophile 2 ydroxylamine R P R + + R 2 P R R R ydroxylamine is too toxic for clinical use Increase selectivity by increasing binding interactions with active site

98 15.4 rganophosphates e) Design of rganophosphate Antidotes 3 Pralidoxime Quaternary is added to bind to the anionic region Side chain is designed to place the hydroxylamine moiety in the correct position relative to phosphorylated serine Pralidoxime 1 million times more effective than hydroxylamine annot act in S due to charge - cannot cross bbb

99 15.4 rganophosphates e) Design of rganophosphate Antidotes 2 R P R SER 2 P R R SER Active Site (Blocked) Active Site (Free)

100 15.4 rganophosphates e) Design of rganophosphate Antidotes ProPAM 3 Prodrug for pralidoxime Passes through BBB as free base xidised in S to pralidoxime

101 15.5 Anticholinesterases as Smart Drugs Act in S Must cross blood brain barrier Used to treat memory loss in Alzheimers disease Alzheimers causes deterioration of cholinergic receptors in brain Smart drugs inhibit Ach hydrolysis to increase activity at remaining receptors

102 15.5 Anticholinesterases as Smart Drugs 2 Et l Rivastigmine (Exelon) (analogue of physostigmine) 2 2 Donepezil Tacrine (ognex) Toxic side effects P l 3 trifonate (organophosphate) Anabaseine (ants and marine worms) S Galanthamine (daffodil and snowdrop bulbs Xanomeline