Chapter 2 Medchem HU PDF

Chapter 2 DRUGS ACTING ON AUTONOMIC NERVOUS SYSTEM  Introduction The nervous system Composed of all nerves in the body with the endocrine system Controls and coordinates various functions of the body Function of the nerve tissues Receive stimuli Transmit the...

Chapter 2 DRUGS ACTING ON AUTONOMIC NERVOUS SYSTEM  Introduction The nervous system Composed of all nerves in the body with the endocrine system Controls and coordinates various functions of the body Function of the nerve tissues Receive stimuli Transmit the stimuli to nerve centers & Initiate response 2  Introduction cont. The Nervous System (NS) is divided into two parts: The Peripheral Nervous System (PNS) It consists of all Afferent (sensory) neurons Efferent (motor) neurons The Central Nervous System (CNS) It consists of The Brain and Spinal cord 3  Nervous System Peripheral Nervous System Central Nervous System (PNS) (CNS) Afferent Efferent Brai Spinal (Sensory) (Motor) n Cord Somatic Nervous System (SNS) Autonomic Nervous System (ANS) Sympathetic Parasympathetic Enteric Nervous System Nervous Nervous System (Craniosacral) System (Thoracolumbar 4  Introduction cont. Peripheral efferent system is further divided into The Autonomic Nervous System (ANC) Controls functions involuntarily to regulate The every day needs & requirements of the body without the conscious participation of the mind Innervates: Smooth muscle Cardiac muscle and Exocrine glands The somatic nervous system Involved in voluntarily controlled functions such as contraction of skeletal muscle & sensory neurons of the skin 5  The Autonomous Nervous System (ANS) The ANS is most important in two situations: In emergencies that cause stress and require us to "fight-or-flight" (run away) In non emergencies that allow us to "rest" and "digest" Hence, it provides almost every organ with a double set of nerves Sympathetic & Parasympathetic These systems generally, not always, work in opposition to each other 6 The Autonomous Nervous System (ANS) cont. The sympathetic system( Thoracolumbar division):  Activates & prepares the body for Vigorous muscular activity Stress & emergencies In evoking fight-or-flight reaction in an emergency The parasympathetic system( Craniosacral division):  Activates & prepares the body Lower activation Operates during normal conditions Permits digestion & Conservation of energy In rest & digestion states 7 8  Hypothalamus activates sympathetic division of nervous system Heart rate, blood pressure and respiration increase Adrenal medulla secretes epinephrine and norepinephrine Blood flow to Stomach Skeletal muscles contractions increases are inhibited Digestion & rest Parasympathetic 9  The Autonomous Nervous System (ANS) cont. The autonomous nervous system Carries nerve impulse from the CNS to the effector organs by ways of two types of efferent neurons: Preganglionic neuron  Emerge from the CNS & make the synaptic connection in ganglia Postganglionic neuron  Has a cell body originating in the ganglia & terminated on effector organs  Carries the impulse from the ganglia to the effector cells Preganglion Postganglion Ganglio ic ic n Somatic nervous system Differs from ANS in that a single myelinated motor neuron, originating in the CNS, travels directly To skeletal muscle without the mediation of ganglia 10 11 Autonomic Neurotransmitters In Peripheral Nervous System (PNS) A chemical neurotransmitter (NT) carries the nerve impulse from neuron to neuron across a synapse O The NT includes HC O 3 NMe HO 2 NH 3  Acetylcholine (Ach), HO H N HO NHR H  Norepinephrine (NE), HO  Epinephrine (E), R= H, NoradrenalineA R = Me, Epinephrine  Serotonin (5-hydroxytryptamine (5-HT)) & others Neurotransmission in the PNS occurs at three major sites: Preganglionic synapses in both parasympathetic and sympathetic ganglia Parasympathetic and sympathetic postganglionic neuroeffector junctions & All somatic motor end plates on skeletal muscle 12 Autonomic Neurotransmitters Cont. Acetylcholine is the transmitter released at all of these sites except for the majority of sympathetic neuroeffector junctions Nerves that release acetylcholine are called cholinergic nerves Cholinergic nerves are part of the Parasympathetic system Somatic motor nerves Preganglionic sympathetic nerves CNS 13 14 Autonomic Neurotransmitters Cont. Norepinephrine is the transmitter released at most sympathetic postganglionic neuroeffector junctions Nerves that release NE are called adrenergic nerves Adrenergic nerves are part of the Postganglionic sympathetic & CNS NB: Not all sympathetic postganglionic neurons are adrenergic The sympathetic postganglionic neurons that innervate the Sweat glands and Some of the blood vessels in skeletal muscle Are cholinergic 15 Autonomous Nervous system fibers 16  Cholinergic Neurons Neurons that release ACh are collectively called cholinergic ACh is released from the presynaptic nerve ending into synapse Interact with specific receptors at postsynaptic site Leads to receptor-mediated response Drugs that mimic the actions of Ach are termed as Cholinomimetic or Parasympathomimetic Those agents that mimic epinephrine and/or norepinephrine are termed as Adrenomimetic or Sympathomimetic 17  Cholinergic Neurons cont. Cholinergic agonists are cholinomimetic agents that act directly at the receptors for Ach Agents that inhibit acetylcholinesterase (AChE), an enzyme responsible for the hydrolysis of ACh, are also Cholinomimetic but are not receptor agonist Cholinergic antagonists possess affinity for cholinergic receptors but exhibit no 18 Synthesis & release of ACh from the cholinergic neurons Neurotransmission in cholinergic neurons involves six steps These are Synthesis Storage Release Binding of the Ach to a receptor Degradation of the NT in the synaptic gap & Recycling of choline 19  Synthesis & release of ACh from the cholinergic neurons cont. Synthesis of ACh Acetylcholine is synthesized in the nerve terminal from acetyl-coenzyme A (AcCoA) & Choline S-Adenosyl Choline-N- Methionine Methyl (Methyl Transferase Donor) Acetyl-S-CoA (Acetyl Donor) 20 Synthesis & release of ACh from the cholinergic neurons cont. Storage of ACh Acetylcholine is stored in synaptic vesicles by active transport process Each vesicle contains approximately 104 ACh molecules, which are released as a single packet 21 Synthesis & release of Ach from the cholinergic neurons cont. Release of ACh When an action potential propagated due to the action of voltage-sensitive sodium channels arrives at the nerve endings Voltage-sensitive calcium channels in the presynaptic membrane open Causing an increase in the concentration of intracellular calcium ion Elevated calcium ion level induces the synaptic vesicles to fuse with the cell membrane & release of ACh into the synapse 22 Synthesis & release of Ach from the cholinergic neurons cont. Acetylcholine release sites Preganglionic nerve fibers of both sympathetic and parasympathetic divisions of the ANS Postganglionic nerves of the Parasympathetic division The sympathetic innervations of sweet glands Neuromuscular junction 23 Synthesis & release of Ach from the cholinergic neurons cont. ACh binding & inactivation Binding ACh crosses the synaptic gap & binds to the cholinergic receptor Leads to biological response within the cell such as Innervations of the nerve impulse in a postganglionic or Activation of specific enzymes in effector cells Acetylcholine inactivation (Degradation) In synaptic cleft, acetylcholinesterase (AChE) breaks it down into acetate and choline 24 Synthesis & release of Ach from the cholinergic neurons cont. Recycling of Choline Choline is recaptured by a sodium-coupled high affinity uptake system  That transports the molecule back into the presynaptic neurons where it is acetylated 25 Synthesis & Release of ACh from the Cholinergic Neuron. AcCoA = Acetyl CoA 26 Intracellular response 27 Cholinergic Receptors (Cholinoceptors) 28 Cholinergic Receptors (Cholinoceptors) Two major families of cholinoceptors: Muscarinic & Nicotinic receptors Both types of receptor can be activated by ACh  These receptors comprise of G-Protein coupled with muscarinic and ligand gated channeled nicotinic receptors  Can be distinguished from each other on the basis of their difference in Composition Location Pharmacological functions & Having specific agonists & antagonists action 29  Cholinergic Receptors Cont. HO Muscarinic Receptors H C CH 3 3 N Located postsynaptically: H C 3 O Muscarine CH 3 Smooth muscle; Cardiac muscle Glands of parasympathetic fibers Effector organs of cholinergic sympathetic fibers Named “muscarinic” because it can be stimulated by the alkaloid muscarine It is a Metabotropic receptor that activates G-protein Muscarinic receptors contains 5 sub-types M1, M2, M3, M4 & M5 30 Recepto Location Mecha Result of r nism Ligand Binding M1 CNS neurons G- Inositol 1, 4, 5-↑ (Neural Sympathetic protein triposphate (IP3), Postganglionic coupled Diacylglycerol ) (DAG) Nerve endings Presynaptic cause membrane - depolarization M2 Myocardium G- ↓ cAMP, activate (Cardia Smooth protein K+ channels c) muscle -cause hyperpolarization M3  Exocrine G- ↑ IP3, DAG (Glandu glands protein  Smooth lar) muscle NN Autonomic Open Depolarization, ganglia Na+, K+ evoke action channels potential 31 Muscarinic Receptor 32  Receptor agonists activate signal transduction pathways NH 3 Acetylcholine (+) PL - G q Muscarinic C receptor PIP2 COOH IP3 Diacylglycerol Increase Ca2+ Activate Protein Kinase C Response 33 34  Cholinergic Receptors cont. Nicotinic Receptors Located in the ganglia of both the Parasympathetic NS and Sympathetic NS Named “nicotinic” because it can be stimulated by the alkaloid nicotine It is Ionotropic receptor that activates Na+ channel The nicotinic acetylcholine receptor, a ligand- gated ion channel H Nicotinic receptors contains 2 sub-typesN N1 (NM) & CH 3 N Nicotine N2 (NN) 35 Nicotinic Receptor 36 Cholinergic Drugs 37  Cholinergic Drugs Cholinergic Agonists Are drugs that can elicit some or all of the effects that ACh produces These drugs can be classified into two groups: Direct acting (True cholinergic drugs ) Are those whose qualitative mode of action is the same as ACh (choline esters) Indirect acting (False cholinergic drugs) Are those that inhibit the hydrolysis of ACh by cholinesterase Also called as acetylcholinesterase inhibitors 38  Directly acting cholinergic drugs Mimic the effects of ACh by binding directly to cholinoceptors Synthetic esters of choline (Choline esters) Such as Carbachol, Bethanechol & Methacholine Naturally occurring alkaloids (Cholinomimetic alkaloids) O CH3 H C CH 3 O Such H 2 CH CHas 3 CH3 Pilocarpine, Muscarine N CH3 Cl & C 2 3 N H2N O Arecholine N CH3 Cl H3C O CH3 O Acetylcholine direct-acting cholinergic drugs have Olonger AllCarbachol Pilocarpine N duration O CH3 of CH3 action than O CHACh CH 3 CH 3 HO H C CH 3 CH3 3 3 N N Cl N Cl CH3 H2N O CH3 H3C O CH3 H3C Methacholine O Muscarine Bethanechol 39 Directly acting cholinergic drugs cont. The directly acting cholinomimetics can be subdivided into agents that exert their effects primarily through Stimulation of muscarinic receptors at parasympathetic neuroeffector junctions Parasympathomimetic drugs Stimulation of nicotinic receptors In autonomic ganglia and At the neuromuscular junction (NMJ) 40 Directly acting cholinergic drugs cont. Acetylcholine The prototypical muscarinic and nicotinic agonist Because it is the physiologic chemical neurotransmitter Has poor therapeutic activity Because of the chemical and physicochemical properties associated with its ester and quaternary ammonium functional groups Undergoes rapid hydrolysis in aqueous solutions O CH3 Which is accelerated in the presence N CH3 Clof catalytic amounts ofH3CeitherO acid Acetylcholine CH3 or base 41   Directly acting cholinergic drugs cont. When administered by parenteral routes, its pharmacological action is fleeting due to enzyme-catalyzed hydrolysis by serum and tissue esterases Quaternary ammonium functional group is poorly absorbed across lipid membranes The pharmacologic effects of ACh are not selective 42  Structure Activity Relationships (SAR) of acetylcholine  The first stage in any drug development is to study the lead compound & to find out which parts of the molecule are important to activity so that they can be retained in future analogues, i.e. structure–activity relationships (SARs)  The chemistry & ease of testing for ACh biological activity have allowed numerous chemical derivatives to be made & studied  To review the SAR, O it is logical to divide CH3the structure of ACh into three components in order CH to 3examine the effects of chemical modification of Neach group H3C Acylo O Ethyle CH3 Quatern xy ne ary Group Bridge Ammoni Structural modificationum of Acetylcholine Group 43  Structure Activity Relationships (SAR) of acetylcholine cont. Modification of the Quaternary Ammonium Group Analogues of ACh in which the nitrogen atom were replaced by As, P, or S have been synthesized Although they exhibited some of the activity of ACh These compounds O wereCHless 3 active & not used clinically N CH3 H3C O CH3 44 Structure Activity Relationships (SAR) of acetylcholine cont. It is concluded that only compounds possessing a positive charge on the atom in the position of the nitrogen had appreciable muscarinic activity The positively charged nitrogen atom is essential to activity. Replacing it with a neutral carbon atom eliminates the activity  Replacing all the three methyl groups on the nitrogen atom by larger alkyl groups were inactive as agonists  When the methyl groups are replaced by three ethyl groups, the resulting Ocompound is cholinergic CH3 antagonist CH3 N  Replacement of only H3C one methyl O group by ethyl an CH 3 or propyl group affords a compound that is active but much less than ACh 45 Structure Activity Relationships (SAR) of acetylcholine cont.  Successive replacement of one, two or three of the methyl groups with hydrogen atoms to obtain a tertiary, secondary or primary amine respectively leads to successive diminishing of muscarinic activity  The size of the quaternary ammonium group and its charge distribution are important to the activity of ACh H3C CH2  Many H2 muscarinic CH3 agonists are tertiary O C N H C 3 amines, for example, pilocarpine, arecoline, N N O O Oand oxotremorine N O Oxotremorine Arecoline N Pilocarpine CH3 O CH3 CH3 N H3C O CH3 46  Structure Activity Relationships (SAR) of acetylcholine cont.  The - onium group is essential for the intrinsic activity and contributes to the affinity of the drug to the receptors: Partially through binding energy and Partially because of its action as detecting & directing group  Thus, the optimum activity is achieved with the H3C CH2 trimethyl ammonium group,H2although CH3 some C exceptions are known (e.g. pilocarpine, N nicotine, & O oxotremorine), CH and itO shows maximal muscarinic 3 O activity N CH 3 Pilocarpine N H3C O CH3 47 Structure Activity Relationships (SAR) of acetylcholine cont.  Molecular modeling data show The binding site to be a negatively charged aspartic acid residue in the 3rd of the 7th transmembrane helixes of the muscarinic receptors Hydrophobic pockets are located in helices 4, 5, 6, and 7 of the muscarinic receptor 48  SAR of Acetylcholine cont. Clearly, there is a tight fit between ACh and its binding site, which leaves little scope for variation. It is proposed that important hydrogen bonding interactions exist between the ester group of ACh and an asparagine residue (Asn). A small hydrophobic pocket exists which can accommodate the methyl group of the ester, but nothing larger. This interaction is thought to be more important in the muscarinic receptor than the nicotinic receptor 49 Structure Activity Relationships (SAR) of acetylcholine cont. Modification of the Ethylene Bridge  The ethylene bridge of the ACh ensures The proper distance b/n the ammonium group and the ester group Therefore, it is critical in binding to the receptor  There should be no more than five atoms b/n the nitrogen and the terminal hydrogen atom for maximal muscarinic potency Five-atom rule O CH3 CH3 N H3C O CH3 Ethyle ne Bridge 50  Structure Activity Relationships (SAR) of acetylcholine cont.  Five-Atom Rule Shortening or lengthening the chain of atoms that separates the ester group from the -onium moiety Reduces muscarinic activity  Replacement of the hydrogen atoms of the Ethylene Bridge by Alkyl groups larger than methyl affords compounds that are much less active than ACh O CH3 CH3 N H3C O CH3 Ethyle ne Bridge 51  Structure Activity Relationships (SAR) of acetylcholine cont. Introduction of a methyl group on the carbon -to the quaternary nitrogen gives acetyl-- methylcholine (methacholine) Has muscarinic potency almost equivalent to that of ACh Has selectivity for muscarinic receptors (possesses much greater muscarinic potency than nicotinic potency) Nicotinic activity is decreased to a greater degree by substitution on the β-position 52  Structure Activity Relationships (SAR) of acetylcholine cont.  Methacholine is used via inhalation as an effective diagnostic agent for the diagnosis of asthma  Bronchoconstrictor  Hydrolysis by AChE is affected more by substitutions on the β-than the α-carbon 53  Structure Activity Relationships (SAR) of acetylcholine cont.  Introduction of a methyl group on the carbon -to the quaternary nitrogen gives acetyl--methylcholine α-substitution on the choline moiety reduces both muscarinic and nicotinic activity relative to ACh Exhibits greater nicotinic than muscarinic potency CH3 54 Structure Activity Relationships (SAR) of acetylcholine cont. Modification of Acyloxy Group  Substitution of the ester group by other functional groups  Increasing the size of the acyl group rapidly  Diminishes the muscarinic activity  But the nicotinic activity remains unaffected  The ether oxygen appears important for the muscarinic activity  Because both choline ethyl ether and - methyl choline ethyl ether have high muscarinic activity 55  Structure Activity Relationships (SAR) of acetylcholine cont.  Replacement of the CH3 group of the acyl moiety by the isosteric NH2 i.e., Carbamylation instead of esterification  Produces a less hydrolizable compounds (Compounds with prolonged mimetic activity) E.g. carbachol and bethanechol  When the acetyl group is replaced by higher homology (i.e., the propionyl or butryl group), the resulting esters are less potent than ACh O CH3 N(CH3)3 H2N O 56  Structure Activity Relationships (SAR) of acetylcholine cont.  Choline esters of aromatic or higher molecular weight acids possess cholinergic antagonistic activity  Chemical instability of acetylcholine can be solved by  Replacing the acetoxy functional group resistant to hydrolysis  This led to the synthesis of the carbamic acid esters of choline e.g. Carbachol, which is a potent Ocholinergic agonist possessing both muscarinic and N(CH3)3 nicotinic activity H2N O 57 Structure Activity Relationships (SAR) of acetylcholine cont.  Carbachol is less readily hydrolyzed  In the GIT or by the acetylcholinesterase than acetylcholine, hence, can be administered orally  Has profound effects on both cardiovascular system & GI system Used in the relief of urinal retention after operation  Used to reduce intraocular tension of glaucoma for narrow angle glaucoma Due to its erratic absorption and O Opronounced nicotinic effects – Its use is limited to the treatment N(CH of 3)3 N(CH3)3 H3C O H2N Oglaucoma 59 Structure Activity Relationships (SAR) of acetylcholine cont.  The same chemical logic was extended to acetyl-- methylcholine as that of carbachol and lead to the synthesis of its carbamate ester, bethanechol  An orally effective potent muscarinic agonist  Its major actions are on – The smooth muscle of the bladder (causing expulsion of urine) & GI tract (increased intestinal motility & tone) – Treatment of certain heart defects by decreasing heart muscle activity and heart rate O CH3 O  Has strong muscarinic activity but little or no N(CH3)3 N(CH3)3 nicotinic H2N O action H3C O 60  Design of acetylcholine analogues  ACh is instable due to neighbouring group participation  The electron withdrawal effect of Nitrogen & carbonyl Oxygen pulls electrons from carbon  Which makes it highly electrophilic, more prone to nucleophilic attack The arrow indicates the inductive pull of oxygen which increases the electrophilicity of the carbonyl carbon  To tackle the inherent instability of acetylcholine, two approaches are possible: Steric hindrance Electronic stabilization (Electronic effects) 61  Design of acetylcholine analogues cont. Steric Hindrance  The principle here can be demonstrated with methacholine Hinder binding to esterase & provide a shield to nucleophilic attack Asymmetric center  It contains an extra methyl groups on the ethylene bridge:  Try and build in a shield for the carbonyl group  The bulky methyl group hinders the approach of any potential nucleophiles and slows down the rate of hydrolysis  It also hinder binding to the esterase enzymes, thus slowing down the enzymatic hydrolysis Q: Why not put bulky group on the acyl half of the molecule, since this would be closer to the carbonyl center and have grater shielding effect? 62  Design of acetylcholine analogues cont. Electronic effect  Carbachol is a long acting cholinergic agent which is resistant to hydrolysis O O NMe3 NMe3 H2N O H3C O  In carbachol, the acyl methyl group of ACh has been replaced by a NH2 group  Which is of comparable size and can therefore fit the receptor  The resistance to hydrolysis is due to the electronic effect of the carbamate group 63  Design of acetylcholine analogues cont.  The resonance structure of carbachol demonstrates how the lone pair electrons form the nitrogen atom is fed into the carbonyl group such that the group’s electrophilic character is eliminated  As a result, the carbonyl is no longer susceptible to nucleophilic attack  The amino group is bioisoster for the methyl group as far as the cholinergic receptors are concerned, but not as far as the esterase enzymes are concerned  Therefore, the inclusion of an electron donating group such as the amino group has greatly increased the chemical and enzymatic stability of cholinergic agonist 64 Design of acetylcholine cont. analogues Combining the steric and electronic activity  The -methyl group increases the stability of acetylcholine analogues through steric effect and has the advantage of introducing some selectivity  It would be interesting to add a β-methyl group to carbachol  The compound obtained is bethanechol which, as expected, is3both stable to CH O selective in its action hydrolysis and  NMe3 H2N O Asymmetric Center 65  Cholinesterase Inhibitors (ChE  Cholinesterase Inhibitors)  Produced in tissues and blood  Hydrolyses acetylcholine  Present in the Central and peripheral nervous systems  Lack of Cholinesterase can cause CNS confusion  There are two types of cholinesterase in humans:  RBC Cholinesterase  Acetylcholinesterase (AChE)-True cholinesterase  Plasma Cholinesterase  Butyrylcholinesterase (BuChE)- Pseudocholinesterase They differ in their location in the body & substrate specificity 66  Cholinesterase Inhibitors (ChE Inhibitors) cont.  Plasma Cholinesterase-Butyrylcholinesterase (BuChE)  Located in human plasma (also called False cholinesterase) Floats freely in plasma  Made by liver Liver disease may affect the levels  Rapidly replaced by new synthesis  Sensitive to most ChE inhibitor pesticide exposures  Broader substrate specificity for esters May hydrolyze dietary esters & drug molecules in the blood  Has similar catalytic properties as AChE 67  Cholinesterase Inhibitors (ChE Inhibitors) cont.  RBC Cholinesterase-Acetylcholinesterase (AChE)  Bound to red blood cells  Made at the same time as the RBC’s  Low RBC count may cause lower levels  Identical to neuronal ChE  Associated with the outside surface of glial cells in the synapse  Catalyzes the hydrolysis of ACh  Inhibition of AChE indirectly provide a cholinergic action by  Prolonging the life time of ACh produced endogenously at the cholinergic nerve endings  This results in increasing the concentration of ACh in the synaptic space & to provide both muscarinic & nicotinic responses 68  Acetylcholinesterase Inhibitors (AChE Inhibitors)  AChE inhibitors are indirect-acting cholinergic agonists  Used in the treatment of myasthenia gravis, atony in the GI tract & glaucoma  Also employed as agricultural insecticides & nerve gas warfare agents  Investigational therapy for Alzheimer’s disease & similar cognitive disorders 69  Acetylcholinesterase Inhibitors (AChE Inhibitors) cont.  Kinetic studies with different substrates & inhibitors suggest that the active center of AChE consists of major domains:  An anionic site To which the trimethylammonium group binds  An esteratic site Which causes hydrolysis of the ester portion of ACh  A Hydrophobic O site Hydrophobic Site Which bind aryl substrates, other H3C O N(CH3)3 uncharged ligands & the alkyl portion of the acyl moiety H of ACh O Esteratic Site Anionic Site 70 Active sites of Acetylcholinesterase  The anionic site was believed to have been formed by the Gamma carboxylate group of a glutamic acid residue But more recent studies suggest that the aromatic moieties of Tryptophan & Phenylalanine residues Bind the quaternary ammonium group of ACh in the anionic site through cationic-π interactions 71 Active sites of Acetylcholinesterase cont.  The location & spatial organization in the esteratic site by  Serine  Histidine &  Glutamic acid residues constitute the esteratic site  The triad of these amino acids residues contributes to the high catalytic efficiency of AChE 72  Categories of Cholinesterase inhibitors  Two basic categories of cholinesterase inhibitors  Reversible  Irreversible  Reversible cholinesterase inhibitors  Are substrates for & react with AChE to form a carbamylated enzyme which is more stable than acetylated enzyme  But still capable of undergoing hydrolytic regeneration  Bind to AChE with greater affinity than ACh  But do not react with the enzyme as a substrate  Are generally short-acting  i.e. they bind to AChE reversibly  They include aryl carbamate (esters of carbamic acid & phenols) such as physostigmine (Eserine)  The classic AChE inhibitor 73  Reversible cholinesterase inhibitors  Physostigmine is an alkaloid obtained from Physostigma venenosum (Calabar bean)  Which provided the lead to this group of compounds Urethane Pyrrolidine or N Carbamat e  Carbamylated at a slow rate but has a high affinity for AChE  Not a quaternary ammonium salt, hence more lipid soluble & can cross the blood brain barrier 74  Reversible cholinesterase inhibitors cont. Physostigmine  Used as an antidote for over dosage of anticholinergics such as: Atropine; Antihistamines; Tricyclic antidepressant; Phenothiazines  Used in the treatment of glaucoma (Topical application) for many years  Along with other cholinomimetic drugs acting in the CNS, has been studied for use in the treatment of Alzheimer’s disease 75 Reversible cholinesterase inhibitors cont. Physostigmine analogues  Use of physostigmine as a prototype of an indirect acting parasympathomimetic drug led to the development of  Stigmine in which the trimethylamine group was placed para to a dimethylcarbamate group in benzene  Better inhibition of ChE was observed when the trimethylamine were placed meta to each other as in neostigmine (Pristigmine)  A more active & useful agent  Neostigmine & pyridostigmine are quaternary amine Neostigmin Pyridostigmi Physostigmin e ne 76 e  Reversible cholinesterase inhibitors Physostigmine analogues cont.  Used as an antidote to non-depolarizing neuromuscular blocking drugs  Commonly used in the treatment of myasthenia gravis  Pyridostigmine (Mestinon) is the most widely used one with fewer side effect  Chemically the analogues are more stable than physostigmine due to dimethylcarbamate moiety with longer duration of action  The most frequent application of neostigmine is to prevent atony  The intestine, skeletal & bladder musculature Pyridostigmi Neostigmin ne e 77 Reversible cholinesterase inhibitors cont.  Carbaryl is a reversible carbamate-derived AChE Edrophonium inhibitor  Has tremendous economic  Used in the diagnosis impact as an insecticide of myasthenia gravis for use on  House plants &  Also exhibit direct vegetables cholinomimetic  As well as for control of fleas & ticks on pets effect on skeletal muscle Carbaryl  Antidote for muscle blockade for surgery 78  Reversible cholinesterase inhibitors cont.  Physostigmine & neostigmine lead to  Carbamylation of cholinesterase which prevents Binding and metabolism of ACh  Edrophonium competes with acetylcholine for binding to cholinesterase to prevent  Binding and metabolism of ACh  The rate of carbamylation follows the following order: Methylcarba Carbamic acid > mic acid > Dimethylcarb amic acid ester ester ester  Regeneration of active AChE by hydrolysis of the carbamylated enzyme is much slower than the  Hydrolysis of the acetylated enzyme 79  Tacrine (Cognex®) is  Donepezil (Aricept®) is a a non-classic non-classic reversible reversible AChE non-competitive AChEI inhibitor  Selective for AChE by 570-  Has been used in 1250X mild to moderate  No hepatotoxicity Alzheimer’s dementia  The drug approved for the treatment of  Rivastigmine (Exelon®) Alzheimer’s with has low hepatotoxicity & ~20% improvement long acting as an inhibitor  But its use is limited due to hepatotoxicity 80 Structure Activity Relationships of Reversible Cholinesterase Inhibitors  The carbamate group is essential to the activity  The benzene ring is important  Involved in some extra hydrophobic bonding with the active site  Alternatively, it may be important in the mechanism of inhibition  Since it provides a good leaving group  The pyrrolidine nitrogen (which is ionized at blood pH) is important  Since it must bind to the anionic binding region of the enzyme  Better inhibition of ChE was observed when dimethylcarbamate & trimethylamine groups were placed meta to each other as in neostigmine  The carbamoyl-enzyme intermediate is stabilized  Because the nitrogen can feed a lone pair of electrons into the carbonyl group  This drastically reduces the electrophilic character & reactivity of carbonyl group 81  Irreversible Inhibitors of Cholinesterase  Both AChE & BuChE are inhibited irreversibly by a group of phosphate esters that are highly toxic  These chemicals are nerve poisons and have been used  In warfare  In bioterrorism, and  As agricultural insecticides  The compounds belongs to a class of organophosphorus esters  They permit ACh to accumulate at nerve endings  Organophosphate insecticides can enter the human body through skin absorption, inhalation and ingestion 82 Irreversible Inhibitors of Cholinesterase cont. A general formula of such compounds: Where R1= alkoxy R2 =alkoxy, alkyl, aryl, aryloxy or tertiary amine X =a good leaving group like F, CN, p- nitrophenoxy A is usually O or S but may also be Se when A is other than O, biologic activation is required before the compound becomes effective as an inhibitor of ChE  The R moiety impacts lipophilicity to the molecule – Contributes to its absorption through the skin 83 Irreversible Inhibitors of Cholinesterase cont.  Some examples of irreversible AChE inhibitors O CH CH3 O H C O 3 3 C2H5 O P S N CH3 (H3C)3C HC O P C2H5 O F O P F Echothiophate CH3 Soman H3C H3C Sarin CH3 O S O C2H5 O O H3C H3C O P S O O P S H3C O O O P F Malathion C O C2H5 Malaoxon H3C O H3C CH3 S O Diisopropylfluorophosphate C2H5 O P O NO2 O P (DFP) C2H5 O O O Parathion Paraoxon 84  Irreversible Inhibitors of Cholinesterase cont. Mammals Insects S Insect Oxidative O Et O S Et O P O NO2 Desulfuration Et O P O NO2 Parathion Et O X P OR1 (Inactive Prodrug) Paraoxon (Active drug) OR2 Mammalian Metabolism S Phosphorylated Enzymes Biotransformation Et O P OH Et O O,O-diethyl O-hydrogen Death phosphorothioate (Inactive & Excreted) Metabolization of Insecticides in Mammals & Insects  Inhibition of AChE by organophosphorus compounds takes place in two steps  Association of enzyme and inhibitor &  The phosphorylation step  Completely analogous to acylation by the substrate  The serine residue in the cholinesterase active site (esteratic site) forms stable phosphoryl ester with the organophosphorus inhibitors 85  Irreversible Inhibitors of Cholinesterase cont.  Although insecticides and nerve gases are irreversible inhibitors of cholinesterase by forming a phosphorylated serine at the active site of the enzyme  It is possible to reactivate the enzyme if action is taken soon after exposure to these poisons has occurred  Several compounds can provide a nucleophilic attack on the phosphorylated enzyme cause regeneration of the free enzyme  Substances such as choline, hydroxylamine & hydroxamic acid have led to the development of more effectiveO cholinesterase reactivators suchI as O N  N-hydroxynicotinamide R1 N R2 H & Pyridine-2-aldoxime N OH methiodide (2-PAM) [Pralidoxime] CH3 Hydroxamic acid 86  Irreversible Inhibitors of Cholinesterase cont.  ChEs that have been exposed to phosphorylating agents (e.g. sarin) becomes refractory to reactivation by ChE reactivators  The process is called aging  Aging occurs by partial hydrolysis of the phosphorylated moiety  Attached to the serine residue at the active site of the enzyme  Aging of the phospho-complex leads to further stable bonds between drug and enzyme that permanently inactivates the enzymatic activity of the cholinesterase  Prior to aging of phospho-enzyme complex strong nucleophiles such as pralidoxime (2-PAM)  Can regenerate the enzyme active site by removing the phosphate group 87 Irreversible Inhibitors of Cholinesterase cont. 88 Irreversible Inhibitors of Cholinesterase cont. The organophosphates bind with high affinity to the serine residue resulting a phosphorylated enzyme complex This complex can undergo aging where one of the O-C bonds is broken and the chemical becomes even more resistant to hydrolysis Pralidoxime (2-PAM) is able to regenerate the active enzyme complex only when aging has not occurred 89 Action of anticholinesterase drugs 90 Cholinergic Blocking Agents 91  Cholinergic Blocking Agents  Cholinergic Blocking agents are also called  Cholinergic antagonists  Anticholinergics drugs  Parasympatholytic Drugs or  Cholinolytic drugs  They are drugs that block or inhibit the actions of ACh in the parasympathetic nervous system (PSNS)  Compete with ACh & block ACh at the muscarinic receptors in the PSNS  ACh is unable to bind to the receptor site and cause a cholinergic effect  Once these drugs bind to receptors, they inhibit nerve transmission at these receptors  Have no intrinsic activity, do not trigger the usual receptor-mediated intracellular effects  i.e. do not produce response 92  Cholinergic Blocking Agents cont. Acetylcholine is the chemical transmitter at the Postganglionic parasympathetic nerve endings At autonomic ganglia and Somatic neuromuscular junctions Different types of anticholinergic drugs antagonize the actions of ACh at the above mentioned three types of peripheral synapses 93  Cholinergic Blocking Agents cont.  Anticholinergic action is apparently dependent upon their ability to  Reduce the number of free receptors that can interact with ACh Hence, decrease the efficiency of the endogenous NT  These drugs antagonize the action of ACh by one of the following mechanisms:  Competitive inhibition Occupy the ACh receptors and prevent its action  Persistent depolarization Cause prolonged depolarization of ACh receptor – Thus, they prevent the excitation of the receptor by the released ACh 94  Cholinergic Blocking Agents cont. Action of an antagonist to block a receptor Antagonist Postsynaptic nerve Ach Ach Cholinergic blockers are competitive antagonists Compete with ACh for binding at the muscarinic receptors of the PSNS, inhibiting nerve transmission This effect occurs at the neuroeffector junctions of  Smooth muscle Cardiac muscle, and 95  Cholinergic Blocking Agents cont. Cholinergic blocking drugs can be classified into  Antimuscarinic agents  Selectively block the muscarinic synapses of the parasympathetic nerves (postganglionic termination)  The effects of parasympathetic innervations are interrupted – And the actions of sympathetic stimulation are left unopposed  Ganglion blocking agents  Block the ACh activity at the ganglia of both sympathetic & parasympathetic nerves  Show a preference for nicotinic receptors of SNS & PSNS of ganglia  Neuromuscular blocking agents  Blocks the ACh activity at the NMJ of the voluntary nervous system  Interfere with the transmission of efferent impulses to skeletal muscle 96 97  Sites of actions of cholinergic antagonists AUTONOMIC SOMATIC Sympathetic innervation Sympathetic Parasympathetic of adrenal medulla Preganglionic Acetylcholine Acetylcholine Acetylcholine Ganglion Sites of action No ganglia ganglionic NR NR NR blockers Adrenal medulla Postganglionic neurons NE NE Acetylcholine Acetylcholine Site of action Site of action of of NM antimuscarinic Adrenergic Adrenergic Muscarinic blockers drugs receptor receptor receptor Nicotinic receptor Effector organs Striated muscle 98  Antimuscarinic Agents  Block muscarinic receptors causing inhibition of all muscarinic functions  Also blocks few exceptional sympathetic neurons that are cholinergic, such as those innervating sweat glands  They do not block nicotinic receptors  Have little or no action at skeletal NMJ or autonomic ganglia 99  Response of Muscarinic Antagonists The response of muscarinic antagonists includes:  Decrease contraction of smooth muscle of the GI tract & urinary tract  Treatment of smooth muscle spasm in GI tract  Reduction of hypermotility states of the urinary bladder  Reduce motility of GI tract  Dilation of the pupils resulting in mydriasis  Unresponsiveness to light  Cycloplegia - loss of movement in the eye muscles that adjust the size of the lens and are used for focusing; e.g. Atropine and cyclopentolate  Reduce gastric secretion e.g. Pirenzepine (M1 antagonist)  Used in enuresis among children  But α-adrenergic agonists may be more effective with few side effects 100  Antimuscarinic Agents cont.  Compounds possessing muscarinic antagonist activity are common components of cold & flu remedies acting to Reduce nasal & upper respiratory tract secretions  All of them are competitive antagonists  Their chemical structures usually contain Ester & basic groups in the same relationship as ACh But they have an aromatic group in place of the acetyl group 101  Antimuscarinic Agents cont.  Antimuscarinic agents may be classified on the basis of their chemical structures into:  Esters  Tropine esters  Natural tropine esters (Solanaceous alkaloids & analogues)  Synthetic esters  Synthetic non-tropine esters  Aminoalcohol Ethers  Used to treat parkinsonism  Aminoalcohols  Have gained their prominence as antiparkinsonism agents  Aminoamides  Used as antisecretory agents  Popular drug for symptomatic relief from cold 102  Tropine Esters Natural Tropine Esters  The two naturally occurring compounds  Atropine [()-hyoscyamine]&  Hyoscine (scopolamine) Alkaloids found in Solanaceous plants They are the earliest antimuscarinic agents They are tropic acid esters of tropine 103  Natural Tropine Esters cont.  The deadly nightshade (Atropa belladonna) mainly contains atropine  Thorn apple (Datura stramoium) mainly contains hyoscine (Scopolamine)  They are tertiary ammonium compounds which make them to be sufficiently lipid soluble 104  Natural Tropine Esters cont. Atropine  Has high affinity for muscarinic receptors  Where it binds competitively preventing ACh from binding to that site  Has both central & peripheral muscarinic blocking effect  Bradycardia (decreased cardiac rate at lower doses)  Tachycardia (increased cardiac rate at higher doses)  Antispasmodic effect to relax GI & bladder  Antisecretory agent Block secretions in the upper & lower respiratory tracts prior to surgery  Drying effect on membrane (xerostomia)  Antidotes for cholinergic agonists 105  Natural Tropine Esters cont. Side effects of atropine Dry mouth, blurring of vision, tachycardia, constipation Effects on the CNS (lower doses cause stimulation but higher dose cause inhibition) include restlessness, confusion, hallucinations, and delirium 106  Natural Tropine Esters cont. Hyoscine (Scopolamine)  Produces peripheral effects similar to atropine  Has greater action on the CNS & longer doA than atropine  One of the most effective antimotion sickness drug available  Has unusual effect of blocking short-term memory  Produce sedation but at higher doses can instead produce excitement (in contrast to atropine)  Used to facilitate endoscopy & gastrointestinal radiology (hyoscine butylbromide)  Anesthetic premedication to dry secretions 107  Natural Tropine Esters cont. Atropi ne 108  Comparison of atropine with ACh  If we superimpose the ACh skeleton onto the Atropine skeleton  The distance b/n the ester & the nitrogen groups are similar in both molecules  The nitrogen atom in atropine is protonated when it binds to the cholinergic receptor  Atropine can be seen to have the two important binding features of ACh  A charged nitrogen (if protonated) and an ester group Me NMe N 3 It is, therefore, able to bind to the receptor, but is unable to 'switchCHit CH2H 2 on' CH OH 2 OO CHCH3 CC O O 109 Natural Tropine Esters cont.  Since atropine is a larger molecule than ACh, it is capable of binding to Other binding groups outside of the acetylcholine binding site As a result, it interacts differently with the receptor and Does not induce the same conformational changes as acetylcholine 110  Structure Activity Relationships Natural Tropine Esters  Atropine, the prototype of anticholinergic agent  Provided structural model that guided the design & synthesis of muscarinic antagonists  The circled portion of the atropine molecule depicts the segment resembling acetylcholine  Although the amine functional group is separated from the ester oxygen by more than two carbons The conformation assumed by the tropanol ring orients these two atoms such that the intervening distance is similar to that in ACh 111  Structure Activity Relationships cont.  Quaternization of tertiary amines such as atropine, homatropine and scopolamine produces agents that block ACh at additional sites  These include the Ganglia of the autonomic Nervous System and At the neuromuscular junction (increases the curariform activity)  These decreases activity at postganglionic site of both Parasympathomimetic and Parasympatholytic activity  Due to increased steric interference at the anionic site 112  Structure Activity Relationships cont.  Tertiary amines are preferred for ophthalmic use because they penetrate the cornea better than their quaternary ammonium derivatives  However, when a drug (e.g. atropine) has a long-lasting mydriatic effect Its recovery period can be shortened by quaternization  The complex ring system of atropine is not necessary for the antagonist activity  Simplification could be carried out  For example, amprotropine has an ester group Et Et separated N from an amine by three carbon atoms H2 H2C C CH2 CH2OH O CH C O 113  Structure Activity Relationships cont.  The tropic acid portion is highly specific for the anticholinergic action & substitution by other acids  Decreases neurotropic potency  Though musculotropic action may increase 114  Structure Activity Relationships cont.  Chain contraction to two carbon atoms can be carried out – without loss of activity & a large variety of active antagonists have been prepared – E.g. Propantheline chloride & Tridihexethyl bromide Most effective & useful agent for the treatment of ulcer & GI tract hypermotility – i.e. reduce the volume & acidity of gastric H Csecretion 3 CH 3 Used in neurology CH to relief enuresis 3 Br O CH CH 2 N 2 Cl O CH3 HO C CH2CH2N(Et)3 CH3 O Tridihexethyl Bromide Propanetheline Chloride 115  Synthetic Tropine esters  Homatropine is an example & differs from atropine in:  The tropine base exists in the boat form, while in atropine it exists in the chair conformer  The ester moiety exists in an equatorial orientation and in the case of atropine it exists in an axial orientation Used in ophthalmology as mydriatic and lack other disadvantages of atropine 116  Synthetic non-tropine esters  Atropine was reviewed as an ester of a complex amino alcohol & subjected to simplification  E.g. Cyclopentolate hydrochloride Used in ophthalmology as mydriatic  Dicyclomine A musculotropic antispasmodic Safe for patients with glaucoma (anticholinergics are CI for glaucoma patients)  Clindinium bromide Marketed alone or in combination with chlordiazepoxide (Librax) which is used for the treatment of Peptic ulcer Hyperchlorhydria Ulcerative or spastic colon 117  Aminoalcohol Ethers  Anticholinergic ethers are useful drugs for treatment of Parkinsonism  They are related to antihistaminics and possess antihistaminic properties  They block centrally than peripherally  Examples for anticholinergic ethers are Orphenadrine Chlorphenoxamine, and Diphenhydramine 118  Aminoalcohol Ethers cont.  Benztropine is used for the treatment of Parkinsonism  Has structural relative of diphenhydramine, though the aminoalcohol portion is tropine  In the structure of benztropine three carbon intervenes between nitrogen and oxygen functions Whereas in others (orphenadrine, chlorphenoxamine and diphenhydramine) a two carbon intervenes However, the rigid ring structure orients the nitrogen and oxygen functions into more nearly the two-carbon chain distance 119  Aminoalcohols Aminoalcohols are centrally acting anticholinergics &used for the treatment of Parkinsonism  They have the following general structure:  Several of them possess bulky groups in vicinity of hydroxyl and cyclic amino functional groups  Are similar to classic aminoester anticholinergics derived from atropine where alcohol group substitutes the carboxyl function  Contain -aminopropanol with three carbons intervening between the hydroxyl and amino function  Are tertiary amines Because the site of action is central and quaternization destroys antiparkinsonian action  Have been quaternized to enhance the anticholinergic activity to produce an antispasmodic and antisecretory compound, such as tridihexyphenidyl chloride 120  Aminoamides  Structurally, aminoamides type of anticholinergics represents the type of molecule as aminoalcohol group  With the exception that the polar amide group replaces the corresponding polar hydroxyl group  Isopropamide  Used as antisecretory  It is one of the active ingredients of ornade & tuss-ornade  Both popular drugs for symptomatic relief from colds 121 General Structure of Muscarinic Antagonists  Substituents R1 and R2 should be carbocyclic or heterocyclic rings for maximal antagonist potency  Both are Bulky groups, there must be some sort of branching in the acyl group  The rings may be identical but the more potent compounds have different rings  Generally, one ring is aromatic and the other saturated or possessing only one olefinic bond 122 General Structure of Muscarinic Antagonists cont.  R1 and R2, however, may be combined into a fused aromatic tricyclic ring system such as that found in propantheline  The size of these substituents is limited  For example, substitution of naphthalene rings for R1 and R2 affords compounds that are inactive apparently owing to steric hindrance of binding of these compounds to the muscarinic receptor 123  General Structure of Muscarinic Antagonists cont.  The R3 substituent may be a  Hydrogen atom, Hydroxyl group (OH), Hydroxylmethyl group (CH2OH) or Carboxamide (CONH2) or  It may be a component of one of the R1 and R2 ring systems  When this substituent is either a hydroxyl group or a hydroxylmethyl group the antagonist is usually  More potent than the same compound without this group  The hydroxyl group presumably increases binding strength by  Participating in a hydrogen bond interaction124  General Structure of Muscarinic Antagonists cont.  The X substituent may be ether, ester or hydrocarbon  In the most potent anticholinergic agents, it is an ester  But, an ester functional group is not an absolute necessity for muscarinic antagonist activity  This substituent may be an ether oxygen or it may be absent completely  The N substituent is a quaternary ammonium salt in the most potent anticholinergic agents  This is not a requirement, however, because tertiary amines also possess antagonist activity presumably by Binding to the receptor in the cationic (conjugated) form  The alkyl substituents on nitrogen are usually 125  General Structure of Muscarinic Antagonists cont.  The distance between the ring-substituted carbon & the amine nitrogen is apparently not critical  Inasmuch as the length of the alkyl chain connecting these may be from two to four carbons  The most potent anticholinergic agents have two methylene units in this chain  Muscarinic antagonists must compete with agonists for a common receptor  Their ability to do this effectively is because the large groups R1 and R2 enhance binding to the receptor 126  General Structure of Muscarinic Antagonists cont..  Antagonists are larger than agonists, this suggests that groups R1 and R2 bind outside the binding site of acetylcholine The area surrounding the binding site of ACh is hydrophobic in nature This accounts for the fact that in potent cholinergic antagonists The group R1 and R2 must be hydrophobic usually phenyl, cyclohexyl or cyclopentyl 127 Therapeutic Actions of Cholinergic Blocking Agents  There are three predictable & clinically useful results from blocking the muscarinic effects of Ach:  Mydriatic & cycloplegia effect  Mydriasis is an excessive dilation of the pupil due to disease or drugs  Although the pupil will normally dilate in the dark, it is usually quite constricted in the light  A mydriatic pupil will remain excessively large, even in a bright environment  Constriction of the pupil is called miosis  Cycloplegia is the paralysis of the ciliary muscle, resulting in a loss of accommodation [loss of movement in the eye muscles that adjust the size of the lens and are used for focusing]  Antispasmodic effect  Lowered tone & motility of the GI tract & the genitourinary tract  Antisecretory effect  Reduced Salivation (antisialagogue), Perspiration (anhidrotic), Acid & gastric secretion 128 Nicotinic Antagonists 129  Nicotinic Antagonists Are chemical compounds that bind to cholinergic nicotinic receptor but have no efficacy All therapeutically useful nicotinic antagonists are competitive antagonists,  i.e. the effects are reversible with ACh There are two subclasses of nicotinic antagonists Skeletal neuromuscular blocking agents & Ganglionic blocking agents Present in nerve synapses at ganglia & at the NM synapse Drugs are able to show some level of selectivity b/n these two sites, mainly due to the distinctive routes which have to be taken to reach them 130 Nicotinic Antagonists cont. Antagonists of ganglionic nicotinic receptor sites are not therapeutically useful since they cannot distinguish between the ganglia of the  Sympathetic nervous system and  Parasympathetic nervous system (Both use nicotinic receptors) Consequently they have many side effects However, antagonists of the neuromuscular junction are therapeutically useful 131  Neuromuscular Blockers Skeletal muscle relaxants characteristically act at different sites: At the neuromuscular junction (anticholinergic skeletal muscle relaxants or neuromuscular blockers) At the spinal cord (central muscle relaxants)  A good muscle relaxation is effected by the use of neuromuscular blocker Which produce blockage of ACh at the neuromuscular junction  Drugs that act at the neuromuscular junction are said to have “curariform” activity 132 Neuromuscular Blockers cont. Curare (1516) and tubocurarine  Curare was first identified when Spanish soldiers in South America found themselves the unwilling victims of poisoned arrows  It was discovered that the Indians were putting poison on to the tips of their arrows  This poison was a crude, dried extract from a plant called Chondrodendron tomentosum and causes paralysis  Curare is a mixture of compounds  The active principle is d-tubocurarine an antagonist of ACh which blocks nerve transmissions from nerve to muscle 133  Neuromuscular Blockers cont. Neuromuscular blockers can be classified into:  Pachycurares (bulky molecules)(Non-depolarizing )  Block ion channels at motor end plate (Antagonist) D-tubocurarine chloride, Dimethyltubocurarine iodide Pancuronium bromide, Gallamine triethiodide, Doxacurium chloride, Mivacurium chloride  Leptocurares (slender molecules)(Depolarizing)  Activates receptor (Agonist) Succinylcholine chloride Decamethonium bromide  Both pachycurares and leptocurares have a distance of about 14 A0 between the quaternary nitrogen atoms 134  Neuromuscular Blockers cont. Using bis-quaternary ammonium structures of tubocurarine as a guide, a large number of compounds were synthesized and evaluated It became apparent that a bis-quaternary ammonium compound, having two quaternary ammonium salts separated by 10 to 12 carbon atoms (similar to the distance between the nitrogen atoms in tubocurarine) – Was a requirement for neuromuscular blocking activity 135  Neuromuscular Blockers cont. The rationale for this structural requirement was that, in contrast to muscarinic receptor, – Nicotinic receptors possessed two anionic binding sites – Both of which had to be occupied for a neuromuscular blocking effect 136 Some neuromuscular junction blocking agents 137  Ganglionic Blockers Stimulation of autonomic ganglia by ACh is the Nicotinic action of the NT Impulse through the ganglion occurs when ACh is released from preganglionic fibers  Activates the N2 nicotinic receptors of the neuronal membrane This triggers an increase in sodium and potassium conductances of a synaptic membrane  Resulting in  An initial excitatory postsynaptic potential  Followed by an inhibitory postsynaptic potential and  Finally, a slowly generating excitatory postsynaptic potential Mechanism of Action Ganglionic nicotinic receptors, like those of the skeletal muscle neuromuscular junction, are subject to both depolarizing and non-depolarizing blockade 138 Depolarizing Ganglionic Blocking Agents These blocking agents are actually ganglionic stimulants Thus, for nicotine, small doses give an action similar to that of the natural neuroeffector ACh, an action known as the “Nicotinic effect of Ach” However, larger amounts of the nicotine bring about a ganglion block characterized  Initially by depolarization  Following by a typical competitive antagonism To conduct nerve impulses, the cell must be able to carry out a polarization & depolarization, & if the depolarized condition is maintained without repolarization, it is obvious that no conduction occurs  Nicotine, carbamoyl choline can produce depolarizing ganglion block Chemicals that cause this type of ganglionic block are not of therapeutic significance 140  Non-depolarizing competitive ganglionic blocking agents  Compounds in this class possess the necessary affinity to attach to the nicotinic receptor sites that are specific for Ach  But lack the intrinsic activity necessary for impulse transmission, i.e., they cannot effect depolarization of the cell  A large enough concentration of ACh, can offset the blocking action by competing for specific receptors  Tetraethylammonium salts, hexamethonium, & a trimethaphan  Mecamylamine possesses a competitive component in its action but is also noncompetitive (dual antagonism) 141 Non-depolarizing Non-competitive ganglionic blocking agents They produce their effect not at the specific ACh receptor site But, at some point further along the chain of events that is necessary for transmission of the nerve impulse When the block has been imposed, increasing the concentration of ACh cannot reverse the blockade i.e. ACh does not act competitively with the blocking agent at the same receptor 142 Non-depolarizing Non-competitive ganglionic blocking agents cont. A few years after the introduction of tetraethyl ammonium compounds, Paton & Zaimis (1949) investigated the usefulness of the Bis- trimethylammonium polymethylene salts Bis-trimethylammonium polymethylene salts indicate that:  A critical distance of ~5 - 6 carbon atoms between onium centers is important for good ganglionic blocking action  Pentamethylene and hexamethylene compounds are effective antidotes against the curare effect of the decamethylene compounds  Hexamethonium bromide and hexamethonium chloride emerged from this research as clinically useful products 143 Adrenergic Drugs 144  Adrenergic Drugs  Are chemical agents that exert their principal pharmacological & therapeutic effects by either  Enhancing or  Reducing the activity of the various components of the Sympathetic division of the ANS  In general, substances that produce effects similar to stimulation of sympathetic nervous activity are called  Sympathomimetics or  Adrenergic stimulants  Those that decrease sympathetic activity are referred to as  Sympatholytics or  Antiadrenergics or  Adrenergic blocking agents 145  Adrenergic Neurotransmitters  Adrenergic nerves release the neurotransmitters  Norepinephrine (NE); Epinephrine (E); Dopamine (D)  NE is released from sympathetic nerve ending into the synaptic cleft  Where it reacts with specific presynaptic & postsynaptic adrenergic receptors  But epinephrine is not released from peripheral sympathetic nerve endings as in NE  Rather synthesized & stored in the adrenal medulla Hence, epinephrine is referred to as a neurohormone (also biosynthesized in certain neurons of the CNS) 146 Adrenergic Neurotransmitters cont.  NE, E & D belongs to the chemical class of substances known as  Catecholamines Because they contain with in their structures both An amine and Ortho dihydroxy benzene (catechol)  Aromatic compounds that contain an arrangement of hydroxyl substituents similar to catechol are highly susceptible to oxidation147 Biosynthesis, storage & release of catecholamines The biosynthesis of the catecholamines, NE , E and D involves a sequence of enzymatic reactions Catecholamine biosynthesis takes place in adrenergic & dopaminergic neurons in the  CNS  Sympathetic neurons of the ANS &  Adrenal medulla The amino acid L-tyrosine is generally considered as  The precursor for the catecholamines The biosynthesis begins with the active transport of L-tyrosine into the axoplasm  Where it is acted upon by tyrosine hydroxylase to form L- dihydroxyphenylalanine (L-dopa) 148 Biosynthesis, storage and release of catecholamines cont. 149 150  Uptake & Metabolism  The action of NE at adrenergic receptor is terminated by a combination of processes, including  Uptake into the neuron & extraneuronal tissues  Diffusion away from the synapse &  Metabolism  Usually the primary mechanism for termination of the action of NE is  Reuptake of the catecholamine into the nerve terminal  This process is termed ‘uptake 1’  Involves a membrane energy-requiring pump system that has a high affinity for NE  This uptake system will also transport certain amines other than NE into the nerve terminal  It is the site of action of cocaine & some of the tricyclic antidepressants  ‘Uptake 2’ is an extraneuronal uptake process  Present in a wide variety of cells, including glial, hepatic & myocardial cells  Has low affinity for NE 152  Uptake & Metabolism cont.  The two principal enzymes involved in catecholamine metabolism are  Monoamine oxidase (MAO) & Catechol-O- methyltransferase (COMT) They are distributed throughout the body with high concentrations found in the liver & kidneys  MAO is associated primary with the outer membrane of the mitochondria  Has a role in the metabolism of intraneuronal catecholamines  COMT is found primarily in the cytoplasm  Not present in sympathetic neurons  Neither of them exhibits a high degree of substrate specificity 153  Uptake & Metabolism cont.  MAO has two types:  MAO-A & MAO-B Exhibits different substrate selectivity  E.g. MAO-A  Shows substrate preference for NE & 5- hydroxytryptamine (5-HT)  MAO-B  Shows substrate selectivity for β-phenylethylamine  COMT catalyzes the methylation of a variety of  Catechol-containing molecules  Methylation occurs almost exclusively on the  meta-hydroxyl group of the catechol regardless of whether the catechol is  NE, E or  One of the metabolic products 154 The main pathways of NE metabolism in the Brain & in the Periphery NM, normetanephrine; VMA, vanillylmandelic acid; DOMA, 3,2- dihydroxymandelic acid; MOPEG, 3- methoxy-4-hydroxyphenylglycol; DOPEG, 3,4-dihydroxyphenylglycol; ADH, aldehyde dehydrogenase; AR, 155 aldehyde reductase Adrenergic Receptors (Adrenoceptors) 156  Adrenergic Receptors (Adrenoceptors)  Ahlquist was the first to propose the existence of two general types of adrenoceptors in mammalian tissues  Alpha (α) and beta (β) Based on the difference relative potency of a series of adrenergic receptor agonists on various smooth muscle preparations α-Adrenergic Receptors  Has two subtypes (α1 and α2)  Based on the second messenger system that is affected  They could be either  Presynaptic or postsynaptic &  Excitatory or inhibitory in their responses 157  α-Adrenergic Receptors cont.  Generally, the catecholamines has the following alpha affinity order:  Epinephrine ≥ Norepinephrine >> Isoproterenol  The α1-adrenergic receptor coupled to the  Enzyme phospholipase C (PLC) via a G-protein (Gq)  When stimulated by activation of the α-adrenergic receptors  PLC hydrolyzes PIP2 (Phosphatidylinositol-4,5- bisphosphate) into IP3 & DAG (2nd messengers)  IP3 stimulates the release of Ca2+ from the sarcoplasmic reticulum  DAG activates protein kinase C, an enzyme that phosphorylates protein  α1-receptor activation also can increase the  Influx of extracellular Ca2+ via  Voltage-dependent &  As well as non-voltage dependent Ca2+ channels 158  G-Protein Transducers and Second Messengers G-Protein Second Messenger System Transducer Family Gs Stimulates adenylyl Cyclase activity and Ca2+ channels Gi Inhibits adenylyl cyclase activity & activates K+ channels Gq Stimulate Phospholipase C (PLC) activity Modulate sodium/hydrogen ion G12 exchanger 159 α-Adrenergic Receptors cont.  Activation of α2-adrenergic receptors leads to a  Reduction in the catalytic activity of adenylyl cyclase Which in turn results in a lowering of intracellular levels of cAMP Inhibition of adenylyl cyclase is regulated by G-protein Gi  Interaction of α2-receptors with agonists such as  NE & E results in: Inhibition of NE release from the neuron Thus, α2-receptors play a great role in the regulation of NE release 160  α-Adrenergic Receptors cont.  α-receptors (of the CNS & peripheral tissues) are involved in  Control of the cardiovascular system E.g. Constriction of vascular smooth muscle by α1 and α2  In heart:-activation of α1-receptors results in a  Selective inotropic response with little or no change in heart rate  In brain:- activation of postjunctional α2-receptors  Reduces sympathetic outflow from the CNS  Which in turn causes a lowering of blood pressure  Both α1 and α2 adrenergic receptors play an important role on the  Regulation of a number of metabolic processes such as insulin secretion & glycogenolysis 161 Signal Transduction of at α1-Receptors α1 162  β-Adrenergic Receptors  Generally, the catecholamines has the following beta affinity order: Isoproterenol > Epinephrine ≥ Norepinephrine  Has three subtypes (β1, β2 & β3) – Based on the rank order of potency of adrenergic receptor agonists NE, E & Isoproterenol  β1 -receptors exhibit the agonist potency order – Isoproterenol > Epinephrine = Norepinephrine  β2 -receptors exhibit the agonist potency order – Isoproterenol > Epinephrine >> Norepinephrine  β3 -receptors exhibit the agonist potency order – Isoproterenol = Norepinephrine > Epinephrine 163  Location & Function of β- Receptors  β1 -receptors are located  Mainly in the heart  Where they mediate the positive inotropic & chronotropic effects of the catecholamines  Also found on the juxtaglomerular cells of the kidneys  Where they are involved in increasing renin secretion  β2 -receptors are located  On smooth muscle throughout the body  Where they are involved in relaxation of the smooth muscle – Producing bronchodilation & vasodilation effects  Also found in the liver, where they promote glycogenolysis  β3-receptor is located on brown adipose tissue  Involved in the stimulation of lipolysis  All the three receptors are coupled to adenylyl cyclase  Which catalyzes the conversion of ATP to cAMP  This coupling is via the guanine nucleotide protein G s  In the absence of agonist, GDP is bound reversely to the G s 164 Signal Transduction at α2- & β-Receptors 165  Adrenergic Receptor Binding Sites Alpha Receptor Site  Important features of the site include:  An anionic site, which binds the positive ammonium group  One hydrogen bonding area  A flat area non-polar area  For the aromatic ring Beta Receptor Site  Important features of the site include:  An anionic site  Shown as Asp anionic negative acid group which binds the positive ammonium group  Two hydrogen bonding areas  Shown as two Serine with OH groups hydrogen bonding to the phenol OH groups of the NE  A flat area non-polar area  For the aromatic ring 166 Sympathomimetic Agents 167  Sympathomimetic Agents  Sympathomimetic agents produce effects resembling those produced by stimulation of the sympathetic nervous system  Catecholamines and sympathomimetic drugs are classified as  Direct acting  Indirect acting or  Mixed acting sympathomimetics Direct-acting sympathomimetic drugs  Act directly on one or more of the adrenergic receptors  These agents may  Exhibit considerable selectivity for a specific receptor subtype (e.g. phenylephrine for α1, terbutaline for β2) or  Have no or minimal selectivity and act on several receptor types (e.g., epinephrine for α1 , α2 , β1 , β2, β3 receptors; norepinephrine for α1, α2 , β1 receptors) 168  Sympathomimetic Agents cont.  Indirect-acting drugs  Increase the availability of Norepinephrine or epinephrine to stimulate adrenergic receptors  This can be accomplished in several ways: By Releasing or displacing norepinephrine from sympathetic nerve varicosities (e.g. Amphetamine) Blocking the transport of norepinephrine into sympathetic neurons (e.g., cocaine) or Blocking the metabolizing enzymes by – Monoamine oxidase (MAO) (e.g., Pargyline) or – Catechol-O-methyltransferase 169 Sympathomimetic Agents cont.  Mixed-acting sympathomimetic drugs Drugs that interact with adrenergic receptors & cause the release of NE (e.g., ephedrine, dopamine) 170 Sympathomimetic Agents cont. 171 Generalized diagram of adrenergic nerve terminal showing sites of drug action 172 Norepinephrine & Adrenergic drugs  Norepinephrine has limited clinical application  It is non-selective  Given only intravenously Due to metabolism by intestinal & liver COMT & MAO  Low lipophilicity  Rapid metabolism limits its duration of action Only 1-2min given by infusion  Epinephrine is more widely used clinically than NE  Although it lacks oral activity for the same reason as NE, it is used to Treat hypotensive crisis Stimulate the heart in cardiac arrest (due to its greater -activity) Given intravenously to inhibit uterine contraction In the form of inhalers to relieve bronchoconstriction in asthma Local haemostatic to stop bleeding in epistaxis 173 Structure-activity relationships of adrenergic  agonists The parent structure for many of the sympathomimetic drugs is β-phenylethanolamine  Because of the basic amino groups, pka range approximately 8.5 to 10  All of these agents are highly positively charged at physiologic pH  Agents in this class have a hydroxyl group on C-1 of the side chain β-to the amine  This hydroxyl substitute carbon must be in the R absolute configuration for maximal direct activity as in the natural neurotransmitter  But most drugs are currently sold as mixtures of both R and S stereoisomers at this position (racemites)  The nature of the other substitutes determines  Receptor selectivity and Duration of action 174  R1–substitution on the amino Nitrogen  R is increase in size from hydrogen in norepinephrine 1  To methyl in epinephrine  To isopropyl in isoproterenol  Activity at -receptors decrease and  Activity at β-receptor increases  The activity at  and β-receptors  Is maximal when R1 is methyl as in epinephrine  But -agonist activity is dramatically decreased  When R1 is larger than methyl  And is negligible when R1 is isopropyl as in isoproterenol, leaving only β-activity 175  R1–substitution on the amino Nitrogen cont.  Presumably, the β-receptors has a large lipophilic binding pocket adjacent  To the amine binding aspartic acid residue » Which is absent in the  receptor  As R1 becomes large than butyl  Affinity for α1-receptor returns but not intrinsic activity » Which means large lipophilic groups can afford compounds with α1-blocking activity » Example: Labetalol 176  R1–substitution on the amino Nitrogen cont.  N-substituent can also provide selectivity for β- receptors  E.g. T-butyl group ( as in Colterol (N-tert- butylnorepinephrine), where R1 = Isobutyl) Selectivity for β2–adrenergic receptors  Colterol is a selective β2–adrenergic agonist  Whereas isoproterenol is a non-selective β- adrenergic agonist Where considering use as a bronchodilator non-selective β2-agonist such as isoproterenol