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Patrick An Introduction to Medicinal Chemistry 3/e Chapter 22 ANTIULCER AGENTS Part 1: Histamine antagonists ©1 Contents Part 1: Histamine antagonists 1. Introduction 1.1. Ulcers 1.2. Therapy of ulcers 1.3. Parietal cells and gastric acid release 2. Histamine 2.1. Properties 2.2. Actions 3. Classical Antihistamines 4. Histamine as a Lead Compound 5. SAR for H1 and H2 Agonists 6. Strategies for converting Agonists to Antagonists (3 slides) 7. Na-Guanylhistamine 7.1. Biological properties 7.2. Structure and chemical properties continued… 8. Binding Theory for Agonists and Antagonists 8.1. Binding regions 8.2. Binding of histamine 8.3. Binding of Naguanylhistamine 9. Chelation Binding Theory 9.1. The proposal 9.2. The evidence 9.3. Binding modes for analogues 10. Chain Extension Strategy 10.1. Aim 10.2. Results 10.3. Proposed binding for 3C extension analogues (2 slides) 10.4. Further evidence (2 slides) ©1 continued… 11. Distinguishing between the Polar Binding Regions 11.1.Strategy 11.2.Rationale 11.3.Method 11.4.SK&F 91581 11.5.Comparison between the thiourea and guanidine groups 12. Chain Extension (2 slides) 13. The Imidazole Ring 13.1. Structures 13.2. Basicity 13.3. Varying basicity 13.4. Tautomer studies (2 slides) 14. Alternative Rationales (2 slides) 15. From Metiamide to Cimetidine (3 slides) 16. Cimetidine (Tagamet) 16.1. Properties 16.2. The cyanoguanidine moiety continued… 16.3. 16.4. 16.5. The cyanoguanidine moiety - tautomers The cyanoguanidine moiety - conformational isomers The cyanoguanidine moiety - binding mode 17. Analogues 17.1. 17.2. The urea analogue Rigid nitropyrrole analogue 18. Desolvation Theory 18.1. The process 18.2. Hydrophobic analogues (3 slides) 19. Dipole Moment Theory 19.1. Proposal 19.2. Dipole-dipole interactions 19.3. QSAR study including dipole-dipole interactions 20. Ranitidine (Zantac) [53 slides] © 1 1. Introduction 1.1 Ulcers • Localised erosions of the mucous membranes of the stomach and duodenum • Potentially fatal if untreated • Caused by stress, infection (H. Pylori) and drugs (NSAIDS) • Aggrevated by gastric acid (HCl) in the stomach 1.2 Therapy of ulcers • Lower the levels of gastric acid -histamine antagonists and proton pump inhibitors • Antibacterial agents vs. H. Pylori • Herbal remedies ©1 1. Introduction 1.3 Parietal cells and gastric acid release Acetylcholine oesophagus Histamine + H2 cAMP Parietal Cells Stomach Gastrin + M3 + + + Cck2 + H+ Cl Stomach HCl Pyloric Sphincter Duodenum • Antrum Receptors Ion channel Proton pump Release of gastric acid is promoted by acetylcholine, gastrin and histamine ©1 2. Histamine 2.1 Properties • A chemical messenger released by cells • Acts as a local hormone NH3 HN Nš NH3 N NH Imidazole ring • • • • • Two possible tautomers pKa for the -NH2 group = 9.80. % ionisation at pH 7.4 = 99.6 pKa for the imidazole ring = 5.74 Imidazole ring is not ionised at blood pH ©1 2. Histamine 2.2 Actions Histamine is released by cell damage Stimulates dilation of blood vessels with increased permeability White blood cells escape blood vessels and access area of tissue damage White blood cells combat infection BUT Also released by allergies, asthma, hay fever and insect bites ©1 3. Classical Antihistamines Commonly used to treat symptoms such as inflammation & itching MeO NMe2 O NMe2 N N Mepyramine • • • • • • • • Diphenhydramine Benadryl But no effect on gastric acid release Casts doubt on histamine receptors being present on parietal cells Histamine may promote gastric acid release indirectly SK&F propose two types of histamine receptor (H1 and H2) H1 - responsible for classical actions of histamine H2 - proposed as the receptor on the parietal cells Claim that H2 receptors are unaffected by classical antihistamines Implies classical antihistamines are H1 specific ©1 4. Histamine as a Lead Compound • No known H2 antagonist at the time - no lead compound • SK&F decide to use histamine itself as the lead compound • Aim is to alter an agonist into an antagonist • Compare development adrenaline • Need to know SAR requirements for H2 agonists • Analogues tested by their ability to promote gastric acid release • Does not prove existence of H2 receptor of propranolol (b-blocker) ©1 from 5. SAR for H1 and H2 Agonists • • Two nitrogen atoms are required for H1 agonist activity All three nitrogen atoms are required for H2 agonist activity NH3 NH3 HN HN N H1 Receptor N H2 Receptor ©1 6. Strategies for converting Agonists to Antagonists • Add extra functional groups to find extra binding interactions with the binding site • Extra binding interactions may result in a different mode of binding resulting in a different induced fit for the receptor • Different induced fit may fail to activate the receptor • A a result, analogue binds but fails to activate the receptor • Analogue likely to bind more strongly than an agonist ©1 6. Strategies for converting Agonists to Antagonists Histamine HN N HN NH3 N Receptor (Inactive) NH3 Induced Fit - Receptor 'Switched on' Extra Functionality HN N NH2 HN N NH2 Receptor (Inactive) Different induced fit ©1 6. Strategies for converting Agonists to Antagonists Examples - extra hydrophobic groups NHR1R2 NHR1R2 N N HN R 3 N Results • • • No antagonist activity observed with extra hydrophobic groups Try adding extra hydrophilic groups instead Aim is to search for extra polar binding regions ©1 7. Na-Guanylhistamine H N HN N NH2 Guanidine moiety H2N 7.1 Biological properties • Partial agonist - promotes HCl release but less strongly than histamine • Prevents histamine from fully promoting the release of HCl • SK&F suggest that N-guanylhistamine is binding to the proposed H2 receptor, resulting in weak activation • Whilst present, N-guanylhistamine blocks histamine from binding ©1 7. Na-Guanylhistamine 7.2 Structure and chemical properties • • • The guanidine group is basic and ionised Different tautomers are possible The positive charge can be delocalised H N HN N NH2 H N HN HN N H2N H2N H N HN N H N NH2 N H2N NH2 H2N The positive charge is more diffuse and can be further away from the imidazole ring ©1 NH2 8. Binding Theory for Agonists and Antagonists 8.1 Binding regions Antagonist binding region Imidazole ring binding region Agonist binding region • • • • • Three binding regions are proposed for the H2 receptor - an imidazole binding region and two polar binding regions Two binding modes are proposed - one for agonists and one for antagonists The imidazole binding region is common to both binding modes One of the polar binding regions is accessed by agonists and the other by antagonists The antagonist polar region is further from the imidazole binding region ©1 8. Binding Theory for Agonists and Antagonists 8.2 Binding of histamine Antagonist binding region Imidazole ring binding region NH3 HN N HN N Agonist binding region No interaction as an antagonist • • • • • Antagonist binding region NH3 Strong interaction as an agonist Histamine has a short chain Charged -nitrogen can only reach the polar agonist region The antagonist binding region is out of range Histamine can only bind as an agonist Histamine acts as a pure agonist ©1 8. Binding Theory for Agonists and Antagonists 8.3 Binding of N-guanylhistamine Antagonist binding region H2N NH2 Imidazole ring binding region NH HN N N Agonist binding region Binding as an antagonist Receptor not activated • • • HN NH NH2 H2N Binding as an agonist Receptor activated Positive charge on the structure is more diffuse and further out Allows N-guanylhistamine to bind in two different modes Structure binds as an agonist in one mode and as an antagonist in the other mode, making it a partial agonist ©1 9. Chelation Binding Theory 9.1 The proposal H H N N HN N N H H Strong interaction O H O Receptor X=NH,S SK&F propose that the guanidine moiety interacts with a carboxylate ion in the antagonist binding region by means of two H-bonds and an ionic interaction 9.2 The evidence S H N NH2 NH2 HN HN N A NH2 N B X X= SMe, Me Structures A and B are both partial agonists, but structure A has greater antagonist properties ©1 9. Chelation Binding Theory 9.3 Binding modes for analogues H S N HN N H N Strong interaction H N N HN H N O H O H Weak interaction O X O Receptor Positive charge is localised further out leading to better interactions with the antagonist binding region • H Receptor X=Me, SMe Only one H-bond is possible with the antagonist binding region. Charge is also directed away from the carboxylate ion weaker antagonist property. The chelation binding theory was eventually disproved but it served a purpose in explaining results and pushing the project forward on rational grounds ©1 10. Chain Extension Strategy 10.1 Aim: To push the polar guanidine group further out and to increase the interaction with the antagonist binding region 10.2 Results: NH2 3C Bridge HN N N H NH2 Guanidine Partial agonist Antagonist activity increases • • NH2 3C Bridge S HN N NH2 Isothiourea Partial agonist Antagonist activity decreases! Antagonist activity of the extended guanidine analogue increases as expected Isothiourea analogue might have been expected to have increased antagonist activity since the charge is further out ©1 10. Chain Extension Strategy 10.3 Proposed binding for 3C extension analogues NH2 HN N NH2 N NH H H S HN N NH H Strong interaction O Weak interaction O O Receptor • O Receptor Different form of hydrogen bonding taking place Compare 2C bridged analogues H X N HN N H N Strong interaction H O H O Receptor X=NH,S ©1 10. Chain Extension Strategy 10.3 Proposed binding for 3C extension analogues Antagonist binding region NH2 HN NH2 HN Imidazole ring binding region N HN N Agonist binding region Good binding as an antagonist NH2 HN NH2 Binding as an agonist ©1 10. Chain Extension Strategy 10.4 Further evidence X HN N N NH H H Partial agonists with good antagonist activity (X= Me or SMe) Binding mode X HN N N NH H H Strong interaction O O Receptor X=NH2, SMe, Me ©1 10. Chain Extension Strategy 10.4 Further evidence NH2 NH2 S HN NH2 HN N N Agonist binding region Poor binding as an antagonist • X HN Agonist binding region Good binding as an antagonist Emphasis now switches to the types of binding interactions at the polar binding regions ©1 11. Distinguishing between the Polar Binding Regions 11.1 Strategy: • Replace the ionic guanidine group with a neutral H-bonding group 11.2 Rationale: • May allow a distinction to be made between the two polar binding regions. • Ionic bonding is known to be crucial for the agonist binding region • It may not be crucial for the antagonist binding region 11.3 Method: • Replace the basic guanidine moiety with a neutral thiourea group ©1 11. Distinguishing between the Polar Binding Regions 11.4 SK&F 91581 HN N Thiourea H N NH2 S No agonist activity, very weak antagonist ©1 11. Distinguishing between the Polar Binding Regions 11.5 Comparison between the thiourea and guanidine groups Similarities - Planarity, geometry, size, polarity, H-bonding ability Differences - Thiourea is neutral while guanidine is basic and ionised Thiourea H N NH2 S e-withdrawing Neutral Guanidine H N NH2 NH2 Basic Conclusions • Agonist polar region involves ionic and H-bonding interactions • Antagonist polar region may not require ionic interactions. H-bonding may be sufficient ©1 12. Chain Extension Strategy • Extend the carbon bridge to 4 carbons • Pushes thiourea group further out • May increase the interaction with the antagonist binding region Results Discovery of burimamide HN N Chain extension N H S NHMe ©1 12. Chain Extension Properties of burimamide • • • 100 times more active as an antagonist compared to Nguanylhistamine No antagonist activity at H1 receptors Activity too low for oral use Conclusions – • • • Chain extension leads to a pure antagonist with good activity Chain extension allows a better overlap of the thiourea group with the antagonist binding region Establishes the existence of H2 receptors ©1 13. The Imidazole Ring 13.1 Structures H N I š N H H N N R N N R H H H • • -H R N š N R II H III Imidazole ring can exist as two tautomers (I) and (II) as well as two ionised forms (III) Which of these is preferred? ©1 13. The Imidazole Ring 13.2 Basicity e-withdrawing HN CH2CH2NH3 N Histamine pKa = 5.74 Ionisation = 3% HN H N Imidazole pKa = 6.80 e-donating HN CH2CH2CH2CH2NHCSNHMe N Burimamide pKa = 7.25 Ionisation = 40% Conclusions • The imidazole ring of histamine is not ionised when it interacts with the imidazole binding region • The ionised form of burimamide is unlikely to bind well • Decreasing the basicity and ionisation of the imidazole ring in burimamide closer to that of histamine may increase the binding interactions to the imidazole binding region ©1 13. The Imidazole Ring 13.3 Varying basicity Strategy Convert the side chain of burimamide to an e-withdrawing group Thiaburimamide HN N Electron withdrawing S S N H NHMe pKa = 6.25 Increase in antagonist activity Non-ionised imidazole is favoured ©1 13. The Imidazole Ring 13.4 Tautomer studies Tautomer I vs tautomer II H Inductive effect N R • Favoured tautomer for histamine is I š N • Side chain is electron withdrawing • Inductive effect decreases with distance • Np is less basic than N • N is more likely to be protonated • Favoured tautomer for thiaburimamide is also tautomer I Strategy • Increase the basicity of N relative to Np to further increase the percentage population of tautomer I vs tautomer II • Add an electron donating group to the imidazole ring closer to N than to Np ©1 13. The Imidazole Ring 13.4 Tautomer studies Metiamide Electron donating HN N Me S S Electron withdrawing • • • • • • • • N H NHMe 10 fold increase in antagonist activity w.r.t burimamide Electron-donating effect of methyl group is more significant at N Increases basicity of N Favours tautomer I over tautomer II Increase in pKa to 6.80 Increase in ionisation to 20% Increase in the population of tautomer (I) outweighs the increase in population of the ionised structures (III) ©1 Unacceptable side effects - kidney damage 14. Alternative Rationales • The increases in activity for thiaburimamide and metiamide may be due to a conformational effect • The thioether link increases the length and flexibility of the side chain • This may lead to increased binding • The methyl substituent may orientate the side chain into the active conformation - i.e. the methyl group acts as a conformational blocker ©1 14. Alternative Rationales Oxaburimamide HN N • S O N H NHMe Less potent than burimamide despite the side chain being electron withdrawing Possible explanations • The ether link is smaller and less flexible • The ether may be involved in a ‘bad’ hydrogen bond • There may be an energy penalty involved in desolvating the oxygen prior to binding ©1 15. From Metiamide to Cimetidine • • • The side effects of metiamide may be due to the thiourea group The thiourea group is not a natural functional group Replacing thiourea with a natural functional group may remove the side effects O S N H NHMe Thiourea Toxic side effects N H NH NHMe Urea Drop in activity N H NHMe Guanidine Drop in activity but no agonist activity! Conclusions • First guanidine analogue to be a pure antagonist • The longer 4C chain pushes the guanidine unit beyond the agonist binding region, but not beyond the antagonist binding region ©1 15. From Metiamide to Cimetidine Binding interactions for the 4C extended guanidine H2N NH2 NH S HN N HN N Agonist binding region S NH NH2 H2N Binding as an antagonist No binding as an agonist ©1 15. From Metiamide to Cimetidine Strategy: • Retain the guanidine group • Guanidine is a natural group present in the amino acid arginine • Increase activity by making the guanidine group neutral • Add a strong electron withdrawing group to decrease basicity (e.g. NO2 or CN) Cimetidine Me S HN N H N NHMe N CN Electron withdrawing cyanide group ©1 16. Cimetidine (Tagamet) 16.1 Properties • Comparable activity to metiamide • Less side effects • Inhibits H2-receptors and lowers levels of gastric acid released • Marketed in 1976 • Biggest selling prescription drug until ranitidine • Metabolically stable • Inhibits cytochrome p450 enzymes • Drug-drug interactions with diazepam, lidocaine and warfarin ©1 16. Cimetidine (Tagamet) 16.2 The cyanoguanidine moiety • H N NHMe N CN Acts as a bio-isostere for the thiourea group H N NHMe S • Both groups are planar and of similar geometry • Both groups are polar but essentially neutral • Both groups have high dipole moments • Both groups have low partition coefficients • The cyanoguanidine group is weakly acidic and weakly basic amphoteric • The cyanogaunidine group is not ionised at pH 7.4 ©1 16. Cimetidine (Tagamet) 16.3 The cyanoguanidine moiety - tautomers • The favoured tautomer is the imino tautomer H RN N CN NHMe I Amino Tautomer • • • N RHN CN NHMe II Imino Tautomer H N RHN CN NMe III Amino Tautomer The electron withdrawing effect of the CN group is an inductive effect The inductive effect is felt most at the neighbouring nitrogen The neighbouring nitrogen is least likely to form a bond to hydrogen ©1 16. Cimetidine (Tagamet) 16.4 The cyanoguanidine moiety - conformational isomers Steric interaction N R N N H Me Z,E • • • CN N H H H N N R Me E,E NC CN Steric interaction H N N N N R H Me R CN N N H H Z,Z E,Z Me The E, E and Z,Z conformations are not favoured - X-ray and nmr evidence Bad news for the chelation bonding theory Chelation to the one carboxylate group requires the E,E or the Z,Z conformation ©1 16. Cimetidine (Tagamet) 16.5 The cyanoguanidine moiety - binding mode NC H E,Z NC N N N R H O O Me Receptor Two H-bonds are not possible for the favoured conformations X H N N N R H Me X Receptor Two separate H-bonds to 2 different H-bond acceptors are more likely ©1 17. Analogues 17.1 The urea analogue Me S HN N H H N NMe O • The preferred conformation for the urea analogue is E,E or Z,Z • Weak antagonist • Unable to bind to two different binding groups in the antagonist binding region ©1 17. Analogues 17.2 Rigid nitropyrrole analogue H Me S HN NO2 N N N H • • • • Unable to adopt the E,E or Z,Z conformation Strongest analogue of cimetidine Locked into the active conformation Can only interact with two separate H-bond acceptors in the antagonist binding region ©1 18. Desolvation Theory 18.1 The process H O H H R G O H H • • • • • • O Desolvation Energy penalty R G R Binding Energy released G H A guanidine unit is highly polar and highly solvated Solvated water must be removed prior to binding An energy penalty is involved The ease of desolvation may affect strength of binding and activity A urea group is more hydrophilic than a cyanoguanidine group May explain lower activity of the urea analogue ©1 18. Desolvation Theory 18.2 Hydrophobic analogues HN N Me X S N H NHMe Aminal system (Z) Strategy • Increase the hydrophobic character of the planar aminal system • Implies less solvation • Implies less of an energy penalty associated with desolvation • Implies easier binding and a stronger activity Result • Antagonist activity of analogues increases as hydrophobic character increases ©1 18. Desolvation Theory 18.2 Hydrophobic analogues log (Act) ‘Outrider’ . HN 6.0 NCN CHNO2 NHMe NHMe NHMe HN HN . S .. HN NCN HN NH2 NH2 5.0 N HN 4.0 . O N H . . HN -1.8 HN . . . HN NNO2 . HN S NHMe NNO2 NH2 O N HN O N H NHMe O NH2 -1.4 -0.6 -1.0 log P of HZ Log (activity) = 2.0 log P + 7.4 ©1 18. Desolvation Theory 18.2 Hydrophobic analogues HN N Me CHNO2 S N H NHMe Greater activity than expected Hydrophilic group should lower activity HN N O Me N S N H N H Lower activity than expected based on the hydrophobicity of the group present ©1 19. Dipole Moment Theory 19.1 Proposal • A dipole-dipole interaction takes places between the drug and the binding site on approach of the drug • The dipoles line up and orientate the drug • Good interaction with the binding site occurs if the binding groups are positioned correctly w.r.t the binding regions results in good activity • Poor interaction occurs if the binding groups are not positioned correctly with respect to the binding regions - leads to poor activity ©1 19. Dipole Moment Theory 19.2 Dipole-dipole interactions Approach and orientation Strong hydrogen bonding O 2N H N NMe R H O 2N H N NMe R H Receptor surface Receptor surface Dipole Moments H-Bonding regions Approach and orientation Weak hydrogen bonding O N N H N R H O N N H N H R Receptor surface Receptor surface ©1 19. Dipole Moment Theory 19.3 QSAR study including dipole-dipole interactions • The orientation of the dipole is more important than its strength • Log (activity) = 9.12 cos q + 0.6 log P -2.71 q 30o Ideal dipole orientation X H • • • • • Observed dipole orientation N N R H Activity increases as hydrophobicity increases (log P) The ideal angle of the dipole moment = 30o At 30o, q= 0o and cos q = 1 At 30o, Log (activity) = 9.12 + 0.6 log P - 2.71 When dipole moment does not equal 30o, cos q < 1 and activity falls ©1 20. Ranitidine (Zantac) Me2N 4 5 3 CHNO2 2 O S N H NHMe • Contains a nitroketeneaminal group • Different heterocyclic ring • Took over from cimetidine as the most widely sold prescription drug in the world ©1