Medicinal Chemistry, Part 2, 3rd Edition PDF

Patrick An Introduction to Medicinal Chemistry 3/e Chapter 19 ANTIBACTERIAL AGENTS Part 3: Other lactams ©1  Carbapenems Thienamycin Imipenem Meropenem Ertapenem Core structure of...

Patrick An Introduction to Medicinal Chemistry 3/e Chapter 19 ANTIBACTERIAL AGENTS Part 3: Other lactams ©1  Carbapenems Thienamycin Imipenem Meropenem Ertapenem Core structure of the carbapenem molecules Doripenem ©1 Newer β-Lactam Antibiotics Thienamycin (Merck 1976) (from Streptomyces cattleya) Acylamino side Opposite chain absent stereochemistry to penicillins OH Plays a role H Carbon in ß-lactamase H resistance H3C NH3 Hydroxyethyl side chain SS N O Double bond leading to CO2 high ring strain and an increase in -lactam ring reactivity Carbapenam nucleus Potent and wide range of activity vs. Gram +ve and Gram -ve bacteria Active vs. Pseudomonas aeruginosa Low toxicity High resistance to β-lactamases Poor stability in solution (ten times less stable than Pen G) ©1  Newer β-Lactam Antibiotics Thienamycin analogues used in the clinic aminomethylideneamino NH H OH HN H N-Formimidoyl derivative of Me S thienamycin N O 1. Produced by the bacteria Streptomyces CO2 cattleya. Imipenem 2. Broad spectrum of activity aerobic and anaerobic Gram positive as well as Gram negative bacteria. 3. It is particularly important for its activity against Pseudomonas aeruginosa and the Enterococcus species. 4. It is not active against methicillin- resistant Staphylococcus aureus Thienamycin (MRSA). ©1 Imipenem dehydropeptidase metabolites Cilastatin is a chemical compound which inhibits the human enzyme dehydropeptidase. Dehydropeptidase is an enzyme found in the kidney DHP= Dehydropeptidases Cilastatin ©1  Meropenem (Merrem) MEROTROL O H CH3 N C H OH H N Me H Me Me S N O CO2 pyrrolidin-2-yl-sulfanyl Carbapenam nucleus 1. Ultra-broad spectrum injectable antibiotic used to treat a wide variety of infections, including meningitis and pneumonia. 2. Penetrates well into many tissues and body fluids including the cerebrospinal fluid, bile, heart valves, lung, and peritoneal fluid ©1 Meropenem MEROTROL ©1  Ertapenem IV First-line treatment for community-acquired infections Effective against Gram negative bacteria. It is not active against MRSA, ampicillin-resistant enterococci, Pseudomonas aeruginosa or Acinetobacter species. Clinically useful activity against pyrrolidin-2-yl-sulfanyl anaerobic bacteria. ©1  Ertapenem A critical 1beta-methyl substituent shields the beta-lactam carbonyl group and serves to reduce dehydropeptidase (DHP)-1 catalyzed hydrolysis of the beta-lactam, enabling ertapenem to be administered without a DHP-1 inhibitor. A meta-substituted benzoic acid substituent increases the molecular weight and lipophilicity pyrrolidin-2-yl-sulfanyl of the molecule, and the carboxylic acid moiety, ionized at physiological pH, results in ertapenem having a net negative charge. As a result, ertapenem is highly protein bound and has an extended half-life, permitting a once-a-day treatment regimen. ©1 ©1 Doribax (Doripenem) ©1  Doripenem and Ertapenem (A) The structures of Doripenem and Ertapenem. (B) The chemical mechanism ©1 of hydrolysis of Ertapenem by the Mycobacterium tuberculosis BlaC. Stability toward dehydropeptidase ©1 Carbapenems ©1  Newer β-Lactam Antibiotics Monobactams Monocyclic β-Lactams: – Nocaridicin – Aztreonam Monobactams are β-lactam compounds wherein the β-lactam ring is alone and not fused to another ring, in contrast to most other β-lactams. They are effective only against aerobic Gram-negative bacteria Only commercially available monobactam antibiotic is aztreonam © Other examples of monobactams are tigemonam, nocardicin A, and tabtoxin 1  Newer β-Lactam Antibiotics Nocardicins (Fujisawa 1975) N OH HO2C O C D H H HC CH2 CH2 C N OH H2N O N Nocardicin A C O H CO2H Monocyclic -lactam ring - monobactams Moderately active in vitro vs narrow group of Gram -ve bacteria Active vs. Pseusomonas aeruginosa Inactive vs. Gram +ve bacteria Different spectrum of activity from penicillins Thought to operate by a different mechanism from penicillins Low toxicity ©1  Newer β-Lactam Antibiotics Clinically useful monobactam Me Me CO2H O N H Me N N H 2N S O Aztreonam N O SO3- Administered by intravenous injection Can be used for patients with allergies to penicillins and cephalosporins No activity vs. Gram +ve or anaerobic bacteria Active vs. Gram -ve aerobic bacteria ©1  β-Lactamase Inhibitors Clavulanic acid (Beechams 1976) (from Streptomyces clavuligerus) Sulphur replaced by O No acylamino O side chain H H H H 9 C N O OH S Me 4 6 5 R 3 7 1 N N 2 Me Acyl side H chain O O H CO2H CO2H Thiazolidine -Lactam -Lactam ring Oxazolidine ring ring Weak, unimportant antibacterial activity Powerful irreversible inhibitor of β-lactamases - suicide substrate Used as a sentry drug for Amoxicillin Augmentin = Amoxicillin + clavulanic acid Allows less Amoxicillin per dose and an increased activity spectrum Timentin = Ticarcillin + Clavulanic acid ©1 β-Lactamase Inhibitors Clavulanic acid - mechanism of action 1 2 NH 2 NH 2 O CH2OH N O CH2OH NH2 O HN O H Base H CO2H O CO2H HO OH OH O O 2-Amino-5-hydroxy-3-oxopentanoic acid 3 4 5 O CH2OH H2N NH NH NH CO2H H CH O CH2OH HC O HN O O O H CO2H O O Irreversibly blocked ©1 ©1 Avibactam (NXL104) non-β-lactam β-lactamase inhibitor Clavulanic acid NXL104 A new drug application for avibactam in combination with ceftazidime3ed (approved by the FDA on February 25, 2015), for treating complicated urinary tract (cUTI) and complicated intra- abdominal infections (cIAI) caused by antibiotic resistant-pathogens, including NXL104 those caused by multi-drug resistant Gram-negative bacterial pathogens ©1  β-Lactamase Inhibitors Penicillanic acid sulfone derivatives O O O O S S 1 Me Me 6 5 6 2 7 N N 3 N N 3 Me O O N CO2 Na CO2 Sulbactam Tazobactam Suicide substrates for -lactamase enzymes Sulbactam has a broader spectrum of activity vs -lactamases than clavulanic acid, but is less potent Unasyn = ampicillin + sulbactam Tazobactam has a broader spectrum of activity vs -lactamases than Clavulanic acid, and has similar potency Tazocin or Zosyn = piperacillin + tazobactam ©1  Olivanic Acids MM 13902 Naturally occurring carbapenem -lactamase inhibitors with antibacterial activity, produced by Streptomyces olivaceus (produced in low yield, isolation difficult) ©1  Other drugs which act on bacterial cell wall biosynthesis D-Cycloserine HO OH OH O O D-Ala-D-Ala H Ligase N O Racemase O H 3C NH2 X H 3C NH 2 X NH2 L-Alanine D-Alanine D-Alanine-D-Alanine D-4-amino-3-isoxazolidone Separated from Streptomyces garyphalu Acts at the cytoplasm It mimics the structure of D-Ala➔ Prevent formation of D-Ala-D-Ala Inhibits the enzyme L-Alanine Racemase and the D-Ala-D-Ala Ligase. (enzymes important in the cytosolic stages of peptidoglycan synthesis) responsible for linking the two d-alanine units together) ©1 Used in the treatment of tuberculosis Mechanism of action - bacterial cell wall synthesis NAM NAG NAM NAG SUGAR BACKBONE L-Ala L-Ala D-Glu D-Glu L-Lys Gly Gly Gly Gly Gly L-Lys Gly Gly Gly Gly Gly D-Ala D-Ala D-Ala D-Ala PENICILLIN TRANSPEPTIDASE D-Al ani ne NAM NAG NAM NAG SUGAR BACKBONE L-Ala L-Ala D-Glu D-Glu L-Lys Gly Gly Gly Gly Gly L-Lys Gly Gly Gly Gly Gly D-Ala D-Ala Cross linking ©1  Glycopeptides: VANCOMYCIN FAMILY Isolated Streptomyces orientalis Used in the prophylaxis and treatment of infections caused by Gram- positive bacteria (narrow-spectrum bactericidal glycopeptide)© 1  Crystal structure James R. Knox and R. F. Pratt in Antimicrobial Agents and Chemotherapy, Year 1990, Volume 34, Issue 7, Pages 1342-1347. The short peptide (L-Lysyl-D-Alanyl-D-Alanine), which is a bacterial cell wall precursor, bound to Vancomycin through five hydrogen bonds indicated by the dotted lines. ©1  VANCOMYCIN: Structure and Mechanism of Action Because vancomycin is a large molecule, it caps the tails and acts as a steric shield, blocking access to the transglycosidase and transpeptidase enzymes. Because vancomycin is such a large molecule, it is unable to cross the outer cell membrane of Gram -ve bacteria, lacks activity against those organisms. It is also unable to cross the inner cell membrane of Gram +ve bacteria, but this is not required as the construction of the cell wall takes place outside the cell membrane ©1  Capping Mechanism The transfer of a sugar residue from one glycoside to another (bond formation, particularly during polysaccharide synthesis) Transglycosylation ©1 Dimerization of VANCOMYCIN 5 H-Bonds 4 H-Bonds 5 H-Bonds ©1  VANOMYCIN: Biosynthesis Reactions involved in the biosynthesis of vancomycin. Vancomycin is derived biosynthetically from a linear heptapeptide containing five aromatic residues. These undergo oxidative coupling with each other to produce three cyclic moieties within the structure. Chlorination, hydroxylation, and the final addition of two sugar complete the structure 1 © units then  Resistance to VANCOMYCIN Modification of the pentapeptide chain leading to resistance Resistance has arisen from a modification of the cell wall precursors where the terminal d-alanine group in the pentapeptide chain has been replaced by d-lactic acid, resulting in a terminal ester link rather than an amide link. This removes one of the NH groups involved in the hydrogen bonding interaction with vancomycin. Vancomycin does not bind to D-Ala-D-Lac, which leads to vancomycin resistance ©1  Teicoplanins: Mixture of 5 major constituents and four minor (named Teicoplanin RS-1 through RS-5) β-D-glucosamine The teicoplanins belong to the vancomycin ©1 family but do not dimarize  Teicoplanin-A2-5 Its mechanism of action is to inhibit bacterial cell wall synthesis. Teicoplanin is marketed by Aventis under the trade name Targocid®. Teicoplanin A2-5 is used in the prophylaxis and treatment of serious infections caused by Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and Enterococcus faecalis. ©1  Binding Mode of Teicoplanin Teicoplanins belong to the vancomycin family but do not dimarize. The long alkyl chain plays an important role is anchoring the antibiotic to the outer surface of the cell membrane where it is perfectly placed to interact with the building blocks for cell wall synthesis. Teicoplanin is used clinically for the treatment of Gram-positive infections and is less toxic than vancomycin. ©1 Eremomycin and LY33328 Semisynthetic glycopeptides New Glycopeptide Antibiotics LY33328 is 1000 times > active than vancomycin ©1  Bactericidal lipoglycopeptide for use in MRSA or other Gram-positive infections. Telavancin is a semi-synthetic derivative of vancomycin Semisynthetic glycopeptides Removal of a tetrahydropyran ring to leave an alcohol group (R4), modification of the hydrophobic tail (R2) and addition of a side chain with a phosphate group (R3), to give Telavancin , which was approved in 2009. ©1  Simplification of Natural Products Simplified analogues of the glycopeptides This lead compound: Capable of binding to D-Ala-D-Ala and D-Ala-D-Lac The complexity of the glycopeptides is an advantage in their targeting and selectivity, it is a problem when it comes to synthesizing analogues. Therefore, work has been carried out to try and prepare simplified analogues of vancomycin which are easier to synthesize, yet retain the desired selectivity. Structures such as those have been prepared which are capable of binding to d-Ala -d-Ala and d-Ala-d-Lac These now represent lead compounds for the development of future antibacterial agents ©1  There are another two mechanisms by which glycopeptides may have an antibacterial activity Firstly, it is possible that glycopeptide dimers disrupt the cell membrane structure. This is supported by the fact that glycopeptide antibacterial agents enhance the activity of aminoglycosides by increasing their absorption through the cell membrane. Secondly, RNA synthesis is known to be disrupted in the presence of glycopeptides. The possibility of three different mechanisms of action explains why bacteria are slow to acquire resistance to the glycopeptides. ©1  Bacitracin Bacitracin is Cyclic polypeptide complex produced Bacillus subtilis. Isolated in 1943 from a knee scrape from a girl named Margaret Tracy Binds to the lipid carrier responsible for transporting the NAM/pentapeptide unit across the cell membrane, thus preventing it from carrying out that role. Bacitracin interferes with the dephosphorylation of the C55- isoprenyl pyrophosphate, a molecule which carries the building blocks of the peptidoglycan bacterial cell wall outside of the inner membrane ©1 ©1  Summary β-Lactamase inhibitors are β-lactam structures that have negligible antibacterial activity but inhibit β-lactamases. They can be administered alongside penicillins to protect them from β-lactamases and to broaden their spectrum of activity. Carbapenems and monobactams are examples of other β-lactam structures with clinically useful antibacterial activity. Glycopeptides, such as vancomycin, bind to the building blocks for cell wall synthesis, preventing their incorporation into the cell wall. They also block the cross-linking reaction for those units already incorporated in the wall. The glycopeptides are the drugs of last resort against drug-resistant strains of bacteria. Bacitracin binds to and inhibits the carrier lipid responsible for carrying the cell wall components across the cell membrane. Cycloserine inhibits the synthesis of D-Ala-D-Ala. ©1  Patrick An Introduction to Medicinal Chemistry 3/e Chapter 19 ANTIBACTERIAL AGENTS  Valinomycin and Gramicidin A (Ionophores) Act as ion carrier The peptides valinomycin and gramicidin A both act as ion- conducting antibiotics (ionophores) and allow the uncontrolled movement of ions across the cell membrane. Ionophore = a substance that is able to transport particular ions across a lipid membrane in a cell.  Valinomycin is a cyclic structure obtained from Streptomyces fermentation It contains three molecules of L-valine, three molecules of D-valine, three molecules of L-lactic acid, and three molecules of D- hydroxyisovalerate. These four components are linked in an ordered fashion such that there is an alternating sequence of ester and amide linking bonds around the cyclic structure. This is achieved by the presence of a lactic or hydroxyisovaleric acid unit between each of the six valine units. Further ordering can be observed by noting that the L and D portions of valine alternate around the cycle, as do the lactate and hydroxyisovalerate units.  Valinomycin acts as an ion carrier and could be looked upon as an inverted detergent. As it is cyclic, it forms a doughnut-type structure where the polar carbonyl oxygens of the ester and amide groups face inwards, while the hydrophobic side chains of the valine and hydroxyisovalerate units point outwards. This is clearly favoured because the hydrophobic side chains can interact via van der Waals interactions with the fatty lipid interior of the cell membrane, while the polar hydrophilic groups are clustered together in the centre of the doughnut to produce a hydrophilic environment This hydrophilic centre is large enough to accommodate an ion and it is found that a ‘naked’ potassium ion (i.e. one with no surrounding water molecules) fits the space and is complexed by the amide carboxyl groups  Valinomycin can, therefore, collect a potassium ion from the inner surface of the membrane, carry it across the membrane and deposit it outside the cell, thus disrupting the ionic equilibrium of the cell. Normally, cells contain a high concentration of potassium ions and a low concentration of sodium ions. The fatty cell membrane prevents passage of ions between the cell and its environment, and ions can only pass through the cell membrane aided by specialized and controlled ion transport systems Valinomycin introduces an uncontrolled ion transport system which proves fatal. Valinomycin is specific for potassium ions over sodium ions The real reason is that sodium ions do not lose their surrounding water molecules very easily and would have to be transported as the hydrated ion. As such, they are too big for the central cavity of valinomycin.  Ionophore Mechanism of action Valinomycin disrupts the ionic equilibrium of a cell Ionophores act on the plasma membrane and result in the uncontrolled movement of ions across the cell membrane, leading to cell death.  Gramicidin 15 Amino Acids: coil in helix: Hydrophobic-out and hydrophilic-in Gramicidin is a heterogeneous mixture of six antibiotic compounds, Gramicidins A, B and C, making up 80%, 6%, and Val = 4 14% respectively Leu = 4 Trp = 4 Ala = 2 Gly = 1  More Ionophores: used in veterinary medicine The ionophores nigericin , monensin A , and lasalocid Function in much the same way as valinomycin and are used in veterinary medicine to control the levels of bacteria  Polymixin B Polypeptide From soil bacteria (Bacillus polymyxa) DAB: α,γ-diaminobutyric acid 1. Can differentiate between different cellular plasma membranes ➔ Selectivity It causes the leakage of SMALL MOLECULES (nucleoside) from the cell rather than ions (valinomycin) 1. Injected intramuscular 2. Active against Pseudomonas strains that are resistant to antibacterial agents 3. Can be used topically 4. Has good activity against G –Ve primarily used for resistant 5. Less effective against G +Ve due to gram negative infections penetration problem (large MW)  Polymyxin B Mechanism of Action 1. Positively charged amino groups in the cyclic peptide portion binds to a negatively charged site in the lipopolysaccharide layer (an electrostatic attraction) ➔ Alters cytoplasmic membrane permeability 1. Fatty acid portion dissolves in hydrophobic region of membrane and disrupts membrane integrity 2. Leakage of cellular molecules, inhibition of cellular respiration. 3. Binds and inactivates endotoxin (Lipopolysaccharides) 4. Relative absence of selective toxicity: nonspecific for cell membranes of any type ➔ highly toxic Polymyxin B operates selectively on the plasma membrane of bacteria and causes the uncontrolled movement of small molecules across the membrane.  Neosporin: Bacitracin, Neomycin, and Polymyxin B Antibiotic ointment used in the prevention of infection and speeding the healing of wounds Neomycin is an aminoglycoside antibiotic  Killer Nanotubes Cyclic peptides with an even number of alternating D,L-α- amino acid residues are known to self-assemble into organic nanotubes Cyclic peptides with self-assemble properties Killer Nanotubes Self assembly of ‘killer nanotubes  Killer Nanotubes ➔ increase membrane permeability, collapse transmembrane ion potentials, and cause rapid cell death The nanotubes would allow molecules to leach out from the cell and cause cell death.  Killer Nanotubes 1. Stacking via hydrogen bonds (HB) 2. Hydrophobic R-groups in exterior 3. R-group modified for selectivity (mammalian vs. bacterial) 4. Polar NH/CO in core 5. Work in progress  Cyclic Lipopeptides New class of antibiotics 1. Daptomycin: derived from Streptomyces roseosporus 2. Disturbs cellular membrane functions 3. Used in treating skin infection 4. Active against MRSA nanotube Decanoic chain  Daptomycin Asp-X-Asp-Gly motif The lipid portion of the molecule is derived from decanoic acid and the yield of product obtained is increased if decanoic acid is added to the fermentation medium Cyclic peptides are being designed which will self-assemble to form nanotubes in the cell membranes of bacteria Cyclic lipopeptides are a new class of antibiotic More Cyclic Lipopeptides Antibacterial Agents that Impair Protein Synthesis (Translation) Stages at which antibacterial agents inhibit translation  Aminoglycosides: Streptomycin & Gentamycin C1a Streptomycin (1944): From Soil bacteria (Streptomyces griseus) first of a class of drugs called aminoglycosides and was the first antibiotic remedy for tuberculosis. 2-deoxystreptamine (2-DOS) 1. Basic Amine: At pH 7.4 the amine is +ve charge 2. The +ve charge help them to penetrate the outer cell wall of Gram -ve. They bind the 30S subunit and prevent the movement of the Ribosome along the mRNA Inhibition of protein biosynthesis by Aminoglycosides  Properties of Aminoglycosides Fast acting BUT might cause some ear and kidney problems. Effective in treating infections caused by aerobic Gram –Ve bacteria. The only compounds active against Pseudomonas aeruginosa Because of high polarity they have to be injected. Not able to cross the BBB  Streptomycin Structure-activity Relationship Reduction of the aldehyde (of α-streptose) to the alcohol results in a compound, dihydrostreptomycin, which has activity similar to STM but with a greater potential for producing delayed severe deafness (major side effect). Oxidation of the aldehyde to a carboxyl group or conversion to Schiff’s base derivatives (oxime, semicarbazone, or phenylhydrazone) results in inactive analogs. Oxidation of the methyl group in α-streptose to a methylene hydroxyl (CH2OH) gives an active analog but with no advantage over STM.  Streptomycin Structure-activity Relationship Modification of the aminomethyl group in the glucosamine portion of the molecule by demethylation or by replacement with larger alkyl groups reduces activity. Removal or modification of either guanidine in the streptidine nucleus results in decreased activity.  Streptomycin Metabolism More Aminoglycosides 1949  General structures of Kanamycins and ring I substituents Glucopyranosyl SAR of Gentamicin  Tetracyclines and Chloramphenicol Chloramphenicol: Tetracyclines: Binds to the 50S subunit and inhibit Inhibit protein synthesis by the movement of the ribosome along binding to 30S subunit and the mRNA, probably by inhibiting the preventing aminoacyl-tRNA peptidyl transferase reaction by which from binding the peptide chain is extended.  Tetracyclines are bacteriostatic antibiotics which have a broad spectrum of activity and are the most widely prescribed form of antibiotic after penicillins. They are also capable of attacking the malarial parasite Tetracyclines cross the outer membrane of gram –ve bacteria by passive diffusion through the porins Commonly used tetracyclines in the clinic are tetracycline, demeclocycline , doxycycline , lymecycline , minocycline , and Oxytetracycline The use of chlortetracycline has decreased over the years because it kills the intestinal flora that produce vitamin K  R5 R4 R3 R2 R1. Chlortetracycline H H OH CH3 Cl Oxytetracycline H OH OH CH3 H Tetracycline H H OH CH3 H Demethylchlortetracycline H H OR H CI Rolitetracycline + H OH CH3 H Metacycline H OH CH2 H Doxycycline H OH H CH3 H Minocycline H H H N(CH3)2 32  Retention of the configuration of the asymmetric centres C-4, C- 4a and C-12a is essential (beta), whereas the configurations at C- 5, C-5a and C-6 may be altered: The amide hydrogen may be replaced with a methyl group, but larger groups have a deleterious effect except for those which are eliminated spontaneously in water The dimethyl amino group may be replaced by a primary amino group without loss of in vitro activity but all other changes so far lead to decreased bacteriostatic action 33  The hydrophobic part of the molecule from C-5 to C-9 may be altered in various ways: Modifications at C-6 and C-7 in particular afford products having greater chemical stability. Increased antibiotic activity and more favourable pharmacokinetics Dehydrogenation to form a double bond between C-5a and C-11a markedly decreases activity Polar substituents at C-5 and C-6 contribute decreased lipid versus water solubility to the tetracycline 34  Side effect: Dermatological:-Skin reactions, photosensitivity GIT:-nausea, vomiting, and diarrhea. CNS:-Dizziness,visual disturbances. Immune System:-allergic reactions. Other:-yellowish-grayish-brown discoloration of the teeth. 35  Drug interaction of tetracyclines Antacids containing aluminum, Impaire the Absorption of tetracyclines calcium, or magnesium, and iron-containing preparations anticoagulant therapy Because tetracyclines have been shown to depress plasma prothrombin activity, patients who are on anticoagulant therapy may require downward adjustment of their anticoagulant dosage. bacteriostatic drugs interfere with the bactericidal action of penicillin, it is advisable to avoid giving tetracycline-class drugs in conjunction with penicillin. Oral contraceptives Concurrent use of tetracyclines with oral contraceptives may render/ make oral contraceptives less effective. 36  Drug interaction of tetracyclines Bile acid sequestrants May decrease tetracycline absorption Iron preparations May decrease absorption of tetracyclines Methoxyflurane when concurrent with tetracycline) may cause fatal nephrotoxicity; concurrent use is contraindicated. Methotrexate Clearance of methotrexate (high- dose therapy) may be decreased by tetracyclines. Ergot alkaloids or their Increased risk of ergotism derivatives are given with 37 tetracyclines.  Chloramphenicol is now prepared synthetically and has two asymmetric centres. Only the R,R -isomer is active. Chloramphenicol binds to the 50S subunit of ribosomes and appears to act by inhibiting the movement of ribosomes along mRNA, probably by inhibiting the peptidyl transferase reaction by which the peptide chain is extended The nitro group and both alcohol groups are involved in binding interactions. The dichloroacetamide group is also important, but can be replaced by other electronegative groups. Chloramphenicol is quite toxic and the nitro substituent is thought to be responsible for this Chloramphenico is responsible for Gray Baby Syndrome  Gray baby syndrome (also termed Gray or Grey syndrome) is a rare but serious side effect that occurs in newborn infants (especially premature babies) following the accumulation of antibiotic chloramphenicol. Toxic levels of chloramphenicol after 2–9 days result in: ✓ Vomiting ✓ Ashen gray color of the skin ✓ Loss of appetite ✓ Cyanosis (blue discolouration of lips and skin) ✓ Hypotension ✓ Abdominal distension ‫انتفاخ في البطن‬ ✓ Hypothermia ✓ Irregular respiration ✓ Cardiovascular collapse ✓ Hypotonia (low muscle tone) ✓ Increased blood lactate ❖ Two pathophysiologic mechanisms are thought to play a role in the development of gray baby syndrome after exposure to the anti-microbial drug chloramphenicol. This condition is due to a lack of glucuronidation reactions occurring in the baby, thus leading to an accumulation of toxic chloramphenicol metabolites: 1. The UDP-glucuronyl transferase enzyme system of infants, especially premature infants, is immature and incapable of metabolizing the excessive drug load. 2. Insufficient renal excretion of the unconjugated drug. Lincosamides Lincosamides consist of a pyrrolidine ring linked to a pyranose moiety (methylthio- lincosamide) via an amide bond Lincosamides are a class of antibiotics, which include lincomycin, clindamycin, and pirlimycin Lincosamides prevent bacterial replication in a bacteriostatic mechanism by interfering with the synthesis of proteins (50S subunit ). Lincosamides Have similar antibacterial properties to macrolides Resistance: Ribosomal methylation Target mutation Antibiotic efflux Drug modification  Macrolides Macrolides are bacteriostatic agents Erythromycins 14-membered macrocyclic lactone Sugars and amino-sugars attached Bind to 50S subunit of the bacterial ribosome Inhibiting translocation!!  Mechanisms of Resistance 1. Methylation of the ribosomal target of the antibiotics. 2. Antibiotic Efflux. 3. Drug Modification.  Acid Instability of Erythromycin Erythromycin is unstable to stomach acids, but can be taken orally in a tablet form. The formulation of the tablet involves a coating Intra-molecular ketal formation in Erythromycin The acid sensitivity is due to the presence of a ketone and two alcohol groups which are set up for the acid catalysed intramolecular formation of a ketal One way to stabilize would be by: 1. Methylation (protection of OH groups e.g. clarithromycin) 2. Increasing the size to 16 –membered ring  Erythromycin 1. Erythromycin and Chloramphenicol bind to the same region of the ribosome ➔ they should not be administered together (ineffective). 2. Erythromycin is used against penicillin-resistant staphylococci. 3. Best described against the legionaries’ disease (atypical pneumonia), diphtheria (inflammation of the mucous membranes), acne. 4. The acid instability of Erythromycin is solved by coated tablet.  Clarithromycin 1. More stable analogue of Erythromycin. 2. Improved oral absorption 3. Used in treatment of ulcers caused by H. Pylori Resistance to Macrolides may be due to: 1) Efficient Efflux (pump the drug back out the cell) 2) Change in the binding site at the ribosome. Methylation of the ribosomal target of the antibiotics (binding is weakened) 3) Modification on the Macrolides by various enzymes  N N N N O HO N HO HO O HO O O N O N O HO O O MeO O O O O O OH O O Increased stability acidic media Increased stability acidic media No intramolec. hemikatalisation No intramolec. hemikatalisation No intramolecular hemiketalizaion Improved ribosome binding, less resistans Increased ribosome affinity Azithromycin Improved ribosome binding contains a 15-membered macrocycle Less Resistance where an N -methyl group has been Increased ribosome affinity incorporated into the macrocycle. It is one of the world’s best-selling drugs. Telithromycin ❑ Telithromycin is a semi-synthetic derivative of erythromycin and reached the European market in 2001. ❑ The cladinose sugar in erythromycin has been replaced with a keto-group and a carbamate ring has been fused to the macrocyclic ring. ❑ The two hydroxyl groups that cause the intramolecular ketal formation in erythromycin have been masked, one as a methoxy group and the other as part of the carbamate ring.  Streptogramins 1. Bind to different regions of 50S 2. The binding of Dalfopristin increases the affinity for Quinopristin. 3. Quinopristin inhibits the peptide chain elongation. Quinopristin 4. Dalfopristin interferes with the transfer of the peptide chain from tRNA to the other. Gram-positive bacteria: Streptococcus pyogenes, Viridans group streptococci, Dalfopristin Streptococcus pneumoniae, Staphylococcus Macrolactone aureus, Some enterococci and especially structures MRSA Parenteral Streptogramins ▪ Pritinamycin is a mixture of macrolactone structures obtained from Streptomyces pristinaespiralis. ▪ Two of the components ( quinupristin and dalfopristin ) have been isolated. ▪ These agents bind to different regions of the bacterial ribosome’s 50S subunit form a complex. ▪ It is found that binding of dalfopristin increases the binding affinity for quinupristin, and so the two agents acting synergy with each other. ▪ Quinupristin inhibits peptide chain elongation, while dalfopristin interferes with the transfer of the peptide chain from one tRNA to the next. ▪ Quinupristin and dalfopristin are protein synthesis inhibitors in a synergistic manner. While each of the two is only a bacteriostatic agent, the combination shows bactericidal activity. Quinupristin Dalfopristin  Oxazolidinones 1. Broad Spectrum 2. Active against O strains that acquired O resistance to other O N N S NH antibacterial agents. H O F 3. They bind at much Morpholine Oxazolidin-2-one earlier stage in the The structure of Linezolide protein synthesis. Both oral and parenteral They bind to 50S subunit and prevent the formation of ribosome complex Peptidyl transferase center (PTC) Oxazolidinones The oxazolidinones are a new class of synthetic antibacterial agents. They inhibit protein synthesis at a much earlier stage than previous agents, and, consequently, do not suffer the same resistance problems Before protein synthesis can start, a 70S ribosome has to be formed by the combination of a 30S ribosome with a 50S ribosome. The oxazolidinones bind to the 50S ribosome and prevent this from happening. As a result, translation cannot even start. Other agents that inhibit protein synthesis do so during the translation process itself Linezolid was the first of this class of compounds to reach the market in 2000, and by 2010, it was netting sales of £716 million per year  The oxazolidinones have a broad spectrum of activity and are active against bacterial strains which have acquired resistance to other antibacterial agents acting against protein synthesis Linezolid has good activity against most clinically important Gram- positive bacteria, including MRSA. It can also be taken orally with 100% uptake from the gastrointestinal tract Unfortunately, there is a high level of side effects related to its use and, as it is a bacteriostatic agent, there is a greater risk of bacterial resistance developing Radezolid is one such structure which binds 10,000 times more strongly as a result of extra binding interactions (extension the structure)  Other oxazolidinones  Thiopeptide Antibiotics: Thiostrepton Thiopeptides are sulfur-rich macrocyclic peptides containing highly-modified amino acids They have antibiotic activity against Gram-positive bacteria, but little or no activity against Gram-negative bacteria They are characterized by a nitrogen-containing six-membered ring (such as piperidine, dehydropiperidine, or pyridine) substituted with multiple thiazole rings and dehydroamino acids  Agents Acting on Nucleic Acid Transcription Quinolones and Fluoroquinolones 1. They inhibit the replication and transcription of bacterial DNA by stabilizing the complex formed between DNA and topoisomerases 2. By forming a ternary complex (Drug-Enzyme-DNA)  Nalidixic Acid O O OH N N First to be discovered-1962 Active against G –ve Short-term UTI Bacteria can develop quickly resistance.  Enoxacin form a zwitterion with the 7-Piperazyl Improvements : carboxylic acid group at position 3 1. Oral absorption O O 2. Tissue distribution F 6 3 OH 3. Metabolic stability 7 8 12 4. Level of activity HN N N N 5. Spectrum of activity (G-ve and P. aeruginosa) 1980s 1. Wider spectrum of activity (G+ve and G-ve) 2. Active against highly resistant P. aeruginosa 3. 6-Flourine: – Increased the activity – Increased the cellular Uptake  Ciprofloxacin O O F 6 3 OH 7 8 12 HN N N Cyclopropy substituent: Replacement Nitrogen at position 8 – Increase the spectrum of with carbon ➔ Reduced side effects activity (S. Aureus) 1. Highly potent against G-ve 2. Used in treatment of wide range of O O infections: urinary, respiratory, GI F 6 3 OH tract, skin, joints. 7 8 12 3. MOST ACTIVE BRAOD HN N N N SPECTRUM ANTIBIOTIC IN THE MARKET Enoxacin  Bacterial Topoisomerase IV is inhibited Forming a ternary complex (Drug-Enzyme-DNA) Topoisomerase IV is predominantly responsible for separation of daughter DNA strands during cell division  1ST and 2nd generation Fluoroquinolone limitations 1. The moderate activity against S. aureus. 2. The quick development of resistance 3. Only marginal activity against anaerobic Streptococcus pneumoniae. Third- and fourth-generation fluoroquinolones, such as ofloxacin, levofloxacin , moxifloxacin, and besifloxacin began to be developed in the early 1990s to tackle these issues  Ofloxacin has an asymmetric centre and is sold as a racemic mixture of both enantiomers, one of which is active and one of which is not. Levofloxacin is the active enantiomer of oflaxacin and is twice as active as the racemate. Ofloxacin 3rd Generation: Trovafloxacin and others 3-azabicyclo[3.1.0]hexyl substituent at the C-7 position, Developed by the 1990s All of them are with improved activity against Streptococcus pneumoniae Fluoroquinolone  Rifamycin Antibiotics The rifamycins are natural products produced by Streptomyces mediterranei. This chemical class is an aliphatic chain forming a bridge between two nonadjacent positions of an aromatic moiety. Semisynthetic derivatives are prepared via conversion of the natural rifamycins to 3-formylrifamycin which is derivatized with various hydrazines to give products such as rifampin and rifapentine. Rifampin and rifapentine have significant benefit over previously investigated rifamycins in that they are orally active, highly effective against a variety of gram-positive and gram-negative organisms, and have high clinical efficacy in the oral treatment of tuberculosis. Rifamycin Antibiotics Semisynthetic derivatives  Rifamycin Antibiotics Mechanism of Action: The rifamycins inhibit bacterial DNA-dependent RNA polymerase (DDRP) by binding to the β-subunit of the enzyme leads to a blocking of the initiation of chain formation in RNA synthesis. Rifamycins are highly active against rapidly dividing intracellular and extracellular bacilli. Rifampin is active against DDRP from both gram-positive and gram- negative bacteria but due to poor penetration of the cell wall of gram- negative organisms by rifampin, the drug has less value in infections caused by such organisms. Structure-activity Relationship: o Free OH groups are required at C-1, 8, 21 and 23 as they are important binding groups for attachment to DDRP. o Acetylation of C-21 and C-23 produces inactive compounds. o Reduction of the double bonds in the macro ring results in a progressive decrease in activity. Structure-activity Relationship: o Opening of the macro ring gives inactive compounds. These latter two changes greatly affect the conformational structure of the rifamycins which in turn decreases binding to DDRP. o Substitution at C-3 or C-4 results in compounds with varying degrees of antibacterial activity. Metabolism: Rifampin and rifapentine are readily absorbed from the intestine although food in the tract may affect absorption. The major metabolism of rifampin and rifapentine is deacetylation which occurs at the C-25 acetate to give desacetylrifampin and desacetylrifapentine, which are still active antibacterial agents. Therapeutic Application: Rifampin (Rifadin, Rimactane) is always used in combination with one or more other antitubercular agents. The drug is potentially hepatotoxic and may produce gastrointestinal disturbances, rash and thrombocytopenic purpura (low levels of platelets that prevents bleeding). Rifampin is known to induce CYP3A4 and CYP2C isoforms and may decrease the effectiveness of oral contraceptives, corticosteroids, Warfarin, quinidine, methadone, zidovudine, clarithromycin, and the azole antifungal agents. Rifapentine is introduced for the treatment of pulmonary tuberculosis and has major advantage over rifampin is the fact that when used in combination therapy rifapentine can be orally administered twice weekly during the "intense" phase of therapy followed by once a week during the "continuous" phase of therapy. In contrast, rifampin is normally administered daily during the "intense" phase of therapy followed by twice a week dosing during the "continuous" phase of therapy. Summary Aminoglycosides, tetracyclines, chloramphenicol, streptogramins, lincosamides, and macrolides inhibit protein synthesis by binding to the bacterial ribosomes involved in the translation process. Resistance can arise from a variety of mechanisms, such as drug efflux, altered binding affinity of the ribosome, altered membrane permeability, and metabolic reactions. Oxazolidinones prevent the formation of the 70S ribosome by binding to the 50S subunit. Quinolones and fluoroquinolones inhibit topoisomerase enzymes, resulting in inhibition of replication and transcription. Rifamycins inhibit the enzyme RNA polymerase and prevent RNA synthesis. In turn, this prevents protein synthesis. Rifampicin is used to treat tuberculosis and staphylococcus infections. Fidaxomicin is a macrocycle which also targets RNA polymerase. 73  Naturally occurring 20-carbon fatty acid derivative Produced in mammalian tissues Unsaturated fatty acids There level is usually very low They are Eicosanoids (Molecules made by enzymatic oxidation of arachidonic acid or other poly unsaturated FA, 20 carbon units in length ) Arachidonic acid 1  EICOSANOIDS Eicosanoids are compounds derived from eicosa- (20-C) polyenoic fatty acids. They include: 1. Prostaglandins (PG) 2. Prostacyclins (PGI) 3. Thromboxanes (TX) 4. Leukotrienes (LT) and 5. Lipoxins. The term “prostaglandins” is often used loosely to include all eicosanoids 2 (Local hormones, key mediators of inflammation, pain, and spasm) 3 Arachidonic acid  Fatty acid Cyclobentane 7 1 9 5 3 8 6 4 2 COOH 10 14 16 18 20 12 13 15 17 19 11 A 20-C cyclopentano-fatty acid Contain carboxylic group (-COOH) positioned as 1 A 5-membered cyclopentane ring C8→C12 A 2-side chains connected to the cyclopentane ring (α and β) and in trans stereochemistry A 15-α-hydroxy group (OH), other oxygen containing groups may present 4 Trans-double bond at C13, other double bonds may present  Importance Of Prostaglandins The prostaglandins, thromboxanes and leukotrienes are products of arachidonic acid metabolism. Many of these substances are involved in important physiologic functions sometimes in a hormone like manner, some of them we do not know its role. Prostaglandins have different 5 pharmachological and biological functions.  Importance Of Prostaglandins Two general physiologic actions for prostaglandins could be defined: - 1. They are potent smooth muscle agonists i.e cause contraction or relaxation of smooth muscles 2. They have a control action on adenohypophyseal trophic hormones or on cells with b-adrenergic receptors for the catecholamines. 6  Functions of PGs: some physiological and others pathological,. Some PGs increase, others decrease, blood pressure. Some PGs contract, other relax, smooth muscles. Some PGs like PGE and PGF cause pain, fever, and inflammation. PGs reduce gastric secretions and regulate kidney function. Clinical applications Drugs are now available that contain PG-derivatives to: 1. Induce labor or abortion 2. Treat gastric ulcers 3. Treat impotence in men (viagra-like). These drugs are used as injections or topically (gels, pessaries). 7 8  Classified into groups by using capital letters A, B, C, D, E, F, G, H, and I. The classification is based on: – The structure of cyclopentane ring – Presence or absence of the double bond – Stereo-chemistry of the oxygen-substituents at C9 and C11 Each group is sub-classified by using subscripts 1, 2, and 3. – Subscripts depends on the number and positions of unsaturation (double bonds) on with side chains 9  Examples – Classes – PGE: Keto function at C9 α-OH at C11 – PGF α-OH at C9 α-OH at C11 Examples – Subscripts – Subscript 1: only one trans double bond at C-13 – Subscript 2: additional cis double bond at C-5 – Subscript 3: a third cis double bond at C-17 – Subscript α: the 9-OH is present at α. Below the plan of the10ring  1 8 COOH 13 15 Prostanoic acid One ketone group at position 9 → Group E An OH group at position 11 → Group E One double bond at position 13 → subclass 1 O COOH HO OH 11, 15-dihydroxy, 9-oxo, 13-trans-prostenoic acid 11  1 8 COOH 13 15 Prostanoic acid One ketone group at position 9 → Group E An OH group at position 11 → Group E One double bond at position 13 → subclass 1 A cis double bond at position 5 → subclass 2 O COOH HO OH 11, 15-dihydroxy, 9-oxo, 5-cis,13-trans-prostadienoic acid 12  1 8 COOH 13 15 Prostanoic acid One OH group at position 9 → Group F An OH group at position 11 → Group F One double bond at position 13 → subclass 1 HO COOH HO OH 9,11, 15-trihydroxy, 13-trans-prostenoic acid 13  1 8 COOH 13 15 Prostanoic acid An OH group at position 9 → Group F An OH group at position 11 → Group F One double bond at position 13 → subclass 1 A cis double bond at position 5 → subclass 2 HO COOH HO OH 9,11, 15-trihydroxy, 5-cis,13-trans-prostadienoic acid 14  1 8 COOH 13 15 Prostanoic acid An OH group at position 9 → Group F An OH group at position 11 → Group F One double bond at position 13 → subclass 1 Two cis double bonds at 5, 17 → subclass 2 HO COOH HO OH 9,11, 15-trihydroxy, 5,17-cis,13-trans-prostatrienoic acid 15  1 8 COOH 13 15 Prostanoic acid HO COOH O COOH HO OH HO HO COOH OH HO O OH HO COOH COOH HO OH HO OH 16  O COOH COX + 2O2 COOH O O OH O COOH O OH HO O COOH COOH HO HO OH OH 17  O O 15-OH-PG COOH dehydrogenase COOH HO OH HO O PG reducatse O Β-oxidative O COOH cleavage COOH HO O HO O ω-oxidation O Metabolic pathways COOH 1. Alcoholic oxidation at C15 2. Reduction of double bond at C13 COOH 3. Oxidative cleavage at β-position HO O 4. ω-oxidation of C20 18  Oxytocic effect on uterus → induce labor – PGE2 – PGF2α Regulate heart rate, pressure and clotting – PGI2 → prevent heart attack and stroke Inflammatory process Inhibit gastric acid secretion → peptic ulcer Affect bronchi – PGE → dilation – PGF → constriction 19 1. Dinoprostone (Prostin E2) 2. Dinoprost tromethamine (Prostin F2α) 3. Carboprost tromethamine (Prostin 15/M) 4. Mesoprostol (cytotec) 20 1. Dinoprostone (Prostin E2) O COOH HO OH 1. It is PGE2 2. Has contractile effect of uterus 3. Used as abortifacient ‫إجهاض‬ 21  2. Dinoprost tromethamine (Prostin F2α) CH2OH H2N C CH2OH CH2OH 2-Amino-2-(hydroxymethyl)propane-1,3-diol Tromethamine 1. It is PGF2 formulated as salt Tris, or tris(hydroxymethyl)aminomethane 2. Suitable for IV infusion (usually known as THAM ) is used as 3. Has contractile effect of uterus alternative to sodium bicarbonate in the 4. Used as abortifacient treatment of metabolic acidosis 22 3. Crboprost tromethamine (Prostin 16/M) HO CH2OH COOH H2N C CH2OH HO HO CH3 CH2OH 1. It is PGF2α formulated as salt 2. The CH3 at C-15 limit its metabolism (tert alcohol is more resistant to metabolism than secondary and primary). 3. Suitable for IV infusion 4. Has contractile effect of uterus 5. Used as abortifacient 23 4. Mesoprostol (Cytotech) Cytotech (Misoprostol) Misoprostol was rapidly de-esterified to yield its free acid; Misoprostol acid (Active) Misoprostol acid 1. It is PGE1 2. Pro-drug: Rapidly de-esterified to misoprostol acid, the biologically active metabolite. The de-esterified metabolite undergoes further oxidation in several body tissues. 3. OH group moved to C-16 4. CH3 added to C-16 5. Has contractile effect of uterus → Used as abortifacient 6. Mainly used for peptic ulcer 24 ❖NSADs are irreversible inhibitors of cyclooxygenase activity, 1 thus they prevent the formation of prostaglandins 2 3  Inhibition of COX-2 by NSAIDs accounts for the anti-inflammatory effects. Inhibition of COX-1 leads to NSAID toxicity and side effects (ulcers, prolonged bleeding time, kidney problems). 4 Classification of NSAIDs A. Non-selective COX inhibitors (traditional NSAIDs) 1. Salicylates: Aspirin, Diflunisal 2. Propionic acid derivatives (Profens): Ibuprofen, Naproxen, Ketoprofen, Flurbiprofen, Fenoprofen, Oxaprozin. 3. Anthranilic acid derivative (Fenamates): Mephenamic acid, Meclofenamic acid 4. Aryl-acetic acid derivatives: Diclofenac, Aceclofenac, Tolmetin, Etodolac 5. Oxicam derivatives (“Enol Acids”): Piroxicam, Tenoxicam, Meloxicam. 6. Pyrrolo-pyrrole derivative: Ketorolac 7. Indole derivative: Indomethacin, Sulindac. 8. Pyrazolone derivative: phenylbutazone, Oxyphenbutazone B. Preferential COX-2 inhibitors: Nimesulide, Meloxicam, Nabumeton. C. Selective COX-2 inhibitors (Coxib): Celecoxib, Etoricoxib, Parecoxib. D. Analgesic-antipyratics with poor antiinflammatory action 1. Para aminophenol derivatives: Paracetamol 2. Pyrazolone derivative: Metamizol, Propiphenazone. 5 3. Benzoxazocine derivative: Nefopam 6 General Structure and Properties of the NSAIDs In general, NSAIDs structurally consist of an acidic moiety (carboxylic acid, enols) attached to a planar, aromatic functionality. Some analgesics also contain a polar linking group, which attaches the planar moiety to an additional lipophilic group. This can be represented as follows: 7 The NSAIDs are characterized by the following chemical/ pharmacologic properties: ❖ All are relatively strong organic acids with pKas in the 3-5 range. Most, but not all, are carboxylic acids (see drug classes). Thus, salts forms can be generated upon treatment with base and all of these compounds are extensively ionized at physiologic pH. The acidic group is essential for COX inhibitory activity! ❖ The NSAIDs differ in their lipophilicities based on the lipophilic character of their aryl groups and additional lipophilic moieties and substituents. ❖ The acidic group in these compounds serves a major binding group (ionic binding) with plasma proteins. Thus all NSAIDs are highly bound by plasma proteins (drug interactions!). ❖ The acidic group also serves as a major site of metabolism by conjugation. Thus a major pathway of clearance for many NSAIDs 8 is glucuronidation (and inactivation) followed by renal elimination. Salicylates Salicylates are derivatives of 2-hydroxybenzoic acid (salicylic acid). The salicylates were discovered in 1838 following the extraction of salicylic acid from willow bark. ‫الصفصاف‬ Salicylic acid was used medicinally as the sodium salt but replaced therapeutically in the late 1800s by the acetylated derivative, acetylsalicylic acid (ASA) or aspirin. Therapeutic utility is enhanced by esterification of the phenolic hydroxyl group as in aspirin, and by substitution of a hydrophobic/lipophilic group at C-5 as in diflunisal: 9 (2-Hydroxybenzoic acid)  Salicylates are strong organic acids and readily form salts with alkaline materials. Note that the carboxyl group is substantially more acidic (and ionizes readily at physiologic pH) than the phenolic hydroxyl: Salicylates inhibit the synthesis of prostaglandin and other mediators in the process of inflammation and have anti-inflammatory, antipyretic and analgesic properties (mild analgesic and antipyretic activities). 10  These compounds are mainly “COX-1 selective” – they are bound with higher affinity by COX-1. Toxicities include GI irritation, hypersensitivity reactions, inhibition of platelet aggregation, and ototoxicity (tinnitus). The therapeutic and certain of the toxic actions (i.e. gut) of aspirin can be related to its ability to inhibit COX in various tissues and participate in transacetylation reactions in vitro. For example, acetylation of COX results in irreversible inhibition of this enzyme and antiinflammatory effects in joints, and adverse effects in the GI tract. Also acetylation of circulating proteins may result in a hypersensitivity response. 11  Structure Activity Relationship of Salicylates 12 SAR 1) Substitution on carboxyl groups may affect the potency and toxicity. 2) Reducing the acidity of the –COOH, retains the analgesic action of salicylic acid , but it is devoid of the antiinflammatory properties. 3) Placing the phenolic hydroxyl group , meta or para to the carboxyl group abolishes the activity. 4) Substitution of halogen atoms on the aromatic ring enhances potency and toxicity. 5) Substitution of aromatic rings at the 5 position of salicylic acid increases anti-inflammatory activity. Aspirin (acetylsalicylic acid) 13 1- Salicylate structure Salicylate (ester) → active moiety → essential If benzoic acid (R1 = H) → less active COO - R1 O - R2 m-OH, p-OH benzoic acid → no activity 14 2- Salts formation Salt formation provides active compounds. COO-Na+ OH – Sodium salicylate COO-Mg+2 OH – Magnesium salicylate - N+ COO HO – Choline salicylate OH 15 3- Ester formation Ester → active compounds COOCH3 OH – Usually longer duration of action – Recommended for topical use Example: Methyl salicylate 16 4- Amide formation Amides COONH2 OH – No anti-inflammatory activity – Good analgesic Example: Salicylamide 17 5- Acetylation of phenolic OH Generally produce active compounds with COOH OCOCH3 – Anti-inflammatory – Analgesic Acetyl salicylic acid – Antipyretic Example: Aspirin 18 6- Halogenated aryl at C-5 Generally produce active compounds with COOH enhanced anti inflammatory activity OH Recommended for arthritis Longer duration of action F F Less side effects Diflunisal Example: Diflunisal 19  The most commonly used analgesic, antipyretic and anti- inflammatory drug → 500 mg dose Used as anti-platelet aggregation → 100 mg dose → block TXA2 Very acidic drug (pKa = 3.5) Synthesis: Kolbe reaction Side effects: gastric ulcer → aspirin salts reduced GIT irritation OH COOH COOH OH OCOCH3 1) CO2/heat/pressure/NaOH 2) HCl Aspirin 20  COOH OH Duration of action > aspirin F Potency > Aspirin F Better tolerated with less GI complication than aspirin Metabolism → dose dependent → glucoronide formation 21 22 (Hydroxyeicosatetraenoic acid, a metabolite of arachidonic acid) 23 Anthranilates [N-Arylanthranilic acid derivatives (Fenamates)] These agents are considered to be N-aryl substituted derivatives of anthranilic acid which is itself a bioisostere of salicylic acid (Fenamates are N containing analogues of salicylates) These agents retain the acidic properties that are characteristic of this class of agents; however, note that while mefenamic acid and meclofenamic acid are derivatives of anthranilic acid, diclofenac is derived from 2-arylacetic acid. 24 Anthranilates The most active fenamates have small alkyl or halogen substituents at the 2’,3’ and/or 6’ position of the N-aryl moiety (meclofenamate is 25 times more potent than mefenamate- see the structures). Among the disubstituted N-aryl fenamates the 2’,3’- derivatives are most active suggesting that the substituents at the 2’,3’-positions serve to force the N-aryl ring out of coplanarity with the anthranilic acid. Hence this steric effect is proposed to be important in the effective interaction of the fenamates at their inhibitory site on cyclooxygenase. 25  The anthranilates have primarily antiinflammatory with some analgesic and antipyretic activity and are non-COX selective. The anthranilates are used as mild analgesics Mefenamic acid as an analgesic for dysmennorhea. The utility of the class of agents is limited by a number of adverse reactions including nausea and vomiting, diarrhea, 26 ulceration, headache, drowsiness and hematopoietic toxicity. Anthranilate Metabolism Both mefenamic acid and meclofenamic acid are metabolized by benzylic oxidation of the ortho methyl group and ring oxidation followed by eventual glucuronidation. Major The anthranilates and their metabolites show more balanced excretion 27 than other NSAIDs, with a greater fraction being eliminated in the feces. NSAIDs - Heteroaryl Acetic Acids and Propionic Acid Derivatives Important class of NSAID drugs, classified according to aryl and heteroaryl acetic acid derivative Typically used for treatment of rheumatoid arthritis ❖ Indole and Indene Acetic Acids (Aryl alkanoic acids) ❖ Propionic Acid Derivatives ❖ Hetero aryl Acetic Acids ❖ Enolic acids ❖ Alkanones: Nabumetone 28 Heteroaryl Acetic Acids and Propionic Acid Derivatives Aryl and Hetero arylacetic Acids compounds are also derivatives of acetic acid, but in this case the substituent at the 2-position is a heterocycle or related carbon cycle. This does not significantly effect the acidic properties of these compounds. The heteroarylacetic acid NSAIDs marketed can be further subclassified as the indene/indoles, the pyrroles and the oxazoles. 29  Ar Where the name came from SAR CH COOH – Ar = aryl or heteroaryl R Arylalkanoic acid – R = H or CH3 – A center of acidity is essential – One carbon distance – Substitution at the α-carbon with CH3 Yields Profeins → increased anti-inflammatory activity Creates chiral center → S-isomer is active Examples – Indomethacin, Sulindac, Tolmetin sodium, Diclofenac sodium, Ibuprofen, Ketoprofen, Naproxen 30  Indene and Indole Acetic Acids (Arylalkanoic acids) Etodolac Indomethacin Sulindac Non-indole, Methylated indole derivative Indomethacin contains a benzoylated indole nitrogen. The methyl group at the 2 position of the indole ring prevents free rotation about the C-N bond and keeps the two aromatic rings in the correct relationship for COX binding and therapeutic activity. Indomethacin is “COX-1” selective” and produces primarily antiinflammatory actions with some analgesic and antipyretic activity. It is used for RA, to suppress uterine contraction (preterm labor), and to promote closure of patent ductus artiosus in 31 neonates (premature infants).  CH2COOH Indomethacin H3CO 5 3 1 N CH3 1-(p-chlorobenzoyl)-5-methoxy- 2-methyl-indol-3-acetic acid O Cl Half life = 5-10h Analgesic, Anti-inflammatory, Antipyretic, Acute gout GI stress Drug-drug interaction → reduced renal blood flow → increases warfarin, furosemide, and lithium toxicity Metabolism O-desmethyl N-deschlorobenzoyl GI ulceration and hemorrhage (these limit use). CNS toxicity ranging from headaches to delusions to psychoses and suicidal tendencies occur along with bone marrow depression: aplastic anemia and thrombocytopenia 32 Sulindac F 5 3 CH2COOH Increases potency 1 CH3 Increases solubility O S CH3 (Z)-5-fluro-2-methyl-1-[(4-methylsulfinyl) phenylmethylene]-1H-indene-3-acetic acid Prodrug with chiral sulfoxide moiety Metabolism: in vivo reduction → forming the active metabolite methyl sulfide (CH3S-) Long duration of action (half life 16.5h) 33 Indomethacin Metabolism The metabolism of indomethacin involves glucuronidation of the carboxyl group along with demethylation (increasing resemblance to 5-HT and CNS toxicity) and glucuronidation of the resulting phenol. In addition, the amide is more susceptible to hydrolysis than may normally be expected due to decreased resonance stabilization. Hydrolysis 34 35 Structure Activity Relationship for Indole Derivatives Carboxyl group is necessary for anti inflammatory activity. If carboxylate group is exchanged with hydroxyl type groups there is a decrease in activity. Antirheumatic activity increases with acidity. Changing aromatic acyl group at position 1 with alkyl, aliphatic acyl or alkyl group lowers activity. Substituting halogen, CF3 or SCH3 groups at para position of 1-benzoyl ring increases activity. Methyl group at position 2 forces the molecule to have a cis conformation therefore has pronounced effect on activity relative to aryl group. The bond at 3-acetic acid chain can freely rotate. Hydrogen or methyl substitution at α-position of the side chain gives similar activity whereas α,α-dimethyl or hydroxyl substitution lowers activity. S isomers are more effective. Substitution at position 5 of the indole ring is feasible and methoxy, dimethylamino, acetyl, methyl and fluoro substituents improve activity. Arylideneindenyl isostere shows similar activity as indole compounds. Cis 36 isomer exhibits higher activity then trans compound. Sulindac This relationship between aromatic rings observed for indomethacin is preserved by restricted rotation about the carbon-carbon double bond in sulindac. In this agent the indole N has been eliminated which reduces the drugs resemblance to 5-HT and therefore fewer CNS side effects are seen. This compound has pharmacologic actions similar to indomethacin (COX-1 selective and antiinflammatory primarily). However, sulindac is a prodrug function; it is reduced to a sulfide which is 50 X more active. (see metabolism later). It is used for RA, OA, AS (ankylosing spondylitis), acute gout and to inhibit uterine contractions. Overall sulindac produces less GI ulceration, probably as a result of its prodrug function. Some CNS toxicity, hepatic damage and prolongs clotting time. 37 Sulindac Metabolism Sulindac is a prodrug and therefore must be converted to an active form. This activation requires reduction to the sulfide which is then capable of inhibiting cyclooxygenase. Alternatively, sulindac may be oxidized to the inactive sulfone. In the case of sulindac, glucuronidation of the carboxyl group may still occur but since the methoxy group has been replaced by a F substituent, ring demethylation does not occur. 38 Etodolac (Lodine) Analogue of indomethacin and similar profile; antiinflammatory mainly with analgesic and antipyretic acitvity and uricosuric action. It is used for RA, OA and as a post-operative analgesic. It may cause GI ulceration and hemorrhage at high doses. This drug is well absorbed and has a half-life 7 hours. Indomethacin 39 Diclofenac sodium Diclofenac- can be classified as Anthranilates or Heteroaryl Acetic Acids Derivatives Diclofenac belongs to a class of drugs called (NSAIDs) that are used for the treatment of mild to moderate pain, fever, and inflammation CH2COO-Na+ NH Cl Cl 2-(2-(2,6-dichloro-phenyl-amino) phenyl) acetic acid One of the most widely prescribed drugs Metabolism: p-aromatic hydroxylation 40 Dicofenac potassium (Cataflam) → faster acting → acute pain and dysmenorrhea  Diclofenac is metabolized by acyl-O-glucuronidation and oxidation of the aromatic rings 41 Propionic Acid Derivatives (“Profens”) Structurally derived from arylacetic acids. These compounds are often referred to as the “profens” based on the suffix of the prototype member, ibuprofen. Like the salicylates these agents are all strong organic acids (pKa = 3-5) and thus form water soluble salts with alkaline reagents. The arylpropionic acids are characterized by the general structure Ar-CH(CH3)-COOH which conforms to the required general structure. 42 Propionic Acid Derivatives (“Profens”) All of these compounds are predominantly ionized at physiologic pH and more lipophilic than ASA or salicylic acid. 43  The α-CH3 substitutent present in the profens increases cyclooxygenase inhibitory activity and reduces toxicity of the profens. The α-carbon in these compounds is chiral and the S-(+)- enantiomer of the profens is the more potent cyclooxygenase inhibitor (S-isomers is more potent than R- 44 isomers).  Most profen products, except naproxen, are marketed as the racemates. In addition to the metabolism (see later), the profens undergo a metabolic inversion at the chiral carbon involving stereospecific transformation of the inactive R-enantiomers to the active S-enantiomers. This is believed to proceed through an activated (more acidic α-carbon) thioester intermediate. Normally only the S-(+) 45 isomer is present in plasma. Metabolic inversion (stereospecific): Transformation of the inactive 46 R-enantiomers to the active S-enantiomers  Ibuprofen H3C COOH ω ω-2 ω-1 2-(P-isobutylphenyl)-propionic acid Introduction of α-CH3 on alkanoic acid causes Enhances anti-inflammatory activity Reduces side effects Metabolism: ω, ω-1, and ω-2 hydroxylation Ibuprofen (Advil) was the first member of the propionicacid class of NSAID’s to come into general use. 47 48  H3 C Ketoprofen COOH H3 C Suprofen COOH O Ketoprofen S O H3C COOH Thiaprofenic acid Ibuprofen Induces GI stress = indomethacin Bioisosteric substitution of any of the phenyl rings by thienyl produce active compounds thiaprofenic acid and suprofen) 2-(6-methoxy-2-naphthyl) propionic acid Half life = 13h Strong protein binding → displaces other drugs 49 Analgesic activity (400 mg = 75-150 mg mepridine) Metabolism of Ketoprofen & Naproxen 50 Oxicams (Enolic Acids) The enolic acids include an oxicam family currently composed of piroxicam, meloxicam and tenoxicam (currently under study as well as others) Oxicams (Piroxicam and Meloxicam) are characterized by the 4- hydroxybenzothiazine heterocycle. The acidity of the oxicams is attributed to the 4-OH with the enolate anion being stabilized by intramolecular H-bonding to the amide N-H group. Piroxicam Meloxicam Tenoxicam 51  Oxicams Also, the presence of the carboxamide substituent at the 3-position of the benzothiazine ring contributes toward acidity by stabilizing the negative charge formed during ionization (resonance stabilization). Although these compounds are acidic (pKa = 6.3), they are somewhat less acidic than carboxylic acids NSAIDs. Yet the oxicams are primarily ionized at physiologic pH and acidity is required for COX inhibitory activity. 3 52  Lack COOH side chain Has an acidic enolic-1,2-benzothiazine Enolate Examples Thiazine ring OH O Ar – Peroxicam N H N – Meloxicam O S O R Oxicams are higher COX-2 selectivity than many other NSAIDs, particularly meloxicam. These agents have utility in treatment of RA and OA. Piroxicam appears to be the equivalent of aspirin, indomethacin, or naproxen for the long-term treatment of rheumatoid arthritis or 53 osteoarthritis. Piroxiam OH O N N H N S CH3 O O Long –lasting antirheumatic agent → Once daily Plasma half time = 50h Can be used for osteoarthritis GI side effects Metabolism: p-hydroxylation → 5-hydroxypiroxicam COX-2 inhibitor Low rate of nephropathy Low rate of GI irritation Least effective in treating rheumatic Metabolism: 5’-hydroxylation , 5’carboxylation → inactive 54  Oxicam Metabolism ❖ Due to the primary difference in their structures, piroxicam and meloxicam are metabolized by different routes. ❖ Piroxicam undergoes pyridine ring oxidation followed by glucuronidation; a small fraction also undergoes hydrolysis. ❖ Meloxicam undergoes slow oxidation of the “benzylic methyl” group of the thiazole side chain. Oxidation Oxidation Hydrolysis 55 ❖ Meloxicam is extensively metabolized in the liver by the enzymes CYP2C9 and CYP3A4 (minor) onto four inactive metabolites. 56 Phenylpyrazolones (Pyrazolone and Pyrazolidinedione Derivatives) This class of agents is characterized by the 1-aryl-3,5- pyrazolidinedione structure. 1-phenyl-2,3-dimethyl-3-pyrazolin-5-one and 1,2- diphenylpyrazolidin-3,5-dione derivatives The presence of a proton which is situated to two electron withdrawing carbonyl groups renders these compounds acidic. The pKa for phenylbutazone is 4.5. Oxyphenbutazone is a hydroxylated metabolite of phenylbutazone. 57 Antipyrine = Phenazone  Where the name came from O N N O R1 O R1 N N R2 R2 Antipyrine = phenazone CH3 N O CH3 N Associated with agranulocytosis 58 1,5-dimethyl,2-phenylpyrazol-3-one 2,3-dimethyl,1-phenylpyrazol-5-one  C4H9 O Phenylbutazone N O N 4-butyl-1,2-diphenyl-pyrazolidine-3,5-dione Developed to treat rheumatoid arthritis and gout Co-administration with aspirin → liver damage Limited for Veterinary Use Associated with aplastic anemia The most common adverse reactions include GI irritation, Na+ and H2O retention and blood dyscrasias. Therapy should be limited to 7-10 days due to bone marrow depression that may develop (aplastic anemia) 59  Phenazone ❑ 1-phenyl-2,3-dimethyl-5-pyrazolin-5-one and 1,2-diphenylpyrazolidin-3,5- dione derivatives 60 SAR for Pyrazolidinediones (and phenylbutazone) The butyl group of carbon 4 may be replaced by propyl or allyl and show similar activity. The meta substitution of the aryl ring are inactive but para s