Antibacterial Agents & Related Drugs PDF

Antibacterial agents & Related Drugs  Contents Introduction -Lactam Antibiotics Tetracyclines Aminoglycosides Macrolides Polypeptides, Polyenes, Lincomycin, Chloramphenicol Sulphonamide & related drugs Antitubercular & Antileprotic Agents U...

Antibacterial agents & Related Drugs  Contents Introduction -Lactam Antibiotics Tetracyclines Aminoglycosides Macrolides Polypeptides, Polyenes, Lincomycin, Chloramphenicol Sulphonamide & related drugs Antitubercular & Antileprotic Agents Urinary Tract Anti-infectives Med Chem. Tadesse B. (PhD) 2  Introduction Bacteria Microscopic organisms whose single cells have Neither a membrane-bounded nucleus nor other membrane-bounded organelles like mitochondria and chloroplasts Properties of Bacteria Prokaryotic (no membrane-enclosed nucleus) No mitochondria or chloroplasts A closed circle of double-stranded DNA Ribosomes differ in their structure from those of eukaryotes Have a rigid cell wall made of peptidoglycan Med Chem. Tadesse B. (PhD) 3  Introduction cont. Properties of Bacteria cont.  The plasma membrane (in Gram positive bacteria) and both membranes in Gram-negative bacteria  Are phospholipid bilayers but contain no cholesterol or other sterols  Many bacteria form a single spore when their food supply runs low Most of the water is removed from the spore and metabolism ceases Spores are so resistant to adverse conditions of dryness and temperature that they may remain viable even after 50 years of dormancy Med Chem. Tadesse B. (PhD) 4  Thought Question Can you think of any difference between a human host and a bacterial pathogen that would be a target for antibacterial agents? Med Chem. Tadesse B. (PhD) 5 Fig. Structure and composition of gram-positive and gram- negative cell walls Med Chem. Tadesse B. (PhD) 6 The cell envelope of a Gram-negative bacterium The outer membrane, a lipid bilayer, is present in Gram-negative but not Gram-positive organisms. It is penetrated by porins, proteins that form channels providing hydrophilic access to the cytoplasmic membrane. The peptidoglycan layer is unique to bacteria and is much thicker in Gram-positive organisms than in Gram-negative ones. The outer membrane & the peptidoglycan layer constitute the cell wall. Penicillin-binding proteins (PBPs) are membrane proteins that cross-link peptidoglycan. Beta-lactamases, if present, reside in the periplasmic space or on the outer surface of the cytoplasmic membrane, where they may destroy β-lactam antibiotics that penetrate the outer membrane Med Chem. Tadesse B. (PhD) 7 PBP activity and inhibition. PBPs have two enzymatic activities that are crucial to synthesis of the peptidoglycan layers of bacterial cell walls: a TP that cross-links amino acid side chains and a GT that links subunits of the glycopeptide polymer. The TP and GT domains are separated by a linker region. The glycosyltransferase is thought to be partially embedded in the membrane Med Chem. Tadesse B. (PhD) 8 Med Chem. Tadesse B. (PhD) 9 Med Chem. Tadesse B. (PhD) 10  ANTIBIOTICS Antibiotics: are substances produced by various species of microorganisms (bacteria, fungi, actinomycetes) that suppress the growth of other microorganisms. o Antibiotics that kill bacteria are called “bactericidal” o The ones that stop the growth of bacteria are called “bacteriostatic” Antimicrobials: are chemical agents (synthetic/natural) used to treat bacterial, fungal and viral infections. – Antimicrobial drug exhibits selective toxicity, i.e. the drug is harmful to the parasite without being harmful to the host. Med Chem. Tadesse B. (PhD) 11  ANTIBIOTICS cont.  The antibiotic age began in the late 1920s when Alexander Fleming saw that the mold, penicillium, inhibiting bacterial growth and when Rene Dubos purified gramicidin as an antibiotic Gramicidin is an antibiotic produced by a soil bacterium; used chiefly as an antiseptic in treating local infections produced by Gram-positive bacteria  Many antibiotic compounds used in modern medicine are produced and isolated from living organisms, such as The penicillin class produced by fungi in the genus penicillium Streptomycin from bacteria of the genus streptomycin Med Chem. Tadesse B. (PhD) 12  Major Targets of Antimicrobial Agents Antimicrobial therapy takes advantage of the biochemical differences that exist between Microorganisms & human beings Different antibiotics have different modes of action, owing to the nature of their structure & degree of affinity to certain target sites within bacterial cells Inhibitors of cell wall synthesis – A drug that targets cell walls can selectively kill or inhibit bacterial organisms – Examples: penicillins, cephalosporins, bacitracin & vancomycin Inhibitors of cell membrane function – Since this structure is found in both eukaryotic & prokaryotic cells, the action of this class of antibiotic are often poorly selective & can often be toxic for systemic use in the mammalian host – Most clinical usage is therefore limited to topical applications – Examples: polymixin B & colistin 13 Major Targets of Antimicrobial Agents…  Inhibitors of Protein Synthesis – Several types of antibacterial agents target bacterial protein synthesis by binding to either the 30S or 50S subunits of the ribosomes – Examples: Aminoglycosides, macrolides, lincosamides, chloramphenicol, tetracyclines  Inhibitors of Nucleic Acid Synthesis – Some antibiotics work by binding to components involved in the process of DNA or RNA synthesis, which causes interference of the normal cellular processes which will ultimately compromise bacterial multiplication & survival – Examples: quinolones, metronidazole, & rifampin  Inhibitors of Other Metabolic Processes – Other antibiotics act on selected cellular processes essential for the survival of the bacterial pathogens – Example: Both sulfonamides & trimethoprim disrupt the folic acid pathway, which is a necessary step for bacteria to produce precursors important for DNA synthesis 14 Med Chem. Tadesse B. (PhD) 15  Classification of Antibiotics Classification Based on Activity  Against Gram positive Most penicillins, erythromycin, bacitracin  Against Gram negative Polymixin, colistin  Broad spectrum Ampicillin, chloramphenicol, tetracyclines, cephalosporins  Antitubercular Streptomycin, cycloserine  Antifungal Griseofulvin, nystatin  Antitumor Puromycin, dactinomycin Med Chem. Tadesse B. (PhD) 16  Classification of Antibiotics… Classification Based on Chemical Structure  β-Lactam Penicillins, cephalosporins, monobactams, carbapenems  Aminoglycosides: Streptomycin, kanamycin, neomycin  Fused rings: Tetracyclines, griseofulvin  Macrolide: Erythromycin, oleandomycin  Polypeptide: Actinomycin, bacitracin, polymixin  Polyene: Nystatin, amphotericin B  Unclassified: Chloramphenicol, cycloserine Med Chem. Tadesse B. (PhD) 17 It includes: β-Lactam antibiotics – Penicillins, Cephalosporins, Carbapenems, Monobactams Glycopeptide antibiotics – Vancomycin, Teicoplanin, Telavancin, Dalbavancin Other cell wall active agents – Cycloserine, Bacitracin, Fosfomycin, Daptomycin 18 β-Lactam Antibiotics  β- Lactam Antibiotics All β-lactam antibiotics have a Four membered ring structure (the β-lactam ring) Fused with 5- or 6- membered heterocyclic ring except monobactams They act by interfering with penicillin binding proteins (PBP) enzymes involved in the synthesis and maintenance of peptidoglycan The ring is very strained and the bond between the carbonyl and the nitrogen in the β-lactam ring is very labile and hence, makes the molecule reactive Video : Peptidoglycan Biosynthesis Med Chem. Tadesse B. (PhD) 20  β- Lactam Antibiotics cont. The group of antibiotics known as the β-lactams include: Penicillin’s Cephalosporin’s Monobactams and Carbapenems Med Chem. Tadesse B. (PhD) 21  β-Lactam Antibiotics… Mechanism of action Interfere with cell wall synthesis by binding to penicillin-binding proteins (PBPs) which are located in bacterial cell walls Inhibition of PBPs leads to inhibition of peptidoglycan synthesis and the cell death Med Chem. Tadesse B. (PhD) 22  β-Lactam Antibiotics… Beta-lactams bind the transpeptidase at the PBP Schematic of normal bacterial cell site, resulting in inhibition of transpeptidation, wall peptidoglycan synthesis thus halting peptidoglycan synthesis transpeptidation reaction Med Chem. Tadesse B. (PhD) 23  PENICILLINS  Antibiotic drugs originally isolated from Penicillium molds Some are biosynthesized or produced synthetically  The term “penicillin” refers to benzyl penicillin (penicillin G), obtained from P.notatum Structure of Penicillins  All penicillins have the same general formula and differ only in the R group Med Chem. Tadesse B. (PhD) 24  Structure of Penicillins  The structure consists of a bicyclic system formed of a highly unstable 4-membered β-lactam ring fused to a 5-membered thiazolidine ring  The penam skeleton is essential for the antibiotic activity  The penam skeleton indicates that it is derived from 2 amino acids: L-cysteine and L-valine Penam skeleton Med Chem. Tadesse B. (PhD) 25 Med Chem. Tadesse B. (PhD) 26  NATURAL PENICILLINS  Benzylpenicillin (Penicillin G) was the first antibiotic isolated from the fungus Penicillium notatum  It will be taken as a prototype to discuss its biologic & chemical properties since these led to the introduction of a variety of new penicillin molecules to medicine Med Chem. Tadesse B. (PhD) 27  Weakness of Benzylpenicillin Molecule Several problems limit the clinical use of penicillin G, the most important are:  Acid sensitive Ineffective orally: it breaks down by the acidity of the stomach  β- Lactamases sensitive  Acylases sensitive Attacking the side chain giving 6-Aminopenicillanic acid (6-APA)  Narrow spectrum Active against Gram-positive and some Gram-negative bacteria  Allergic reactions  Deactivation by metal ions, oxidizing and reducing agents Med Chem. Tadesse B. (PhD) 28  NATURAL PENICILLINS cont. Instability of Benzylpenicillin  The instability of benzylpenicillin towards nucleophilic and electrophilic attacks is due to:  Ring Strain In β-lactam, the N is highly strained The carbonyl becomes more electrophilic than usual and more susceptible to attack by nucleophiles Med Chem. Tadesse B. (PhD) 29  PENICILLINS cont. Penicillin Sensitivity to β-Lactamases Med Chem. Tadesse B. (PhD) 30  PENICILLINS cont.  Influence of the Acyl Side Chain  The acyl side chain renders the molecule acid sensitive (electrophilic attack by gastric acidity) Med Chem. Tadesse B. (PhD) 31  Penicillin Analogs  The production of different semisynthetic penicillins from natural penicillins is usually achieved by Hydrolyzing penicillin G or V with penicillin acylase enzyme to give 6-APA (6-aminopenicillanic acid) Which is then acylated with either acid chloride (RCOCl) under mild conditions or With acid (RCOOH) in presence of a dehydrating agent Med Chem. Tadesse B. (PhD) 32  Synthesis of Penicillins Penicillin G can be enzymatically converted into 6- aminopenicillanic acid (6-APA) 6-APA serves as a convenient starting material for the synthesis of other penicillins Med Chem. Tadesse B. (PhD) 33  Penicillin Analogs cont. I. Acid Stable Semisynthetic Penicillins (sensitive to β-lactamases) II. β-Lactamase Resistant Semisynthetic Penicillins  Methicillin (Meticillin)  It is acid sensitive and has to be administered by injection  Inactive against Gram negative bacteria and with poor activity against some streptococci (a gram-positive bacteria) Med Chem. Tadesse B. (PhD) 34  Penicillin Analogs cont. III. Acid & penicillinase resistant (administered orally as sodium salts)  The halogen they comprise influences the pharmacodynamic properties  Cloxacillin is better absorbed through the gut wall than oxacillin  Flucloxacillin is less bound to plasma protein, resulting in higher level of the free drug in the blood  These drugs are of narrow spectrum and inactive against Gram negative bacteria Med Chem. Tadesse B. (PhD) 35  Penicillin Analogs cont. IV. Broad Spectrum Semisynthetic Penicillins  Gram positive bacteria have a simple cell wall that is easily penetrated by penicillin  Gram negative bacteria have a coating on the outside of cell wall which consists of fats, sugars and proteins This outer coating contains protein channels that permit hydrophilic molecules and water to pass through to the interior The outer fatty portion coating acts as a barrier to polar hydrophilic penicillin molecules The outer surface of the coating has an overall of negative or positive charge depending on its triglycerides constituent  Penicillins with ionized carboxylic are repelled by the negative charges on the wall Med Chem. Tadesse B. (PhD) 36  Penicillin Analogs cont. Broad Spectrum Semisynthetic Penicillins Classified into: Broad Spectrum Antibiotics with an Amino Function Ampicillin Esters (Improvement of Absorption through the Gut Wall) Acyl Ampicillin Derivatives: Effective against P. aeruginosa Broad Spectrum Antibiotics with Acidic Functions Carbenicillin Esters (Improvement of Absorption through Gut Wall) Med Chem. Tadesse B. (PhD) 37  Broad Spectrum Antibiotics with an Amino Function Ampicillin  Due to lack of stability towards β-lactamases; it is better used with clavulanic acid or sulbactam (Called Unacyn)  Inactive against Pseudomonas aeruginosa (particularly resistant species)  Acid resistant due to protonation of NH2 group in gastric juice and acquires electron withdrawing property  The presence of COOH and NH2 groups on the same molecule leads to Zwitter ions formation Med Chem. Tadesse B. (PhD) 38 Broad Spectrum Antibiotics with an Amino Function cont. Amoxicillin  Created to increase the overall acidity (COOH+ p-OH) of ampicillin molecule over the alkalinity (NH2) so as to be to the acid side Oral absorption is increased to 80%  Best oral absorption leads to fewer disturbances of the GIT flora and less drug-induced diarrhea  Coadministered with clavulanic acid as in case of ampicillin as augmentin Med Chem. Tadesse B. (PhD) 39 Ampicillin Esters (Improvement of Absorption through the Gut Wall)  The prodrugs used comprise a relatively bulk esters formed far away from the penicillin skeleton Which is exposed to attack by esterases and liberate ampicillin spontaneously Example: Bacampicillin, Talampicillin, Pivampicillin Med Chem. Tadesse B. (PhD) 40  Acyl Ampicillin Derivatives  The following ampicillin derivatives are effective against P. aeruginosa; Example: Piperacillin, Azlocillin, Mezlocillin Mezlocillin Piperacillin Azlocillin Azlocillin synthesis Med Chem. Tadesse B. (PhD) 41  Broad Spectrum Antibiotics with Acidic Functions Carbenicillin: is a benzyl penicillin analog in which one of the methylene hydrogens of the side chain has been substituted with a carboxylic acid moiety The introduction of the side-chain carboxyl produces enhanced anti– gram-negative activity. The drug is susceptible to β-lactamases and is acid unstable and thus must be given by injection Ticarcillin: is a sulphur-based bioisostere of carbenicillin that cannot decarboxylate as the carboxyl group of carbenicillin does. This agent is somewhat more potent against pseudomonas than is indanyl carbenicillin. When potassium clavulanate is added to ticarcillin (Timentin), the combination has enhanced spectrum due to its enhanced stability to lactamases Med Chem. Tadesse B. (PhD) 42  V. Long Acting Penicillins  Prolongation of action of benzylpenicillin is achieved through a salt formation with an amino compound to decrease water solubility  Thus, leading to slow release of the active drug following intramuscular injection  Long acting forms are used in the treatment of patients requiring prolonged therapy i) Benzathine Penicillin ii) Procaine Penicillin Med Chem. Tadesse B. (PhD) 43  Penicillins… They are bactericidal but can act only on dividing cells They are not toxic to animal cells which have no cell wall The structure common to all penicillins is a β-lactam ring fused with a thiazolidine nucleus Fig.: Structure of penicillins and products of their enzymatic hydrolysis Med Chem. Tadesse B. (PhD) 44  Structural Activity Relationship of Penicillin Penicillins have been well studied and scientist were able to determine the following SAR:  Position 1: When the sulfur atom of the thiazolidine ring is oxidized to a sulfone or sulfoxide, it improves acid stability, but decreases the activity of the agent  Position 2: No substitutions allow at this position, any change will lower activity. – The methyl groups are necessary  Position 3: The carboxylic acid of the thiazolidine is required for activity. If it is changed to an alcohol or ester, activity is decreased  Position 4: The nitrogen is a must  Position 5: No substitutions allowed Med Chem. Tadesse B. (PhD) 45  SAR of Penicillin cont.  Position 6: Substitutions are allowed on the side chain of the amide. An electron withdrawing group added at this position will give the compound better acid stability because this substitution will make the amide oxygen less nucleophilic A bulky group added close to the ring will make the compound more resistant to -lactamases Steric hindrance provides protect to the -lactam ring The spectrum of activity becomes broader when a polar group is added to this position allowing the compound to pass through the porins of the Gram negative bacteria cell wall Med Chem. Tadesse B. (PhD) 46  SAR of Penicillin cont.  Position 7: The carbonyl on the -lactam ring is a must  These structural activity relationships have helped scientist design more effective penicillins to combat many different prokaryotes Med Chem. Tadesse B. (PhD) 47  β-lactamase Inhibitors It inactivate β-lactamases, thereby preventing the destruction of β-lactam antibiotics that are substrates for these enzymes It includes: Traditional β-lactamase inhibitors (clavulanic acid, sulbactam, & tazobactam) and the newly approved Avibactam – The traditional β-lactamase inhibitors resemble β-lactam molecules but they have very weak antibacterial action. – Inhibits bacterial -lactamases (penicillinase) β-lactamase is family of enzyme produced by bacteria that inactivate β-lactam antibiotics – Most active against -lactamase produced by Staphylococcus aureus, Hemophilus infleunza, some enterobacteriaceae, Bacteroid spp – Most active against plasmid-encoded β-lactamases (including the enzymes that hydrolyze ceftazidime and cefotaxime) Med Chem. Tadesse B. (PhD) 48  β-Lactamase Inhibitors… Clavulanic acid  It is produced by Streptomyces clavuligerus  An Oxapenam derivative  It has poor intrinsic antimicrobial activity but it is a “suicide” inhibitor that irreversibly binds β-lactamases produced by a wide range of gram-positive and gram-negative microorganisms.  Combined with amoxicillin for oral administration (AUGMENTIN) and with ticarcillin for parenteral administration (TIMENTIN) Med Chem. Tadesse B. (PhD) 49  β-Lactamase Inhibitors…  Sulbactam o May be given orally or parenterally along with a β-lactam antibiotics o Coadministered with ampicillin for IV or IM use (Unacyn) Same spectrum as augmentin: used in mixed infection o Made from 6,6-dibromopenicillanic acid sulfone on catalytic hydrogenation Med Chem. Tadesse B. (PhD) 50  β-Lactamase Inhibitors…  Tazobactam o Derivative of the penicillin nucleus and is a penicillanic acid sulfone o Added to the extended spectrum beta-lactam antibiotic piperacillin as a parenteral preparation to produce Tazocin or Zosyn or Piprataz Equivalent or superior to 3rd generation cephalosporin like ceftriaxone Med Chem. Tadesse B. (PhD) 51 Med Chem. Tadesse B. (PhD) 52  Cephalosporins They are semisynthetic antibiotics derived from products of various microorganisms, including Cephalosporium and Streptomyces They have a 7-aminocephalosporanic acid composed of a dihydrothiazine ring fused to a β-lactam ring β-lactam ring: is associated with the antibacterial activity Substitution of various groups on the basic molecule result in different pharmacological, pharmacokinetic & antibacterial activity of the cephalosporins Cephalosporins also vary in acid stability and β-lactamase susceptibility Resistance to cephalosporins results from: – Penicillinases & cephalosporinases – Modification on PBPs Med Chem. Tadesse B. (PhD) 53  CEPHALOSPORINS  The first isolated cephalosporin was cephalosporin C (1948)  The structure of cephalosporin C has similarities to that of penicillin G  Has a bicyclic system containing a four β-lactam ring fused with a six membered dihydrothiazine ring Med Chem. Tadesse B. (PhD) 54  CEPHALOSPORINS cont. The larger 6-membered ring relieves the strain in the bicyclic system to some extent (compared to thiazolidine of penicillin), but is still a reactive system Cephalosporin skeleton reveals that it can be derived from the same biosynthetic precursors as penicillin: cysteine and valine Med Chem. Tadesse B. (PhD) 55  Properties of Cephalosporin C  Highly polar side chain (Difficult to isolate & purify)  Low potency (1/1000 of Pen. G)  Not absorbed orally  Non toxic  Relatively stable to acid hydrolysis compared to penicillins  More stable to penicillinase than penicillin G  There is no cross penicillin allergenicity Med Chem. Tadesse B. (PhD) 56  Nomenclature of Cephalosporins  They can be nomenclated following any of the following skeletons  7-ACA is modified at limited positions to obtain semisynthetic cephalosporins  In the preparation of semisynthetic cephalosporins, the following improvements are sought:  Increased acid stability  Improved pharmacokinetic properties, particularly better oral absorption  Broadened antimicrobial spectrum  Increased activity against resistant microorganisms (as a result of resistance to enzymatic destruction, improved penetration, increased receptor affinity etc.)  Decreased allergenicity and  Increased tolerance after parenteral administration With Replacement of the acetoxy group by a suitable non-leaving group Med Chem. Tadesse B. (PhD) 57  Nomenclature of Cephalosporins cont.  Unlike penicillins, it is not possible to obtain semisynthetic analogs by fermentation  Similarly, it is possible to obtain 7-ACA from cephalosporin C by chemical means, not by fermentation or enzymatic hydrolysis  7-ADCA (7-amino-3-deacetoxycephalosporanic acid) is a useful starting material and is prepared from phenoxymethylpenicillin after undergoing several steps o 7-ADCA is a key intermediate for the synthesis of cephalosporins antibiotics and their intermediates. o The current process for producing 7-ADCA involves a multistep chemical ring expansion of penicillin G to phenylacetyl-7-ADCA, from which the aromatic side chain is removed using a penicillin acylase. Med Chem. Tadesse B. (PhD) 58  Nomenclature of Cephalosporins cont. Fig. Chemical preparation of 7-aminocephalosporanic acid (7-ACA) and 7-amino-3-deacetoxycephalosporanic acid (7-ADCA) Med Chem. Tadesse B. (PhD) 59 Fig. Conversion of cephalosporin C into 7-ACA Med Chem. Tadesse B. (PhD) 60  Structure-Activity Relationships  Saturation of the double bond leads to inactivation  2-Cephem derivatives are inactive Due to the β-lactam lacks the necessary ring strain to be sufficiently reactive  Removal of COOH at C-4 leads to inactivation  Replacement of S with O, CH2 does not affect activity  Replacement of 7α–H by a methoxy group increases resistance against β-lactamases Med Chem. Tadesse B. (PhD) 61  Structure-Activity Relationships cont. The acetoxy group at C-3 is an easy leaving group It is hydrolyzed in vivo to a hydroxyl methyl group which undergoes lactonization with the adjacent 4-COOH group yielding inactive product Med Chem. Tadesse B. (PhD) 62  Structure-Activity Relationship cont.  Replacement of the acetoxy group by a suitable non-leaving group may lead to:  Increase in acid stability  The drug can be administered orally  Broadening of the spectrum  Increase in activity against resistant strains by increasing penetration  Decrease in allergenicity Med Chem. Tadesse B. (PhD) 63  Structure-Activity Relationship cont. The -lactam ring is crucial to the mechanism The carboxylic acid at position 4 is important to binding The bicyclic system is important in increasing ring strain Stereochemistry is important The acetoxy substituent is important to the mechanism Med Chem. Tadesse B. (PhD) 64  Possible modifications 7-Acylamino side chain 3-Acetoxymethyl side chain Extra substitution at C-7 Med Chem. Tadesse B. (PhD) 65  Breaking News The U.S. Food and Drug Administration approved the first COVID-19 vaccine on Aug. 23, 2021. The vaccine has been known as the Pfizer-BioNTech COVID- 19 Vaccine, and will now be marketed as Comirnaty, for the prevention of COVID-19 disease in individuals 16 years of age and older. The vaccine also continues to be available under emergency use authorization (EUA), including for individuals 12 through 15 years of age and for the administration of a third dose in certain immunocompromized individuals. – Since Dec. 11, 2020, the Pfizer-BioNTech COVID-19 Vaccine has been available under EUA in individuals 16 years of age and older, and the authorization was expanded to include those 12 through 15 years of age on May 10, 2021. Med Chem. Tadesse B. (PhD) 66  Generations of Cephalosporins Cephalosporins are grouped into “5 generations“ based on their time of discovery and their antimicrobial properties Each newer generation has greater Gram-negative antimicrobial activity than the preceding generation o In most cases decreased Gram-positive antimicrobial activity Each newer generation shows enhanced resistance to β-lactamases From 1st generation to 3rd generation – Increasing activity against Gram negative & anaerobes – Increasing resistance to destruction to -lactamase – Increasing ability to reach cerebrospinal fluid (CSF) Fourth generation cephalosporins considered as true broad spectrum activity Med Chem. Tadesse B. (PhD) 67  First Generation Cephalosporins (1960-1970) Have greater efficacy against gram positive bacteria Moderate spectrum agents, not significantly active against Gram-negative organisms Poor ability to penetrate cerebrospinal fluid Inactive against Pseudomonas – Oral: Cephalexin (usually 0.5 g twice to four times daily), Cefadroxil, & Cephradine Absorbed from the gut to a variable extent Excretion is mainly by glomerular filtration and tubular secretion Probenecid may increase serum levels substantially – Parenteral: Cephradine (IV, IM, & PO also) Cefazolin, Cephalothin, Cephapiril, Cephaloglycin: IV or IM Cefazolin penetrates well into most tissues – It is a drug of choice for surgical prophylaxis – Does not penetrate the central nervous system Med Chem. Tadesse B. (PhD) 68  First Generation Cephalosporins cont.  Oral First Generation Cephalosporins Parenteral First Generation Cephalosporins Cefazolin Med Chem. Tadesse B. (PhD) 69 Second Generation Cephalosporins (1970–1980) Have greater Gram-negative spectrum and some activity against Gram-positive Cocci More resistant to β-lactamase Some drugs can pass the cerebrospinal fluid Med Chem. Tadesse B. (PhD) 70  Second Generation Cephalosporins cont.  Parenteral Second Generation Cephalosporins The 3-acetoxymethyl side chain is replaced by different groups as  3-carbamoyloxymethyl group or more different complex groups A methoxy group may be attached to the 7-position or in a side chain(make it resistance to hydrolysis by -lactamase) The 7-amino group is attached to a diversity of side chain  Cefoxitin (Mefoxin) Cefamandole (Mandole, cefam) Cefonicid Cefuroxime (Zinnat, Ceforim) NH2 stabilizes neighbouring carbonyl group Med Chem. Tadesse B. (PhD) 71  Second Generation Cephalosporins cont. Parenteral Second Generation Cephalosporins Oral Second Generation Cephalosporins Med Chem. Tadesse B. (PhD) 72  Third Generation Cephalosporins (1980) Broader spectrum against Gram-negative organisms and less Gram-positive activity Some drugs have high activity against Pseudomonas aeruginosa Many drugs are for parenteral use Better resistance to β-lactamases Med Chem. Tadesse B. (PhD) 73  Third Generation Cephalosporins cont.  Replacing the furan ring of the aforesaid oximinocephalosporins with an aminothiazole ring enhances the penetration of cephalosporins through the outer membrane of Gram-negative bacteria It also increase affinity for the transpeptidase enzyme As a result, third generation cephalosporins containing aminothiazole ring have a marked increase in activity against gram-negative bacteria  Examples: Ceftazidime, cefotaxime, ceftizoxime and ceftriaxone Cefuroxime is a 2nd Generation Med Chem. Tadesse B. (PhD) 74 Third Generation Cephalosporins cont.  Parenteral Third Generation Cephalosporins Ceftazidime (Fortax, Taxidime); Cefotaxime (Claforan) Ceftriaxone (Rocephin); Cefoperazone (Cefobid) Med Chem. Tadesse B. (PhD) 75 Third Generation Cephalosporins cont.  Oral Third Generation Cephalosporins Cefixime (Suprax) Cefpodoxime (Vantin, Orelox) Cefpodoxime Proxetil A cephalosporin prodrug esters on the 4-carboxylic group Med Chem. Tadesse B. (PhD) 76  Fourth Generation Cephalosporins  They are zwitter ions that can penetrate the outer membrane of Gram-negative bacteria  They have a greater resistance to -lactamases than the third- generation cephalosporins  Many can cross the blood-brain barrier and are effective in meningitis  They are also used against Pseudomonas aeruginosa Examples:- o Cefepime (Maxipime, Axepin, Cepimex, Neomed, Maxcef); o Cefpirome (Cefir, Cefrom, Broact) Med Chem. Tadesse B. (PhD) 77 Fourth Generation Cephalosporins cont.  Cefepime hydrochloride is a semi-synthetic, broad spectrum, cephalosporin antibiotic for parenteral administration Med Chem. Tadesse B. (PhD) 78  Fifth generation’ Cephalosporins Ceftaroline – Active against Methicillin-resistant Staphylococcus aureus (MRSA) and Streptococcus species that are resistant to penicillin antibiotics Ceftobiprole – Approved in Canada and EU – Has antipseudomonal activity and appears to be less susceptible to development of resistance. Ceftolozane – an option for the treatment of complicated intra- abdominal infections and complicated urinary tract infections. – It is combined with the β-lactamase inhibitor tazobactam Med Chem. Tadesse B. (PhD) 79 Ceftobiprole (ceftobiprole medocaril) Zeftera Med Chem. Tadesse B. (PhD) 80  Ceftaroline fosamil acetate (Teflaro) Starting point: cefozopran Prodrug: Ceftaroline fosamil acetate Rapid biotransformation in Plasma Med Chem. Tadesse B. (PhD) 81  Cephalosporins Combined with β-lactamase Inhibitors Novel cephalosporin-β-lactamase inhibitor combinations have been developed to combat resistant Gram-negative infections. Ceftolozane-tazobactam & ceftazidime-avibactam were both FDA- approved for the treatment of complicated intra-abdominal infections & urinary tract infections. Both agents have potent in vitro activity against Gram-negative organisms, including P aeruginosa and extended-spectrum β-lactamase producing Enterobacteriaceae. While neither agent is active against organisms producing metallo-β- lactamases, ceftazidime-avibactam may be an option for carbapenemase- producing organisms. Due to limited activity against anaerobic pathogens, both should be combined with metronidazole when treating complicated intra-abdominal infections. Both agents have short half-lives of 2-3 hours and are dosed every 8 hours. Both are primarily renally excreted and require dose adjustment in patients with impaired renal clearance Med Chem. Tadesse B. (PhD) 82  Structure-Activity Relationships of Ceftaroline Starting point: cefozopran Med Chem. Tadesse B. (PhD) 83 Med Chem. Tadesse B. (PhD) 84 OTHER β-LACTAM ANTIBIOTICS Med Chem. Tadesse B. (PhD) 85 Carbapenems-Newer -Lactam Antibiotics Thienamycin (Merck 1976)(from Streptomyces cattleya)  Potent and wide range of activity against Gram positive and Gram negative bacteria  Active against Pseudomonas aeruginosa  Low toxicity  High resistance to -lactamases  Poor stability in solution (ten times less stable than Pen G) Med Chem. Tadesse B. (PhD) 86  Monobactams Contain a monocyclic beta-lactam ring (e.g. Aztreonam). Aztreonam interacts with PBPs of susceptible microorganisms. The antimicrobial activity of aztreonam differs from those of other β- lactam antibiotics and more closely resembles that of an aminoglycoside It is resistant to many of the β-lactamases that are elaborated by most gram-negative bacteria. Narrow spectrum (Gram negative aerobic bacteria: Hemophilus influenza, Nisseria meningitides, & Pseudomonas) Unlike other β-lactam antibiotics, they have no activity against Gram positive bacteria and anaerobic organisms No cross sensitivity with other -lactam It penetrates well into the cerebrospinal fluid. Activity against Enterobacteriaceae is excellent, as is that against P. aeruginosa. Penicillin-allergic patients tolerate aztreonam without reaction. Med Chem. Tadesse B. (PhD) 87 Glycopeptide (Polypeptide) Antibiotics Med Chem. Tadesse B. (PhD) 88  Glycopeptide (Polypeptide) Antibiotics Several bacterial species produce antibiotics of cyclic or open chain peptides Obtained from either Streptomyces or Bacillus Spp. Mainly active against Gram-positive bacteria Some are active against Gram-negative bacteria as polymixins, colistins and gramicidins Usually water-soluble and are not readily metabolized Their molecular weights are usually over 1000 Med Chem. Tadesse B. (PhD) 89  Glycopeptide (Polypeptide) Antibiotics cont. Produce neuro- and nephrotoxicity in humans So their systemic use is limited to microorganisms sensitive to them and after failure of other drugs Bacteria are rarely able to develop significant resistance to these antibiotics Examples of the antibiotic of this group are: Vancomycin, viomycin, capreomycin, bacitracin, polymixin B, Colistin, gramicidins, novobiocin, tyrothricin, tyrocidins and amphomycin Med Chem. Tadesse B. (PhD) 90 Med Chem. Tadesse B. (PhD) 91  Glycopeptide (Polypeptide) Antibiotics: Vancomycin It is a glycopeptide antibiotic produced by Amycolatopsis orientalis Spectrum: – It is primarily active against gram-positive bacteria due to its large molecular weight and lack of penetration through Gram-negative cell membranes. – S. aureus (MRSA), S. pneumoniae, Bacillus spp., Clostridium difficile Essentially all species of gram negative bacilli & Mycobacteria are resistant to vancomycin MOA: Vancomycin inhibits the synthesis of the cell wall – The drug is bactericidal for Gram-positive bacteria in concentrations of 0.5–10 mcg/mL – Bind to D-alanyl-D-alanine in the growing bacterial cell wall→ preventing the elongation of peptidoglycan strands & halting cell wall synthesis – Does not interfere with plasma binding protein Med Chem. Tadesse B. (PhD) 92  Telavancin Vancomycin Med Chem. Tadesse B. (PhD) 93  Other Glycopeptide antibiotics… Teicoplanin – It is very similar to vancomycin in MOA & antibacterial spectrum – Unlike vancomycin, it can be given IM as well as IV – It has a long half-life (45-70 hours), permitting once-daily dosing Telavancin – A semisynthetic lipoglycopeptide derived from vancomycin – Has dual MOA: i) Similar to vancomycin results in improved activity against bacteria with reduced susceptibility to vancomycin and ii) it disrupts the bacterial cell membrane potential and increases membrane permeability. – The half-life of is approximately 8 hours, which supports once-daily intravenous dosing Dalbavancin and Oritavancin – They are semisynthetic lipoglycopeptides derived from teicoplanin. – Same MOA as vancomycin & teicoplanin; oritavancin works by additional mechanisms, including disruption of cell membrane permeability & inhibition of RNA synthesis. – They have very long half-lives (> 10 days) permits once-weekly IV administration – More active than vancomycin 94  Other Cell Wall- Or Membrane-Active Agents DAPTOMYCIN It is a novel cyclic lipopeptide fermentation product of Streptomyces roseosporus Its spectrum of activity is similar to that of vancomycin except that it may be active against vancomycin-resistant strains of enterococci and S aureus. It is a cyclic lipopeptide antibiotic with potent bactericidal activity against most Gram-positive organisms including multiple antibiotic- resistant and -susceptible strains The precise mechanism of action is not fully understood, but it is known to bind to the cell membrane via calcium-dependent insertion of its lipid tail. This results in depolarization of the cell membrane with potassium efflux and rapid cell death  Binding & insertion to the cytoplasmic membrane Oligomerization Membrane & Channel depolarization & formation K+ Ion efflux Proposed mechanism of action of daptomycin. Finally, it is followed by arrest of DNA, RNA, & protein synthesis resulting in cell death. Cell lysis does not occur.  Other Cell Wall- Or Membrane-Active Agents… Bacitracin It is a cyclic peptide mixture first obtained from the Tracy strain of Bacillus subtilis in 1943. It is active against Gram-positive microorganisms. Inhibits cell wall formation by interfering with dephosphorylation in cycling of the lipid carrier that transfers peptidoglycan subunits to the growing cell wall. There is no cross-resistance between bacitracin and other antimicrobial drugs. Highly nephrotoxic when administered systemically and is only used topically & it is poorly absorbed & topical application results in local antibacterial activity.  Other Cell Wall- Or Membrane-Active Agents… CYCLOSERINE It is an antibiotic produced by Streptomyces orchidaceous. It is water soluble and very unstable at acid pH. Inhibits many Gram-positive & Gram-negative organisms but it is used almost exclusively to treat tuberculosis caused by strains of M. tuberculosis resistant to first-line agents. Inhibiting cell-wall biosynthesis in bacteria It is a structural analog of D-alanine and inhibits the incorporation of D- alanine into peptidoglycan pentapeptide by inhibiting alanine racemase, which converts L-alanine to D-alanine and D-alanyl-D-alanine ligase. D-alanyl-D-alanine D-alanine Bacterial Protein Synthesis Inhibitors Med Chem. Tadesse B. (PhD) 99  Bacterial Protein Synthesis Inhibitors Inhibit bacterial protein synthesis by binding to and interfering with ribosomes Divided into two groups – Bacteriostatic: Chloramphenicol, Macrolides, Clindamycin & Tetracyclines – Bactericidal: Aminoglycosides Med Chem. Tadesse B. (PhD) 100 Video: Protein Synthesis Inhibition Med Chem. Tadesse B. (PhD) 101 Tetracyclines Med Chem. Tadesse B. (PhD) 102  Tetracyclines  They are a family of broad spectrum antibiotics  Some tetracyclines are obtained Naturally from Streptomyces species Others are semisynthetically prepared  Basic system octahydronaphthacene with annulated 6– membered ring group named -“Tetracyclic system’’  Nomenclature:  1,4,4a,5,5a,6,11,12a-Octahydronaphthacene Med Chem. Tadesse B. (PhD) 103  Protein synthesis inhibitors: Tetracyclines Are broad spectrum antibiotics which are synthesized by a number of streptomyces species It includes: Tetracycline Oxytetracycline Tigecycline Chlortetracycline Minocycline Demeclocycline Doxycycline Their principal difference is pharmacokinetics Med Chem. Tadesse B. (PhD) 104  Tetracyclines… They are broad spectrum bacteriostatic antibiotics that inhibit protein synthesis – Active against many Gram-positive & Gram-negative bacteria, including certain anaerobes, rickettsia, chlamydia, and mycoplasmas. Mechanism of Action:- Act by binding to the 30s ribosome o They are specific inhibitors of bacterial protein synthesis o The bacterial ribosome consists of both a large (50s) and small (30s) subunits that come together to form the intact, 70s ribosome o They bind to a ribosomal subunit (30S) o Preventing the aminoacyl tRNA from binding to its site on the ribosomal subunit leading to termination of the peptide chain growth Med Chem. Tadesse B. (PhD) 105 Mechanism of Action of Tetracyclines… o Both tetracyclines and the aminoacyl tRNA binding procedures require magnesium ions o The active site of the ribosome is composed of three subsites: o The aminoacyl site (A), the eptidyltransfer site (P), and the exit site (E) o Translation is initiated with the recognition and binding of mRNA, which encodes a specific protein, to the o 30s subunit in the presence of an initiator transfer RNA (tRNA) and of initiation factor 2 (IF2) o Binding of the initiator tRNA in the P-site is followed by binding of the 50s subunit, generating a translation competent ribosome particle Med Chem. Tadesse B. (PhD) 106 Med Chem. Tadesse B. (PhD) 107  Classification of TTCs  Short acting (half-life 6-8 hours) o Tetracycline, chlortetracycline, Oxytetracycline  Intermediate acting: (half-life 16 hours): o Demeclocycline  Long acting: (half-life18-24 hours) o Doxycycline and minocycline  Longest acting: (half-life-36 hours) o Tigecycline; the newest TTC Med Chem. Tadesse B. (PhD) 108  Chemical Properties of Tetracycline (model structure)  The carbon atoms 4, 4a, 5a, 6 and 12a are asymmetric depending on the substitution  They are yellow amphoteric compounds forming water soluble salts with acids or bases  The 4-dimethylamino moiety is basic, and the phenolic 3-OH is acidic  In neutral solution it exists mainly as Zwitter ion between C-3 and C-4 Med Chem. Tadesse B. (PhD) 109  Chemical Properties of Tetracycline cont. An interesting property of tetracycline is their ability to undergo epimerization at C-4 in solution of intermediate pH range These isomers are called epitetracyclines Under acidic conditions, an equilibrium is established in about 1 day & consists of approximately equal amounts of the isomers The 4-epitetracyclines have been isolated & characterized They exhibit much less activity than the “natural” isomers, thus accounts for a decrease in therapeutic value of aged solutions Med Chem. Tadesse B. (PhD) 110  Chemical Properties of Tetracycline cont.  Strong acids & strong bases attack the tetracyclines with a hydroxyl group on C-6, causing a loss in activity through modification of the C ring  Effect of Strong Acids (Inactive 5,6-Anhydrotetracycline)  Strong acid produce a dehydration through a reaction involving the 6-OH group & 5a-H  The double bond is formed between positions 5a & 6 induces a shift in the position of the double bond between C-11a & C-12 to a position between C- 11 & C-11a, forming the more energetically favored resonant system of naphthalene group found in the inactive anhydrotetracyclines Med Chem. Tadesse B. (PhD) 111  Chemical Properties of Tetracycline cont.  Effect of Bases (Ring “C” Cleavage and Inactive Isotetracycline) In alkaline solution at above pH 8.5, they promote a reaction between the 6-hydroxyl group & the ketone group at the 11- position, Causing the bond between the 11 & 11a atoms to cleave and to form the lactone ring, isotetracycline, which is inactive Med Chem. Tadesse B. (PhD) 112  Chemical Properties of Tetracycline cont.  The hydrochloride salt is used as capsules for oral administration  Form an insoluble chelates Tetracycline forms, at neutral pH, insoluble chelates with polyvalent metal ions as Fe2+, Al3+, Ca2+, Mg2+ Therefore, tetracyclines are incompatible when co- administered with Multivalent ion-rich antacids, Hematinics and Products rich in calcium ions Med Chem. Tadesse B. (PhD) 113  Contraindications of Tetracyclines  Tetracyclines are contraindicated to children It react with calcium of newly forming bones and teeth Yielding tetracycline-calcium phosphate complexes Disposition in teeth causes yellow discoloration that darkens over time  Tetracyclines are contraindicated to pregnant or lactating mothers  Since they cross the placental barrier into the fetus and distribute into milk of lactating mothers Med Chem. Tadesse B. (PhD) 114 Structure of Natural Tetracyclines Med Chem. Tadesse B. (PhD) 115  Structure of Semisynthetic Tetracyclines  They are highly stable towards acids and bases  Doxycycline is the tetracycline of choice because it causes  Fewer gastrointestinal disturbances and not participating in degradation process due to absence of 6-hydroxy group  Minocycline is primarily used to treat acne and other skin infections  Sancycline: the structure is the minimal requirement for activity Med Chem. Tadesse B. (PhD) 116  Structure-Activity Relationships of Tetracyclines  The 4 fused rings are essential for the activity  The 4-dimethylamino group is essential for activity & must have α-orientation  Removal of the 6-OH group leads to increase in stability and no change in activity  Removal of the 6-CH3 group leads to no change in activity  Conjugation between C-10 to C-12 is essential for biological activity Med Chem. Tadesse B. (PhD) 117  Structure-Activity Relationships of Tetracyclines cont.  A 7-Cl (EWG as in Demeclocycline) or a 7-N(CH3)2 (EDG as in Minocycline) increases activity  Replacement of amino at C-2 with other functions, such as aldehyde or nitrile reduces or abolishes activity  A (Cis–A/B ring fusion with a -hydroxyl) group at C-12 is essential  Esters of 12a-OH is active, except formyl ester  Alkylation at C-11a leads to inactive compound Med Chem. Tadesse B. (PhD) 118 AMINOGLYCOSIDE ANTIBIOTICS Med Chem. Tadesse B. (PhD) 119  AMINOGLYCOSIDE ANTIBIOTICS They comprise a large group of naturally occurring and semisynthetic antibiotic drugs Are antibiotics that contain an aminocyclitol moiety (1,3-diguanidinoinositol) to which aminosugars are glycosidically linked They may be more correctly called aminocyclitol antibiotics Med Chem. Tadesse B. (PhD) 120  AMINOGLYCOSIDE ANTIBIOTICS cont.  They are a class of antibiotics that contains a pharmacophoric 1,3-diamino derivatives of inositol namely: Streptamine 2-deoxystreptamine Spectinamine Streptidine or Fortamine Streptidine and Streptamine can be called 1,3- diguanidino and 1,3-diamino inositol, respectively Med Chem. Tadesse B. (PhD) 121  AMINOGLYCOSIDE ANTIBIOTICS cont.  These derivatives (1,3-diamino derivatives of inositol ) are chemically attached to an aminosugar in glycosidic linkage  Contains cyclohexanes with several substituted or unsubstituted amino and hydroxyl groups, which bring them high water solubility  All have an aminohexose as the amino sugar and some have a pentose as an extra sugar  The various aminoglycoside antibiotics include:  Streptomycins, Gentamicins, Neomycins, Paromomycins, Kanamycins, Spectinomycins, Tobramycin, Netelmicin and Sisomicin Med Chem. Tadesse B. (PhD) 122  AMINOGLYCOSIDE ANTIBIOTICS cont.  They are produced mainly from Streptomyces species and some are from Micromonospora species  They are freely water-soluble  They are basic in nature and form salts with acids  When given orally, their action is primarily locally Since they are not well absorbed from the gastrointestinal tract  They are more commonly given intramuscularly and are excreted mainly in the urine  Their clinical use is limited, due to their high toxicity  They can also lead to ototoxicity causing hearing loss and vertigo, and  Their use may lead to kidney tubular necrosis Med Chem. Tadesse B. (PhD) 123 AMInoGLYCosIDe AntIBIotICs Cont. Med Chem. Tadesse B. (PhD) 124  AMINOGLYCOSIDE ANTIBIOTICS cont. Mechanism of Action  The aminoglycosides are bactericidal agents  Inhibition of protein biosynthesis initiation upon attachment to 30s portion of ribosomes  Misreading mutation of the genetic code and the synthesis of nonsense proteins, which are not normal proteins so they cannot take part in cellular activities Nonsense proteins disturb the semi-permeability of the bacterial cell  Aminoglycoside molecules enter the cell easily and kill it Med Chem. Tadesse B. (PhD) 125 MOA of Aminoglycosides… Med Chem. Tadesse B. (PhD) 126  AMINOGLYCOSIDE ANTIBIOTICS cont.  Bacterial Resistance against Aminoglycosides Resistant strains have emerged against streptomycin, kanamycin and gentamycin in clinic “R-factor mediated enzymes” are responsible for the production of aminoglycoside deactivating enzymes, preventing ribosomal binding: Acetyl transferases (AAC) Phosphotransferases (APH), Nucleotidyl transferases (ANT) These enzymes transfer to hydroxyl and amino groups of the drug Med Chem. Tadesse B. (PhD) 127  AMINOGLYCOSIDE ANTIBIOTICS cont. Aminoglycoside Deactivating Enzymes  AAC acetylates 3-NH2 of the ring II, and 2`, 6`- NH2 of the ring I  APH phosphorylates 3`-OH of the ring I and 2”-OH of the ring III  ANT adenylates 2”,4”-OH of the ring III and 4`-OH of the ring I Med Chem. Tadesse B. (PhD) 128  Structure Activity Relationships of Aminoglycosides  Ring I is very necessary for broad-spectrum antibacterial activity  2` and 6`-NH2 groups are specially important  Exchanging of one of them in kanamycin B with hydroxyl group decreases the activity (kanamycin A, C)  Methylation of C-6` or 6`-NH2 doesn’t alter the antibacterial activity but increases the resistance against AAC Med Chem. Tadesse B. (PhD) 129  Structure Activity Relationships of Aminoglycosides cont.  Omitting the 3`-OH and/or 4`-OH in kanamycin  Doesn’t decrease the antibacterial activity  But increases the resistance against Phosphotransferases (APH)  3`,4`-dideoxykanamycin B (Dibekacin)  The same is true for gentamycin  Omitting the 3`-OH and 4`-OH and the addition of a double bond between C- 4` and C-5`has the same effect Med Chem. Tadesse B. (PhD) 130 Structure Activity Relationships of Aminoglycosides cont. Ring II is flexible toward changes  1-NH2 in kanamycin can be acylated and the antibacterial activity remains almost unchanged  But resistance against deactivating enzymes increases-(Amikacin)  1-NH2 ethylation of sisomycin saves the antibacterial activity  Increases the enzymatic resistance-(Netilmycin) Med Chem. Tadesse B. (PhD) 131  Structure Activity Relationships of Aminoglycosides cont. Ring III functional groups are less sensitive to modifications  2”-deoxy gentamicins are less active than 2”-OH ones but 2”-NH2 derivative (seldomycin) are very active  3”- NH2 can be primary or secondary  4”-OH can be axial or equatorial but the former is resistant against the deactivating enzymes nucleotidyl transferases (ANT) Med Chem. Tadesse B. (PhD) 132 AMINOGLYCOSIDE ANTIBIOTICS cont. Med Chem. Tadesse B. (PhD) 133 Macrolides Med Chem. Tadesse B. (PhD) 134  Macrolides  Macrolides are a group of closely related antibiotics produced by Streptomyces  Have three common chemical characteristics  A large lactone ring (so termed as macrolide antibiotics) ketone group  Usually the lactone ring has 12, 14, 15 or 16 atoms in it and often unsaturated with olefinic group conjugated with ketone function The most common drugs have 14 atoms  A glycosidically linked amino sugar (Usually cladinose & Desosamine)  Dimethyl amino group on sugar moiety, base that form salt (pka =6-9) Med Chem. Tadesse B. (PhD) 135  Macrolides… Fig. Ketolides & Macrolides relationship Med Chem. Tadesse B. (PhD) 136  Macrolides cont.  The clinically used drugs of this group are: erythromycins, clarithromycin, azithromycin, oleandomycin, spiramycin, leucomycins and josamycin  They usually have 2 sugars attached to the ring  One sugar of them carries a substituted amino group So these drugs are weakly basic and form salts with acids  The macrolides are chemically unstable in acid due to the formation of inactive internal cyclic ketal  The 6-OH group is essential to the process, and is initiated by protons (H+) It is one of the causes of intestinal cramping that is encountered with macrolide use Med Chem. Tadesse B. (PhD) 137  Macrolides cont. Antimicrobial Activity  They have a narrow antibacterial spectrum of activity  They are often used for the treatment of upper and lower respiratory tract infections caused by Gram-positive microorganisms and Are also used in some sexually transmitted disease as gonorrhea  Some derivatives induce the stimulation of gastrin production resulting in hyperperistalsis This causes gastrointestinal cramps in some patients Med Chem. Tadesse B. (PhD) 138  Macrolides cont.  Mechanism of Action  They inhibit bacteria by interfering with programmed ribosomal protein biosynthesis  Resistance  Resistant bacteria possess “R-factor enzymes”, which can methylate a guanine residue on their own ribosomal RNA  Making them less efficient at protein synthesis and inhibit binding to some 14-membered ring macrolides and azalides  But not 16-membered ring macrolides or ketolides macrolides Med Chem. Tadesse B. (PhD) 139  14-Membered Ring Macrolides  Includes : Erythromycin A, Clarithromycin, Troleandomycin, Oleandomycin  Erythromycin A  Erythromycin is a complex of 6 components (A-F) and is produced from Saccharopolyspora erythraea (formerly Streptomyces erythreus)  Erythromycin A is only clinically used and is available for oral administration Med Chem. Tadesse B. (PhD) 140  14-Membered Ring Macrolides cont. Clarithromycin  A semisynthetic derivative of erythromycin A by converting the 6-OH group to a -OCH3 group, thus preventing internal ketal formation through this group  Conversion to the 6-OCH3 leads to better blood levels and better absorption  Internal ketalization and inactivation may occur b/n 9-keto group and 12-OH group  Clarithromycin is associated with less gastric upset Med Chem. Tadesse B. (PhD) 141  15-Membered Ring Macrolides Azithromycin (An azalide)  Semisynthetic derivative of erythromycin prepared by removal of “O” on C-9 and a ring expansion by inserting an N-CH3 group between C-9 and C-10, and given the number 9a  More stable to acid degradation (lacks 9-keto function) than erythromycin Med Chem. Tadesse B. (PhD) 142  15-Membered Ring Macrolides cont. Azithromycin (An azalide)  Has long half-life due to great tissue penetration, thus allowing once-a-day dosage  Has higher activity against Gram negative organisms than erythromycin and clarithromycin  The drug should be taken on an empty stomach 16-Membered Ring Macrolides  Includes: Spiramycin, Leucomycins, Josamycin Spiramycin Med Chem. Tadesse B. (PhD) 143  Ketolides Fig. Structure of the Telithromycin is a semi-synthetic erythromycin ketolide telithromycin derivative. It is created by substituting a keto-group for the cladinose sugar and adding a carbamate ring in the lactone ring. An alkyl-aryl moiety is attached to this carbamate ring. Furthermore, the oxygen at the 6 position is methylated, as is the case with clarithromycin, to achieve better acid-stability. Med Chem. Tadesse B. (PhD) 144  Ketolides… The ketolides, of which telithromycin is the first to reach clinical development, represent a new generation of antimicrobials that have been developed with a view to overcoming the problem of macrolide resistance. Telithromycin is structurally derived from macrolides and possesses several distinguishing features that are important for its improved microbiological profile. The L-cladinose at position C3 of the macrolactone ring has been replaced with a keto function. – This modification enables telithromycin to bind to its target without tripping the inducible resistance to macrolide–lincosamide–streptograminB (MLSB) drugs that many groups of pathogens exhibit. The C6 position has been modified by the addition of a methoxy group. – This helps prevent hemiketalization of the C6 position with the 3- and 9- keto groups, thereby conferring excellent acid stability, particularly at gastric pH values. Med Chem. Tadesse B. (PhD) 145  Ketolides… Telithromycin is differentiated from other ketolide compounds by the addition of a large aromatic N-substituted carbamate extension from positions C11/C12. This carbamate extension improves binding of the drug to its target, the 50S ribosomal subunit, as demonstrated in in vitro experiments. Telithromycin binds to wild-type ribosomes with 10-fold greater affinity than erythromycin A and 6-fold greater affinity than clarithromycin; its affinity for MLSB-resistant ribosomes is > 20 times that of both macrolides. Med Chem. Tadesse B. (PhD) 146 POLYENE ANTIBIOTICS Med Chem. Tadesse B. (PhD) 147  POLYENE ANTIBIOTICS  There are about 60 antibiotics under this class and mainly used for mycosis  The molecule contains a macrocyclic lactone ring with 4 to 7 conjugated double bonds  The ring in many cases is glycosidically linked to an aminosugar  Polyenes antibiotics are active against fungal, algal and protozoal cells  Continuous use may lead to many side effects as  Hypokalemia, anemia, azotemia and renal complications that may end with permanent renal damage  The mode of action is to combine to  Sterols present in cell membrane (e.g. ergosterol in fungi), leading to alteration in permeability and loss of essential cell constituents  Mammalian cell membrane is affected (contains cholesterol), but bacterial cell is not affected Med Chem. Tadesse B. (PhD) 148  POLYENE ANTIBIOTICS cont.  The most common polyenes are: Tetraenes as nystatin and pimaricin Pentaenes as filipin Hexaenes as endomycin Heptaenes as amphotericin B, candicidin, hamycin and trichomycin  Heptaenes are the most active and cause less damage to host cell membranes  Amphotericin B is used by IV injection in systemic Candidiasis and mycosis Nystatin is poorly absorbed, and has no systemic effect It used orally, topically or intravaginally for different kinds of candidiasis Med Chem. Tadesse B. (PhD) 149 MISCELENEOUS ANTIBIOTICS Med Chem. Tadesse B. (PhD) 150  Chloramphenicol (CAF)  Originally produced by fermentation of Streptomyces venezuelae, but it is now totally synthesized  Has 2 asymmetric centers, resulting in 4 diastereoisomers but one isomer (1R, 2R) D-threo is significantly active  When given orally it is rapidly absorbed but it has a short half-life  Active against a wide range of Gram negative and Gram positive bacteria, some rickettsiae and viruses  Mainly excreted in urine in the form of its metabolites Med Chem. Tadesse B. (PhD) 151  Chloramphenicol cont.  Diffuses into cerebrospinal fluid, so used in meningitis  Penetrates lymph and mesenteric ganglia, so used in typhoid and paratyphoid fever  The C-3 palmitate ester is prodrug used to mask its bitter taste for use as an oral pediatric suspension  The C-3 hemisuccinate ester sodium salt is water soluble prodrug for parenteral use  Cleaved by esterases to produce the active drug in lungs, liver, kidneys and blood  Mechanism of Action  Chloramphenicol inhibits bacterial protein biosynthesis by binds to 50 S r-RNA and inhibit formation of peptide bond Med Chem. Tadesse B. (PhD) 152  Structure Activity Relationship Chloramphenicol Replacement of phenyl group by other aromatic systems or cyclic systems e.g. cyclohexyl, furyl, naphthyl, pyridyl or thienyl results in loss of activity Replacement of NO2 by NH2, NHR, OH, SO2R, CN results in loss of activity Shifting of NO2 from para-position leads to loss of activity The propanediol moiety should be in D-(-) threo-isomer. Other isomers are inactive Med Chem. Tadesse B. (PhD) 153  Structure Activity Relationship Chloramphenicol… Replacement of OH & extension or suppression of terminal CH2OH abolishes the activity Replacement of nitro group by other electron withdrawing groups gives active compounds as:CH3SO2 (Thiamphenicol), CH3CO (Cetophenicol) Replacement of dichloro group by azido group gives active compounds as Azidamphenicol Med Chem. Tadesse B. (PhD) 154  Prodrugs of Chloramphenicol Chloramphenicol has poor water solubility and is intensively bitter The bitter taste can be masked for use as a pediatric oral suspension by use of the C-3 palmitate, which has extremely low solubility. – The ester is cleared in the duodenum to liberate the drug The conversion to the 3-hemisuccinyl ester overcome the poor water solubility of the drug, which forms a water-soluble sodium salt suitable for parental preparation. – This is cleaved in the body to produce active chloramphenicol. Chloramphenicol succinate ester Chloramphenico palmiate Med Chem. Tadesse B. (PhD) 155  Toxicity of Chloramphenicol When used for a long time → reversible agranulocytosis and thrombocytopenia Hematoxicity is due to the formation of nitroso and hydroxyl amines due to the reduction of the aromatic nitro group (reversible when the drug is discontinued) Gray baby syndrome: A syndrome due to toxicity of the antibiotic CAF in the new-born, especially the premature new-born: – Because of lack of the necessary liver enzymes to metabolize the drug. Chloramphenicol accumulates in the baby causing – Hypotension – Cyanosis (blue coloring of lips, nail beds, & skin from lack of oxygen in the blood), – Death Chloramphenicol is therefore usually not given to new-borns or premature babies. Med Chem. Tadesse B. (PhD) 156  Bacterial Resistance of CAF Bacterial resistance to chloramphenicol arises from the ability of certain strains of bacteria to produce chloramphenicol acetyltransferase, an enzyme that acetylates OH at C-1 & C-3 of the propanol moiety to produce 1-acetoxy and 3-acetoxy derivatives, respectively, which are devoid of any activity 1-Acetoxy derivative 3-Acetoxy derivative Med Chem. Tadesse B. (PhD) 157  Lincomycins  Are sulfur containing antibiotic isolated from S. lincolnensis  They are the most active & medically useful of the compounds obtained from fermentation  Extensive efforts to modify lincomycin structure to improve its antibacterial and pharmacological properties resulted in  preparation of 7-chloro-7-deoxy derivative clindamycin  Resemble macrolides in Antibacterial spectrum and biochemical mechanism of action  Binds to 50s ribosomal to inhibit protein synthesis Med Chem. Tadesse B. (PhD) 158  Lincomycins cont.  Structure contains a basic function  Pyrrolidine nitrogen, by which H2O soluble salts with Pka of 7.6 may be formed  When subjected to hydrazinolysis, lincomycin is cleaved at its amide bond into Trans-L-4-n-propylhygric acid (pyrrolidine moiety) and  Methyl α-thiolincosamide (sugar moiety) Med Chem. Tadesse B. (PhD) 159  SAR of Lincomycins  Lincomycin – related antibiotics have been synthesized  These antibiotics differ in structure at one or more of three position of lincomycin structure  N-methyl group of hygric acid moiety is substituted by a hydrogen  Propyl group of hygric acid moiety is substituted by an ethyl group  Thiomethyl ether of the α-thiolincosamide moiety is substituted by a thioethyl ether H Med Chem. Tadesse B. (PhD) 160  SAR of Lincomycins cont.  Replacement of 7(R)–OH group of lincomycin by chlorine with inversions of configuration resulted in compounds with Enhanced antibacterial activity in vitro Improved absorption and higher tissue levels due to higher partition coefficient Cl Med Chem. Tadesse B. (PhD) 161 Sulphonamides & Related Drugs Med Chem. Tadesse B. (PhD) 162 Sulfonamides and folate reductase inhibitors General structure The general term “sulphonamide“ has been used for derivatives of p-aminobenzene sulphonamide (sulfanilamide, R=H) Whereas specific compounds are described as N1- or N4-substituted sulfanilamides depending on whether the substitution is on the amido or aromatic amino group, respectively Med Chem. Tadesse B. (PhD) 163  Sulfonamides (Sulphonamides)  Group of related compounds collectively called sulfa drugs  Inhibit growth of Gram-positive and Gram-negative organisms Through competitive inhibition of enzyme that aids in production of folic acid  Structurally similar to para-aminobenzoic acid (PABA) PABA is a substrate in folic acid pathway  Human cells lack specific enzyme in folic acid pathway Basis for selective toxicity  Develop resistance due to plasmid codes for enzyme that has lower affinity to drug Med Chem. Tadesse B. (PhD) 164  Discovery of sulphonamides  The antibacterial activity of sulphonamides was first discovered by the observation that Azo dye prontosil caused remarkable cure of Streptococcal infections of mice However, Prontosil was inactive on bacterial cultures  Prontosil is inactive in vitro, but has excellent activity in vivo Confirmed that sulphonamide portion of prontosil is the responsible for the observed antibacterial action Med Chem. Tadesse B. (PhD) 165  Mode of Action  Folic acid is essential for the growth. of many bacteria  PABA is essential for the synthesis of folic acid  Sulfonamides are PABA analogs and thus competitively inhibit the dihydropteroate synthetase Preventing the addition of PABA to pteridine diphosphate and blocking the net biosynthesis of folate coenzyme This action arrests the bacterial growth and cell division, which are bacteriostatic Med Chem. Tadesse B. (PhD) 166  Trimethoprim  Inhibits folic acid production interferes with activity of enzyme following enzyme inhibited by sulfonamides  Often used synergistically with sulfonamide  Does not have high affinity for the malaria protozoan’s folate reductase, but it does have high affinity for bacterial folate reductase However, it also has some affinity for the mammalian folate reductase, which cause toxicity Med Chem. Tadesse B. (PhD) 167 Mode of Action Med Chem. Tadesse B. (PhD) 168 Structural relationships among sulfonamides, para-aminobenzoic acid Sites of action of sulfonamides and (PABA), and folic acid trimethoprim Tadesse B. (PhD) 169 Med Chem. Tadesse B. (PhD) 170  Classification of Sulphonamides A) Systemic sulfonamides B) Intestinal sulfonamides C) Sulfonamides for ophthalmic infections & D) Topical sulfonamides for burn therapy A) Systemic sulfonamides They are used in the treatment of systemic infections & has three subclasses based on the duration of action Short acting sulfonamides:  Intermediate acting sulfonamides Long acting sulfonamides Med Chem. Tadesse B. (PhD) 171  Short acting sulfonamides  Rapidly absorbed and excreted with biological half lives ranging from 4-7 hours  Examples: sulfamethazine, sulfamethizole, sulfapyridine, and sulfisoxazole Med Chem. Tadesse B. (PhD) 172  Intermediate acting sulfonamides Absorbed and excreted some what more slowly than the short acting sulfonamides Half-lives range from 10-12 hours given twice daily The main representatives are: sulfadiazine, sulfamethoxazole & sulfaphenazone Med Chem. Tadesse B. (PhD) 173  Long acting sulfonamides Rapidly absorbed and slowly excreted Half-lives are 35-40hrs e.g. Sulfadoxine has a half life of 179 hrs The slow exertion is related to an optimal lipophilicity Med Chem. Tadesse B. (PhD) 174  B) Intestinal sulfonamides  Prodrugs designed to be poorly absorbable  In the large intestine, N4 protecting groups are cleaved, releasing the free sulfonamide antibacterial agent Examples: sulfasalazine  Sulfasalazine used in treatment of acute exacerbation of ulcerative colitis after being broken down in the body to 5-aminosalicylic acid and Sulfapyridine Med Chem. Tadesse B. (PhD) 175  C) Sulfonamides for ophthalmic infections Sulfacetamide – as sodium solution for repeated topical applications in the local management of ophthalmic infections Sulfisoxazole diolamine – used in IV and IM preparations Med Chem. Tadesse B. (PhD) 176  D) Topical sulfonamides for burn therapy  Mafenide acetate: a homologue of sulfanilamide molecule Do not inhibited by PABA (not a true sulfanilamide type compound) Its antibacterial action involves a mechanism that differs from that of true sulfanilamide type compounds Effective against Clostridium welchii in topical application but ineffective orally Currently used alone or with antibiotics in the treatment of slow healing, infected wounds  Silver sulfadiazine: effective topical antimicrobial agent, especially against Pseudomonas species Med Chem. Tadesse B. (PhD) 177  Crystalluria and the pka of sulfonamides  Can cause severe renal damage by crystallizing in the kidneys  Sulfanilamide’s and their metabolites (usually acetylate at N4) are excreted almost entirely in the urine  The pka of the sulfonamide group of sulfanilamide is 10.4  Unless pH is above the pka, little of the water soluble salt is present  Urine –pH 6 (and potentially lower during bacterial infections) All of the sulfonamide is relatively insoluble, non-ionized form in the kidneys Brings crystalluria Med Chem. Tadesse B. (PhD) 178  Structure Activity Relationships (SAR) of sulfonamides  R’ =H or acyl  The SO2 group must attach to the benzene ring  The para nitrogen (N4) is required for activity and must be a free amino group However, prodrugs are available which must be converted to the free amine for activity Major route of metabolism is acetylation of N4 that actually reduces water solubility The free amino group is essential for activity, if it is replaced by groups which can be converted in vivo to free amino group Activity is maintained in vivo but not in vitro such as NO2, NO, NHOH, -N=N-, NHCOR Med Chem. Tadesse B. (PhD) 179  SAR of sulfonamides cont.  Compounds with alkylated aromatic amino groups showed no activity If the amino group is replaced by an alkyl or alkoxy or other functional groups no activity is observed  If the amino group is changed to the 2/6 or 3/5 position on the benzene ring, inactive compounds result  N4 -acylation with dicarboxylic acids such as succinic acid or phthalic acid Yields sulfonamide that is not absorbed in the small intestine, but hydrolyzed in the large intestine to free sulfonamides (intestinal sulfonamides) Med Chem. Tadesse B. (PhD) 180  SAR of sulfonamides cont.  Substitution on the amidic nitrogen with various groups results in a wide fluctuation in activity and this is the most important type of modification that has been used in the design of antibacterial sulfonamides  Substitution on the aromatic ring of the sulfonamides either  Diminishes or completely destroys the activity  Also replacement of the benzene ring by other ring systems decreases the activity Med Chem. Tadesse B. (PhD) 181  SAR of sulfonamides cont.  N1 is acidic and must have one free H attached (cannot be tertiary)  Increasing acidity (decreasing pka)  Increases enzyme affinity  Increases water solubility which has less risk of crystalluria formation and shorter duration due to more renal excretion unchanged  So, N1 substituents should be electron withdrawing  The more electron withdrawing the lower the pka  With a pka of 10.4, the drug is mostly unionized at urinary pH and so crystalluria is a problem  With a pka of 4.9, the drug is mostly ionized at urinary pH and variety of studies shown that the active form is N1-ionized salt Med Chem. Tadesse B. (PhD) 182 Urinary Tract Anti-infectives Med Chem. Tadesse B. (PhD) 183  Introduction The urinary tract (UT) is an organ that produces, stores & eliminates urine In humans, it includes the  Two kidneys  Two ureters  The urinary bladder  Two sphincter muscles &  The urethra Urinary tract infections (UTIs) are bacterial infections that affect any part of the urinary tract Urinary tract anti-infectives are agents used to treat UTIs Med Chem. Tadesse B. (PhD) 184  Introduction cont. UT anti-infectives generally fail to achieve adequate concentrations in the plasma or tissues for the treatment of systemic infections following oral or parenteral administrations They are concentrated in urine & are effective for eradicating UTIs A number of compounds (drugs) used as urinary tract anti- infectives Med Chem. Tadesse B. (PhD) 185  Quinolones  Are a family of broad spectrum antibiotics  Comprises a group of synthetic substance possessing in common an N-1-alkylated-3-carboxy pyrid-4-one ring fused to another aromatic rings  The parent of the group is Nalidixic acid Carry 4-oxo-1,4-dihydroquinoline system The first quinolone to be marketed in 1965 Primarily reserved for oral treatment of uncomplicated UTIs caused by susceptible microorganisms usually Escherichia coli Useful in the treatment of UTIs in which gram negative bacteria predominate Med Chem. Tadesse B. (PhD) 186  Quinolones cont.  Isosteric heterocyclic groupings in this class includes the Quinolones e.g. (Norfloxacin, Ciprofloxacin) Naphthyridines (Nalidixic acid and Enoxacin) and Cinnolines (e.g. Cinoxacin)  Analog design of nalidixic acid lead first generation quinolones: Oxolinic acid, cinoxacin & enoxacin but are not popular today Med Chem. Tadesse B. (PhD) 187  Quinolones cont.  Enoxacin retains the 1,8-naphthyridine nucleus with fluoro- & N- piperazinyl substitution on position 6 & 7, respectively  Isosteric replacement of naphthyridine’s nitrogen in position 8 With –CH group (with 4- quinolone compound) & Tying up position 6 & 7 with a methylenedioxy bridge led to oxolinic acid Med Chem. Tadesse B. (PhD) 188  Quinolones cont.  Further modification of oxolinic acid Replacing the –CH group at position 2 with isosteric N developed a cinnoline product, cinoxacin Cinoxacin Has an increased potency & broadened the spectrum of activity against Gram negative bacteria Well absorbed orally & highly protein bound which brings its long half-life Has antibacterial activity similar to nalidixic acid  Further research in 1980s introduced a fluoroquinolone, second generation (Norfloxacin) Med Chem. Tadesse B. (PhD) 189  Generation of Fluoroquinolones 1st Generation 2nd generation 3rd generation 4th Generation Cinoxacin Ciprofloxacin Levofloxacin Trovafloxacin Nalidixic Ofloxacin Sparfloxacin Acid Oxolinic Lomefloxacin Gemifloxacin acid Norfloxacin Gatifloxacin Enoxacin Moxifloxacin Increased gram positive coverage with each generation Med Chem. Tadesse B. (PhD) 190  Generation Fluoroquinolone The newer fluoroquinolones have – Broad-spectrum bactericidal activity, – Excellent oral bioavailability – Good tissue penetration and – Favorable safety and tolerability profiles. A new fourth generation classification of the quinolone drugs takes into account the expanded antimicrobial spectrum of the more recently introduced fluoroquinolones and their clinical indications The quinolones can be differentiated within classes based on their pharmacokinetic properties First-generation drugs (e.g., nalidixic acid) achieve minimal serum levels Second-generation quinolones (e.g., ciprofloxacin) have increased gram- negative and systemic activity Third-generation drugs (e.g., levofloxacin) have expanded activity against gram-positive bacteria and atypical pathogens Fourth-generation quinolone drugs (currently only trovafloxacin) add significant activity against anaerobes Med Chem. Tadesse B. (PhD) 191  Quinolones cont.  Properties of Quinolones Has low incidence of CNS effects Phototoxicity with those containing a halogen at position 8 e.g. Sparfloxacin, Lomefloxacin Can be classified into classes based on their dissociation properties: Those containing only 3-carboxylic acid group as ionizable functionality e.g. Nalidixic acid, Oxolinic acid, Cinoxacin Broad spectrum fluoroquinolones (3-COOH, a basic 7- piperazine & 6-fluoro) e.g. Ciprofloxacin, Norfloxacin, Enoxacin Highly polar quinolones enter bacterial cells through densely charged porin channels in the outer membrane Med Chem. Tadesse B. (PhD) 192  Quinolones cont.  Mechanism of Action  Quinolones and fluoroquinolones are bactericidal agents, actively killing bacteria  Quinolones inhibit the bacterial DNA gyrase (topoisomerase II) & topoisomerase IV  There by inhibiting DNA replication and transcription  DNA gyrase is an enzyme that is responsible for  Introducing negative supercoils into circular duplex DNA  Negative supercoiling relieves the torsional stress of helical DNA Facilitates unwinding & thereby allowing transcription & replication to occur  In many cases, bacterial strains that have developed resistance to other antibacterial antibiotics, such as Penicillin resistant Gonococcus Methicillin resistant S. aureus, and Aminoglycoside resistant P. aeruginosa Are susceptible to the quinolones Med Chem. Tadesse B. (PhD) 193 Structure-Activity Relationship of quinolones Briefly, the basic nucleus of quinolones can be modified at the N-1 position and at the C-6, C-7, and C-8 positions. Med Chem. Tadesse B. (PhD) 194  The structure-activity relationship of quinolones The C-4 ketone together with C-3 COOH are essential for activity basic scaffold, must 6 position substitution is be combined with aryl importent for their activities or heterocyclic rings F>Cl>CN>NH2>H, O O F HN OH N N substituted with N(CH3)2,CH3 or X will C2H5 substituted by R, R=C2H5; increase theantibacterial spectrum, in CH2CH2F;cyclopropal have which the piperazine substitution is the can be subs tit uted by any beeter antibacterial activity best one group i ncluding heteroc ycli c rings but F substi tut ionis most powerf ul Med Chem. Tadesse B. (PhD) 195 Structure Activity Relationships of Fluoroquinolones  The 1, 4-dihydro-4-oxo pyridine -3-carboxylic acid moiety is essential for antibacterial activities  The pyridone system must be annulated with aromatic ring  Alteration of 3-COOH group produce compounds with decreased activity Quinolones bind to the DNA gyrase via carboxyl group at position-3 in quinolone ring  Isosteric replacement of nitrogen for carbon atoms at position 2 (Cinnolines) 5 (1,5–naphthyridines) 6 (1, 6-naphthyridines) 8 (1, 8-naphthyridines) Are consistent with retention of activity  Fluorine atom at position 6 is associated with significantly enhanced antibacterial activity Med Chem. Tadesse B. (PhD) 196  SAR of Fluoroquinolones cont.  Alkyl substitution at position 1 (R2) is essential for activity  Lower alkyl (methyl, ethyl, cyclopropyl) compounds possess progressively greater potency  Aryl substitution retains antibacterial activity (2,4-difluorophenyl group is optimal)  Position 5,6,7 (esp.) & 8 of the annulated ring may be substituted with good effect  E.g. Piperazinyl & 3-aminopyrrolidinyl substitutions at position 7 enhanced activity against P. aeruginosa  Ring condensation at 1,8; 5,6; 6,7; & 7,8 position leads to active compounds  E.g. Ofloxacin (1,8)- an alternative to ciprofloxacin Med Chem. Tadesse B. (PhD) 197  SAR of Fluoroquinolones cont.  Addition of fluorine atom at position 6 enhances DNA gyrase inhibitory activity & furthermore, extends activity against Staphylococci & Piperazine group at position 7 extends antibacterial activity against Staphylococci & P. aeruginosa E.g. Ciprofloxacin  Additional fluorine atom at position 8 provides better absorption & longer half-life (Decreases rate of degradation) E.g. Lomefloxacin Med Chem. Tadesse B. (PhD) 198  SAR of Fluoroquinolones cont.  Substitution of methyl group for piperazine group results in better absorption & longer half-life E.g. Lomefloxacin & Sparfloxacin  Addition of cyclopropyl group at N-1, amino group at C-5 & Fluorine at C-8 extends spectrum against Mycoplasma & Chlamydia  E.g. Sparfloxacin Video: Mechanism of action & resistance of fluoroquinolones Med Chem. Tadesse B. (PhD) 199  Other Urinary Tract Anti- infectives  Nitrofurans  Are the first nitro heterocyclic compounds to be introduced into chemotherapy  The nitrofurans are derivatives of 5-nitro-2-furaldehyde formed on a reaction with appropriate hydrazine or amine derivative  Antimicrobial activity is present only when the nitrogen is in the 5- position  Mechanism of action  Nitrofurans inhibit microbial DNA synthesis  It has broad-spectrum of activity against G+ and G bacteria but it is not effective against fungi  Nitrofurantoin  Active against many G+ & G- UT pathogens  Antibacterial activity is higher in acidic urine Med Chem. Tadesse B. (PhD) 200  Methenamine and its salts  Methenamine The activity of methenamine depends on the liberation of formaldehyde Methenamine is used internally as a urinary antiseptic for treatment of chronic urinary tract infections The free base has practically no bacteriostatic power so formaldehyde release at the lower PH of the kidney is required Med Chem. Tadesse B. (PhD) 201  Urinary Analgesics  Pain and discomfort frequently accompany bacterial infections of the urinary tract  For this reason, certain analgesic agents, such as salicylates or phenazopyridine, which concentrate in urine because of their water solubility properties, are combined with urinary tract anti- infectives Phenazopyridine It is used as a local analgesic on the mucosa of UTs Med Chem. Tadesse B. (PhD) 202 Thank You Med Chem. Tadesse B. (PhD) 203