Antibiotics Unit 7 Past Paper PDF

Antibiotics Unit 7 PCHE 0301 Fall 2024-25  History of Antibiotics Father of Chemotherapy 1. Bacteria discovered- 1670- Leeuwenhoek 2. Pasteur- 3. Lister (phenol antiseptic)- surgical survival rate ,‘Germ theory of disease’ 4. Koch- TB, Cholera, typhoid 5....

Antibiotics Unit 7 PCHE 0301 Fall 2024-25  History of Antibiotics Father of Chemotherapy 1. Bacteria discovered- 1670- Leeuwenhoek 2. Pasteur- 3. Lister (phenol antiseptic)- surgical survival rate ,‘Germ theory of disease’ 4. Koch- TB, Cholera, typhoid 5. Paul Ehrlich; “Magic bullets” Arsenic compounds that killed microbes (1910). Selective toxicity, replaced by penicillin in 1945. 6. In 1935; Pronotosil dye, a prodrug of sulfanilamide; effective against systemic bacterial infections. 2  O O N S O Metabolism H2N S O H2N N NH2 NH2 Prontosil Sulfanilamide NH2  Sir Alexander Fleming (1929) accidently discovered Penicillin, it was introduced in 1940???. [isolation and purification problem] Florey and Chain introduced Penicillin in therapy (1938), isolated by freeze drying, & chromatography, Dorothy Hodgkins, 1945; X- ray crystal structure In 1942, Walksman proposed the definition of AB Definition: Antibiotics are metabolites derived from microorganism or their synthetic analogues having ability to prevent the growth of microorganism in low concentration. 1957: Sheehan proposed synthetic method for Penicillin Alexander Fleming, 1928 Chain & Florey 1938 Dorothy Hodgkins, 1945 Selman K Waksman 1952 5 6 Other antibiotics: 1944: Streptomycin from soil organisms 1947: Chloramphenicol 1948: Tetracyclines 1952: Erythromycin 1952: Isoniazid : AntiTB 1955: Cephalosporins 1962: Nalidixic acid and 1987: Ciprofloxacin (FLQs) 8 9 Mechanism of Antibiotic action  Mechanism of Antibiotic action Cell wall cross linking- Penicillin , cephalosporin. mucopeptide synthesis Bacitracin, Vancomycin peptide synthesis- Cycloserine bactericidal Cytoplasmic membrane- Polyenes (fungicidal) Protein synthesis- 50 S subunit Ribosome- Erythromycin, chloramphenicol 30 S subunit Ribosome- Aminoglycoside, Tetracyclines Nucleic acid synthesis- Actinomycin DNA/mRNA synthesis- Mitomycin, Rifampin 11  Chemical Classification Βeta-Lactam antibiotics: Penicillin (derived from amino acids) Cephalosporin Aminoglycosides Streptomycin Neomycin Kanamycin Gentamicin Tobramycin Amikacin Polyenes (Antifungal) Amphotericin B Nystatin 12  Tetracycline Doxycycline, Minocycline Macrolides (a large lactone ring) Erythromycin Clarithromycin Azithromycin Polypeptides Bacitracin Polymyxin Unclassified: Chloramphenicol Fusidic acid Griseofulvin Rifamycins Cycloserine 13  History of penicillin By Chance discovery (Serendipity); good fortune Bacterial culture left for several weeks in open to air became infected with Fungal colony- Flemming, 1928 Fungal spores from another lab thru open window Weather (cold (fungus) to warm (bacteria) Disposed plates stacked in disinfectant but above level Observation  Βeta-Lactam Antibiotics: Penicillin Penicillium notatum and P. chrysogenum are the mould producing penicillin. Penicillins have a β-lactam (a four-membered cyclic amide) ring structure fused with thiazolidine ring and a Acyl side chain. Potent and rapid bactericidal activity It inhibits the biosynthesis of dipeptidoglycans that provide strength and rigidity to cell wall. B-lactam Thiazolidine ring Acyl chain 15  Mechanism of action Target- Cell Wall Synthesis The bacterial cell wall is made up of cross-linked polymer called peptidoglycan which allows a bacteria to maintain its shape. If the peptidoglycan fails to crosslink, the cell wall will lose its strength which results in cell lysis. All β-lactams antibiotics disrupt the synthesis of the bacterial cell wall by interfering with the transpeptidase which catalyzes the cross-linking process. 16  Peptidoglycan or Murein Peptidoglycan is a carbohydrate with protein (sugars and amino acids) composed of alternating units of N-acetyl muramic acid (NAMA) and N-acetylglucosamine (NAG). The NAMA units have a peptide side chain which can be cross linked from the L-Lys residue to the terminal D-Ala- D-Ala link on a neighboring NAMA unit. This is done directly in Gram (-) bacteria and via a pentaglycine bridge on the L-lysine residue in Gram (+) bacteria. 17 DAP: diaminopimelic acid  Transpeptidase- PBP The cross-linking reaction is catalyzed by a class of transpeptidases known as penicillin binding proteins. A critical part of the process is the recognition of the D-Ala- D-Ala sequence of the NAMA peptide side chain by the PBP. Interfering with this recognition disrupts the cell wall synthesis. β-lactams mimic the structure of the D-Ala-D-Ala link and bind to the active site of PBPs, disrupting the cross-linking process. 20 22  Mechanism of β-Lactam Drugs The amide of the β-lactam ring is unusually reactive due to ring strain and a conformational arrangement which does not allow the lone pair of the nitrogen to interact with the double bond of the carbonyl. β-Lactams acylate the hydroxyl group on the serine residue of PBP active site in an irreversible manner. This reaction is further aided by the oxyanion hole, which stabilizes the tetrahedral intermediate and thereby reduces the transition state energy. 24  Mechanism of β-Lactam Drugs The hydroxyl attacks the amide and forms a tetrahedral intermediate. R S Me Me N OH O COOH SER Transpeptidase reside H 25  Mechanism of β-Lactam Drugs The tetrahedral intermediate collapses, the amide bond is broken, and the nitrogen is reduced. R S Me Me N O O COOH SER H 26  Which groups/structure of Penicillin is/are required for activity? (SAR of penicillin) Why is Benzyl penicillin not given orally? Name the penicillin product obtained after hydrolysis by penicillinase/ β- lactamase or acid. How will u Classify Penicillins? Which groups should be added to benzyl penicillin to make it acid resistant or β- lactamase resistant? Name the important biosynthetic precursor of synthetic penicillins. 6-aminopenicillanic acid 6 5 Penicillin molecule contains three chiral carbon atoms 3 (C-3, C-5, C-6)  Cis- stereochemistry essential Amide Essential Lactam Essential Free carboxylate Essential Bicyclic system Essential SAR of Penicillins  Classification of penicillins Natural and semisynthetic ones (antistaphylococcal, extended spectrum penicillins etc) 1. Natural penicillins: Extracted from the cultural solution of penicillia. Prototype is penicillin G; pH sensitive, therefore not given orally. Effective against Gram-positive cells; Susceptible to penicillinase 2. Semisynthetic penicillins: Have broader spectrum. Are effective against Gram-negative cells, too. BUT Not resistant to penicillinases Acid-stable penicillins (eg. penicillin V) Penicillinase-resistant penicillins (eg. oxacillin der, Carbapenems) Extended-spectrum penicillins (eg. ampicillin) Antistaphylococcal penicillins (eg. Nafcillin). Penicillin Penicillinase ( Lactamase)  NH S CH3 O N CH3 Benzyl O OH O Benzyl Penicillin CH3 Penicillin G S Methyl CH3 NH N OH O O O O Phenoxy Phenoxymethyl penicillin Penicillin V Penicillin G or Benzyl Penicillin 37 38 Long-acting Penicillin G Procaine 39 Long-acting Penicillin G 40  Phenoxypenicillin (Penicillin V)- Orally active Orally effective developed by fermentation technology where phenoxyacetic acid is added while production. It is acid resistant given orally to treat infection caused by Gm+ cocci. Dose: 500mg 6 hourly as tablets or dry powder for oral syrup 41  Semisynthetic Penicillins Molecular modifications were attempted in Penicillin G in order to produce 1. Acid resistant orally effective analogues 2. Extended spectrum of activity –broad spectrum and effective towards dreaded Gram-negative infections and 3. β-lactamase resistant penicillin. 42  1. Broad spectrum orally effective: Ampicillin, amoxicillin 2. β-Lactamase Resistant Penicillin: -Methicillin, cloxacillin, oxacillin, dicloxacillin, - Carbapenems: very broad spectrum -Monobactam: Gram negative 3. Extended spectrum: Carbenicillin, ticarcillin, mezlocillin, piperacillin 43  Penicillins- Antistaphylococcal Penicillins which have bulky side groups can block the β-Lactamases which hydrolyze the lactam ring. O NH S Me Me N O COOH 44  Penicillins- Antistaphylococcal These lactamases are prevalent in S. aureus and S. epidermidis, and render them resistant to Penicillin G and V. Methicillin was the first penicillin developed with this type of modification, and since then all bacteria which are resistant to any type of penicillin are designated as methicillin resistant. (MRSA- methicillin-resistant S. aureus) 45 H3C O CH3 S CH3 Cloxacillin, NH Dicloxacillin N O Floxacillin, O Flucloxacillin O O HO CH3 CH3 Methicillin S N CH3 NH O N OH O CH3 O O Oxacillin 46 Penicillins- Antistaphylococcal 47 Penicillins- Aminopenicillins H NH2 H C N R C S Me O Me N O COOH 48 Penicillins- Aminopenicillins 49 Penicillins- Extended Spectrum 50  Penicillins- Extended Spectrum Extended spectrum penicillins are similar to the aminopenicillins in structure but have either a carboxyl group or urea group instead of the amine O R2N NH H CO2R H H N C N S R C R C S Me Me O Me N O Me N O O COOH COOH 51 Penicillins- Extended Spectrum 52  Carbapenems Carbapenems are a potent class of β-lactams which attack a wide range of PBPs, have low toxicity, and are much more resistant to β-lactamases than the penicillins or cephalosporins. 2-aminoethylthioether 1-hydroxyethyl Inhibits beta lactamase 3 2 Thienamycin (naturally occurring) β-lactamase Inhibitors, class II 53  Carbapenems Thienamycin, discovered by Merck in the late 1970’s, is one of the most broad spectrum antibiotics ever discovered. It uses import porins unavailable to other β-lactams to enter Gram (-) bacteria. Due to its highly unstable nature in sol form, this drug and its derivatives are created through synthesis, not bacterial fermentation. Optimum stability at pH 6-7 54  Carbapenems Thienamycin was slightly modified and marked as Imipenem (1980). Due to its rapid degradation by renal dehydropeptidase-1 it is administered with an inhibitor called cilastatin under the name Primaxin. Imipenem may cause seizures or severe allergic reactions. IV Stable against lactamase 55  Other modifications of Thienamycin have produced superior carbapenems called Meropenem and Ertapenem, which are not as easily degraded by renal peptidase and do not have the side effects of Imipenem. 2nd gen, more effective against G-ve, IV DHP-1 resistant Meropenem  Carbapenems analogous Thienamycin, Imipenem Ertapenem Meropenem 57  Monobactams The only clinically useful monobactam is aztreonam. While it resembles the other β-lactam antibiotics and targets the PBP of bacteria. It inhibits mucopeptide synthesis in the bacterial cell wall. It is highly effective in treating Gram (-) bacteria and is resistant to many β-lactamases. poorly absorbed when given orally, so it must be administered as an IV or IM or inhaled by nebulizer 58  CH3 S CH3 NH N OH H2N O O O HO Ampicillin CH3 S CH3 NH N OH H2N O O O Amoxicillin 59  Ampicillin Its trihydrate is water soluble It is acid stable Administered orally as capsule or suspension. 500mg 6 hourly. Oral absorption is 60-65% only, given along with food. 60  Ampicillin Sodium salt is water soluble and used as IM/IV injection 1-2 g 6 hourly. Uses: To treat Respiratory tract infection and urinary tract infection caused by both Gm + ve and G–ve organisms. Not resistant to penicillinase producing bacteria and it produces allergic reactions Hetacillin, an acetone condensation product and Bacampicillin, ethoxycarbonyloxyethyl ester of ampicillin are used to improve bioavailability 61 Amoxicillin 62  Penicillinase resistant Penicillins Methicillin: Semi-synthetic analogue, 2,6-dimethoxybenzoyl derivative of 6-APA. Its sodium salt is given as injection and is effective against penicillinase producing S. aureus and Bacillus cereus Incidence of nephritis and hypersensitivity reactions are high with methicillin sodium Dose: 1 gm IM/IV 6 hourly Nafcillin is 2-ethoxynaphthyl analogue of methicillin orally effective. 63  H3C O CH3 S CH3 NH N O O O O HO CH3 CH3 S CH3 Methicillin N 3 NH O N OH 5 O CH3 O O Oxacillin Nafcillin 64  Oxacillin sodium It is an Oxazole derivative , highly resistant to inactivation by penicillinase. It is relatively resistant to acid and therefore given orally as capsule. (500mg 6 hourly) Patients who are sensitive to penicillin, oxacillin shall be given with caution. Undergoes first pass metabolism Its 5-hydoxymethyl derivative is also active. 65  Halogenated Cloxacillin, Dicloxacillin and Oxacillins Floxacillin, Flucloxacillin: They have better tissue penetration and are orally effective penicillinase resistant penicillins. Cl S CH3 They are more bound to plasma Cl CH3 O N NH protein and have narrow spectrum N O OH of activity. CH3 O O Dose: 500mg capsule 6-8 hourly Dicloxacillin 66 Extended spectrum O O COOH N Carbenicillin NH CH3 HOOC S CH3 Carbenicillin sodium: carboxyl group is substituted in the α-carbon of benzyl side chain. Not orally effective and administered as IV injection It is effective against Pseudomonas and Proteus species. 67  β-Lactamase Inhibitors The combination of β-lactamase inhibitors with ampicillin or carbenicillin is recommended to treat infections of susceptible organisms There are two groups of inhibitors- – Clavulanic acid and – Sulbactam –is penicillanic acid sulfone derivative. Exp: Tazabactam has anti-psuedomonal activity. 68  O O S O OH N N O O CO2H CO2H sulbactam Clavulinic acid Tazabactam The β-lactamase binds reversibly with the inhibitors for more time when compared with penicillins  β-Lactam/Inhibitor combinations Aminopenicillins: Ampicillin-sulbactam = Unasyn Amoxicillin-clavulanate = Augmentin Extended-Spectrum Penicillins Piperacillin-tazobactam = Zosyn Ticarcillin-clavulanate = Timentin 70 71 Cephalosporins 72  Other Inhibitors of Cell Wall Synthesis dihydrothiazine ring Two side chains Cephalosporins – 2nd, 3rd, and 4th generations more effective against G-ve Cephalosporins were discovered shortly after penicillin entered into widespread product, but not developed till the 1960’s.  Cephalosporins H R N S dihydrothiazine ring C O N OAc O CO2H 5-thia-1-azabicyclo[4.2.0]oct-2-ene Two side chains 74 Semi-synthetic Cephalosporin have following improvements: - Increased acid stability Betalactam rings – Improved pharmacokinetics – Broad spectrum of activity S S – Increased activity towards resistant N N organisms – Increased tolerance after parenteral Cepham Penam administration All cephalosporins are named 3- cephems or ∆3- cephems, 3 designate the position of double bond in dihydrothiazine ring 75  Cephalosporins- Classification 1st generation (G+, G-, enterobacteriaceae) Cefalexin,Cefradine, Cefadroxil, cefazolin, cephaprine, cephalothin 2nd generation (Aerobic, anaerobic bacilli, G-) Cefuroxime, cefaclor, cefamandole 3rd generation (G- enterobacteriaceae) Cefotaxime,Ceftazidime, ceftriaxone 4th generation (-lactamase, G- anerobes) Cefepime, cefpirome 5th generation: Ceftaroine fosamil (-lactamase MRSA)  Cephalosporins- Classification Cephalosporins are classified into four generations based on their activity. Ceftazidime (3rd gen) in particular can cross blood brain barrier and is used to treat meningitis. Later generations are reserved against penicillin resistant infections to prevent the spread of cephalosporin resistant bacteria. 77  Oral Cephalosporins Cephalexin: analogue of ampicillin having aminophenylacetamido group at 7th position and methyl group at 3rd position. Capsules 250 mg and 500mg and oral suspension are available. Drug of choice in URTI and UTI 500mg 6 hourly. NH2 NH S O N O CH3 HO O 79  Cephadrine/cefradine: related to cephalexin but the phenyl ring is partially hydrogenated.. Used in URTI and UTI in dose level of 500mg 6 hourly. It is absorbed almost completely from oral route and is available as parenteral dosage form also. Cefaclor is orally affective chloro analogue of cephadrine. 80  NH2 NH S HO O N Cefadroxil O CH3 HO O 81 Parenteral Cephalsporins 82  Cafazolin sodium - first generation parenteral cephalosporins used in current clinical practice (short acting). 83 Cefuroxime 84 Cefotaxime 85  Ceftazidime IM/IV treat lower respiratory tract, skin, urinary tract, blood- stream, joint, and abdominal infections, and meningitis. Broad spectrum including anaerobes. 86 Cefepime 87 Macrolide Antibiotic  Products Erythromyin Clarithromycin Azithromycin Roxithromycin Dirithromycin Troleandomycin  Erythromycin keto group amino sugar Desosamine at C-5 dimethylamino cladinose (C-3) 14 membered lactone First macrolide isolated from Streptomyces erythraeus. Desosamine confers a basic character. Mechanism of action Clarithromycin Azithromycin Inhibition of Protein Synthesis by Antibiotics Figure 20.4  Aminoglycoside antibiotics Chemically they contain amino sugars, glycosidically linked to aglycone. Aminoglycoside (Aminocyclitol) contains a pharmacophoric 1,3- diaminoinositol moiety; either streptamine , deoxystreptamine or spectinamine All have at least one aminohexose sugar They are strongly basic compounds and available as sulfate salts. Distribute well into body fluids but not into the CNS, bone or fatty or connective tissues.  Aminoglycoside antibiotics They are obtained from various species of Streptomyces. They are poorly absorbed from GIT following oral administration and therefore administered as IM / IV. Broad spectrum The important side effects are ototoxicity, nephrotoxicity  Aminoglycoside antibiotics Mechanism of action: They bind with 30S ribosomal subunit to form a complex that can not initiate proper amino acid polymerization. Streptomycin and kanamycin are effective against M. tuberculosis.  Products Streptomycin Neomycin Kanamycin Paromomycin Gentamicin Tobramycin Netilmicin Amikacin (semisynthetic kanamycin derivative) Streptomycin  Its Aqueous solution may be stored at room temperature for 1 week without any loss of potency, most stable at pH 4.5- 7. Solution decompose if sterilized by heating, so sterile solutions are prepared by adding sterile dist. Water to sterile powder. Neutrotoxiciy, ototoxicity and nephrotoxcity Streptomycin  Gentamicin sulfate Obtained from Micromonospora purpuea. Broad spectrum of activity. Given as injection 80 mg thrice daily to treat Gm+ve and Gm-ve infections It is used topically to treat infection as ointment and eye/ear drops. Gentamicin Tobramycin: It is active against most strains of P. aueroginosa 80 mg twice or thrice daily as IM. Netilmicin: A gentamicin analgoue used in gentamicin resistant infections. Amikacin: Semi-synthetic, powerful aminoglycoside antibiotic from Kanamycin used in treating resistant stains of bacterial infections. Less ototoxic than kanamycin or gentamicin. Dose 500 mg IM injection twice daily