Lecture 3 - PenicillinS PDF

Antibiotics Definition  1942, Waksman “an antibiotic is a substance produced by microorganisms which has the capacity of inhibiting the growth & destroying other microorganisms”.  Later “ any substance produced by a living organism that is capable of inhibiting the growth or survival of one o...

Antibiotics Definition  1942, Waksman “an antibiotic is a substance produced by microorganisms which has the capacity of inhibiting the growth & destroying other microorganisms”.  Later “ any substance produced by a living organism that is capable of inhibiting the growth or survival of one or more species of microorganisms in low concs.  medicinal chemists to modify the natural antibiotics to prepare new synthetic analogs so we now have semisynthetic & synthetic derivatives in the definition. Classification of Antibiotics -Lactam Penicillins & Cephalosporins Aminoglycoside: Streptomycin & Kanamycin Tetracyclines Non- -Lactam Macrolides: Erythromycin & Oleandomycin Polypeptide: Polymixin Polyene: Amphotericin Miscellaneous : Chloramphenicol 1 Mechanism of action NAM NAM Penicillin NAM NAM L-Ala L-Ala L-Ala L-Ala D-Glu D-Glu D-Glu D-Glu Transpeptidase L-Lys-Gly-Gly-Gly-Gly-Gly L-Lys L-Lys-Gly-Gly-Gly-Gly-Gly L-Lys D-Ala D-Alanine D-Ala D-Ala D-Ala D-Ala D-Ala D-Ala Cross linking  Cell wall is peptidoglycan structure [peptide & sugar]  Consist of parallel chains of N-acetylmuramic acid NAM + N-acetylglucosamine NAG  Peptide contain D-amino a (human have only L amino a)  Baceteria have racemase enzyme (L-amino a → D-amino a)  Cross –linking of peptide chains take place by transpeptidase enzyme & catalysed by Penicillin binding proteins 2 -lactame O N O N Monobactam O 5. Oxapenam S S N N N N O O O O 1. Penam 2. Penem 3. Carbapenam 4. Carbapenem S O N N N O O O 6. Cephem 7. Carbacephem 8. Oxacephem  Penam No.1 is the backbone of all penicillins & No. 2-5 are isosteric forms.  Cephem No.6 is the backbone of all cephalosporins & No. 7, 8 are isosteric forms 3  O N O 1- Structure name is …….. 2- Draw 3 different isosteric forms. 4 Discovery of Penicillin  1877 Pasteur: certain moulds produce toxic substances that kill bacteria  1928 Fleming penicillium notatum could inhibit bacterial growth (but too unstable)  1941 Florey & Chain use freeze-drying & chromatography in isolation  Why Fleming not able to isolate penicillin: its highly strained H H R HN S A = -Lactam ring. C 6 5 4 CH3 x x B = Five member thiazolidine ring. A B 3 O N x CH3 X = Asymmetric center O 7 1 2 COOH Penicillins contain a highly unstable bicyclic system consisting of a four- membered -lactam ring fused to a five-membered thiazolidine ring (penam). 5 Medical Literature It uses the name 6-aminopenicillanic acid (6-APA) H H H2N S CH3 N CH3 O COOH 6-Aminopenicillanic acid (6-APA) Trivial Nomenclature It uses the name penicillin for 6-carbonylamino-penicillanic acid and distinguishes penicillins according to the R-group of the acyl portion. H H H e.g. Ampicillin R N S Me 6 D(-)--aminobenzylpenicillin O N Me O COOH 6 Classification Natural Penicillins H H H N S e.g: Benzylpenicillin CH3 6 O N CH3 O COOH 6-Phenylacetylamino-2,2-dimethylpenam-3-carboxylic acid Penicillin structural features & requirements for antibacterial activity  Penicillins have similar structures: thiazolidine ring (A) fused to β-lactam ring (B)  A + B constitute the 6-APA nucleus which is required for antibacterial activity.  Cleaving the β-lactam ring by penicillinases (β-lactamases) results in loss of antibacterial properties.  Penicillins may also be inactivated by amidases  Substituents are attached to the amino group (R). 7 Structural Requirement 4 6 cis-stereochemistry Sulfur is usual not essential H H H R N S Me acylamino side chain 1, 2, 3, 4 & 5 are essential O N Me 1 O B-lactam COOH 2 Free carboxylic gp 3 bicyclic system 5 1. Acylamino side chain is essential (except for thienamycin, see later). 2. Strained -lactam ring. 3. Free carboxylic group. 4. The cis-stereochemistry of C-6 & C-7 5. The bicyclic system 6. Sulfur is not essential. 8 Design a new penicillin derivative, that expected to have antimicrobial activity 9 Stability of Benzylpenicillin R R1 R R1 S S N N X N N O R2 O R2 O O impossible strained flat system Tertiary amide Penam (Resonance stabilized) (No resonance stabilization)  The -lactam N is unable to feed its lone pair of electrons into the C=O because it require the bicyclic ring to adopt an impossible strained flat system.  So, lone pair of electrons is localized on N atom & C=O gp is far more electrophilic  Therefore, penicillins are easily attacked by acids (Electrophilic) or bases (Nucleophilic), also by amidase or -lactamase (penicillinase) enzyme. 10 Discuss instability of pencillin 11 Electrophilic Attack (By Gastric Acidity) R H C N S N S N S Me + O H R R.. N N N Me O.. O O O O H COOH.. + COOH COOH H HS Me HOOC C CH S N CH Me Me N + H COOH N Me O O R COOH R penicillic acid penicillenic acid Disadvantages of penicillin G include Acid sensitive β-lactamases sensitive (so ineffectiveness against gram-ve organisms) Narrow spectrum. hypersensitivity reactions (in some patients). Has short duration of action 12 Penicillins Sensitive to -Lactamases  -Lactamases are enzymes which catalyze the hydrolysis of -lactam ring.  There are about 50 different known types of such enzymes.  Production of -lactamases is the most important factor in the development of penicillin-resistant strains of bacteria. O H H R CO HN S R C N S CH3 CH3 N CH3 O H N CH3 Nu O COOH Nu : OH2 COOH + H -lactamase -lactamase R COHN S CH3 O HN CH3 OH COOH Nu -lactamase 13 Semi synthetic Penicillins Obtained from natural penicillins by two steps 1. Hydrolyzing benzylpenicillin using penicillin acylase to give ► 6-APA. 2. Acylation of 6-APA with different acid chlorides in presence of triethylamine COOH H H H H H H2N N S Me S Me Penicillin acylase 6-APA O N + N Me Me (1) O O COOH COOH Benzylpenicillin (penicillin G) H H H O R N S Me R Cl (2) O N Me Et3N O COOH 14 Types of Semi synthetic Penicillins A. Acid Stable Semi synthetic Penicillins By introduction of an electron withdrawing group in the -position of the amide side chain decreases the nucleophilicity of the side chain amide carbonyl oxygen towards participating in -lactam ring opening to form penicillenic acid. Examples are: O Phenoxymethylpenicillin H H O CH2 C N (Penicillin V) O Azidocillin O H H CH C N It is D-(-)--Azidobenzylpenicillin N3 It is more active than penicillin V But it still affected by -lactamases. O 15 B. -Lactamase Stable Penicillins  Using steric shield (bulk gp) strategy on the side chain  The bulky group would sterically block the attack and prevent the fitting of the - lactamase enzyme on the -lactam ring.  These gps must be large enough to prevent fitting to β-lactamase active site and small enough to allow interaction with transpeptidase active site  e.g. two ortho-positioned methoxy groups to shield the lactam ring. Methicillin OMe O H H H C N -Lactamase Stable but it has no electron S CH3 withdrawing group on the side chain, so it is OMe N CH3 acid sensitive and has to be administered by O COOH injection. 2,6-Dimethoxyphenylpenicillin 16 Oxacillins R O H Oxacillin R = R`= H C N S Cloxacillin R = Cl, R`= H R` N Dicloxacillin R = R`= Cl CH3 N O O Flucloxacillin R = Cl, R`= F COOH Both isoxazol ring + substituted phenyl ring provide 1. Steric shield ► give β-lactamase resistant derivatives 2. Electron withdrawing effect ► give acid stable derivatives OCH3 Temocillin CH CONH S CH3 COOH S N CH3 O COOH  Introduction of 6--methoxy group to penicillins ►give highly resistant derivatives to bacterial -lactamases  Not absorbed orally & used as Na salt for parenteral administration. 17 C. Extended-Spectrum Penicillins  The introduction of -amino group on the side chain of benzylpenicillin confers a high degree of acid stability with enhanced activity against Gram -ve bacteria.  The -amino group is protonated in the gastric juice and acquire electron withdrawing property and the molecule becomes acid resistant.  -NH2 creates new asymmetric center; D-isomer is 8 times more active > L-isomer  They are sensitive to penicillinase enzyme so combined with -lactamase inhibitors e.g. Ampicillin NH2 H N S Me O N Me O Ampicillin COOH  It is D-(-)--Aminobenzylpenicillin.  It is administered orally as the trihydrate and parenterally as soluble sodium salt  Formulations with sulbactam are also available. 18  NH3 NH2 H H N S Me N S Me O N Me O N Me O O Zwitterions O Ampicillin esters O O OR  The existence of NH2 & COOH in the same molecule renders it amphoteric & forms Zwitterions that its solubility becomes difficult in both acid & alkaline media.  Only 40 % of the dose of Ampicillin is absorbed orally from the GIT To overcome this bad absorption:  Esterification of the COOH gp give prodrugs with ↑ lipid solubility & improves oral bioavailability.  The esters are lipophilic compounds e.g. Pivampicilline, bacampicillin,  They are devoid of antibacterial activity (inactive) but hydrolyzed by tissue esterases & liberate ampicillin 19 Ampicillin esters used to improve absorption through GIT Bacampicillin H H H CH CO N S CH3 NH2 N CH3 CH3 O COO CH 1-Ethoxycarbonyloxyethylester of Ampicillin O COOEt Acyl derivatives of Ampicillin Against Ps. aeruginosa Piperacillin Et N N CONH CH CO O O  The acyl derivatives of ampicillin that are active against Ps. aeruginosa  include the acylureidopenicillin; azocillin & piperacillin. 20 Amoxicillin H H HO CH CO NH S Me NH2 N Me O COOH D-(-)--Amino-p-hydroxybenzylpenicillin.  Used as trihydrate for oral administration & as Na salt for parenteral use.  Used also with clavulanic acid  It has superior oral absorption producing around 2.5 times the plasma peak concentration achieved by comparable doses of ampicillin.  It’s absorption from G.I.T is unaffected by food. 21 E) Compounds liberating benzylpenicillin (Long acting penicillins) Uses: 1. Long acting forms for treatment of gonorrhea & syphilis, that require prolonged therapy 2. In prophylaxis, particularly in rheumatic fever. Procaine Penicillin H Et + H2N COOCH2 CH2 N Benzylpenicillin Et  It is poorly soluble (1:200 in H2O) equimolecular compound of penicillin & procaine  Used I.M. as a suspension of crystals which slowly dissolves at the injection site  The salt releases the active antibiotic over a period of about 24 hours 22 ß-Lactamase Inhibitors the production of β-lactamases increase the threat of bacterial resistance. so how can we combat this dangerous enemy? -lactamase inhibitors  Suicidal inhibitors with weak antibacterial activity,  but have higher capacity to inactivate many -lactamases.  Used in combination with penicillins to overcome resistance problems  e.g. clavulanic acid & sulbactam. i. Sulbactam O O CH3 S CH3 N O COOH  available as the sodium salt.  It is coadministered with Ampicillin (Unasyn IM or IV) 23 ii. Clavulanic Acid H 4 6 5 O OH 3 Oxapenam produced by Streptomyces clavulingerus. N 2 Used as K salt for oral use with Amoxicillin & for 1 O COOH intravenous use with Ticarcillin. H MOA.  inhibits the -lactamase enzyme by irreversible binding to the enzyme & allows the penicillin molecule to attack cell wall & destroy bacterial cell.  Co-Amoxiclav is a mixture of K Clavulanate & Amoxicillin trihydrate (oral formulation) or Amoxicillin Na (parenteral formulation). 24 Chemically, what is the difference between -Lactam and non -Lactam antibiotics 25  The role of racemase and transpeptidase enzymes. Which one of them is human type enzyme? 26 Role and source of these enzymes : penicillinases (β-lactamases) amidases 27 References: 1- Foye's principles of medicinal chemistry. Philadelphia, Lippincott Williams & Wilkins. 2-Wilson and Gisvold's textbook of organic medicinal and pharmaceutical chemistry. Philadelphia, Lippincott-Raven. 28