Antibiotics 3
β-lactam Antibiotics-penicillins
and
β-lactamase inhibitors
Contents
• Chemical degradation of penicillins – various pathways
• Study of individual penicillins including structures and specific uses
• β-lactamase inhibitors – classification
• Mode of action of β-lactamase inhibitors
• Study of individual compounds used as β-lactamase inhibitors
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Learning Objectives
At the end of this lecture, student will be able to
• Discuss the chemical degradation of Penicillin
• Discuss the structures, specific uses and side effects of penicillins
• Classify β-lactamase inhibitors
• Explain the mode of action of β-lactamase inhibitors
• Discuss the structures, specific uses β-lactamase inhibitors
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β-lactam Antibiotics – Pencillins Contd…
• Chemical Degradation of Penicillins:-
• The main cause of deterioration of penicillins is the reactivity of the
strained β-lactam ring, particularly by hydrolysis.
• The course of the hydrolysis and the nature of the degradation
products are influenced by the pH of the solution.
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β-lactam Antibiotics – Pencillins Contd…
I. The β-lactam carbonyl group of penicillin readily undergoes
Nucleophilic attack by water or hydroxide ion to form inactive
‘Penicilloic acid’ which is reasonably stable in neutral to alkaline
solutions but readily undergoes de-carboxylation and further
hydrolytic reactions in acidic solutions.
• Other Nucleophiles as hydroxyl amines, alkyl amines and alcohols –
open the β-lactam ring to form the corresponding hydroxamic
acids, amides and esters.
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β-lactam Antibiotics – Pencillins Contd…
II. In strongly acidic solution (pH < 3), penicillin undergoes a
complex series of reactions forming a variety of inactive
degradation products. The first step in the rearrangement to the
penicillanic acid. This process is initiated by protonation of the β-
lactam nitrogen, followed by Nucleophilic attack of the acyl
oxygen atom on the β-lactam carbonyl carbon.
• Subsequent opening of the β-lactam ring destabilizes the
thiazolidine ring, suffers acid-catalyzed ring opening to form
penicillanic acid.
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β-lactam Antibiotics – Pencillins Contd…
• Penicillanic acid is very unstable and undergoes TWO major
degradation pathways.
• ‘I Path’ is hydrolysis of oxozolone ring to form unstable penamaldic
acid, an enamine easily undergoes hydrolysis to penicillamine (a
major degradation product) and penaldic acid.
• ‘II Path’ involves a complex rearrangement of penicillanic acid to
penillic acid.
• Penillic acid (an imidazoline -2-carboxylic acid) readily
decarboxylates and on hydrolytic ring opening to form major end
product penilloic acid.
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β-lactam Antibiotics – Pencillins Contd…
• Penicilloic acid (cannot be detected as intermediate), the major
product formed. Weakly acidic (neutral?) to alkaline hydrolytic
conditions (also enzymatic conditions), exists in equilibrium with
penamaldic acid, undergoes decarboxylation to give penilloic acid.
• The third major product of the degradation is penicilloaldehyde
formed by decarboxylation of penaldic acid (a derivative of
malonaldehyde) .
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Chemical degradation of penicillins
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β-lactam Antibiotics – Pencillins Contd…
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β-lactam Antibiotics – Pencillins Contd…
➢ Individual Compounds:
• Penicillin G: [Benzyl penicillin]
• Agent of choice for the treatment of different kinds of bacterial
infections than any other antibiotic.
• Inactive orally. But by combining antacids as calcium carbonate,
aluminium hydroxide and magnesium trisilicate or a strong buffer as
sodium citrate and by giving large doses, as it is poorly absorbed
from intestinal tract, will increase the oral activity of the drug.
• Water soluble potassium or sodium salts are used orally and
parenterally to achieve high plasma concentration of pencillin G
rapidly.
All pencillins should be administered with caution due to allergic side
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β-lactam Antibiotics – Pencillins Contd…
• Penicillin V [Phenoxy methyl Penicillin]
• It is resistant to hydrolysis by gastric acid and it produces uniform
concentration in blood, (when administered orally).
• For parenteral solution, potassium salt is usually used.
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β-lactam Antibiotics – Pencillins Contd…
• Cloxacillin:[3-(o-chlorophenyl)-5-methyl-4-isoxasolyl] Penicillin
sodium.
• Oxacillin, cloxacillin and dicloxacillin are highly resistant to
inactivation by penicillinase. The steric effects of 3-phenyl and 5-
phenyl groups prevent binding to the B- lactamase active site.
• Prescence of Cl in ortho position causes increase in activity due to
increase in oral absorption. It attains high plasma levels
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β-lactam Antibiotics – Pencillins Contd…
• Naficillin sodium
• [6-(2-ethoxy-1-naphthyl) penicillin sodium]
• Ethoxy group and second ring of naphthalene group increase the
stability against penicillinase.
• stable to acid so it can be given by oral route
• used in infection caused solely by penicillin G-resistant
staphylococci or streptococci.
• also effective against pneumococci & group a – β-hemolytic
streptococci.
• should be administered with care, due to its allergic side effects.
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β-lactam Antibiotics – Pencillins Contd…
• Ampicillin: 6-[D-α-aminophenyl acetamido] Penicillanic acid or D-α-
amino benzyl penicillin
• It has an anti-bacterial spectrum broader than that of Penicillin G.
• Active against Gram –Ve organism that are susceptible to other
penicillins
• Acid resistant
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β-lactam Antibiotics – Pencillins Contd…
• More active against Gram –ve bacteria and enterococci than other
penicillins
• Not resistant to penicillinases
• Use:- Particularly useful for the treatment of acute urinary tract
infections caused by E.Coli or Proteus mirailis.
• It is the agent of choice against Haemophilic influenzae infections
• Used in combination with Probenicid for the treatment of
Gonorrhea.
• Effective in treating Salmonellosis and Shigellosis.
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β-lactam Antibiotics – Pencillins Contd…
• Amoxicillin: 6-[D-α-amino-p-hydroxy phenyl acetamido] Penicillanic
acid (semi synthetic penicillin)
• Its antibacterial spectrum is same as that of ampicillin (resistant to
acid, susceptible to alkaline and β-lactamase hydrolysis).
• More complete gastro-intestinal absorption to give higher plasma
and urine levels.
• Less diarrhea.
• food on absorption.
Little or no effect of www.DuloMix.com 17
β-lactamase inhibitors (Suicidal substrates)
• The β-lactamase inhibitors, such as sulbactam and tazobactam and
natural occurring β-lactams, such as the thienamycins, inhibit both
β-lactamases and interact with penicillin binding protein (PBP)
present in the bacterial cell wall.
• β-lactamase inhibitors are given along with β-lactamase sensitive
penicillin, so that they competitively bind to the enzyme and
protect the penicillin from destruction.
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β-lactamase inhibitors (Suicidal substrates)
• β-lactamase inhibitors are of 2 classes.
• Class. I – inhibitors:- have a heteroatom at position 1
• Eg. Clavulanic acid & sulbactam.
Clavulanic acid Sulbactam
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β-lactamase inhibitors (Suicidal substrates)
• Class. II – inhibitors:- do not have a heteroatom at position 1
Eg. Carbapenams – as Thienamycin
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β-lactamase inhibitors (Suicidal substrates)
Mode of action of β-lactamase inhibitors :-
• Inactivation of β-lactamases is done by mechanism-based
inhibitors, which act by reacting with the enzyme in the same way
as that of the substrate.
• An acyl enzyme intermediate is formed by the reaction of the β-
lactam with an active-site serine hydroxyl group of the enzyme.
• For normal substrates (Penicillins), the acyl enzyme intermediate
readily undergoes hydrolysis, destroying the substrate and freeing
the enzyme to attack more substrate.
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β-lactamase inhibitors (Suicidal substrates)
• For mechanism based-inhibitor, the acyl enzyme intermediate
formed is diverted by tautomerism to a more stable imine that
undergo hydrolysis more slowly to eventually free the enzyme
(transient inhibitors).
• Because these inhibitors are also substrates for the enzymes
that they inactivate, they are sometimes called as ‘Suicidal
substrates’
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β-lactamase inhibitors (Suicidal substrates)
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β-lactamase inhibitors (Suicidal substrates)
Individual Compounds:-
• Clavulanate potassium:-
• Clavulanic acid is isolated from Streptomyces Clavuligeris
• *it is 1-oxapenam
• *lacks 6-acylamino side chain
• *but possess 2-hydroxyethylidene moiety at C-2.
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β-lactamase inhibitors (Suicidal substrates)
• It has very weak anti-bacterial activity
• It is a potent inhibitor of S.aureus β-lactamase and plasmid-mediated
β-lactamases produced by Gram-ve bacilli.
• Combination with Amoxacillin and potassium salt of Clavulanic acid
(Augmentin) is intended for the treatment of skin, respiratory, ear
and urinary tract infections caused by β-lactamase producing
bacterial strains- the oral bioavailability of both are the same
• It is effective against strains which are resistant to Amoxacillin alone
• It is stable to acid
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β-lactamase inhibitors (Suicidal substrates)
• Combination of potassium clavulanate and ticarcillin sodium
(extended–spectrum penicillin) is recommended for Septicemia,
lower respiratory tract infections and urinary tract infections, bone
and joint infections, skin & structure infections caused by β-
lactamase producing strains of S.aureus, Klebsiella, E.coli,
P.aeruginosa and other pseudomonas spp. Citrobacter spp.
Enterobacter spp. Serratia Marcescens, etc
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β-lactamase inhibitors (Suicidal substrates)
• Ticarcillin disodium:-
Ticarcillin disodium ‘α-carboxy-3-thienyl penicillin’
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β-lactamase inhibitors
Class II inhibitors – Carbepenams
• Class of highly effective antibiotic agents commonly used for the
treatment of severe or high-risk bacterial infections. This class of
antibiotics is usually reserved for known or suspected multidrug-
resistant (MDR) bacterial infections.
• Broadest coverage of antibacterial activity
• Including Gm+, and Gm- (especially drug resistant species), anaerobic
coverage -Cover MSSA, Enterococcus, streptococcus spp.
• Drugs of choice for ESBL infections (ESBL-producing bacteria can’t be killed by
many of the antibiotics ) www.DuloMix.com 28
β-lactamase inhibitors (Suicidal substrates)
Carbapenams:
Thienamycin
• First isolated from of Streptomyces Cattleya.
• ONLY two structural features of thienamycins are shared with the
penicillins and cephalosporins.
• A fused bicyclic ring system containing a β-lactam.
• An equivalently attached 3-carboxyl group.
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β-lactamase inhibitors (Suicidal substrates)
• The presence of double bond between C-2 and C-3 in the bicyclic
structure creates a considerable ring strain and increases the reactivity of
the β-lactam to ring opening reactions.
• It has a 1-hydroxylethyl gp at 6th position (not the acyl amino side chain)
& this is oriented to the α ring
• It has broad spectrum antibacterial properties-active against most Gram+
& gram- bacteria and resistant to activation by most β-lactamases (could
be because of hydroxyl ethyl side chain)
• It is more susceptible to acid & alkaline hydrolysis coz the strain nature of
the fused ring system. Stable at pH between 6-7
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β-lactamase inhibitors (Suicidal substrates)
• Imipenam
• Very stable to most β-lactamases. It is an inhibitor of β-lactamases
from certain Gram+ve & Gram-ve bacteria resistant to other β-
lactamam antibiotics
• It is used for the treatment of a wide variety of bacterial infections
of the skin & tissues, lower respiratory tract, bones & joints and
genitourinary tract infections.
• They are also used for septicemia & endocarditis caused by β-
lactamases producing strains
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Summary
• Chemical degradation of penicillins – various pathways
• Study of individual penicillins including structures and specific uses
• β-lactamase inhibitors – classification
• Mode of action of β-lactamase inhibitors
• Study of individual compounds used as β-lactamase inhibitors
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Thank You
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