The document discusses various β-lactam antibiotics including penicillins and cephalosporins. It describes the chemical structure of penicillins including the β-lactam ring. It discusses the classes of penicillins, their history, mechanisms of action, degradation pathways, structure-activity relationships, resistance issues and classification. Specific penicillins discussed include benzylpenicillin, phenoxymethyl penicillin, methicillin, ampicillin, amoxicillin, oxacillin, cloxacillin and dicloxacillin.

1. β-lactam Antibiotics
(Penicillins and Cephalosporins)
Dr. Ejaz Ali

2. Introduction to β-lactam
The cyclic amides are known as lactams.

3. Introduction to β-lactam
• α-lactam is a cyclic amide with three atoms in its ring.
• β-lactam is a cyclic amide with four atoms in its ring.

4. β-lactam Antibiotics
The β-lactam antibiotics have β-lactam ring as the main part of their
chemical structures and pharmacophore.

5. β-lactam Antibiotics

6. β-lactam Antibiotics
β-Lactam antibiotics include two major classes;
1. Penicillins
2. Cephalosporins

7. History of Penicillins
• Discovered by Alexander Fleming in 1928.
• Was isolated from fungus Penicillium notatum which is now
prepared from Penicillium chrysogenum.
• Florey and Chain isolated penicillins using freeze drying and
chromatography in 1944.

8. Chemistry of Penicillins
• Penicillins is a subclass of β-lactam antibiotics containing β-
lactam ring fused with 5 membered S containing heterocyclic ring
known as thiazolidine.
• The bicyclic heterocyclic ring formed by this fusion is known as
6-amino penicillanic acid (6-APA) nucleus.

9. Nomenclature of Penicillins

10. Chemistry of Penicillins

11. Chemistry of Penicillins
• The C-3, 5 and 6 are asymmetric which interfere with the
planarity of lactam bond.
• Lack of planarity prevents resonance of the lactam nitrogen
with its carbonyl group.
• Consequently, the β-lactam ring is more strained and unstable
due to lack of resonance.
• Therefore, β-lactam ring is reactive and more sensitive to
nucleophilic attack compared to normal planar amides.

12. Degradation of Penicillins
• The most unstable β-lactam amide bond in penicillin molecule
undergoes hydrolysis slowly in water.
• But heating, presence of alkali and lactamase enzyme accelerate
the hydrolysis. The hydrolysis produces penicilloic acid which is
decarboxylated to form penilloic acid.
• Penicillins also undergo acid catalyzed hydrolysis. The main end
products of acid catalyzed hydrolysis are penicillamine, penilloic
acid and penalloaldehyde.

13. Degradation of Penicillins

14. Degradation of Penicillins

15. Prevention of Degradation of Penicillins
• In-vitro degradation of penicillins can be retarded by keeping pH
of solutions 6-6.8 and by refrigerating them.
• Metal ions such as Hg, Zn and Cu catalyze the degradation, and
should be avoided as packing materials.

16. Structure Activity Relationship
• The chemical constituents attached to penicillin nucleus can
greatly influence the stability of the penicillins and spectrum
of activity.

17. Structure Activity Relationship

18. Structure Activity Relationship
• Substitution of side chain R with an electron-withdrawing group
decreases the electron-density on the side chain carbonyl and
protects penicillins from acid degradation.
• This property has clinical implication because these compounds
survive passage through the stomach and can be given orally for
systemic activity.

19. Structure Activity Relationship
• The more lipophilic chain attached to the side chain, the more
protein bound is the antibiotic.
• It prevents the enzymatic degradation of penicillins and
increases the half life.
• However, it reduces the effective bactericidal concentration of
penicillins.

20. Structure Activity Relationship

21. Structure Activity Relationship
If acyl amino chain is attached with a bulkier group, penicillins
become stable towards penicillinase enzyme.
Methicillin

22. Structure Activity Relationship
The introduction of an ionized or polar group into the -position of the
side chain benzyl carbon atom of penicillin G confers activity against
Gram-negative bacilli. Hence, derivatives with an ionized -amino
group, such as ampicillin and amoxicillin, are generally effective
against such Gram-negative genera as Escherichia, Klebsiella,
Haemophilus, Salmonella, Shigella, and non–indole-producing
Proteus.

23. Structure Activity Relationship
Furthermore, activity against penicillin G–sensitive, Gram-positive
species is largely retained. The introduction of an -amino group in
ampicillin (or amoxicillin) creates an additional chiral center.
Extension of the antibacterial spectrum brought about by the
substituent applies only to the D-isomer, which is 2 to 8 times more
active than either the L-isomer or benzylpenicillin (which are
equiactive) against various species of the aforementioned genera of
Gram-negative bacilli.

24. Mechanism of Action-Cell Wall Functions
The bacterial cells are covered by an outer most layer called cell
wall which serves the following functions;
• Semipermeable barrier for the selective passage of substances.
• Strong barrier to protect the bacterial cell changes in osmotic
pressure.
• To prevent bacterial cell from digestion by host enzymes.

25. Mechanism of Action-Cell Wall Composition
• The cell wall is a spongy gel forming layer consisting of
alternating sugars N-acetyl glucosamine (NAG) and N-acetyl
muramic acid (NAM) linked in a polymer chain.
• A complex polymeric sheet is formed by many such
peptidoglycan chains.
• The polymeric sheets are crossed linked to form a thick cell wall.
• A small peptide chain consisting of L-alanyl-D-glutamyl-L-lysyl-
D-alanin is attached to NAM unit by a peptide bond.

26. Mechanism of Action of Penicillins

27. Mechanism of Action-Cell Wall Composition
• The protruding peptide chain cross links the two polymeric sheets
through a peptide bond between the terminal D-alanyl unit and of
one peptide chain to lysyl unit of an adjacent tetrapeptide strand
through a pentaglycine unit.
• Cross linking is catalyzed by enzyme called transpeptidase found
in the inner part of cell membrane.
• Penicillins bind to this enzyme and hence are also called as
penicillin binding proteins (PBP).

28. Mechanism of Action
• There are many types of PBPs (PBP-1a, PBP-1b, PBP-2, PBP-2a,
PBP-3) which are involved in construction and repair of cell wall.
• Penicillins bind to PBPs and inhibit their enzymatic properties
and prevent the construction and repair of cell wall.
• Prevention of cell wall synthesis exposes the dividing and young
bacteria to survive, resulting in killing of bacterial cells.
• As a result, penicillins act as bactericidal antibiotics.

29. Mechanism of Action

30. Allergic Reactions to Penicillins
• The origin of allergy is hepatic reaction which involves formation
of antigenic penicilloyl proteins due to reaction of nucleophilic
group of β-lactam ring with host proteins.
• Hence, this side effect is caused by the pharmacophore and is
unlikely to overcome with the molecular manipulations.
• Allergy to penicillins is expressed as mild drug rash or itching and
is of delayed onset.
• Topical wheal-and-flare test may be performed.

31. Allergic reactions to Penicillins
• Occasionally the reaction is immediate and profound.
• It may include the cardiovascular collapse and shock.
• Erythromycin and clindamycin are useful alternatives for therapy
in many cases of penicillin allergy.
• Penicillins are prepared in facilities separate from those used to
prepare other drugs to prevent cross-contamination and possible
sensitization.

32. Resistance to Penicillins
• The first mode of resistance is due to enzymatic hydrolysis of the
beta-lactam ring.
• If the bacterium produces the enzymes beta-lactamase or
penicillinase, these enzymes will break open the beta lactam ring
rendering the antibiotic ineffective.
• 2nd mode of beta-lactam resistance is due to possession of altered
penicillin-binding-proteins (PBPs).

33. Resistance to Penicillins
• Beta-lactams cannot bind effectively to these PBPs as a result these
antibiotics become less effective in disrupting the cell wall.
• Resistance to beta lactamase can be reduced by carrying out some
structural modifications in the parent compound.

34. Classification of Penicillins
• Penicillins can be classified based on their sources, chemistry,
pharmacokinetic properties, resistance to enzymatic spectrum of
activity, and clinical uses.
• Penicillins may be biosynthetic, semisynthetic, or synthetic; acid-
resistant or not; orally or (only) parenterally active; and resistant to
lactamases (penicillinases) or not.
• They may have a narrow, intermediate, broad, or extended spectrum
of antibacterial activity and may be intended for multipurpose or
limited clinical use.

35. Classification of Penicillins

37. Benzylpenicillin (Penicillin G)
Benzyl group

38. Benzylpenicillin (Penicillin G)
• Penicillin G is unstable under acidic conditions of stomach
• Parenteral route of administration.
• Self destructive mechanism in its structure because of influence of
acyl side chain.
• Effective mainly against Gram positive cocci such as streptococci
and staphylococci.
• It is also effective against Neisseria gonorrhoeae and
Haemophilus influenza
• Many once sensitive bacteria are now resistant.
• Used in upper and lower RTIs, genitourinary tract infections

39. Phenoxymethyl penicillin (Penicillin V)
Phenoxy methyl group
More electronegative
O-atom inhibits β-lactam
bond hydrolysis

40. Phenoxymethyl penicillin (Penicillin V)
• Produced by fungus in a medium rich in phenoxy acetic acid
• Can also be prepared by semi-synthesis and is comparatively more
stable than penicillin G
• Stability is due to electronegative oxygen atom at C-7 amide side
chain inhibiting participation in beta-lactam bond hydrolysis
• It was the first oral penicillin
• Antimicrobial spectrum is roughly same as that of penicillin G.
• Same sensitivity to beta-lactamases as penicillin G and almost same
allergenicity.

41. Methicillin
Dimethoxy benzoyl group

42. Methicillin
• Although it is not used today but methicillin was first penicillinase
resistant penicillin used clinically.
• Unstable is gastric acid (half life = 5 min at pH = 2)
• Increased bulk resulting from the addition of dimethoxy benzoyl
group to 6-APA leads to methicillin (beta-lactamase resistant)
• Methicillin has significantly narrower antimicrobial spectrum so it
was limited to use clinically only for infections caused by beta-
lactamase producing Staphylococcus aureus and few other infections

43. Methicillin
• MRSA refers to methicillin resistant staphylococcus aureus.
• Resistance mechanism includes altered PBPs.
• Methicillin is also an effective inducer of penicillinases.

44. Ampicillin
• Ampicillin is an amino benzylpenicillin and is a semisynthetic
penicillin.
• D-Ampicillin, is significantly more active than L-ampicillin.

45. Ampicillin
• Antibacterial spectrum broader than that of penicillin G.
• This product is active against the same Gram-positive organisms
that are susceptible to other penicillins, and it is more active
against some Gram-negative bacteria and enterococci than are
other penicillins.
• Amino group plays an important role in the broader activity.
• Amino group confers an ability to cross cell wall barriers that are
impenetrable to other penicillins.

46. Amoxicillin
• Amoxicillin, is the p-hydroxy analog of ampicillin.
• Amoxicillin is a fine, white to off-white, crystalline powder that is
sparingly soluble in water.
• It is available in various oral dosage forms

47. Amoxicillin
• Its antibacterial spectrum is nearly identical with that of
ampicillin.
• It is resistant to acid, susceptible to alkaline and lactamase
hydrolysis, and weakly protein bound.
• Orally administered amoxicillin possesses significant advantages
over ampicillin, including more complete GI absorption to give
higher plasma and urine levels, less diarrhea, and little or no effect
of food on absorption.

48. Amoxicillin
• Thus, amoxicillin has largely replaced ampicillin for the
treatment of certain systemic and urinary tract infections for
which oral administration is desirable.
• However, amoxicillin is less effective than ampicillin in the
treatment of dysentery, presumably because of its greater GI
absorption.
• Oral absorption of amino benzyl penicillins (e.g., ampicillin and
amoxicillin) and cephalosporins is carrier mediated thus
explaining their generally superior oral activity.

49. Oxacillin
• Oxacillin sodium, (5-methyl3-phenyl-4-isoxazolyl) penicillin
sodium monohydrate, is the salt of a semisynthetic penicillin
that is highly resistant to inactivation by penicillinase.
• The steric effects of the 3-phenyl and 5-methyl groups of the
isoxazolyl ring prevent the binding of this penicillin to the
lactamase active site and, thereby, protect the lactam ring from
degradation in much the same way as has been suggested for
methicillin.

50. Cloxacillin
• It is also relatively resistant to acid hydrolysis and, therefore,
may be administered orally with good effect.
• The chlorine atom ortho to the position of attachment of the
phenyl ring to the isoxazole ring enhances the activity of
cloxacillin sodium over that of oxacillin, not by increasing its
intrinsic antibacterial activity but by enhancing its oral
absorption, leading to higher plasma levels.
• In almost all other respects, it resembles oxacillin.

51. Dicloxacillin
• The substitution of chlorine atoms on both
carbons ortho to the position of attachment of
the phenyl ring to the isoxazole ring further enhance the stability of
dicloxacillin sodium and produce high plasma concentrations.
• Progressive halogen substitution, however, also increases the
fraction bound to protein in the plasma, potentially reducing the
concentration of free antibiotic in plasma and tissues.
• Its medicinal properties and use are the same as those of
cloxacillin sodium.