Myself Gaurav Chaudhary, Assistant Professor, I.T.S. College of Pharmacy. The Slideshare contains complete Notes of Unit-1 Medicinal Chemistry-III BP601T It contains the Classification, stereochemistry chemical degradation, and Structure-activity relationship of Antibiotics(Penicillin, Cephalosporins, Monobactum, Aminoglycosides). The study material is authentic.

1. Antibiotics
Mr. Gaurav Chaudhary
Assistant Professor,
ITS College of Pharmacy

3. Content
 Introduction
 History
 Classification
 Beta-lactum antibiotics
 Penicillin
 Cephalosporins
 Beta-lactamase Inhibitors
 Monobactums
 Tetracyclines
 Aminoglycosides

4. Introduction
 Antibiotics are the chemical compounds produced by or derived from various
micro-organisms, such as fungi, actinomycetes, bacteria, yeast and molds.
 Depending upon the type of action, these are called bactericidal or bacteriostatic
antibiotics.
 Bactericidal are those which kill micro-organisms while bacteriostatic only inhibit
their growth.
 Some act upon specific micro-organism and are known as ‘narrow spectrum’
antibiotics whereas others may act on several kinds and known as ‘broad
spectrum’ antibiotics.

5. History
 There are three phases to explain the history of chemotherapy, such as Empirical
period, Ehrlich's phase, and the modern phase.
 In empirical period, Moulded curd of soybean was used by Chinese for boils and
carbuncles. During these phases, Hindus used chaulmoogra oil for the treatment of
leprosy. Paracelsus used mercury for the treatment of syphilis(16th century).
Cinchona bark was used for fevers(17th century).

7.  In Ehrlich's phase, it was revealed that certain dyes produced toxicity and killed some
microorganisms. So, neoarsphenamine was developed by Ehrlich for the treatment of
syphilis. Ehrlich's coined the term 'chemotherapy.
 The word antibiosis was coined after the killing of anthrax bacilli when grown in
culture media with other bacteria during the 18th century.
 The modern phases demonstrated the therapeutic effect of prontosil (by Domagk) in
pyogenic infections in 19th century.
 In 1929, Sir Alexander Fleming accidentally discovered the antibacterial properties of
penicillin by destroying the staphylococcus in culture plate.
Continued..

8. Continued..
 Chain and Florey followed up this observation in 1939 and later penicillin was
clinically used during 1941.
 In 1942, Waksman proposed the widely cited definition that “an antibiotic or
antibiotic substance is a substance produced by microorganisms, which has the
capacity of inhibiting the growth and even of destroying other microorganisms.”
 Later proposals have sought both to expand and to restrict the definition to include
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
concentrations.

9.  The isolation of the antibacterial antibiotic tyrocidin from the soil bacterium
Bacillus brevis by Dubois suggested the probable existence of many antibiotic
substances in nature and provided the impetus for the search for them.
 An organized search of the order Actinomycetales led Waksman and associates to
isolate streptomycin from Streptomyces griseus. The discovery that this antibiotic
possessed in vivo activity against Mycobacterium tuberculosis in addition to
numerous species of Gram-negative bacilli was electrifying.
Continued..

10. Continued..
 It was now evident that soil microorganisms would provide a rich source of
antibiotics. Broad screening programs were instituted to find antibiotics that might
be effective in the treatment of infections which are until now resistant to existing
chemotherapeutic agents, as well as to provide safer and more effective
chemotherapy.
 The discoveries of broad-spectrum antibacterial antibiotics such as
chloramphenicol and the tetracyclines, antifungal antibiotics such as nystatin and
griseofulvin, and the ever-increasing number of antibiotics that may be used to
treat infectious agents that have developed resistance to some of the older
antibiotics.

11. Classification
Depending upon spectrum of antimicrobial activity:
 Narrow spectrum antibiotics: The antibiotics having high degree of specificity,
i.e., they are selectively effective either on gram positive bacteria or gram
negative bacteria or certain fungi or yeast, are called narrow spectrum antibiotics,
e.g. benzyl penicillin.
 Broad spectrum antibiotics: The antibiotics which are effective on a large
number of pathogens, not only gram positive and gram negative bacteria, but also
affect intracellular organisms like viruses and rickettsiae, are called broad
spectrum antibiotics. e.g. streptomycin, chloramphenicol, tetracycline etc.

12. Continued..***
Classification on the basis of mechanism of action:
 Cell wall synthesis Inhibitors: Penicillins, Cephalosporins, Vancomycin, Beta-lactmase
Inhibitors, Carbapenems, Aztreonam, Polymycin, Bacitracin, etc.
 Inhibit Protein Synthesis: Bind to 30S Ribosomal Subunit: Aminoglycosides (Gentamicin),
Tetracyclines; Bind to 50S Ribosomal Subunit: Chloramphenicol, Lincosamides, Macrolides,
Clindamycin, Streptogramins.
 Inhibit Nucleic Acid Synthesis (DNA synthesis Inhibitors): Quinolones, Fluoroquinolones.
 Inhibit Metabolic Pathways (Folic acid pathway inhibitors): Sulfonamides, Trimethoprim.
 RNA synthesis Inhibitors: Rifampin
 Mycolic Acid synthesis inhibitors: Isoniazid

13. Beta-lactum antibiotics
 β-lactam is a cyclic amide with four atoms (3-carbon and 1-nitrogen) in its ring. β-lactam
ring is a reactive moeity and, therefore, more sensitive to nucleophilic attack when
compared with normal planar amides.
Chemical Structure of Beta Lactum ring
 Penicillin-G or benzyl penicillin (natural) and phenoxymethyl penicillin (penicillin-V)
remain the agents of choice for the treatment of infections caused by most species of
Gram-positive bacteria. Cephalosporin is the discovery of a second major group of β-
lactam antibiotics.

14. Continued..
 Chemical modifications of naturally occurring penicillins and cephalosporins have
provided semisynthetic derivatives that are effective against various bacterial
species known to be resistant to penicillin, in particular, penicillinase-producing
staphylococci and Gram-negative bacilli. Thus, apart from a few strains that have
either inherent or acquired resistance, almost all bacterial species are sensitive to
one or more of the available antibiotics.

15. Mechanism of action of β-Lactam antibiotics
 β-Lactam antibiotics act by interfering with proteins essential for synthesis of
bacterial cell wall, and in the process either kills or inhibits their growth.
 Bacterial enzymes, penicillin binding protein (PBP) are responsible for cross
linking peptide units during synthesis of peptidoglycan that provide strength and
rigidity to the cell wall.
 Members of β-lactam antibiotics are able to bind themselves to these PBP
enzymes, and in the process, they interfere with the synthesis of peptidoglycan
resulting to lysis and cell death.

16. Penicillin
 The first antibiotic, penicillin, which was first discovered and reported in 1929 by Sir
Alexander Fleming.
 Penicillins belongs to a class of diverse group of compounds, most of which end in the
suffix-cillin.
 They are β-lactam compounds containing a nucleus of 6-animopenicillanic acid (lactam
plus thiazolidine) ring and other ring side chains.
 The side chain determines, in large part, the antibacterial spectrum and pharmacologic
properties.

17. Classification

18. Continued..

19. Continued..

20. Nomenclature
 The nomenclature of penicillins is somewhat complex and very cumbersome. Two
numbering systems for the fused bicyclic heterocyclic system exist.
 The Chemical Abstracts system initiates the numbering with the sulfur atom and
assigns the ring nitrogen the 4-position.
 The numbering system adopted by the USP is the reverse of the Chemical
Abstracts procedure, assigning number 1 to the nitrogen atom and number 4 to the
sulfur atom.

21. Continued..
 Three simplified forms of penicillin nomenclature have been adopted for general use.
The first uses the name “penam” for the unsubstituted bicyclic system, including the
amide carbonyl group. Thus, penicillins generally are designated according to
chemical abstracts system as 6acyloamino-2,2-dimethylpenam-3-carboxylic acids.
 The second, uses the name “penicillanic acid” to describe the ring system with
substituents that are generally present.
 A third form, uses trivial nomenclature to name the entire 6-carbonylaminopenicillanic
acid portion of the molecule penicillin and then distinguishes compounds on the basis
of the R group of the acyl portion of the molecule. Thus, penicillin G is named
methicillin is 2,6-dimethoxyphenylpenicillin, and so on.

22. Mechanism
 The penicillins cause the lysis of growing bacteria. They bind to the enzymes involved
in the biosynthesis of the bacterial cell wall.
 The penicillins and the other β-lactam antibiotics have a structure that closely
resembles that of acylated D-alanyl-D-alanine.
 Penicillins bind to a number of receptor proteins, transpeptidases and
carboxypeptidases called penicillin binding proteins (PBPs), the enzymes that catalyze
the synthesis of peptidoglycan, which is a critical component of the bacterial cell wall.
 This leads to interuption of cell wall synthesis, consequently leading to bacterial cell
growth inhibition and cell lysis.

23. Stereochemistry
 The penicillin molecule contains three chiral carbon atoms (C-3, C-5, and C-6).
 All naturally occurring and microbiologically active synthetic and semisynthetic
penicillins have the same absolute configuration about these three centers.
 The atom bearing the acylamino group (C-6) has the L configuration, whereas the
carbon to which the carboxyl group is attached has the D configuration. Thus, the
acylamino and carboxyl groups are trans to each other, with the former in the α and the
latter in the β orientation relative to the penam ring system.
 The atoms composing the 6-aminopenicillanic acid (6-APA) portion of the structure
are derived biosynthetically from two amino acids, L-cysteine (S-1, C-5, C-6, C-7, and
6-amino) and L-valine (2,2-dimethyl, C-2, C-3, N-4, and 3-carboxyl). The absolute
stereochemistry of the penicillins is designated 3S:5R:6R.

24. Structure Activity Relationship of Penicllin

25. Continued..
Substitution on C-6 Amino group(R group)
 Electron withdrawing groups decreases the electron density on Carbonyl & protect
penicillin from acid degradation. They can survive passage through GI tract & can be given
orally for systemic purposes.
 α- aryloxy alkyl side chain gives increased acid stability & oral absorption. Increasing the
steric hindrance with bulky group at α-carbon increased the resistance to Beta Lactamse.
 Polar or ionised group like amino, hydroxy & carboxyl increases the gram negative
activity.
 The D-isomer is 2- 8 times more potent than the L-isomer.
 When an aromatic ring is directly attached to carbonyl & both ortho positions of aromatic
ring are substituted with methoxy group. It gives a very potent compound. If any methoxy
group is replaced by H, it decreases the activity.

26. Continued..
Substitution in the thiazolidine ring
 Derivatization of C-2 carboxylic acid is not tolerated unless it leads to free penicillin and
carboxylic acid can be generated. Many esters of carbonyl group attached to C-3 have been
prepared as prodrug to increaese lipophilicity & acid stability of the drug.
 Dimethyl group at C-3 is essential & characteristic of penicillins. One methyl group should
be above the plane and the other below the plane. One methyl group at C-3 replaced with
3,4 diacetoxy benzoic acid may produce better activity.
 The sulphur of thiazolidine ring can be replaced by O and CH2 leads to broad spectrum
anti-bacterial activity.
 The N-1 at ring junction is vital for anti-bacterial activity. So, it should be left unsubstituted
or replaced.

27. Chemical Degradation

28. Continued..
 The deterioration of penicillin occurred due to reactivity (hydrolysis) of the strained
lactam ring and is influenced by the pH of the solution. The β-lactam carbonyl group of
penicillin readily undergoes nucleophilic attack by water (-OH-) to form the inactive
penicilloic acid.
 In strongly acidic solutions (pH < 3), penicillin undergoes a complex series of reactions
leading to a variety of inactive degradation products. The first step appears to involve
rearrangement to the penicillenic 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. The subsequent opening
of the β-lactam ring destabilizes the thiazoline ring, which then also suffers acid-catalyzed
ring opening to form the penicillenic acid.

29. Continued..
 The later is very unstable and experiences two major degradation pathways. The most
easily understood path involves hydrolysis of the oxazolone ring to form the unstable
penamaldic acid. Because it is an enamine, penamaldic acid easily hydrolyzes to
penicillamine (a major degradation product) and penaldic acid.
 The second path involves a complete rearrangement of penicillanic acid to a penillic acid
through a series of intramolecular processes that remain to be elucidated completely.
Penillic acid (an imidazoline-2-carboxylic acid) readily decarboxylates and suffers
hydrolytic ring opening under acidic conditions to form a second major end product of
acid-catalyzed penicillin degradant, penilloic acid.
 The third product of the degradation is penicilloaldehyde formed by decarboxylation of
penaldic acid.

30. Continued..
 Oxidizing agents also inactivate penicillins, but reducing agents have little effect on them.
Temperature affects the rate of deterioration: although the dry salts are stable at room
temperature and do not require refrigeration, prolonged heating inactivates the penicillins.
 Acid-catalyzed degradation in the stomach contributes strongly to the poor oral absorption
of penicillin. Thus, efforts to obtain penicillins with improved pharmacokinetic and
microbiological properties have focused on acyl functionalities that would minimize
sensitivity of the β-lactam ring to acid hydrolysis while maintaining antibacterial activity.

31. Cephalosporins
 The cephalosporins are β-lactam antibiotics isolated cephalosporium species or
prepared semi-synthetically. Members of this group of antibiotics are similar to
penicillin in their structure and mode of action.
 The first known member of this group of antibiotics was first isolated by Guiseppe
Brotzu in 1945 from the fungus Cephalosporium acremonium which inhibited the
growth of wide varieties of Gram-positive and Gram-negative bacteria.
 Cephalosporins contain 7-aminocephalosporanic acid (7-ACA) nucleus and side
chain containing 3,6-dihydro-2 H-1,3- thiazane ring.

32. Continued..
 Cephalosporins are used in the treatment of bacterial infections and diseases
arising from penicillinase-producing, methicillin-susceptible Staphylococci and
Streptococci, Proteus mirabilis, some Escherichia coli , Klebsiella pneumonia,
Haemophilus influenza, Enterobacter aerogenes and some Neisseria.
 They are subdivided into generations (1st to 5th) in accordance to their target
organism, but later versions are increasingly more effective against Gram-negative
pathogens. Cephalosporins have a variety of side chains that enable them to get
attached to different penicillin-binding proteins (PBPs), to circumvent blood brain
barrier, resist breakdown by penicillinase producing bacterial strains and ionize to
facilitate entry into Gram-negative bacteria cells.

33. Classification of Cephalosporins
First Generation Cephalosporins:
 Oral: Cephalexin, Cefadroxil
 Parentral: Cephalothin, Cephapirin, Cefazolin
 Oral & Parentral: Cephradrine
Second Generation Cephalosporins:
 Oral: Cefaclor, Cefprozil, Cefpodoxime
 Parentral: Cefamandole, Cefonicid, Ceforanide, Cefoxitin, Cefotetan, Cefmetazole
 Oral & Parentral: Cefuroxime
Third Generation Cephalosporins:
 Oral: Cefixime, Ceftibuten
 Parentral: Cefotaxime, Ceftizoxime, Ceftazidime
Fourth Generation Cephalosporins:
 Parentral: Cefepime, Cefpirome

34. Parenteral Cephalosporins

35. Continued..

36. Oral Cephalosporins

37. Continued..

38. Continued..

39. Nomenclature
 The chemical nomenclature of the cephalosporins is slightly more complex than even that
of the penicillins because of the presence of a double bond in the dihydrothiazine ring.
 The fused ring system is designated as 5-thia-1-azabicyclo[4.2.0]oct-2-ene. In this system,
cephalothin is 3-(acetoxymethyl)- 7-[2(thienylacetyl) amino)- 8-oxo-5-thia-1-
azabicyclo[4.2.0]oct2-ene-2-carboxylic acid.
 The Chemical Abstracts procedure names the saturated bicyclic ring system with the
lactam carbonyl oxygen cepham (cf., penam for penicillins).

40. Continued..
 According to this system, all commercially available cephalosporins and cephamycins are
named 2-cephems to designate the position of the double bond. (Interestingly, all known 2-
cephems are inactive, presumably because the ß-lactam lacks the necessary ring strain to
react sufficiently.)
 The trivialized forms of nomenclature of the type that have been applied to the penicillins
are not consistently applicable to the naming of cephalosporins because of variations in the
substituent at the 3-position.
 Thus, although some cephalosporins are named as derivatives of cephalosporanic acids,
this practice applies only to the derivatives that have a 3-acetoxymethyl group.

41. Mechanisms
 The cephalosporins are believed to act in a manner analogous to that of the
penicillins.
 Cephalosporins bind with receptor proteins, transpeptidases and
carboxypeptidases called penicillin binding proteins (PBPs), the enzymes that
catalyze the synthesis of peptidoglycan, which is a critical component of the
bacterial cell wall.
 This leads to the interruption of cell wall synthesis, consequently leading to
bacterial cell growth inhibition and cell lysis.

42. Structure Activity Relationship

43. Modification of Thiazine Ring
At position 1
 Most of the cephalosporins have the thiazine ring attached to β-lactum ring, but in some
cases the S-1 maybe replaced with O to give oxacephems with increased anti-bacterial
activity due to its acylating power.
 S-1 can be replaced with methylene group gives greater stability & longer half-life.
 Oxidation of S-1 will greatly affects the activity of the drug, generally reducing the
activity, but some agents with sulphoxide or sulphone may have antibacterial activites.

44. Continued..
At C-3 position
 Substitution at C-3 influences pharmacokinetic, pharmacological as well as anti-bacterial activity.
 If R2 is acetoxymethyl (-CH2-CO-CH3), it degrades in the presence of acid into inactive compound.
 So, modification at C-3 may reduce the inactivation (degradation) of Cephalosporins. Increases acid stability & Oral
activity too.
 Amide & Ester have a broader spectrum of activity.
 The benzoyl ester, pyridine & imidazole improved gram +ve activity but decreases gram –ve activity & improves
activity against P. aeruginosa.
 Aromatic thiols enhance activity against gram –ve bacteria withimproved pharmacokinetics.
 Azide ion yield low gram –ve activity.
 Replacement of acetoxymethyl group with simply methyl, chloro & venyl results in good oral absorption.

45. Continued..

46. Continued..
Modification at C-4 Carboxyl group:
 Carboxyl group at C-4 converted into esster prodrug to increase bioavailability & also
improves oral activity.
β-lactum Ring
 β-lactum Ring is essential for the activity of β-lactum antibiotics, the C=O at C-8 is
susceptible to electrophile attack.
 Aminoacyl substitution at C-7 is essential for activity while the stereochemistry at C-7 and
C-6 is also essential, generally H should be below the plane at C-7 and C-6.
 Introduction of methoxy group at C-7 produce steric hindrance & show higher resistance to
β-lactamase.

47. Continued..
Modification at 7-Aminoacyl group
 Acylation of amino group gives gram +ve activity but decrease gram –ve activity.
 R1 maybe aromatic ring along with substitution, increases activity as phenyl group by
increasing lipophilicity of the compound.
 The phenyl ring in the side chain can be replaced with other heterocycles (thiopene,
tetrazole, furan) with improved spectrum of activity & pharmacokinetic properties.
 Amino-thiazole increases gram +ve activity.
 Lipophilicity can also be increased by the introduction of α- substitution in side chain on α-
carbon with –OH, -NH2, =N-O-CH3, -COOH, -SO3H.

48. Chemical Degradation of Cepalosporins

49. Continued..
 Cephalosporins experience a variety of hydrolytic degradation reactions whose specific nature
depends on the individual structure. Among 7-acylaminocephalosporanic acid (7-ACA)
derivatives, the 3-acetoxylmethyl group is the most reactive site. In addition to its reactivity to
nucleophilic displacement reactions, the acetoxyl function of this group readily undergoes
solvolysis in strongly acidic solutions to form the des-acetylcephalosporin derivatives.
 The later lactonizes to form the desacetylcephalosporin lactones, which are virtually inactive.
The 7-acylamino group of some cephalosporins can also be hydrolyzed under enzymatic
(acylases) and, possibly, non-enzymatic conditions to give 7-ACA (or 7-ADCA) derivatives.
Following hydrolysis or solvolysis of the 3-acetoxymethyl group, 7-ACA lactonizes under
acidic conditions.

50. Continued..
 The reactive functionality common to all cephalosporins is the B-lactam. Hydrolysis of the
B-lactam of cephalosporins is believed to give initially cephalosporanic acids or possibly
anhydro-desacetylcephalosporanic acids (for the 7-acylaminocephalosporanic acids). It has
not been possible to isolate either of these initial hydrolysis products in aqueous systems.
Apparently, both types of cephalosporanic acids undergo fragmentation reactions that have
not been characterized fully.

51. Beta-Lactamase Inhibitors
 β-Lactamases (also known as penicillinases or cephalosporinases) are plasmid or
chromosomally encoded bacterial enzymes that catalyze hydrolysis of the amide bond of
the β-lactam ring, producing acidic derivatives with no antibacterial activity. Production of
β-lactamases is the most common mechanism of bacterial resistance to penicillins and
cephalosporins.
 β-lactamase mediated resistance to β-lactam antibiotics is an increasing threat to clinical
antimicrobial chemotherapy. Resistance to β-lactams is primarily because of bacterially
produced β -lactamase enzymes that hydrolyze the β-lactam ring, thereby inactivating the
drug. The newest effort to circumvent resistance is the development of novel broad
spectrum β-lactamase inhibitors that work against many problematic β-lactamases.

52. Continued..
 The discovery of the naturally occurring, mechanism based inhibitor clavulanic acid, which
causes potent and progressive inactivation of β-lactamases, has created renewed interest in β-
lactum combination therapy. This interest has led to the design and synthesis of additional
mechanism based β-lactamase inhibitors, such as sulbactam and tazobactam and the isolation of
naturally occurring β-lactums, such as the thienamycins, which both inhibit β-lactamases and
interact with Penicllin binding proteins(PBPs).
 The combinations of β-lactam antibiotics and β-lactamase inhibitors (such as sulbactam,
tazobactam and clavulanic acid) have been successfully used for β-lactamase-mediated
resistance.
 Beta-lactamase inhibitors inactivate β-Lactamases enzyme and, in association with beta-lactums,
offer wide spectrum bactericidal action (Gram negative bacilli, anaerobic germs, methicillin-
sensitive staphylococci and streptococci) and are used as first-line treatment in certain
community-acquired and nosocomial infections.

53. Monobactums
 Fermentation of unusual microorganisms led to the discovery of a class of monocyclic B-
lactam antibiotics, named monobactams. The discovery of this class of antibiotics was first
reported by Skyes and co-workers.
 The antibiotic was obtained from the bacterium Chromobacterium violaceum. They are
part of B-lactam compounds but unlike most other B-lactams, the B-lactam ring of
monobactams stand alone and is not fused to another ring.
 The monobactams are not effective against Gram-positive bacteria or anaerobes.
Monobactams work only against aerobic gram-negative bacteria (Neisseria, Pseudomonas).
Monobactams work by inhibiting the peptidoglycan synthesis process (a
process essential to maintain bacterial cell wall integrity); as a result, the bacteria lose the
ability to resist and burst, leading to cell death. Monobactam antibiotics are administered
intravenously or intramuscularly. They are used as injectables and inhalers.

54. Tetracyclines
 Among the most important broad-spectrum antibiotics are members of the tetracycline
family. Nine such compounds—tetracycline, rolitetracycline, oxytetracycline,
chlortetracycline, demeclocycline, meclocycline, methacycline, doxycycline, and
minocycline-have been introduced into medical use. The tetracyclines are obtained by
fermentation procedures from Streptomyces spp. or by chemical transformations of the
natural products.
 Their chemical identities have been established by degradation studies and confirmed by
the synthesis of three members of the group, oxytetracycline, 6-demethyl-6-
deoxytetracycline, and anhydrochlortetracycline, in their (α) forms. The important
members of the group are derivatives of an octahydronaphthacene, a hydrocarbon system
that comprises four annulated six-membered rings. The group name is derived from this
tetracyclic system.

55. Stereochemistry
 The stereochemistry of the tetracyclines is very complex. Carbon atoms 4, 4a, 5, 5a, 6, and 12a are
potentially chiral, depending on substitution. Oxytetracycline and doxycycline, each with a 5a-
hydroxyl substituent, have six asymmetric centers; the others, lacking chirality at C-5, have only
five. Determination of the complete, absolute stereochemistry of the tetracyclines was a difficult
problem. Detailed x-ray diffraction analysis established the stereochemical formula for the natural
and semisynthetic tetracyclines.
 The tetracyclines are amphoteric compounds, forming salts with either acids or bases. In neutral
solutions, these substances exist mainly as zwitterions. The acid salts, which are formed through
protonation of the enol group on C-2, exist as crystalline compounds that are very soluble in water.
The hydrochloride salts are used most commonly for oral administration and usually are
encapsulated because they are bitter. Water-soluble salts may be obtained also from bases, such as
sodium or potassium hydroxides, but they are not stable in aqueous solutions. Water-insoluble salts
are formed with divalent and polyvalent metals.

56. Mechanism
 Tetracyclines are specific inhibitors of bacterial protein synthesis. They bind to the 30S
ribosomal subunit and, thereby, prevent the binding of aminoacyl tRNA to the mRNA-
ribosome complex. Both the binding of aminoacyl tRNA and the binding of tetracyclines at
the ribosomal binding site require magnesium ions. Tetracyclines also bind to mammalian
ribosomes but with lower affinities, and they apparently do not achieve sufficient
intracellular concentrations to interfere with protein synthesis. The selective toxicity of the
tetracyclines toward bacteria depends strongly on the self-destructive capacity of bacterial
cells to concentrate these agents in the cell. Tetracyclines enter bacterial cells by two
processes: passive diffusion and active transport. The active uptake of tetracyclines by
bacterial cells is an energy dependent process that requires adenosine triphosphate (ATP)
and magnesium ions.

57. Structure Activity Relationship of Tetracyclines

58. Continued..
Modification at C1 & C3
 Keto-enol tautomersim between C1 & C3 is highly delocalised, extremely slow to react.
 Substitution on C1 & C3 may block the system & thus loss of antibacterial activity.
 Equilibrium between non-ionized & zwitter ionic structures of tetracyclines is also essential.
Modification at C2:
 Amide carbonyl appears essential for tetracyclines but not the amide nitrogen.
 The amide group left un-substituted or mono-substituted with alkylation gives potent
compound.
 This types of substitution gives drugs which are freely water soluble in physiological pH range
& thus hydrolyse slowly.
 Larger group substitution may decreases activity due to disturbing keto enol tautomerism.

59. Continued..
Modification at C4:
 All naturally occurring tetracyclines contain α C4 dimethyl amino group, which is essential
for activity.
 This helps in formation of an equilibrium between unionized and zwitter ion.
 Substitution or removal of this dimethyl amino group leads to loss of activity.
Modification at C4a and C5a:
 Hydrogen at C4a & C5a is essential for activity in all tetracyclines
Modification at C5:
 Many natural tetracyclines have an un-substituted C5. However, oxytetracycline have α-
hydroxyl group.

60. Continued..
Modification at C6:
 Many tetracyclines contain α-methyl & β-hydroxy group at C6.
 The hydroxy group at C6 like natural tetracylcines makes molecule unstable to both acids & bases.
Removal of OH gives stable compounds.
 6-Methylene (CH2) group also produces excellent antibacterial activity & stability.
Modification at Ring D:
 Groups like halogens, nitro, amino at C7 produces potent compounds.
 Nitro at C7 may produce toxicity while halo substitution is less active. Most important substitution is
dimethyl amino, eg: Minocycline.
 Ring D should be aromatic & strong electron withdrawing groups at C7 produces potent compounds.

61. Continued..
 C9 can also be substituted but less activity than C7 substitutions.
 C10 hydroxy group activated C7 & C9 position & this phenolic moiety is essential for
activity.
Modification at C11, C12, C11a & C12a:
 11, 12 β-diketone system is essential.
 Hydroxy group at C12a is essential.
 Substitution at these positions results in loss of activity.

63. Tetracycline

64. Continued..
 Tetracycline is (4S,6S,12aS)-4-(dimethylamino) -1,4,4a,5,5a,6,11,12a-octahydro-
3,6,10,12,12a-pentahydroxy -6-methyl-1,11-dioxonaphthacene-2-carboxamide.
 It is a bright yellow, crystalline salt that is stable in air but darkens on exposure to strong
sunlight.
 It is stable in acid solutions with a pH above 2.
 Tetracycline inhibits protein synthesis by binding to the 30S and 50S subunit of microbial
ribosomes. Thus, it prevents the formation of a peptide chain.
 Tetracycline are used to treat a number of infections, including acne, cholera, plague,
brucellosis, malaria, and syphilis.

65. Oxytetracycline
• Oxytetracycline is (4S,4aR,5S,5aR,6S,12aS)-4-(Dimethylamino)-3,5,6,10,11,12a-
hexahydroxy-6-methyl-1,12-dioxo-1,4,4a,5,5a,6,12,12a-octahydrotetracene-2-carboxamide
• Oxytetracycline hydrochloride is a pale yellow, bitter, crystalline compound. The amphoteric
base is slightly soluble in water and slightly soluble in alcohol. It is odorless and stable in air
but darkens on exposure to strong sunlight.

66. Continued..
 Oxytetracycline hydrochloride is also used for parenteral administration
(intravenously and intramuscularly).
 Oxytetracycline inhibits cell growth by inhibiting translation. It binds to the 305
ribosomal subunit and prevents the amino-acyl tRNA from binding to the A site of
the ribosome. The binding is reversible in nature. Oxytetracycline is lipophilic and
can easily pass through the cell membrane or passively diffuses through porin
channels in the bacterial membrane.
 Oxytetracycline is a tetracycline class of antibiotics, commonly used for the
treatment of various infectious diseases like anthrax, Chlamydia, cholera, typhus,
relapsing fever, malaria, plaque, syphilis, respiratory infection, streptococcal
infection, and acne.

67. Chlortetracycline
 The hydrochloride salt is a crystalline powder with a bright yellow color. It
is stable in air but slightly photosensitive and should be protected from
light. It is odorless and bitter.

68. Continued..
 Oral and parenteral forms of chlortetracycline are no longer used because of the poor
bioavailability and inferior pharmacokinetic properties of the drug. It is still marketed in
ointment forms for topical and ophthalmic use.
 Chlortetracycline, like other tetracyclines, competes for the amino-acyl site of the bacterial
ribosome. This binding competes with tRNA carrying amino acids preventing the addition
of more amino acids to the peptide chain.
 A combination cream with triamcinolone acetonide(corticosteroid) is available for the
treatment of infected allergic dermatitis in humans. In veterinary medicine,
chlortetracycline is commonly used to treat conjunctivitis in cats, dogs and horses. It is also
used to treat infected wounds in cattle, sheep and pigs, and respiratory tract infections in
calves, pigs and chickens.

69. Minocycline
 Minocycline is (2E,4S,4aR,5aS,12aR)- 2-(Amino –hydroxy -methylidene) -4,7-
bis(dimethylamino)- 10,11,12a-trihydroxy-4a,5,5a,6- tetrahydro-4H-tetracene-1,3,12-trione
 Minocycline is the most potent tetracycline currently used in therapy, because minocycline,
lacks the 6-hydroxyl group, it is stable in acids and does not dehydrate or rearrange to anhydro
or lactone forms.

70. Continued..
 Minocycline is well absorbed orally to give high plasma and tissue levels. It has a very
long serum half-life, resulting from slow urinary excretion and moderate protein
binding.
 As with other tetracyclines, the mechanism of action of minocycline involves attaching
to the bacterial 30 S ribosomal subunit and preventing protein synthesis.
 Minocycline is used to treat infections caused by bacteria including pneumonia and
other respiratory tract infections, certain infections of the skin, eye, lymphatic, intestinal,
genital, and urinary systems, and certain other infections that are spread by ticks, lice,
mites, and infected animals.
 During the last 30 years, minocycline, a wide-spectrum antimicrobial agent, has been
effective against MDR Gram-positive and Gram-negative bacterial infections.

71. Doxycycline
 Doxycycline is (4S,4aR,5S,5aR,6R,12aS)-4-(Dimethylamino)-3,5,10,12,12a-
pentahydroxy-6-methyl-1,11-dioxo-1,4,4a,5,5a,6,11,12a-octahydrotetracene-2-
carboxamide
 Doxycycline is available as a hydrate salt which is sparingly soluble in water and
is used in a capsule.

72. Continued..
 Absence of the 6-hydroxyl group produces a compound that is very stable in acids and
bases and that has a long biological half-life. In addition, it is absorbed very well from the
GI tract, thus allowing a smaller dose to be administered, High tissue levels are obtained
with it, and unlike other tetracyclines, doxycycline apparently does not accumulate in
patients with impaired renal function. Therefore, it is preferred for uremic patients with
infections outside the urinary tract. Its low renal clearance may limit its effectiveness,
however, in urinary tract infections.

73. Continued..
 Doxycycline is a broad spectrum antibiotic. It inhibits the synthesis of bacterial
proteins by binding to the 30S ribosomal subunit, which is only found in bacteria.
This prevents the binding of transfer RNA to messenger RNA at the ribosomal
subunit meaning amino acids cannot be added to polypeptide chains and new
proteins cannot be made. This stops bacterial growth giving the immune system
time to kill and remove the bacteria.
 It is used to treat bacterial pneumonia, acne, chlamydia infections, Lyme disease,
cholera, typhus, and syphilis. It is also used to prevent malaria in combination
with quinine.

74. Aminoglycosides
 Aminoglycosides are so named because their structures consist of amino sugars linked
glycosidically. The amino-glycosides consists of two or more amino sugars joined by glycosidic
linkage with a substituted cyclohexane ring.
 Aminoglycoside antibiotics were the first drugs discovered by systematic screening of natural
product sources for antibacterial activity. The laboratory of Waksman reported the discovery and
isolation of the aminoglycoside antibiotic streptomycin from soil bacteria in 1944.
 Despite their long history as antibacterial drugs, the present aminoglycosides remain important
antibiotics for the treatment of serious Gram-negative pathogens. Resistance developed against the
aminoglycosides as well as their relative toxicity has encouraged for the development of improved
derivatives.

75. Mechanism
 The aminoglycosides are bactericidal. They act directly on the bacterial ribosome
to inhibit the initiation of protein synthesis and to interfere with the translation of
the genetic message. They bind to the 16S rRNA portion of the 30S ribosomal
subunit to form a complex that cannot initiate proper amino acid polymerization
and thus impairing function ribosome.
 Aminoglycosides are used in the treatment of severe infections of the abdomen
and urinary tract, as well as bacteremia and endocarditis. They are also used for
prophylaxis, especially against endocarditis.

76. SAR of Aminoglycoside (Kanamycin)
Kanamycin A: R1=NH2; R2=OH
Kanamycin B: R1=R2=NH2
Kanamycin C: R1=OH; R2=NH2

77. Ring 1 modification:
 Position 6’ & 2’ are most important.
 Bacterial inactivating enzymes targets 6’ & 2’ position.
 Amino group at both 6’ & 2’ positions as in kanamycin B makes it the most active, as
compared to kanamycin A & C.
 Methylation at 6’ carbon or 6’ amino increases the resistance to bacterial enzyme & also
have good activity.
 Removal of 3’ hydroxyl or 4’ hydroxyl or both does not reduce antibacterial activity; E.g.:
3’,4’-dideoxy kanamycin & Gentamycin.
 4’ & 5’ double bond gives potent compound; E.g.: Netilmicin & Sisomicin.
 3’ hydroxyl phosphorylation reduces binding to 30S ribosomal subunit.

78. Diamino cyclo hexane ring modification
 Many amino glycosides contain 2- deoxy streptamine as central ring.
 The acylation of 1-amino group produces active compound like amikacin.
 1-N-ethyl sisomicin (Netilimicin) retains the activity of sisomicin & more resistant to
bacterial enzymes.

79. Ring 2 modification
 2”- deoxy gentamycins are less active.
 2”-amino (seldomycin) are highly active.
 3”-amino group of gentamycins maybe primary or secondary with high activity.
 2”-hydroxy sisomicin is claimed to be resistant to bacterial strains that adenylate 2”-
hydroxyl group of ring 2.
 3”-deamino sisomicin axhibit good activity.
 4”- position of ring 2 can be methylated to give active products like sisomicin and its
derivatives.
 4”-hydroxyl group maybe axial or equatorial with little change in potency.

80. Streptomycin

81. Continued..
 Streptomycin is a white, odorless powder that is hygroscopic but stable toward light and
air.
 Streptomycin is 5-(2,4-diguanidino- 3,5,6-trihydroxy-cyclohexoxy)- 4-[4,5-dihydroxy-6-
(hydroxymethyl) -3-methylamino-tetrahydropyran-2-yl] oxy-3-hydroxy-2-methyl -
tetrahydrofuran-3-carbaldehyde.
 It works by blocking the ability of 30S ribosomal subunits to make proteins, which results
in bacterial death.
 It is used to treat other serious infections (such as Mycobacterium avium complex-MAC,
tularemia, endocarditis, plague) along with other medications.

82. Neomycin

83. Continued..
 Neomycin is (2RS,3S,4S,5R)- 5-Amino- 2-(aminomethyl)- 6-((2R,3S,4R,5S) -5-
((1R,2R,5R,6R)-3,5-diamino-2-((2R,3S,4R,5S)-3-amino-6-(aminomethyl)-4,5-
dihydroxytetrahydro-2H-pyran-2-yloxy)-6-hydroxycyclohexyloxy)-4-hydroxy-2-
(hydroxymethyl)tetrahydrofuran-3-yloxy)tetrahydro-2H-pyran-3,4-diol
 Neomycin is bactericidal in action. Similar to other aminoglycosides, it inhibits bacterial
protein synthesis through irreversible binding to the 30 S ribosomal subunit of susceptible
bacteria.
 Neomycin, an antibiotic, is used to prevent or treat skin infections caused by bacteria. It is
not effective against fungal or viral infections.

84. Kanamycin

85. Continued..
 Kanamycin is 2-(aminomethyl)- 6-[4,6-diamino-3- [4-amino- 3,5-dihydroxy-6-
(hydroxymethyl) tetrahydropyran-2-yl]oxy- 2-hydroxy- cyclohexoxy]- tetrahydropyran-
3,4,5-triol
 It inhibits protein synthesis by tightly binding to the conserved A site of 16S rRNA in the
30S ribosomal subunit.
 It is used in infections of the intestinal tract and to systemic infections arising from Gram
negative bacilli.