4. General Structure
O
O
O
CH3
R1
H3C
CH3
CH3
O
H3C
OH
H3C
CH3
OH
O
O
HO
CH3
N
CH3
CH3
O OH
CH3
CH3
OR2
1 3
5
9
12
1`
1``
Erythromycin
Glycon
Aglycone
They all contain three characteristics parts in the molecule:
A highly substituted macrocyclic lactone: aglycone.
A ketone group.
An amino desoxysugar: glycon, and in some of the macrolides, a
neutral desoxysugar which are glycosisically attached to the
aglycone ring.
5. The lactone ring usually has 12, 14, or 16 atoms and is
often unsaturated
macrolide antibiotics are weak bases and different
salts with pKa range of 6.0-9.0 can be formed on the
amino group.
Macrolides are water-insoluble molecules. Salts
prepared by glucoheptonic and lactobionic salts are
water soluble, whereas stearic acid and laurylsulfuric
acid salts are water-insoluble.
Macrolides are stable in aqueous solutions at or below
room temperature. They are unstable in acidic or basic
conditions or at high temperatures.
6. It has been the subject of chemical manipulations to:
a) Increase the water solubility of the drug for
parenteral dosage forms.
b) Increase the lipid solubility and hence chemical
stability of the drug against aqueous acidic
conditions as well as increase in oral absorption and
masking the bitter taste of the drug.
7. Chemical Instability of Macrolide Antibiotics
O
O
CH3
CH3
O
H3C
CH3
O
O
CH3
1 3
12 6
89
O
H3C
HO
H3C
Anhydroerythromycin
6,9;9,12-spiroketal
O OH
CH3
CH3
OR2
1``
O
HO
CH3
N
CH3
CH3
1`
Macrolides are unstable under acidic conditions and
undergo an intramolecular reaction to form an
inactive cyclic ketal.
8. Chemical Instability..
The cyclic ketal is the cause of intestinal cramp
which is reported after the use of erythromycin.
Water-insoluble salts and enteric coated dosage
forms of macrolides have less such a side effect.
Water insoluble forms cannot take part in the
reactions which occur in aqueous solutions.
Stearate salt is an example of insoluble salts of
erytromycin.
11. Mechanism of Action
Macrolides attach to the 50s portion of bacterial
ribosomes and inhibit the protein synthesis.
Prevent translocation during elongation of protein
synthesis
Their binding site is either identical or in close
proximity to that for clindamycin and chloramphenicol.
13. Spectrum of Antibacterial Activity
Macrolides are similar to penicillins regarding their
spectrum of activity.
They are effective against penicillin-resistant strains.
Macrolides are effective against most of the G(+)
bacteria, cocci or bacillus, they have antibiotic
activity against G(-) cocci ,especially Neisseria Spp
too.
Macrolide antibiotics are effective against
Mycoplasma, Chlamydia, Campylobacter and
Legionella in contrast to penicillins.
They are less effective against G(-) bacteria, though
some strains of H. influenza and Brucella are
sensitive to the antibacterial activity of this class of
antibiotics.
15. Bacterial Resistance
Methylation of a guanine residue on ribosomal RNA leads
to lower affinity toward macrolides
An active efflux system
presence of a plasmid-associated erythromycin esterase.
Clarithromycin and azithromycin show cross-resistance
with erythromycin, but telithromycin can be effective
against macrolide-resistant organisms.
Lack of cell wall permeability to macrolides is the reason
why G(-) bacteria are resistant to antibacterial effects of
these agents.
16. Pharmacokinetics
Absorption: Enteric coated preparations protect the
antibiotic from gastric acid destruction allowing oral
absorption
Also when stable esterified salts are used.
Fate: Widely distributed to all tissues except CNS.
Excretion: Metabolized by liver and excreted by bile .
Tylosin and telmicosin excreted unchanged in bile and
urine.
19. Inhibition of the cytochrome P450 system by erythromycin,
clarithromycin, and telithromycin.
20. Adverse Effects
Epigastric distress: Common with erythromycin
Cholestatic jaundice: Especially with the estolate form of
erythromycin
Ototoxicity: Transient deafness associated with
erythromycin, at high dosages.
Contraindications: Patients with hepatic dysfunction.
Interactions: Inhibit the hepatic metabolism of a number
of drugs
22. Clinical Application
of Erythromycin
It is used to treat
The upper part of the respiratory tract infections,
Soft tissue G(+) infections,
Mycoplasma pneumonia caused pneumonia,
Campylobacter jejuni enteritis,
Chlamydia infections.
Gonorrhoea.
It is a good choice for penicillin-sensitive cases.
26. Lincosamide
First lincosamide to be discovered is lincomycin, isolated from
Streptomyces lincolnensis
Lincosamides are derivatives of an amino acid and a sulfur-
containing octose.
Lincosamides, macrolides, and chloramphenicol, although not
structurally related, seem to act at this same site.
The lincosamides are bacteriostatic or bactericidal depending on
the concentration.
Activity is enhanced at an alkaline pH.
Lincomycin has been superseded by clindamycin, which exhibits
improved antibacterial activity.
27. Clindamycin
Mechanism same as that of erythromycin
Good for anaerobic organisms (Bacteroids)
Resistance like erythromycin
Well absorbed by the oral route.
It distributes well into all body fluids except the CSF.
Penetration into bone occurs even in the absence of
inflammation.
The drug is excreted into the bile or urine .
Adverse effect:
In addition to skin rashes, serious adverse effect is fatal
pseudomembranous colitis caused by C. difficile, which is
treated by metronidazole or vancomycin .
29. Ketolides
Ketolides are antibiotics belonging to the macrolides
group.
Ketolides are derived from erythromycin by
substituting the cladinose sugar with a keto-group and
attaching a cyclic carbamate group in the lactone ring.
These modifications give ketolides much broader
spectrum than other macrolides.
30.
31.
32. Ketolides are effective against macrolide-resistant
bacteria as well as having a structural modification
that makes them poor substrates for efflux-pump
mediated resistance.
Ketolides block protein synthesis by binding to
ribosomal subunits and may also inhibit the formation
of newly forming ribosomes.
The only ketolide on the market at this moment is
Telithromycin, which is sold under the brand name
of Ketek.
Other ketolides in development include Cethromycin
and Solithromycin.
Editor's Notes
Macrolides and ketolides inhibit protein synthesis by the same mode of action. They bind within the exit tunnel of the large ribosomal subunit, thus blocking the exit of nascent polypeptides. The large ribosomal subunit consists of 2 pieces of rRNA (23S and 5S) and 31 ribosomal proteins. Macrolides and ketolides bind to specific residues of 23S rRNA—namely, the adenine at positions 2058 (A2058) and 2059 (A2059). These specific adenine residues are on domain V of 23S rRNA. In addition, telithromycin binds, via the 11,12 carbamate bridge containing the alkyl-aryl extension, to a specific adenine (A752) on domain II of the 23S rRNA, a region near domain V in the 3-dimensional structure of the ribosome (figure 2). In susceptible organisms, telithromycin binds 10-fold more tightly to ribosomes than erythromycin does, because of the binding to domain II