Transcript Bacterial Protein Synthesis Inhibitors (Antimicrobials)

Bacterial Protein Synthesis
Inhibitors (Antimicrobials)
Reference: Gareth Thomas
Week 14
prof. aza
12. Bacterial Protein Synthesis
Inhibitors Antimicrobials)
• Many protein inhibitors inhibit protein
synthesis in both prokaryotic and
eukaryotic cells (Table 10.2).
• This inhibition can take place at any
stage in protein synthesis.
• However, some inhibitors have a
specific action in that they inhibit
protein synthesis in prokaryotic cells
but not in eukaryotic cells, or vice versa.
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• Consequently, a number of useful drugs
have been discovered that will inhibit
protein synthesis in bacteria but either
have no effect or a very much reduced
effect on protein synthesis in mammals.
• The structures and activities of the
drugs that inhibit protein synthesis are
quite diverse.
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• Consequently, only a few of the
more commonly used drugs and
structurally related compounds will
be discussed in greater detail in
this section.
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Table 10.2. Examples of drugs that
inhibit protein synthesis.
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12.1. Aminoglycosides
• Streptomycin (Figure 10.15) is a member
of a group of compounds known as
aminoglycosides.
• These compounds have structures in
which amino sugar residues in the form
of mono- or polysaccharides are
attached to a substituted 1 ,3diaminocyclohexane ring by modified
glycosidic type linkages.
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• The ring is either streptidine
(streptomycin) or deoxy
streptamine (kanamycin,
neomycin, gentamicin and
tobramycin).
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Figure 10.18.
The structures of
(a) streptomycin and (b) neomiycin C.
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• Streptomycin is as the first
aminoglycoside discovered (Schatz
and co-workers. 1944) from
cultures of the soil Actinomycetes
Streptomyces griseus.
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• It acts by interfering with the initiation
of protein synthesis in bacteria.
• The binding of streptomycin to the 30S
ribosome inhibits initiation and also
causes some amino acid-tRNA
complexes to misread the mRNA codons.
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• This results in the insertion of
incorrect amino acid residues into
the protein chain, which usually
leads to the death of the bacteria.
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• The mode of action of the other
aminoglycosides has been assumed
to follow the same pattern even
though most of the investigations
into the mechanism of the
antibacterial action of the
aminoglycocides have been carried
out
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• The clinically used aminoglycosides
have structures closely related to
that of streptomycin.
• They are essentially broadspectrum antibiotics although the
are normally used to treat serious
Gram-negative bacterial infections
(see section 4.2.5.1).
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• Aminoglycosidic drugs are very water
soluble. They are usually administered
as their water-soluble inorganic salts
but their polar nature means that they
are poorly absorbed when administered
orally.
• Once in the body they are easily
distributed into most body fluids.
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• However, their polar nature means that
they do not easily penetrate the central
nervous system (CNS), bone, fatty and
connective tissue.
• Moreover, aminoglycosides tend to
concentrate in the kidney where they
are excreted by glomerular filtration.
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Figure 19. Kanamycin.
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• Aminoglycoside-drug-resistant strains
of bacteria are not recognised as a
serious medical problem.
• They arise because dominant bacteria
strains have emerged that possess
enzymes that effectively inactivate the
drug.
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• These enzymes act by catalysing
the acylation, phosphorylation and
adenylation of the drug (see section
6.13). This results in the formation
of inactive drug derivatives.
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• The activity of the aminoglycosides is
related to the nature of their ring
substituents.
• Consequently, it is convenient to discuss
this activity in relation to the changes
in the substituents of individual rings
but, in view of the diversity of the
structures of aminoglycosides, it is
difficult to identify common trends.
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• As a result, this discussion will be
largely limited to kanamycin (Figure
10.19).
• However, the same trends are often
true for other aminoglycosides whose
structures consist of three rings,
including a central deoxystreptamine
residue.
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• Changing the nature of the amino
•
substituents at positions 2’ and 6’ of ring I
has the greatest effect on activity.
For example, kanamycin A, which has a
hydroxy group at position 2’, and
kanamycin C, which has a hydroxy group
at position 6’, are both less active than
kanamycin B, which has amino groups at
the 2’ and 6’ positions.
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• However, the removal of one or
both of the hydroxy groups at
positions 3’ and 4’ does not have any
effect on the potency of the
kanamycins.
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• Modifications to ring II (the
deoxystreptamine ring) greatly reduce
the potency of the kanamycins.
• However, N-acylation and alkylation of
the amino group at position I can give
compounds with some activity
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• For example, acylation of kanamycin
A gives 1-N-(L (-)-4- amino-2hydroxybutyryl) kanamycin A
(amikacin), which has a potency of
about 50% of that of kanamycin A
(Figure 10.20).
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• In spite of this, amikacin is a useful
drug for treating some strains of Gramnegative bacteria because it is resistant
to deactivation by bacterial enzymes.
Similarly, 1-N-ethylsisomicin (netilmicin)
is as potent as its parent aminoglvcoside
sisomicin.
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• Changing the substituents of ring III
does not usually have such a great
effect on the potency of the drug as
similar changes in ring I and II.
• For example, removal of the 2” hydroxy
group of gentamicin results in a
significant drop in activity.
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• However, replacement of the 2”
hydroxy group of gentamicin (Figure 21)
by amino groups gives the highly active
seldomycins.
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Figure 20. An outline of the chemistry involved in the synthesis of the antibiotics
amikacin and netilmicin. Cbz is frequently used as a protecting group for amines
because it is easily removed by hydrogenation.
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Figure 10.21. The structures of gentamicin.
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12.2. Chloramphenicol
Chloramphenicol was first isolated
from the microorganism
Streptomyces venezuela by
Ehrlich and co-workers in 1947.
It is a broad-spectrum antibiotic
whose structure contains two
asymmetric centres. However. only
the D(-)-threo form is active.
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• Chloramphenicol can cause serious
side effects and so it is
recommended that it is only used
for specific infections. It is often
administered as its palmitate in
order to mask its bitter taste.
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• The free drug is liberated from this
ester by hydrolysis in the duodenum
chloramphenicol has a poor water
solubility (2.5 g/dm-3 ) and So it is
sometimes administered in the form of
its sodium hemisuccinate salt (see
section 3.7.4.2), which acts as a
prodrug.
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• Chloramphenicol acts by inhibiting
the elongation stage in protein
synthesis in prokaryotic cells.
• It binds reversibly to the 50S
ribosome subunit and is thought to
prevent the binding of the
aminoacyl-tRNA complex to the
ribosome.
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• However, its precise mode of action is
not understood.
• Investigation of the activity of
analogues of chloramphenicol showed
that activity requires a para-electronwithdrawing group.
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• However, substituting the nitro group
with other electron-withdrawing groups
gave compounds with a reduced activity.
Furthermore, modification of the side
chain, with the exception of the
difluoro derivative, gave compounds
that had a lower activity than
chloramphenicol (Table 10.3).
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• These observations suggest that D(-)-
threo chloramphenicol has the optimum
structure of those tested for activity.
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• Investigation of the activity of
analogues of chloramphenicol showed
that activity requires a para-electronwithdrawing group. However,
substituting the nitro group with other
electron-withdrawing groups gave
compounds with a reduced activity.
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• Furthermore, modification of the side
chain, with the exception of the
difluoro derivative, gave compounds
that had a lower activity than
chloramphenicol (Table 10.3).
• These observations suggest that D(-)threo chloramphenicol has the optimum
structure of those tested for activity.
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Table 10.3. The activity against Escherichia coli of some
analogues chloramphenicol relative to chloramphenicol.
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12.3 TetracycLines
• Tetracyclines are a family of natural
and semisynthetic antibiotics isolated
from various Streptomyces species, the
first member of the group
chlortetracycline being obtained in 1945
by Duggar from Streptomyces
aureofaciens. A number of highly active
semisynthetic analogues have also been
prepared from the naturally occurring
compounds (Table 10.4).
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Table 10.4.The structures of the tetracyclines.
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• The tetracvclines are a broad-spectrum
group of antibiotics active against many
Gram-positive and Gram-negative
bacteria, rickettsiae, mycoplasmas,
chlamydiae and some protozoa that
cause malaria. A number of the natural
and semisynthetic compounds are in
current medical use.
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• The structures of the tetracyclines
are based on four-ring system.
Their structures are complicated
by the presence of up to six chiral
carbons in the fused-ring system.
These normally occur at positions 4,
4a, 5, 5a, 6 and 12a, depending on
the symmetry of the structure.
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• The configurations of these centres in the
active compounds have been determined by Xray crystallography (Table 10.4). This
technique has also confirmed that C1 to C3
and C11 to C12 were conjugated structures.
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Table 10.4.The structures of the tetracyclines.
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amphoteric,
• Tetracyclines are amphoteric,
forming salts with acids and bases.
They normally exhibit three pKa.
ranges of 2.5 —3.4 (pKa1 ), 7.2-7.5
(pKa2.) and 9.1—9.7 (pKa3.), the
last being the range for the
corresponding ammonium salts.
These values have been assigned by
Leeson and co-workers to the
structures shown in Table 10.4.
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forming stable chelates
• These assignments have been supported
by the work of Rigler and collegues.
However, the assignments for pKa2, and
pKa3, are opposite to those suggested
by Stephens and collegues.
• Tetracyclines also have a strong affinity
for metal ions, forming stable chelates
with calcium, magnesium and iron ions.
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affinity for metals
• These chelates are usually soluble in
water, which accounts for the poor
absorption of tetracyclines in the
presence of drugs and foods that
contain these metal ions.
• However, this affinity for metals
appears to play an essential role in the
action of tetracyclines.
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by preventing protein elongation
• Tetracyclines are transported into the
bacterial cell by passive diffusion and
active transport. Active transport
requires the presence of Mg2 ions and
ATP possibly as an energy source.
• Once in the bacteria, tetracyclines act
by preventing protein elongation by
inhibiting the binding of the aminoacyltRNA to the 30S subunit of the
prokaryotic ribosome.
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• This binding has also been shown to
require magnesium ions.
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• Tetracyclines also penetrate
mammalian cells and bind to
eukaryotic ribosomes.
• However, their affinity for
eukaryotic ribosomes is lower than
that for prokaryotic ribosomes and
so they do not achieve a high
enough concentration to disrupt
eukaryotic protein synthesis.
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bacterial resistance
• Unfortunately, bacterial resistance to
tetracyclines is common. It is believed
to involve three distinct mechanisms,
namely: active transport of the drug out
of the bacteria by membrane spanning
proteins; enzymic oxidation of the drug;
and ribosome protection by
chromosomal protein determinants.
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• The structure-activity relationships
of tetracyclines have been
extensively investigated and
reported.
• Consequently, the following
paragraphs give only a synopsis of
these relationships.
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• This synopsis only considers general
changes to both the general
structure of the tetracyclines
(Figure 10.22) and the substitution
patterns of their individual rings.
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• Activity in the tetracyclines
requires four rings with a cis A/B
ring fusion.
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• Changes to the 4-dimethylamino group
also usually reduce activity. This group
must have an a-configuration and partial
conversion of this group to its β-epimer
under acidic conditions at room
temperature significantly reduces
activity.
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Figure 10.22. General structure activity relationships
in the tetracyclines.
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• In addition, either removal of the adimethylamino group at position 4 or
replacement of one or more of its
methyl groups by larger alkyl groups
also reduces activity.
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• Ester formation at C-12a gives inactive
esters, with the exception of the
formyl ester, which hydrolyses in
aqueous solution to the parent
tetracycline. Alkylation of C-11a also
gives rise to a loss of activity.
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• Modification of the substituents at
positions 5, 5a. 6. 7. 8 and 9 may lead to
similar or increased activity.
• Minor changes to the substituents at
these positions tend to change the
pharmacokinetic properties rather than
activity (Table 10.5).
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Table 10.5. The pharmacokinetic properties of tetracycline hyrochlorides.
The values given are representative values only because variations
between individuals can be quite large.
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• A number of active derivatives have
been synthesized by electrophilic
substitution of C-7 and C-9 but the
effect of introducing substituents at C8 has not been studied because this
position is difficult to substitute.
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