Lecture 7 Antibacterial drugs targeting protein synthesis Flashcards
Name an example of a pre-ribosomal stage inhibitor
Mupirocin
Name examples of a early stage protein synthesis inhibitor
linezolid
tertracyclines
Name examples of mid stage protein synthesis inhibitors
aminoglycosides chloramphenicol lincosamides macrolids streptogramins
Name an example of a late stage protein synthesis inhibitor
fusidic acid
What are the 3 stages of translation?
1) Initiation
2) Elongation cycle
3) Termination
Describe the elongation stage of translation.
methionine-carrying tRNA starts out in the middle slot of the ribosome, called the P site.
Next to it, a fresh codon is exposed in another slot, called the A site. The A site will be the “landing site” for the next tRNA, one whose anticodon is a perfect (complementary) match for the exposed codon.
Once the matching tRNA has landed in the A site, it’s time for the action: that is, the formation of the peptide bond that connects one amino acid to another. This step transfers the methionine from the first tRNA onto the amino acid of the second tRNA in the A site.
We now have two amino acids, a (very tiny) polypeptide! The methionine forms the N-terminus of the polypeptide, and the other amino acid is the C-terminus.
Once the peptide bond is formed, the mRNA is pulled onward through the ribosome by exactly one codon. This shift allows the first, empty tRNA to drift out via the E (“exit”) site. It also exposes a new codon in the A site, so the whole cycle can repeat.
[see slide for images]
Why is targeting ribosomes selectively toxic to bacteria?
Differences in human and bacterial ribosomes
Bacterial ribosomes are smaller and made from different RNA
What makes mupirocin selectively toxic?
Structural differences between eukaryotic and prokaryotic isoleucyl-tRNA synthatases
What makes fusidic acid and tetracycline selectively toxic?
difference in uptake of antibiotic into eukaryotic and prokaryotic cells - preferential to bacteria
What is meant by ‘mechanism-based toxicity’ and how does this occur with ribosome inhibitors?
Side effect due to the mechanism of action of the drug
Due to inhibition of mitochondrial protein synthesis which shares similarities with bacterial protein synthesis
Describe the spectrum of mupirocin
Narrow spectrum
Gram +ve
Describe the mechanism of action of mupirocin
Bacteriostatic
Acts as a substate analogue to inhibit isoleucyl tRNA synthetase
Does not interfere with the ribosome directly
Isoleucine cannot be incorporated into the chain = inhibition of protein synthesis
Works as a competitive inhibitor that is 8000x more potent
In what form is mupirocin given?
Topically
Due to rapid de-esterification in kidney and liver = producing metabolites lacking antibiotic activity (hydrolysis of ester bond between monic acid and 9-hydroxynonanoic acid)
Due to topical treatment it is well tolerated
What bacteria are sensitive to mupirocin?
staphylococcal and streptococcal skin infections
nasal carriage of MRSA
Which antibiotics target specifically the 50s subunit?
Macrolids Lincosamides Streptogramins Oxaxolidinones Aminoglycosides (and also 30s) Chloramphenicol
Which antibiotics target specifically the 30s subunit?
Aminoglycosides (and also 50s)
Tetracyclines
Name an example of an antibiotic in the oxazolidinone class and its spectrum of action
Linezolid and Tedizolid (2014)
gram +ve due to being removed by efflux in gram -ve’s
Describe the mechanism of action of the oxazolidinone class
Only fully synthesis drug to target RNA
Binds to the 50s subunit at the site where it normally interacts with the 30s subunit = full subunit not formed
Other theory:
Probably interferes with binding or correct positioning of aminoacyl-tRNA in the ribosomal A site
What is linezolid used for?
oral/iv treatment of pneumonia or skin/soft tissue infections
What are the side effects of linezolid?
Skin reactions e.g. rashes, dermatitis
GI e.g. vomiting
Abnormalities in liver function
Reversible thrombocytopenia and anaemia (mechanism based toxicity)
Why is Tedizolid and improvement to linezolid?
Reduced risk of myelosupression
Increased bacterial potency
Describe the spectrum of tetracyclines
Broad
Lipophilic members e.g. doxycycline more active than hydrophobic members e.g. tetracycline
Describe the mechanism of action of tetracyclines
Bind to 30s subunit
Prevents the association of aminoacyl-tRNA with the ribosomes (same as oxazolidinone but binds to a different subunit)
What are the clinical uses for tetracyclines?
Mostly given orally
Skin, soft tissue and intra-abdominal infections
What are the adverse effect of tetracyclines?
GI disturbances - for oral products
Photosensitivity
CNS effects - mainly for minocycline
Deposition in developing teeth and bones - due to collation of metal ions - therefore contraindicated for young children/late in pregnancy
What is the mechanism of action of aminoglycosides?
Most bactericidal
Either binds to 30s (streptomycin, spectinomycin) or both 30s and 50s subunits (kanamycin, tobramycin)
Effects the proof reading by causing a mis-read of mRNA = production of abnormally folded proteins
Insertion of abnormal proteins into the cytoplasmic membrane = membrane destabilisation and bacterial cell death
What are the clinical uses of aminoglycosides?
Mostly given IV/IM
Serious gram -ve infections mostly
Given alone for:
Severe sepsis caused my enterobactericeae and Ps.aeruginosa
Plague and tulaemia
Enterococcal endocarditis
Used in combo for:
Gram +ve and anaerobic infections
TB
What are the adverse side effects of aminoglycosides?
Toxicity - dependent on the specific aminoglycoside
Ototoxicity - effects on vestibular and cochlear branches of the 8th nerve and destruction of inner hair cells, effects up to 10% of patients
Nephrotoxicity
Neuromuscular blockade
What is the mechanism of action of chloramphenicol?
Broad spectrum
Binds to 50s subunit
Inhibits peptidyl transferase activity = stops peptide bond formation
What are the clinical uses of chloramphenicol?
Clinical use restricted due to toxicity
Still has a role in the treatment of typhoid and paratyphoid fever caused by salmonella
What are the adverse effects from chloramphenicol?
Bone marrow depression - can lead to aplastic anaemia , probably from inhibition of protein synthesis in human marrow cell mitochondria
Grey syndrome (infants and babies) - due to build up of the drug, life threatening and presents as cyanosis ('grey skin'), abdominal distention, vomiting and circulatory collapseName one of the worlds best selling macrolid
Azithromycin
Describe the clinical use of azithromycin
Chlamydia
Respiratory infections - e.g. haemophilus influenzae, moraxella catarrhalis
What is the mode of action of macrolids?
Binds to the 23s rRNA at the peptide exit site in the 50s subunit exit tunnel of the nascent chain = chain cannot leave the ribosome and protein cannot be made (premature dissociation of the peptidyl-tRNA from the P-site)
What adverse effects are associated with macrolids?
Generally well tolerated
Can cause GI issues, jaundice and ototoxicity (rare)
What is the mode of action of streptogramins?
2 structurally unreleated molecules acting synergistically
Quinupristin (30%) and dalfopristin (70%) = Synercid
Both bid to the 50s subunit but at different sites = form a stable complex
Quinupristin inactivates peptidyltransferase
Dalfopristin prevents positioning of peptidyl-tRNA at P site, inhibiting peptide bond formation
What is the clinical use of streptogramins?
IV drug for vancomycin resistant E. faecium bacteremias
Skin and soft tissue infections caused by S. aureus
What are the adverse effects associated with streptogramins?
Generally well tolerated
Name examples of lincosamides
Clindamycin
Lincomycin
What is the mechanism of action of lincosamides?
Binds to 50s subunit
Inhibits peptidyl transferase - but not the same binding site as chloramphenicol
What is the spectrum of activity for lincosamides?
Unusual spectrum
active against gram +ve aerobes but not gram -ve aerobes
Very potent against anaerobes
What are the clinical uses for lincosamides?
gram +ve infections, particularly staphylococcal infections
Combined to to treat anaerobic infections
What adverse effects are associated with lincosamides?
Rashes
Diarrhoea
Pseudomembranous colitis - caused by C.Diff
What is the mechanism of action of fusidic acid?
Normally EF-G is an enzyme that catalyses translocation of tRNA and mRNA down the ribosome at the end of each elongation
Uses energy from the hydrolysis of GTP to GDP
Fusidic acid binds to EF-G and locks in the protein at the translocation stage
[see slide for image]
[still unsure how this works properly - see textbook]
What is the spectrum of activity of fusidic acid?
Limited to gram +ve organisms
What are the clinical uses of fusidic acid?
topical, oral and IV therapy for staphylococcal infections
Usually given in combination with other agents to reduce risk of resistance
What are the adverse effects caused by fusidic acid?
Side effects rare
Rashes
Jaundice
