Antibacterials

• Antibiotics


  Drugs that inhibit bacterial growth or kill bacteria
• Mechanisms of antibacterial action

  • Inhibition of bacterial cell wall synthesis
  • Inhibition of protein synthesis
  • Inhibition of nucleic acid transcription and replication
  • Injury to plasma membrane
  • Inhibition of synthesis of essential metabolites
  • Miscellaneous

  
• How antibiotics kill bacteria

  • Damaging their cell membranes
  • Disrupting essential bacterial metabolic pathways (antimetabolites)
  • Preventing them from making proteins
• Transfer ribonucleic acid (tRNA)


  A type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein

  
• Translation


  The process of synthesizing proteins from the genetic code in mRNA

  
• Transcription


  The process of synthesizing mRNA from the genetic code in DNA

  
• Tetracyclines


  • Bacteriostatic broad spectrum normally prescribed after β-lactam
  • First isolated from Strepomyces aureofaciens
  • Bind to 30S preventing aminoacyl-tRNA from binding to the acceptor site on the mRNA-ribosome complex, stopping further addition of amino acids and protein release

  
• 30S ribosomal unit


  • Discriminates against aminoacyl-tRNAs that do not match the codon of mRNA, ensuring accuracy in translation
  • Works with the 50S subunit to move the tRNAs and associated mRNA by precisely one codon, in a process called translocation

  
• Tetracycline binding


  • Binds to sugar phosphates of 16S rRNA
  • Involves Magnesium ion
  • Critical functional groups on 'Southern' and 'Eastern' faces
• Tetracyclines are incompatible with

  • Multivalent ion rich antacids
  • Dairy products rich in Ca
• Tetracyclines can complex Ca2+ from teeth/bones, so should be avoided in children
• Tetracyclines are incompatible in acidic/basic conditions
• Widespread resistance to tetracyclines due to overuse in animal husbandry of young animals
• Tetracycline epimerisation


  • The alpha-stereo orientation of C-4 is essential for bioactivity
  • Presence of tricarbonyl systems of ring A allows for enolisation
  • Reprotonation can take place from the top or the bottom of the molecule
  • Reprotonation from the top regenerates Tetracycline
  • Reprotonation from the bottom produces inactive 4-Epietracycline
  • At equilibrium, the mixture nearly consists of equal quantities of the two diastereiomers

  
• Tetracycline dehydration


  • Most natural tetracyclines have tertiary and benzylic OH at C-6
  • Has ideal geometry for acid catalysed dehydration
  • Resulting product biologically inactive and deeper in colour
  • All can arrive at 4-Epianhydrotetracycline which is nephrotoxic causing less reabsorption of materials from tubular fluid, can be fatal
  • Tetracyclines with no C-6 OH groups can't dehydrate, free of this toxicity

  
• Mechanisms of tetracycline resistance

  • Drug efflux proteins, which actively pump the drug out of the cell
  • Ribosomal protection proteins: (i) reversibly distort the structure of ribosomes to prevent the binding of tetracycline, dislodge tetracycline, (ii) or still allow tRNA to bind to the ribosomes irrespective of tetracycline binding

  
• Tetracycline adverse effects


  • Tooth Staining
  • Phototoxicity – most notibly with C7-Cl (visible absorber)
  • Kidney damage
  • Nausea
  • Vomiting
  • Diarrhea
  • CNS effects (vertigo, dizziness)
  • Distinguish imperfectly between bacterial 70S and mammalian 80S ribosomes
  • Inducers of CYP increase metabolism therefore dose may need adjustment (can cause azotemia)
  • Typically administered orally due to thrombophlebitis

  
• Macrolides


  • Clarithromycin, Erythromycin, Fidaxomicin
  • Bacteriostatic
  • Inhibit ribosomal protein biosynthesis by binding to 23S rRNA polypeptide (in the 50S subunit) exit tunnel adjacent to the peptidyl tRNA centre in the 50S ribosomal subunit
  • Prevent the transfer of the tRNA bound at the A site of the rRNA complex to the P site, inhibiting translocation and addition of incoming tRNA and amino acid to the nascent polypeptide chain

  
• Clinically important macrolides


  • Clarithromycin
  • Erythromycin
  • Fidaxomicin

  
• Macrolide drug interactions


  • Responsible for significant drug interactions
  • Metabolised by CYP 3A4 into nitrosoalkanes
  • CYP 3A4 responsible for 50% biotransformation of all therapeutic agents
  • Then complex with the enzyme, destroying the isoenzyme, subsequently affecting the metabolism of all drugs metabolised by CYP 3A4

  
• Drugs affected by macrolide interactions

  • Benzodiazepines
  • Neuroleptics (Clozapine)
  • Statins
  • Theophylline
  • Carbamazepine
  • Anti-arrhythmics (Quinidine)
• Macrolide resistance


  • Transcription modification of 23S rRNA or mutation caused by erm genes (erm-methyltransferases)
  • Demethylation of A2058 residue in 23s rRNA decreases drug affinity

  
• Chloramphenicol


  • Discovered (1947) from Streptomyces venzuala
  • Unusual chlorinated derivative, diol, chiral centres
  • Bacteriostatic
  • Inhibits protein synthesis through binding large ribosome subunit 50S at the peptidyl transferase centre A site preventing binding of next charged tRNA (peptidyl transferase)
  • Postulated similar structure to an aminoacylated nucleoside
  • Binding through H-bonding and interaction with Mg2+ in catalytic site
  • Quite toxic, thought to be due to nitro group
  • Binds to same region as Marcrolides and Lincosamides

  
• Chloramphenicol drug interactions


  • Inhibits CYP 2C19 and 3A4, elevating plasma levels of various drugs
• Drugs with elevated levels due to chloramphenicol

  • Tricyclic antidepressants
  • Selective serotonin reuptake inhibitors
  • Antiepilectic drugs
  • Proton pump inhibitors
  • Azole antifungals
  • Macrolide antibiotics
  • Calcium channel blocking agents
  • Clopidrogel, Gliclazide, Propanolol, Tetrahydrocannabinol
• Chloramphenicol resistance


  • Efflux mechanisms, mutation in the target site, permeability barriers, and inactivation of an enzyme phosphotransferases by acetylation
• Chloramphenicol acetyltransferases


  Enzymes that inactivate chloramphenicol by acetylation
• Lincosamides


  • Similar mechanism of action as Macrolides, binding to the 50S rRNA of the large bacterial ribosome subunit
  • Prolong the effects of neuromuscular-blocking drugs
  • Should not be given simultaneously with Macrolides or Chloramphenicol, as this causes antagonism and possible cross-resistance

  
• Lincosamide resistance


  • Transcription modification of 23S rRNA or mutation caused by erm genes (erm-methyltransferases), permeability barriers
  • Demethylation of A2058 residue in 23s rRNA decreases drug affinity

  
• Oxazolidinones


  • Linezolid, a new synthetic class of antibiotics
  • Bacteriostatic/bacteriocidal against Gram (+)
  • Inhibit protein synthesis by binding to 50S subparticle preventing formation of the 70S functional initiation complex including initiator tRNA, N-formylmethionine-tRNAfMet (fMet-tRNAfMet) to begin the first cycle of polypeptide elongation
  • Distorts the binding site for initiator tRNA which overlaps both 30S and 50S
  • Inhibit much earlier stage, have less resistance problems
  • Compete with Chloramphenicol in binding studies with the 50S unit but do not have the same mode of action

  
• Oxazolidinone drug interactions


  • Linezolid acts as weak monoamine oxidase inhibitor, can cause Serotonin Syndrome with SSRIs
  • Caution with other sympathomimetic drugs eg pseudoephedrine
  • Rare but serious side effects: myelosuppression, peripheral neuropathy, lactic acidosis
• Oxazolidinone resistance


  • Target modification and due to cfr (chloramphenicol-florfenicol resistant) genes
  • Mutation caused by Guanine to Uracil substitution of 23S rRNA at A2058 position inactivates oxazolidinones

  
• DNA Gyrase (Type II topoisomerases)


  Enzymes that alter the conformation of DNA by catalysing double strand cuts, passing uncut portion through the gap and resealing the molecule

  
• DNA topoisomerase IV


  Enzymes that untie enchained daughter DNA molecules

  
• Quinolones and Fluoroquinolones


  • Levofloxacin, Ofloxacin
  • Using energy generated by ATP hydrolysis, DNA is wound about itself as a supercoil
  • Inhibition of DNA Gyrase or topoisomerase IV makes cells DNA inaccessible, leading to cell death

  
• DNA replication produces two identical replicas of DNA from one original DNA molecule

  
• During DNA replication, the two strands of the original DNA molecule are separated, and each strand serves as a template for the production of its counterpart

  
• DNA reversal


  In the absence of ATP, reversal takes place relaxing the molecule
• DNA


  • It must be partially unwound for cell to access genetic information
• DNA reversibility


  1. Allows for changes to be stored properly

  2. Unwound

  3. Replicated

  4. Repaired

  5. Transcribed