Infections and Microorganisms

Cards (37)

  • HIV: human immunodeficiency virus.
  • AIDS: acquired immune deficiency syndrome.
  • There is no human reverse transcriptase, so is an ideal target for treating HIV. There are two major classes:
    • Nucleoside reverse transcriptase inhibitors (NRTIs): bind the active site and inhibit the enzyme and cause chain termination
    • Non-nucleoside reverse transcriptase inhibitors (NNRTIs): bind to an allosteric site and are non-competitive
  • The allosteric site that NNRTIs bind to is close to the active site, inducing a conformational change that reduces enzyme activity. NNRTIs are vulnerable to resistance when used alone.
  • Human protease active sites are not symmetrical whereas HIV protease is a symmetrical homodimer with two identical protein subunits (is an aspartyl protease).
  • HIV protease inhibitors must be transition state inhibitors that mimic the tetrahedral transition state of the mechanism. This must be stable to hydrolysis e.g., hydroxyethylamine isosteres.
  • HIV protease inhibitor design:
    • Mimic tetrahedral transition state
    • C2 symmetry
    • Benzyl group retained for strong binding to S1
    • Symmetrical reaction intermediate
    • Remove alcohols to stabilise the molecule
    • Addition of valines increases activity
    • Axis of symmetry through a bond increasing separation of NH groups
  • Ritonavir is an improved HIV drug with features such as:
    • Pyridine bioisosteres to improve polarity and water solubility - thiazolyl ring to avoid metabolism
    • Use R-OH instead of R-SH to increase hydrogen bonding
  • Avoiding HIV resistance:
    • Administration of a single agent only works for a short time before resistance occurs
    • High active anti-retroviral therapy (HAART) is an approach consisting of at least three drugs against two different targets
    • Generally two NRTIs plus a protease inhibitor
  • HIV is hard to eradicate because:
    • HIV infects human host cells - integration of viral DNA into the human DNA means that the virus can reactivate at any time
    • Reverse transcriptase is an error prone polymerase, allowing HIV to mutate quickly
  • Antibiotic: an agent that either kills or inhibits bacterial growth.
  • Bactericidal: kills bacteria by inhibition of cell wall biosynthesis.
  • Bacteriostatic: prevents bacteria from growing and replicating.
  • Bactericidal agents:
    • Penicillins
    • Cephalosporins
    • Vancomycin
    • Rifampicin
  • Bacteriostatic agents:
    • Chloramphenicol
    • Sulphonamides
    • Tetracyclines
  • 5 mechanisms of antibiotics:
    • Inhibition of cell metabolism (sulphonamides)
    • Inhibition of cell wall synthesis (β-lactams)
    • Interactions with plasma membrane (polymyxins)
    • Disruption of protein synthesis (chloramphenicol)
    • Inhibition of nucleic acid transcription and replication (rifamycins)
  • Bacterial wall biosynthesis:
    • Made from peptidoglycan (branched polymer)
    • Amide linkages catalysed by peptidoglycan transpeptidase
    • Structure varies according to bacterial strain
  • D-ala-D-ala has a similar structure to beta-lactam rings.
  • Ring strain of beta-lactams makes the carbonyl group more reactive, especially when fuse to a five-membered ring (introduces torsional effects).
  • Beta-lactamases are mutated from transpeptidases. They contain a serine residue and are able to hydrolyse the ester link.
  • Beta-lactams are not exceptionally reactive acylating agents, but they do not randomly acylate biomolecules in vivo.
  • Beta-lactamase inhibitors are known as suicide inhibitors.
  • Disadvantages of beta-lactams:
    • Activity over a wide spectrum of bacteria
    • Sensitive to beta-lactamases
    • Some are sensitive and destroyed in the stomach
  • Penicillin G is sensitive to acid for three main reasons:
    1. Strain in the four-membered ring (increased by the presence of the fused five-membered ring)
    2. Reactive carbonyl group (little resonance with lactam nitrogen)
    3. Neighbouring group participation from the acyl side chain
  • Cephalosporins: inhibition of cell wall by inhibition of peptidoglycan transpeptidase, but are resistant to acid hydrolysis and lactamases.
  • Cephalosporin modifications:
    1. 7-acylamino side chain
    2. 3-acetoxymethylene side chain
    3. Substitution at C7
  • 1st gen cephalosporins
    • Cephalothin: poorly absorbed from gut, administered by IV
    • Cephaloridine: pyridine ring stable to metabolism with a good leaving group, zwitterion so soluble in water but poorly absorbed through the gut wall (IV)
  • Generation of 7-aminocephalosporic acid (7-ACA):
    • Need to hydrolyse the unreactive secondary amide in the presence of a labile β-lactam ring
  • Subsequent generations of cephalosporins:
    • Oximino-cephalosporins, significantly increases stability to some β-lactamases
    • Cefuroxime: much greater stability, resistant to esterases due to urethane group
    • Aminothiazole ring enhances penetration across the outer membrane of bacteria
    • May increase affinity for transpeptidase enzyme
    • Reserved for resistant infections
  • Nucleoside: base with sugar attached.
    Nucleotide: base, sugar and phosphate.
  • Chain terminators: modified nucleoside triphosphates that block the extension of the new DNA strand.
  • Azidothymidine (AZT) is used in the treatment of HIV.
    • Phosphorylated to a triphosphate in the body
    • Two mechanisms of action; inhibits a viral enzyme (reverse transcriptase) and is added to growing DNA to act as a chain terminator
  • Acyclovir: chain terminator made of an incomplete sugar ring (deoxyguanosine molecule) used to treat herpes and shingles.
  • Nucleotides cannot be administered directly due to the charged phosphate group reducing cellular uptake.
    • Cellular enzymes dephosphorylate the 5' phosphate.
    • A blocked nucleotide or 'protide' can bypass the dependence on nucleotide kinases
    • This allows passive entry
  • Protides require blocking groups on the phosphate group.
    • These groups must be lipophilic, stable in plasma, hydrolysable and have non-toxic by-products
    • Hydrolysis can be done either by phosphoramidase or esterases, based on phosphoramidate diesters that include an aromatic moiety and an amino acid
    • R groups can be modified to provide thousands of combinations, allowing optimisation for each nucleoside drug
  • R groups on protides:
    • < or = 6 carbons: little or no antiviral activity
    • C7 and C9 = moderate activity
    • C8 and C10 = optimal for potent antiviral activity
  • Prodrug conclusions:
    • They often bypass the rate limiting first phosphorylation step required for nucleoside drug activation
    • They can enter cells readily (in some cases better than parent nucleosides)
    • Effective anticancer and antiviral drugs