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 peptidoglycantranspeptidase
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:
Strain in the four-membered ring (increased by the presence of the fused five-membered ring)
Reactive carbonyl group (little resonance with lactam nitrogen)
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:
7-acylamino side chain
3-acetoxymethylene side chain
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 (reversetranscriptase) 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)