Antiviral Chemotherapy 2 (DONE) Flashcards
HIV replication
Fusion of HIV to the host cell surface
HIV RNA, reverse transcriptase, integrase and other viral proteins enter the host cell
Viral DNA is formed by reverse transcription
Viral DNA is transported across the nucleus and integrates into the host DNA
New viral RNA is used as genomic RNA and to make viral proteins
New viral RNA and proteins move to the cell surface and a new, immature HIV forms
Virus is released, viral protease cleaves new polyproteins to create mature, infectious virus
NNRTIs
NNRTIs target allosteric site to NTPs, causing conformational change, while NTPS target catalytic site
The structures of the NNRTIs are very diverse, but are all in the shape of U or a butterfly
Tend to be lipophilic to bind allosteric pocket of enzyme
TSAOs
Discovered by accident: the spiro ring at C3’ was not the intended product, and the silyl groups are just temporary protecting groups- just there for the chemistry
But: if the silyl groups are removed, all NNRTI activity is lost. Like most NNRTIs, the TSAOs are very lipophilic
HEPTs
Again, these were designed as nucleoside analogues, intended to act (after 5’ phosphorylation) as NRTIs
But: they act as NNRTIs, not as nucleosides, despite their structural resemblance to nucleosides
How could we establish that such a compound (HEPT) were acting as an NNRTI, not a NRTI?
Would observe direct inhibition if incubated with HIV RT (nucleosides are inactive, until 5’-phosphorylated)
Activity is retained on (chemical) removal of 5’-OH
Would be HIV-1 specific (NNRTIs don’t work on HIV-2)
Limitations with NRTIs
Emergence of resistant virus
Cross-resistance possible
No activity in resting cells
Clinical toxicities common/can be severe
Limitations with NNRTIs
Rapid emergence of resistant virus Cross-resistance to several NNRTIs common Clinical efficacy alone is limited No activity in resting cells Entirely HIV-1 specific
Viral protease inhibitors
Most viruses translate their genome into a large polyprotein precursor that needs to be cleaved into individual (functional) viral proteins
This crucial cleavage step is often carried out by viral proteases
HIV protease role
The viral genome is incorporated into the human genome (by the integrase)
The pro-viral DNA is transcripted into mRNA
HIV genes are expressed to give large polyproteins
HIV protease cleaves these polypeptides into individual functional proteins
HIV protease
An essential enzyme for the HIV life cycle
Cleaves specific amide bond of polypeptides at at least nine different cleavage sites
Belongs to the aspartic acid protease family
Structurally related to human proteases, such as renin and pepsin
HIV protease structure
The HIV protease has been cloned and over-expressed in E. coli
It has been crystallised and its structure has been solved
The crystal structure and genetic data have been used to design inhibitors
HIV protease inhibitors: drug design
Initial design of inhibitors focused on isosteres of the Phe-Pro or Tyr-Pro amide bond, in particular of the high energy tetrahedral intermediate formed during the cleavage of the amide bond
It has been postulated that enzymes in general have higher affinity for these intermediates than they have for either the substrates or the products
Inhibitors based on these transition states (TS inhibitors) should therefore be particularly potent
HIV protease inhibitors conclusions
The hydroxyethylene isostere is a good mimic
These compounds act as competitive inhibitors of the enzyme
The stereochemistry at the hydroxyl is crucial for the activity (S active, R inactive)
However, such peptide structures have very poor PK
Issues with HIV protease inhibitors
Viral resistance
Common side effects- increase in blood sugar, change in fat distribution, increase of fat in the blood, liver toxicity
Poor bioavailability
Short half life- dosing two to three times a day
Complex chemical synthesis
HCV proteases
Hepatitis C RNA encodes for a large polyprotein precursor 3000 amino acids long
This polyprotein precursor is cleaved by viral and host proteases to form 10 structural and non-structural proteins
The non-structural proteins are further processed by two viral proteases into their mature forms
HCV NS3 protease inhibitors
Two types:
Non-covalent product based inhibitors- bind to the active site of the NS3 serine protease, blocking its action and shutting down viral replication e.g. carboxylic acid containing inhibitors, acid sulphonamide containing inhibitors
Covalent reversible inhibitors- form a reversible covalent bond with serine in the active site. Acting as a serine trap e.g. boceprevir
Issues with HCV protease inhibitors
Emergence of viral resistance- not suitable as monotherapy
Side effects- rash, anaemia
Short half life, requiring frequent dosing (2-3 times per day)
Complex chemical synthesis
HIV integrase
Critical enzyme that incorporates the HIV genome into the human genome. This step is essential for the viral replication. Two classes of integrase inhibitors have been developed against HIV:
Integrase strand transfer inhibitors (INSTIs), in clinical use, interfere with strand transfer reaction
Integrase binding inhibitors (INBIs), under clinical evaluation, inhibit binding of the integrase to the viral DNA
CD4 analogues
HIV entry requires an interaction between gp120 on the virus and CD4 on the target cell
Shortened soluble versions of the CD4 polypeptide (rsCD4) appear to have some protective anti-HIV effect in-vitro, presumably by binding to and deactivating free virus
One problem with these therapies is the very short plasma half life of the rsCD4 (<5 min). Other non-peptide analogues are under study
Fuzeon (Enfuvirtide)
A 36- amino acid peptide that links to HIV gp41 and prevents entry, now approved for use
Has to be dose IV due to its peptide structure
Twice daily dosing
CCR5
HIV entry requires docking onto CD4 using either CCR5 or CXCR4
Approved in 2007, maraviroc is an entry inhibitor that works by blocking the CCR5 attachment
HIV penetration/absorption inhibitors
HIV absorption/penetration may be blocked by a wide variety of poly-anionic compounds. The original materials show little antiviral selectivity.
Later compounds included heparin sulphates. Their activity varies with molecular weight and charge. Some appear to have good antiviral selectivity, but they suffer from very poor bio-availability
HIV glucosidase inhibitors
HIV maturation requires post-translational glucosidation of viral polypeptides
Specific inhibitors of HIV glucosidation have been sought and to some extent found. Alpha-glucosidase 1 appears to be the best target for achieving highest anti-HIV specificity. Most inhibitors of this system are mimics of natural sugars (notably glucose).
