Discovery and development of HIV-protease inhibitors
In this prospective, the discovery of small-molecule ligands, like protease inhibitors, that can modulate catalytic activities has an enormous therapeutic effect.
[7] In 1985, HIV was identified as the causative agent of acquired immune deficiency syndrome (AIDS) and its complete genome was immediately available.
[13] Following entry into the cell the RNA of the virus is reverse-transcribed to DNA by the first virally encoded enzyme, the reverse transcriptase.
A very critical step is the proteolytic cleavage of the polypeptide precursors into mature enzymes and structural proteins catalyzed by HIV protease.
These drugs prevent proteolytic cleavage of HIV Gag and Pol polyproteins that include essential structural and enzymatic components of the virus.
Theories considering the effect of protease inhibitors on insulin-stimulated glucose uptake have also been linked to the lipodystrophic syndrome.
It is possible that protease inhibitors can cause a decrease in insulin-stimulated tyrosine phosphorylation of IRS-1, representing inhibition of early steps in insulin signaling.
Decreased adiponectin secretion and induced expression of interleukin-6 associated with HIV protease inhibitors may also contribute to inhibition of insulin-stimulated glucose uptake.
An extended beta-sheet region on the monomers, known as the flap, constitutes in part the substrate binding site with the two aspartyl residues lying on the bottom of a hydrophobic cavity.
Ile50 remains at the tip of the turn and when the enzyme is unliganded a water molecule makes hydrogen bonds to the backbone of Ile50 on each monomer.
[17] HIV proteases catalyze the hydrolysis of peptide bonds with high sequence selectivity and catalytic proficiency.
[12] The water molecule seems to play a role in the opening and closing of the flaps as well as increasing the affinity between enzyme and substrate.
Some improvements were made, for example the terminal phenyl residues were removed and pyridyl groups put instead to add water solubility.
[19] Significant gastrointestinal side effects and a large pill burden are ritonavir's main drawbacks and is therefore not used as a single treatment.
[6][18] The design of indinavir was guided by molecular modeling and the X-ray crystal structure of the inhibited enzyme complex.
This structure results in fewer chiral centers, that makes it easier to synthesize and gives it enhanced aqueus solubility.
[6] Lopinavir was marketed in 2000[18] and was originally designed to diminish the interactions of the inhibitor with Val82 of the HIV-1 protease, a residue that is often mutated in the drug resistant strains of the virus.
[18] Unlike other HIV protease inhibitors on the market, tipranavir was developed from a nonpeptidic coumarin template and its antiprotease activity was discovered by high-throughput screening.
[25] Darunavir reached the market in 2006[18] and is a nonpeptidic analogue of amprenavir, with a critical change at the terminal tetrahydrofuran (THF) group.
[16][27] Hydrogen bonds between the water molecule, which is linked to Ile50 and Ile50', and carbonyl groups of the peptidomimetic inhibitors seem to connect them with the flap regions.
[19] On the other hand, on the nonpeptidic inhibitors, there is a proton acceptor which replaces the tetracoordinated water molecule and interacts directly with the two Ile50 residues on the flap of the enzyme.
[28] Specific pockets in the binding site of the HIV protease, often referred to as S1, S1',S2 and S2', recognize hydrophobic amino acids on natural substrates.
[29] Some residues in the enzyme binding site are capable of forming hydrogen bonds with hydrophilic groups on the inhibitor, for example with the THF moieties on amprenavir and darunavir.
[31] The non-active site mutations are considered to affect by other mechanisms, like influencing dimer stability and conformational flexibility.
Researches directed towards development of new therapies to cure AIDS are focused on avoiding cross-resistance to drugs that are already on the market.
[38] In 2006, GlaxoSmithKline discontinued the phase II clinical development of brecanavir, an investigational protease inhibitor for the treatment of HIV, due to insurmountable issues regarding formulation.
Pre-clinical studies have demonstrated that this modification fully retains the antiviral potency but can evidently slow hepatic metabolism and thereby increase the half life and plasma trough levels.
