Over the past 10 years,
protease inhibitors have been a key component in antiretroviral
therapies for HIV/
AIDS. While the vast majority of HIV/
AIDS cases in the world are due to HIV-1, HIV-2
infection must also be addressed. HIV-2 is endemic to Western Africa, and has also appeared in European countries such as Portugal, Spain, and Estonia. Current
protease inhibitors have not been optimized for treatment of HIV-2
infection; therefore, it is important to assess the effectiveness of currently FDA-approved
protease inhibitors against the
HIV-2 protease, which shares only 50% sequence identity with the
HIV-1 protease. Kinetic inhibition assays were performed to measure the inhibition constants (K(i)) of the
HIV-1 protease inhibitors
indinavir,
nelfinavir,
saquinavir,
ritonavir,
amprenavir,
lopinavir,
atazanavir,
tipranavir, and
darunavir against the
HIV-2 protease.
Lopinavir,
saquinavir,
tipranavir, and
darunavir exhibit the highest potency with K(i) values of 0.7, 0.6, 0.45, and 0.17 nm, respectively. These K(i) values are 84, 2, 24, and 17 times weaker than the corresponding values against the
HIV-1 protease. In general, inhibitors show K(i) ratios ranging between 2 and 80 for the HIV-2 and HIV-1
proteases. The relative drop in potency is proportional to the affinity of the inhibitor against the
HIV-1 protease and is related to specific structural characteristics of the inhibitors. In particular, the potency drop is high when the maximum cap size of the inhibitors consists of very few atoms. Caps are groups located at the periphery of the molecule that are added to core structures to increase the specificity of the inhibitor to its target. The caps positioned on the
HIV-1 protease inhibitors affect selectivity through interactions with distinct regions of the binding pocket. The flexibility and adaptability imparted by the higher number of rotatable bonds in large caps enables an inhibitor to accommodate changes in binding pocket geometry between HIV-1 and
HIV-2 protease.