Interactions between different generation HIV-1 fusion inhibitors and the putative mechanism underlying the synergistic anti-HIV-1 effect resulting from their combination.

We previously reported that the combinatorial use of T20 and T1144, the first and next generations of HIV fusion inhibitors, containing different functional domains resulted in synergistic anti-HIV-1 effect, but this effect diminished when T20 and T1144 were covalently linked together. To elucidate the mechanism underlying this synergistic anti-HIV-1 effect, we studied the interactions between T20 and T1144 either in a mixture state or in a covalently linked state. T20 alone in solution was largely featureless, while T1144 alone was in α-helical trimeric conformation. When mixed in solution, T20 and T1144 showed a loose and transient interaction, with a moderate 10% α-helical content increase, but this interaction was greatly enhanced in the linked state, and T20 and T1144 showed ∼100% α-helical content. These results suggested that the loose and transient interaction between T20 and T1144 may destabilize the T1144 trimer, which makes its otherwise shielded binding sites more accessible to N-terminal heptad repeat (NHR) and increases its associating rate, thus increasing its anti-HIV-1 potency against the temporarily exposed target in NHR and causing the synergistic anti-HIV-1 effect. However, the strong interaction between T20 and T1144 in the covalently linked state may shield their NHR-binding sites, resulting in reduction of the synergistic effect.