Anti-HIV-1 activity of calanolides used in combination with other mechanistically diverse inhibitors of HIV-1 replication

The natural product (+)-calanolide A, a unique nonnucleoside reverse transc riptase inhibitor (NNRTI) of HIV-1 replication, is currently being evaluate d in clinical trials in the USA. (+)-Calanolide A, the congeners costatolid e and dihydrocostatolide, and (+)-2-oxo(+)-calanolide A, were evaluated in combination with a variety of mechanically diverse inhibitors of HIV replic ation to define the efficacy and cellular toxicity of potential clinical dr ug combinations. These assays should be useful in prioritizing the use of d ifferent combination drug strategies in a clinical setting. The calanolides exhibited synergistic antiviral interactions with other nucleoside and non -nucleoside reverse transcriptase inhibitors and protease inhibitors. Addit ive interactions were also observed when the calanolides were used with rep resentative compounds from each of these classes of inhibitors. No evidence of either combination toxicity or antagonistic antiviral activity was dete cted with any of the tested compounds. The combination antiviral efficacy o f three-drug combinations involving the calanolides, and the efficacy of tw o- and three-drug combinations using a (+)-calanolide A-resistant challenge virus (bearing the T1391 amino acid change in the reverse transcriptase), was also evaluated in vitro. These assays suggest that the best combination of agents based on in vitro anti-HIV assay results would include the calan olides in combination with lamivudine and nelfinavir, since this was the on ly three-drug combination exhibiting a significant level of synergy. Combin ation assays with the (+)-calanolide A-resistant strain yielded identical r esults as seen with the wild-type virus, although the concentration of the calanolides had to be increased.