Background:  We have previously demonstrated that styrylquinolines derivatives (SQL) are true integrase inhibitors that act at a preintegration step of the viral replication cycle, most likely by impairing preintegration complex formation. In addition we demonstrated that integrase mutations leading to virus resistance against SQL do not alter diketo acids (DKA) inhibitory effect. Furthermore, SQL were fully active against reverse transcriptase inhibitors (RTI) and DKA resistant viruses. The absence of cross-resistance between RTI or DKA and SQL led us to investigate the potential synergism between them.

Methods:  Synergistic interactions between SQL (FZ41) and either RTI (zidovudine and nevirapine) or DKA (L731-988) were tested on wild-type virus (NL43) on HeLa P4 cells. Inhibition by combination of drugs was assessed at 3 fixed molar ratios (1:1, 2:1, 1:2). Interactions were calculated using the multiple drug-effect equation of Chou and Talalay (CalcSyn ®), yielding a combination index (CI) at inhibition rates of 50%, 75%, and 90%.

Results:  Synergy was considered significant when the 95% confidence interval of the CI from several independent experiments was < 1. Combination of FZ41 with either zidovudine or nevirapine demonstrated synergy at respectively ED₉₀ and both ED₅₀ and ED₉₀ at 1:1 ratio. More interestingly, combination of the 2 integrase inhibitors also led to synergic effect. The combination indexes were between 0.57 and 0.71 for the ED₉₀ at 1:1 ratio and between 0.47 and 0.62 for the ED₉₀ at 2:1 ratio. Together with both the absence of cross-resistance and the different mechanisms of inhibition in vitro, these results reinforce the fact that SQL and DKA are antiviral compounds that target the viral integrase at different sites.

Conclusions:  SQL showed synergism with both strand transfert inhibitors and RTI inhibitors, first evidencing an additional advantage in developing this new class of integrase inhibitors and second suggesting that SQL could be used in combination with other antiretroviral drugs within multi therapy regimen.

Keywords: styrylquinolines; integrase inhibitors; synergy