Inibithion of the HIV-1 Ribonuclease H activity by alizarine derivatives.

The HIV-1 reverse transcriptase (RT) ribonuclease H (RNase H) associated activity is responsible for the hydrolysis of the RNA component of the heteroduplex RNA:DNA replication intermediate, it is essential for viral replication and it is a validated target for drug development. Nevertheless, until now only few compounds have been able to inhibit selectively the HIV-1 RNase H function. Anthraquinones are common secondary metabolites occurring in bacteria, fungi, lichens and higher plants which have been reported to have diverse biological activities. In particular, some of them have been reported to inhibit the HIV-1 RT associated polymerase activity and the integrase activity in biochemical assays. Given the structural similarities between integrase and RNase H proteins and since diketo acid derivatives that inhibit the first has been found to inhibit also the latter, we synthesized and tested a series of alizarine derivatives. Results showed that some of them were able to inhibit in biochemical assays the HIV-1 RNase H function. The most potent derivative, 1,2-O-Bis-benzoyl-9,10-anthraquinone (K49), showed an IC50 value of 10 μM. Mechanism of action studies showed that K49 does not inhibit the HIV-1 RT associated polymerase activity at 100 µM concentrations, it does not intercalate into DNA, it is not cytotoxic and, differently from the diketo acids, it does not chelate the divalent cofactor Mg2+. Kinetic studies demonstrated that K49 is a non-competitive inhibitor and that it does not bind to the classical non-nucleoside RT inhibitors (NNRTI) binding site. In fact, the Yonetani-Theorell graphical model revealed that K49 binds to a site which is not overlapping to the nevirapine binding site. Overall, these results demonstrated that anthraquinone derivatives may selectively inhibit the HIV-1 RNase H function with a mechanism of action different from the one shown by the diketo acid derivatives.