Less Cytotoxic Protoflavones as Antiviral Agents: Protoapigenone 1′-O-isopropyl Ether Shows Improved Selectivity Against the Epstein-Barr Virus Lytic Cycle

Protoflavones, a rare group of natural flavonoids with a non-aromatic B-ring, are best known of their antitumor properties. The protoflavone B-ring is a versatile moiety that may be explored for other pharmacological purposes, but common cytotoxicity of these compounds is a limitation to such efforts. Protoapigenone was previously found to be active against the lytic cycle of Epstein-Barr virus (EBV). Further, the 5-hydroxyflavone moiety is a known pharmacophore against HIV-integrase. The aim of this work was to prepare a series of less cytotoxic protoflavone analogs, and to study their antiviral activity against HIV and EBV. Twenty-seven compounds including 18 new derivatives were prepared from apigenin through oxidative de-aromatization and subsequent continuous-flow hydrogenation, deuteration, and/or 4′-oxime formation. One compound was active against HIV at the micromolar range, and 3 compounds showed significant activity against the EBV lytic cycle at the medium-low nanomolar range. Among these, protoapigenone 1′-O-isopropyl ether (6) was identified as a promising lead due to its 73-times selectivity of its antiviral over its cytotoxic effect, which exceeds that of protoapigenone by 2.4-times. Our results open new opportunities to design new, potent and safe anti-EBV agents based on the natural protoflavone moiety.