Structural optimization of phthalazine derivatives for anti-HBV activities to improve oral bioavailability

The hepatitis B virus (HBV) capsid protein forms a protective nucleocapsid essential for viral replication, establishing capsid assembly modulation as a promising therapeutic strategy. Based on previous mechanistic studies, we identified phthalazine derivatives as potent HBV capsid assembly modulators (CAMs), with Yhhu6517 exhibiting submicromolar antiviral activity in vitro. However, its clinical translation was hindered by poor oral pharmacokinetics (PK), due to rapid first-pass metabolism of oxidation-prone primary alcohol groups. Through metabolic stability-guided structure-activity relationship (SAR) studies involving systematic replacement of primary alcohols with non-primary alcohol-derived hydrophilic groups, we optimized the fragments to yield compound 2p. This optimized candidate 2p maintained a potent anti-HBV activity (IC₅₀ = 0.016 μM in HepG2.2.15 cells) while demonstrating improved an oral bioavailability (F = 80.6 % in mice) and enhanced plasma exposure (AUC_0-24h = 10.3 μg·h/mL). These findings confirm phthalazine-based anti-HBV agents, with compound 2p emerging as a candidate for the further development.