Description_x000D_ Biochem/physiol Actions_x000D_ BAY41-4109 is a heteroaryldihydropyrimidine (HAP)-based capsid assembly modulator (CAM) that destabilizes hepatitis B virus (HBV) capsid assembly. BAY41-4109 inhibits HBV replication in HepG2.2.15 cultures with little host cytotoxicity (IC50 = 53 nM vs. CC50 = 7 μM in 8 days). Despite its relatively short half-life in vivo (plasma t1/2 = 5 h/dog, 2 h/rat, ≤1 h/mouse), BAY41-4109 shows greater antiviral efficacy than 3TC in Tg [HBV1.3 fsX-3'5'] mice (HBV DNA reduction = 52%/BAY vs. 36%/3TC in liver via 30 mg/kg b.i.d. p.o.; 70% vs. 30% in plasma with 15 mg BAY/kg or 30 mg/kg 3TC b.i.d. p.o., respectively) with good oral availability (F/tmax =31%/mouse, 58%/rat & dog).