Human ACAT inhibitory effects of shikonin derivatives from Lithospermum erythrorhizon

Three naphthoquinones were isolated by bioassay-guided fractionation from the CHCl₃ extracts of roots of Lithospermum erythrorhizon. They were identified as acetylshikonin (1), isobutyrylshikonin (2), and β-hydroxyisovalerylshikonin (3) on the basis of their spectroscopic analyses. The compounds 1-3 were tested for their inhibitory activities against human ACAT-1 (hACAT-1) or human ACAT-2 (hACAT-2). Compound 2 preferentially inhibited hACAT-2 (IC₅₀ = 57.5 μM) than hACAT-1 (32% at 120 μM), whereas compounds 1 and 3 showed weak inhibitory activities in both hACAT-1 and -2. To develop more potent hACAT inhibitor, shikonin derivatives (5-11) were synthesized by semi-synthesis of shikonin (4), which was prepared by hydrolysis of 1-3. Among them, compounds 5 and 7 exhibited the strong inhibitory activities against hACAT-1 and -2. Furthermore, we demonstrated that compound 7 behaved as a potent ACAT inhibitor in not only in vitro assay system but also cell-based assay system.