Fig. 4.

IC₅₀ shift plot for silybin A, silybin B, and silibinin. HLMs (A), HIMs (B), and rCYP3A4 (C) were incubated with silybin A (circles), silybin B (diamonds), or silibinin (squares) (0.1–200 μM) in the presence (open symbols) or absence (solid symbols) of NADPH (1 mM). The primary reaction mixture was diluted 5-fold to initiate the secondary reaction, which contained NADPH (1 mM) and midazolam (4 μM). Midazolam 1′-hydroxylation activity in the presence of vehicle control [0.1% (v/v) DMSO] was 2000 ± 90 pmol/min/mg (HLMs), 350 ± 10 pmol/min/mg (HIMs), and 7.9 ± 1.0 pmol/min/pmol (rCYP3A4). Symbols and error bars denote means and S.D.'s, respectively, of triplicate incubations. Open symbols denote observed data when NADPH was present in the primary incubation; solid symbols denote observed data when NADPH was absent in the primary incubation. Curves denote nonlinear least-squares regression of observed data using WinNonlin (version 5.3).