ABSTRACT − Scutellariae Radix is widely used in the traditional herbal medicine for the treatment of fever, cough, dys-entery, hepatitis and hypertension in Korea, China and Japan. In this study, we investigated the effects of 70% ethanolicextract of Scutellariae Radix (SRE) on CYP450-mediated drug metabolism in the in vitro systems using human livermicrosomes and hepatocytes. The microsomal incubation assay showed that SRE inhibited the drug metabolism reactionscatalyzed by CYP1A2, CYP2C8 and CYP2C9 in a dose-dependent manner. In particular, SRE was shown to strongly inhibitthe metabolic activity of CYP1A2 with an IC 50 value of 4.6 µ g/mL. When SRE was evaluated for its effect on the inductionof CYP450 enzyme activities in cryopreserved human hepatocytes, SRE did not exhibit any effect.Key words − Scutellariae Radix, CYP450, Herb-drug interaction, Human liver microsomes, Human hepatocytes Scutellariae Radix, dried roots of Scutellaria baicalensisGeorgi, is a well-known traditional medicine used for inflam-mation, allergy, diarrhea, and hepatitis in Asian countriesincluding Korea, China and Japan (Lin & Shieh, 1996). Baica-lin, baicalein, wogonin and wogonoside are known to be themajor bioactive components of Scutellariae Radix (Li-Weber,2009). Scutellariae Radix is one of the most frequently pre-scribed medicinal plants in traditional medicine and thus likelyto be taken with other prescription drugs. Therefore, the inves-tigation of the herb-drug interactions which could be caused byScutellariae Radix should be required.Cytochrome P450s (CYPs) constitute a superfamily ofhemeproteins that play an important role in the metabolism ofxenobiotics, including a variety of drugs (Ortiz de Montellano,1995). Thus, if a certain substance has any effects on CYPenzyme activities, it is likely to affect the CYP-mediated drugmetabolism to cause drug interactions. Several investigatorshave reported the effect of Scutellariae Radix on CYP enzymeactivities. For example, Kim et al. (2001) reported that thewater extract of Scutellariae Radix inhibited the production ofaflatoxin M1 by CYP1A1/2 in rats. Several Scutellaria fla-vonoids such as baicalein, wogonin, and oroxylin A have beenreported to inhibit CYP isozyme activities (Kim et al., 2002).Baicalin has known to modulate CYP enzymes through induc-tion of CYP gene expression (Li et al., 2010; Fan et al., 2009).However, to our knowledge, there has been no report eval-uating collectively the possibility of herb-drug interactions byScutellariae Radix including both CYP inhibition and induc-tion.In this study, we investigated the effect of 70% ethanolicextract of Scutellariae Radix (SRE) on human CYP enzymeactivities to predict the likelihood of herb-drug interactions.We characterized the inhibitory effect of SRE on CYPisozymes using human liver microsomes and also inspected forits induction effect on CYP isozymes in cryopreserved humanhepatocytes.