Bioactivation of 3-n-Butylphthalide via Sulfation of Its Major Metabolite 3-Hydroxy-NBP: Mediated Mainly by Sulfotransferase 1A1

3-n-Butylphthalide (NBP) [(+/-)-3-butyl-1(3H)-isobenzofuranone] is an anti-cerebral-ischemia drug. Moderate hepatotoxicity has been observed in clinical applications. One of the major metabolites, 3-N-acetylcysteine-NBP, has been detected in human urine, indicating the formation of a reactive metabolite. We elucidated the formation mechanism of the reactive metabolite and its association with the hepatotoxicity of NBP. The in vitro incubations revealed that 3-glutathione-NBP (3-GSH-NBP) was observed only in fresh rat liver homogenate rather than in liver microsomes, liver cytosol, or liver 9,000g supernatant supplemented with NADPH and GSH. We also detected 3-GSH-NBP when 3'-phosphoadenosine-5'-phosphosulfate was added in GSH-fortified human liver cytosol (HLC). The formation of 3-GSH-NBP was 39.3-fold higher using 3-hydroxy-NBP (3-OH-NBP) as the substrate than NBP. The sulfotransferase (SULT) inhibitors DCNP (2,6-dichloro-4-nitrophenol) and quercetin suppressed 3-GSH-NBP formation in HLC by 75 and 82%, respectively, suggesting that 3-OH-NBP sulfation was involved in 3-GSH-NBP formation. Further SULT phenotyping revealed that SULT1A1 is the major isoform responsible for the sulfation. Dose-dependent toxicity was observed in primary rat hepatocytes exposed to 3-OH-NBP, with an IC50 of approximately 168 mu M. Addition of DCNP and quercetin significantly increased cell viability, whereas L-buthionine-sulfoximine (a GSH depleter) decreased cell viability. Overall, our study revealed the underlying mechanism for the bioactivation of NBP is as follows. NBP is first oxidized to 3-OH-NBP and further undergoes sulfation to form 3-OH-NBP sulfate. The sulfate spontaneously cleaves off, generating highly reactive electrophilic cations, which can bind either to GSH to detoxify or to hepatocellular proteins to cause undesirable side effects.