Metabolism and Pharmacokinetics of 3-n-Butylphthalide (NBP) in Humans: The Role of Cytochrome P450s and Alcohol Dehydrogenase in Biotransformation

3-n-Butylphthalide (NBP) is a cardiovascular drug currently used for the treatment of cerebral ischemia. The present study aims to investigate the metabolism, pharmacokinetics, and excretion of NBP in humans and identify the enzymes responsible for the formation of major metabolites. NBP underwent extensive metabolism after an oral administration of 200 mg NBP and 23 metabolites were identified in human plasma and urine. Principal metabolic pathways included hydroxylation on alkyl side chain, particularly at 3-, ω-1-, and ω-carbons, and further oxidation and conjugation. Approximately 81.6% of the dose was recovered in urine, mainly as NBP-11-oic acid (M5-2) and glucuronide conjugates of M5-2 and mono-hydroxylated products. 10-Keto-NBP (M2), 3-hydroxy-NBP (M3-1), 10-hydroxy-NBP (M3-2), and M5-2 were the major circulating metabolites, wherein the areas under the curve values were 1.6-, 2.9-, 10.3-, and 4.1-fold higher than that of NBP. Reference standards of these four metabolites were obtained through microbial biotransformation by Cunninghamella blakesleana. In vitro phenotyping studies demonstrated that multiple cytochrome P450 (P450) isoforms, especially CYP3A4, 2E1, and 1A2, were involved in the formation of M3-1, M3-2, and 11-hydroxy-NBP. Using M3-2 and 11-hydroxy-NBP as substrates, human subcellular fractions experiments revealed that P450, alcohol dehydrogenase, and aldehyde dehydrogenase catalyzed the generation of M2 and M5-2. Formation of M5-2 was much faster than that of M2, and M5-2 can undergo β-oxidation to yield phthalide-3-acetic acid in rat liver homogenate. Overall, our study demonstrated that NBP was well absorbed and extensively metabolized by multiple enzymes to various metabolites prior to urinary excretion.