Comparison of Honokiol Contents in Different Plant Parts of Magnolia obovata Collected at Various Times and Cultivation Sites, Using Honokiol Exhibiting Antiproliferative Activity against Cancer Stem Cells Derived from Human Glioblastoma

The bark of Magnolia obovata has long been used as the traditional crude drug “Koboku,” and its major constituents, the neolignans honokiol (1) and magnolol (2), exhibit various pharmacological activities, including anticancer effects. We confirmed that 1 exerted antiproliferative activity against cancer stem cells derived from the human glioblastoma cell line U-251 MG. However, because only the bark is utilized as Koboku, its supply is limited, raising concerns regarding production sustainability. Therefore, we quantified 1 and 2 in plant parts, including leaves collected at various times and cultivation sites, to evaluate whether non-bark tissues could serve as supplementary sources of these compounds. Analysis of leaves categorized by length and petioles classified by color in May revealed that shorter leaves (5–10 cm) contained the highest level of 1, exceeding half of that in commercially available Koboku. Quantitative analysis of the bark, leaves, petioles, buds, and seeds from two cultivation sites in September and November showed that the bark consistently contained the highest amounts of 1 and 2. In the examined leaf samples, relatively high levels of both compounds were observed in May, particularly in small leaves, whereas the size-dependent differences among leaves were less evident in September and November. Collectively, these findings suggest that small, early expanded leaves of M. obovata may have potential as a supplementary source of honokiol (1).