Cytotoxic principles and α-pyrone ring-opening derivatives of bufadienolides from Kalanchoe hybrida

Purification of the cytotoxic fractions of the methanol extracts of Kalanchoe hybrida leads to three new compounds with the basic skeleton of α-pyrone ring-opening products of bufadienolides, namely, kalanhybrin A–C (1–3). Moreover, four bufadienolides (4–7) and eight other compounds were also characterized from the title plant. The isolated compounds (1–7) were evaluated for their cytotoxicity toward MCF-7, NCI-H460, and SF-268 tumor cell lines. Among them, compounds 4 and 6 displayed significant cytotoxicity toward MCF-7 and NCI-H460 tumor cell lines at the tested concentration. In addition, the biosynthetic pathway of these α-pyrone ring-opening derivatives was also proposed.