Ajmaline, Oxindole, and Cytotoxic Macroline–Akuammiline Bisindole Alkaloids from <i>Alstonia penangiana</i>

Examination of the EtOH extract of the Malayan <i>Alstonia penangiana</i> resulted in the isolation of 10 new alkaloids, comprising two ajmaline (<b>1</b>, <b>2</b>), four macroline oxindole (<b>3</b>–<b>6</b>), and four macroline–akuammiline bisindole alkaloids (<b>7</b>–<b>10</b>). The structures of these alkaloids were determined based on analysis of the spectroscopic data and, in the case of the oxindole <b>6</b> and the bisindole alkaloid <b>7</b>, also confirmed by X-ray diffraction analysis. The bisindole alkaloids <b>7</b> and <b>8</b> showed pronounced in vitro growth inhibitory activity against an array of human cancer cell lines, including KB, vincristine-resistant KB, PC-3, LNCaP, MCF7, MDA-MB-231, HT-29, HCT 116, and A549 cells with IC₅₀ values in the 0.3–8.3 μM range