(±)-Peharmaline A: A Pair of Rare β-Carboline–Vasicinone Hybrid Alkaloid Enantiomers from Peganum harmala

(+/-)-Peharmaline A (1), a pair of rare beta-carboline-vasicinone hybrid alkaloid enantiomers with the hitherto unknown hybrid dimeric system, was isolated from the seeds of Peganum harmala L. Their structures, including absolute configurations, were determined by extensive spectroscopic analyses and electronic circular dichroism (ECD) calculations. Plausible biogenetic pathways involving Mannich/Pictet-Spengler-type and intermolecular Michael addition reactions are briefly discussed. Compound 1 exhibited significant cytotoxic activities against HL-60, PC-3, and SGC-7901 cancer cell lines with median inhibitory concentration values of 9.2, 21.6, and 25.4 mu M, respectively. However, two biosynthetically related precursors, harmaline and vasicinone, were inactive. This cytotoxic beta-carboline-vasicinone hybrid dimeric alkaloid may give some insight into the discovery of new lead compounds for cancer therapy.