β-Carboline-1-carboxylic acid is an alkaloid that has been found in P. quassioides and has diverse biological activities. It reduces LPS-induced increases in MCP-1, TNF-α, IL-6, and IL-1β levels in RAW 264.7 cells when used at a concentration of 15 µg/ml and inhibits the epithelial-to-mesenchymal transition (EMT) induced by TGF-β1 in A549 cells. β-Carboline-1-carboxylic acid induces cytotoxicity in CT26.WT, K562, and SGC-7901 cells (IC50s = 14.96, 22.11, and 19.7 µg/ml, respectively) but not HepG2 or A549 cells (IC50s = 36.41 and 41.51 µg/ml, respectively). It also inhibits cAMP phosphodiesterase with an IC50 value of 96 µM.