Tetrandrine (Sinomenine A) is a natural, bis-benzylisoquinoline alkaloid isolated from the root of the plant. Tetrandrine non-selectively inhibits calcium channel activity and induces G1 blockade of the G1 phase of the cell cycle and apoptosis in various cell types, resulting in immunosuppressive, anti-proliferative and free radical scavenging effects.

In mice, the LD50 of Tetrandrine administrated via intravenous injection is 158.9 mg/kg, and when administered via intraperitoneal injection, the LD50 is 365 mg/kg. In rats, the LD50 of Tetrandrine administered via oral gavage is 646 mg/kg.

In isolated rat pituitary nerve terminals, Tetrandrine inhibits voltage-gated Ca2+ currents (IC50=10.1 mM) and acts as a high-affinity blocker of the type II maxi-Ca(2+)-activated K+ channels (IC50=0.21 mM) present in these terminals. Moreover, Tetrandrine can suppress Ca(2+)-activated Cl- currents (IC50=5.2 mM). Additionally, in human leukemia HL-60 cells, Tetrandrine is capable of inhibiting cell proliferation and inducing apoptosis.