Huperzine A

Huperzine A may present with mild cholinergic side effects such as nausea, vomiting, and diarrhea.^[7] Slight muscle twitching and slurred speech might also occur, as well as hypersalivation and sweating. The use of huperzine A during pregnancy and lactation is not recommended due to the lack of sufficient safety data.^[8]

Huperzine A is a potent, highly specific, reversible acetylcholinesterase inhibitor,^{[9][10][11][12]} with IC₅₀ binding affinity of ~82 nM.^[13] It is also a weak NMDA receptor antagonist,^[14] with IC₅₀ of ~65,000–82,000 nM^[13] (65–82 μM); due to this relatively low affinity, huperzine A is unlikely to produce significant NMDA receptor blockade at clinically relevant concentrations in humans. Huperzine A readily crosses the blood–brain barrier and demonstrates central nervous system activity at therapeutic doses as low as 100 μg in humans.^[15]

Acetylcholinesterase is an enzyme that catalyzes the breakdown of choline-based neurotransmitters, particularly acetylcholine (ACh), which plays a critical role in memory, learning, and behavior. The structure of the complex of huperzine A with acetylcholinesterase has been determined by X-ray crystallography (PDB code: 1VOT; see the 3D structure).^[16]

Huperzine A may have additive effects if taken with drugs causing bradycardia, such as beta blockers,^[17] which may decrease heart rate. Theoretically, there may be possible additive cholinergic effects if huperzine A is taken with other acetylcholinesterase inhibitors or cholinergic agents.^[18]

Huperzine A, in spite of the possible cholinergic side effects, seems to have a wide margin of safety. Toxicology studies show huperzine A to be non-toxic even when administered at 50–100 times the human therapeutic dose. The extract is active for 6 hours at a dose of 2 μg/kg with no remarkable side effects.^[19]

Two scalable and efficient total syntheses of huperzine A have been reported.^[20][21]

In 1989, a research study found^[22] that the chemical structure of the alkaloid selagine reported in 1960 from a study of Lycopodium slago L.^[23] (analyzed using 60-MHz NMR) was identical to that of Huperzine A.