Huperzine A

Huperzine A is a naturally-occurring sesquiterpene alkaloid compound found in the firmoss Huperzia serrata and in varying quantities in other food Huperzia species, including H. elmeri, H. carinat, and  H. aqualupian. Huperzine A has been investigated as a treatment for neurological conditions such as Alzheimer's disease, but a 2013 meta-analysis of those studies concluded that they were of poor methodological quality and the findings should be interpreted with caution. Huperzine A inhibits the breakdown of the neurotransmitter acetylcholine (ACh) by the enzyme acetylcholinesterase. It is also an antagonist of the NMDA-receptor. It is commonly available over the counter as a nutritional supplement and marketed as a memory and concentration enhancer.

Pharmacological effects
Huperzine A is extracted from Huperzia serrata. It is a reversible acetylcholinesterase inhibitor and NMDA receptor antagonist that crosses the blood–brain barrier. Acetylcholinesterase is an enzyme that catalyzes the breakdown of the neurotransmitter ACh and other choline esters that function as neurotransmitters. The structure of the complex of huperzine A with acetylcholinesterase has been determined by X-ray crystallography (PDB code: 1VOT; see the 3D structure).

Huperzine A has been investigated as a possible treatment for diseases characterized by neurodegeneration such as Alzheimer's disease, and there is some evidence from small-scale studies that it can benefit cognitive functioning, global clinical status, and ability to engage in activities of daily living (ADLs) among individuals with the disease. In a 2016 systematic review of systematic reviews, huperzine A was associated with a standardized mean difference of 1.48 (95% CI, 0.95-2.02) compared to placebo on measures of ADL among people with dementia, but the evidence was very low-quality and uncertain. In a 2022 umbrella review, huperzine A was associated with broad benefits to dementia patients' cognitive functioning, but the degree of heterogeneity in measurements and outcomes of the reviewed studies indicated publication bias toward huperzine A benefit.

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

Drug interactions
Huperzine A may have additive effects if taken with drugs causing bradycardia, such as beta-blockers, 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.

Safety
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.

Other possible uses
Huperzine A might be useful in the treatment of organophosphate nerve agent poisoning by preventing damage to the central nervous system caused by such agents.

Synthesis
Two scalable and efficient total syntheses of huperzine A have been reported.