As part of a phase Ib clinical trial to determine the tolerability and safety of the highly specific
acetylcholinesterase (AChE) inhibitor
huperzine A, twelve (12) healthy elderly individuals received an escalating dose regimen of
huperzine A (100, 200, 300, and 400 microg doses, twice daily for a week at each dose), with three (3) individuals as controls receiving a placebo. Using the WRAIR whole blood
cholinesterase assay, red blood cell AChE and plasma
butyrylcholinesterase (BChE) were measured in unprocessed whole blood samples from the volunteers following each dose, and then for up to 48h following the final and highest (400 microg) dose to monitor the profile of inhibition and recovery of AChE. Significant inhibition of AChE was observed, ranging from 30-40% after 100 microg to >50% at 400 microg, and peaking 1.5h after the last dose. Gradual recovery of AChE activity then occurs, but even 48 h after the last dose red blood cell AChE was about 10% below control (pre-dose) values.
Huperzine A levels in plasma peaked 1.5h after the final 400 microg dose (5.47+/-2.15 ng/mL). Plasma BChE was unaffected by
huperzine A treatment (as expected). Aliquots of
huperzine A-containing (from three individuals) and placebo blood samples were exposed ex vivo to the irreversible
nerve agent soman (GD) for 10 min, followed by removal of unbound huperzine and
soman from the blood by passing through a small C(18) reverse phase spin column. Eluted blood was diluted in
buffer, and aliquots taken at various time intervals for AChE and BChE activity measurement to determine the time taken to achieve full return in activity of the free
enzyme (dissociation from the active site of AChE by
huperzine A), and thus the proportion of AChE that can be protected from
soman exposure.
Huperzine A-inhibited red blood cell (RBC) AChE activity was restored almost to the level that was initially inhibited by the
drug. The increased doses of
huperzine A used were well tolerated by these patients and in this ex vivo study sequestered more red blood cell AChE than has been previously demonstrated for
pyridostigmine bromide (PB), indicating the potential improved prophylaxis against
organophosphate (OP)
poisoning.