IDENTIFICATION OF A MORE POTENT ANALOG OF THE NATURALLY-OCCURRING ALKALOID HUPERZINE-A - PREDICTIVE MOLECULAR MODELING OF ITS INTERACTION WITH ACHE

Huperzine A (HA), a potent reversible inhibitor of acetylcholinesteras e (AChE), is an important psychotherapeutic agent for improving cognit ive function in Alzheimer's patients through the enhancement of centra l cholinergic tone. This molecule takes on added value in that it has recently been shown to exhibit neuroprotective properties (glutamate t oxicity blocking activity) in vitro. Based upon our cumulative SAR inf ormation and to some extent the predicted binding site of HA within To rpedo AChE, we chose to investigate the synthesis and biology of certa in C-10 substituted analogues. The important finding was made that int roduction of an axial methyl group into the C-10 position of huperzine A increased the potency for AChE inhibition 8-fold; the corresponding equatorial isomer was about 1.5-fold less active than huperzine A. Th e introduction of substituents larger than methyl resulted in a drop i n activity. For example, the ethyl analogue was found to be about 100- fold less active than huperzine A, indicating that while it is still c apable of binding to Torpedo AChE, some steric interaction with the '' walls'' of the active site gorge must result. Through the use of molec ular modeling methods involving the docking of these analogues to the reported X-ray crystal structure of Torpedo AChE, it is clearly eviden t that the C-10 axial methyl group points into a hydrophobic region of the enzyme, while the equatorial methyl group is directed to a less f avorable hydrophilic region. Substituents larger than methyl were foun d to result in a conformational energy penalty. The ready explanation of this structure-activity relationship data provides further evidence in support of our modeling studies aimed at establishing huperzine A' s binding site in AChE. This knowledge should facilitate the identific ation of other structural analogues of huperzine A likely to exhibit a n improved therapeutic profile.