FABAD J. Pharm. Sci., 28, 51-58, 2003 SCIENTIFIC REVIEWS Acetylcholinesterase Inhibitors from Natural Resources ‹lkay ORHAN*°, Bilge fiENER* Acetylcholinesterase Inhibitors from Natural Resources Do¤al Kaynaklardan Elde Edilen Asetilkolinesteraz ‹nhibitörleri Summary : Acetylcholinesterase inhibitors prevent reduction of Özet : Asetilkolinesteraz inhibitörleri, asetilkolini sal›nd›¤› sinir acetylcholine via inhibiting acetylcholinesterase enzyme which ucunda hidroliz eden asetilkolinesteraz enzimini inhibe etmek hydrolyzes acetylcholine in the neuronal end from which it is suretiyle, asetilkolin miktar›n›n azalmas›n› önlemektedir. Asetil- released. Acetylcholinesterase inhibitors play an important ro- kolinesteraz inhibitörleri, Alzheimer hastal›¤›n›n yan›s›ra, le in the treatment of Alzheimer’s Disease as well as Myasthe- myastenia gravis, glokom ve helmintiyazis gibi hastal›klar›n te- nia Gravis, Glaucoma and Helminthiasis together with the davisi ile insektisit ilaçlar›n etki mekanizmalar›nda da önemli mechanism of action of insecticide drugs. rol oynayan bilefliklerdir. In this review, some compounds obtained from natural resour- Bu derlemede, do¤al kaynaklardan elde edilen asetilkolineste- ces that have acetylcholinesterase inhibitory activity are evalu- raz inhibitör aktiviteye sahip baz› bileflikler de¤erlendirilmekte- ated. dir. Key Words: Acetylcholinesterase, Alzheimer’s Disease, acetylc- Anahtar kelimeler: Asetilkolinesteraz, Alzheimer hastal›¤›, ase- holinesterase inhibitory activity, plant. tilkolinesteraz inhibitör aktivite, bitki. Received : 9.12.2002 Revised : 5.3.2003 Accepted : 21.3.2003 INTRODUCTION method for screening biological sources. AD is one of the most common mental problems in the aged Many new natural product-originated bioactive population1-3. The basal forebrain and brainstem compounds effective in treating several diseases ha- cholinergic systems also play an important role in ve been isolated from different plants, fungi and the regulation of cortical and thalamic electrical acti- microorganisms. They are unknown complex mix- vity4. The findings from experimental animals, tures having potentially large number of secondary aging and AD research have provided an experi- metabolites. Sensitive assays have been developed mental foundation for the cholinergic hypothesis of to screen these extracts from natural sources. The learning and memory5-7. Based on the cholinergic simplest assays are the ones based on the mecha- hypothesis, AD results from a defect in the choliner- nism of action of a known drug. The assays have al- gic system. One goal in the treatment for AD is to so been incorporated into efficient testing schemes increase the acetylcholine level in the brain. Therefo- that are useful for high-throughput screening (HTS). re, AChE inhibitors are being developed for the tre- For example; one assay used for Alzheimer’s Dise- atment of this disease. ase (AD) is based on the inhibition of acetylcholines- terase (AChE). The development of new leads of Because of the side effects of the present drugs, re- AChE inhibitors has been realized by the Ellman cently, galanthamine isolated from Amaryllidaceae * Gazi University, Faculty of Pharmacy, Department of Pharmacognosy, 06330, Ankara, Turkey. ° Corresponding author 51 Orhan, fiener plants has been approved by the FDA for the treat- perzia serrata (Thunb.) Trev) (Lycopodiaceae). This ment of AD. However, the research for new AChE plant, called as "Qing Ceng Ta", has been used in tra- inhibitors is still of interest and natural products are ditional Chinese medicine for its memory-enhan- an important source of these compounds. cing property for centuries9. Based on the documented memory enhancing and Over 100 alkaloids, a number of which are of the se- antiaging activities in folk medicine, the following ries of huperzin A-R, have been isolated from the ge- plants and isolated compounds have been chronolo- nus Lycopodium which is very rich in alkaloid con- gically described as potential leads for the develop- tent10. Of them, only huperzin A possessed remar- ment of new drugs for the treatment of AD. kable acetylcholinesterase inhibitory activity9. Acetylcholinesterase inhibitors from plants Physostigma venenosum Physostigmine (1), the first discovered-compound within this class of compounds, is an alkaloid isola- ted from Physostigma venenosum L. (Fabaceae). La- ter on, physostigmine (Synapton®), has been a mo- (2) del for some drugs with acetylcholinesterase activity such as rivastigmine (Exelon®), which was synthesi- The activity of Huperzin A has been found to be as zed later8. Although, the results of the first clinical high as physostigmine, galanthamine, donepezil trials with physostigmine were promising, short ac- and tacrine, the commercial drugs already used aga- tion duration and cholinergic side effects of physos- inst AD, or (even greater) than. In various in vivo tigmine have limited its therapeutic use. and ex vivo experiments, it has been shown to inhi- bit acetylcholinesterase reversibly and also to pre- vent oxidative cell damage induced by β-amiloid plaques11-15. In Ainge et al.’s work, Huperzin A isolated from L. varium showed a potent insecticidal activity against (1) the insects Anthrenocerus australis, Lucilia cuprina and Tineda bisselliella. Its total synthesis was comp- Lycopodium species leted and now this compound is in the stage-III cli- nical trial in China16-20. Huperzin A (2) has a special significance among the compounds with acetylcholinesterase inhibitory ac- Related to this subject, in our ongoing research on tivity, isolated from natural resources. Huperzin A, investigation of acetylcholinesterase inhibitory acti- [(5R, 9R, 11E)-5-amino-11-etilidin- 5,6,9,10-tetrahid- vity of some plants growing in Turkey, we screened ro-7-metil-5,9-metanosikloocta-[b]-piridin-2 (1H)- five Lycopodium species (L. annotinum, L. alpinum, on], is an alkaloid isolated in 1986 by researchers of L. clavatum, L. complanatum subsp. chamaecypa- the Shanghai Institute of Materia Medica from the rissus and L. selago) of the Lycopodiaceae family re- clubmoss Lycopodium serratum Thunb. (syn. Hu- garding their acetylcholinesterase inhibitory activity 52 FABAD J. Pharm. Sci., 28, 51-58, 2003 using the Ellman method, which is a spectrophoto- Corydalis ternata metric, in vitro robotic screening method, and deter- mined the responsible compound for the activity as In a screening study by Kim et al., the methanolic α-onocerin (3), a triterpene-type compound, from extract prepared from tubers of Corydalis ternata Lycopodium clavatum that showed ca. 50 % (Papaveraceae) was found to have potent inhibitory activity21. activity by the Ellman method. Bioactivity-directed fractionation of this extract afforded protopine (6), an alkaloid-type compound, by the Ellman method . This result was supported by passive avoidance test, which is used to measure antiamnesic activity, in male mice23. (3) Salvia species Perry et al. studied the acetylcholinesterase inhibi- tory activity of essential oils of Salvia lavandulaefo- (6) lia and S. officinalis (Lamiaceae), the plants knowns to be used as memory-enhancing in European folk Evodia rutaecarpa medicine, and the monoterpenes called (+) - α - pi- nen, α - and β - terpineol, citronellal, δ - terpinen, R In another screening study performed in South Ko- - (+) - limonen, 1,8-cineol, 1R-(+)-camphor, linalol, rea24, Park et al. investigated 87 extracts prepared 1S-(-)-β-pinen and geraniol, the constituents of these from 29 plants in total by the Ellman method with essential oils analyzed by GC-MS, were tested on regard to anticholinesterase activity and found that human erythrocyte acetylcholinesterase by the Ell- 9 of the extracts showed over 40 % inhibitory acti- man method. As a result; the essential oils of S. la- vity. These extracts and their inhibition rates are as vandulaefolia ile S. officinalis as well as camphor, follows: Poncirus trifoliata (dichloromethane ext- 1,8-cineol, and α-pinen inhibited the enzyme in a ract, 91.0 %), Evodia rutaecarpa (dichloromethane dose-dependent manner. When compared to the extract, 84.3 %), Coptis chinensis (methanol extract, standard drugs physostigmine and tacrine, the most 83.3 %), Coptis chinensis (dichloromethane extract, active monoterpenes were 1,8-cineol (4) (IC50= 0.67 76.9 %), Saussurea lappa (dichloromethane extract, 22 mM) and α-pinen (5) (IC50= 0.63 mM) . 70.5 %), Angelica sinensis (dichloromethane extract, 65.5 %), Notopterygium incisium (dichloromethane extract, 50.3 %), Evodia rutaecarpa (methanol ext- ract, 43.8 %), Polygala tenuifolia (dichloromethane extract, 40.0 %). Among them, the dichloromethane extract of Evo- dia rutaecarpa displayed inhibitory activity in the (4) (5) passive avoidance test in rats (Sprague-Dawley) with scopolamine-induced memory loss, and 53 Orhan, fiener dehydroevodiamine HCl (7) was isolated as the ac- acetylcholinesterase inhibitory activity by the Ell- tive component through bioactivity-directed fracti- man method against both acetylcholinesterase and onation25,26. butyrylcholinesterase which also plays a role pos- sibly in the pathology of AD and 27 steroidal alkalo- ids of pregnane-type, 10 of which were new, were isolated by bioactivity-directed fractionation. The al- kaloids inhibited both enzymes in a dose-dependent manner at IC50 values between 5.21-22.7 µM for acetylcholinesterase and 2.18-38.36 µM for butyrylc- holinesterase29. (7) Areca catechu Buxus species In a study performed by Gilan et al., a hydroalcoho- lic extract