Structure-activity studies on acetylcholinesterase inhibition in the lycorine series of Amaryllidaceae alkaloids

Article abstract of DOI:10.1016/j.bmcl.2010.06.130

The synthesis of differentially functionalized analogs of the Amaryllidaceae alkaloid lycorine, accessed via a concise chemoselective silylation strategy, is described uncovering two of the most potent inhibitors of acetylcholinesterase (AChE) identified to date in this series. Important elements of this novel pharmacophore were elucidated through structure-activity relationship (SAR) studies.

Full text of DOI:10.1016/j.bmcl.2010.06.130

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5290–5294
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Structure–activity studies on acetylcholinesterase inhibition in the lycorine
series of Amaryllidaceae alkaloids
a
a
a
b
James McNulty a, , Jerald J. Nair , Jessamyn R. L. Little , John D. Brennan , Jaume Bastida
*
a Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S 4M1
b Departament de Productes Naturals, Facultat de Farmacia, Universitat de Barcelona, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of differentially functionalized analogs of the Amaryllidaceae alkaloid lycorine, accessed via
a concise chemoselective silylation strategy, is described uncovering two of the most potent inhibitors of
acetylcholinesterase (AChE) identified to date in this series. Important elements of this novel pharmaco-
phore were elucidated through structure–activity relationship (SAR) studies.
Received 6 May 2010
Revised 22 June 2010
Accepted 25 June 2010
Available online 1 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
We dedicate this paper to the memory of
our late friend and colleague Dr. Don
Hughes
Keywords:
Amaryllidacea
Alkaloids
Lycorine
Alzheimer’s disease
Acetylcholine esterase
Alzheimer’s disease (AD) is currently implicated in approxi-
mately 70% of the overall cases of dementia.1 AD is a progressive
neurodegenerative disorder, it is currently the sixth leading cause
of death in the United States with the number of AD associated
deaths increasing dramatically by 47.1% between 2000 and
2006.1 Approximately 5.3 million Americans currently suffer from
AD, with an estimated cost of $180 billion per annum on the health
care system.1b Promising experimental approaches towards treat-
ment of AD have been advanced recently involving the develop-
ment of small molecules and antibodies to block the amyloid-b
protein, as well as inhibition of the hyperphosphorylation and neu-
rofibrillary tangle association of tau-protein. Experimental drugs
such as rember (methylene blue), GSK2/3b inhibitors and others
are at various stages of clinical development.1b,2 Current clinical
treatment of AD involves the use of reversible inhibitors of the en-
zyme acetylcholinesterase (AChE). In addition to the positive role
of acetylcholine in neuronal transmission, AChE has been impli-
cated in the production of amyloid fibrils, characteristic in the
pathology of the brain cells of patients with AD.3 A major focus
in clinical treatment of AD therefore involves the use of reversible
inhibitors of AChE including the approved drugs aricept (donepe-
zil) 1, rivastigmine 2 and galanthamine (reminyl) 4, Scheme 1.
A significant number of naturally occurring compounds have
been found to inhibit AChE.4 The most significant of these are the
Amaryllidaceae alkaloids galanthamine 4 and sanguinine 55 as well
as the Lycopodium alkaloids huperzine A (not shown) and B 3.6 The
AChE inhibitor galanthamine hydrobromide (reminyl) is the first of
the Amaryllidaceae alkaloids to be approved as a prescription drug
in the treatment of AD. Although the related alkaloid sanguinine is
a more potent inhibitor of AChE,7,8 its low natural abundance has
precluded its further development.
Recently, structure–activity relationships (SAR) studies was
undertaken by Elgorashi et al. on AChE inhibition amongst a struc-
turally diverse library of Amaryllidaceae alkaloids assembled from
plant species endemic to the southern African region.9a,b The col-
lection of natural alkaloids used in this work,9a,b and natural and
synthetic analogs described herein are collected in Scheme 2, 6
to 34. In addition to the confirmation of the AChE inhibitory activ-
ities of galanthamine and sanguinine, this work also revealed the
naturally occurring alkaloid 1-acetyllycorine 28 to be a potent
AChE inhibitor. These researchers also showed that the parent
alkaloid lycorine 25 as well as 1,2-diacetyllycorine 29 and 2-acet-
yllycorine exhibited significantly diminished AChE inhibitory
activities and ascribed the selectivity of 28 to its ability to function
as a hydrogen-bond acceptor, analogous to the hydroxyl group of
galanthamine. The lower AChE inhibitory activity of 1,2-diac-
etyllycorine 29 is difficult to explain given the narrow structural
* Corresponding author. Tel.: +1 905 525 9140x27393; fax: +1 905 522 2509.
E-mail address: jmcnult@mcmaster.ca (J. McNulty).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.06.130

J. McNulty et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5290–5294
5291
OR1
O
OMe
OMe
H
N
O
N
O
O
H
N
O
H
NMe
NMe2
R2O
2. Rivastigmine
3. Huperzine B
N
Ph
4. R1=H, R2=Me (Galanthamine)
5. R1=H, R2=H (Sanguinine)
1. Aricept
Scheme 1. Reversible inhibitors of AChE including the approved drugs aricept, rivastigmine and galanthamine as well as the Lycopodium alkaloid huperzine B.
O
OR1
OMe
HO
R2
HO
HO
HO
O
O
O
O
N
N
N
H
6. Norbelladine
OMe
R4
R3
7. R1=R2=R3=R4=H (Crinine)
11. 1,2-Epoxyambelline
8. R1=Me, R2=OH, R3=H, R4=OMe (Ambelline)
9. R1=H, R2=H, R3=OH, R4=OMe (6α-Hydroxycrinamidine)
R1
10. R1=OMe, R2=H, R3=OH, R4=OMe (6α-Hydroxyundulatine)
R1
R2
OR1
OH
HO
11
N
O
O
O
O
O
12
N
N
O
12. R1=α-OH (Amabiline)
R3
21. R1=H (Dihydrohamayne)
22. R1=Me (Dihydrocrinamine)
13. R1=β-OAc (Joesephinine)
14. R1=α-OH, R2=OH, R3=H (Hamayne)
15. R1=α-OMe, R2=OH, R3=H (Crinamine)
16. R1=α-OMe, R2=OAc, R3=H (Acetylcrinamine)
17. R1=β-OH, R2=H, R3=H (Vittatine)
OR1
OR2
2
18. R1=β-OH, R2=OH, R3=H (11-Hydroxyvittatine)
19. R1=β-OMe, R2=OH, R3=H (Haemanthamine)
20. R1=β-OMe, R2=OH, R3=OH (Haemanthidine)
R1O
1
3
3
2
1
4
H
10
7
10b
4a
4
10b
9
8
4a
O
11
12
12
6
10a
OH
H
O
11
10a
9
NMe
N
O
6a
HO
6
10
6a
7
H
25. R1=H, R2=H (Lycorine)
R2O
HO
26. R1=H, R2=TBS (2-TBS-lycorine)
8
H
4. R1=H, R2=Me (Galanthamine)
27. R1=Ac, R2=TBS (1-Acetoxy-2-TBS-lycorine)
28. R1=Ac, R2=H (1-Acetoxylycorine)
29. R1=Ac, R2=Ac (1,2-Diacetoxylycorine)
N
5. R1=H, R2=H (Sanguinine)
MeO
23. R1=Ac, R2=H (Acetylsanguinine)
24. Pseudolycorine
OH
OH
H
H3C
N
HO
H
OH
OH
OH
OH
H
C
O
O
H3CO
H3CO
O
O
H
A
B
O
O
NH
NH
O
OH
O
OH
OCH3
32. Homolycorine
OCH3
OH
30. Pancratistatin
31. Narciclasine
H
N
CH3
H3CO
H3CO
O
O
OH
O
H
N
34. Montanine
33. Tazettine
Scheme 2. Structurally-diverse library of natural and semisynthetic alkaloids of the Amaryllidaceae screened for AChE inhibitory activity in the present study.
diversity screened within this particular sub-set of lycorine
derivatives.
24) was found to be cytotoxic, substitution of either or both of
the hydroxyl groups at C1 and C2 diminished this activity signifi-
cantly.10 In terms of CYP3A4 inhibitory activity, lycorine 25 was
found to be inactive.11 Substitution at C2 with the bulky, lipophilic
tert-butyldimethylsilyl (TBS) substituent 26 introduced potent
inhibition of CYP3A4 which was maintained when a C1-acetyl sub-
stituent was introduced 27, while removal of the C2 substituent to
We have recently documented the selective anticancer activity
of lycorine derivatives10 as well as their interactions with human
cytochrome P450 3A4.11 In this work, a series of 1,2-differentially
substituted lycorine derivatives were prepared and first screened
for anticancer activity. While lycorine 25 (and pseudolycorine

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J. McNulty et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5290–5294
give 1-acetyllycorine 28 resulted in a 20-fold reduction in inhibi-
zoyl-2-acetyllycorine 37 which had the H2 resonance at d 5.40 (dd,
J = 3.5, 2.1 Hz).
tory activity.11 Thus a small, polar H-bond acceptor at C1 of lyco-
rine in conjunction with
a
lipophilic substituent at C2 are
The entire library of compounds 4, 5 and 7–37 (Schemes 2
and 3), constituted of both natural and semi-synthetic representa-
tives of seven structural groups within the Amaryllidaceae, was
then screened for AChE inhibition utilizing the standard Ellman
assay.14 It is immediately apparent that only representatives of
the galanthamine and lycorine groups exhibited significant AChE
inhibition (Table 1). The inhibitory activity of galanthamine 4 (Ki =
important pharmacophoric elements for the CYP3A4 inhibition.11
The relatively high natural abundance of the alkaloid lycorine is
a strong incentive to consider the development of an analog as a
potentially selective AChE inhibitory lead. Cytotoxicity and inhibi-
tion of the critical enzyme CYP3A4 are two important factors that
require consideration to the advancement of a selective AChE
inhibitor. In this Letter, we report the preparation of several novel
structural analogs of lycorine, their AChE inhibitory activity and
the important finding that introduction of lipophilic substituents
at C1 and C2 results in potent AChE inhibitory activity. This data,
in conjunction with the lower cytotoxicity11 and selective CYP3A4
interaction of C1/C2 substituted lycorines indicate lycorines 27 and
28 as the most potent and selective AChE inhibitors identified so
far in the lycorine series.
0.30
AChE isolated from Torpedo californica (Tc),15 showed that the
active site accommodates catalytic triad (Ser200, His440,
l
M) is striking, as noted by others.7–9 X-ray crystallography of
a
Glu327) located at the bottom of a deep and narrow pocket lined
with aromatic residues and a subsite including Trp84 located near
the bottom of the cavity. Trp84 has been identified as the binding
site for acetylcholine. The structure of the complex of (À)-galan-
thamine and TcAChE showed that galanthamine binds at the base
of the active site gorge, interacting with both the acyl-binding
pocket and the indole ring of Trp84.16 While the tertiary amine
The potency of C1 acetyl-functionalized lycorine compounds as
inhibitors of AChE9 as well as their noted lower cytotoxicity11
prompted us to consider the semi-synthesis of further differentially
substituted C1 and C2 analogs from the readily available alkaloid
lycorine 25 in order to further probe the AChE pharmacophore.
Previous studies in this regard have highlighted the difficulties
attending the differential functionalization of the hydroxyl groups
present within the lycorine series.12 Work in our group has shown
that highly selective mono-silylation of 1,2-diols can be achieved
under certain conditions.13 Our approach towards the selective
functionalization of the 1,2-dihydroxy groups in lycorine 25 uti-
lizes the greater reactivity of the allylic pseudoequatorial 2-hydro-
xy group over the axially-orientated homoallylic C1 hydroxyl, in
addition to the bulkiness of the incoming tert-butyldimethylsilyl
chloride (TBSCl) electrophile. The reaction of lycorine with TBSCl
in pyridine (Scheme 3) afforded the novel monosilyl ether 26 in
56% isolated yield (81% based on recovery of lycorine). Interest-
ingly, no bis-silylated product was detected during the course of
the reaction.
Direct chemoselective access to the tert-butyldimethylsilyl
ether 26 allowed us to introduce a benzoate group at C1 as in 35,
obtained in near quantitative yield from 26 upon benzoylation
with benzoyl bromide and pyridine, and its 1H NMR (see Supple-
mentary data section) spectrum showed the expected deshielding
effect on H1 (d 5.82, dd, J = 3.5, 1.4 Hz). Desilylation of 35 was
straightforward with tetrabutylammonium fluoride (TBAF) in tet-
rahydrofuran (THF) affording 1-benzoyllycorine 36, the proton
spectrum of which indicated a slight upfield shift for H2 (d 4.35,
dd, J = 3.5, 2.1 Hz) compared to 35. Acetylation of 36 utilizing acetic
anhydride and pyridine then provided in 98% isolated yield 1-ben-
group of galanthamine did not interact with Trp84, a p–p interac-
tion was observed for the double bond of the cyclohexene ring
with Trp84.16
Based on these remarkable findings, the search and develop-
ment of synthetically-derived analogs of the drug with improved
activities has gained increasing impetus.17 Interestingly, it is note-
worthy that the activity of natural products within this group, such
as galanthamine, sanguinine and 11-hydroxygalanthamine are
generally more potent than those of synthetic origin in terms of
AChE inhibitory ability.7–9,17–19
As mentioned earlier, SAR9a followed by quantitative-structure–
activity relationship (QSAR)9b studies by Elgorashi et al. first re-
vealed 1-acetyllycorine 28 as a potent inhibitor of AChE while lyc-
orine, 2-acetyllycorine, and 1,2-diacetyllycorine 29 in the same
study exhibited no inhibitory activity. The AChE activity of 1-acet-
yllycorine 28 (Ki = 0.43
selective silylation strategy (Scheme 3)11 is comparable to that of
galanthamine 4 (Ki = 0.30 M) and we confirm that both lycorine
lM) (Table 1), here accessed via the chemo-
l
25 and 1,2-diacetylycorine 29 are devoid of inhibition (Table 1).
Nonetheless, silylation at C2 (Scheme 3) afforded the novel deriv-
ative 26, which to our delight inhibited AChE at 0.86 lM, contrast-
ing sharply with the inactive C2 acetate. With silyl ether 26 as a
new lead, a mini-panel of C1 and C2 functionalized lycorine ana-
logs was then assembled (Scheme 3) to probe this new discovery
on the pharmacophore. Conversion of 26 to the C1 acetate and ben-
zoate (both novel compounds) produced a ꢀ2.5-fold increase in
activity as seen for 27 (Ki = 0.34 lM) and 35 (Ki = 0.39 lM). Desily-
lation of the TBS group to yield the respective 1-acyl functionalities
OH
OTBS
N
OTBS
N
HO
C
HO
BzO
O
O
O
O
O
O
a
b
A
B
D
N
35
25
26
OAc
N
OH
N
BzO
BzO
O
O
O
c
d
O
37
36
Scheme 3. Chemoselective silylative discrimination of the 1,2-dihydroxy groups in lycorine. Reagents and conditions: (a) TBSCl, pyridine, rt, 12 h, 56%; (b) BzCl, py, DCM, rt,
3 h, 96%; (c) TBAF, THF, rt, 1 h, 95%; (d) Ac2O, py, DCM, rt, 3 h, 98%.

J. McNulty et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5290–5294
5293
Table 1
philic effect observed for 26, 27 and 35. Acylation at C-2 had a pro-
nounced effect on inhibition since diacetate 29 was void of activity,
Inhibitory activity against the biotransformation of acetylthiocholine by eel acetyl-
cholinesterase (AChE)
and 37 (Ki = 0.97
lM) was almost half as potent as the hydroxy
Compound
Kia
(lM)
benzoate 36. These results also vindicate the requirement for a
lipophilic interaction in the C2 area of the molecule.
Galanthamine 4
Crinine 7
Crinamine 15
Pseudolycorine 24
Lycorine 25
26
27
28
0.30 ( 0.06)
na
na
na
In conclusion, a more comprehensive view has emerged from
this SAR study of the lycorine pharmacophore as a potent inhibitor
of AChE. The screen of a library of Amaryllidaceae alkaloids com-
prising seven different structural-types here showed the galantha-
mine and lycorine groups, as noted by others,7–9 to be superior at
AChE inhibition. A chemoselective silylation strategy was em-
ployed to install the TBS group at C2, providing access to new C1
and C2 functionalized analogs in the lycorine subset. A pronounced
spike in activity is observed on proceeding from the inactive parent
na
0.86 ( 0.03)
0.34 ( 0.08)
0.43 ( 0.02)
na
na
na
29
Homolycorine 32
Tazettine 33
Montanine 32
na
35
36
37
0.39 ( 0.03)
0.54 ( 0.03)
0.97 ( 0.10)
lycorine 24 to the 2-TBS analog 26 (Ki = 0.86 lM), which is rational-
ized in terms of lipophilic binding at the enzyme active site. Acyla-
tion (acetylation or benzoylation) of 26 then afforded 27 (Ki = 0.34
a
Values are means of three experiments, standard error is given in parentheses
(na = not active at 10 M).
l
M) and 35 (Ki = 0.39 lM) respectively, which are here uncovered
l
as the most potent inhibitors of AChE to date within the lycorine
series. The lipophilic effect of the C2 silyl group again becomes
apparent upon desilylation to the alcohols 28 (Ki = 0.43
lM) and
effected a slight decrease in activity as evident for acetate 28
(Ki = 0.43 M) and benzoate 36 (Ki = 0.54 M). Acylation then
afforded diacetate 29, which was inactive, and 1-benzoate-2-ace-
tate 37 (Ki = 0.97 M), respectively. This AChE inhibitory pattern
36 (Ki = 0.54 M) and subsequent acetylation to compounds 29
l
l
l
and 37, seen to be the least efficacious of the series. While the sim-
ilarities between the galanthamine and lycorine pharmacophores
as discussed above are striking, it is speculated here and else-
where9b that they may have similar interactions at the enzyme ac-
tive site. More tangible evidence for this is required, such as has
been afforded by X-ray studies of galanthamine complexation with-
in TcAChE.16 The effect of lipophilic substitution at C2 and polar H-
bond acceptor groups at C1 is additive in that the most potent ana-
logs (27 and 35) have both of these functionalities. These spectacu-
lar findings should be of value in the design of other novel
derivatives and provide direction in the development of a selective
AChE clinical candidate from the lycorine series. Further refinement
of the lycorine AChE inhibitory pharmacophore, in conjunction
with cytotoxicity studies and mapping the CYP3A4 interactions of
these new compounds, is currently in progress in our laboratories.
l
of activity within the lycorine series of compounds (Table 1) high-
lights several core features of this fascinating pharmacophore. An
intact pyrrolo-phenanthridine nucleus appears to be necessary
for high potency. Recent work identified semi-synthetic N-alkyl-
ated ring D seco-lycorine analogs, possessing a planar dihydrophe-
nanthridine nucleus, as inhibitors of AChE, however the most
potent of these exhibited an IC50 value of 2.66 l
M.18 This is in stark
contrast to the natural phenanthridine variants pancratistatin 30
and narciclasine 31, the most potent anticancer agents within the
Amaryllidaceae,20 shown here to have no observable effect on
AChE. Aromatization of ring C is seen to potentiate AChE inhibition,
compared to the partially saturated lycorine 25 precursor, which
was demonstrated for natural, aromatized ring C lycorine variants
such as assoanine (IC50 3.87 l
M).8 This enhanced activity was
rationalized in terms of the planarity generated through aromati-
zation of ring C. Modification of ring A is seen to have a negligible
effect as both pseudolycorine 24 and lycorine 25 are inactive (at
Acknowledgments
We thank NSERC and McMaster University for financial support
of this work. We are grateful to Dr. D. Hughes for obtaining high
field NMR data.
10
lM) (Table 1). The dramatic spike in activity in going from lyc-
orine to analog 26 (Ki = 0.86
lM) suggests that the C2 area of the
molecule must occupy a bulky, lipophilic pocket within the active
site to accommodate the TBS group. This substitution contrasts
sharply with the inactive 2-acetyllycorine,9 suggesting that a small,
polar H-bond acceptor at C2 may impede inhibition. Acylation at
C1 had a pronounced effect on inhibition (Table 1), affording com-
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.06.130.
pounds 27 (Ki = 0.34 lM) and 35 (Ki = 0.39 lM), which are now ad-
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