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10 Carbohydrates and Carbohydrate-Based Therapeutics in Alzheimer’s Disease
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326
Scheme 10.12 Synthesis of the N9 and N7 nucleosides 92 and 93, respectively. Reagents/
solvent and yield: (a) BSA, 2-acetamido-6-chloropurine, TMSOTf, (CH
Cl)2, 85 °C, 25% for 92
2
and 37% for 93. Source: Adapted from Marcelo etal.[88].
inhibitors, namely donepezil, rivastigmine, and galantamine, which help in control­ling the symptoms of AD and do not treat the underlying disease or delay its progres­sion[87]. In this perspective, Rauter and coworkers explored purine nucleosides as ChE inhibitors. Marcelo etal.[88], developed the first synthesis of 2-acetamidopurine nucleosides 92 and 93 starting from compound 91 incorporating a tetrahydrofuran ring fused to the pyranose (Scheme10.12). The bicyclic sugar 94 was also modified to generate the elongated carboxylic ester appendage bearing the azido group at position 6 (97a,b, Scheme 10.13). For the purpose, Swern oxidation followed by a Grignard reaction afforded the allyl alcohols 95a,b. Ozonolysis followed by sodium chlorite oxi- dation in tBuOH/H
O/2-methylbut-2-ene (2:2:1) afforded the corresponding car-
2
boxylic acids, which were esterified to give the benzyl esters 96a,b. Triflation and
Scheme 10.13 Synthesis of the (6′S)- and (6′R)-configurated N7 nucleosides 98a and 98b. Reagents/solvent and yield: (a) 1. (COCl) steps; (b) 1. O (2:2:1); 3. BnBr, KHCO
NaN
, DMF, 72% for 97a, 86% for 97b both over two steps; (d) BSA, 2-acetamido-6-
3
chloropurine, TMSOTf, CH
, DMS, DCM; 2. NaClO2, NaH2PO4.H2O, tBuOH/H2O/2-methylbut-2-ene
3
, Bu4NI, DMF, 68% over three steps; (c) 1. Tf2O, pyridine, DCM; 2.
3
CN, 60% for 98a and 55% for 98b. Source: Adapted from Marcelo
3
, DMSO, Et3N; 2. CH2=CHMgBr, THF, 65% over two
2
etal.[88].
10.5 Cholinesterase Inhibitors
nucleophilic substitution with sodium azide afforded the diastereoisomers 97a and 97b in high yield. With these two bicyclic intermediates, coupling with silylated
9
2-acetamido-6-chloropurine catalyzed by TMSOTf afforded a mixture of N
- and N7-
linked (98a and 98b) nucleosides, which were screened for AChE and BChE inhibi-
7
tion. While none of the compounds tested inhibited AChE, the N
nucleosides showed
potent inhibition toward BChE (Table10.7). Nanomolar inhibition was obtained for
Table10.7  Inhibition (%) of BChE for different concentrations of the compounds tested and IC
values.
50
327
Compound nr Concentration (μg ml−1) BChE inhibition (%) BChE IC50± SEM
92 100.00 48*** -----
10.00 11
93 100.00 84*** 0.76 ± 0.05
10.00 85***
1.00 70***
0.10 20***
97a 100.00 91*** 4.20 ± 0.05
10.00 64***
1.00 36***
0.10 15*
97b 100.00 98*** 13.40 ± 0.80
10.00 73***
1.00 14**
98a 100.00 81*** 22.00 ± 1.60
10.00 34***
1.00 9
98b 100 91*** 0.14 ± 0.01
10 79***
1 77***
0.1 63***
0.01 10
Rivastigmine
b
100 100*** 0.17 ± 0.01
10 100***
1 100***
0.1 76***
0.01 NI
a
a) IC50 compound concentration inhibiting 50% of enzyme activity; values are expressed as mean ± standard error of the mean (SEM). b) Rivastigmine is a drug to treat AD patients and was used as positive control. ***P< 0.001, **P< 0.01, *P< 0.05, NI: no inhibition. Source: Adapted from Marcelo etal.[88].
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328
Scheme 10.14 Synthesis of the N9 and N7 nucleosides 99 to 122 as reported by Schwarz
etal.[89] Reagents/solvent and yield: (a) BSA, 6-chloropurine (CP) or 2-acetamido-6­chloropurine (ACP), TMSOTf, CH R
=βN7CP, 20%; 100: d-Glc, R1=Ac, R2=βN9CP, 41%; 101: d-Glc, R1=Bn, R2=αN7CP, 6%; 102:
2
d-Glc, R
=Bn, R2=βN7CP, 17%; 103: d-Glc, R1=Bn, R2=αN9CP, 10%; 104: d-Glc, R1=Bn, R2=βN9C P,
1
45%; 105: R
2
110:
R
2
d-Gal, R
R
2
120:
R
1
d-Gal, R
=Bn, R2=β-N7CP, 12%; 106: d-Gal, R1=Bn, R2=αN9CP, 7%; 107: d-Gal, R1=Bn,
1
=β-N9CP, 43%; 108: d-Man, R1=Bn, R2=αN7CP, 17%; 109: d-Man, R1=Bn, R2=βN7CP, 14%;
d-Man, R
=Bn, R2=αN9CP, 27%; 111: d-Man, R1=Bn, R2=βN9CP, 10%; 112: d-Glc, R1=Bn,
1
=αN7ACP, 7%; 113: d-Glc, R1=Bn, R2=βN7ACP, 34%; 114: d-Glc, R1=Bn, R2=βN9ACP, 29%; 115:
=Bn, R2=αN7ACP, 3%; 116: d-Gal, R1=Bn, R2=βN7ACP, 44%; 117: d-Gal, R1=Bn,
1
=αN9ACP, 10%; 118: d-Gal, R1=Bn, R2=βN9ACP, 41%; 119: d-Man, R1=Bn, R2=αN7ACP, 8%;
d-Man, R
=Bn, R2=βN7ACP, 28%; 121: d-Man, R1=Bn, R2=αN9ACP, 24%; 122: d-Man,
1
=Bn, R2=βN9ACP, 3%. Source: Adapted from Schwarz etal.[89].
CN, microwave irradiation (150 W); 99: d-Glc, R1=A c ,
3
compound 98b competing well with rivastigmine, a drug currently in use for the treat­ment of AD[88]. Experimental results showed that the presence of benzyl groups on
7
the carbohydrate scaffold and the N
-linked purine nucleobase were required for the strong BChE inhibition[84]. The preliminary evaluation of the acute cytotoxicity of the elongated bicyclic sugar precursors and nucleosides was also performed indicat­ing low values, in the same order of magnitude as those of rivastigmine[88].
In 2014, Rauter and coworkers[89] presented a novel microwave-assisted synthesis and anticholinesterase activity of a series of 24new purine nucleosides incorporating 6-chloropurine or 2-acetamido-6-chloropurine linked to -glucosyl, -galactosyl and -mannosyl residues, with very interesting results compared to those previously obtained by Marcelo etal.[88]. Optimization of the reaction conditions adapted to formation of the new nucleoside structures was achieved by the use of microwave irradiation (150 W, 65 °C), reducing the reaction time from 2 hours to 15 minutes (Scheme10.14)[89]. Compound structure was designed aiming at ChE (AChE and BChE) inhibition efficiency and selectivity by investigating sugar stereochemistry,
7
purine structure, and N
or N9-linked purine ligation to the sugar (Table10.8). The outcome of this work showed that α-anomers were the most active compounds. Regarding BChE inhibition, the nucleosides 110, 112, 119, and 120 were noticeably more potent than the drug galantamine, and the most promising competitive and
7
selective BChE inhibitor, the N showed a K
of 50 nM and a selectivity factor of 340-fold for BChE over AChE[89]. To
i
the best of our knowledge, compound 119 shows the lowest K
-linked 2-acetamido-α--mannosylpurine 119,
for BChE, being one of
i
the best candidates toward the study of BChE selective inhibition against AD, when compared to other newly reported carbohydrate-based[90, 91] and noncarbohydrate­based compounds[82]. Interestingly, selectivity over AChE or BChE can be tuned by
10.5 Cholinesterase Inhibitors
Table10.8  Inhibitory constant Ki for nucleosides 99–123 as determined by Elman's assay with BChE and AChE in comparison to galantamine hydrobromide.
329
Compound
BChE Ki (μM)
AChE Ki (μM) Compound BChE Ki (μM)
AChE Ki (μM)
99 β-Glc-N7CP >100 28.9 ± 2.1 111 β-Man-N9CP >20 >20 100 β-Glc-N 101 α-Glc-N 102 β-Glc-N 103 α-Glc-N 104 β-Glc-N 105 β-Gal-N 106 α-Gal-N 107 β-Glc-N 108 α-Man-N 109 β-Man-N 110 α-Man-N
Galantamine hydrobromide
Source: Adapted from Schwarz etal.[89].
9
CP >100 17.5 ± 2.6 112 α-Glc-N7ACP 2.5 ± 0.3 >10
7
CP >20 9.6 ± 2.3 113 β-Glc-N7ACP >100 23.0 ± 1.9
7
CP >2 >2 114 β-Glc-N9ACP >2 >2
9
CP >100 17.0 ± 2.0 115 α-Gal-N7ACP 50.0 ± 7.0 42.0 ± 9.2
9
CP >100 16.0 ± 2.7 116 β-Gal-N7ACP >20 >20
7
CP >20 >20 117 α-Gal-N9ACP >2 >2
9
CP >10 5.6 ± 0.8 118 β-Gal-N9ACP >2 >2
9
CP >100 25.6 ± 2.1 119 α-Man-N7ACP 0.05 ± 0.01 18.1 ± 4.8
7
CP 30.9 ± 2.2 23.4 ± 3.6 120 β-Man-N7ACP 1.4 ± 0.1 3.0 ± 0.3
7
CP 9.6 ± 0.7 28.5 ± 6.5 121 α-Man-N9ACP 10.3 ± 2.6 17.1 ± 2.5
9
CP 2.8 ± 0.3 2.4 ± 0.3 122 β-Man-N9ACP >10 >10
9.4 ± 0.7 0.5 ± 0.0 Galantamine
9.4 ± 0.7 0.5 ± 0.0
hydrobromide
structural features. Indeed, glucosyl and galactosyl linkage to chloropurine favor the
7
formation of selective AChE inhibitors, while α-glucosyl and α-mannosyl N
ligation
to 2-acetamido-6-chloropurine favors selective BChE inhibitors.
Noteworthy, in 2021, Jasiecki etal. [92] revised and discussed the relationship between BChE levels and iron concentration in the brain, highlighting that produc­tion of BChE by glial cells is iron-dependent. Moreover, AD patients exhibit higher iron brain levels than healthy individuals, which contributes to the overexpression of BChE and subsequent decrease in ACh levels[89].
Several reports indicate that increased Aβ plaque formation is observed when Aβ is expressed in a context of increased AChE levels[93–95], with the neurotoxicity induced by AChE-Aβ complexes being greater than when promoted by Aβ alone[96]. Regarding BChE and Aβ interactions, however, much remains unclear. Although BChE is found in amyloid-β plaques and NFTs, its role in the amyloid hypothesis of AD is yet not fully understood. Some studies imply that BChE has a catalytic role in Aβ aggregation, while others point to a different scenario, where BChE is actually able to inhibit Aβ aggregation[97, 98]. In 2012, the results of a study published by Darvesh etal. suggested that BChE is associated with a subpopulation of Aβ depos­its and could take part in AD plaque maturation[99]. On the other hand, in 2016, Kumar etal. reported that low levels of BChE (0.02–0.004 μM–cerebrospinal fluid concentrations in AD patients) increased the formation of Aβ42 fibrils by 20–30%; yet, higher concentrations (0.2 μM) of BChE attenuated the kinetics of amyloid-β fibrillization process [100]. Furthermore, when present simultaneously with
10 Carbohydrates and Carbohydrate-Based Therapeutics in Alzheimer’s Disease
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330
apolipoprotein E4 (ApoE4), highly stable and soluble complexes between Aβ and BChE are formed (BAβACs), leading to a reduction in Aβ amyloid aggregation[100]. Also, it seems that Aβ interacts with a presumed activation site in the catalytic tun­nel of BChE, which results in an increased ACh influx to the catalytic site, with a consequent increase in the rate of ACh hydrolysis[100]. Altogether, these data sug­gest that further studies are required, aiming to clarify the role of BChE in Aβ aggre­gation processes.
10.6 Fyn Kinase Inhibitors
Fyn kinase, a member of the Sec family of tyrosine kinases, is increasingly recog­nized as a promising disease-modifying therapeutic target against AD[11]. Over the past decade, evidence has shown that soluble amyloid β oligomers (Aβo) bind to the
C
at the neuronal surface of the postsynaptic neuron, resulting in the activation
PrP of the metabotropic glutamate receptor 5 (mGluR5), with subsequent Fyn kinase phosphorylation, followed by phosphorylation of downstream substrates, including the tau protein. Chronic exposure to soluble Aβo hence contributes to the accumu­lation of hyperphosphorylated Tau, which composes the NFTs responsible for syn­aptic dysfunction in the brain of AD patients. What’s more, Fyn kinase can also phosphorylate tau directly. By taking part in this signaling pathway, Fyn arises as a unique linker of the two major hallmarks of AD pathology[11] (Figure10.8).
APP is sequentially cleaved by BACE-1, and γ-gamma secretase, to produce Aβ monomers, including Aβ1–40 and Aβ1–42, at the surface of the postsynaptic neu­ronal membrane. These monomers aggregate into oligomers, which in turn bind to
C
PrP
APP
BACE-1
γ-Secretase
Aβ Monomers
Phosphorylation of
downstream signalling
molecules
P
P
P
P
P
P
P
P
P
P
P
P
hyperphosphorylated
Tau
Aβo
NMDA
NR2B
P
2+
Ca
Synaptic dysfunction
Fyn
mGluR5
Postsynaptic neuron - CYTOPLASM
P
EXTRACELLULAR
Figure10.8  Simplified illustration of Aβo-induced Fyn activation leading to Tau hyperphosphorylation.
MEDIUM
10.6 Fyn Kinase Inhibitors
their high-affinity partner, the PrPC. Once in complex with Aβo, PrPC interacts phys­ically with the mGluR5 and, in the cytoplasm, Fyn kinase binds to mGluR5 and is phosphorylated. This activated complex then results in the phosphorylation of the NR2B subunit of the N-methyl--aspartate receptor (NMDA), which then leads to a calcium influx that triggers the phosphorylation of downstream signaling mole­cules. With chronic exposure to Aβo at the neuronal surface, this cascade of events ultimately culminates in Tau hyperphosphorylation, which is responsible for synap­tic dysfunction and cognitive decline in AD patients.
Not many compounds have been identified to this point as Fyn kinase inhibitors with proven concomitant downstream effects. In 2015, Kaufman etal.[101] investi­gated the brain-permeable Src family kinase inhibitor AZD0530, also known as saracatinib (123, Scheme10.15), as a compound with potential against AD.
Saracatinib is a non-glycosylated C-5-substituted anilinoquinazoline and was originally discovered as an anticancer agent by Hennequin etal.[103] in 2006, along with a series of analogs investigated in structure–activity relationship (SAR) studies.
1
Biological assays showed that, when given at 5 mg kg
per day, this Fyn kinase inhibitor prevented Aβo-induced signaling, Tau phosphorylation, and deposition, while being able to rescue special memory and synaptic depletion in AD transgenic mice[98]. In another study by Tang etal.[104], later in 2020, the same dose of sara­catinib promoted similar improvements in phosphorylated Tau accumulation in the
331
Scheme 10.15 Synthesis of saracatinib (123)[102]. Reagents/solvent and yield: (a)
formamidine acetate, 2-methoxyethanol, DIPEA, then IPA (52%); (b) BnOH, NaH, DMA; (c) HCl, 98%; (d) anhydrous toluene, POCl 92%; (g), Ph (j) distillation, NaOH, 89%; (k) 2-(4-methylpiperazin-1-yl)ethanol, Ph IPA, 86%.
P, DTAD, tetrahydropyran-4-ol, toluene; (h) HCl, dioxane, then IPA, 80%; (i) TFA;
3
, DIPEA; (e) anhydrous toluene, 69%; (f) pyridine, HCl,
3
P, DTAD, THF, then
3
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332
hippocampus, accompanied by memory improvements in mice with transgenic and traumatic tauopathy.
The original 100
mg-scale synthesis of saracatinib[103] included 17linear steps, 7 of which were protection/deprotection reactions. With some practical concerns raised for a potential scale-up synthesis, four years later, an optimized route for the kilogram manufacture of saracatinib was developed [102]. This route started from the difluoro quinazolinone 125, accessed in 52% yield from ester 124 by using forma- midine acetate in 2-methoxyethanol (Scheme10.15), followed by addition of isopro­panol and crystallization. Then, after a double S
Ar reaction to allow the replacement
N
of both fluorine groups with benzyloxy moieties by reaction with benzyl alcohol in the presence of sodium hydride and dimethylacetamide (DMA), intermediate 126 was chlorinated in the presence of phosphorous oxychloride and DIPEA but 127 was not isolated and reacted with 128 to give anilinoquinazoline 129, which was obtained as a hydrochloride salt in 69% yield. Selective debenzylation gave the 5-hydroxy inter­mediate 130 in 92% yield. Compound 131 was then accessed as a hydrochloride salt in 80% yield by reaction with tetrahydropyran-4-ol under Mitsunobu conditions, with subsequent addition of hydrochloric acid in 1,4-dioxane, followed by crystalli­zation. After deprotection of the remaining benzyl group, followed by distillation to dryness and neutralization with sodium hydroxide, the final Mitsunobu coupling of intermediate 132 with 2-(4-methylpiperazin-1-yl)ethanol gave saracatinib 123 which, after crystallization, was obtained as a difumarate in 86% yield.
To the best of our knowledge, saracatinib (123) was the first Fyn kinase inhibitor with proven downstream effects in an AD model and with adequate pharmacoki­netic properties. However, the very first sugar-based Fyn kinase inhibitors were not published until 2020[103]. Indeed, Rauter and coworkers identified a new family of nature-inspired glucosylpolyphenols (121–124, Figure10.9) that were shown to dis­tinctively interfere with the Aβo-Fyn-Tau neuronal signaling cascade, with the advantage of having a much easier, straightforward, and efficient synthesis com­pared to the route described for saracatinib.
Synthesis of the three per-O-methylated C-glucosyl polyphenols 133–135 was conducted by Matos et al.[105] in only two reaction steps, in good to very good overall yield (Scheme10.16). Briefly, after full methylation of the commercially available methyl α--glucopyranoside (137) with sodium hydride and methyl iodide, the TMSOTf-promoted C-glucosylation reaction of each polyphenol in the presence of drierite gave the desired final compound. This final reaction step afforded com­pound 133 in 53% yield, compound 134 in 37% yield, and compound 135 in 45% yield.
Synthesis of compound 136 required a few additional reaction steps (Scheme10.17) but is still simpler than that of saracatinib (123). Firstly, the C-glycosylation reaction carried out as previously described afforded the intermediate 139 in 63% yield. It is interesting to note that this reaction occurred via a Friedel–Crafts-type mechanism, being the first exception to the Fries-type rearrangement ever reported in the C-glycosylation of unprotected phenols. Compound 139 was then submitted to a benzoylation reaction with benzoyl chloride, imidazole, and dichloromethane as a solvent to afford its derivative 140 in 88% yield, which was deprotected to give the
MeO
OMe
MeO
OMe
MeO
135
136
OMe
OMe
10.6 Fyn Kinase Inhibitors
333
MeO
MeO
HO
O
H
H
OH
133
OMe
OH
OHHO
OMe
O
OH
O
MeO
HO
HO
HO
O
H
OH
134
OH
OH
O
H
O
O
O
O
Figure10.9  Structure of Fyn kinase inhibitor 134 and of the inhibitors of Aβo induced Fyn activation 133, 135, and 136.
desired compound 136 in 8% yield. The low yield of this final step was due to the hydrolysis of the ester groups and could potentially be further optimized.
In terms of their bioactivity, the per-O-methylated C-glucosyl phloroglucinol 133 and the per-O-methylated C-glucosyl hydroquinone 134 (Figure10.9) were found to significantly inhibit Fyn kinase activity at 10 μM (p< 0.01 vs. untreated controls) in hiPSC-derived neural progenitor cells. On the other hand, compounds 133, 135, and 136 (Figure 10.9) were able to inhibit Aβo-induced Fyn activation at the same
–
Scheme 10.16 Synthesis of per-O-methylated C-glucosyl polyphenols 133–135[105]]. Reagents/solvent and yield: (a) DMF, NaH, MeI, 91%; (b) dry CH
CN, hydroquinone,
3
phloroglucinol or acetophloroglucinol, drierite, TMSOTf. Source: Adapted from Matos etal.[105].
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334
Scheme 10.17 Synthesis of per-O-methylated C-glucosyl polyphenols[103]. Reagents/
solvent and yield: (a) DMF, NaH, MeI, 91%; (b) dry CH imidazole, BzCl, 88%; (d) DCM, BBr
.
SMe2, 8%.
3
CN, drierite, TMSOTf, 63%; (c) DCM,
3
concentration (p < 0.001 vs. untreated Aβo controls). Importantly, all four com­pounds (133–136) reduced Aβo-induced Tau hyperphosphorylation (at 10 levels below controls, which once more supports Fyn kinase as a therapeutic tar­get for AD.
Even though other analog molecules also exhibited bioactivity in the SAR study conducted by the authors, this group of glycoconjugates displayed the adequate physicochemical features (optimal log D and effective permeability in a PAMPA assay) without any relevant cytotoxic effects at 50
μM[103], which makes them can-
didates for further development as CNS-active therapeutic agents.
μM) to
10.7 Amyloid Protein–Protein Interaction Inhibitors
As previously discussed, the interactions between Aβo and its high-affinity partner
C
at the neuronal cell surface are crucial in triggering Fyn activation and subse-
PrP quent events leading to tau hyperphosphorylation in AD (Figure10.10). Hence, when thinking of protein–protein interaction inhibition strategies aiming to directly disrupt the involvement of Aβ in the development of this signaling cascade, two main approaches arise: (i) tackling the aggregation process that turns Aβ monomers (Aβm) into Aβo and (ii) inhibiting the binding between Aβo and the PrP (Figure10.10).
In the context of approach (i), a group of flavonoids was investigated for their abil­ity to inhibit Aβ aggregation[106]. The authors highlighted the ability of catechol­type flavonoids, such as the natural compound quercetin (141, Figure 10.11), to completely abolish Aβ aggregation by acting as Michael acceptors once oxidized into the corresponding o-quinones. This process is characteristic of Pan Assay
C
10.7 Amyloid Protein–Protein Interaction Inhibitors
HO
143
144
OH
335
(i)
(iI)
Aβ
Aβ
m
o
Aβ
o
Aβo + PrP
c
Figure10.10  Two major processes to be targeted by amyloid protein–protein interaction inhibitors in the context of AD.
Interference Compounds (PAINS) with the catechol motif, which are able to indis­criminately interfere with many other molecular targets. Hence, highly promiscu­ous compounds such as quercetin (141) should not be regarded as ideal candidates for further optimization. Yet, they ultimately uncovered the potential of chrysin (142, Figure10.11), another natural compound, to act as an anti-aggregation com­pound. Even though ThT fluorescence assays did not reveal a significant change in the presence of amyloid aggregates by the effects of this compound (tested in con­centrations from 0.5 to 100 μM), Atomic Force Microscopy (AFM) experiments showed that, compared with structurally similar compounds, chrysin preferably
OH
OH O
142
OH
O
OH O
O
OH
N
O
N
HO
OH O
141
OH O
O
OH
N
O
N
HO
HO
HO HO
H
Figure10.11  Amyloid anti-aggregation agents (141, 142) and Aβo-PrPC disrupting agents (143, 144).