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M. Prinz et al. / European Journal of Pharmaceutical Sciences 49 (2013) 603–613
regarding the amyloid precursor protein (APP) or the presenilins,
resulting in increased Aß levels. The second form is the sporadic
form affecting older people. It is followed by the normal aging pro-
cess (Graeber et al., 1997; Hardy and Selkoe, 2002).
tein is the glycogen-synthase kinase (GSK) 3b an inhibition
of GSK-3b would slow down the phosphorylation (Berg
et al., 2012; Dermaut et al., 2005; Hanger et al., 1992).
(g) Another possibility would be an immunotherapy. Although
the concentration of immunoglobuline G (IgG) in brain is
only 0.1% of the concentration in plasma IgG can serve as a
target: The antibody–antigen-binding can provoke phagocy-
tosis (Bard et al., 2000) or the antibody can bind to Ab fibrils
and convert them into monomers again (Solomon et al.,
1996). There is the feasibility of an active immunization (vac-
cination) by peptides containing B- and T-cell epitopes or a
passive immunization by monoclonal antibodies (Morgan,
2011).
Today’s medication for AD is based on acetylcholinesterase
inhibitors (AChEIs) like donepezil, rivastigmine, and galantamine,
as well as the N-methyl-D-aspartate-receptor (NMDA) antagonist
memantine. The first ones address the loss of cholinergic activity
in an AD brain, which can be compensated by AChEIs, but the ex-
pected anti-AD effect is small. The AChEIs can only be used to treat
the symptoms of moderate AD. Memantine slows down the neuro-
degeneration, but cannot cure the disease (Massoud and Gauthier,
2010). All compounds can be regarded as disease-modifying anti-
Alzheimer drugs only. As these ‘‘old’’ drugs do not show the desired
effect, new drugs have to be developed. A large variety of possible
targets is currently being discussed:
(h) Amyloid plaques are the second main characteristic of AD.
They are built of amyloid b, generated from the amyloid pre-
cursor protein (APP). APP can be cleaved in two ways, patho-
genically and non-pathogenically. In the non-pathogenic
(a) Based on the cholinergic hypothesis the concentration of ACh
is decreased in AD brain. There is, hence, an ongoing develop-
ment of AChEIs, selective allosteric agonists of the musca-
rinic M1 receptor, M2 selective antagonists, and
butyrylcholinesterase inhibitors (BuChEIs) (Decker and Hol-
zgrabe, 2012; Fisher, 2012; Massoud and Gauthier, 2010).
Whereas the AChE level decreases in AD to 15%, the BuChE
level increases. Since the ratio of BuChE/AChE changes from
0.2 in normal brain to 11 in AD brain (Greig et al., 2005)
the BuChEIs are important too.
way,
so that the toxic protein is eliminated. In the pathogenic
way, b-secretase cleaves APP and in a second step, -secre-
tase releases the amyloid b peptide, which forms first oligo-
mers, then protofibrils and finally the non-soluble plaques.
Possibilities of intervention might be the inhibition of b-
a-secretase cleaves APP within the amyloid b domain,
c
secretase, the activation of
a-secretase, or the inhibition of
amyloid b assembly (Campagna et al., 2011; Ghosh et al.,
2012; Nitsch et al., 1997; Postina, 2012; Scarpini et al.,
2003; Tjernberg et al., 1999; Wolfe, 2012). However, none
of these approaches has so far succeeded in clinical trials.
(i) Due to the multifactorial character of the disease, the thera-
peutic tools should be of similar complexity, resulting in a
‘‘multiple target drug’’ as demonstrated for ladostigil and
memoquin (Cavalli and Bolognesi, 2012; Cavalli et al.,
2008) and many other drugs (Brunhofer et al., 2012; Catto
et al., 2012; Fernández-Bachiller et al., 2012; Wang et al.,
2012).
(b) According to the genetic hypothesis, AD patients have an
increased probability of the e4 allel of apolipoprotein E. This
could be a risk factor, and researchers try to reduce the toxic
effects of APOE4 (apolipoprotein) and to increase the APOE3
expression (Fagan, 2002; Huang and Mucke, 2012; Mahley
et al., 2006).
(c) The metabolic hypothesis is based on the correlation of
increased levels of APOE4 and cholesterol, as observed in
AD brain. The use of statins to decrease the level of choles-
terol in brain is difficult, as the therapeutic index for neuro-
protection is very small (Fonseca et al., 2010).
Recently, bisquarternary type-A compounds (see Scheme 1A)
were found to be good AChEIs but they do not ideally fit into the
catalytic gorge of AChE due to their length. As ditopic inhibitors
they interact with both the active and the peripheral binding site
(Alptuzun et al., 2010; Aydin et al., 2010). Systematic shortening
of these compounds resulted in a series of compounds (see
Scheme 1B) inhibiting the AChE in a high nanomolar range of con-
centration. Additionally, they are able to inhibit the formation of
the amyloid b fibrils (Alptuzun et al., 2003). Since this class of com-
pounds is permanently positively charged, it might be difficult to
cross the blood–brain barrier (BBB), especially if the compound
cannot make use of an active transporter. Therefore, the pyridine
ring was replaced by differently connected piperidine rings, (see
Scheme 1C and D) resulting in a lower inhibitory activity. In order
to regain activity, a conjugated system was introduced containing a
dihydropyridine moiety. Compound E can be either non-protonated
(transport form) to cross the BBB, or protonated for inhibition of the
AChE. Additionally, E has a broad conjugated system for interaction
(d) Anti-inflammatory hypothesis: Some studies showed
a
reduced prevalence of AD upon a long-term treatment with
non-steroidal anti-inflammatory drugs (NSAIDs) (Landi
et al., 2003; Vlad et al., 2008). However, COX-2 (cyclooxygen-
ase) inhibitors as well as the COX-1 selective inhibitor
naproxen did not show a positive effect (Imbimbo, 2009;
Landi et al., 2003; Vlad et al., 2008)
(e) Reactive oxygen species (ROS) were found to be involved in
AD: oxidative stress, generated by dysfunctional mitochon-
dria, comprising reduced mitochondrial membrane potential,
decreased ATP levels, impaired activity of complexes of the
respiratory chain, and deteriorated mitochondrial morphol-
ogy, is a very early event in AD. It appears before amyloid
plaques and neurofibrillary tangles are formed (Dumont
and Beal, 2011; Leuner et al., 2012a,b). ROS seem to contrib-
ute to the AD development. Thus, protecting an AD brain
from oxidative stress and improving mitochondrial function
might relieve the brain and reduce the pathogenic factors
(Benzi and Moretti, 1995; Christen, 2000; Leuner et al.,
2012a,b).
with amyloid b fibrils (see E) by
2010). The inhibition of AChE can easily be explained by the
AChE-inhibitor interactions found via molecular modeling: A
interaction of the benzyl substituent at the hydrazine site and
Trp84 (active site), a face-to-face interaction ( and cation–p,
p–p-stacking (Alptuzun et al.,
p–p
(f) The
tioned neurofibrillary tangles. These consist mainly of the
-protein, which stabilizes microtubules in healthy people.
s-protein hypothesis is related to the previously men-
p–
p
respectively) of the pyridinium ring with either Tyr334 or Phe332
of AChE, and a face-to-face interaction between the substituent of
the pyridine rest and Trp279 (peripheral side). The latter substitu-
ent can be a second pyridinium ring (Kapková et al., 2003) or an
aromatic moiety. Even though no compound was found with a per-
fect interaction of the phenyl ring and Trp278, the skeleton was
considered to be a new lead structure.
s
In the case of AD, the protein is hyperphosphorylated, the
structure of microtubules destroyed, and mitochondrial
function impaired (Schulz et al., 2012). In order to reduce
the tangle accumulation, the hyperphosphorylation has to
be reduced. As the main kinase phosphorylating the s-pro-
Prinz, Michaela
