Alzheimer’s disease (AD) is an alarming non-communicable, multi-factorial, and non-treatable
disease. Its underlying neurodegenerative events have not yet been fully explained
and its early diagnosis is very difficult. The appearance of the disease is associated
with clinical features such as the degeneration of several cholinergic nuclei of the
brain, causing lower levels of the neurotransmitter acetylcholine and the formation
of protein aggregates in the inter-synaptic space (amyloid plaques) or inside the
cells (neurofibrillary tangles, Brunetti et al., 2020).
Current therapies focus on enhancing the cholinergic system. The canonical drugs that
have been used over the years are cholinesterase inhibitors and memantine (N-methyl-d-aspartate
receptor blocker). The efficacy of these molecules concerns just the symptoms and
is limited to the first stages of the disease (Brunetti et al., 2020, 2022; Carocci
et al., 2022).
Considering the rising prevalence of this pathology and, consequently, its high economic
and social costs, large efforts are currently being made both by public institutions
and private companies for the development of novel disease-modifying therapeutics.
Some attempts in this direction have led to the study of monoclonal antibodies such
as Aducanumab, whose actual utility for AD therapy has not yet been unanimously demonstrated
(Carocci et al., 2022). Recently, other approaches have focused on joining well-established
therapeutic targets to the control of endocannabinoid and inflammatory pathways according
to new “metabolic” hypotheses, but none of the tested molecules seem suitable for
clinical trials (Brunetti et al., 2020, 2022; Carocci et al., 2022).
These findings highlight the lack of a single theory to explain the onset and progression
of AD. For this reason, researchers are increasingly focusing their attention on the
development of new drugs such as multi-target directed ligands (MTDLs), capable of
interacting with multiple therapeutic targets to fight both the symptoms (like currently
marketed drugs such as donepezil and rivastigmine do by inhibiting cholinesterases)
and the progression of the disease itself (Poliseno et al., 2021; Brunetti et al.,
2022a; Carocci et al., 2022).
In this context, hundreds of papers have been published over the last fifteen years
adapting an old but effective strategy: the three main drugs used in the symptomatic
therapy of AD, namely tacrine (now discontinued due to hepatotoxicity), rivastigmine,
and donepezil, have been repurposed as templates for the design of multifunctional
hybrids. The novelty in each work was the combination of different structural motifs,
through efficient synthetic strategies such as merging, fusing, or linking, thus endowing
the final MTDL with additional biological properties beyond cholinesterase inhibition
(Poliseno et al., 2021; Carocci et al., 2022; Brunetti et al., 2022a). The most common
mechanisms studied to define the etiology of AD have most frequently been used to
define biological targets for the design of MTDLs, namely the inhibition of aggregation
of amyloid proteins and neurofibrillary tangles, the inhibition of enzymes such as
monoaminoxidases and beta secretase and fatty acid amide hydrolase (Brunetti et al.,
2020, 2022a), but also the reduction of oxidative stress (Carocci et al., 2022), the
chelation of heavy metals such as iron, copper and zinc (Poliseno et al., 2021; Brunetti
et al., 2022a) and the modulation of glutamatergic, serotonergic, and dopaminergic
receptors (De Deurwaerdère et al., 2021). Recently, innovative pathways have also
been frequently included in test panels (Piemontese, 2019; Leuci et al., 2022).
An important source of inspiration for the design of hybrid drugs can be secondary
metabolites produced by plants or microorganisms (Leuci et al. 2021; Poliseno et al.,
2021). In particular, microfungi have historically provided numerous substances useful
for medicinal applications, especially as antibiotics (penicillins and in general
many beta-lactams, still widely used drugs, can be produced in fermenters using the
work of these microorganisms, Leuci et al., 2021). Moreover, recently, other fungal
metabolites showed their importance to the pharmaceutical industry, particularly for
their effects in the treatment of cardiovascular disorders. This is the case of monacolin
K, which is the active substance found in red yeast rice-based food supplements and
which can be defined as a “case study” from a legislative point of view (it is a real
drug and yet is included in food supplements, with different, rapidly evolving legislation
in many European countries, Leuci et al., 2021). However, the implications on consumer
perception are also remarkable, considering the widespread belief that “natural is
better” that leads patients to prefer functional foods (Piemontese et al., 2022) or
food supplements (Leuci et al., 2021) to standard drug formulations, which, in turn,
contain much cheaper and, probably, safer active ingredients equally obtained from
microorganisms (Leuci et al., 2021; Piemontese et al., 2022). In fact, these “more
natural” remedies can be contaminated with harmful chemical substances such as mycotoxins,
pesticide residues, or heavy metals (Leuci et al., 2021; Piemontese et al., 2022).
As far as neurodegenerative diseases are concerned, many other natural metabolites
might exert neuroprotective effects thanks to their pharmacological properties, including
anti-inflammatory and antioxidant activities (Leuci et al. 2021).
As an example, based on the known chemical structures, my research group recently
selected a series of secondary metabolites of natural origin with suitable characteristics
to evaluate their potential future role as chemical scaffolds in the design of anti-Alzheimer’s
agents with innovative profiles (Piemontese et al., 2018; Leuci et al., 2021). In
a preliminary in vitro screening, some of them showed interesting activities against
classic AD targets, chiefly acetylcholinesterase inhibition (IC50 = 6.86–86.0 μM),
while proving to have other interesting properties such as the ability to chelate
metals, e.g., zinc, copper, and iron, which are closely associated with oxidative
stress and to the formation of neurotoxic protein aggregates, such as amyloids (Piemontese
et al., 2018). Using as a template the structure of one of these scaffolds (tenuazonic
acid) and modifying the amino acid used in the reactions, we carried out in the recent
past a partial and preliminary structure-activity-relationship total synthesis study
(Poliseno et al., 2021) with good results in terms of acetylcholinesterase inhibition
(IC50 = 42–57 μM), antioxidant activity (EC50 = 6.3–10.8 μM), Aβ aggregation inhibition
(up to 63.8% at 40 μM), and metal chelation (pFe3+ = 16.6, pCu2+ = 10.6 and pZn2+
= 6.0). The so obtained compounds were then part of the design and preparation of
synthetic hybrids with potential multi-target anti-AD activity, with the aim of improving
anticholinesterase activity (IC50 = 16–24 μM), through the binding with a moiety inspired
by the donepezil structure (Poliseno et al., 2021).
Our next goal is even more ambitious.
Very often the preparation of molecules including nature-inspired moieties is hindered
by the presence of one or more stereocenters and therefore by the difficulty of chemical
and optical resolution of the reaction mixtures; tenuazonic acid itself has two chiral
carbons (Piemontese et al. 2018; Poliseno et al., 2021). So, why not use nature itself
to obtain these compounds? Fungi and bacteria are able, through their metabolism,
to be much more effective than enantio- or diastereo-selective syntheses!
However, a critical point for the synthesis of MTDLs is the availability of appropriate
heterocyclic nuclei which must bear functional groups suitable for derivatization,
such as a carboxylic acid or a primary amine. Very often, natural secondary metabolites
do not have these attachment points, or they are “masked” by the metabolic processes
of the microorganism, probably to prevent their potentially harmful reactivity. In
this context, the use of genomic techniques can be crucial: fungi, for example, could
be induced to produce different metabolites, perhaps intermediates, through the deletion
of some genes or by modifying ecophysiological growth factors (Cervini et al., 2020).
Furthermore, through chemical or biotechnological techniques, isolated secondary metabolites
could be modified to obtain structures that bear desirable functional groups useful
for subsequent derivatizations or, more simply, to confer new biological properties
(Ji et al., 2016).
The use of these secondary metabolites as therapeutics or as intermediates in semi-synthetic
chemical pathways still requires optimized strategies for their biosynthesis, as well
as for their extraction and purification. In many cases, it is not possible to use
liquid cultures of microfungi or it is more convenient to exploit specifically inoculated
food matrices. On the other hand, if the secondary metabolite is derived from plants,
it goes without saying that it is necessary to select the producer species, optimize
its biosynthetic capacity, and design a strategy for the isolation of suitable quantities
with sufficient purity. These processes may require a long time and high energy consumption
(for example high temperatures, which can also damage biologically active substances).
Furthermore, they often require the use of large quantities of chemical solvents.
To overcome this difficulty, more economical and environmentally friendly extraction
methods, which use ultrasound and microwaves, are increasingly being studied. Very
interesting advancements have also been made in recent years in the use of alternative
extraction media, such as deep eutectic solvents, at least for the extraction step
(Brunetti et al., 2022b).
These innovative solvents have recently also been used for the development of sustainable
organic synthesis. As an example, several moieties inspired by the structure of donepezil,
which were included in hybrid compounds previously developed in my research group,
have already been synthesized under green chemistry conditions (Piemontese et al.,
2020). This can certainly be a starting point for the optimization of green synthesis
of other moieties as well, which could be combined with natural compounds in MTDL
design approaches. The industrial scale-up of these intriguing synthetic techniques
is also an important breakthrough to be expected in the coming years.
These practices are strongly linked to the objectives of the so-called European Green
Deal, whose goal is the achievement of climate neutrality by 2050. It should be added
that the European Commission is particularly sensitive to the problem of Alzheimer’s
disease. In fact, AD is an integral part of the “Healthier Together - EU Non-Communicable
Diseases Initiative” program, promoted in June 2022, which describes numerous interventions
aimed at managing priorities from the point of view of public health and social problems
related to it (EU Commission, 2022).
The lack of a cure for AD underlies the entire program, and all related interventions
were planned around this fact. With great effort and large investments by public institutions
and private companies, the gloomy predictions about AD incidence and mortality could
be corrected for the better, improving the life expectancy and quality of life of
hundreds of millions of people. And perhaps nature, thanks to (maybe) the serendipity
and (for sure) ingenuity of thousands of researchers, could once again provide the
key to solving this long-standing problem, just as it did at the dawn of the history
of medicinal chemistry.