AD has been in the line of fire with research on disease-modifying treatments for years. In 2020, at least 121 agents have been under evaluation in clinical trials for AD and many others are under investigation in preclinical studies [33]. Over the last few years, an increasing number of drug candidates against non-amyloid targets, such as anti-tau or anti-inflammatory drugs or compounds targeting synapses, vascular factors and neurogenesis have been proposed. However, until now all these approaches have failed to efficiently halt disease onset and progression. Even the most advanced and promising strategy based on the use of monoclonal antibodies designed to bind to β-amyloid were inefficient in providing the intended clinical benefits on cognitive and functional measures in AD patients. Recently, one anti-amyloid antibody (aducanumab) was approved under the accelerated approval pathway by FDA as new disease-modifying compound in AD. However, there is still uncertainty about the real efficacy and safety of aducanumab, even taking into account the previous attempts with other monoclonal antibodies, and both academic institutions and pharmaceutical companies are actively in search of innovative treatments [34, 35]. The reasons for the glaring failure of the therapeutic strategies against AD are most likely multiple and include, among others, (i) the rationale of the therapeutic approaches - almost exclusively based on theoretical assumptions or in vitro evidence [36, 37] – (ii) the timing of therapeutic intervention - which should be very early along the natural history of the disease – (iii) the phenotypic variability of AD [38] – that deeply affects and diversifies the responsiveness to treatments – and (iv) the complexity of disease pathogenesis suggesting that more than one target should be locked by therapeutic strategies to be successful in tackling the illness [7, 39, 40].
In the recent past, genetic studies have discovered rare mutations with putative protective effects against AD providing an exciting background for the development of novel investigational compounds for the treatment of the disease [18, 20, 41–43]. In this context, a bioinspired approach based on a genetic variant already existing in nature and harboring multiple protective effects against the disease may offer a solid basis for the development of a successful therapeutic strategy. Following this approach, we have generated a novel compound that promises to replicate the natural protection occurring in the human heterozygous carriers of the AβA2V variant [12, 14, 44–46].
The treatment of the APPSwe/PS1dE9 transgenic mouse model of AD with the Aβ1-6A2V(D) through the intranasal route allowed us to avoid the counteracting effects of the TAT carrier[19]. Indeed, unlike Aβ1-6A2V-TAT(D), Aβ1-6A2V(D) alone did not alter APP processing and retained the ability to reduce Aβ oligomers and amyloid burden, and preserve synaptic integrity even in a long-term treatment schedule.
Concerning the effects of Aβ1-6A2V(D) on synapses, extensive data from scientific literature support the relevance of AMPA receptors in the synaptopathy associated with AD [47]. Indeed, high concentrations of soluble oligomeric Aβ cause endocytosis and removal of AMPA receptors [48]. Further evidence from in vivo studies indicate that the disruption of PSD-95 - a postsynaptic scaffold protein of excitatory synapses that binds to NMDA and AMPA receptors - is associated with cognitive and learning deficits [49]. Reduced expression of PSD-95 is a recurrent feature in brain tissue from AD subjects and murine models of AD [50]. It is also known that Aβ1−42 inhibits synaptic plasticity [51] by enhancing endocytosis of NMDARs with consequent reduction of NMDARs expression at the postsynaptic level [52]. NR2A and NR2B levels are indeed decreased in susceptible regions of the human AD brain, such as the hippocampus and the cortex [53]. Overall, these findings reflect the disruptive actions of soluble Aβ on synaptic plasticity in AD, provide keys to interpreting the effects of the Aβ1-6A2V(D) – based strategy on AD-related synaptopathy and support the use of PSD-95, AMPA and NMDA receptor levels as indicators of efficacy for therapeutic strategies against AD aimed at preserving synaptic integrity [54].
In our opinion, these data provide a solid rationale for the use of Aβ1-6A2V(D) in preventing amyloidogenesis and its deleterious effects on synaptic function and cognition in AD. The approach based on D-peptides is very promising because of their characteristics including high resistance to protease digestion, stability, and bioavailability [37], which make them optimal molecular prototypes for the development of drugs for the treatment of neurological disorders [55]. Actually, Aβ1-6A2V(D) may be included in the class of ‘amyloid β-targeted peptide inhibitors’, which have special properties, including high selectivity, low accumulation in tissues, low side-effects and toxicity, and different chemical and biological synthesis routes when compared with other compounds used for therapeutic purposes [36, 56, 57].
On the other hand, the intranasal administration route is increasingly being used as a noninvasive method to bypass the BBB for drug delivery of therapeutics in a number of neurological diseases, including AD [58, 59]. It is known that small fractions of nasally applied macromolecules may reach the brain directly via olfactory and trigeminal nerve components of the nasal mucosa or by bulk flow and diffusion within perineuronal channels, perivascular spaces, or lymphatic channels directly connected to brain tissue [60, 61] and appear to rapidly distribute within the brains of rodents and primates. In our study, intranasal delivery of Aβ1-6A2V(D) resulted in high concentration and good distribution of the six-mer peptide in brain tissue of APPSwe/PS1dE9 mice, providing grounds for a highly compliant treatment for AD in humans, similar to other therapeutics already used in clinical practice [62, 63].
In a nutshell, the AβA2V-based strategy has at least two main advantages compared to the previous therapeutic approaches for AD [64–67]. First, it stems from a “protective” model already existing in nature: APP-A673V heterozygous carriers which are protected from AD occurrence. Second, it engages combined mechanisms of action and results in multiple allied effects on AD pathogenesis, involving inhibition of oligomer generation, fibril formation and amyloid accumulation, and interference with Aβ-dependent neurotoxicity and synaptic dysfunction that may delay cognitive impairment in animal models.
Moreover, the use of Aβ1-6A2V (D) promises to be less expensive than other pharmacological treatments for AD - such as monoclonal antibodies - and guarantee high compliance for AD patients if the treatment can be performed through the intranasal route.
Most importantly, the timing of the treatment with Aβ1-6A2V(D) - which in our preclinical study on APPSwe/PS1dE9 mice started in an early phase of their disease, when the first amyloid deposits appear in the brain - suggests that the bio-inspired AβA2V-based strategy may be either a preventive or a curative approach to AD. Further studies are needed to set the most efficient treatment schedule and to exclude potential side effects of Aβ1-6A2V(D) that in any case have not been observed in mice.
In conclusion, this study stems from the discovery of a protective genetic variant of β-amyloid offering grounds for the development of AD therapeutics. This bio-inspired approach provides, in our opinion, a novel compound for the prevention and/or cure of AD and, in a more general context, opens the way for innovative therapeutic strategies based on the identification of naturally occurring genetic variants having protective effects in humans [12, 18, 19, 68–71]. Such strategies should be aimed at replicating in AD patients the condition occurring in the human carriers who are protected against the disease to prevent or halt the progression of AD.