**3. New outlook for Alzheimer's disease**

#### **3.1 What are peptides?**

*Neuroprotection - New Approaches and Prospects*

Alpha secretase as a therapeutic target for AD offers a novel approach of upregu-

Targeting BACE1 for therapeutic development in AD is ideal, as it is the determining step in the generation of Aβ fragments. Inhibition of BACE1 has shown to decrease levels of Aβ plaques. Studies in mouse models have proven that by removing BACE1 there is no generation of Aβ fragments, and subsequently no neurode-

Since it was discovered to play a role in AD in 1999, BACE1 has been thoroughly researched as a potential target for AD. BACE1 has been an elusive target for inhibitors, its location in the brain, size of the active site, and similarity to other aspartic proteases making it difficult for the ideal therapeutic to be developed [52]. Initial inhibitors of BACE1 were non-cleavable peptide-based analogues, designed on the amino acid sequence of APP, which showed excellent inhibitory effects on BACE1. However, the size was too large to exhibit *in vivo* benefits, although ideal for the active site [53]. The development of SMEs for BACE1 renewed hope in the use of the aspartic protease as a target, hoping to increase blood–brain barrier (BBB) penetration and bioavailability that were identified as issues with the first-generation BACE1 inhibitors. From there, the hunt for a BACE1 inhibitor began with multiple classes of inhibitors being developed in an attempt to find the ideal therapeutic. In a similar pattern to other amyloid therapies, BACE1 inhibitors in other trials were halted or discontinued due to lack of efficacy or off-target effects. Only two BACE1 inhibitors were in the 2019 cohort of clinical trials: CNP520, discontinued in July 2019 due to worsening of cognitive function, and E2609, discontinued in September 2019 due to unfavourable risk–benefit ratio [54, 55]. Both compounds joining the list of lessons learnt from BACE1 inhibitors, along with Lanabecestat, Atabecestat and Verubecestat. All of which proved excellent in reducing Aβ; however, translation into clinical trials was not as smooth, lacking efficacy or displaying

With no current curative treatments for AD available, previous cohorts of clinical trials are missing something vital. The types of therapeutics used and targets

lating cleavage of APP rather than preventing it. By cleaving APP within the Aβ domain, alpha secretase prevents the generation of Aβ fragments instead releasing non-toxic p3 peptide following gamma secretase cleavage [45]. Modulation of alpha secretase is expected to increase its activity and reduce levels of Aβ, potentially increasing the levels of a neuroprotective product of alpha secretase cleavage of APP, sAPPα [46]. Alpha secretases belong to the 'a disintegrin and metalloprotease' (ADAM) family, which are found to play roles in cell adhesion, migration, proteolysis and signalling [47]. ADAM10 was found to be the alpha secretase relevant to APP cleavage in neurons, making it the target of modulation in AD [48]. Two therapeutics that have undergone clinical trials showing potential as alpha secretases enhancers are etazolate (EHT-0202) and bryostatin-1. Both stimulate alpha secretase to increase generation of sAPPα [49]. The potential of alpha secretase enhancers as a therapeutic for AD is likely. However, studies into the effects of enhancers on the other substrates of ADAM10 are required to identify any possible

*2.1.2.3.2 Alpha secretases*

adverse effects [50].

generation and loss in cognitive abilities [51].

*2.1.2.3.3 BACE1*

**26**

off-target effects [56].

**2.2 Improving therapeutics or target choice**

Peptides are small molecular biologicals that play a major role in the body as signalling molecules. Naturally occurring peptides in humans are commonly called hormones, acting as messengers utilising the blood stream and other extracellular spaces to regulate the many biological processes that keep us going [57]. Two of the most well-known peptides are glucagon and insulin, both playing large roles in homeostasis of blood-glucose levels. These hormones act on blood-glucose levels by targeting accessory organs and stimulating glucose production or glycogen storage, respectively. The action of glucose and insulin is a classic example of how peptides work in the body with high specificity and rapid onset of effect. Although commonly linked to hormones, peptides are also used as neurotransmitters, antiinfectives and growth factors [58].

Peptides range in length from 10 to 50 amino acids long, and can have a mass of up to 5 kDa, putting them between SMEs and proteins in terms of size and weight. *In vivo*, natural peptides are highly efficacious and selective with limited off-target effects, transient at most for those that exist [59]. Their ability to act as signalling molecules both extracellularly and intracellularly displays the range of therapeutic opportunity that peptides exhibit. Following the discovery of peptides playing large roles in homeostasis in the body, research turned towards identifying and isolating certain peptides that were linked to diseases. To continue with the example of insulin, the development of insulin as a therapeutic comes from the identification of individuals lacking a "pancreatic secretion" in the early 1900s, where insulin was isolated from the pancreas of stray dogs and calves and used to treat a child with type I diabetes [57]. This discovery only fuelled the fire for further discovery and isolation of other natural peptides that were found to be involved in diseases, leading to the identification of over 7000 naturally occurring peptides. Although identified, not all can be used directly as a therapeutic due to unbeneficial properties such as poor bioavailability and short half-life [58].

#### **3.2 Peptides as therapeutic options**

The nature of tasks that peptides perform in the body makes them an enticing molecule, as an opportunity to control biological processes in a similar way that hormones and other natural peptides control everyday life. Many consider peptides to be the inferior option for therapeutic development as they display low oral bioavailability and a tendency to be metabolised by proteolytic enzymes in the local environment leading to a short half-life *in vivo* [57]. These unfavourable traits are mitigated in the body through close proximity of targets to the site of release, sometimes in high concentrations for when multiple targets exist. For peptides to be successful in therapeutic applications, an intense intravenous dosing regimen for the patient is required to maintain an adequate load of the therapeutic. Although hindered by poor bioavailability and short half-life, the biological nature of peptides offers plenty of properties that would make them an ideal therapeutic for complex diseases where specificity and toxicity are of concern [57].

The specific nature of peptides is due to their ability to cover a larger area of the target site compared to SMDs, decreasing the risk of off-target effects that have halted previous clinical trials into AD therapeutics [60]. A benefit of peptides over SMDs is the relative inability to build-up toxicity due to the metabolic instability of the amide bonds that hold the peptide together, resulting in the release of amino acids that can be utilised by various systems [61]. These qualities of peptides are what make them ideal therapeutics for most biological process disorders, specifically those found in the CNS. The delicate environment of the CNS requires therapeutics that are highly specific so as not to affect the normal functioning of the brain, but also produce minimal toxicity to prevent damage to nearby neurons.

#### **3.3 Peptides approved for therapeutic use**

In the 36 months that spanned 2016–2018, 8 peptide therapeutics were approved by the FDA making up just over 6% of the drugs approved in that time [62]. This can be looked at in both an optimistic and pessimistic view. However, looking at cumulative FDA approvals given since 1980, there is no denying that peptides have a place in therapeutic development. In 2018, there were over 70 peptides available for medical use in the United States, Europe and Japan, and more than 150 in clinical studies [63]. The most commonly used therapeutic peptides target biological processes in a similar way that biologics, such as proteins, do, replacing molecules that stimulate PPIs. Crucial hormones that were approved for therapeutic use are vasopressin, oxytocin, insulin, glucagon and corticotropin, all of which were approved last century yet still play a pivotal role in HRTs [63].

Currently approved peptides cover a large range of therapeutic areas, such as oncology, metabolic diseases, haematology, respiratory disorders and gastroenterology. Of the peptides approved by the FDA, only three are approved for CNS indications: corticotropin, approved for use in inflammatory diseases; glatiramer, approved for use in MS; and taltirelin, approved for use in spinocerebellar degeneration. After the discovery of taltirelin in 2000, no other peptides have been approved for CNS indications, even though there have been over 30 new peptides approved for other indications since [63]. This begs the question on whether research has moved away from peptides for CNS indications due to their difficulty passing through the BBB, or whether the technology is only now catching up.

**29**

metic therapeutics.

*An Alternate View of Neuroprotection with Peptides in Alzheimer's Disease*

**3.4 Future considerations for peptide therapeutics for use in CNS indications**

With a variety of unfavourable characteristics, peptides require modification prior to clinical testing. The field of peptide synthesis has improved in the past two decades contributing to a rise in more effective peptide therapeutics available for clinical trials [64]. Many traits of peptides that were initially unfavourable have been resolved with new techniques in peptide synthesis. However, there remains the large issue of bioavailability that is restricting the use of peptides as therapeutics for the CNS. The biological nature of peptides reduces their bioavailability, their size making it difficult to cross membrane barriers and the structure of their bonds increasing the rate of degradation in the gastrointestinal (GI) tract and plasma. Due to these features, most approved peptide therapeutics are parenterally administered, involving either intravenous or subcutaneous injections. Parenteral administration allows for the systemic distribution of a relatively large dose of the peptide providing high concentration of the therapeutic when it reaches its target, without having to cross any membrane barriers. The oral route does not allow for this as the conditions are acidic and tight mucosal barriers exist to protect the body from

Administration directly into the blood stream works for many indications where the target is easily reached through diffusion across capillary walls; however, CNS indications are protected from standard blood flow by the BBB. Peptides targeting the CNS endure this extra barrier that acts as a neuroprotective wall, preventing unwanted molecules from entering the sterile and sensitive environment [65]. Studies in transport of drugs across the BBB have shown that there are multiple ways that can be exploited to deliver drugs to the CNS, specifically using transporter pathways that shuttle hormones such as insulin into the CNS [66]. Delivery of previous therapeutics for AD in clinical trials involved either disruption of the BBB, increasing lipid solubility of the molecules or using pre-existing transport systems, with mouse model studies showing effectiveness of the latter two [67]. An alternative route through the olfactory pathway may provide hope for delivering peptides to the CNS; however, intranasal delivery has demonstrated limited progress in clinical settings. Offering an attractive opportunity to bypass the BBB, intranasal delivery presents similar patterns in degradation to other routes of

Although an issue present in the delivery of peptides to the CNS, transport into the CNS is secondary to proteolytic degradation in terms of bioavailability of peptides, with a large proportion of peptide load being degraded before it can reach the target site. Widely accepted as techniques that decrease degradation is conjugation or the production of peptidomimetics, techniques used in peptide synthesis today. The most common conjugate for increasing bioavailability of a peptide is polyethylene glycol (PEG), a molecule that has shown to help prevent clearance of therapeutics. PEG increases the overall size of peptide therapeutics, making it too large for renal clearance and hindering proteolytic cleavage in plasma [30]. Peptidomimetics are a modified form of the peptide that is biologically similar while containing unnatural amino acids or modified peptide bonds [69]. Through the addition of unnatural amino acids and altered peptide bonds, proteolytic enzymes are incapable of cleaving peptidomimetics due to the unnatural nature of the molecule. The process of screening the effects of multiple modifications to the structure of the peptide has improved with the development of simple screening assays, increasing the output of peptidomi-

*DOI: http://dx.doi.org/10.5772/intechopen.91065*

external threats [29].

delivery [68].

*Neuroprotection - New Approaches and Prospects*

**3.3 Peptides approved for therapeutic use**

The nature of tasks that peptides perform in the body makes them an enticing

The specific nature of peptides is due to their ability to cover a larger area of the target site compared to SMDs, decreasing the risk of off-target effects that have halted previous clinical trials into AD therapeutics [60]. A benefit of peptides over SMDs is the relative inability to build-up toxicity due to the metabolic instability of the amide bonds that hold the peptide together, resulting in the release of amino acids that can be utilised by various systems [61]. These qualities of peptides are what make them ideal therapeutics for most biological process disorders, specifically those found in the CNS. The delicate environment of the CNS requires therapeutics that are highly specific so as not to affect the normal functioning of the brain, but also produce minimal toxicity to prevent damage to nearby neurons.

In the 36 months that spanned 2016–2018, 8 peptide therapeutics were approved by the FDA making up just over 6% of the drugs approved in that time [62]. This can be looked at in both an optimistic and pessimistic view. However, looking at cumulative FDA approvals given since 1980, there is no denying that peptides have a place in therapeutic development. In 2018, there were over 70 peptides available for medical use in the United States, Europe and Japan, and more than 150 in clinical studies [63]. The most commonly used therapeutic peptides target biological processes in a similar way that biologics, such as

proteins, do, replacing molecules that stimulate PPIs. Crucial hormones that were approved for therapeutic use are vasopressin, oxytocin, insulin, glucagon and corticotropin, all of which were approved last century yet still play a pivotal role

Currently approved peptides cover a large range of therapeutic areas, such as oncology, metabolic diseases, haematology, respiratory disorders and gastroenterology. Of the peptides approved by the FDA, only three are approved for CNS indications: corticotropin, approved for use in inflammatory diseases; glatiramer, approved for use in MS; and taltirelin, approved for use in spinocerebellar degeneration. After the discovery of taltirelin in 2000, no other peptides have been approved for CNS indications, even though there have been over 30 new peptides approved for other indications since [63]. This begs the question on whether research has moved away from peptides for CNS indications due to their difficulty passing through the BBB, or whether the technology is only now

molecule, as an opportunity to control biological processes in a similar way that hormones and other natural peptides control everyday life. Many consider peptides to be the inferior option for therapeutic development as they display low oral bioavailability and a tendency to be metabolised by proteolytic enzymes in the local environment leading to a short half-life *in vivo* [57]. These unfavourable traits are mitigated in the body through close proximity of targets to the site of release, sometimes in high concentrations for when multiple targets exist. For peptides to be successful in therapeutic applications, an intense intravenous dosing regimen for the patient is required to maintain an adequate load of the therapeutic. Although hindered by poor bioavailability and short half-life, the biological nature of peptides offers plenty of properties that would make them an ideal therapeutic for complex diseases where specificity and toxicity are of

**3.2 Peptides as therapeutic options**

concern [57].

**28**

in HRTs [63].

catching up.
