**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 external threats [29].

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 delivery [68].

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 peptidomimetic therapeutics.
