“…Nevertheless, while these compounds seemed very promising in preclinical experiments, they proved ineffective in clinical trials [ 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 ]. This could be partly due to the fact that peptides are degraded by the gastrointestinal enzymes when administered orally, and that they fail to penetrate the blood–brain barrier when injected IV [ 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 ]. Therefore, in order to increase the efficacy of these compounds, they should be administered in form of nasal spray, and eventually administered together with enzyme inhibitors and blood–brain barrier transporters [ 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 ].…”