Central nervous system (CNS) diseases are among the most difficult to treat, mainly because the vast majority of the drugs fail to cross the blood-brain barrier (BBB) or to reach the brain at concentrations adequate to exert a pharmacological activity. The obstacle posed by the BBB has led to the in-depth study of strategies allowing the brain delivery of CNS-active drugs. Among the most promising strategies is the use of peptides addressed to the BBB. Peptides are versatile molecules that can be used to decorate nanoparticles or can be conjugated to drugs, with either a stable link or as pro-drugs. They have been used to deliver to the brain both small molecules and proteins, with applications in diverse therapeutic areas such as brain cancers, neurodegenerative diseases and imaging. Peptides can be generally classified as receptor-targeted, recognizing membrane proteins expressed by the BBB microvessels (e.g., Angiopep2, CDX, and iRGD), “cell-penetrating peptides” (CPPs; e.g. TAT47-57, SynB1/3, and Penetratin), undergoing transcytosis through unspecific mechanisms, or those exploiting a mixed approach. The advantages of peptides have been extensively pointed out, but so far few studies have focused on the potential negative aspects. Indeed, despite having a generally good safety profile, some peptide conjugates may display toxicological characteristics distinct from those of the peptide itself, causing for instance antigenicity, cardiovascular alterations or hemolysis. Other shortcomings are the often brief lifetime in vivo, caused by the presence of peptidases, the vulnerability to endosomal/lysosomal degradation, and the frequently still insufficient attainable increase of brain drug levels, which remain below the therapeutically useful concentrations. The aim of this review is to analyze not only the successful and promising aspects of the use of peptides in brain targeting but also the problems posed by this strategy for drug delivery.
Peptides are a marvelous resource, but not all that glitters is gold. Like anything else, they need to be handled with caution, and they are not yet the cure-all for delivery problems, or, more specifically, for trans-BBB delivery problems. In most studies providing this type of information, the amount reaching the brain remained below par, which cannot be considered a satisfactory state of affairs even though enough active principle may have reached the brain to have an impact on the CNS pathology under study. In our opinion, peptides remain however a key component of the so-far elusive solution of the brain delivery problem. The search for more efficient sequences, the use of “stabilized” and/or “decorated” (e.g., glycosylated) peptides, the further development of cleverly engineered nanovehicles, and the ongoing exploration of innovative delivery routes (e.g., the nose-to-brain pathway) offer the perspective of steady progress toward the eventual implementation of a peptide-based technology affording the needed concentration of the drug in brain parenchyma. (Mol Pharm . 2022 Nov 7;19(11):3700-3729.)