Role of the primary structure in the antitumor mechanism of CIGB-552.
Biostability of peptides in blood and serum is an important issue for development of peptides as clinically drugs. In vitro degradation of peptides in serum and plasma is considered the primary methodology for studying the degradation pattern of peptides (Werle, 2006). For that reason, our group examined the in vitro metabolic stability of CIGB-552 in serum during 120 minutes. A typical serine-proteases degradation pattern was suggested for this peptide sequence (Vallespi et al., 2014). The main metabolites of CIGB-552 showed no great differences regarding hydrophobicity, hydrophilicity, and isoelectric point. However, the analysis of the cytotoxic effect, cell-penetrating capacity, antitumor mechanism and interaction with its intracellular target, COMMD1, demonstrated a loss of antitumor capacity for the metabolites compared to CIGB-552 (Figure 4). These findings suggest the importance of C-terminal amino acidic residues (Lys18, Phe19 and Trp20) in the antitumor activity of CIGB-552 (Astrada et al., 2016). The role of these residues in the peptide-membrane interaction was analyzed by molecular dynamic simulation (Astrada et al., 2016). CIGB-552 and the 17 amino acid metabolite (named as 5) have a conserved structural motif. Two tryptophan residues and a tyrosine create a hydrophobic cluster that brings together the C-terminal carboxylate moieties and one arginine residue, forming a stable salt bridge interaction that leads to a looped conformation. Indeed, tryptophan–tryptophan and arginine–C-terminal carboxylate distances in both systems display very stable interactions.
Altogether, these results demonstrate that the primary structure of CIGB-552 is considered the minimum active sequence needed to produce a potent antitumor effect in cell and animal models. In this sense, the protection of CIGB-552 from proteolitic degradation is one of the most important challenges that face this peptide in its development as an anticancer drug. The use of different encapsulation systems such as liposomes, multilayer emulsions or nanoparticle vehicles could be an alternative to deal with this issue.