Aim: Evaluate the selective cytotoxic effect of the palindromic sequence RWQWRWQWR and its analogues obtained by replacement of L-amino acids by D-amino acids or the functionalization by adding the RGD (integrin ligand motif) to the peptide. Methods: Peptides were obtained by SPPS, characterized by RP-HPLC and ESI-QTOF and its biological activity was evaluated using MTT assays. Evaluation of mechanism associated to the cytotoxic effect were carried out by flow cytometry, RT-qPCR, wound healing, transwell and zymography. Results: The peptides with replacements of D-amino acid showed a lesser cytotoxic effect against breast cancer cell lines, regardless it was one or several residues modified which suggested a possible specific interaction between the peptide and the cancer cell membrane besides its initial electrostatically contact. On the other hand, addition of the RGD sequence to the palindromic peptide in the N-terminal end resulted in a greater cytotoxic effect against cell lines derived from the three mainly diagnosed breast cancer molecular subtypes. An approximation on mechanisms associated to this effect was evaluated on MCF-7 cells, it shows that the peptide induced apoptosis by activating intrinsic and extrinsic pathway, which correlates with the possibility of a specific interaction, and induces mitochondrial depolarization with release of oxygen reactive species. Also, this peptide induces a reduction in migration and invasion associated with a diminish in metalloprotease 9 activity and a lesser release of IL-6, IL-10 and arginase cytokines. Conclusions: Our results suggest that this promising peptide can be considered for preclinical evaluation in the development of drugs to treat breast cancer and thus mitigate the impact of this disease.

Previous studies show that the palindromic peptide RWQWRWQWR, derived from bovine lactoferricin (LfcinB), exhibits selective in vitro cytotoxicity against cervical, breast, and prostate cancer cells.

Palindromic antimicrobial peptides (PAMs) constitute versatile scaffolds for the design and optimization of anticancer agents with applications in therapy, diagnosis, and/or monitoring. In the present study, fluorolabeled peptides derived from the palindromic sequence RWQWRWQWR containing fluorescent probes, such as 2-Aminobenzoyl, 5(6)-Carboxyfluorescein, and Rhodamine B, were obtained. RP-HPLC analysis revealed that the palindromic peptide conjugated to Rhodamine B (RhB-RWQWRWQWR) exhibited the presence of isomers, likely corresponding to the open-ring and spiro-lactam forms of the fluorescent probe. This equilibrium is dependent on the peptide sequence, as the RP-HPLC analysis of dimeric peptide (RhB-RRWQWR-hF-KKLG)₂K-Ahx did not reveal the presence of isomers. The antibacterial activity of the fluorescent peptides depends on the probe attached to the sequence and the bacterial strain tested. Notably, some fluorescent peptides showed activity against reference strains as well as sensitive, resistant, and multidrug-resistant clinical isolates of E. coli, S. aureus, and E. faecalis. Fluorolabeled peptides 1-Abz (MIC = 62 µM), RhB-1 (MIC = 62 µM), and Abz-1 (MIC = 31 µM) exhibited significant activity against clinical isolates of E. coli, S. aureus, and E. faecalis, respectively. The RhB-1 (IC₅₀ = 61 µM), Abz-1 (IC₅₀ = 87 µM), and RhB-2 (IC₅₀ = 35 µM) peptides exhibited a rapid, significant, and concentration-dependent cytotoxic effect on HeLa cells, accompanied by morphological changes characteristic of apoptosis. RhB-1 (IC₅₀ = 18 µM) peptide also exhibited significant cytotoxic activity against breast cancer cells MCF-7. These conjugates remain valuable for elucidating the possible mechanisms of action of these novel anticancer peptides. Rhodamine-labeled peptides displayed cytotoxicity comparable to that of their unlabeled analogues, suggesting that cellular internalization constitutes a critical early step in their mechanism of action. These findings suggest that cell death induced by both unlabeled and fluorolabeled peptides proceeds predominantly via apoptosis and is likely contingent upon peptide internalization. Functionalization at the N-terminal end of the palindromic sequence can be evaluated to develop systems for transporting non-protein molecules into cancer cells.