Peptides 3 and 4 were found to be the least potent amongst the tested compounds, whereas peptide 5 showed good potency in all tested cell lines. In fact, peptide 5 was designed by engineering the sequence of compound 3 and 4 via replacement of the leucine with cyclohexylalanine residues, which led to an increased cytotoxicity. Cyanine dye-labelled constructs 6-9 were found to be significantly more potent (p < 0.0001) than the peptides alone (3-5) in the tested cell lines -the only exception being the cytotoxicity of compound 8 in KB cells, which was not found to be significantly different (p = 0.11) to that of its native sequence. Conjugates 10 and 11 bearing a folate component showed enhanced toxicity in KB cells when compared to MCF7 and HEK293 cells. Likewise, cytotoxicity of compound 12 in KB cells was found to be significantly higher (p < 0.0001) than in the other tested cell lines. Nevertheless, potency of 10-11 in cells not overexpressing folate receptor α is comparable to that of their native peptides. Lastly, dual labelled conjugates 13-15 showed enhanced potency compared to the parent sequences and to compounds 10-12 (p < 0.05), although no selectivity was observed toward KB cells.