Cytotoxicity results, obtained by the CellTiter-Blue assay, are shown in Table . The low activity of peptides 3 and 4 in the tested cell lines (EC50 > 300 µM) is likely due to their low cell permeability. In contrast, peptide 5 was engineered to be more hydrophobic than compounds 3 and 4 via a leucine to cyclohexylalanine mutation; this modification likely led to an increased membrane permeability. Indeed, the cytotoxicity data of 5 obtained here is comparable to the reported values, reinforcing the idea that increased hydrophobicity may lead to a better mitochondrial targeting peptide. Delocalized lipophilic cations, such as triphenylphosphonium group and the cyanine dyes used here, preferentially localize within mitochondria. While triphenylphosphonium group is widely used to generate mitochondrial-targeting entities, conjugation of peptide 3 triphenylphosphonium group showed negligible toxicity in mammalian cells. In contrast, cyanine dye-labelled constructs 6-9 were all found to be more potent than the peptides alone (3-5) in the tested cell lines. The enhanced potency of these constructs may be due to the increased mitochondrial targeting ability and membrane permeability upon conjugation with cyanine dyes 1 and 2. Compounds 10-12, which bear a folate component on the side chain of the N-terminal lysine, show a potency comparable to the parent sequences 3-5 towards MCF7 and HEK293 cells. Nevertheless, those constructs all show increased selectivity for KB cells, as envisioned.