Diastereomeric peptides with a net positive charge greater than +1, and cyclic derivatives thereof, are provided, having at least 13 amino acid residues, comprising histidine and one or more hydrophobic amino acid residues, optionally esterified or amidated at the C-terminus and/or acylated at the N-terminus. The peptides may contain other amino acid residues including non-natural amino acids. The peptides are particularly useful in the treatment of cancer.