An anti-cancer agent is provided comprising a tumour homing peptide having, or having been modified to present, two cysteine residues, with an arsenic atom between, such that the tumour homing peptide cyclises to give an arsenic- containing anti-cancer agent. This allows for selection of an appropriate tumour homing peptide for treatment of a given cancer whereby the subsequent agent provides for a more targeted delivery of arsenic to the tumour microenvironment.