An anti-cancer agent comprising a cyclised tumour homing peptide having an arsenic atom bonded to two cysteine residues; wherein the anti-cancer agent has a structure as shown in formula I or formula II: wherein, in formula I: the sulfur atoms shown and to which the arsenic is bonded are the thiol sulfurs of the respective cysteine residues; the cysteine residues in formula I are separated by less than 20 amino acid residues represented by the curved line; and R is a stabilising group selected from the group consisting of hydroxyl, C 1 to C 6 alkyl, C 2 to C 6 alkenyl, C 2 to C 6 alkynyl, C 1 to C 6 alkoxy, aryl, cycloalkyl, heterocyclic, and short chain peptides, each of which groups may be substituted or unsubstituted; wherein, in formula II: the atoms and groups are as described for formula I; the capping group is formed from an amino reactive compound; and W is between 5 to 20 amino acid residues; and wherein, in formula I and II the tumour homing peptide to which the arsenic atom is bonded between two cysteine residues is selected from the group consisting of: 1. CAYHRLRRC; 2. CDCRGDCFC; 3. CPIEDRPMC; 4. CNRRTKAGC; 5. CGTKRKC; 6. CRGDGWC; 7. CVSNPRWKC; 8. CHVLWSTRC; 9. CLDGGRPKC; 10. CVNHPAFAC; 11. CRGDRGPDC; 12. CRGDKTTNC; 13. CRGDHAGDC; 14. CLSYYPSYC; 15. CTPSPPFSHC; 16. CPHSKPCLC; 17. CSDSWHYWC; 18. CSDWQHPWC; 19. CSDYNHHWC; 20. CSDGQHYWC; 21. CYDSWHYWC; 22. CFDGNHIWC; 23. CTDFPRSFC; 24. CTQDRQHPC; 25. CLSRYLDQC; 26. CGNSNPKSC; 27. CPHNLTKLC; and wherein the arsenic atom, when in the formed anti-cancer agent, is bonded to and bridged between the N-terminal and C-terminal cysteine residues of the above peptide sequences. wherein the anti-cancer agent has a structure as shown in formula I or formula II: wherein, in formula I: the sulfur atoms shown and to which the arsenic is bonded are the thiol sulfurs of the respective cysteine residues; the cysteine residues in formula I are separated by less than 20 amino acid residues represented by the curved line; and R is a stabilising group selected from the group consisting of hydroxyl, C 1 to C 6 alkyl, C 2 to C 6 alkenyl, C 2 to C 6 alkynyl, C 1 to C 6 alkoxy, aryl, cycloalkyl, heterocyclic, and short chain peptides, each of which groups may be substituted or unsubstituted; the sulfur atoms shown and to which the arsenic is bonded are the thiol sulfurs of the respective cysteine residues; the cysteine residues in formula I are separated by less than 20 amino acid residues represented by the curved line; and R is a stabilising group selected from the group consisting of hydroxyl, C 1 to C 6 alkyl, C 2 to C 6 alkenyl, C 2 to C 6 alkynyl, C 1 to C 6 alkoxy, aryl, cycloalkyl, heterocyclic, and short chain peptides, each of which groups may be substituted or unsubstituted; wherein, in formula II: the atoms and groups are as described for formula I; the capping group is formed from an amino reactive compound; and W is between 5 to 20 amino acid residues; and the atoms and groups are as described for formula I; the capping group is formed from an amino reactive compound; and W is between 5 to 20 amino acid residues; and wherein, in formula I and II the tumour homing peptide to which the arsenic atom is bonded between two cysteine residues is selected from the group consisting of: 1. CAYHRLRRC; 2. CDCRGDCFC; 3. CPIEDRPMC; 4. CNRRTKAGC; 5. CGTKRKC; 6. CRGDGWC; 7. CVSNPRWKC; 8. CHVLWSTRC; 9. CLDGGRPKC; 10. CVNHPAFAC; 11. CRGDRGPDC; 12. CRGDKTTNC; 13. CRGDHAGDC; 14. CLSYYPSYC; 15. CTPSPPFSHC; 16. CPHSKPCLC; 17. CSDSWHYWC; 18. CSDWQHPWC; 19. CSDYNHHWC; 20. CSDGQHYWC; 21. CYDSWHYWC; 22. CFDGNHIWC; 23. CTDFPRSFC; 24. CTQDRQHPC; 25. CLSRYLDQC; 26. CGNSNPKSC; 27. CPHNLTKLC; and 1. CAYHRLRRC; 2. CDCRGDCFC; 3. CPIEDRPMC; 4. CNRRTKAGC; 5. CGTKRKC; 6. CRGDGWC; 7. CVSNPRWKC; 8. CHVLWSTRC; 9. CLDGGRPKC; 10. CVNHPAFAC; 11. CRGDRGPDC; 12. CRGDKTTNC; 13. CRGDHAGDC; 14. CLSYYPSYC; 15. CTPSPPFSHC; 16. CPHSKPCLC; 17. CSDSWHYWC; 18. CSDWQHPWC; 19. CSDYNHHWC; 20. CSDGQHYWC; 21. CYDSWHYWC; 22. CFDGNHIWC; 23. CTDFPRSFC; 24. CTQDRQHPC; 25. CLSRYLDQC; 26. CGNSNPKSC; 27. CPHNLTKLC; and wherein the arsenic atom, when in the formed anti-cancer agent, is bonded to and bridged between the N-terminal and C-terminal cysteine residues of the above peptide sequences.