A cyclic peptide, capable of modulating the activity of poly (ADP-ribose) polymerase 1 (PARP-1), useful for the treatment of cancer, comprises the formula: Â Â Â Â [X1-X2-X3-X4-X3-X4-X3] Wherein X1 is a peptide moiety capable of inhibiting the cleavage of PARP-1, X2 may absent, or Val or Ser, X3 is selected from Trp-Trp or Ar1-Ar2, and X4 is selected from Arg-Arg or a dipeptide comprising two homocysteic acid residues or two guanidinophenylalanine residues, wherein Ar1 and Ar2 are amino acids comprises an aryl side chain comprising a napthyl, 1,2-dihydronapthyl or 1,2,3,4-tetrahydronapthyl group. The X1 group which inhibits PARP-1 cleavage may be, for example, -Pro-Arg-Gly-Pro-Arg-Pro-, or derivatives thereof with the Arg residues substituted by acidic residues, or -Gly-Asp-Glu-NMeAsp-MeGly-Val (where NMeAsp is N-methyl aspartic acid). The cyclic peptides may be administered together with an aerobic glycolysis inhibitor, such as 2-deoxyglucose.
