Abstract: The present invention relates to a cyclic inhibitor of a kallikrein protease comprising or consisting of (I) the peptide (X1 (X2)(X3)R(X4)(X5)(X6)(X7)(X8)(X9)(X10)(X11) (Formula (X)), wherein (X1) is present or absent and, is preferably present, and if present, is an amino acid, and is most preferably D-alanine or G; (X2) is an amino acid with a side chain; (X3) is an amino acid with a polar uncharged side chain, is preferably with a polar uncharged side chain comprising a hydroxyl group, is more preferably T or S and is most preferably T; (X4) is an amino acid, preferably citrulline, Q or E; (X5) is an amino acid, preferably an amino acid with a hydrophobic side chain; (X6) is present or absent, is preferably absent, and, if present, is an amino acid with a negatively charged side chain, preferably D; (X7) is present or absent, is preferably present, and, if present, is an amino acid, more preferably an amino acid with a side chain comprising a pyrrole of indole, even more preferably P, hydroxyl-proline, (R)