Compound of the Formula I, or pharmaceutically acceptable salts thereof: wherein R 1 is an 4-, 5- or 6-membered heterocyclic ring, the ring having one or more heteroatoms which are selected from the group consisting of O, S or N, wherein the heterocyclic ring is optionally substituted with an oxo; thioxo; C 1 -C 5 alkyl; C 1 -C 5 alkenyl; C 1 -C 5 alkynyl; C 1 -C 5 alkoxy; aryl; halo; halo C 1 -C 5 alkyl; nitro; azido; cyano; hydroxyl; hydroxyl C 1 -C 5 alkyl; a SO n R 14 group, where R 14 is alkyl and n = 0, 1 or 2; a carboxyl or ester group of the formula -COOR 15 wherein R 15 is H or C 1 -C 5 alkyl and which may be in ionic form -COO -; and wherein X 1 is -CO-, -CS-, -CH 2 CO- or CH(R 4), wherein R 4 is H or optionally substituted C 1 -C 5 alkyl, - COOH or -COOR 11, wherein R 11 is optionally substituted C 1 -C 5 alkyl; X 2 is-CO- or -CS-; Z is -CH 2 -, -S-, -O- or -NH-; R 7 and R 8, which may be the same or different, are H, or optionally substituted C 1 -C 5 alkyl; R 9 and R 10, which may be the same or different, are H, optionally substituted C 1 -C 5 alkyl, or an optionally substituted carbocyclic ring, wherein the optional substituted C 1 -C 5 alkyl groups are halo, nitro or cyano; W represents a side chain of an amino acid residue, natural or un-natural, wherein the side chain is: in which Q is O or S; and R 5 and R 6, may be the same or different and are H, a C 1 -C 5 alkyl or W represents the side chain of an amino acid residue selected from the group consisting of: asparagine, histidine, leucine, thienylalanine and phenylalanine; X represents 1 to 20 amino acids, independently selected from the group consisting of: phenylalanine, tyrosine and tryptophan, which are in the D-configuration, and wherein the C-terminal amino acid residue of X is optionally substituted with an amino group or 7-amino-4-methyl coumarin. R 1 is an 4-, 5- or 6-membered heterocyclic ring, the ring having one or more heteroatoms which are selected from the group consisting of O, S or N, wherein the heterocyclic ring is optionally substituted with an oxo; thioxo; C 1 -C 5 alkyl; C 1 -C 5 alkenyl; C 1 -C 5 alkynyl; C 1 -C 5 alkoxy; aryl; halo; halo C 1 -C 5 alkyl; nitro; azido; cyano; hydroxyl; hydroxyl C 1 -C 5 alkyl; a SO n R 14 group, where R 14 is alkyl and n = 0, 1 or 2; a carboxyl or ester group of the formula -COOR 15 wherein R 15 is H or C 1 -C 5 alkyl and which may be in ionic form -COO -; and wherein X 1 is -CO-, -CS-, -CH 2 CO- or CH(R 4), wherein R 4 is H or optionally substituted C 1 -C 5 alkyl, - COOH or -COOR 11, wherein R 11 is optionally substituted C 1 -C 5 alkyl; X 2 is-CO- or -CS-; Z is -CH 2 -, -S-, -O- or -NH-; R 7 and R 8, which may be the same or different, are H, or optionally substituted C 1 -C 5 alkyl; R 9 and R 10, which may be the same or different, are H, optionally substituted C 1 -C 5 alkyl, or an optionally substituted carbocyclic ring, wherein the optional substituted C 1 -C 5 alkyl groups are halo, nitro or cyano; W represents a side chain of an amino acid residue, natural or un-natural, wherein the side chain is: in which Q is O or S; and R 5 and R 6, may be the same or different and are H, a C 1 -C 5 alkyl or W represents the side chain of an amino acid residue selected from the group consisting of: asparagine, histidine, leucine, thienylalanine and phenylalanine; X represents 1 to 20 amino acids, independently selected from the group consisting of: phenylalanine, tyrosine and tryptophan, which are in the D-configuration, and wherein the C-terminal amino acid residue of X is optionally substituted with an amino group or 7-amino-4-methyl coumarin.