A compound of formula or a pharmaceutically acceptable salt thereof, R 1 - K -P-Y wherein: R 1 is the group attached to the N-terminal of the first amino acid of the sequence P, optionally via the ligand X, and is selected from the group consisting of H, CH 3 C(=O)-, and maleimide; X is a biradical selected from the group consisting of -NH-(CH 2) r -C(=O)-, -C(=O)-(CH 2) r -C(=O)-, -S(CH 2)r-, -S-(CH 2) r -C(=O)-, -O-(CH 2) r -, -S-CH 2 -CH(NH 2)-C(=O)-, -O-(CH 2) r -C(=O)-, -(CH 2) r -C(=O)-, -NH-O-CH 2 -C(=O)-NH-(CH 2) r -CH -C(=O)-, -(CH 2) r -C(=O)-NH-(CH 2) r -CH(NH 2)-C(=O)-, and -NH-(CH 2) r -CH(NHC(=O)CH 2 NH 2)-C(=O)-; wherein the biradical X is attached to R 1 and to the N of the sequence P as follows: R 1 -NH-(CH 2) r -C(=O)-N m -, R 1 -C(=O)-(CH 2) r -C(=O)-N m -, R 1 -S-(CH 2) r -N m -, R 1 -S-(CH 2) r -C(=O)-N m -, R 1 -O-(CH 2) r -N m -, R 1 -S-CH 2 -CH(NH 2)-C(=O)-N m, R 1 -O-(CH 2) r -C(=O)-N m -, R 1 -(CH 2) r -C(=O)- N m, R 1 -NH-O-CH 2 -C(=O)-NH-(CH 2) r -CH(NH 2)-C(=O)-N m, R 1 -(CH 2) r -C(=O)-NH- r -CH -C(=O)-N m, and R 1 -NH-(CH 2) r -CH(NHC(=O)CH 2 NH 2)-C(=O)-N m; r is an integer from 1 to 5; P is a biradical of an amino acid sequence comprising the sequence D-Pro-D-Trp-D-Val-D-Pro-D-Ser-D-Trp-D-Met-D-Pro-D-Pro-D-Arg-D-His-D-Thr (SEQ ID NO: 1); Y is the group attached to the C-terminal of the last amino acid of the sequence P, and is selected from the group consisting of -NH 2, -OH, -OR 2 and -NHR 2; R 2 is a radical selected from the group consisting of (C 1 -C 6)-alkyl and (CH 2) 2 -NH-C(=O)-CH 2 -O-NH 2; k is an integer from 0 to 2; m is an integer from 0 to 1; with the proviso that when the biradical X is -C(=O)(CH 2) r C(=O)-, then R 1 is H; with the proviso that when the N of the amino acid of the sequence P to which is attached the biradical X is a biradical -NH-, then m is 1, and when is a biradical -N-, then m is 0; and with the proviso that when R 1 is maleimide then the biradical X is -C(=O)-(CH 2) r -C(=O)-, -CH(NH 2)-C(=O)-, -(CH 2) r -C(=O)-, and -(CH 2) r -C(=O)-NH-(CH 2) r -CH(NH 2)-C(=O)-. R 1 is the group attached to the N-terminal of the first amino acid of the sequence P, optionally via the ligand X, and is selected from the group consisting of H, CH 3 C(=O)-, and maleimide; X is a biradical selected from the group consisting of -NH-(CH 2) r -C(=O)-, -C(=O)-(CH 2) r -C(=O)-, -S(CH 2)r-, -S-(CH 2) r -C(=O)-, -O-(CH 2) r -, -S-CH 2 -CH(NH 2)-C(=O)-, -O-(CH 2) r -C(=O)-, -(CH 2) r -C(=O)-, -NH-O-CH 2 -C(=O)-NH-(CH 2) r -CH -C(=O)-, -(CH 2) r -C(=O)-NH-(CH 2) r -CH(NH 2)-C(=O)-, and -NH-(CH 2) r -CH(NHC(=O)CH 2 NH 2)-C(=O)-; wherein the biradical X is attached to R 1 and to the N of the sequence P as follows: R 1 -NH-(CH 2) r -C(=O)-N m -, R 1 -C(=O)-(CH 2) r -C(=O)-N m -, R 1 -S-(CH 2) r -N m -, R 1 -S-(CH 2) r -C(=O)-N m -, R 1 -O-(CH 2) r -N m -, R 1 -S-CH 2 -CH(NH 2)-C(=O)-N m, R 1 -O-(CH 2) r -C(=O)-N m -, R 1 -(CH 2) r -C(=O)- N m, R 1 -NH-O-CH 2 -C(=O)-NH-(CH 2) r -CH(NH 2)-C(=O)-N m, R 1 -(CH 2) r -C(=O)-NH- r -CH -C(=O)-N m, and R 1 -NH-(CH 2) r -CH(NHC(=O)CH 2 NH 2)-C(=O)-N m; r is an integer from 1 to 5; P is a biradical of an amino acid sequence comprising the sequence D-Pro-D-Trp-D-Val-D-Pro-D-Ser-D-Trp-D-Met-D-Pro-D-Pro-D-Arg-D-His-D-Thr (SEQ ID NO: 1); Y is the group attached to the C-terminal of the last amino acid of the sequence P, and is selected from the group consisting of -NH 2, -OH, -OR 2 and -NHR 2; R 2 is a radical selected from the group consisting of (C 1 -C 6)-alkyl and (CH 2) 2 -NH-C(=O)-CH 2 -O-NH 2; k is an integer from 0 to 2; m is an integer from 0 to 1; with the proviso that when the biradical X is -C(=O)(CH 2) r C(=O)-, then R 1 is H; with the proviso that when the N of the amino acid of the sequence P to which is attached the biradical X is a biradical -NH-, then m is 1, and when is a biradical -N-, then m is 0; and with the proviso that when R 1 is maleimide then the biradical X is -C(=O)-(CH 2) r -C(=O)-, -CH(NH 2)-C(=O)-, -(CH 2) r -C(=O)-, and -(CH 2) r -C(=O)-NH-(CH 2) r -CH(NH 2)-C(=O)-.