Peptide-based compounds

1. A compound of formula I

or a pharmaceutically acceptable salt thereof,

wherein

G represents glycine;

D represents aspartic acid;

R 1 represents —(CH 2 ) n — or —(CH 2 ) n —C 6 H 4 — wherein n represents a positive integer 1 to 10, and h represents a positive integer 1 or 2;

X 1 represents a bond or 1 to 5 amino acid residues wherein each amino acid residue independently optionally can be derivatized with a functional side-chain chosen from C 1-22 alkyl, C 1-22 perfluoroalkyl or polyethyleneglycol;

X 2 and X 4 represent independently an amino acid residue capable of forming a disulfide bond;

X 3 represents arginine, N-methylarginine or an arginine mimetic;

X 5 represents a hydrophobic amino acid;

X 6 represents a thiol-containing amino acid residue;

k represents a positive integer 1 to 10;

X 7 represents an amino acid residue; and

X 8 is absent.

2. A pharmaceutical composition comprising an effective amount of the compound of general formula (I), according to claim 1 , or a pharmaceutically acceptable salt thereof, together with one or more pharmaceutically acceptable adjuvants, excipients or diluents.

3. A compound as claimed in claim 1 for the manufacture of a contrast medium for use in a method of diagnosis involving administration of said contrast medium to an animate subject and generation of an image of at least part of said subject.