Metallofullerene contrast agents

1. An endohedral metallofullerene compound represented by the formula (III): A k X n N i @C m ′,

wherein A and X represent identical or different elements in the periodic table of elements, provided that at least one of A and X is paramagnetic;

C m ′ represents a fullerene having one or more missing carbon atoms and/or bonds, wherein the fullerene is derived from C m , which may optionally be substituted;

C m represents a fullerene molecule having m carbon atoms;

k represents an integer from 1 to 3;

n represents an integer from 0 to 2, provided that 1≦k+n≦3;

i represents an integer of 0 or 1; and

m represents an even integer from about 60 to about 200.

2. The compound of claim 1 , which in water has a relaxivity of about 30 mM −1 ·S −1 to about 300 mM −1 ·S −1 and forms a dispersion in water with entities having an average hydrodynamic radius of less than 20 nm.

3. The endohedral metallofullerene compound of claim 1 , wherein at least one of A and X represents a rare earth element or a group IIIB element in the periodic table of elements.

4. The endohedral metallofullerene compound of claim 1 , wherein at least one of A and X is selected from the group consisting of scandium, yttrium, lanthanum, gadolimium, holmium, erbium, thulium, dysprosium, terbium and ytterbium.

5. The endohedral metallofullerene compound of claim 1 , wherein A k X n N i @C m ′ represents Gd@C 60 ′, Gd@ C 82 ′ or Gd 3 N@C 80 ′.

6. The endohedral metallofullerene compound of claim 1 , wherein the substituent(s) in C m ′ further comprise one or more targeting species selected from the group consisting of cholesterol, L-DOPA, tropane or its derivatives, tetrabenazine, urushiol, transferrin, muc-1, antibodies, cytokines, saccharides, polysaccharides, glutamate and aminolevulinic acid.

7. The endohedral metallofullerene compound of claim 1 , wherein the C m ′ is optionally substituted by:

(a) one or more groups which are directly bonded to C m via an atom selected from the group consisting of N, O, S and P, and/or

(b) one or more groups comprising a hydrophilic moiety.

8. The endohedral metallofullerene compound of claim 1 , wherein C m ′ is substituted with at least one selected from the group consisting of an oxygen atom and a —COOH group.

9. A composition comprising the endohedral metallofullerene compound of claim 1 .