Iodine radiolabelling method

A method of radioiodination of a biological targeting moiety, said method comprising:

• (i) provision of a compound of Formula (I):
• (ii) reaction of said compound of Formula (I) with a compound of Formula (IIa) or Formula (IIb):
• I*—Y1-L2-N3 (IIa)
• I*—Y1-L3-C≡N+—O− (IIb)
• in the presence of a click cycloaddition catalyst, to give a conjugate of Formula (IIIa) or (IIIb) respectively:
• wherein:
• I* is a radioisotope of iodine suitable for in vivo imaging, chosen from 123I, 124I or 131I;
• L1, L2 and L3 are each independently a linker group which may be present or absent;
• Y1 is —Ar1— or —X1—;
• where —Ar1— is C3-10 arylene, and
• —X1— is —CH═CH—(CH2)n—(Ar1)j— or —CH═CH—(Ar1)j—(CH2)n—
• where j is 0 or 1, and n is an integer of value 0 to 4,
• wherein when Y1 is —X1—, I* is attached to —CH═CH terminus of X1;
• BTM is the biological targeting moiety.