Labelling of various macromolecules using positron emitting 76Br and 68Ga : Synthesis and characterisation

Abstract: Different prosthetic groups containing a trialkylstannyl- and an electrophilic group have been synthesised and labelled with the accelerator produced 76Br (T1/2=16 h) through oxidative bromination. The labelled prosthetic groups were conjugated to amino-containing macromolecules such as proteins and 5´-modified oligonucleotides.N-Succinimidyl 4-[76Br]bromobenzoate 14 was synthesised in 65 % radio-chemical yield and was conjugated to 5´-hexylamino-modified phosphodiester and phosphorothioate oligonucleotides in 12-19 % isolated radiochemical yield. The stability of the 76Br-oligonucleotide-conjugates in vivo in rats was investigated. No degradation from the 5´-end, resulting in labelled, low molecular weight compounds was detected. Compound 14 has also been used for labelling of different proteins in 23-61% radiochemical yield.N-Succinimidyl-5-[76Br]bromo-3-pyridinecarboxylate 17 and methyl-4-[76Br]bromo-benzimidate 15 were synthesised from the corresponding trimethylstannyl-compound in 25% and 40 % yield respectively. Compounds 14 and 17 were conjugated to ε-Boc-octreotide in 55 and 50% isolated radiochemical yield respectively after microwave heating. Compound 15 did not react with octreotide under the conditions investigated. The two 76Br-labelled octreotide derivatives showed different lipophilicity and different binding-properties to tissue from meningiomas.Hyaluronic acid, a polysaccharide, was modified with tyramine and labelled by oxidative bromination using 76Br in 10% radiochemical yield.The generator produced 68Ga (T1/2=68 min) was used to label octreotide and oligonucleotides modified with the metal chelating group 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA). 68Ga-DOTA-octreotide was isolated in 65% radiochemical yield and a phosphorothioated 68Ga-DOTA-oligonucleotide was isolated in 35% radio-chemical yield after 30 min synthesis time.Compound 14 was reacted with 3-aminomethylbenzylamine to give compound 18. The specific radioactivity of 18 was determined to be 36 GBq/µmol by measuring the ratio between the mass-peaks for the 76Br and 79Br-compounds using packed-capillary LC-MS.