Labelling of octreotide using 76Br‐prosthetic groups

A method for labelling the octapeptide octreotide (D–Phe–Cys–Phe–D–Trp–Lys–Thr–Cys–Thr(ol)) with the positron emitting 76Br (T1/2=16 h) is presented. ϵ‐Boc‐protected octreotide was conjugated to N–succinimidyl 4‐[76Br]bromobenzoate 1 and N–succinimidyl 5‐[76Br]bromo‐3‐pyridinecarboxylate 3 using microwave heating. The conjugates 4 and 5 were isolated in 50–55% radiochemical yield after the removal of the protecting Boc‐group. Compound 3 was synthesised from the corresponding trimethylstannyl‐precursor in 25% radiochemical yield. The synthesis of methyl‐4‐[76Br] bromobenzimidate 8 in 40% radiochemical yield from the precursor methyl‐4‐trimethylstannylbenzimidate is also described. Experiments were performed to react 8 with Boc‐octreotide but no product was obtained. The binding‐properties of 76Br‐conjugates 4 and 5 to meningiomas were investigated using frozen section autoradiography. Compound 5 showed better binding properties than 4.