3-¹²⁵I-4-HO-Phenylacetyl-[Lys-Psi(CH₂=NH)-Arg-Phe¹¹-t-Leu¹²-]NT(10-13): Radiosynthesis and characterisation

4-HO-Phenylacetyl-[Lys-Psi(CH₂=NH)-Arg-Phe¹¹-t-Leu¹²-]NT(10-13) is a Neurotensin (NT) derivative with a Ki of 25 nM and is a good candidate for labelling with radio iodine. A fast and efficient radio iodination procedure by electrophilic substitution is proposed. 0.1 µmol of peptide together with 200 MBq of Na¹²⁵I in 1 ml of PBS of pH 7.4 were added to an Iodogen coated vial. Labelling was performed at 20°C for 5 min. QC by HPLC and Sep Pak showed labelling yields >98%. The pure n.c.a. (75 GBq/mmol) compound was obtained after semipreparative HPLC purification followed by recovery from a RPC-18 Bonda Pak column. Overall synthesis yields were over 80%. Biodistribution of 150 kBq of the n.c.a. compound in HT29 bearing nude mice revealed tumour uptake to ±5% of the I.D./g 10 min p.i. which could be inhibited for 75% by competition with native NT. The in vitro half-life of the pure peptide in human blood is 67 h. Due to its excellent biological half-life and its acceptable biological properties, the proposed ¹²³I labelled peptide is a good candidate as a new peptidergic SPECT tracer for NT receptor expressing pathologies.