Technetium-99m somatostatin analogues: effect of labelling methods and peptide sequence

In this; paper the preclinical evaluation of the somatostatin analogue RC160 labelled with technetium-99m using bifunctional chelators (BFCs) based on the hydrarinonicotinamide (HYNIC) and N3S system is described and a comparison made with [Tyr(3)]-octreotide (TOC). Conjugates of both peptides with HYNIC, and of RC160 with benzoyl-MAG(3) and an N3S-adipate derivative were prepared and radiolabelling performed at high specific activities using tricine, tricine/nicotinic acid and ethylenediamine-N,N'-diacetic adic (EDDA) as co-ligands for HYNIC conjugates. All conjugates and Tc-99m-labelled peptides showed preserved binding affinity for the somatostatin receptor (IC50, Kd<5 nM). The biodistribution was markedly dependent on the BFC and co-ligand used, with the amidothiol ligands showing a greater degree of hepatobiliary clearance, the HYNIC/tricine complex higher blood levels and the HYNIC/EDDA complex the highest level of renal excretion and lowest blood levels. All peptide conjugates showed receptor-mediated uptake in tumour xenografts, but tumour uptake was significantly lower for the Tc-99m-RC160 derivatives compared with Tc-99m-EDDA/HYNIC- [Tyr(3)]-octreotide (0.2%-3.5%ID/g vs 9.7%ID/g) and correlated well with the reduced internalisation rate for RC160 derivatives. Our results show that the selection of the labelling approach as well as the right choice of the peptide structure are crucial for labelling peptides with Tc-99m to achieve complexes with favourable biodistribution. Despite the relatively low tumour uptake compared with 99mTc-EDDA/HYNIC-[Tyr(3)]-octreotide, Tc-99m-RC160 could play a role in imaging rumours that do not bind octreotide derivatives.