Stabilized 111In-Labeled sCCK8 Analogues for Targeting CCK2-Receptor Positive Tumors: Synthesis and Evaluation

Radio labeled cholecystokinin-8 (CCK8) peptide analogues can be used for peptide receptor radionuclide imaging and therapy for tumors expressing CCK2/gastrin receptors. Earlier findings indicated that sulfated CCK8 (sCCK8, Asp-Tyr(OSO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2) may have better characteristics for peptide receptor radionuclide therapy (PRRT) than gastrin analogues. However, sCCK8 contains an easily hydrolyzable sulfated tyrosine residue and two methionine residues which are prone to oxidation. Here, we describe the synthesis of stabilized sCCK8 analogues, resistant to hydrolysis and oxidation. Hydrolytic stability was achieved by replacement of the Tyr(OSO3H) moiety by a robust isosteric sulfonate, Phe(p-CH2SO3H). Replacement of methionine by norleucine (Nle) or homopropargylglycine (HPG) avoided undesired oxidation side-reactions. The phenylalanine analogue Phe(p-CH2SO3H) of L-tyrosine, synthesized by a modification of known synthetic routes, was incorporated in three peptides: sCCK(8)[Phe(2)(p-CH2SO3H),Met(3,6)], sCCK8[Phe(2)(p-CH2SO3H),Nle(3,6)], and sCCK8[Phe(2)(p-CH7SO3H),HPG(3,6)]. All peptides were N-terminally conjugated with the macrocyclic chelator DOTA (1,4,7,10-tetraftzacyclododecane-N,N',N '',N'''-tetraacetic acid) and radiolabeled with In-111. In vitro binding assays on CCK2R-expressing HEK293 cells revealed that all three peptides showed specific binding and receptor-mediated internalization, with binding affinity values (IC50) in the nanomolar range. In vitro oxidation studies demonstrated that peptides with Nle or HPG indeed were resistant to oxidation. In vivo targeting studies in mice with AR42J tumors showed that tumor uptake was highest for In-111-DOTA-sCCK8 and In-111-DOTA-sCCK8[Phe(2)(p-CH2SO3H),Nle(3,6)] (4.78 +/- 0.64 and 4.54 +/- 1.15% ID/g, respectively, 2 h p.i.). The peptide with the methionine residues replaced by norleucine (In-111-DOTA-sCCK8[Phe(2)(p-CH2SO3H), Nle(3,6)]) showed promising in vivo characteristics and will be further investigated for radionuclide imaging and therapy of CCK2R-expressing tumors.