Radiolabeled 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(OSO(3)H)-Met-Gly-Trp-Met-Asp-Phe-NH(2)) 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(OSO(3)H) moiety by a robust isosteric sulfonate, Phe(p-CH(2)SO(3)H). Replacement of
methionine by
norleucine (Nle) or
homopropargylglycine (HPG) avoided undesired oxidation side-reactions. The
phenylalanine analogue Phe(p-CH(2)SO(3)H) of
l-tyrosine, synthesized by a modification of known synthetic routes, was incorporated in three
peptides: sCCK8[Phe(2)(p-CH(2)SO(3)H),Met(3,6)], sCCK8[Phe(2)(p-CH(2)SO(3)H),Nle(3,6)], and sCCK8[Phe(2)(p-CH(2)SO(3)H),HPG(3,6)]. All
peptides were N-terminally conjugated with the macrocyclic
chelator DOTA (1,4,7,10-tetraazacyclododecane-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 (IC(50)) 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 (111)In-DOTA-sCCK8 and (111)In-
DOTA-sCCK8[Phe(2)(p-CH(2)SO(3)H),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 ((111)In-
DOTA-sCCK8[Phe(2)(p-CH(2)SO(3)H), Nle(3,6)]) showed promising in vivo characteristics and will be further investigated for
radionuclide imaging and
therapy of CCK2R-expressing
tumors.