Synthesis and in vitro evaluation of pharmacokinetically optimized 99mTc-labeled cholecystokinin-2 receptor-targeted peptides for improved SPECT imaging

Background Hitherto, clinically applied 99m Tc-based cholecystokinin-2 receptor (CCK-2R) ligands suffer from limited in vivo stability. Therefore, we focused on the development of pharmacokinetically optimized 99m Tc-labeled CCK-2R ligands for improved imaging of CCK-2R-overexpressing malignancies via single-photon emission computed tomography (SPECT). The novel CCK-2R ligands were designed based on the recently published compounds CCK-66, rhCCK-18 and rhCCK-84. Results CCK-2R ligands were prepared by solid-phase peptide synthesis (SPPS) and 99m Tc-labeled via the tetraamine (N 4 ) chelator (95 °C, 15 min) with final radiochemical purities ≥ 95% (radiochemical yields > 99%, n = 39), as determined by radio-RP-HPLC. Affinity (IC 50,inverse ) and internalization experiments were performed using CCK-2R + AR42J cells. Lipophilicity (logD 7.4 ), human serum albumin (HSA) binding and stability in human serum (4 h, 37 °C) was determined as well. Four of the seven novel CCK-2R ligands, i.e. [ 99m Tc]Tc-N 4 -CCK-66 and [ 99m Tc]Tc-N 4 -CCK-103 to -105, exhibited similar CCK-2R affinities compared to the reference ligands [ 177 Lu]Lu-DOTA-PP-F11N and [ 177 Lu]Lu-DOTA-CCK-66 (IC 50,inverse : 28.2–40.3 nM). [ 99m Tc]Tc-N 4 -CCK-66 and [ 99m Tc]Tc-N 4 -CCK-104 demonstrated high CCK-2R-mediated internalization (223.8 ± 0.1% and 145.2 ± 0.1% compared to [ 177 Lu]Lu-DOTA-PP-F11N, respectively), high stability in human serum (> 95%) and similar lipophilicity (log D 7.4 : − 2.09 to − 2.42) and HSA binding (47.5–53.2%) compared with [ 177 Lu]Lu-DOTA-CCK-66. 4-(Di- tert -butyl(hydroxy)silyl)benzoic acid (SiOH-BA)-bearing compounds [ 99m Tc]Tc-N 4 -CCK-100 and [ 99m Tc]Tc-N 4 -CCK-103 showed an elevated lipophilicity (logD 7.4 : − 1.89 to − 0.95) and HSA binding (78.1–89.1%) as well as a reduced stability in human serum (87.0–93.0%), but higher internalization than [ 177 Lu]Lu-DOTA-PP-F11N (145.0 ± 0.1% and 219.5 ± 0.1%, respectively). Conclusions Several 99m Tc-labeled CCK-2R ligands with very favorable in vitro characteristics were successfully developed in this study. In particular, [ 99m Tc]Tc-N 4 -CCK-66 and [ 99m Tc]Tc-N 4 -CCK-104 are highly recommended for further evaluation in in vivo studies to examine their potential as 99m Tc-based SPECT agents for improved imaging of CCK-2R-expressing malignancies. Furthermore, a possible radiotheranostic use of these N 4 -conjugated compounds could be explored in future studies using 186/188 Re.