In order to optimize elevated kidney retention of previously reported minigastrin derivatives, we substituted (R)-DOTAGA by DOTA in (R)-DOTAGA-rhCCK-16/-18. CCK-2R-mediated internalization and affinity of the new compounds were determined using AR42J cells. Biodistribution and µSPECT/CT imaging studies at 1 and 24 h p.i. were carried out in AR42J tumor-bearing CB17-SCID mice. Both DOTA-containing minigastrin analogs exhibited 3- to 5-fold better IC50 values than their (R)-DOTAGA-counterparts. natLu-labeled peptides revealed higher CCK-2R affinity than their natGa-labeled analogs. In vivo, tumor uptake at 24 h p.i. of the most affine compound, [19F]F-[177Lu]Lu-DOTA-rhCCK-18, was 1.5- and 13-fold higher compared to its (R)-DOTAGA derivative and the reference compound, [177Lu]Lu-DOTA-PP-F11N, respectively. However, activity levels in the kidneys were elevated as well. At 1 h p.i., tumor and kidney accumulation of [19F]F-[177Lu]Lu-DOTA-rhCCK-18 and [18F]F-[natLu]Lu-DOTA-rhCCK-18 was high. We could demonstrate that the choice of chelators and radiometals has a significant impact on CCK-2R affinity and thus tumor uptake of minigastrin analogs. While elevated kidney retention of [19F]F-[177Lu]Lu-DOTA-rhCCK-18 has to be further addressed with regard to radioligand therapy, its radiohybrid analog, [18F]F-[natLu]Lu-DOTA-rhCCK-18, might be ideal for positron emission tomography (PET) imaging due to its high tumor accumulation at 1 h p.i. and the attractive physical properties of fluorine-18.
Keywords: 18F-labeled minigastrin analogs; MTC; cholecystokinin-2 receptor (CCK-2R); cholecystokinin-B receptor (CCK-BR); radiohybrid; rhCCK.