Radiolabeled gastrin/CCK analogs in tumor diagnosis: towards higher stability and improved tumor targeting.

Cholecystokinin subtype 2 receptors (CCK2R) are overexpressed in several human cancers, including medullary thyroid carcinoma. Gastrin and cholecystokinin (CCK) peptides that bind with high affinity and specificity to CCK2R can be used as carriers of radioactivity to CCK2R-expressing tumor sites. Several gastrin and CCK related peptides have been proposed for diagnostic imaging and radionuclide therapy of primary and metastatic CCK2R-positive human tumors. Their clinical application has been restricted to a great extent by their fast in vivo degradation that eventually compromises tumor uptake. This problem has been addressed by structural modifications of gastrin and CCK motifs, which, however, often lead to suboptimal pharmacokinetic profiles. A major enzyme implicated in the catabolism of gastrin and CCK based peptides is neutral endopeptidase (NEP), which is widely distributed in the body. Coinjection of the NEP inhibitor phosphoramidon (PA) with radiolabeled gastrin and other peptide analogs has been recently proposed as a new promising strategy to increase bioavailability and tumor-localization of radiopeptides in tumor sites. Specifically, co-administration of PA with the truncated gastrin analog [(111)In-DOTA]MG11 ([((111)In-DOTA)DGlu(10)]gastrin(10-17)) impressively enhanced the levels of intact radiopeptide in mouse circulation and has led to an 8-fold increase of CCK2R-positive tumor uptake in SCID mice. This increased tumor uptake, visualized also by SPECT/CT imaging, is expected to eventually translate into higher diagnostic sensitivity and improved therapeutic efficacy of radiolabeled gastrin analogs in CCK2R-expressing cancer patients.

Optimised labeling, preclinical and initial clinical aspects of CCK-2 receptor-targeting with 3 radiolabeled peptides.

Medullary thyroid carcinoma (MTC) expresses CCK-2 receptors. (111)In-labeled DOTA-DGlu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH(2) (DOTA-MG11), DOTA-DAsp-Tyr-Nle-Gly-Trp-Nle-Asp-Phe-NH(2) (DOTA-CCK), and (99m)Tc-labeled N(4)-Gly-DGlu-(Glu)(5)-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH(2) ((99m)Tc-Demogastrin 2) are analogs developed for CCK-2 receptor-targeted scintigraphy. All 3 radiolabeled analogs were selected on the basis of their high CCK-2 receptor affinity and their good in vitro serum stability, with in vitro serum t(1/2) values of several hours. Radiolabeling of DOTA-peptides with (111)In requires a heating procedure, typically in the range of 80 degrees -100 degrees C up to 30 min. Following this procedure with DOTA-MG11 resulted in a >98 % incorporation of (111)In, however, with a radiochemical purity (RCP) of <50 %. The decrease in RCP was found to be due to oxidation of the methionine residue in the molecule. Moreover, this oxidized compound lost its CCK-2 receptor affinity. Therefore, conditions during radiolabeling were optimised: labeling of DOTA-MG11 and DOTA-CCK with (111)In involved 5 min heating at 80 degrees C and led to an incorporation of (111)In of >98 %. In addition, all analogs were radiolabeled in the presence of quenchers to prevent radiolysis and oxidation resulting in a RCP of >90 %. All 3 radiolabeled analogs were i.v. administered to 6 MTC patients: radioactivity cleared rapidly by the kidneys, with no significant differences in the excretion pattern of the 3 radiotracers. All 3 radiolabeled analogs exhibited a low in vivo stability in patients, as revealed during analysis of blood samples, with the respective t(1/2) found in the order of minutes. In patient blood, the rank of radiopeptide in vivo stability was: (99m)Tc-Demogastrin 2 (t(1/2) 10-15 min)>(111)In-DOTA-CCK (t(1/2) approximately 5-10 min)>(111)In-DOTA-MG11 (t(1/2)<5 min).

Characterization and preliminary evaluation of ester-modified technetium-99m SNS/S mixed ligand complexes as potential brain perfusion agents.

Two novel [99mTc](SNS/S) mixed ligand complexes carrying a pendant ester function on the monothiolate coligand were synthesized. The corresponding oxorhenium and [99gTc]oxotechnetium complexes prepared at the macroscopic level and chemically characterized were used for structure assignment of [99mTc](SNS/S) complexes prepared at the nanomolar level. Enzymatic hydrolysis of the pendant ester group of [99mTc](SNS/S) mixed ligand complexes by esterase was investigated in vitro and compared with that of the ethyl cysteinate dimer, [99mTc]ECD. Preliminary biodistribution data in mice shows that the complexes are lipophilic and exhibit significant initial uptake in rodent brain.

Glutathione-mediated metabolism of technetium-99m SNS/S mixed ligand complexes: a proposed mechanism of brain retention.

Two series of [99mTc](SNS/S) mixed ligand complexes each carrying the N-diethylaminoethyl or the N-ethyl-substituted bis(2-mercaptoethyl)amine ligand (SNS) are produced at tracer level using tin chloride as reductant and glucoheptonate as transfer ligand. The identity of [99mTc](SNS/S) complexes is established by high-performance liquid chromatographic (HPLC) comparison with authentic rhenium samples. The para substituent R on the phenylthiolate coligand (S) ranges from electron-donating (-NH2) to electron-withdrawing (-NO2) groups, to study complex stability against nucleophiles as a result of N- and R-substitution. The relative resistance of [99mTc](SNS/S) complexes against nucleophilic attack of glutathione (GSH), a native nucleophilic thiol of 2 mM intracerebral concentration, is investigated in vitro by HPLC. The reaction of [99mTc](SNS/S) complexes with GSH is reversible and advances via substitution of the monothiolate ligand by GS- and concomitant formation of the hydrophilic [99mTc](SNS/GS) daughter compound. The N-diethylaminoethyl complexes are found to be more reactive against GSH as compared to the N-ethyl ones. Complex reactivity as a result of R-substitution follows the sequence -NO2 >> -H > -NH2. These in vitro findings correlate well with in vivo distribution data in mice. Thus, brain retention parallels complex susceptibility to GSH attack. Furthermore, isolation of the hydrophilic [99mTc](SNS/GS) metabolite from biological fluids and brain homogenates provides additional evidence that the brain retention mechanism of [99mTc](SNS/S) complexes is GSH-mediated.