Targeting of the Cholecystokinin-2 Receptor with the Minigastrin Analog ¹⁷⁷Lu-DOTA-PP-F11N: Does the Use of Protease Inhibitors Further Improve In Vivo Distribution?

Abstract

Patients with metastatic medullary thyroid cancer (MTC) have limited systemic treatment options. The use of radiolabeled gastrin analogs targeting the cholecystokinin-2 receptor (CCK2R) is an attractive approach. However, their therapeutic efficacy is presumably decreased by their enzymatic degradation in vivo. We aimed to investigate whether the chemically stabilized analog Lu-DOTA-PP-F11N (Lu-DOTA-(DGlu)6-Ala-Tyr-Gly-Trp-Nle-Asp-Phe-NH2) performs better than reference analogs with varying in vivo stability, namely Lu-DOTA-MG11 (Lu-DOTA-DGlu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2) and Lu-DOTA-PP-F11 (Lu-DOTA-(DGlu)6-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2), and if the use of protease inhibitors further improves CCKR2-targeting. First human data on Lu-DOTA-PP-F11N are also reported. In vitro stability of all analogs was assessed against a panel of extra- and intra-cellular endoproteases, while their in vitro evaluation was performed using the human MTC MZ-CRC-1 and the transfected A431-CCK2R(+) cell lines. Biodistribution without and with the protease inhibitors phosphoramidon (PA) and thiorphan (TO) was assessed 4h post-injection in MZ-CRC-1 and A431-CCK2R(+) dual xenografts. Autoradiography of Lu-DOTA-PP-F11N (without and with PA) and nanoSPECT/CT images were performed. SPECT/CT images ofLu-DOTA-PP-F11N in a metastatic MTC patient were also acquired. natLu-DOTA-PP-F11N is less of a substrate for neprilysines than the other analogs, while intracellular cysteine proteases, like cathepsins-L, might be involved in the degradation of gastrin analogs. The uptake of all radiotracers was higher in MZ-CRC-1 tumors, compared to A431-CCK2R(+), apparently due to the higher number of binding sites on MZ-CRC-1 cells.Lu-DOTA-PP-F11N has the same biodistribution as Lu-DOTA-PP-F11, however, the uptake in the MZ-CRC-1 tumors is almost double (20.7±1.71 vs 11.2±2.94 %IA/g, = 0.0002). Co-administration of PA or TO increases significantly the Lu-DOTA-MG11 uptake in the CCK2R(+) tumors and stomach. Less profound is the effect onLu-DOTA-PP-F11, while no influence or even reduction is observed for Lu-DOTA-PP-F11N (20.7±1.71 vs 15.6±3.80 (with PA) %IA/g, p<0.05 in MZ-CRC-1 tumors). First clinical data show high Lu-DOTA-PP-F11N accumulation in the tumors, stomach, kidneys and colon. The performance of Lu-DOTA-PP-F11N without protease inhibitors is as good as the performance ofLu-DOTA-MG11 in the presence of inhibitors. The human application of single compounds without unessential additives is preferable. Preliminary clinical data spotlight stomach as a potential dose-limiting organ beside the kidneys.

Cholecystokinin 2 Receptor Agonist 177Lu-PP-F11N for Radionuclide Therapy of Medullary Thyroid Carcinoma: Results of the Lumed Phase 0a Study

“…The evaluation of the different radiolabeled minigastrin derivatives differing in both, the Cterminal peptide binding moiety and the structural characteristics of the linker, revealed that the compounds with six D-glutamic acid residues within the amino acid chain belong to the most promising substances with an reasonable balance of tumor uptake and decreased kidney reabsorption (11,12,16). The pronounced hydrophilicity of such compounds is most likely the reason for the fast renal excretion and shorter blood half-life of 177 Lu-PP-F11N in comparison to…”

“…Another approach to reduce nephrotoxicity is to modify the compound itself: Amino acid chains with more than 5 glutamic acids in their sequence play an important role in the kidney reuptake mechanism and may reduce kidney uptake (10,11). Such an approach was implemented by several groups and resulted in the development of a library of improved radiolabeled minigastrin analogues (12)(13)(14)(15)(16)(17). Some of these new compounds show higher tumor uptake, higher tumor-to-kidney uptake ratios and higher serum stability, compared to the previously developed minigastrin analogues.…”

“…(DOTA-(DGlu) 6 -Ala-Tyr-Gly-Trp-Nle-Asp-PheNH 2 ) which can be labeled with Lutetium-177, a widely used radionuclide that has not only excellent therapeutic properties but also a gamma ray component that allows acquisition of good quality images for a personalized treatment approach (theranostics), as well as dosimetry studies (16).…”

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

“…Already in 2002, [ 90 Y]Y-DTPA-MG0 was studied in 8 patients [234] with limited therapeutic success and severe nephrotoxic side effects, a reason why this study was not continued. Recently, pharmacokinetic properties, dosimetry, and the targeting ability of [ 177 Lu]Lu-DOTA-PP-F11N was tested in an initial clinical study for potential use of CCK2 receptor-based endoradiotherapy [235]. Tumour uptake seems sufficient for successful therapeutic use, with the stomach as the main dose-limiting organ.…”

New Developments in Peptide Receptor Radionuclide Therapy

“…PRRT with radiolabeled gastrin analogs is therefore an attractive treatment option for patients with recurrent or metastatic medullary thyroid cancer. Of large libraries, the analogs 177 Lu-PP-F11N (Table 2) and 111 In-CP04 (PP-F11, Table 2), as a surrogate for 177 Lu-or 90 Y-labeled CP04, are currently in early prospective trials (LUMED/ NCT02088645 and GRAN-T-MTC/NCT03246659, respectively) to determine safety, maximum tolerated dose, biodistribution, and dosimetry (24,25). Preliminary clinical data identified the stomach as a potential dose-limiting organ, whereas the renal absorbed dose was rather low.…”