Enhanced CCK2R targeting for theragnostic use in nuclear medicine

Radiolabelled minigastrin (MG) analogues with specific binding to the Cholecystokinin-2 receptor (CCK2R) provide a very sensitive molecular imaging technique for the detection of medullary thyroid carcinoma (MTC) and other CCK2R expressing malignancies. When radiolabelled with beta emitting radionuclides these analogues could also be used for targeted radiotherapy. The different radiolabelled MG analogues developed so far either show a very high retention in the kidneys or have a limited metabolic stability in vivo hindering especially the therapeutic application. Within this project new stabilisation strategies will be explored to improve the pharmacokinetic profile and tumour targeting properties of MG analogues. By applying specific amino acid substitutions to four specific positions in the linear peptide sequence novel MG analogues with improved stability and retained receptor binding will be developed. These modifications will focus mainly on the stabilisation of the C-terminal receptor specific peptide sequence, but will include also additional substitutions in the N-terminal region. Up to twelve different new peptide analogues will be synthesised and derivatised with the macrocyclic chelator DOTA which enables radiolabelling with different radioisotopes, such as In-111 for single-photon emission computed tomography, Ga-68 for positron emission tomography, and Lu-177 for theragnostic use. The DOTA-peptides developed within this will undergo an extensive evaluation in vitro and in vivo to identify one most promising candidate for future clinical studies. This MG analogue will show a high tumour uptake and tumour retention, as well as low kidney retention and high tumour to kidney activity ratio. The preclinical evaluation in vitro and in vivo will include the characterisation of the stability in serum and tissue homogenates, the evaluation of CCK2R binding and cell internalisation on CCK2R expressing cell lines, as well as biodistribution studies in an animal tumour model including small animal imaging and dosimetry studies. The development of a radiolabelled probe for the molecular imaging and targeted therapy of MTC and other CCK2R expressing malignancies will be relevant for the clinical management of cancer patients. Additionally, the gained knowledge in stabilisation strategies for linear peptide sequences will be valuable also for the development of other peptide based molecules targeting alternative receptors. abstract page 1 of 1

Radiolabelled molecules can be used in nuclear medicine to detect cancer lesions and to selectively irradiate and destroy cancer cells. Cancer cells often express specific target structures on the cell surface, which can be selectively targeted with specific substances. For example, medullary thyroid carcinomas and small cell lung carcinomas often exhibit the cholecystokinin-2 receptor on the cell surface. The endogenous peptide hormone gastrin binds to this protein structure. This lock-and-key principle was used in this project to develop a new diagnostic procedure and a new therapeutic concept for these rare diseases. The main focus was laid on specific properties of the substance. In addition to low accumulation in the kidneys to avoid undesired side effects in therapy, special attention was paid to stabilization against degradation by endogenous enzymes to improve the tumour uptake. Specific stabilizing chemical modifications were introduced to develop new gastrin derivatives with increased stability against enzymatic degradation, but with retained receptor binding. Radiolabelling was achieved by incorporation of the chelator DOTA, which is widely used in nuclear medicine. DOTA forms stable complexes with various radiometals, enabling radiolabelling with indium-111 for single-photon emission tomography, gallium-68 for positron emission tomography, as well as yttrium-90 and lutetium-177 for therapeutic application. The radiolabelled gastrin derivatives developed were evaluated in extensive preclinical studies. This studies included the characterization of stability in serum and tissue homogenates, the evaluation of the specific receptor binding and of the receptor-specific cell uptake. Using an animal tumor model the biodistribution, excretion and tumour targeting was investigated, and preclinical diagnostic imaging studies were performed on a dedicated small animal gamma camera. The generated results allowed to select a new gastrin derivative with promising properties for diagnostic and therapeutic clinical application. A first clinical trial could be initiated evaluating the substance radiolabelled with gallium-68 for high sensitive positron emission tomography imaging. The development of this new radiolabelled gastrin derivative opens new perspectives in the diagnosis and treatment of patients with cholecystokinin-2 receptor expressing tumours. The introduced stabilizing chemical modifications investigated within the project can be also applied to peptide hormones targeting other receptors. The scientific work performed thus makes an important contribution to molecular imaging and targeted therapy of cancer in nuclear medicine.