Medullary thyroid cancer (MTC) is a rare disease and represents 3.5 % of all thyroid cancers. MTC is a neuroendocrine tumour which in comparison to other thyroid cancers does not derive from thyroid follicular cells, while its metastases does not take up radioiodine. Thus, the management of patients with MTC is more demanding. The majority of MTC patients presents with the advanced disease at the time of diagnosis, since it is metastasizing before the onset of the symptoms. The currently available therapeutic options (surgery, conventional chemotherapy, latest tyrosine kinase inhibitors) are only partially effective and connected to side effects. The search for effective treatment is still a challenge for many research groups. Nuclear medicine plays an important part in the field of oncology for imaging of tumour cells and/or targeted radionuclide therapy. Radiolabelled peptides are an important class of radiopharmaceuticals in imaging and therapy of tumours, overexpressing receptors for regulatory peptides. These molecules have high affinity and specificity for receptors and enable visualisation and/or targeted radionuclide therapy. Overexpression of CCK2R on MTC cells is the molecular basis for the development of radiolabelled CCK/gastrin analogues. Thus, nuclear medicine can play a major role in both diagnosis (scintigraphy, PET/CT) and therapy of advanced MTC. Besides CCK2R, MTC cells also overexpress somatostatin receptors. Receptor co-expression enables the use of multiple radiolabelled ligands in the management of MTC and development of multireceptor (hybride) targeting molecules. The main goal of dissertation was design, synthesis, radiolabelling and characterisation of minigastrin and somatostatin analogues. The radiolabelling of these molecules with different radionuclides (indium-111, gallium-68, lutetium-177, yttrium-90, bismuth-213) and their characterisation enables translation of these molecules into clinical practice. We have designed the research work in such a way that we also can influence the daily clinical practice regarding preparation of radiopharmaceuticals and patient management. The results from the study in chapter one are the basis for the development of novel radiolabelled minigastrin analogues. Nephrotoxicity is the main side effect in targeted radionuclide therapy due to renal reuptake and accumulation of radiolabelled peptides. The main mechanism of high kidney retention of radiolabelled minigastrin analogues has been related to N-terminal glutamic acids (the overall charge of molecule) and can be reduced by coinjection of polyglutamic acids or gelofusine. The comparison of two radiolabelled minigastrin analogues with the same chemical composition and charge, differing only in the stereochemistry of the amino acids that form the spacer, showed the difference in enzymatic stability and large difference in their kidney uptake and retention. We also showed the influence of secondary structure on the pharmacokinetics of the radiopeptides. In the first part of the research, we synthesised and characterised several novel, indium-111 radiolabelled minigastrin analogues. We developed a series of minigastrin analogues with mixed (non-ionic/ionic) amino acids that form the spacer. The introduction of mixed ionic/non-ionic spacers lead to a lower enzymatic stability in vitro in comparison to non-ionic spacer (6 D-Gln) analogue, whereas, relative to the charged analogue with 6 D-Glu as a spacer, there was no significant change in the stability. All radiopeptides tested show similar internalisation rate in cell lines and have similar binding affinity for CCK2R in nM range. Methionine in the C-terminal part of the peptide is oxidation-prone, especially during the radiolabelling step at high temperature. The binding affinity for the CCK2R is lost, if the methionine residue is oxidized (sulphones, sulphoxides), which was also demonstrated in the experiment in vivo. To overcome the problem of oxidation, we introduced isosteric replacement (norleucine) of the oxidation-prone methionine in the C-terminus of the analogues. In contrast to literature data, direct comparison between methionine and norleucine congeners in vitro and in vivo showed no significant difference in the behaviour of these analogues. We also showed that these analogues are quickly accumulated in the tumour and retained there for up to 4h p.i. Good tumour retention is a prerequisite for the use of these analogues in PRRT. The main advantages of norleucine congeners are more effective formulation for radiolabelling, better radiolabelling results and fewer side products which, in our case, can lead to safer and more effective radiopharmaceuticals. Radiolabelling of the same molecule with different radionuclides (diagnostic – 111In and 68Ga and therapeutic - 177Lu, 90Y, 213Bi) enables so called theranostic approach to patient management. 68Ga is a generator-based PET radionuclide. The full automation of the processes in everyday clinical and research practice is required when handling the high-energy PET or β- emitters. In the second part of the research, we used a novel, self-shielded radiosynthesis box for fully automated preparation of radiopharmaceuticals and determined the shielding properties of the modular synthesis system. We developed radiolabelling methods of novel minigastrin analogues and optimized methods for radiolabelling of clinical relevant somatostatin analogues with 68Ga, 90Y and 177Lu. We radiolabelled minigastrin analogues with 213Bi and compared in vitro characteristics with 111In labelled ones. There was no significant difference between two analogues in vitro. We showed the feasibility of 213Bi radiolabelling and thus the potential use of these analogues in alpha radionuclide therapy. Co-expression of both cholecystokinin and somatostatin receptors on the surface of MTC cells is a basis for multireceptor targeting of these tumour cells. We developed minigastrin analogues with improved characteristics and developed radiolabelling methods for novel minigastrin analogues and optimized radiolabelling of clinical relevant somatostatin analogues. The optimisation of both radioligands is a prerequisite for a possible multireceptor targeting of MTC. We also designed a hybride molecule with both minigastrin and somatostatin moiety. We successfully synthesised hybride molecule employing principles of “click” chemistry. However, the conjugation of bifunctional chelator to the molecule, which enables radiolabelling and further characterization of the molecule, was not successful. The importance of correct measurements of radioactivity in radionuclide calibrator is described in the last chapter. Imprecise measurement leads to administration of either excessive or too low activity to the patient. Excessive activity administered is not in agreement with patient radioprotection and in the worst case can lead to toxicity. On the other hand, too low activity administered can result in insufficient quality of diagnostic scans or inadequate response in case of targeted radionuclide therapy. Type of radiation, geometry and volume of radiation influence the measurement of radioactivity. Measurements of 90Y and 111In in different radionuclide calibrators showed significant difference between measured and true activity, which can be as much as 25%, by either overestimation or underestimation of true activity. Measuring true activity is especially critical for efficient and safe targeted PRRT with β- emitters.
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