Peptide Receptor Radionuclide Therapy in Neuroendocrine Neoplasms : Aspects of tumour characteristics, receptor recycling and peptide mass

Abstract: Neuroendocrine neoplasm (NEN) can arise in any part of the body, but most commonly in the lungs, bronchi, and the gastrointestinal tract including the pancreas. They combine neuroendocrine and tissue-of-origin-specific characteristics; explaining different symptoms depending on the organ of origin. NEN is divided into slow-growing neuroendocrine tumours (NETs) and the rarer aggressive neuroendocrine cancers (NECs). Some hormone producing NETs give rise to symptoms (functioning), generally detected earlier than the non-functioning NETs, which often are larger and metastatic at diagnosis. NETs commonly express an abundance of somatostatin receptors (SSTR). Synthetic copies of somatostatin (somatostatin analogues, SSA), supress hormonal symptoms such as diarrhoea and flush. The SSA-SSTR ligand-receptor complex interaction instantly internalises into the cells, separate, and the SSTR re-surface. Gallium-68 (68Ga)-labelled SSAs are used for PET/CT-camera visualisation of NETs, and SSA labelled with a therapeutic radionuclide, provide a means for internal radio-therapy, peptide receptor radionuclide therapy (PRRT).The aim of the thesis was to compare the tumour response to PRRT in small intestinal NET (SI-NET) and pancreatic NET (P-NET). Study I, II and IV are retrospective and include patients who underwent PRRT with 177Lu-DOTA-TATE at the Uppsala University Hospital. Study I, quantified and related the radiation dose in 25 SI-NETs to tumour shrinkage using two- and three-dimensional measurements, although no dose-response relationship was demonstrated. A relationship between tumour shrinkage and the total administered activity was however found. Study II compared the tumour response between SI-NETs from study I with P-NETs included in an earlier report, now re-evaluated by adding more tumour parameters, and with longer observation time. There radiation dose in P-NETs was the same as in SI-NETs. The radiation dose in P-NETs was highest at the first PRRT cycles, and then decreased significantly in consecutive cycles, which was not observed in SI-NETs.The prospective study III, mapped the recirculation time of SSTR in SI-NETs and normal organs. Twelve tumours were measured at repeated 68Ga-DOTA-SSA-PET examinations. Larger tumours (>4 cm) showed a faster SSTR turn-over rate than small tumours, demonstrating a turnover resembling that in the normal organs. These results open the possibility that pre-treatment could protect normal tissues during PRRT, and probably increase radioactivity tumour uptake and hence, the radiation dose.The retrospective study IV investigated the effects of various amounts of SSA delivered in the PRRT preparation, although the absorbed radiation dose to tumours and normal tissues, was unrelated to the amount of peptide and to the patient’s total tumour burden.