Radiopharmaceuticals used for diagnosis and therapy of NETs.

The first description of the in vivo visualization of somatostatin receptor-positive tumors in patients was based on the use of a radioiodine (¹²³I) labelled somatostatin analogue (Krenning et al. 1989). In the years that followed an Indium-111 (¹¹¹In) labelled somatostatin analogue, chelated with diethylenetriaminepentaacetic acid (DTPA), was successfully developed. Subsequently, ¹¹¹In-OctreoScan was introduced worldwide. In the years to come ^99mTc-Tektrotyde became commercially available with easy access. In the last decade, with the increasing use of positron emission tomography (PET) imaging, somatostatin analogues have been labelled with various positron-emitting isotopes, such as Gallium-68 (⁶⁸Ga) and Copper-64 (⁶⁴Cu) (Lewis et al. 1999, Schottelius et al. 2004, Gabriel et al. 2007) e.g ⁶⁸Ga-DOTATOC, ⁶⁸Ga-DOTATATE ⁶⁸Ga-DOTANOC and ⁶⁸Cu-DOTATATE. Scintigraphy with these investigational compounds display encouraging good imaging quality amd improved sensitivity in tumor site detection compared with SPECT scintigraphy. Also, other PET radiopharmaceuticals were developed, such as ¹⁸F-dihydroxy-phenyl-alanine (¹⁸F-DOPA) and ¹¹C-labelled 5-hydroxytryptophan (¹¹C-5-HTP) with encouraging results in terms of visualization of GEP-NETs (Koopmans et al. 2008). After the successful introduction of SRS in the diagnosis and staging of NETs, the next logical step was to increase the administered activity so that the radiopharmaceutical can induce tumor shrinkage in patients who had inoperable and/or metastasized NENs. Therefore, the first peptide receptor radionuclide therapy (PRRT) was performed with high administered activity of [¹¹¹In-DTPA0] octreotide (Krenning et al. 1994a). To make significant advancements in the treatment of somatostatin receptor-positive metastatic disease, more efficient radiolabelled somatostatin analogues were developed with higher affinity to the somatostatin receptor. Treatment with radiolabelled peptides or PRRT is a promising new therapeutic option in the management of inoperable or metastasized NETs. Symptomatic control can be achieved with all ¹¹¹In-, ⁹⁰Y- and ¹⁷⁷Lu-labelled somatostatin analogue-based PRRT. For objective response and long-lasting duration of response, ⁹⁰Y-DOTATOC and ¹⁷⁷Lu-DOTATATE are the most promising radiopharmaceuticals. Side effects of PRRT are few and mild, if adequate kidney protective measures are taken and dose-limits are respected. In a minority of patients, when SRS fails to identify neuroendocrine disease, MIBG scintigraphy and subsequent ¹³¹I-MBG therapy might be an alternative treatment option. Targeted alpha-particle therapy (TAT) has emerged as an alternative treatment option to beta emitters in PRRT. The use of alpha emitters for cancer therapy has two advantages over beta emitter PRRT. The short range of alpha particles of only a few cell diameters (<0.1mm) allows for selective ablation of the target cancer cells, while sparing the surrounding healthy tissue. In addition, the higher linear energy transfer (LET), when compared to conventional beta emitters, results in the formation of complex DNA double-strand and DNA cluster breaks, which ultimately lead to cell death.(Lassmann M et al. Ann ICRP. 2018) Putative radiopharmaceuticals that can be considered for metastatic NEN treatment include Actinium-225 (²²⁵Ac)-DOTATATE and Bismuth-213 (²¹³Bi)-DOTATOC. There was evidence of partial response using both radiopharmaceutical agents without significant hematological, renal, or hepatotoxicity. Future studies should consider longer term, randomized controlled trials investigating the role of TAT, in particular, ²²⁵c-DOTATATE, in the treatment of metastatic NENs. Nuclear medicine plays a pivotal role in the imaging and treatment of neuroendocrine tumors (NETs). New techniques in somatostatin receptor imaging include the use of different radiolabelled somatostatin analogues with higher affinity and different affinity profiles to the somatostatin receptor subtypes. Considerable advances have been made in the imaging of NETs, but to find the ideal imaging method with increased sensitivity and better topographic localization of the primary and metastatic disease remains the ultimate goal of research.