Radiopharmaceuticals for the Diagnosis and Therapy of Neuroendocrine Differentiated Prostate Cancer.

ABSTRACT

Neuroendocrine differentiation of prostate cancer (PCa) is a relatively frequent event, generally understudied, that carries important prognostic information. It is the most frequently observed during the advanced stages of disease, when PCa has lost its sensitivity to androgen deprivation therapy or to chemotherapy, moderate to diffuse bone metastatic spread dominates the imaging scenario and it is responsible for painful clinical symptomatology. However, evidences indicate that neuroendocrine differentiation is a progressive phenomenon that starts at the very early part of the pathogenesis of cancer transformation contributing to it. Neuroendocrine tumor phenotypes have reduced capability to secrete the prostate specific antigen (PSA) and therefore PSA does not represent a reliable marker to follow-up neuroendocrine differentiation. Tumor progression may be monitored by measuring plasma concentration of neuroendocrine tumor markers, primarily chromogranin A and neuron-specific enolase. Several nuclear medicine tracers are available for studying different biochemical properties of tumor cells with neuroendocrine differentiation. Single photon computed emission tomography (SPECT) with [111In-diethylenetriaminepentaacetic acid] ([111In-DTPA0])- octreotide (Octreoscan) has been extensively used in the past. However, the development of the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), which in comparison to DTPA allows higher affinity bindings for beta-emitting radionuclides and for somatostatin (SST) analogues, and the increased availability of the Germanium-68/Gallium-68 (68Ge/68Ga)-generator, which enables positron emission tomography/computed tomography (PET/CT) imaging, have allowed the synthesis of several PET tracers for different SST receptors. The receptor of the bombesin/ gastrin releasing peptide (GRP), which is overexpressed in PCa with neuroendocrine differentiation, also represents an innovative research field with diagnostic and therapeutic applications through, respectively, positron and beta emitters. At the moment, however, we observe some discrepancy between the high number of preclinical studies and the small number of clinical studies, most likely related to competing and, at the moment, more effective radiopharmaceuticals for imaging and for radiometabolic therapy, such PET/CT with radiolabeled choline and prostate-specific membrane antigene (PSMA)-ligands, the latter being labeled either with 68Ga for imaging or with Lutetium-177 for therapy. Radium-223 dichloride has also been recently successfully introduced for palliative therapy of bone metastases in PCa. For these reasons, while the development of radiopharmaceuticals for diagnosis and therapy (theranostics concept) of neuroendocrine differentiated PCa is scientifically stimulating, the ultimate clinical impact remains presently difficult to predict. Similar effectiveness in comparison to other forms of diagnostic and radiometabolic radiopharmaceuticals that have already gained convincing acceptance among referring clinicians needs to be demonstrated.