What can gallium-68 PET add to receptor and molecular imaging?

Abstract

In the last decade there has been a significant increase in the development of radiolabelled peptides for diagnostic applications, especially due to simplified methods of purification. Peptides have fast clearance, rapid tissue penetration, and low antigenicity and can therefore be produced easily and inexpensively. In addition, if the diagnostic scan is positive, the peptides can be labelled with therapeutic radionuclides (yttrium-90, lutetium-177) and used for therapy [1]. Most efforts at labelling peptides have targeted somatostatin and its receptors. Somatostatin is a regulatory peptide widely distributed in the human body. Its action is mediated by membrane-bound receptors (SSTR) that are present in normal human tissues, such as thyroid, brain, gastrointestinal tract (GIT), pancreas, spleen and kidney [2]. They are also abundant in a variety of human tumours, notably neuroendocrine tumours (NET) [3] of which carcinoid tumour and phaeochromocytoma are encountered most in clinical practice. SSTR are also expressed, with variable abundance, in renal cell carcinoma, small cell lung cancer, breast cancer, prostate cancer and malignant lymphoma [4]. Somatostatin itself has a short half-life and is rapidly degraded by enzymes; therefore analogues have been developed which mimic its effects but are resistant to enzyme degradation. There are 5 somatostatin receptor subtypes but only subtypes 2 (SSTR2) and 5 (SSTR5) and to a lesser extent receptor subtype 3 (SSTR3) have a high affinity for commercially available synthetic analogues and even these differ in their affinity for the various receptor subtypes [5].