Figure 5.

Schematic overview of molecular or pathophysiologic targets for well-established and novel radiotracers: some of them address membrane-bound cell receptors or transporters like somatostatin receptors (SSTR), adenosine receptors A₃ (ADORA3), or L-amino-acid transporters (LAT1). Other tracers such as [¹¹C]choline are taken up by transporters as the choline transporter-like protein (CTL1) and incorporated into the phospholipids of proliferating cells. The enzyme substrates [³H]thymidine and [¹⁸F]FLT target proliferating cells by incorporation into the DNA via for example the cytosolic thymidine kinase (TK-1). Enzyme inhibitors, portrayed here by [¹⁸F]3, a radiolabeled selective cyclooxygenase 2 inhibitor, in turn accumulate in areas of high enzyme concentration and activity. The radiolabeled glucose derivative [¹⁸F]FDG visualizes glucose turnover by glucose transport (GLUT-1) and hexokinase (HK) reaction and consequently the metabolic activity and viability of cells, which is high in the normoxic proliferating regions of the spheroid and lower in the hypoxic core. In contrast, [¹⁸F]FMISO binds to macromolecules in a hypoxic environment and thus accumulates in the hypoxic core (Table 4).