Beyond FDG: novel PET tracers for cancer imaging

Abstract

Despite the excellent clinical performance of fluorodeoxyglucose (FDG) as a cancer-imaging agent for positron emission tomography (PET), false positive and false negative results can be problematic in some clinical settings. Radiopharmaceutical development has recently focussed on the search for new PET tracers that could complement or replace FDG in such settings. Due to the general availability and favourable physical properties of fluorine-18, much effort has been directed to fluorinated compounds. The most promising of these are discussed.

Figure 1

Baseline FLT imaging in this patient with metastatic malignant melanoma demonstrated splenic (S), peritoneal (P) and BM metastases. After 14 days of treatment with a novel antiangiogenesis agent the upper abdominal peritoneal deposit (vertical arrow) had substantially decreased activity while the lower abdominal focus could no longer be visualised. Uptake at sites of baseline abnormality in the spleen (horizontal arrow) and right femoral BM (oblique arrow) were relatively photopaenic compared to adjacent normal tissues. FDG–PET scanning (not shown) demonstrated no change over the same period.

Figure 2

Most clinical studies of hypoxia imaging have utilized the nitroimadozole, [¹⁸F]fluoromisonidazole (FMISO). Slow blood pool clearance and high lipophilicity contribute to significant background activity and relatively low contrast between hypoxic and normal tissues. A new agent [¹⁸F]fluoro-azomyacinarabinofuranoside (FAZA) has lower lipophilicity as demonstrated by low brain uptake in the left panel. More rapid blood clearance with similar absolute uptake in hypoxic tissue leads to higher contrast as demonstrated in this comparative study of FAZA (left) and FMISO (right) scans in a patient with locally advanced retropharyngeal cancer. These images are normalised for background activity.