Redox trapping in hypoxia
A nitroimidazole is reduced inside the cell to a radical anion. Oxygen, if present, re-oxidises it and the tracer leaves. Where there is no oxygen, reduction continues and the product binds covalently to whatever is nearby.
The switch is the point: retention is not proportional to anything, it is gated by pO₂. That gives a genuinely functional readout of the property that makes tumours radioresistant, and it is the basis of dose-painting research.
One constraint follows from the mechanism and is easy to forget: the reduction is enzymatic, so trapping requires viable cells. Necrotic tissue is profoundly hypoxic and completely unlabelled. Hypoxia imaging shows you cells that are alive and starving, not cells that are dead.
The agents
F-18 FMISOfluoromisonidazole · research
PETF-18 · t½ 110 min · image at 2–4 h
- Handle
- Nitroreductase, gated by pO₂
- Trapping
- Reduced to a radical anion; re-oxidised and released if oxygen is present, further reduced and covalently bound if it is not. A tumour-to-blood ratio above about 1.2–1.4 defines hypoxia.
- Use
- Radiotherapy dose-painting research, glioma, head and neck cancer.
- Pitfall
- Slow kinetics and high background give poor lesion contrast — the reason it has never entered routine practice despite thirty years of interest. Not FDA-approved.
F-18 FAZAfluoroazomycin arabinoside
PETF-18 · t½ 110 min
- Handle
- Same nitroreductase mechanism
- Trapping
- More hydrophilic than FMISO, so background clears faster and contrast improves.
- Use
- Hypoxia imaging in trials, particularly head and neck.
- Pitfall
- Still investigational; absolute uptake remains low, so quantification is noise-limited.
Cu-64 ATSM—
PETCu-64 · t½ 12.7 h
- Handle
- Intracellular reducing environment
- Trapping
- Different chemistry: the Cu(II) complex is reduced to Cu(I) in low-oxygen cells and dissociates, stranding the copper inside.
- Use
- Faster imaging than FMISO, with better contrast.
- Pitfall
- Retention depends on cellular redox state as much as on pO₂ itself, which complicates the claim that the image is a hypoxia map. Tumour-type dependent.