¹⁸F-FMISO or fluoromisonidazole is a radiopharmaceutical used for PET imaging of

hypoxia Hypoxia means a lower than normal level of oxygen, and may refer to: Reduced or insufficient oxygen * Hypoxia (environmental), abnormally low oxygen content of the specific environment * Hypoxia (medical), abnormally low level of oxygen in the tis ...

. It consists of a 2- nitroimidazole molecule labelled with the positron-emitter fluorine-18. Hypoxia is considered a negative prognostic marker for many

solid tumours A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

, and therefore an agent to detect and quantify it is highly desirable. FMISO was one of the first such agents, after initial synthesis in the late 1980s. It remains among the most popular agents for investigation of hypoxia imaging.

Mechanism

On entering a viable cell, the nitro group of the FMISO nitroimidazole is reduced. In non-hypoxic cells, the reduced FMISO molecule can be oxidised, and therefore diffuses out of the cell to circulate freely and ultimately be excreted. In hypoxic tumour cells however this oxidation cannot take place and the FMISO molecules accumulate. Their location can then be quantitatively imaged using

positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in Metabolism, metabolic processes, and in other physiological activities including bl ...

Clinical use

Large scale clinical trials with FMISO have not been carried out, however there is some evidence from small-scale early-stage imaging trials that PET-measured hypoxia (using FMISO, and the alternative radiotracer FAZA) is linked to overall survival and loco-regional control in head and neck cancer patients. Similar correlations have been found in other cancers, including breast cancer and

brain tumour A brain tumor occurs when abnormal cells form within the brain. There are two main types of tumors: malignant tumors and benign (non-cancerous) tumors. These can be further classified as primary tumors, which start within the brain, and secondary ...

s. Direct impacts on patient care has not yet been conclusively demonstrated however. The use of hypoxia imaging to guide

radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...

treatments is an area of active research. Despite some positive early results further research is required to characterise the

specificity and sensitivity In medicine and statistics, sensitivity and specificity mathematically describe the accuracy of a test that reports the presence or absence of a medical condition. If individuals who have the condition are considered "positive" and those who do ...

of FMISO, and exactly how hypoxia levels should influence treatment planning decisions. Similarly, hypoxia imaging could be used to screen patients before the prescription of hypoxic guided drugs. It may also be useful as a post-treatment measure of effectiveness for both radiotherapy and chemotherapy. Outside of oncology, there is interest in cardiac hypoxia imaging. FMISO has had limited interest for this purpose, in part due to low target-to-background contrast and long injection to imaging delays (due to slow blood clearance) requiring high injected activities.

References

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External links

18F-Fluoromisonidazole
Molecular Imaging and Contrast Agent Database (MICAD) PET radiotracers Nitroimidazoles