|Systematic (IUPAC) name|
|Mol. mass||181.15 g/mol
182.15 g/mol (FDG)
|Melt. point||170–176 °C (338–349 °F)|
|Bioavailability||N/A (given i. v.)|
|Metabolism||6-phosphorylation; normal glycolysis after decay|
|Half life||75%: 110 min; 20%: 16 min.|
|Excretion||20% radioactivity renally excreted in 2 hours; remainder decays in place to CO2 and H20|
|Pregnancy cat.||X(AU) X(US)|
|Legal status||? (US) Ethical pharmaceutical for nuclear medicine|
Fludeoxyglucose (18F) (INN) or fluorodeoxyglucose (18F), commonly abbreviated 18F-FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). Chemically, it is fludeoxyglucose (FDG), a glucose analog, with the positron-emitting radioactive isotope fluorine-18.
After 18F-FDG is injected into a patient, a PET scanner can form images of the distribution of FDG around the body. The images can be assessed by a nuclear medicine physician or radiologist to provide diagnoses of various medical conditions.
FDG, as a glucose analog, is taken up by high-glucose-using cells such as brain, kidney, and cancer cells, where phosphorylation prevents the glucose from being released again from the cell, once it has been absorbed. The 2' hydroxyl group (—OH) in normal glucose is needed for further glycolysis (metabolism of glucose by splitting it), but FDG is missing this 2' hydroxyl. Thus, in common with its sister molecule 2-deoxy-D-glucose, FDG cannot be further metabolized in cells. The 18F-FDG-6-phosphate formed when 18F-FDG enters the cell thus cannot move out of the cell before radioactive decay. As a result, the distribution of 18F-FDG is a good reflection of the distribution of glucose uptake and phosphorylation by cells in the body.
After 18F-FDG decays radioactively, however, its 2'-fluorine is converted to 18O–, and after picking up a proton H+ from a hydronium ion in its aqueous environment, the molecule becomes glucose-6-phosphate labeled with harmless nonradioactive "heavy oxygen" in the hydroxyl at the 2' position. The new presence of a 2' hydroxyl now allows it to be metabolized normally in the same way as ordinary glucose, producing non-radioactive end-products.
Although in theory all 18F-FDG is metabolized as above with a radioactivity elimination half-life of 110 minutes (the same as that of fluorine-18), clinical studies have shown that the radioactivity of 18F-FDG partitions into two major fractions. About 75% of the fluorine-18 activity remains in tissues and is eliminated with a half-life of 110 minutes (presumably by being metabolized in-place as glucose-6-phosphate, then eliminated in carbon dioxide and water); another fraction representing about 20% of the total fluorine-18 activity of an injection is eliminated renally by two hours after a dose of 18F-FDG, with a rapid half-life of about 16 minutes (this portion makes the renal-collecting system and bladder prominent in a normal PET scan). This short biological half-life indicates that this 20% portion of the total fluorine-18 tracer activity is eliminated pharmacokinetically (through the renal system) much more quickly than the isotope itself can decay. The urine of a patient undergoing a PET scan may therefore be especially radioactive for several hours after administration of the isotope.
All radioactivity of 18F-FDG, both the 20% which is rapidly excreted in the first several hours of urine which is made after the exam, and the 80% which remains in the patient, decays with a half-life of 110 minutes (just under 2 hours). Thus, within 24 hours (13 half-lives), the radioactivity in the patient and in any initially-voided urine which may have contaminated bedding or objects after the PET exam, will have decayed to 2-13 = 1/8200th of the initial radioactivity of the dose.
In PET imaging, 18F-FDG can be used for the assessment of glucose metabolism in the heart, lungs, and the brain. It is also used for imaging tumours in oncology, where usually dynamic images are analysed in terms of Standardized Uptake Values. 18F-FDG is taken up by cells, phosphorylated by hexokinase (whose mitochondrial form is greatly elevated in rapidly-growing malignant tumours), and retained by tissues with high metabolic activity, such as most types of malignant tumours. As a result FDG-PET can be used for diagnosis, staging, and monitoring treatment of cancers, particularly in Hodgkin's disease, non-Hodgkin's lymphoma, colorectal cancer, breast cancer, melanoma, and lung cancer. It has also been approved for use in diagnosing Alzheimer's disease.
In body-scanning applications in searching for tumor or metastatic disease, a dose of 18F-FDG in solution (typically 5 to 10 millicuries or 200 to 400 MBq) is typically injected rapidly into a saline drip running into a vein, in a patient who has been fasting for at least 6 hours, and who has a suitably low blood sugar. (This is a problem for some diabetics; usually PET scanning centers will not administer the isotope to patients with blood glucose levels over about 180 mg/dL = 10 mmol/L, and such patients must be re-scheduled). The patient must then wait about an hour for the sugar to distribute and be taken up into organs which use glucose — a time during which physical activity must be kept to a minimum, in order to minimize uptake of the radioactive sugar in muscles (this causes unwanted artifacts when the organs of interest are inside the body). Then, the patient is placed in the PET scanner for a series of one or more scans which may take from 20 minutes to as long as an hour (often, only about one quarter of the body length may be imaged at a time).
In the 1970s, Tatsuo Ido and Al Wolf at the Brookhaven National Laboratory were the first to describe the synthesis of 18F-FDG. The compound was first administered to two normal human volunteers by Abass Alavi in August, 1976 at the University of Pennsylvania. Brain images obtained with an ordinary (non-PET) nuclear scanner demonstrated the concentration of 18F-FDG in that organ (see history reference below).
Because the high energy particle bombardment conditions in the medical cyclotron which is used to produce 18F would destroy organic molecules like deoxyglucose or glucose, the radioactive 18F must be made first as fluoride in the cyclotron. This may be accomplished by bombardment of neon-20 with deuterons, but usually is done by proton bombardment of 18O-enriched water, causing a (p,n) reaction (sometimes called a "knockout reaction"—a common type of nuclear reaction with high probability) in the 18O. This produces "carrier-free" dissolved 18F-fluoride (18F–) ions in the water. The 109.8 minute half-life of 18F makes rapid and automated chemistry necessary after this point.
To do this chemistry, the 18F– is normally collected by ion exchange, followed by a phase transfer reaction that attaches the radioactive 18F atoms to the deoxyglucose in an automated series of chemical reactions in a "hot cell" (a radioisotope chemistry preparation chamber). Following this, the labeled 18F-FDG compound (still having a half-life only 109.8 minutes, or slightly less than 2 hours), is rapidly shipped to points of use by the fastest possible mode. Due to transport regulations for radioactive compounds, this is normally done by specially licensed road transport, but transport may also include dedicated small commercial jet services, to extend the reach of PET scanning to centers hundreds of miles away from the cyclotron and laboratory which produce the radioisotope-labeled compound.
Recently, on-site cyclotrons with integral shielding and portable chemistry stations for making 18F-FDG have accompanied PET scanners to remote hospitals. This technology holds some promise in the future, for replacing some of the scramble to transport FDG from site of manufacture to site of use.