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Biodistribution

The ultimate test of the quality of a radiopharmaceutical is its biodistribution. Often the first indication that something might be wrong with a radiopharmaceutical is an unexpected pattern of biodistribution found during an imaging procedure. The example shows stomach uptake on a bone scan due to high levels of 99mTc –pertechnetate in the preparation.


Fluorine-18 Micro-PET image of a rat skeleton

Digital autoradiograph of a slice of a rat brain

Biodistribution studies in animals may be performed by manufacturers of licensed radiopharmaceuticals as part of their Quality Assurance procedure before a batch of kits, for example, is released for sale. These studies may identify shortcomings in the quality of a product, especially when the precise chemical composition of the compound is uncertain, 99mTc-methylene diphosphonate for example. However, when the exact structure of the complex is known, as is the case for most recently developed products then physicochemical methods of analysis will normally provide all the information required on the quality of a new batch of the product.

Animal biodistribution studies are nearly always performed during the development of a novel radiopharmaceutical before it is first administered to man. These studies may be done in animals with normal phenotypes but more increasingly are being performed in transgenic animals which have certain characteristics which mirror those of the ultimate human recipients of the tracer.

Biodistribution studies may be performed by imaging the animals on single-photon or PET imaging devices or by killing the animals at certain times after administration of the radiopharmaceutical and measuring the distribution of the radioactivity in tissues by counting or autoradiographic techniques.