click to enlarge Figure 1. [18F]-FDG PET image of a HCT-116 human colorectal tumor xenograft in a nu/nu mouse, showing relatively high uptake in normal tissue (background), compared with the tumor (highlighted by arrow). (Source: Charles River)

Positron emission tomography (PET) is an imaging technology that produces three-dimensional images of positron-emitting radioisotope distribution, typically after an intravenous injection of a radioisotope-labeled tracer. PET imaging is used both preclinically and clinically, with tracers normally targeted to specific aspects of physiology and molecular and cellular signaling. In this way, an image of the tracer distribution and degree of tracer uptake can be used for identifying disease, due to the significantly different molecular signatures of diseased and normal cells, and for monitoring therapeutic response, by observing changes in tracer uptake in diseased tissue over time during a treatment schedule.

These strategies have been used effectively in oncology. The most widely used PET tracer for this purpose is [¹⁸F]-fluorodeoxyglucose ([¹⁸F]-FDG), a glucose analog that enters the cell and the glycolytic pathway and is trapped after phosphorylation. [¹⁸F]-FDG can therefore be used as a biomarker for metabolism and viability. While [¹⁸F]-FDG has become a clinical standard—particularly for clinical cancer diagnosis and treatment staging—it is actively taken up by normal active cells. This leads to a high-image background that can reduce the sensitivity and specificity of the intended readout. These limitations can also limit the utility for [¹⁸F]-FDG PET-based characterization of therapeutic response.

click to enlarge Figure 2. [18F]-FLT PET image of a HCT-116 human colorectal tumor xenograft in a nu/nu mouse, showing relatively high specificity of the tracer for the tumor (highlighted by arrow), compared with surrounding normal tissues. These properties can lead to more accurate assessment of therapeutic efficacy in drug development. (Source: Charles River)

Currently, a spectrum of new PET tracers are in development that not only allow characterization of an array of disease-critical molecular processes, but also better address the concepts of tracer specificity and uptake sensitivity to the disease process in question.¹ Of these, [¹⁸F]-3?-deoxy-3?-L-fluorothymidine ([¹⁸F]-FLT) and [¹⁸F]-1-(29-deoxy-29-fluoro-b-D-arabinofuranosyl) thymine ([¹⁸F]-FMAU) are two of the most promising candidates in oncology.^2-3

[¹⁸F]-FLT and [¹⁸F]-FMAU: Biomarkers for Cellular Proliferation
Abnormal proliferation is a general property of cancer cells and therefore a potentially broad and specific target for assessment of tumor progression and response to therapy. Thymidine, a pyrimidine analog, is incorporated in DNA during cell proliferation. The thymidine analogs [¹⁸F]-FLT and [¹⁸F]-FMAU are therefore potential biomarkers for proliferation.

[¹⁸F]-FLT can enter the cell and—like thymidine—is phosphorylated by thymidine kinase 1 (TK1). Unlike thymidine, [¹⁸F]-FLT does not go on to be incorporated into DNA as it lacks the 3’-hydroxyl group, but being a substrate for TK1, it is still trapped in the cell as [¹⁸F]-FLT-monophosphate. Since TK1 is a key enzyme in the salvage pathway for DNA synthesis, the degree of uptake of [¹⁸F]-FLT in specific tissue can therefore be used as a biomarker for proliferation.

[¹⁸F]-FMAU, having a 3’-hydroxyl group, can be incorporated into DNA in the mitochondria, regulated by TK2. [¹⁸F]-FMAU is also a poor substrate for TK1 and therefore can be used as a biomarker for proliferation via the mitochondrial DNA synthesis pathway.

Following intravenous injection of [¹⁸F]-FLT or [¹⁸F]-FMAU in a patient or research animal, a PET image can be interpreted as a three-dimensional map of cellular proliferation in specific tissues such as a tumor, providing a method for assessing treatment response for both patient care and for assessing efficacy in preclinical and clinical drug development.

References
1. Dunphy MPS, Lewis JS. J Nucl Med. 2009; 50:106S–121S.
2. Been LB, Suurmeijer AJH, Cobben DCP, Jager PL, Hoekstra HJ, Elsinga PH. Eur J Nucl Med Mol Imaging. 2004; 31:1659–1672.
3. Bading JR, Shields AF. J Nucl Med. 2008; 49:64S–80S.