Exploring Metabolism In Vivo Using Endogenous ¹¹C Metabolic Tracers

Cancer and other diseases are increasingly understood in terms of their metabolic disturbances. This thinking has revolutionized the field of ex vivo metabolomics and motivated new approaches to detect metabolites in living systems, including proton magnetic resonance spectroscopy (¹H-MRS), hyperpolarized ¹³C MRS, and PET. For PET, imaging abnormal metabolism in vivo is hardly new. Positron-labeled small-molecule metabolites have been used for decades in humans, including ¹⁸F-FDG, which is used frequently to detect upregulated glycolysis in tumors. Many current ¹⁸F metabolic tracers including ¹⁸F-FDG have evolved from their ¹¹C counterparts, chemically identical to endogenous substrates and thus approximating intrinsic biochemical pathways. This mimicry has stimulated the development of new radiochemical methods to incorporate ¹¹C and inspired the synthesis of a large number of ¹¹C endogenous radiotracers. This is in spite of the 20-minute half-life of ¹¹C, which generally limits its use in patients to centers with an on-site cyclotron. Innovation in ¹¹C chemistry has persisted in the face of this limitation, because (1) the radiochemists involved are inspired, (2) the methods of ¹¹C incorporation are diverse, and (3) ¹¹C compounds often show more predictable in vivo behavior, thus representing an important first step in the validation of new tracer concepts. In this mini-review we will discuss some of the general motivations behind PET tracers, rationales for the use of ¹¹C, and some of the special challenges encountered in the synthesis of ¹¹C endogenous compounds. Most importantly, we will try to highlight the exceptional creativity used in early ¹¹C tracer syntheses, which used enzyme-catalyzed and other "green" methods before these concepts were commonplace.