¹⁸F: Labeling Chemistry and Labeled Compounds

Abstract

Positron emission tomography (PET) is a unique tool for the investigation, localization, and quantification of physiological activities in vivo by tracing the involved or accompanying biochemical processes. Because of its nuclear and chemical properties, fluorine-18, which is commonly produced by a cyclotron using the ¹⁸O(p,n)¹⁸F or the ²⁰Ne(d,α)¹⁸F nuclear process, is a nearly ideal positron emitting radionuclide. Its half-life of 109.7 min permits tracer syntheses and imaging protocols extending over hours and allows distribution of ¹⁸F-radiopharmaceuticals to hospitals and facilities lacking a cyclotron. The low maximum positron energy of 635 keV results in low radiation doses, short ranges in tissue, and therefore in excellent imaging resolution. Introduction of ¹⁸F-fluorine, either via nucleophilic strategies using [¹⁸F]F⁻ or electrophilic routes using molecular [¹⁸F]F₂, permits the synthesis of a broad spectrum of compounds within a time compatible with the half-life. Although fluorine is only slightly larger than a hydrogen atom, changes in the physiological behavior of bioactive compounds as a result of alteration in metabolic stability, lipophilicity, affinity to the target, or other structures, etc., are often observed even after F-for-H or F-for-OH substitutions. In this chapter, an overview of the scope and limitations of the ¹⁸F-chemistry is given. Fluorination strategies, routes, and synthetic aspects are exemplified, as far as possible, by established and selected ¹⁸F-radiopharmaceuticals with clinical relevance or with potential for further clinical application.

Further Reading

A review of the clinical use and the potential of ¹⁸F-labeled radiopharmaceuticals are by far beyond the scope of this chapter. Excellent reviews have been published, e.g., on PET tracers for mapping cardiac nervous system (Langer and Halldin 2002), on the production (Stöcklin 1995) and future of clinical ¹⁸F-labeled radiopharmaceuticals (Stöcklin 1998; Shiue and Welch 2004) for the heart and brain (Stocklin 1992; Halldin et al. 2001), for the 5-HT system (Crouzel et al. 1992), on targeting peptide receptors by PET (Lundquist and Tolmachev 2002; Okarvi 2001, 2004; Schottelius and Wester 2009; Tolmachev and Stone-Elander 2010) on general aspects in “working against time” during rapid radiotracer synthesis (Fowler and Wolf 1997), on radiotracers for endogenous competition (Laruelle and Huang 2001), on molecular imaging of cancer with PET (Gambhir 2002), on hypoxia PET tracers (Grierson and Patt 1999), on ¹⁸F-fluorine chemistry (Lasne et al. 2002; Stöcklin and Pike 1993; Stöcklin 1995; Ametamey et al. 2008; Miller et al. 2008; Cai et al. 2008), on microwave heating (Stone-Elander and Elander 2002), or on fluorinase-based ¹⁸F-labeling (Onega et al 2009), to mention only a few.