Synthesis of [18F]F-γ-T-3, a Redox-Silent γ-Tocotrienol (γ-T-3) Vitamin E Analogue for Image-Based In Vivo Studies of Vitamin E Biodistribution and Dynamics

Abstract

Vitamin E, a natural antioxidant, is of interest to scientists, health care pundits and faddists; its nutritional and biomedical attributes may be validated, anecdotal or fantasy. Vitamin E is a mixture of tocopherols (TPs) and tocotrienols (T-3s), each class having four substitutional isomers (α-, β-, γ-, δ-). Vitamin E analogues attain only low concentrations in most tissues, necessitating exacting invasive techniques for analytical research. Quantitative positron emission tomography (PET) with an F-18-labeled molecular probe would expedite access to Vitamin E's biodistributions and pharmacokinetics via non-invasive temporal imaging. (R)-6-(3-[¹⁸F]Fluoropropoxy)-2,7,8-trimethyl-2-(4,8,12-trimethyltrideca-3,7,11-trien-1-yl)-chromane ([¹⁸F]F-γ-T-3) was prepared for this purpose. [¹⁸F]F-γ-T-3 was synthesized from γ-T-3 in two steps: (i) 1,3-di-O-tosylpropane was introduced at C6-O to form TsO-γ-T-3, and (ii) reaction of this tosylate with [¹⁸F]fluoride in DMF/K₂₂₂. Non-radioactive F-γ-T-3 was synthesized by reaction of γ-T-3 with 3-fluoropropyl methanesulfonate. [¹⁸F]F-γ-T-3 biodistribution in a murine tumor model was imaged using a small-animal PET scanner. F-γ-T-3 was prepared in 61% chemical yield. [¹⁸F]F-γ-T-3 was synthesized in acceptable radiochemical yield (RCY 12%) with high radiochemical purity (>99% RCP) in 45 min. Preliminary F-18 PET images in mice showed upper abdominal accumulation with evidence of renal clearance, only low concentrations in the thorax (lung/heart) and head, and rapid clearance from blood. [¹⁸F]F-γ-T-3 shows promise as an F-18 PET tracer for detailed in vivo studies of Vitamin E. The labeling procedure provides acceptable RCY, high RCP and pertinence to all eight Vitamin E analogues.

Keywords: F-18 positron emission tomography (F-18 PET); F-γ-T-3; Vitamin E; Vitamin E analogues; [18F]F-γ-T-3; antioxidants; nutraceuticals; radiofluorination; tocopherol (TP); tocotrienol (T-3); γ-T-3.

Figure 1

Chemical structures of the Vitamin E analogues.

Figure 2

Tocotrienol (T-3) chemical structure and molecular biology of its major structural domains (adapted from [2]).

Scheme 1

Roadmap for the synthesis of F-γ-T-3 and [¹⁸F]F-γ-T-3. Reagents and conditions for F-γ-T-3: MsF, Et₃N, DCM, r.t., 30 min, (92.7% yield), followed by CsCO₃, DMF, r.t., overnight, (61% yield); for TsO-γ-T-3, CsCO₃, DMF, r.t., overnight (48% yield); and for radiosynthesis of [¹⁸F]F-γ-T-3, K [¹⁸F]F, K₂₂₂, CH₃CN, 100 °C, 10 min (12% RCY, >99% RCP).

Figure 3

A representative HPLC radio-uv co-chromatogram of the reaction mixture after radiofluorination. The 291 and 254 nm peaks eluting at 14.4 min, and the radioactive peak (14.4 min; red trace) represent the co-mixture of authentic F-γ-T-3 (small blue peaks under the radioactive peak) and product [¹⁸F]F-γ-T-3 in the reaction mixture. Absorption peaks in the blue trace (291 nm) correspond to the major UV absorption band of the methyl-substituted 6-chromanol (benzopyran) ring system (e.g., F-γ-T-3, TsO-γ-T-3), and the dark green trace (254 nm) is a less selective indicator of aryl compounds. The main UV absorption peaks eluting at 10.1 min (291 and 254 nm) represent unconsumed TsO-γ-T-3; the absorption peaks eluting at earlier times are unidentified. The isocratic reverse phase HPLC system is described in the text.

Figure 4

F-18 PET images showing the biodistribution of radioactivity in a mouse following i.v. tail veil injections of [¹⁸F]F-γ-T-3 (left image) and reference [¹⁸F]FDG (right image). The [¹⁸F]F-γ-T-3 image was captured 2 h after injection, and the [¹⁸F]FDG was obtained 90 min post-injection, using the same animal 24 h following the [¹⁸F]F-γ-T-3 study.

Figure 5

NMR numbering of target compounds F-γ-T-3 and [¹⁸F]F-γ-T-3.

Figure 6

The partial ¹H-NMR spectrum of F-γ-T-3 reveals the distinctive coupling pattern of the fluoropropyloxy substituent at C6-O. These resonances are well separated from other aliphatic-H resonances and are therefore useful in distinguishing F-γ-T-3 from non-substituted T-3s.