Pentafluorosulfanyl (SF5) as a Superior 19F Magnetic Resonance Reporter Group: Signal Detection and Biological Activity of Teriflunomide Derivatives

Abstract

Fluorine (¹⁹F) magnetic resonance imaging (MRI) is severely limited by a low signal-to noise ratio (SNR), and tapping it for ¹⁹F drug detection in vivo still poses a significant challenge. However, it bears the potential for label-free theranostic imaging. Recently, we detected the fluorinated dihydroorotate dehydrogenase (DHODH) inhibitor teriflunomide (TF) noninvasively in an animal model of multiple sclerosis (MS) using ¹⁹F MR spectroscopy (MRS). In the present study, we probed distinct modifications to the CF₃ group of TF to improve its SNR. This revealed SF₅ as a superior alternative to the CF₃ group. The value of the SF₅ bioisostere as a ¹⁹F MRI reporter group within a biological or pharmacological context is by far underexplored. Here, we compared the biological and pharmacological activities of different TF derivatives and their ¹⁹F MR properties (chemical shift and relaxation times). The ¹⁹F MR SNR efficiency of three MRI methods revealed that SF₅-substituted TF has the highest ¹⁹F MR SNR efficiency in combination with an ultrashort echo-time (UTE) MRI method. Chemical modifications did not reduce pharmacological or biological activity as shown in the in vitro dihydroorotate dehydrogenase enzyme and T cell proliferation assays. Instead, SF₅-substituted TF showed an improved capacity to inhibit T cell proliferation, indicating better anti-inflammatory activity and its suitability as a viable bioisostere in this context. This study proposes SF₅ as a novel superior ¹⁹F MR reporter group for the MS drug teriflunomide.

Keywords: DHODH; MRI; MRS; SF5; fluorine; teriflunomide.

Figure 1

Chemical structures of the synthesized teriflunomide derivatives.

Figure 2

¹⁹F MR spectra of teriflunomide and its derivatives. Signal acquired (normalized amplitudes) from compounds dissolved in DMSO (upper panels) or human serum (lower panels) obtained by a global single-pulse spectroscopy (TR = 1000 ms and TA = 8000 ms).

Figure 3

Biological and ¹⁹F MR reporter activities of TF compounds. (A) Selecting the best ¹⁹F TF derivative and corresponding MR acquisition method. Drug compounds with the best inhibitory capacity (inverse IC₅₀ normalized to TF in percentage) and best SNR efficiency in serum (normalized to TF in percentage using a RARE sequence) obtained using RARE, bSSFP, and UTE sequences are shown in the upper right quadrant (orange region). (B)¹⁹F MR reporter activity of TF in the stomach of a C57BL/6 mouse ex vivo using an optimized ¹⁹F UTE MR sequence. Left panel: ¹H anatomical image of the stomach, middle panel: ¹⁹F MRI of TF in the stomach, and right panel: ¹⁹F/¹H overlay. ¹H: RARE (TR/TE = 2000 ms/10 ms, TA = 1 min, 4 s) and ¹⁹F: UTE (TR/TE = 100 ms/0.27 ms, TA = 2 h, 30 min, FA = 25 °). (C) ¹⁹F MR reporter activity of SF₅-TF in the stomach of a C57BL/6 mouse ex vivo using an optimized ¹⁹F UTE MR sequence. Left panel: ¹H anatomical image of the stomach, middle panel: ¹⁹F MRI of SF₅-TF in the stomach, and right panel: ¹⁹F/¹H overlay. ¹H: RARE (TR/TE = 2000 ms/10 ms, TA = 1 min, 4 s) and ¹⁹F: UTE (TR/TE = 100 ms/0.27 ms, TA = 2 h, 30 min, FA = 42 °). SNR is indicated by the color bars.