Studying tracer metabolism by LC-MS: (+)-[18F]flubatine and (S)-[18F]fluspidine – two different radioligands showing similar metabolic pathways in vitro and in vivo


Studying tracer metabolism by LC-MS: (+)-[18F]flubatine and (S)-[18F]fluspidine – two different radioligands showing similar metabolic pathways in vitro and in vivo

Ludwig, F.-A.; Fischer, S.; Smits, R.; Hoepping, A.; Houska, R.; Patt, M.; Hesse, S.; Wünsch, B.; Sabri, O.; Brust, P.; Steinbach, J.

Objectives: Radiometabolites can affect PET imaging dramatically due to their expected different properties. Therefore identification of radiometabolites is an important step to understand the metabolic fate of a radioligand. The approach presented demonstrates how LC-MS supports in vitro experiments and contributes to explore the metabolic profile of two tracers recently studied in human brain [1, 2].
Methods: (+)-[18F]Flubatine ([18F]1) and (S)-[18F]Fluspidine ([18F]2) (Figure 1), as well as nonradioactive references were incubated with human liver microsomes (HLM) in presence of NADPH and/or activated glucuronic acid (UDPGA) at 37°C. Radiometabolite patterns were monitored by radio-HPLC and structures were identified by LC-MS of non-radioactive incubations using different MS-methods (EPI, MS3). Plasma (30 min p.i.) and urine (90 min p.i.) from human subjects receiving [18F]1 or [18F]2 during clinical studies were investigated and compared with results von microsomal incubations.
Results: During HLM incubations in presence of NADPH, mono-hydroxylation was predominant for both, 1 and 2, beside debenzylation of 2. In presence of UDPGA 1 and 2 underwent glucuronidation, but only after previous hydroxylation. Corresponding in vitro radiometabolites were detected by radio-HPLC and assigned regarding their structure. Samples obtained from humans showed high stability of both tracers, whereby [18F]1 (97.0% in plasma 30 min p.i, n=6) proved to be more stable than [18F]2 (85.3% in plasma 30 min p.i, n=3). However, hydroxylation and subsequent glucuronidation was found to be the major metabolic pathway of both tracers.
Conclusions: Using in vitro studies and LC-MS, in vivo radiometabolites could be identified. Beside high metabolic stability, [18F]1 and [18F]2 show similar major pathways, namely glucuronidation after previous hydroxylation.
Acknowledgements: Supported by the Helmholtz Validation Fund (HVF) and the German Research Foundation (DFG).
References: [1] Sattler et al. (2015), J Nucl Med, 56, suppl. 3, 1020; [2] Sattler et al. (2016), J Nucl Med, 57, suppl. 2, 1022.

Keywords: PET; LC-MS; Microsomes; Flubatine; Fluspidine; Metabolism; Fluorine-18

  • Lecture (Conference)
    XIV Turku PET symposium, 27.-30.5.2017, Turku, Finnland

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Publ.-Id: 25633