LC-MS supported studies on the metabolism of the sigma-1 receptor ligand (–)-(S)-[18F]fluspidine

Objectives: In an ongoing clinical study (–)-(S)-[18F]fluspidine (1, 2) is being investigated for imaging of sigma-1 receptors in patients with major depression by PET. Beside estimation of the fraction of unchanged radiotracer in plasma and urine, radiometabolites formed should be detected and their structures elucidated.
Methods: (–)-(S)-[18F]Fluspidine or unlabelled reference were incubated with human liver microsomes (HLM) in presence of NADPH and/or activated glucuronic acid (UDPGA) at 37°C. Metabolites were detected by radio-HPLC or LC-MS and characterized by aid of reference compounds or structural elucidation using different MS methods (EPI, MS3). Plasma (10, 20, 30 min p.i) and urine (90 min p.i.) samples of human subjects receiving 268 (245-290) MBq (–)-(S)-[18F]fluspidine i.v. were investigated and compared to results from HLM incubations.
Results: By incubation with HLM mainly 3 hydroxylation products and one debenzylation product were formed. The product hydroxylated at the alkyl side chain was glucuronidated subsequently. Plasma samples (10, 20, 30 min p.i.) obtained from human showed 97.7%, 93.8%, and 88.6% (SD=2.6-7.9, n=6-8) of unchanged tracer. In urine (90 min p.i.) the fraction of tracer was 0–7.8% (average of 2.3%, n=10) and up to 3 main radiometabolites were detected (Figure 1). The one with the highest intensity, also found in plasma, matched the glucuronide formed in vitro. Debenzylation of the parent tracer was detected with a small proportion.
Conclusions: The results demonstrate an appropriate metabolic stability of (–)-(S)-[18F]fluspidine for the application as PET ligand. The structure of the major radiometabolite found in plasma and urine could be assigned by means of in vitro studies. Routes of metabolism are currently investigated in more detail. 302
References:
[1] German Clinical Trial Register, DRKS-ID: DRKS00008321
[2] Fischer et al. Eur. J. Nucl. Med. Mol. Imaging 2011, 38, 540–551