BAF312 (siponimod) is a sphingosine-1-phosphate (S1P) receptor modulator in clinical development for the treatment of multiple sclerosis, with faster organ/tissue distribution and elimination kinetics than its precursor FTY720 (fingolimod). Our aim was to develop a tracer to better quantify the penetration of BAF312 in the human brain, with the potential to be labeled for positron emission tomography (PET) or single-photon emission computed tomography (SPECT). Although the PET radioisotopes 11 C and 18 F could have been introduced in BAF312 without modifying its structure, they do not have decay kinetics compatible with the time required for observing the drug's organ distribution in patients. In contrast, the SPECT radioisotope 123 I has a longer half-life and would suit this purpose.

 

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