TZ4877 is a ligand for the sphingosine-1-phosphate (S1P) receptor 1 in the CNS, developed and radiolabeled with fluorine-18. TZ4877 is the standard (non-radioactive compound) which is used for HPLC analysis of the S1P1 PET tracer, [¹⁸F]TZ4877. TZ50153 is the radiolabeling precursor of the PET tracer, [¹⁸F]TZ4877.