Development of [ 18 F]MIPS15692, a radiotracer with in vitro proof-of-concept for the imaging of MER tyrosine kinase (MERTK) in neuroinflammatory disease.

MER tyrosine kinase (MERTK) upregulation is associated with M2 polarization of microglia, which plays a vital role in neuroregeneration following damage induced by neuroinflammatory diseases such as multiple sclerosis (MS). Therefore, a radiotracer specific for MERTK could be of great utility in the clinical management of MS, for the detection and differentiation of neuroregenerative and neurodegenerative processes. This study aimed to develop an [ ¹⁸ F] ligand with high affinity and selectivity for MERTK as a potential positron emission tomography (PET) radiotracer. MIPS15691 and MIPS15692 were synthesized and kinase assays were utilized to determine potency and selectivity for MERTK. Both compounds were shown to be potent against MERTK, with respective IC ₅₀ values of 4.6 nM and 4.0 nM, and were also MERTK-selective. Plasma and brain pharmacokinetics were measured in mice and led to selection of MIPS15692 over MIPS15691. X-ray crystallography was used to visualize how MIPS15692 is recognized by the enzyme. [ ¹⁸ F]MIPS15692 was synthesized using an automated iPHASE FlexLab module, with a molar activity (A _m ) of 49 ± 26 GBq/μmol. The radiochemical purity of [ ¹⁸ F]MIPS15692 was >99% and the decay-corrected radiochemical yields (RCYs) were determined as 2.45 ± 0.85%. Brain MERTK protein density was measured by a saturation binding assay in the brain slices of a cuprizone mouse model of MS. High levels of specific binding of [ ¹⁸ F]MIPS15692 to MERTK were found, especially in the corpus callosum/hippocampus (CC/HC). The in vivo PET imaging study of [ ¹⁸ F]MIPS15692 suggested that its neuroPK is sub-optimal for clinical use. Current efforts are underway to optimize the neuroPK of our next generation PET radiotracers for maximal in vivo utility.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: L.V. reports NHMRC MRFF funding for unrelated projects, research funding from Biogen, Eisai and LMI for unrelated projects, and personal fees from Biogen. J.F. is a current employee of CSL Innovation Ltd. S.V.F. is a founder, stockholder, and board member of the UNC start-up Meryx Inc. which is commercializing MERTK related UNC intellectual property and conducting a phase 1 clinical trial of MERTK inhibitors. X.W. also holds stock in Meryx Inc.. S.V.F. & X.W. report patent for UNC2876A from the University of North Carolina.
(Copyright © 2021. Published by Elsevier Masson SAS.)