Synthesis and Preclinical Positron Emission Tomography Imaging of the p38 MAPK Inhibitor [11C]Talmapimod: Effects of Drug Efflux and Sex Differences

by Selleckchem | Jun 27 2023

Stress-activated kinases are targets of interest in neurodegenerative disease due to their involvement in inflammatory signaling and synaptic dysfunction. The p38α kinase has shown clinical and preclinical promise as a druggable target in several neurodegenerative conditions. We report the radiosynthesis and evaluation of the first positron emission tomography (PET) radiotracer for imaging MAPK p38α/β through radiolabeling of the inhibitor talmapimod (SCIO-469) with carbon-11. [11C]Talmapimod was reliably synthesized by carbon-11 methylation with non-decay corrected radiochemical yields of 3.1 ± 0.7%, molar activities of 38.9 ± 13 GBq/μmol, and >95% radiochemical purity (n = 20). Preclinical PET imaging in rodents revealed a low baseline brain uptake and retention with standardized uptake values (SUV) of ∼0.2 over 90 min; however, pretreatment with the P-glycoprotein (P-gp) drug efflux transporter inhibitor elacridar enabled [11C]talmapimod to pass the blood-brain barrier (>1.0 SUV) with distinct sex differences in washout kinetics. Blocking studies with a structurally dissimilar p38α/β inhibitor, neflamapimod (VX-745), and displacement imaging studies with talmapimod were attempted in elacridar-pretreated rodents, but neither compound displaced radiotracer uptake in the brain of either sex. Ex vivo radiometabolite analysis revealed substantial differences in the composition of radioactive species present in blood plasma but not in brain homogenates at 40 min post radiotracer injection. Digital autoradiography in fresh-frozen rodent brain tissue confirmed that the radiotracer signal was largely non-displaceable in vitro, where self-blocking and blocking with neflamapimod marginally decreased the total signal by 12.9 ± 8.8% and 2.66 ± 2.1% in C57bl/6 healthy controls and 29.3 ± 2.7% and 26.7 ± 12% in Tg2576 rodent brains, respectively. An MDCK-MDR1 assay suggests that talmapimod is likely to suffer from drug efflux in humans as well as rodents. Future efforts should focus on radiolabeling p38 inhibitors from other structural classes to avoid P-gp efflux and non-displaceable binding.

Comments:

The passage you provided discusses the development and evaluation of the first positron emission tomography (PET) radiotracer for imaging MAPK p38α/β, which are stress-activated kinases implicated in neurodegenerative diseases. The radiotracer was synthesized by radiolabeling the p38α/β inhibitor talmapimod (SCIO-469) with carbon-11 ([11C]talmapimod).

The synthesis of [11C]talmapimod was successful, yielding a radiochemical purity greater than 95% with a non-decay corrected radiochemical yield of 3.1 ± 0.7% and molar activities of 38.9 ± 13 GBq/μmol. Preclinical PET imaging in rodents showed low baseline uptake and retention of the radiotracer in the brain, with standardized uptake values (SUV) of approximately 0.2 over 90 minutes.

To enable [11C]talmapimod to cross the blood-brain barrier, pretreatment with the P-glycoprotein (P-gp) drug efflux transporter inhibitor elacridar was used. This pretreatment increased the brain uptake of the radiotracer to over 1.0 SUV, and distinct sex differences were observed in the washout kinetics.

Attempts were made to block radiotracer uptake in the brain using a structurally dissimilar p38α/β inhibitor called neflamapimod (VX-745), as well as displacement imaging studies with talmapimod itself. However, neither compound successfully displaced the radiotracer uptake in the brain of either sex.

Ex vivo radiometabolite analysis revealed significant differences in the composition of radioactive species present in blood plasma but not in brain homogenates at 40 minutes post radiotracer injection. Digital autoradiography in fresh-frozen rodent brain tissue confirmed that the radiotracer signal was largely non-displaceable in vitro, with marginal decreases in total signal observed when blocking with neflamapimod.

An MDCK-MDR1 assay suggested that talmapimod is likely to be subjected to drug efflux by P-gp in both humans and rodents. Therefore, future efforts should focus on radiolabeling p38 inhibitors from other structural classes to avoid P-gp efflux and achieve more displaceable binding.

In summary, this study successfully synthesized [11C]talmapimod as a PET radiotracer for imaging MAPK p38α/β. However, the radiotracer exhibited low baseline brain uptake, and attempts to displace its binding using other p38 inhibitors were unsuccessful. Further research is needed to develop radiotracers with improved binding characteristics for imaging p38α/β in neurodegenerative diseases.