[ ¹²⁴ I]I-cobimetinib PET enables noninvasive tracking of MEK inhibition in MAPK-driven cancers

Abstract We report the synthesis, radiolabeling, and preclinical evaluation of [ 124 I]I-cobimetinib, a novel PET radiotracer for imaging MAPK pathway-driven tumors. Cobimetinib was converted to its boron pinacol ester (Bpin-cobimetinib) in 82 % yield, with >98 % purity confirmed by NMR, HRMS, and elemental analysis. Copper-mediated radioiodination of the Bpin precursor afforded [ 124 I]I-cobimetinib with >98 % radiochemical purity and 64 ± 8 % isolated radiochemical yields ( n = 6, range: 56–74 %) across multiple syntheses. The radiotracer showed high lipophilicity (logD = 3.4 ± 0.2) and excellent in vitro stability in PBS (90.1 ± 0.8 % intact at 72 h) and serum (81.2 ± 0.5 % intact at 72 h). In vitro , [ 124 I]I-cobimetinib displayed potent MEK-dependent cytotoxicity, with IC 50 values of 0.30 ± 0.08 μM in KRAS/BRAF-mutant lines versus 2.76 ± 0.55 μM in MAPK-low cells, and exhibited 2.5–4.5-fold higher cellular uptake in MAPK-driven models. In tumor-bearing mice, [ 124 I]I-cobimetinib rapidly accumulated in tumors (2.9 ± 0.48 %ID/g at 2 h, 6.4 ± 0.6 %ID/g at 48 h) with high tumor-to-blood (7.2) and tumor-to-muscle (14.9) ratios, while blocking studies confirmed MEK-specific uptake. Low thyroid uptakes (1.2–2.3 %ID/g over 96 h) indicated limited in vivo deiodination. PET/CT imaging showed intense tumor localization with minimal background, demonstrating robust in vivo retention (SUVmean = 3.8 ± 0.4; SUVmax = 5.6). These results establish [ 124 I]I-cobimetinib as a selective, stable PET tracer for noninvasive imaging of MEK-activated tumors, supporting its potential for tumor characterization and therapy monitoring in MAPK pathway driven cancers.