Synthesis and Evaluation of RG7388-Based Fluorinated MDM2 Inhibitors for Developing 18F-Labeled Probes for PET Imaging

MDM2 (murine double minute 2) is a negative regulator of the tumor suppressor protein p53 and is frequently overexpressed in sarcomas. MDM2 inhibition with small-molecule and peptide inhibitors has been shown to increase p53 levels and induce therapeutic effects in wild-type p53 cancers. MDM2 is considered a promising drug target, with several MDM2 inhibitors currently in clinical development. Hence, a noninvasive imaging tool to evaluate MDM2 expression in tumors would be highly valuable for guiding MDM2-targeted therapies. Herein, we report the synthesis and preliminary evaluation of RG7388-based fluorinated MDM2 inhibitors as potential 18F-labeled PET probes for imaging MDM2. RG7388 was modified at the carboxylic acid or methoxy functions to synthesize 3 fluorinated analogues for each series. Based on inhibition data and their comparable potency (IC50) towards MDM2, we selected RG7388 analogues 1 and 6, both bearing a fluoroethyl-1,2,3-triazole moiety, for 18F-labeling via click chemistry. From these studies, the carboxy-modified [18F]1 was identified as a promising lead compound, with an IC50 of 16.8 nM and high uptake and specificity in MDM2-expressing osteosarcoma cell lines, SJSA-1 and U2OS. [18F]1 showed favorable biodistribution characteristics in non-tumor-bearing mice, and preliminary PET/CT imaging studies revealed a higher uptake of [18F]1 in SJSA-1 xenografts compared to muscle at 1 h post-injection. Western blot analysis of SJSA-1 cells and immunohistochemical staining of SJSA-1 tumor sections confirmed high MDM2 expression and its localization in the nuclei of tumor cells, corroborating the PET imaging data.