Preclinical\nEvaluation of a Radiolabeled Pan-RAF Inhibitor\nfor RAF-Specific PET/CT Imaging

Abnormalities in the RAS–RAF signaling pathway\noccur in\nmany solid tumors, leading to aberrant tumor proliferation, invasion,\nand metastasis. Due to the elusive pharmacology of RAS, RAF inhibitors\nhave become the main targeted therapeutic drugs. Naporafenib (LXH-254)\nis a high-affinity pan-RAF inhibitor with FDA Fast Track Qualification.\nWe sought to develop an <sup>18</sup>F-labeled molecular probe from\nLXH-254 for PET imaging of tumors overexpressing RAF to noninvasively\nscreen patients for susceptibility to targeted RAF therapy. To reduce\nthe lipid solubility, LXH-254 was designed with triethylene glycol\ndi(<i>p</i>-toluenesulfonate) (TsO-PEG<sub>3</sub>-OTs)\nto obtain the precursor (LXH-254-OTs) and a nucleophilic substitution\nreaction with <sup>18</sup>F to obtain the tracer ([<sup>18</sup>F]F-LXH-254).\n[<sup>18</sup>F]F-LXH-254 exhibited good molar activity (7.16 ±\n0.81 GBq/μmol), radiochemical purity (>95%), and stability.\nMicro-PET imaging revealed distinct radioactivity accumulation of\n[<sup>18</sup>F]F-LXH-254 in tumors in the imaging groups, whereas\nin the blocked group, the tumor radioactivity level was consistent\nwith the background tissue, illustrating the affinity and specificity\nof [<sup>18</sup>F]F-LXH-254 in targeting RAF. Overall, [<sup>18</sup>F]F-LXH-254 is a promising radiotracer for screening and diagnosing\npatients with RAF-related disease and monitoring their treatment.\nThis is the first attempt at using an <sup>18</sup>F-labeled RAF-specific\nradiotracer.