Preclinical Evaluation of a Radiolabeled Pan-RAF Inhibitor for RAF-Specific PET/CT Imaging

Abnormalities in the RAS-RAF signaling pathway occur in many solid tumors, leading to aberrant tumor proliferation, invasion, and metastasis. Due to the elusive pharmacology of RAS, RAF inhibitors have become the main targeted therapeutic drugs. Naporafenib (LXH-254) is a high-affinity pan-RAF inhibitor with FDA Fast Track Qualification. We sought to develop an ¹⁸F-labeled molecular probe from LXH-254 for PET imaging of tumors overexpressing RAF to noninvasively screen patients for susceptibility to targeted RAF therapy. To reduce the lipid solubility, LXH-254 was designed with triethylene glycol di(<i>p</i>-toluenesulfonate) (TsO-PEG₃-OTs) to obtain the precursor (LXH-254-OTs) and a nucleophilic substitution reaction with ¹⁸F to obtain the tracer ([¹⁸F]F-LXH-254). [¹⁸F]F-LXH-254 exhibited good molar activity (7.16 ± 0.81 GBq/μmol), radiochemical purity (>95%), and stability. Micro-PET imaging revealed distinct radioactivity accumulation of [¹⁸F]F-LXH-254 in tumors in the imaging groups, whereas in the blocked group, the tumor radioactivity level was consistent with the background tissue, illustrating the affinity and specificity of [¹⁸F]F-LXH-254 in targeting RAF. Overall, [¹⁸F]F-LXH-254 is a promising radiotracer for screening and diagnosing patients with RAF-related disease and monitoring their treatment. This is the first attempt at using an ¹⁸F-labeled RAF-specific radiotracer.