Tritium-Labeled Compounds in PET Tracer Discovery? A Case Study from Roche's Internal Monoacylglycerol Lipase Program

The discovery and development of PET tracers is a complex and costly process requiring direct access to ¹¹C or ¹⁸F, radiochemistry infrastructure, as well as an imaging facility equipped with PET tomographs. In the absence of access to a fully equipped PET center facility, a valid alternative for the selection of promising PET tracer candidates is the use of tritium as a surrogate radioisotope in the early stages of development. Tritium-labeled compounds enable detailed binding or displacement assays on tissues and ex vivo radioligand performance assessments, such as rodent experiments, while offering unique advantages including a long half-life and superior spatial resolution. Furthermore, the radiation safety requirements for handling tritium-labeled compounds are less stringent compared with those needed for working with short-lived PET nuclides, such as ¹¹C or ¹⁸F, due to the lower energy emissions of tritium. Candidates are selected based on critical attributes, including low half-maximal inhibitory concentration (IC₅₀), brain penetration, and binding to target proteins, ensuring optimal properties for PET imaging. This approach minimizes reliance on a specialized infrastructure while accelerating the identification and optimization of radioligand candidates. As an example, to illustrate this workflow using tritiated compounds, an internal program targeting monoacylglycerol lipase (MAGL) in the CNS is presented. From a pool of 617 in-house MAGL inhibitors, five compounds were selected for tritium labeling and evaluated alongside literature-reported PET tracers. Among the newly synthesized ligands, compound 9 demonstrated a favorable pharmacological profile, while autoradiography experiments highlighted superior properties in the chemotype of T-401, originating from the literature. This study reports on the synthesis and characterization of tritium-labeled MAGL ligands as critical steps in PET tracer development. Promising candidates identified in this workflow will subsequently be handed over to PET centers for further evaluation in imaging applications, bridging preclinical research with clinical translation.