Imaging of Cancer γ-Secretase activity using an Inhibitor-Based PET Probe

Abstract

Purpose: Abnormal Notch signaling promotes cancer cell growth and tumor progression in various cancers. Targeting g-secretase, a pivotal regulator in the Notch pathway, has yielded numerous g-secretase inhibitors (GSIs) for clinical investigation in the last 2 decades. However, GSIs have demonstrated minimal success in clinical trials in part due to the lack of specific and precise tools to assess γ-secretase activity and its inhibition in vivo. Experimental Design: Wedesigned an imaging probe based onGSI Semagacestat structure and synthesized the radioiodine-labeled analogues [131I]- or [124I]-PN67 from corresponding trimethyl-tin precursors. Both membrane- and cell-based ligand-binding assays were performed using [131I]-PN67 to determine the binding affinity and specificity for γ-secretase in vitro.Moreover, we evaluated [124I]-PN67 byPETimaging in mammary tumor and glioblastoma mouse models. Results: The probe was synthesized through iodo-destannylation using chloramine-T as an oxidant with a high labeling yield and efficiency. In vitro binding results demonstrate the high specificity of this probe and its ability for target replacement study by clinical GSIs. PET imaging studies demonstrated a significant (P < 0.05) increased in the uptake of [124I]-PN67 in tumors versus blocking or sham control groups across multiple mouse models, including 4T1 allograft, MMTV-PyMT breast cancer, and U87 glioblastoma allograft. Ex vivo biodistribution and autoradiography corroborate these results, indicating γ-secretase specific tumor accumulation of [124I]-PN67. Conclusions: [124I]-PN67 is a novel PET imaging agent that enables assessment of γ-secretase activity and target engagement of clinical GSIs.