Imaging of Cancer γ-Secretase Activity Using an Inhibitor-Based PET Probe.

Abstract

Abnormal Notch signaling promotes cancer cell growth and tumor progression in various cancers. Targeting γ-secretase, a pivotal regulator in the Notch pathway, has yielded numerous γ-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 <i>in vivo</i>. We designed an imaging probe based on GSI 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 <i>in vitro</i>. Moreover, we evaluated [¹²⁴I]-PN67 by PET imaging in mammary tumor and glioblastoma mouse models. The probe was synthesized through iodo-destannylation using chloramine-T as an oxidant with a high labeling yield and efficiency. <i>In vitro</i> 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 (<i>P</i> < 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. <i>Ex vivo</i> biodistribution and autoradiography corroborate these results, indicating γ-secretase specific tumor accumulation of [¹²⁴I]-PN67. [¹²⁴I]-PN67 is a novel PET imaging agent that enables assessment of γ-secretase activity and target engagement of clinical GSIs.