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

Purpose: Abnormal Notch signaling promotes cancer cell growth and tumor progression in various cancers. Targeting gamma-secretase, a pivotal regulator in the Notch pathway, has yielded numerous gamma-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 gamma-secretase activity and its inhibition in vivo. Experimental design: We designed an imaging probe based on GSI Semagacestat structure and synthesized the radioiodine-labeled analogues [I-131]- or [I-124]-PN67 from corresponding trimethyl-tin precursors. Both membrane- and cell-based ligand-binding assays were performed using [I-131]-PN67 to determine the binding affinity and specificity for gamma-secretase in vitro. Moreover, we evaluated [I-124]-PN67 by PET imaging 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 [I-124]-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 gamma-secretase specific tumor accumulation of [I-124]-PN67. Conclusions: [I-124]-PN67 is a novel PET imaging agent that enables assessment of gamma-secretase activity and target engagement of clinical GSIs.

Automated summary

The study developed a novel PET imaging agent, [I-124]-PN67, based on GSI structure for assessing gamma-secretase activity and target engagement of clinical GSIs. In vitro binding assays demonstrated the probe's high specificity, and PET imaging studies in mouse models showed increased uptake in tumors, indicating gamma-secretase specific tumor accumulation. The results suggest that [I-124]-PN67 could be a valuable tool for evaluating the effectiveness of GSIs in cancer treatment.