Orit Jacobson

Characterization of SEZ6 expression, binding affinity, and cellular internalization in H524 cells

Small cell lung cancer (SCLC) remains one of the most lethal malignancies, with a 5-year survival of <10% despite advances in treatment. Seizure-related gene 6 (SEZ6), a type I transmembrane protein preferentially expressed in neuroendocrine tumors, has recently emerged as a promising therapeutic target, supported by clinical activity of SEZ6-targeted antibody–drug conjugates in relapsed SCLC. In this study, we developed and evaluated a humanized anti-SEZ6 monoclonal antibody for SEZ6-targeted positron emission tomography imaging and α-particle radioimmunotherapy. Immunohistochemistry of patient tissues confirmed strong, stage-independent SEZ6 expression. Cell-based studies demonstrated high SEZ6 receptor densities, supporting high-affinity antibody binding and rapid receptor-mediated internalization. [89Zr]Zr-DFO-SEZ6-Ab administered at low antibody mass exhibited rapid hepatosplenic clearance with minimal tumor uptake despite high intratumoral levels. This pattern was consistent with active ectodomain shedding and local accumulation of soluble antigen within the tumor microenvironment, creating an extracellular antigen sink and limiting effective target engagement. Increasing the antibody mass overcame this barrier, dramatically increasing tumor uptake in xenografts. Treatment with [225Ac]Ac-Macropa-SEZ6-Ab induced complete tumor regression, whereas coadministration of the ATR inhibitor berzosertib enhanced efficacy at lower radioactivity doses without added toxicity. Together, these findings identify tumor-localized SEZ6 shedding as a biological barrier to SEZ6-targeted therapy, define dosing strategies to overcome it, and demonstrate potent antitumor activity of SEZ6-directed α-particle radioimmunotherapy in SCLC. Significance:Dosing strategies that overcome antigen sink effects induced by a tumor-localized ectodomain shedding enable SEZ6-based imaging and theranostics in small cell lung cancer.

γH2AX and TUNEL staining in control and Berzosertib-treated tumors

PET imaging of recurrent tumor using [89Zr]Zr-DFO-SEZ6-Ab at 90 days following radioligand therapy with 4.62 kBq [225Ac]Ac-Macropa-SEZ6-Ab

Immunofluorescence imaging of regrown tumors stained with SEZ6 antibody following radioligand therapy with 4.62 kBq [225Ac]Ac-Macropa-SEZ6-Ab

Immunofluorescence imaging of regrown tumors stained for Ki-67 following radioligand therapy with 4.62 kBq [225Ac]Ac-Macropa-SEZ6-Ab

Antitumor activity and toxicity profile of the SEZ6-targeting ADC (ABBV-706) in H1341 xenografts

Representative PET images of [89Zr]Zr-DFO-SEZ6-Ab administered at low antibody mass

Stability assessment of [89Zr]Zr-DFO-SEZ6-Ab by radio-HPLC

In vivo PET imaging and SEZ6 expression analysis in H524 xenograft model