Development and Preclinical Evaluation of [Ga]BMSH as a New Potent Positron Emission Tomography Tracer for Imaging Programmed Death-Ligand 1 Expression.

Immunotherapy targeting the programmed death-ligand 1 (PD-L1)/programmed cell death protein 1 (PD-1) pathway has shown remarkable efficacy against various cancers, but the overall response rate (ORR) is still low. PD-L1 expression in tumors may predict treatment response to immunotherapy. Indeed, ongoing clinical studies utilize a few PD-L1 radiotracers to assess PD-L1 expression as a predictive biomarker for immunotherapy. Here, we present a novel positron emission tomography (PET) radiotracer called [Ga]BMSH, which is derived from a small molecule inhibitor specifically targeting the binding site of PD-L1. The inhibitor was modified to optimize its in vivo pharmacokinetic properties and enable chelation of Ga. In vitro evaluation revealed [Ga]BMSH possessed a strong binding affinity, high specificity, and rapid internalization in PD-L1 overexpressing cells. Biodistribution studies showed that PD-L1 overexpressing tumors had an uptake of [Ga]BMSH at 4.22 ± 0.65%ID/g in mice, while the number was 2.23 ± 0.41%ID/g in PD-L1 low-expressing tumors. Micro-PET/CT imaging of tumor-bearing mice further confirmed that, compared to [F]FDG, [Ga]BMSH can specifically identify tumors with varying levels of PD-L1 expression. Our findings suggest that the [Ga]BMSH is a PD-L1 radioligand with ideal imaging properties, and its further application in the clinical screening of PD-L1 overexpressing tumors may improve ORR for immunotherapy.