Preclinical evaluation of PSMA-targeted ultrasound contrast agents in an orthotopic model of prostate cancer in rabbits.

Abstract

The localization of prostate cancer by ultrasound remains limited by the lack of B-mode conspicuity and the confinement of clinically approved microbubbles (MBs) to the vasculature. This precludes differentiating viable tumor, necrotic tissue, and margin-associated disease. We investigated prostate-specific membrane antigen (PSMA)-targeted lipid-shelled perfluorocarbon nanobubbles (PSMA-NBs) in an orthotopic rabbit model using a clinical contrast-enhanced ultrasound (CEUS) system. We implanted PSMA-positive PC3pip-GFP tumors into the prostates of immunosuppressed New Zealand White rabbits and performed transabdominal imaging with PSMA-NBs, MBs, and Plain-NBs using identical protocols. To address tumor heterogeneity and ultrasound boundary ambiguity, regions of interest were defined from baseline B-mode images and segmented into the tumor core, rim, and a peritumoral area. Pixel-wise parametric and decorrelation time (DT) maps were generated and compared with whole-slide histology (H&E) and, in an exploratory and non-specific analysis, with PSMA IHC. Compared to MBs at the doses used, PSMA-NBs exhibited higher peak intensities in the tumor core and rim (1.60-fold and 1.50-fold, respectively) and improved retention (mean transit time [MTT]: 4.20 to 5.40-fold higher) for up to 10 min in the tumor and peritumoral areas. In an exploratory analysis constrained by cohort size, PSMA-NB kinetics, notably MTT, tracked histology-defined tumor viability, and DT mapping showed spatially heterogeneous retention at the tumor periphery. Compared to Plain-NBs, PSMA-NBs also exhibited improved retention (MTT +21% overall) in the rim and peritumoral areas. This study demonstrates the potential of PSMA-NBs to characterize prostate cancer by molecularly targeted CEUS beyond that achieved with MBs at the doses used.