Androgen-independent Molecular Imaging Vectors to Detect Castration-Resistant and Metastatic Prostate Cancer

Abstract

Prostate-specific promoters are frequently employed in gene-mediated molecular imaging and therapeutic vectors to diagnose and treat castration-resistant prostate cancer (CRPC) that emerges from hormone ablation therapy. Many of the conventional prostate-specific promoters rely on the androgen axis to drive gene expression. However, considering the cancer heterogeneity and varying androgen receptor status, we herein evaluated the utility of prostate-specific enhancing sequence (PSES), an androgen-independent promoter in CRPC. The PSES is a fused enhancer derived from the prostate-specific antigen (PSA) and prostate-specific membrane antigen gene regulatory region. We augmented the activity of PSES by the two-step transcriptional amplification (TSTA) system to drive the expression of imaging reporter genes for either bioluminescent or positron emission tomography (PET) imaging. The engineered PSES-TSTA system exhibits greatly elevated transcriptional activity, androgen independency, and strong prostate specificity, verified in cell culture and preclinical animal experimentations. These advantageous features of PSES-TSTA elicit superior gene expression capability for CRPC in comparison with the androgen-dependent PSA promoter-driven system. In preclinical settings, we showed robust PET imaging capacity of PSES-TSTA in a castrated prostate xenograft model. Moreover, intravenous administrated PSES-TSTA bioluminescent vector correctly identified tibial bone marrow metastases in 9 of 9 animals, whereas NaF- and FDG-PET was unable to detect the lesions. Taken together, this study showed the promising utility of a potent, androgen-independent, and prostate cancer-specific expression system in directing gene-based molecular imaging in CRPC, even in the context of androgen deprivation therapy.