Development of novel site-specific imaging probes for prostate cancer detection

Abstract

Molecular imaging is believed to play a pivotal role in decreasing cancer mortality and improving prognosis in the new era. It relies on molecular probes to image cellular or molecular abnormalities in living body. The primary objective of this research is to develop novel multimodal molecular imaging probes, via nanotechnology and peptide conjugation chemistry, for site-specific targeting and imaging of human prostate cancers. A novel nanoparticle based bimodal MRI/fluorescence molecular imaging probe was developed, characterized, and studied for its capacity to target gastrin releasing peptide receptor (GRPr) which is overexpressed in human prostate cancer. Synthesized USPIO(Cy7.5)-BBN nanoparticles have a small core size (5 nm), a high MRI relaxivity, and a superior binding affinity and specificity to the GRPr overexpressed on human prostate cancer in vitro and in vivo. Furthermore, SIO-AF750-BBN, a silica-coated iron oxide nanoparticle loaded with near infrared fluorescence (NIRF) dyes and bombesin, was developed as a tri-modal photoacoustic (PA), NIRF and MRI imaging probe. SIO-AF750-BBN displayed a high binding affinity and specificity to the GRPr expressed prostate cancer cells in vitro and in vivo. SIO-AF750-BBN demonstrated a significant photoacoustic contrast enhancement in tumor tissues, i.e. 3 and 5-fold as compared to the receptor-blocking control group and the peptide analog AF750-BBN without attachment to a nanoparticle, respectively, after intravenous tail vein injection in tumor bearing mice. In addition to bombesin agonist as the targeting moiety, bombesin antagonist is also explored. A series of bombesin antagonists with three different NIRF dyes and three pharmacokinetic modifiers were synthesized and characterized. Of those, three bombesin antagonist analogs, AF750-G-pip-Sta-BBN, AF750-GSG-Sta-BBN, and AF750-6Ahx-Sta-BBN, exhibited high NIRF imaging efficacy, high binding specificity and affinity, and antagonism to human PC-3 prostate cancer cells both in vitro and in vivo. In conclusion, this Ph.D. work addresses the issues of discovery and development of novel site-specific and multi-modality molecular probes for cancer targeted imaging. USPIO(Cy7.5)-BBN displayed the capacity to generate imaging contrast enhancement for prostate tumors in living bodies on MRI/NIRF. The SIO-AF750-BBN demonstrated tumor specific photoacoustic signal enhancement, as well as near-infrared fluorescent imaging enhancement after intravenous administration of this agent in human prostate cancer animal model. Finally, AF750 labeled bombesin antagonist analogs were developed and demonstrated to have a high binding affinity and selectivity to the GRPr and a desirable in vivo bio-distribution profile, indicating their high potentials to be translated to clinical use for imaging-guided surgery and therapy.