Bimodal MRI/fluorescence imaging contrast agent targeting human prostate cancer

Objectives We aimed to develop novel site-specific nanoparticles targeting the BB2 receptors overexpressed in aggressive prostate cancers.

Methods Water soluble ultra-small superparamagnetic iron oxide (USPIO) nanoparticles with a core size of 5 nm were prepared via glucose and casein coating reactions. Cy7.5 was conjugated to K-8AOC-QWAVGHLMNH₂ at a molar ratio of 1:1 followed by the conjugation with the COOH groups on USPIO. The Cy7.5-BBN was purified using RP-HPLC and characterized by mass spectroscopy. The USPIO-Cy7.5-BBN nanoparticles were purified via 100 KDa membrane filtration and characterized by transmission electron microscope (TEM), dynamic light scattering (DLS) and Fourier transform infrared (FTIR) spectroscopy. MRI r₂ relaxivity was measured at 7T. In vitro binding affinity was performed in PC-3 cells. In vivo T₂ MRI contrast enhancement in PC-3 tumors was evaluated after tail vein injections of nanoparticles in mice at a dose of 87 µg Fe/mouse on 7T MRI. In vivo near infrared fluorescence (NIRF) imaging was performed using IVIS-Spectrum. Mice were assigned to three groups: targeting group were injected with USPIO-Cy7.5-BBN, blocking group were pre-injected with 50 µg unlabeled BBN[1-14] before the injection of USPIO-Cy7.5-BBN, and control group were injected with USPIO-Dylit800.

Results USPIO-Cy7.5-BBN has a core diameter of 5.1±0.5 nm and a hydrodynamic diameter of 15.4±3.6 nm. The r₂ relaxivity was 70.2±2.5 s^-1mM^-1 at 7T. In vivo MRI contrast enhancement ratio of tumor to muscle was significantly increased in the uptake group at 4hr (31.1±3.4%) and 24hr (25.7±2.1%) post injection compared to the blocking group (4hr: 15.3±2.0% and 24hr: 2.8±6.8%). In vivo and ex vivo NIRF imaging showed a significantly increased fluorescence on the tumors in the uptake group compared to the blocking group.

Conclusions We synthesized a bimodal USPIO-Cy7.5-BBN nanoparticle with a small core size and high relaxivity. In vitro and in vivo data showed high affinity and specificity to the BB2r expressed by PC-3 cells.