Peptide targeted high-resolution molecular imaging of prostate cancer with MRI

Abstract

Non-invasive accurate detection of prostate cancer is critical for clinical management of the disease. Molecular MRI has a potential for accurate detection of prostate cancer with high spatial resolution. Fibronectin is a hallmark of epithelial-mesenchymal transition occurred in aggressive prostate cancer and highly expressed in malignant tumors. A pentapeptide CREKA targeted contrast agent CREKA-dL-(DOTA-Gd)4 was synthesized and evaluated to target fibrin-fibronectin complexes in tumor extracellular matrix for molecular MRI of prostate cancer. The contrast agent was synthesized by solid-phase peptide synthesis. The T1 relaxivity of CREKA-(DOTA-Gd)4 at 1.5 T was 33.2 mM(-1)s(-1) per molecule (8.3 per Gd). The fluorescence imaging showed that CREKA specifically bound to orthotopic PC3 prostate tumor in athymic nude mice. Strong enhancement was observed in the tumor tissue injected with CREKA-(DOTA-Gd)4 in the first 5 minutes post-injection before MR signal became visible in the bladder at a low dose of 0.03 mmol-Gd/kg. The targeted contrast agent exhibited minimal Gd retention in the main organs and tissues 2 days after injection. The peptide targeted contrast agent CREKA-(DOTA-Gd)4 is promising for high-resolution molecular MRI of prostate cancer.

Keywords: MRI; Molecular imaging; peptide; prostate cancer; targeted contrast agent.

Figure 1

Synthesis of CREKA-dL-(Gd-DOTA)₄.

Figure 2

Plots of R₁ (1/T ₁) and R₂ (1/T ₂) versus the Gd(III) concentration of CREKA-dL-(Gd-DOTA)₄ and KAREC-dL-(Gd-DOTA)₄.

Figure 3

(A) Immunohistochemical staining of fibronectin in PC3 tumor tissue (A) and normal mouse muscle (B).

Figure 4

Tumor binding of CREKA-TR (A) and KAREC-TR (B) in athymic nude mice bearing orthotopic PC3-GFP prostate tumor 2 hours after intravenous injection at a dose of 10 nmol/mouse. CREKA-TR resulted in 5.2 times higher red fluorescence intensity in the tumor than KAREC-TR (C, n = 3, *: p < 0.05). 1, tumor; 2, liver; 3, lung; 4, testicle; 5, brain; 6, heart; 7, spleen; 8, muscle.

Figure 5

Representative T₁-weighted axial 2D MR images of orthotopic PC-3 human prostate tumor before and at different time points after intravenous injection of CREKA-dL-(Gd-DOTA)₄ (A) and KAREC-dL-(Gd-DOTA)₄ (B) at 0.03 mmol-Gd/kg in nu/nu nude mice.

Figure 6

Tumor enhancement ratio of CREKA-dL-(Gd-DOTA)₄ and KAREC-dL-(Gd-DOTA)₄ at 0.03 mmol-Gd/kg in nu/nu nude athymic mice (n = 6) bearing orthotopic human prostate tumor.

Figure 7

Biodistribution of Gd(III) in the major organs and tissues of the mice at 48 h after intravenous injection of CREKA-dL-(Gd-DOTA)₄ and KAREC-dL-(Gd-DOTA)₄ at a dose of 0.03 mmol-Gd/kg in nu/nu nude athymic mice (n = 6) bearing orthotopic human prostate tumor (*p < 0.05).