Introduction

Magnetic resonance imaging (MRI) is a non-invasive powerful diagnostic technique. The contrast of the resulting image often can be enhanced by injection of paramagnetic agents. Though, gadolinium (Gd) is the popular choice amongst the paramagnetic metals due to its suitable magnetic properties. However, Gd based imaging probes are associated to the initiation of nephrogenic systemic fibrosis (NSF).¹ In addition, recently a number of reports demonstrated that intravenously administered Gd accumulates in the brains of the patients with normal renal function.² In this context, progressive approaches have been made based on non-lanthanide metals, particularly less toxic and bio-friendly manganese are receiving special attention. Herein, we report the design of a novel Mn²⁺ complex based on an ethylenediaminetetraacetic acid (EDTA) coordination cage comprising an ethoxybenzyl (EOB) moiety with high chelation stability for use as a hepatobiliary agent.

Materials and Methods

All reagents were purchased from commercial sources and used without further purification unless otherwise stated. Solvents were purified and dried using standard procedures. Deionized water was used for all experiments. T₁ measurements were carried out using an inversion recovery method with a variable inversion time (TI) at 1.5 T (64MHz, GE Healthcare, Milwaukee, WI, USA). For T₂ measurements, the CPMG (Carr-Purcell-Meiboon-Gill) pulse sequence was adapted for multiple spin-echo measurements. T₁ and T₂ relaxation times were obtained from the non-linear least squares fit of the mean pixel values for the multiple spin-echo measurements at each TI value and echo time. Relaxivities (R₁ and R₂) were then calculated as an inverse of relaxation time per mM. MR images of anaesthetized mice were obtained pre- and post-Mn-EDTA-EOB (0.05 mmol Mn/kg) injection by tail vein with a 1.5 T MR unit (GE Healthcare, Milwaukee, WI, USA) using home-made small animal RF coil. The imaging parameters for SE (Spin echo) were as follows: repetition time (TR) = 300 ms; echo time (TE) = 12 ms; 8 mm field of view (FOV); 192×128 matrix size; 1.2 mm slice thickness; number of acquisition (NEX) = 8.

Results and Discussion

The synthesis method of Mn-EDTA-EOB complex are depicted in Scheme 1. The formation of new compounds was confirmed by microanalysis and spectroscopic methods such as, ¹H NMR, HR-FAB-MS. Its R₁ relaxivity (6.3 mM^-1s^-1) in human serum albumin (HSA) solution is higher than MRI CAs such as MnDPDP^® (5.2 mM^-1s^-1) indicating an interaction between HSA and Mn-EDTA-EOB chelate. The kinetic inertness of Mn-EDTA-EOB complex is much higher than that of commercially available MRI contrast agent Gd-DTPA. T₁-weighted MR images were obtained by 8-week-old male Institute of Cancer Research (ICR) mice after a bolus injection of Mn-EDTA-EOB through tail vein (Figure 1). It shows strong signal enhancement in liver, kidney, gallbladder and heart. In combination with bio distribution study, in vivo MR images confirm that Mn-EDTA-EOB is excreted through both the liver and kidneys, which is similar to that of the clinically used liver-specific agent Gd-DTPA-EOB and Gd-BOTPA.³ In the hepatobiliary pathway, Mn-EDTA-EOB is taken up by varying degrees of functioning hepatocytes and excreted via the biliary trees, confirming hepatobiliary uptake. Finally, In a liver cancer model, Mn-EDTA-EOB shows significantly improved tumor detection and characterization, suggesting this new complex can be a prominent MR imaging agent for liver cancer (Figure 2).

Conclusions

In this study, we successfully synthesized and biological evaluation of Mn-EDTA-EOB chelate as a new family of stable hepatobiliary MRI contrast agent. This new complex can be suitable MR imaging agent for liver cancer.

Acknowledgements

No acknowledgement found.