Magnetic resonance energy transfer for in vivo glutathione susceptibility weighted imaging

Abstract

Glutathione (GSH) plays a vital role in maintaining biological redox homeostasis. Accordingly, accurate imaging of glutathione in vivo is of great significance. Herein, we propose a magnetic resonance energy transfer (MRET) strategy based on a distance-dependent magnetic exchange coupling effect (MECE), which can realize GSH detection within tumors in vivo by susceptibility weighted imaging (SWI). Fe₃O₄ nanoparticles (NPs) and CoFe₂O₄ NPs linked with cystamine (Fe₃O₄–S–S–CoFe₂O₄) have been successfully designed as SWI nanoprobes. After the disulfide bonds are broken by excess GSH in the tumor, the increase in the distance between Fe₃O₄ NPs and CoFe₂O₄ NPs will induce a decrease of MECE and magnetic susceptibility. As a result, the changes in the SWI signals are used for tumor GSH detection in vivo. Experimental results in vitro and in vivo demonstrate that the Fe₃O₄–S–S–CoFe₂O₄ SWI nanoprobe can sensitively detect concentrations of GSH in tumors. Hence, this strategy not only improves the sensitivity of the GSH response in SWI but also provides a powerful basis for the design of other responsive functional MRI nanoprobes.

Introduction

The tumor microenvironment (TME) is closely related to the development and metastasis of tumors and plays a vital role in the diagnosis and treatment of tumors [[1], [2], [3]]. Generally, the TME has the following characteristics: hypoxia, acidic pH, excess H₂O₂ and GSH, etc. [[4], [5], [6]] Especially, GSH is an important antioxidant that plays a crucial role in maintaining biological redox homeostasis and can increase the resistance to various kinds of therapies, such as radiotherapy, chemotherapy, photo-dynamic therapy, and so on [[7], [8], [9]]. Thus, it is very important to accurately monitor the changes in the GSH concentration within tumors in vivo.

Recently, GSH-responsive fluorescent probes have been widely developed and used for the detection of the GSH concentration within tumors [[10], [11], [12], [13]]. However, fluorescence imaging cannot be used for deep tissue detection because of the fluorescence tissue penetration limitation [14,15]. Therefore, it is necessary to develop a type of high sensitivity imaging probe to be used for the in situ detection of GSH within tumors in vivo. Magnetic resonance imaging (MRI) is a noninvasive imaging technique that has been widely applied for diagnosing and detecting tumors due to its high spatial resolution, non-ionizing radiation and excellent tissue penetration [[16], [17], [18]]. Thus, a series of GSH-responsive MRI nanoprobes has been developed [[19], [20], [21]]. Unfortunately, these reported GSH-responsive MRI probes are still structural MRI probes with low sensitivity (such as T₁WI or T₂WI). As we all know, the sensitivity of structural MRI is low compared to functional MRI [[22], [23], [24]]. Hence, it is still difficult for GSH-responsive structural MRI probes to realize the in situ sensitive detection of the GSH concentration within tumors in vivo. Fortunately, there is a functional MRI technique termed susceptibility-weighted imaging (SWI) based on magnetic susceptibility differences between different tissues. Functional SWI exhibits higher sensitivity compared to structural MRI [25,26]. Consequently, there is an urgent need, but also still a significant challenge, to design functional SWI probes for the in situ sensitive detection of the GSH concentration within a tumor in vivo.

Hence, we propose an innovative magnetic resonance energy transfer (MRET) strategy based on the distance-dependent magnetic exchange coupling effect (MECE) and designed a GSH-responsive functional SWI nanoprobe, which has been successfully used for the highly sensitive detection of GSH within the TME in vivo. As shown in Scheme 1, the probe, termed Fe₃O₄–S–S–CoFe₂O₄, is engineered by linking magnetic Fe₃O₄ NPs and magnetic CoFe₂O₄ NPs with disulfide bonds [27]. After Fe₃O₄–S–S–CoFe₂O₄ nanoprobes are delivered to the tumor site by intravenous injection, cleavage of the disulfide bond triggered by excess GSH in the TME will induce an increased distance between Fe₃O₄ NPs and CoFe₂O₄ NPs, which will lead to a reduction of the total magnetic susceptibility and change in the SWI signal [28,29]. As a result, the GSH concentration in the TME can be reflected by variation of the SWI signal value. Experimental results in vitro and in vivo demonstrate that the Fe₃O₄–S–S–CoFe₂O₄ SWI nanoprobe has successfully achieved highly sensitive in situ detection of GSH within tumors in vivo. Moreover, the experimental results prove that the critical distance for the occurrence of MECE is 9 nm. Therefore, this strategy not only improves the sensitivity and accuracy of GSH-responsive MRI probes but also provides new inspiration for the design of responsive functional MRI probes.