Introduction

Melanin is a pigment that includes free radicals and is widely distributed in living animals. Malignant melanoma is one of the most progressive tumors in humans with increasing incidence worldwide, and has shown resistance to chemotherapy, resulting in high mortality at the metastatic stage. In general, melanoma involves the abnormal accumulation of melanin pigment produced by malignant melanocytes. Electron paramagnetic resonance (EPR) spectroscopy and imaging is a powerful technique to directly visualize melanomas using endogenous free radicals in the melanin pigment. Because melanin radicals have a large linewidth, the low spatial resolution of EPR imaging results in blurred images and a lack of anatomical information. Dynamic nuclear polarization (DNP)-MRI is a noninvasive imaging method to obtain the spatio-temporal information of free radicals with MRI anatomical resolution. Proton signals in tissues, including free radicals, can be dramatically enhanced by EPR irradiation at the resonance frequency of the free radical prior to applying the MRI pulse sequence. However, the DNP effects of free radicals in the pigment of living organisms is unclear. Therefore, if endogenous free radicals in melanin pigment could be utilized as a bio-probe for DNP-MRI, this will be an advantage for the specific enhancement of melanoma tissues and might allow the separate noninvasive visualization of melanoma tissues without the need for probe administration. Here, we report that biological melanin pigment induced a in vivo DNP effect by interacting with water molecules. In addition, we demonstrated in vivo melanoma imaging based on the DNP effects of endogenous free radicals in the melanin pigment of living mice.

Methods

To demonstrate the capabilities of melanin radical imaging with DNP-MRI, a seven-tube phantom (200 mL, 5.4-mm diameter and 9-mm length) was prepared, where each tube was filled with various concentrations of eumelanin in 0.2% agarose gel water solution. The phantoms were measured by DNP-MRI with or without EPR irradiation at 456 MHz using a surface coil. B16F0 cells (1.0 × 106/mouse) were subcutaneously administered to the right leg of C57BL/6 mice. B16F0 bearing mice were used for the ex vivo and DNP-MRI and EPR experiments. In vivo free radical imaging was performed with a low field (15mT) DNP-MRI system (Keller) obtained from Japan Redox Inc. (Fukuoka Japan). In the in vivo experiments, mice were anesthetized with 2% isoflurane at 3, 7, 10, and 14 days after the subcutaneous injection of B16F0 cells. During the procedure, the body temperature of the mice was kept at 37 ± 1°C with a heating pad. The scanning conditions for the DNP-MRI experiment were as follows: power of EPR irradiation, 7 W; flip angle, 90°; repetition time (TR) × echo time (TE) × EPR irradiation time (TEPR), 500 × 25 × 250 ms; number of acquisitions, 10; slice thickness, 64 mm including the whole thickness of the mouse; phase-encoding steps, 32; field of view (FOV), 40 × 40 mm; and matrix size, 64 × 64 after reconstruction.

Results and Discussion

The concentration of free radicals in melanin pigment increased linearly as a function of the eumelanin concentration. Figure 1 shows the DNP-MRI image of the gel phantom with various concentrations of eumelanin (2–20 mg/mL). Although melanin radicals have a large line width (4.4 ± 0.1 G) and low radical concentration in the pigment (6–58 mM), the DNP-MR image with EPR irradiation (EPR ON) showed an enhanced MRI signal and difference images showed a dose-dependent enhancement with eumelanin concentration. These experiments suggested that melanin free radicals in the pigment might interact with water molecules to induce the DNP effect. We performed the DNP-MR imaging of B16F0 bearing mice with and without EPR irradiation at 456 MHz (Figure 1). The DNP-MRI image of the coronal and sagittal planes and a difference map calculated using the EPR ON and OFF images. Regular MR images (EPR OFF) demonstrated the outline of mice and the negative enhancement in EPR ON images was clearly observed in the tumor region of both planes(Figure 1). Difference images identified the melanoma region separately and the melanoma regions obtained by DNP-MRI corresponded to that of 1.5T MRI images, although the borderline tumor was not clear in the T1W MR images. The DNP-MR image of the sagittal plane showed the depth of tumor invasion in the leg of mice. After the imaging study, tumor tissues were extracted and frozen sections were generated for HE staining to confirm melanin pigments by microscopy. Dark pigments in the melanoma tissue were distributed throughout the whole melanoma region. These experiments successfully demonstrated that melanin radicals in the melanoma showed a DNP effect under in vivo conditions and that our system directly visualized the melanoma including melanin free radicals with a high spatial resolution without the need for contrast agents.

Conclusion

We demonstrated in vivo melanoma imaging using endogenous free radicals in melanin pigment using DNP-MRI. Our DNP-MRI technique might be applied to other free radical molecules as a bio-probe for pigments or to crystals for non-probe bio or medical imaging, if the tissues induce a DNP effect by interactions between free radicals and water molecules.

Acknowledgements

This work was supported by the Medical Research and Development Programs Focused on Technology Transfer, Development of Advanced Measurement and Analysis Systems (SENTAN) from the Japan Agency for Medical Research and Development, AMED Grant Number 162128; Health Labour Sciences Research Grant (Research on Publicly Essential Drugs and Medical Devices) from the Ministry of Health, Labour and Welfare of Japan; and Special Coordination Funds for Promoting Science and Technology (SCF funding program “Innovation Center for Medical Redox Navigation”). This work was also supported by JSPS KAKENHI (Grant Number 16H05079 and 16H05113). We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

References

Hyodo F, Naganuma T, Eto H, Murata M, Utsumi H, Matsuo M. In vivo melanoma imaging based on dynamic nuclear polarization enhancement in melanin pigment of living mice using in vivo dynamic nuclear polarization magnetic resonance imaging. Free Radic Biol Med. 2019 Apr;134:99-105