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Detection of tumor hypoxic region using pH-activatable nanoparticles containing manganese contrast agent
Daisuke Kokuryo1,2, Peng Mi3,4,5, Horacio Cabral6, Tsuneo Saga1, Ichio Aoki1, Nouhiro Nishiyama3,4, and Kazunori Kataoka4,6,7

1Department of Molecular Imaging and Theranostics, National Institute for Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Chiba, Japan, 2Graduate School of System Informatics, Kobe University, Kobe, Japan, 3Institute of Innovative Research, Tokyo Institute of Technology, Japan, 4Innovation Center of Nanomedicine (iCONM), Kawasaki Institute of Industry Promotion, Kawasaki, Japan, 5State Key Laboratory of Biotherapy and Cancer Center, Sichuan University, People's Republic of China, 6Graduate School of Engineering, The University of Tokyo, Japan, 7Graduate School of Medicine, The University of Tokyo, Japan

Synopsis

Assessing the environment inside a tumor would aid the development of an effective therapeutic strategy. Our group has recently developed a pH-activatable nanoparticle containing MR contrast agents, which is called MnCaP micelle. For an in vivo tumor model, a specific and strong enhancement of MR signal in the tumor was obtained after the administration. The enhanced area was agreed with the high lactate region found with 1H-MRS and which corresponds to the hypoxic region inside tumor. We conclude that MnCaP micelle detects hypoxic regions in the tumor clearly and therefore it has potential to provide important information for tumor therapy.

Purpose

Evaluation of the tumor microenvironment, including vasculature, hypoxia and inflammation, is one of most important factors in determining whether a treatment strategy will succeeded or not [1, 2]. Much research into detecting the hallmarks of the tumor microenvironment has been performed using imaging techniques like PET and MRI [3-5]. For example, our group reported that nanoparticles containing MR contrast agents can be utilized to detect the small tumors and early-angiogenesis in the tumor region [6, 7]. Recently, we have also developed a pH-activatable calcium phosphate (CaP) nanoparticle containing manganese as a MR contrast agent, called MnCaP micelle to detect and evaluate differences in the tumor microenvironment [8]. We present here studies evaluating in vivo MR signal changes in tumor using MnCaP micelles to detect the low pH area related to the hypoxic region.

Materials and Methods

Nanoparticle: MnCaP micelles was comprised of Mn2+ encapsulated CaP nanoparticles made a poly(ethylene glycol) (PEG) shell and poly(ethylene glycol)-b-poly(glutamic acid) (PEG-b-P(Glu)) block copolymers (Fig. 1 (a)). The Mn ions were trapped in the CaP core with polycrystal structures of hydroxyapatite, and released from the CaP core at pH 7.0 (Fig. 1 (b)) [8]. The average diameter of the nanoparticle was controlled to be 60 nm.

MR experiments: All imaging experiments were performed at the National Institute for Radiological Sciences, Chiba, Japan. To detect and evaluate signal changes in the low-pH tumor region, MR experiments using female BALB/c nude mice transplanted with colon26 murine cancer cell (1.0 × 106 cells/50 μl) were performed. The mice were maintained in accordance with the guidelines of the institute, and all experiments were reviewed and approved by the institute's Committee for Care and Use of Laboratory Animals. The accumulation of MnCaP micelle in in vivo tumor was evaluated with T1-weighted spin-echo MRI using a 1.0 Tesla preclinical MR scanner (Icon, Bruker Biospin, Ettlingen, Germany). A 0.225 mmol/kg MnCaP micelle dose was administered to the tail vein of the tumor model mice. The same concentration of Gd-DTPA was administrated to a control group. The imaging parameters were as follows: TR/TE = 400/11.5 ms; FOV = 44.0 × 44.0 mm2; Matrix size = 256 × 256; Slice thickness = 1.0 mm; Number of slices = 10; Number of acquisition = 4. The chemical shift image of lactates was acquired using a 7.0 Tesla preclinical MR scanner (BioSpec, Bruker Biospin) with a cryogenic probe. The point-resolved spectroscopic (PRESS) sequence was used with the following parameters: TR/TE = 3000/20 ms; FOV = 14.1 × 14.1 mm2; slice thickness = 1.5 mm; acquired data number = 10 × 10 and matrix size = 16 × 16.


Results and Discussion

Figure 2 presents MR images before and after the administration of the MnCaP micelle and Gd-DTPA. The tumor signal was enhanced at 2 hours after MnCaP micelle administration. On the other hand, there was almost no changes to the signal after the Gd-DTPA administration. Occasionally, there is a specific signal enhanced area within the tumor as shown in the upper-right image of Figure 2. Figure 3 shows changes to the signal ratio after MnCaP micelle and Gd-DTPA administration. The signal ratio after MnCaP micelle administration was 1.5 times higher than the ratio before administration. For the area with specific enhancement the signal ratio after MnCaP micelle administration was higher than for areas without specific enhancement, and this was maintained or increased from about 1 hour after the administration. It is likely that the specific enhancement is associated with the release of Mn ion from the CaP core and the subsequent binding with protein. Thus, the specific enhanced area is an indicator of the low-pH area. Figure 4 shows that the pattern of specific signal enhancement using MnCaP micelle was similar to the concentration pattern for lactate. The overproduction of lactate is related to an insufficient O2 supply and the acidified the interstitial pH. Therefore, this MnCaP micelle can detect hypoxic regions within the tumor.

Conclusion

Our MnCaP micelle nanoparticle produced over 60% signal enhancements in the hypoxic tumor regions, and it therefore has the potential to provide the important information for tumor therapy.

Acknowledgements

This research was financially supported by the Center of Innovation Program stream from the Japan Science and Technology Agency and the Funding Program for World-Leading Innovative R&D on Science and Technology from the Japan Society for the Promotion of Science. We thank Sayaka Shibata and Nobuhiro Nitta for assistance with MRI experiments, and also thank Jeff Kershaw for proofreading.

References

[1] Hanahan D., et al: Cell, 144 (5): 646-674, 2011.

[2] Swartz M. A., et al: Cancer Res., 72(10): 2473–2480, 2012.

[3] Wehrl H. F., et al: J Nucl Med., 55 (5): 11S-18S, 2014.

[4] Heidari P., et al: J Nucl Med., 56 (8): 1246-1251, 2015.

[5] Zheng X., et al: Nat Commun., 5: 5834, 2015.

[6] Kokuryo D., et al: J Control Release, 169(3): 220-7, 2013.

[7] Kawamura W., et al: Sci. Technol. Adv. Mater, 16: 035004, 2015.

[8] Mi P., et al: Nature Nanotechnology, 11 (8): 724–730, 2016.

Figures

Schematic illustration of a MnCaP micelle. Mn ions are held in the CaP-based core (a). When the pH is lower than 7.0, the CaP core breaks and releases the Mn ions (b).

Typical T1 weighted images before and at 2 hours after the administration of MnCaP micelle and Gd-DTPA. In the tumor region (dashed ellipse), the MR signals are enhanced after MnCaP micelle administration. Specific signal enhancements is confirmed near the red arrows.

Changes to the signal from the tumor normalized by the signal in muscle before administration. The signal begins to change immediately after MnCaP micelle administration. For areas with specific enhancement in the tumor, the ratio is maintained or increased from about 1 hour after MnCaP micelle administration. Significant difference: *p < 0.05.

(a) T1-weighted MRI after MnCaP micelle administration. (b) Chemical shift image for lactate superimposed on the T1-weighted image. The areas with high (red arrows) and low (yellow arrows) lactate concentration correspond to signal enhancement of T1-weighted image.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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