Jeffrey M. Gaudet

Magnetic particle imaging (MPI), using superparamagnetic nanoparticles as an imaging tracer, is touted as a quantitative biomedical imaging technology, but MPI signal properties have never been characterized for magnetic nanoparticles undergoing biodegradation. We show that MPI signal properties can increase or decrease as iron oxide nanoparticles degrade, depending on the nanoparticle formulation and nanocrystal size and the degradation rate and mechanism. Further, we show that long-term in vitro MPI experiments only roughly approximate long-term in vivo MPI signal properties. Further, we demonstrate, for the first time, an environmentally sensitive MPI contrast mechanism, opening the door to smart contrast paradigms in MPI.

Nanoparticle tracers with small sizes and large magnetization are critical for biomedical imaging and especially for magnetic particle imaging (MPI). Small size is important for accessing future intracellular and neurological in vivo applications Here, we show <15 nm nanoparticles made of zero valent iron cores, iron oxide shells and coated with a strongly binding brush co-polymer are effective MPI tracers. The small nanoparticle cores create a hydrodynamic diameter that is half of the state-of-the-art iron oxide tracers while the strongly magnetic zero valent iron maintains similar MPI signal magnitude and resolution.

Purpose Magnetic particle imaging (MPI) is an emerging molecular imaging technique that directly detects iron nanoparticles distributed in living subjects. Compared with imaging iron with magnetic resonance imaging (MRI), MPI signal can be measured to determine iron content in specific regions. In this paper, the detection of iron-labeled macrophages associated with cancer by MRI and MPI was compared. Procedures Imaging was performed on 4T1 tumor-bearing mice 16–21 days post-cancer cell implantation, 24 h after intravenous injection of Ferucarbotran, a superparamagnetic iron oxide (SPIO) or Ferumoxytol, an ultra-small SPIO. Images of living mice were acquired on a 3T clinical MRI (General Electric, n = 6) or MPI (Magnetic Insight, n = 10) system. After imaging, tumors and lungs were removed, imaged by MPI and examined by histology. Results MRI signal voids were observed within all tumors. In vivo, MPI signals were observed in the tumors of 4 of 5 mice after the administration of each contrast agent and in all excised tumors. Signal voids visualized by MRI were more apparent in tumors of mice injected with Ferumoxytol than those that received Ferucarbotran; this was consistent with iron content measured by MPI. Signal voids relating to macrophage uptake of iron were not detected in lungs by MRI, since air also appears hypointense. In vivo, MPI could not differentiate between iron in the lungs vs the high signal from iron in the liver. However, once the lungs were excised, MPI signal was detectable and quantifiable. Histologic examination confirmed iron within macrophages present in the tumors. Conclusions MPI provides quantitative information on in vivo iron labeling of macrophages that is not attainable with MRI. The optimal iron nanoparticle for MPI in general is still under investigation; however, for MPI imaging of macrophages labeled in vivo by intravenous administration, Ferumoxytol nanoparticles were superior to Ferucarbotran.

A major hurdle in the advancement of cell-based cancer immunotherapies is the inability to track in vivo therapeutic cell migration. With respect to dendritic cell (DC)-based cancer immunotherapies, this lack of knowledge represents an even greater hurdle as the quantity of tumor-antigen specific DC reaching a secondary lymphoid organ post injection is predictive of the magnitude of the ensuing tumor-specific immune response. We propose fluorine-19 (F-19) cellular magnetic resonance imaging (MRI) as a suitable and non-invasive imaging modality capable of detecting and quantifying DC migration in vivo and thus, serving as a surrogate marker of DC-based immunotherapeutic effectiveness.

Background: Islet transplantation (Tx) represents the most promising therapy to restore normoglycemia in type 1 diabetes (T1D) patients to date. As significant islet loss has been observed after the procedure, there is an urgent need for developing strategies for monitoring transplanted islet grafts. In this report we describe for the first time the application of magnetic particle imaging (MPI) for monitoring transplanted islets in the liver and under the kidney capsule in experimental animals. Methods: Pancreatic islets isolated from Papio hamadryas were labeled with superparamagnetic iron oxides (SPIOs) and used for either islet phantoms or Tx in the liver or under the kidney capsule of NOD scid mice. MPI was used to image and quantify islet phantoms and islet transplanted experimental animals post-mortem at 1 and 14 days after Tx. Magnetic resonance imaging (MRI) was used to confirm the presence of labeled islets in the liver and under the kidney capsule 1 day after Tx. Results: MPI of labeled islet phantoms confirmed linear correlation between the number of islets and the MPI signal (R²=0.988). Post-mortem MPI performed on day 1 after Tx showed high signal contrast in the liver and under the kidney capsule. Quantitation of the signal supports islet loss over time, which is normally observed 2 weeks after Tx. No MPI signal was observed in control animals. In vivo MRI confirmed the presence of labeled islets/islet clusters in liver parenchyma and under the kidney capsule one day after Tx. Conclusions: Here we demonstrate that MPI can be used for quantitative detection of labeled pancreatic islets in the liver and under the kidney capsule of experimental animals. We believe that MPI, a modality with no depth attenuation and zero background tissue signal could be a suitable method for imaging transplanted islet grafts.

A ¹⁹Fluorine (¹⁹F) perfluorocarbon cell labeling agent, when employed with an appropriate cellular MRI protocol, allows for in vivo cell tracking. ¹⁹F cellular MRI can be used to non-invasively assess the location and persistence of cell-based cancer vaccines and other cell-based therapies. This study was designed to determine the feasibility of labeling and tracking peripheral blood mononuclear cells (PBMC), a heterogeneous cell population. Under GMP-compliant conditions human PBMC were labeled with a ¹⁹F-based MRI cell-labeling agent in a manner safe for autologous re-injection. Greater than 99% of PBMC labeled with the ¹⁹F cell-labeling agent without affecting functionality or affecting viability. The ¹⁹F-labeled PBMC were detected in vivo in a mouse model at the injection site and in a draining lymph node. A clinical cellular MR protocol was optimized for the detection of PBMC injected both at the surface of a porcine shank and at a depth of 1.2 cm, equivalent to depth of a human lymph node, using a dual ¹H/¹⁹F dual switchable surface radio frequency coil. This study demonstrates it is feasible to label and track ¹⁹F-labeled PBMC using clinical MRI protocols. Thus, ¹⁹F cellular MRI represents a non-invasive imaging technique suitable to assess the effectiveness of cell-based cancer vaccines.

Tumor associated macrophages (TAMs) are associated with tumor growth and metastasis. MRI can detect TAMs labeled with iron oxide (USPIO) or perfluorocarbon (PFC) agents. This study compared these two cell tracking approaches for imaging TAMs in vivo. 4T1 tumors were imaged with MRI at 4 days or 3 weeks post cell implantation after intravenous (i.v.) administration of either USPIO or PFC. Signal loss was detected within tumors at both time points post USPIO. Images acquired at 4 days demonstrated signal loss encompassing the entire tumor and around the periphery at 3 weeks. Number of black voxels suggested higher numbers of TAMs in the tumor at the later time point. After PFC administration, Fluorine-19 (¹⁹F) signal was detected in a similar spatial distribution as signal loss post USPIO. ¹⁹F signal quantification revealed that the number of ¹⁹F spins was not significantly different at the two time points, suggesting a similar number of TAMs were present in tumors but accumulated in different regions. ¹⁹F signal was higher centrally in tumors at 4 days and heterogenous around the periphery at 3 weeks. This study revealed that ¹⁹F-based cell tracking methods better represent TAM density and provides additional information not achievable with iron-based methods.

Cellular magnetic resonance imaging (MRI) is an evolving field of imaging with strong translational and research potential. The ability to detect, track, and quantify cells in vivo and over time allows for studying cellular events related to disease processes and may be used as a biomarker for decisions about treatments and for monitoring responses to treatments. In this review, we discuss methods for labeling cells, various applications for cellular MRI, the existing limitations, strategies to address these shortcomings, and clinical cellular MRI.

MUC1 is a glycoprotein expressed on the apical surface of ductal epithelial cells. Malignant transformation results in loss of polarization and overexpression of hypoglycosylated MUC1 carrying truncated carbohydrates known as T or Tn tumor antigens. Tumor MUC1 bearing Tn carbohydrates (Tn-MUC1) represent a potential target for immunotherapy. We evaluated the Tn-MUC1 glycopeptide in a human phase I/II clinical trial for safety that followed a preclinical study of different glycosylation forms of MUC1 in rhesus macaques, whose MUC1 is highly homologous to human MUC1. Either unglycosylated rhesus macaque MUC1 peptide (rmMUC1) or Tn-rmMUC1 glycopeptide was mixed with an adjuvant or loaded on autologous dendritic cells (DC), and responses were compared. Unglycosylated rmMUC1 peptide induced negligible humoral or cellular responses compared with the Tn-rmMUC1 glycopeptide. Tn-rmMUC1 loaded on DCs induced the highest anti-rmMUC1 T-cell responses and no clinical toxicity. In the phase I/II clinical study, 17 patients with nonmetastatic castrate-resistant prostate cancer (nmCRPC) were tested with a Tn-MUC1 glycopeptide-DC vaccine. Patients were treated with multiple intradermal and intranodal doses of autologous DCs, which were loaded with the Tn-MUC1 glycopeptide (and KLH as a positive control for immune reactivity). PSA doubling time (PSADT) improved significantly in 11 of 16 evaluable patients (P = 0.037). Immune response analyses detected significant Tn-MUC1–specific CD4⁺ and/or CD8⁺ T-cell intracellular cytokine responses in 5 out of 7 patients evaluated. In conclusion, vaccination with Tn-MUC1–loaded DCs in nmCRPC patients appears to be safe, able to induce significant T-cell responses, and have biological activity as measured by the increase in PSADT following vaccination. Cancer Immunol Res; 4(10); 881–92. ©2016 AACR.