In vivo and ex vivo 19-fluorine magnetic resonance imaging and spectroscopy of beta-cells and pancreatic islets using GLUT-2 specific contrast agents

First-in-Human Evaluation of Positron Emission Tomography/Computed Tomography With [18F]FB(ePEG12)12-Exendin-4: A Phase 1 Clinical Study Targeting GLP-1 Receptor Expression Cells in Pancreas

Pancreatic β-cell mass (BCM) has a central importance in the pathophysiology of diabetes mellitus. Recently, pancreatic β-cell-specific imaging, especially positron emission tomography (PET) with exendin-based probes, has emerged for non-invasive evaluation of BCM. We developed a novel exendin-based probe labeled with fluorine-18, [¹⁸F]FB(ePEG12)12-exendin-4 (¹⁸F-Ex4) for PET imaging. We subsequently conducted a first-in-human phase 1 study of ¹⁸F-Ex4 PET/computed tomography (CT) and investigated the safety and utility for visualizing the pancreas. Six healthy male subjects were enrolled in this study. A low dose (37.0 MBq) of ¹⁸F-Ex4 PET/CT was administered (first cohort: n = 2), and subsequently a higher dose (74.0 MBq) was administered (second cohort: n = 4). In the first and second cohorts, 38.6 ± 4.8 and 71.1 ± 4.8 MBq of ¹⁸F-Ex4 were administered, respectively. No serious adverse events were observed in both groups. Only one participant in the first cohort showed transient hypoglycemia during the PET scans. ¹⁸F-Ex4 PET/CT successfully visualized the pancreas in all participants. The mean standardized uptake value of the pancreas was found to be higher than that in the surrounding organs, except for the bladder and kidney, during the observation. Dosimetry analyses revealed the effective systemic doses of ¹⁸F-Ex4 as 0.0164 ± 0.0019 mSv/MBq (first cohort) and 0.0173 ± 0.0020 mSv/MBq (second cohort). ¹⁸F-Ex4 PET/CT demonstrated the safety and utility for non-invasive visualization of the pancreas in healthy male subjects. ¹⁸F-Ex4 is promising for clinical PET imaging targeting pancreatic β cells.

Islet Transplantation Imaging in vivo

Although islet transplantation plays an effective and powerful role in the treatment of diabetes, a large amount of islet grafts are lost at an early stage due to instant blood-mediated inflammatory reactions, immune rejection, and β-cell toxicity resulting from immunosuppressive agents. Timely intervention based on the viability and function of the transplanted islets at an early stage is crucial. Various islet transplantation imaging techniques are available for monitoring the conditions of post-transplanted islets. Due to the development of various imaging modalities and the continuous study of contrast agents, non-invasive islet transplantation imaging in vivo has made great progress. The tracing and functional evaluation of transplanted islets in vivo have thus become possible. However, most studies on contrast agent and imaging modalities are limited to animal experiments, and long-term toxicity and stability need further evaluation. Accordingly, the clinical application of the current achievements still requires a large amount of effort. In this review, we discuss the contrast agents for MRI, SPECT/PET, BLI/FI, US, MPI, PAI, and multimodal imaging. We further summarize the advantages and limitations of various molecular imaging methods.

A Trimodal Imaging Platform for Tracking Viable Transplanted Pancreatic Islets In Vivo: F-19 MR, Fluorescence, and Bioluminescence Imaging

PURPOSE

Combining specific and quantitative F-19 magnetic resonance imaging (MRI) with sensitive and convenient optical imaging provides complementary information about the distribution and viability of transplanted pancreatic islet grafts. In this study, pancreatic islets (PIs) were labeled with positively charged multimodal nanoparticles based on poly(lactic-co-glycolic acid) (PLGA-NPs) with encapsulated perfluoro-15-crown-5-ether and the near-infrared fluorescent dye indocyanine green.

PROCEDURES

One thousand and three thousand bioluminescent PIs were transplanted into subcutaneous artificial scaffolds, which served as an alternative transplant site. The grafts were monitored using in vivo F-19 MR, fluorescence, and bioluminescence imaging in healthy rats for 2 weeks.

RESULTS

Transplanted PIs were unambiguously localized in the scaffolds by F-19 MRI throughout the whole experiment. Fluorescence was detected in the first 4 days after transplantation only. Importantly, in vivo bioluminescence correlated with the F-19 MRI signal.

CONCLUSIONS

We developed a trimodal imaging platform for in vivo examination of transplanted PIs. Fluorescence imaging revealed instability of the fluorescent dye and its limited applicability for longitudinal in vivo studies. A correlation between the bioluminescence signal and the F-19 MRI signal indicated the fast clearance of PLGA-NPs from the transplantation site after cell death, which addresses a major issue with intracellular imaging labels. Therefore, the proposed PLGA-NP platform is reliable for reflecting the status of transplanted PIs in vivo.

Tri-modal In vivo Imaging of Pancreatic Islets Transplanted Subcutaneously in Mice

PURPOSE

Transplantation of pancreatic islets (PIs) is a promising therapeutic approach for type 1 diabetes. The main obstacle for this strategy is that the outcome of islet engraftment depends on the engraftment site. It was our aim to develop a strategy for using non-invasive imaging techniques to assess the location and fate of transplanted PIs longitudinally in vivo.

PROCEDURES

In order to overcome the limitations of individual imaging techniques and cross-validate findings by different modalities, we have combined fluorine magnetic resonance imaging (F-19 MRI), fluorescence imaging (FLI), and bioluminescent imaging (BLI) for studying subcutaneously transplanted PIs and beta cell-like cells (INS-1E cell line) in vivo. We optimized the transduction (using lentiviral vectors) and labeling procedures (using perfluoro crown ether nanoparticles with a fluorescence dye) for PIs and INS-1E cell imaging.

RESULTS

The feasibility of using the proposed imaging methods for PI assessment was demonstrated both in vitro and in vivo. Our data suggested that F-19 MRI is suitable for high-resolution localization of transplanted cells and PIs; FLI is essential for confirmation of contrast localization by histology; and BLI is a reliable method to assess cell viability and survival after transplantation. No significant side effects on cell viability and function have been observed.

CONCLUSIONS

The proposed tri-modal imaging platform is a valuable approach for the assessment of engrafted PIs in vivo. It is potentially suitable for comparing different transplantation sites and evaluating novel strategies for improving PI transplantation technique in the future.

Challenges for labeling and longitudinal tracking of adoptively transferred autoreactive T lymphocytes in an experimental type-1 diabetes model

OBJECTIVE

Tracking the autoreactive T-cell migration in the pancreatic region after labeling with fluorinated nanoparticles (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-[3-(2-pyridyldithio)propionate]-perfluoro-15-crown-5-ether nanoparticles, PDP-PFCE NPs) in a diabetic murine model using ¹⁹F MRI.

MATERIALS AND METHODS

Synthesis of novel PDP-PFCE fluorine tracer was performed for in vitro labeling of T cells. Labeling conditions were optimized using different PDP-PFCE NPs concentrations. For in vivo ¹⁹F MRI, mice were longitudinally followed after adoptive transfer of activated, autoreactive, labeled T cells in NOD.SCID mice.

RESULTS

Established MR protocols were used for challenging T cell labeling to track inflammation in a model of diabetes after successful labeling of CD4+ and CD8+ T cells with PDP-PFCE NPs. However, T cells were difficult to be detected in vivo after their engraftment in animals.

DISCUSSION

We showed successful in vitro labeling of T cells using novel fluorinated liposomal nanoparticles. However, insufficient and slow accumulation of labeled T cells and subsequent T cell proliferation in the pancreatic region remains as limitations of in vivo cell imaging by ¹⁹F MRI.

Comparison of different compressed sensing algorithms for low SNR 19 F MRI applications-Imaging of transplanted pancreatic islets and cells labeled with perfluorocarbons

Transplantation of pancreatic islets is a possible treatment option for patients suffering from Type I diabetes. In vivo imaging of transplanted islets is important for assessment of the transplantation site and islet distribution. Thanks to its high specificity, the absence of intrinsic background signal in tissue and its potential for quantification, ¹⁹ F MRI is a promising technique for monitoring the fate of transplanted islets in vivo. In order to overcome the inherent low sensitivity of ¹⁹ F MRI, leading to long acquisition times with low signal-to-noise ratio (SNR), compressed sensing (CS) techniques are a valuable option. We have validated and compared different CS algorithms for acceleration of ¹⁹ F MRI acquisition in a low SNR regime using pancreatic islets labeled with perfluorocarbons both in vitro and in vivo. Using offline simulation on both in vitro and in vivo low SNR fully sampled ¹⁹ F MRI datasets of labeled islets, we have shown that CS is effective in reducing the image acquisition time by a factor of three to four without seriously affecting SNR, regardless of the particular algorithms used in this study, with the exception of CoSaMP. Using CS, signals can be detected that might have been missed by conventional ¹⁹ F MRI. Among different algorithms (SPARSEMRI, OMMP, IRWL1, Two-level and CoSAMP), the two-level l₁ method has shown the best performance if computational time is taken into account. We have demonstrated in this study that different existing CS algorithms can be used effectively for low SNR ¹⁹ F MRI. An up to fourfold gain in SNR/scan time could be used either to reduce the scan time, which is beneficial for clinical and translational applications, or to increase the number of averages, to potentially detect otherwise undetected signal when compared with conventional ¹⁹ F MRI acquisitions. Potential applications in the field of cell therapy have been demonstrated.

Pre-Microporation Improves Outcome of Pancreatic Islet Labelling for Optical and 19F MR Imaging

Background

In vitro labelling of cells and small cell structures is a necessary step before in vivo monitoring of grafts. We modified and optimised a procedure for pancreatic islet labelling using bimodal positively charged poly(lactic-co-glycolic acid) nanoparticles with encapsulated perfluoro crown ethers and indocyanine green dye via microporation and compared the method with passive endocytosis.

Results

Pancreatic islets were microporated using two pulses at various voltages. We tested a standard procedure (poration in the presence of nanoparticles) and a modified protocol (pre-microporation in a buffer only, and subsequent islet incubation with nanoparticles on ice for 10 min).

We compared islet labelling by microporation with labelling by endocytosis, i.e. pancreatic islets were incubated for 24 h in a medium with suspended nanoparticles.

In order to verify the efficiency of the labelling procedures, we used ¹⁹F magnetic resonance imaging, optical fluorescence imaging and confocal microscopy.

The experiment confirmed that microporation, albeit fast and effective, is invasive and may cause substantial harm to islets. To achieve sufficient poration and to minimise the reduction of viability, the electric field should be set at 20 kV/m (two pulses, 20 ms each).

Poration in the presence of nanoparticles was found to be unsuitable for the nanoparticles used. The water suspension of nanoparticles (which served as a surfactant) was slightly foamy and microbubbles in the suspension were responsible for sparks causing the destruction of islets during poration. However, pre-microporation (poration of islets in a buffer only) followed by 10-min incubation with nanoparticles was safer.

Conclusions

For labelling of pancreatic islets using poly(lactic-co-glycolic acid) nanoparticles, the modified microporation procedure with low voltage was found to be safer than the standard microporation procedure. The modified procedure was fast, however, efficiency was lower compared to endocytosis.