Hexamethyldisiloxane-based nanoprobes for (1) H MRI oximetry

pO2 reporter composite hydrogel macroencapsulation devices for magnetic resonance imaging oxygen quantification

Hydrogel cell encapsulation devices are a common approach to reduce the need for chronic systemic immunosuppression in allogeneic cell product transplantation. Macroencapsulation approaches are an appealing strategy, as they maximize graft retrievability and cell dosage within a single device; however, macroencapsulation devices face oxygen transport challenges as geometries increase from preclinical to clinical scales. Device design guided by computational approaches can facilitate graft oxygen availability to encapsulated cells in vivo but is limited without accurate measurement of oxygen levels within the transplant site and graft. In this study, we engineer pO2 reporter composite hydrogels (PORCH) to enable spatiotemporal measurement of oxygen tension within macroencapsulation devices using the proton Imaging of siloxanes to map tissue oxygenation levels (PISTOL) magnetic resonance imaging approach. We engineer two methods of incorporating siloxane oximetry reporters within hydrogel devices, an emulsion and microbead-based approach, and evaluate PORCH cytotoxicity on co-encapsulated cells and accuracy in quantifying oxygen tension in vitro. We find that both emulsion and microbead PORCH approaches enable accurate in situ oxygen quantification using PISTOL magnetic resonance oximetry, and that the emulsion-based PORCH approach results in higher spatial resolution.

Evaluations of an Early Change in Tumor Pathophysiology in Response to Radiotherapy with Oxygen Enhanced Electron Paramagnetic Resonance Imaging (OE EPRI)

PURPOSE

Electron Paramagnetic Resonance Imaging (EPRI) can image the partial pressure of oxygen (pO2) within in vivo tumor models. We sought to develop Oxygen Enhanced (OE) EPRI that measures tumor pO2 with breathing gases of 21% O2 (pO221%) and 100% O2 (pO2100%), and the differences in pO2 between breathing gases (ΔpO2). We applied OE EPRI to study the early change in tumor pathophysiology in response to radiotherapy in two tumor models of pancreatic cancer.

PROCEDURES

We developed a protocol that intraperitoneally administered OX071, a trityl radical contrast agent, and then acquired anatomical MR images to localize the tumor. Subsequently, we acquired two pO221% and two pO2100% maps using the T1 relaxation time of OX071 measured with EPRI and a R1-pO2 calibration of OX071. We studied 4T1 flank tumor model to evaluate the repeatability of OE EPRI. We then applied OE EPRI to study COLO 357 and Su.86.86 flank tumor models treated with 10 Gy radiotherapy.

RESULTS

The repeatability of mean pO2 for individual tumors was ± 2.6 Torr between successive scans when breathing 21% O2 or 100% O2, representing a precision of 9.6%. Tumor pO221% and pO2100% decreased after radiotherapy for both models, although the decreases were not significant or only moderately significant, and the effect sizes were modest. For comparison, ΔpO2 showed a large, highly significant decrease after radiotherapy, and the effect size was large. MANOVA and analyses of the HF10 hypoxia fraction provided similar results.

CONCLUSIONS

EPRI can evaluate tumor pO2 with outstanding precision relative to other imaging modalities. The change in ΔpO2 before vs. after treatment was the best parameter for measuring the early change in tumor pathophysiology in response to radiotherapy. Our studies have established ΔpO2 from OE EPRI as a new parameter, and have established that OE EPRI is a valuable new methodology for molecular imaging.

Primary trifluoroborate-iminiums enable facile access to chiral α-aminoboronic acids via Ru-catalyzed asymmetric hydrogenation and simple hydrolysis of the trifluoroborate moiety

This work describes the first preparation and application of primary trifluoroborate-iminiums (pTIMs) as a new, easily accessible and valuable class of organoboron derivatives. An array of structurally diverse pTIMs was prepared from potassium acyltrifluoroborates in excellent yields. Highly efficient and enantioselective [(R,R)-TethTsDpen-RuCl] complex-catalyzed hydrogenation of pTIMs provided direct access to chiral primary trifluoroborate-ammoniums (pTAMs). Moreover, facile synthesis of a series of structurally diverse chiral α-aminoboronic acids from chiral pTAMs was accomplished through novel, operationally simple and efficient conversion using hexamethyldisiloxane/aqueous HCl. Using no chromatography at any point, this work allowed easy access to chiral α-aminoboronic acids, as exemplified by the synthesis of optically pure anti-cancer drugs bortezomib and ixazomib.

Starting with potassium acyltrifluoroborates (KATs), N-unprotected chiral α-aminoboronic acids are prepared in three simple steps without chromatography. This facile methodology will tap the broad potential of these valuable compounds.

More bullets for PISTOL: linear and cyclic siloxane reporter probes for quantitative 1H MR oximetry

Tissue oximetry can assist in diagnosis and prognosis of many diseases and enable personalized therapy. Previously, we reported the ability of hexamethyldisiloxane (HMDSO) for accurate measurements of tissue oxygen tension (pO₂) using Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL) magnetic resonance imaging. Here we report the feasibility of several commercially available linear and cyclic siloxanes (molecular weight 162–410 g/mol) as PISTOL-based oxygen reporters by characterizing their calibration constants. Further, field and temperature dependence of pO₂ calibration curves of HMDSO, octamethyltrisiloxane (OMTSO) and polydimethylsiloxane (PDMSO) were also studied. The spin-lattice relaxation rate R₁ of all siloxanes studied here exhibited a linear relationship with oxygenation (R₁ = A′ + B′*pO₂) at all temperatures and field strengths evaluated here. The sensitivity index η( = B′/A′) decreased with increasing molecular weight with values ranged from 4.7 × 10⁻³–11.6 × 10⁻³ torr⁻¹ at 4.7 T. No substantial change in the anoxic relaxation rate and a slight decrease in pO₂ sensitivity was observed at higher magnetic fields of 7 T and 9.4 T for HMDSO and OMTSO. Temperature dependence of calibration curves for HMDSO, OMTSO and PDMSO was small and simulated errors in pO₂ measurement were 1–2 torr/°C. In summary, we have demonstrated the feasibility of various linear and cyclic siloxanes as pO₂-reporters for PISTOL-based oximetry.

A faster PISTOL for 1 H MR-based quantitative tissue oximetry

Quantitative mapping of oxygen tension (pO₂ ), noninvasively, could potentially be beneficial in cancer and stroke therapy for monitoring therapy and predicting response to certain therapies. Intracellular pO₂ measurements may also prove useful in tracking the health of labeled cells and understanding the dynamics of cell therapy in vivo. Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL) is a relatively new oximetry technique that measures the T₁ of administered siloxanes such as hexamethyldisiloxane (HMDSO), to map the tissue pO₂ at various locations with a temporal resolution of 3.5 minutes. We have now developed a siloxane-selective Look-Locker imaging sequence equipped with an echo planar imaging (EPI) readout to accelerate PISTOL acquisitions. The new tissue oximetry sequence, referred to as PISTOL-LL, enables the mapping of HMDSO T₁ , and hence tissue pO₂ in under one minute. PISTOL-LL was tested and compared with PISTOL in vitro and in vivo. Both sequences were used to record dynamic changes in pO₂ of the rat thigh muscle (healthy Fischer rats, n = 6), and showed similar results (P > 0.05) as the other, with each sequence reporting a significant increase in pO₂ (P < 0.05) under hyperoxia compared with steady state normoxia. This study demonstrates the ability of the new sequence in rapidly and accurately mapping the pO₂ changes and accelerating quantitative ¹ H MR tissue oximetry by approximately 4-fold. The faster PISTOL-LL technique could enable dynamic ¹ H oximetry with higher temporal resolution for assesing tissue oxygentation and tracking the health of transplanted cells labeled with siloxane-based probes. With minor modifications, this sequence can be useful for ¹⁹ F applications as well.

Nanoparticle-Based Therapeutics for Brain Injury

Brain injuries affect a large patient population with major physical and emotional suffering for patients and their relatives; at a significant cost to the society. Effective diagnostic and therapeutic options available for brain injuries are limited by the complex brain injury pathology involving blood-brain barrier (BBB). Brain injuries, including ischemic stroke and brain trauma, initiate BBB opening for a short period of time, which is followed by a second reopening for an extended time. The leaky BBB and/or the alterations in the receptor expression on BBB may provide opportunities for therapeutic delivery via nanoparticles (NPs). The approaches for therapeutic interventions via NP delivery are aimed at salvaging the pericontusional/penumbra area for possible neuroprotection and neurovascular unit preservation. The focus of this progress report is to provide a survey of NP strategies employed in cerebral ischemia and brain trauma and finally provide insights for improved NP-based diagnostic/treatment approaches.

Oxygenation Imaging by Nuclear Magnetic Resonance Methods

Oxygen monitoring is a topic of exhaustive research due to its central role in many biological processes, from energy metabolism to gene regulation. The ability to monitor in vivo the physiological distribution and the dynamics of oxygen from subcellular to macroscopic levels is a prerequisite to better understand the mechanisms associated with both normal and disease states (cancer, neurodegeneration, stroke, etc.). This chapter focuses on magnetic resonance imaging (MRI) based techniques to assess oxygenation in vivo. The first methodology uses injected fluorinated agents to provide quantitative pO₂ measurements with high precision and suitable spatial and temporal resolution for many applications. The second method exploits changes in endogenous contrasts, i.e., deoxyhemoglobin and oxygen molecules through measurements of T ₂* and T ₁, in response to an intervention to qualitatively evaluate hypoxia and its potential modulation.

Siloxane Nanoprobes for Labeling and Dual Modality Functional Imaging of Neural Stem Cells

Cell therapy represents a promising therapeutic for a myriad of medical conditions, including cancer, traumatic brain injury, and cardiovascular disease among others. A thorough understanding of the efficacy and cellular dynamics of these therapies necessitates the ability to non-invasively track cells in vivo. Magnetic resonance imaging (MRI) provides a platform to track cells as a non-invasive modality with superior resolution and soft tissue contrast. We recently reported a new nanoprobe platform for cell labeling and imaging using fluorophore doped siloxane core nanoemulsions as dual modality ((1)H MRI/Fluorescence), dual-functional (oximetry/detection) nanoprobes. Here, we successfully demonstrate the labeling, dual-modality imaging, and oximetry of neural progenitor/stem cells (NPSCs) in vitro using this platform. Labeling at a concentration of 10 μL/10(4) cells with a 40%v/v polydimethylsiloxane core nanoemulsion, doped with rhodamine, had minimal effect on viability, no effect on migration, proliferation and differentiation of NPSCs and allowed for unambiguous visualization of labeled NPSCs by (1)H MR and fluorescence and local pO2 reporting by labeled NPSCs. This new approach for cell labeling with a positive contrast (1)H MR probe has the potential to improve mechanistic knowledge of current therapies, and guide the design of future cell therapies due to its clinical translatability.

A review of responsive MRI contrast agents: 2005-2014

This review focuses on MRI contrast agents that are responsive to a change in a physiological biomarker. The response mechanisms are dependent on six physicochemical characteristics, including the accessibility of water to the agent, tumbling time, proton exchange rate, electron spin state, MR frequency or superparamagnetism of the agent. These characteristics can be affected by changes in concentrations or activities of enzymes, proteins, nucleic acids, metabolites, or metal ions, or changes in redox state, pH, temperature, or light. A total of 117 examples are presented, including ones that employ nuclei other than (1) H, which attests to the creativity of multidisciplinary research efforts to develop responsive MRI contrast agents.

Dual-modality, dual-functional nanoprobes for cellular and molecular imaging

An emerging need for evaluation of promising cellular therapies is a non-invasive method to image the movement and health of cells following transplantation. However, the use of a single modality to serve this purpose may not be advantageous as it may convey inaccurate or insufficient information. Multi-modal imaging strategies are becoming more popular for in vivo cellular and molecular imaging because of their improved sensitivity, higher resolution and structural/functional visualization. This study aims at formulating Nile Red doped hexamethyldisiloxane (HMDSO) nanoemulsions as dual modality (Magnetic Resonance Imaging/Fluorescence), dual-functional (oximetry/detection) nanoprobes for cellular and molecular imaging. HMDSO nanoprobes were prepared using a HS15-lecithin combination as surfactant and showed an average radius of 71±39 nm by dynamic light scattering and in vitro particle stability in human plasma over 24 hrs. They were found to readily localize in the cytosol of MCF7-GFP cells within 18 minutes of incubation. As proof of principle, these nanoprobes were successfully used for fluorescence imaging and for measuring pO₂ changes in cells by magnetic resonance imaging, in vitro, thus showing potential for in vivo applications.