Jesús Pacheco-Torres

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia‐inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of 19F or 1H indicators, such as perfluorocarbons or their 1H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy‐ and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. 19F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser‐enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes. Copyright

Dendritic MRI contrast agents: An efficient prelabeling approach based on CuAAC

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) allows the efficient and complete functionalization of dendrimers with preformed Gd chelates (prelabeling) to give monodisperse macromolecular contrast agents (CAs) for magnetic resonance imaging (MRI). This monodispersity contrasts with the typical distribution of materials obtained by classical routes and facilitates the characterization and quality control demanded for clinical applications. The potential of a new family of PEG-dendritic CA based on a gallic acid-triethylene glycol (GATG) core functionalized with up to 27 Gd complexes has been explored in vitro and in vivo, showing contrast enhancements similar to those of Gadomer-17, which reveals them to be a promising platform for the development of CA for MRI.

Proton imaging of siloxanes to map tissue oxygenation levels (PISTOL): A tool for quantitative tissue oximetry

Hexamethyldisiloxane (HMDSO) has been identified as a sensitive proton NMR indicator of tissue oxygenation (pO2) based on spectroscopic spin‐lattice relaxometry. A rapid MRI approach has now been designed, implemented, and tested. The technique, proton imaging of siloxanes to map tissue oxygenation levels (PISTOL), utilizes frequency‐selective excitation of the HMDSO resonance and chemical‐shift selective suppression of residual water signal to effectively eliminate water and fat signals and pulse‐burst saturation recovery 1H echo planar imaging to map T1 of HMDSO and hence pO2. PISTOL was used here to obtain maps of pO2 in rat thigh muscle and Dunning prostate R3327 MAT‐Lu tumor‐implanted rats. Measurements were repeated to assess baseline stability and response to breathing of hyperoxic gas. Each pO2 map was obtained in 3½ min, facilitating dynamic measurements of response to oxygen intervention. Altering the inhaled gas to oxygen produced a significant increase in mean pO2 from 55 Torr to 238 Torr in thigh muscle and a smaller, but significant, increase in mean pO2 from 17 Torr to 78 Torr in MAT‐Lu tumors. Thus, PISTOL enabled mapping of tissue pO2 at multiple locations and dynamic changes in pO2 in response to intervention. This new method offers a potentially valuable new tool to image pO2 in vivo for any healthy or diseased state by 1H MRI. Copyright

Functional MRI in mice lacking IP3-dependent calcium signaling in astrocytes

Functional magnetic resonance imaging (fMRI) is a fundamental tool to investigate human brain networks. However, the cellular mechanisms underlying fMRI signals are not fully understood. One hypothetical mechanism is the putative vascular control exerted by cytosolic calcium in perivascular astrocytes. We have performed combined fMRI-electrophysiology experiments in mice lacking the inositol 1,4,5-triphosphate-type-2 receptor, with the primary pathway of cytosolic calcium increase eliminated into astrocytes. Our results show that evoked electrophysiologic activity and fMRI signals acquired during either transient or sustained neuronal activations occur independently of these large calcium signals. This result challenges the suggested intermediary role of astrocytic calcium surges in fMRI-signal generation.

Image guided drug release from pH-sensitive Ion channel-functionalized stealth liposomes into an in vivo glioblastoma model

UNLABELLED Liposomal drug delivery vehicles are promising nanomedicine tools for bringing cytotoxic drugs to cancerous tissues selectively. However, the triggered cargo release from liposomes in response to a target-specific stimulus has remained elusive. We report on functionalizing stealth-liposomes with an engineered ion channel and using these liposomes in vivo for releasing an imaging agent into a cerebral glioma rodent model. If the ambient pH drops below a threshold value, the channel generates temporary pores on the liposomes, thus allowing leakage of the intraluminal medicines. By using magnetic resonance spectroscopy and imaging, we show that engineered liposomes can detect the mildly acidic pH of the tumor microenvironment with 0.2 pH unit precision and they release their content into C6 glioma tumors selectively, in vivo. A drug delivery system with this level of sensitivity and selectivity to environmental stimuli may well serve as an optimal tool for environmentally-triggered and image-guided drug release. FROM THE CLINICAL EDITOR Cancer remains a leading cause of mortality worldwide. With advances in science, delivery systems of anti-cancer drugs have also become sophisticated. In this article, the authors designed and characterized functionalized liposomal vehicles, which would release the drug payload in a highly sensitive manner in response to a change in pH environment in an animal glioma model. The novel data would enable better future designs of drug delivery systems.

Mesenchymal Stem Cells Improve Motor Functions and Decrease Neurodegeneration in Ataxic Mice

The main objective of this work is to demonstrate the feasibility of using bone marrow-derived stem cells in treating a neurodegenerative disorder such as Friedreichs ataxia. In this disease, the dorsal root ganglia of the spinal cord are the first to degenerate. Two groups of mice were injected intrathecally with mesenchymal stem cells isolated from either wild-type or Fxntm1Mkn/Tg(FXN)YG8Pook (YG8) mice. As a result, both groups presented improved motor skills compared to nontreated mice. Also, frataxin expression was increased in the dorsal root ganglia of the treated groups, along with lower expression of the apoptotic markers analyzed. Furthermore, the injected stem cells expressed the trophic factors NT3, NT4, and BDNF, which bind to sensory neurons of the dorsal root ganglia and increase their survival. The expression of antioxidant enzymes indicated that the stem cell-treated mice presented higher levels of catalase and GPX-1, which are downregulated in the YG8 mice. There were no significant differences in the use of stem cells isolated from wild-type and YG8 mice. In conclusion, bone marrow mesenchymal stem cell transplantation, both autologous and allogeneic, is a feasible therapeutic option to consider in delaying the neurodegeneration observed in the dorsal root ganglia of Friedreichs ataxia patients.

Dynamic oxygen challenge evaluated by NMR T1 and T2* – insights into tumor oxygenation

There is intense interest in developing non‐invasive prognostic biomarkers of tumor response to therapy, particularly with regard to hypoxia. It has been suggested that oxygen sensitive MRI, notably blood oxygen level‐dependent (BOLD) and tissue oxygen level‐dependent (TOLD) contrast, may provide relevant measurements. This study examined the feasibility of interleaved T2*‐ and T1‐weighted oxygen sensitive MRI, as well as R2* and R1 maps, of rat tumors to assess the relative sensitivity to changes in oxygenation. Investigations used cohorts of Dunning prostate R3327‐AT1 and R3327‐HI tumors, which are reported to exhibit distinct size‐dependent levels of hypoxia and response to hyperoxic gas breathing. Proton MRI R1 and R2* maps were obtained for tumors of anesthetized rats (isoflurane/air) at 4.7 T. Then, interleaved gradient echo T2*‐ and T1‐weighted images were acquired during air breathing and a 10 min challenge with carbogen (95% O2–5% CO2).

Stem cell injection in the hindlimb skeletal muscle enhances neurorepair in mice with spinal cord injury

AIMS To develop a low-risk, little-invasive stem cell-based method to treat acute spinal cord injuries. methods: Adult mice were submitted to an incomplete spinal cord injury, and mesenchymal stem cells injected intramuscularly into both hindlimbs. Behavior tests and MRI of the spinal cord were periodically performed for up to 6 months, along with immunohistochemical analysis. Immunohistochemical and PCR analysis of the muscles were used to detect the grafted cells as well as the soluble factors released. RESULTS The stem cell-treated mice presented significant improvements in their motor skills 5 months after treatment. Spinal cord repair was detected by magnetic resonance and immunohistochemistry. In the hindlimb muscles, the stem cells activated muscle and motor neuron repair mechanisms, due to the secretion of several neurotrophic factors. CONCLUSION Bone marrow mesenchymal stem cell injection into hindlimb muscles stimulates spinal cord repair in acute spinal cord lesions.

Multi-modal MRI classifiers identify excessive alcohol consumption and treatment effects in the brain

Robust neuroimaging markers of neuropsychiatric disorders have proven difficult to obtain. In alcohol use disorders, profound brain structural deficits can be found in severe alcoholic patients, but the heterogeneity of unimodal MRI measurements has so far precluded the identification of selective biomarkers, especially for early diagnosis. In the present work we used a combination of multiple MRI modalities to provide comprehensive and insightful descriptions of brain tissue microstructure. We performed a longitudinal experiment using Marchigian–Sardinian (msP) rats, an established model of chronic excessive alcohol consumption, and acquired multi‐modal images before and after 1 month of alcohol consumption (6.8 ± 1.4 g/kg/day, mean ± SD), as well as after 1 week of abstinence with or without concomitant treatment with the antirelapse opioid antagonist naltrexone (2.5 mg/kg/day). We found remarkable sensitivity and selectivity to accurately classify brains affected by alcohol even after the relative short exposure period. One month drinking was enough to imprint a highly specific signature of alcohol consumption. Brain alterations were regionally specific and affected both gray and white matter and persisted into the early abstinence state without any detectable recovery. Interestingly, naltrexone treatment during early abstinence resulted in subtle brain changes that could be distinguished from non‐treated abstinent brains, suggesting the existence of an intermediate state associated with brain recovery from alcohol exposure induced by medication. The presented framework is a promising tool for the development of biomarkers for clinical diagnosis of alcohol use disorders, with capacity to further inform about its progression and response to treatment.

Gd-Si Oxide Nanoparticles as Contrast Agents in Magnetic Resonance Imaging

We describe the synthesis, characterization and application as contrast agents in magnetic resonance imaging of a novel type of magnetic nanoparticle based on Gd-Si oxide, which presents high Gd3+ atom density. For this purpose, we have used a Prussian Blue analogue as the sacrificial template by reacting with soluble silicate, obtaining particles with nanorod morphology and of small size (75 nm). These nanoparticles present good biocompatibility and higher longitudinal and transversal relaxivity values than commercial Gd3+ solutions, which significantly improves the sensitivity of in vivo magnetic resonance images.