Dynamic Thermal Mapping of Localized Therapeutic Hypothermia in the Brain

Methylated tetra-amide derivatives of paramagnetic complexes for magnetic resonance biosensing with both BIRDS and CEST

Paramagnetic agents that utilize two mechanisms to provide physiological information by magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) are described. MRI with chemical exchange saturation transfer (CEST) takes advantage of the agent's exchangeable protons (e.g., -OH or -NH_x , where 2 ≥ x ≥ 1) to create pH contrast. The agent's incorporation of non-exchangeable protons (e.g., -CH_y , where 3 ≥ y ≥ 1) makes it possible to map tissue temperature and/or pH using an MRSI method called biosensor imaging of redundant deviation in shifts (BIRDS). Hybrid probes based upon 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate chelate (DOTA^4- ) and its methylated analog (1,4,7,10-tetraazacyclododecane-α, α', α″, α‴-tetramethyl-1,4,7,10-tetraacetate, DOTMA^4- ) were synthesized, and modified to create new tetra-amide chelates. Addition of several methyl groups per pendent arm of the symmetrical chelates, positioned proximally and distally to thulium ions (Tm³⁺ ), gave rise to favorable BIRDS properties (i.e., high signal-to-noise ratio (SNR) from non-exchangeable methyl proton peaks) and CEST responsiveness (i.e., from amide exchangeable protons). Structures of the Tm³⁺ probes elucidate the influence of methyl group placement on sensor performance. An eight-coordinate geometry with high symmetry was observed for the complexes: Tm-L1 was based on DOTA^4- , whereas Tm-L2 and Tm-L3 were based on DOTMA^4- , where the latter contained an additional carboxylate at the distal end of each arm. The distance of Tm³⁺ from terminal methyl carbons is a key determinant for sustaining BIRDS temperature sensitivity without compromising CEST pH contrast; however, water solubility was influenced by introduction of hydrophobic methyl groups and hydrophilic carboxylate. Combined BIRDS and CEST detection of Tm-L2, which features two high-SNR methyl peaks and a strong amide CEST peak, should enable simultaneous temperature and pH measurements for high-resolution molecular imaging in vivo.

High-resolution pH imaging using ratiometric chemical exchange saturation transfer combined with biosensor imaging of redundant deviation in shifts featuring paramagnetic DOTA-tetraglycinate agents

Chemical exchange saturation transfer (CEST) and biosensor imaging of redundant deviation in shifts (BIRDS) methods differ respectively by detecting exchangeable and nonexchangeable proton signals by magnetic resonance. Because CEST contrast depends on both temperature and pH, simultaneous CEST and BIRDS imaging can be employed to separate these contributions. Here, we test if high-resolution pH imaging in vivo is possible with ratiometric CEST calibrated for temperature variations measured by BIRDS. Thulium- and europium-based DOTA-tetraglycinate agents, TmDOTA-(gly)₄^- and EuDOTA-(gly)₄^- , were used for high-resolution pH mapping in vitro and in vivo, using BIRDS for temperature adjustments needed for a more accurate ratiometric CEST approach. Although neither agent showed pH dependence with BIRDS in vitro in the pH range 6 to 8, each one's temperature sensitivity was enhanced when mixed because of increased redundancy. By contrast, the CEST signal of each agent was affected by the presence of the other agent in vitro. However, pH could be measured more accurately when temperature from BIRDS was detected. These in vitro calibrations with TmDOTA-(gly)₄^- and EuDOTA-(gly)₄^- enabled high-resolution pH imaging of glioblastoma in rat brains. It was concluded that temperature mapping with BIRDS can calibrate the ratiometric CEST signal from a cocktail of TmDOTA-(gly)₄^- and EuDOTA-(gly)₄^- agents to provide temperature-independent absolute pH imaging in vivo.

Targeted temperature management and early neuro-prognostication after cardiac arrest

Targeted temperature management (TTM) is a recommended neuroprotective intervention for coma after out-of-hospital cardiac arrest (OHCA). However, controversies exist concerning the proper implementation and overall efficacy of post-CA TTM, particularly related to optimal timing and depth of TTM and cooling methods. A review of the literature finds that optimizing and individualizing TTM remains an open question requiring further clinical investigation. This paper will summarize the preclinical and clinical trial data to-date, current recommendations, and future directions of this therapy, including new cooling methods under investigation. For now, early induction, maintenance for at least 24 hours, and slow rewarming utilizing endovascular methods may be preferred. Moreover, timely and accurate neuro-prognostication is valuable for guiding ethical and cost-effective management of post-CA coma. Current evidence for early neuro-prognostication after TTM suggests that a combination of initial prediction models, biomarkers, neuroimaging, and electrophysiological methods is the optimal strategy in predicting neurological functional outcomes.

Imaging the transmembrane and transendothelial sodium gradients in gliomas

Under normal conditions, high sodium (Na⁺) in extracellular (Na⁺_e) and blood (Na⁺_b) compartments and low Na⁺ in intracellular milieu (Na⁺_i) produce strong transmembrane (ΔNa⁺_mem) and weak transendothelial (ΔNa⁺_end) gradients respectively, and these manifest the cell membrane potential (V_m) as well as blood–brain barrier (BBB) integrity. We developed a sodium (²³Na) magnetic resonance spectroscopic imaging (MRSI) method using an intravenously-administered paramagnetic polyanionic agent to measure ΔNa⁺_mem and ΔNa⁺_end. In vitro ²³Na-MRSI established that the ²³Na signal is intensely shifted by the agent compared to other biological factors (e.g., pH and temperature). In vivo ²³Na-MRSI showed Na⁺_i remained unshifted and Na⁺_b was more shifted than Na⁺_e, and these together revealed weakened ΔNa⁺_mem and enhanced ΔNa⁺_end in rat gliomas (vs. normal tissue). Compared to normal tissue, RG2 and U87 tumors maintained weakened ΔNa⁺_mem (i.e., depolarized V_m) implying an aggressive state for proliferation, whereas RG2 tumors displayed elevated ∆Na⁺_end suggesting altered BBB integrity. We anticipate that ²³Na-MRSI will allow biomedical explorations of perturbed Na⁺ homeostasis in vivo.

Resilience to Injury: A New Approach to Neuroprotection?

Despite thousands of neuroprotectants demonstrating promise in preclinical trials, a neuroprotective therapeutic has yet to be approved for the treatment of acute brain injuries such as stroke or traumatic brain injury. Developing a more detailed understanding of models and populations demonstrating "neurological resilience" in spite of brain injury can give us important insights into new translational therapies. Resilience is the process of active adaptation to a stressor. In the context of neuroprotection, models of preconditioning and unique animal models of extreme physiology (such as hibernating species) reliably demonstrate resilience in the laboratory setting. In the clinical setting, resilience is observed in young patients and can be found in those with specific genetic polymorphisms. These important examples of resilience can help transform and extend the current neuroprotective framework from simply countering the injurious cascade into one that anticipates, monitors, and optimizes patients' physiological responses from the time of injury throughout the process of recovery. This review summarizes the underpinnings of key adaptations common to models of resilience and how this understanding can be applied to new neuroprotective approaches.

Extracellular pH mapping of liver cancer on a clinical 3T MRI scanner

PURPOSE

To demonstrate feasibility of developing a noninvasive extracellular pH (pH_e ) mapping method on a clinical MRI scanner for molecular imaging of liver cancer.

METHODS

In vivo pH_e mapping has been demonstrated on preclinical scanners (e.g., 9.4T, 11.7T) with Biosensor Imaging of Redundant Deviation in Shifts (BIRDS), where the pH_e readout by 3D chemical shift imaging (CSI) depends on hyperfine shifts emanating from paramagnetic macrocyclic chelates like TmDOTP^5- which upon extravasation from blood resides in the extracellular space. We implemented BIRDS-based pH_e mapping on a clinical 3T Siemens scanner, where typically diamagnetic ¹ H signals are detected using millisecond-long radiofrequency (RF) pulses, and ¹ H shifts span over ±10 ppm with long transverse (T₂ , 10² ms) and longitudinal (T₁ , 10³ ms) relaxation times. We modified this 3D-CSI method for ultra-fast acquisition with microsecond-long RF pulses, because even at 3T the paramagnetic ¹ H shifts of TmDOTP^5- have millisecond-long T₂ and T₁ and ultra-wide chemical shifts (±200 ppm) as previously observed in ultra-high magnetic fields.

RESULTS

We validated BIRDS-based pH in vitro with a pH electrode. We measured pH_e in a rabbit model for liver cancer using VX2 tumors, which are highly vascularized and hyperglycolytic. Compared to intratumoral pH_e (6.8 ± 0.1; P < 10^-9 ) and tumor's edge pH_e (6.9 ± 0.1; P < 10^-7 ), liver parenchyma pH_e was significantly higher (7.2 ± 0.1). Tumor localization was confirmed with histopathological markers of necrosis (hematoxylin and eosin), glucose uptake (glucose transporter 1), and tissue acidosis (lysosome-associated membrane protein 2).

CONCLUSION

This work demonstrates feasibility and potential clinical translatability of high-resolution pH_e mapping to monitor tumor aggressiveness and therapeutic outcome, all to improve personalized cancer treatment planning.