First-In-Human Study in Cancer Patients Establishing the Feasibility of Oxygen Measurements in Tumors Using Electron Paramagnetic Resonance With the OxyChip

Hand-held electron spin resonance scanner for subcutaneous oximetry using OxyChip

PURPOSE

Electron spin resonance (ESR) is used to measure oxygen partial pressure (pO2) in biological media with many clinical applications. Traditional clinical ESR involves large magnets that encompass the subject of measurement. However, certain applications might benefit from a scanner operating within local static magnetic fields. Our group recently developed such a compact scanner for transcutaneous (surface) pO2 measurements of skin tissue. Here we extend this capability to subsurface (subcutaneous) pO2 measurements and verify it using an artificial tissue emulating (ATE) phantom.

METHODS

We introduce a new scanner, tailored for subcutaneous measurements up to 2 mm beneath the skin's surface. This scanner captures pulsed ESR signals from embedded approximate 1-mm oxygen-sensing solid paramagnetic implant, OxyChip. The scanner features a static magnetic field source, producing a uniform region outside its surface, and a compact microwave resonator, for exciting and receiving ESR signals.

RESULTS

ESR readings derived from an OxyChip, positioned approximately 1.5 mm from the scanner's surface, embedded in ATE phantom, exhibited a linear relation of 1/T2 versus pO2 for pO2 levels at 0, 7.6, 30, and 160 mmHg, with relative reading accuracy of about 10%.

CONCLUSION

The compact ESR scanner can report pO2 data in ATE phantom from an external position relative to the scanner. Implementing this scanner in preclinical and clinical applications for subcutaneous pO2 measurements is a feasible next phase for this development. This innovative design also has the potential to operate in conjunction with artificial skin graft for wound healing, combining therapeutic and pO2 diagnostic features.

Surface dielectric resonator for in vivo EPR measurements

This research report describes a novel surface dielectric resonator (SDR) with a flexible connector for in vivo electron paramagnetic resonance (EPR) spectroscopy. Contrary to the conventional cavity or surface loop-gap resonators, the newly developed SDR is constructed from a ceramic dielectric material, and it is tuned to operate at the L-band frequency band (1.15 GHz) in continuous-wave mode. The SDR is designed to be critically coupled and capable of working with both very lossy samples, such as biological tissues, and non-lossy materials. The SDR was characterized using electromagnetic field simulations, assessed for sensitivity with a B1 field-perturbation method, and validated with tissue phantoms using EPR measurements. The results showed remarkably higher sensitivity in lossy tissue phantoms than the previously reported multisegment surface-loop resonators. The new SDR can provide potential new insights for advancements in the application of in vivo EPR spectroscopy for biological measurements, including clinical oximetry.

Highly Sensitive Detection of Melanin in Melanomas Using Multi-harmonic Low Frequency EPR

PURPOSE

Low frequency EPR can noninvasively detect endogenous free radical melanin in melanocytic skin lesions and could potentially discriminate between benign atypical nevi and malignant melanoma lesions. We recently succeeded in demonstrating the ability of clinical EPR to noninvasively detect the endogenous melanin free radical in skin lesions of patients. However, the signal-to-noise ratio (SNR) was extremely low warranting further research to boost the sensitivity of detection. In the present study, we assessed the performance of a clinical EPR system with the capability to perform multi-harmonic (MH) analysis for the detection of melanin.

PROCEDURES

The sensitivity of MH-EPR was compared with a classical continuous wave (CW)-EPR (1st harmonic) detection in vitro in melanin phantoms, in vivo in melanoma models with cells implanted in the skin, in lymph nodes and having colonized the lungs, and finally on phantoms placed at the surface of human skin.

RESULTS

In vitro, we observed an increase in SNR by a factor of 10 in flat melanin phantoms when using MH analysis compared to CW combined with an increase in modulation amplitude. In B16 melanomas having grown in the skin of hairless mice, we observed a boost in sensitivity in vivo similar to that observed in vitro with the capability to detect melanoma cells at an earlier stage of development. MH-EPR was also able to detect non-invasively the melanin signal coming from melanoma cells present in lymph nodes as well as in lungs. We also observed a boost of sensitivity using phantoms of melanin placed at the surface of human skin.

CONCLUSIONS

Overall, our results are paving the way for new clinical trials that will use MH clinical EPR for the characterization of pigmented skin lesions.

Sensitive simultaneous measurements of oxygenation and extracellular pH by EPR using a stable monophosphonated trityl radical and lithium phthalocyanine

The monitoring of acidosis and hypoxia is crucial because both factors promote cancer progression and impact the efficacy of anti-cancer treatments. A phosphonated tetrathiatriarylmethyl (pTAM) has been previously described to monitor both parameters simultaneously, but the sensitivity to tackle subtle changes in oxygenation was limited. Here, we describe an innovative approach combining the pTAM radical and lithium phthalocyanine (LiPc) crystals to provide sensitive simultaneous measurements of extracellular pH (pHe) and pO2. Both parameters can be measured simultaneously as both EPR spectra do not overlap, with a gain in sensitivity to pO2 variations by a factor of 10. This procedure was applied to characterize the impact of carbogen breathing in a breast cancer 4T1 model as a proof-of-concept. No significant change in pHe and pO2 was observed using pTAM alone, while LiPc detected a significant increase in tumor oxygenation. Interestingly, we observed that pTAM systematically overestimated the pO2 compared to LiPc. In addition, we analyzed the impact of an inhibitor (UK-5099) of the mitochondrial pyruvate carrier (MPC) on the tumor microenvironment. In vitro, the exposure of 4T1 cells to UK-5099 for 24 h induced a decrease in pHe and oxygen consumption rate (OCR). In vivo, a significant decrease in tumor pHe was observed in UK-5099-treated mice, while there was no change for mice treated with the vehicle. Despite the change observed in OCR, no significant change in tumor oxygenation was observed after the UK-5099 treatment. This approach is promising for assessing in vivo the effect of treatments targeting tumor metabolism.

Re-examining What the Results of "a Measurement of Oxygen Level in Tissues" Really Mean

Within this special issue, many eminent investigators report on measurements of oxygen (O2) levels in tissues. Given the complexities of spatial and temporal heterogeneities of O2 in tissues and its many sources, this commentary draws attention to what such measurements do and do not actually assess regarding O2 levels in tissues. Given this limitation, it also discusses how these results can be used most effectively. To provide a convenient mechanism to discuss these issues more fully, this analysis focuses on measurements using EPR oximetry, but these considerations apply to all other techniques. The nature of the delivery of O2 to tissues and the mechanisms by which O2 is consumed necessarily result in very different levels of O2 within the volume of each voxel of a measurement. Better spatial resolution cannot fully resolve the problem because the variations include O2 gradients within each cell. Improved resolution of the time-dependent variation in O2 is also very challenging because O2 levels within tissues can have fluctuations of O2 levels in the range of milliseconds, while most methods require longer times to acquire the data from each voxel. Based on these issues, we argue that the values obtained inevitably are complex aggregates of averages of O2 levels across space and time in the tissue. These complexities arise from the complex physiology of tissues and are compounded by the limitations of the technique and its ability to acquire data. However, one often can obtain very meaningful and useful results if these complexities and limitations are taken into account. We illustrate this, using results obtained with in vivo EPR oximetry, especially utilizing its capacity to make repeated measurements to follow changes in O2 levels that occur with interventions and/or over time.

SOX71, A Biocompatible Succinyl Derivative of the Triarylmethyl Radical OX071 for In Vivo Quantitative Oxygen Mapping Using Electron Paramagnetic Resonance

PURPOSE

This study aimed to develop a biocompatible oximetric electron paramagnetic resonance (EPR) spin probe with reduced self-relaxation, and sensitivity to oxygen for a higher signal-to-noise ratio and longer relaxation times at high oxygen concentration, compared to the reference spin probe OX071.

PROCEDURES

SOX71 was synthesized by succinylation of the twelve alcohol groups of OX071 spin probe and characterized by EPR at X-Band (9.5 GHz) and at low field (720 MHz). The biocompatibility of SOX71 was tested in vitro and in vivo in mice. A pharmacokinetic study was performed to determine the best time frame for EPR imaging. Finally, a proof-of-concept EPR oxygen imaging was performed on a mouse model of a fibrosarcoma tumor.

RESULTS

SOX71 was synthesized in one step from OX071. SOX71 exhibits a narrow line EPR spectrum with a peak-to-peak linewidth of 66 mG, similar to OX071. SOX71 does not bind to albumin nor show cell toxicity for the concentrations tested up to 5 mM. No toxicity was observed after systemic delivery via intraperitoneal injection in mice at twice the dose required for EPR imaging. After the injection, the probe is readily absorbed into the bloodstream, with a peak blood concentration half an hour, post-injection. Then, the probe is quickly cleared by the kidney with a half-life of ~ 45 min. SOX71 shows long relaxation times under anoxic condition (T1e = 9.5 µs and T2e = 5.1 µs; [SOX71] = 1 mM in PBS at 37 °C, pO2 = 0 mmHg, 720 MHz). Both the relaxation rates R1e and R2e show a decreased sensitivity to pO2, leading to twice longer relaxation times under room air conditions (pO2 = 159 mmHg) compared to OX071. This is ideal for oxygen imaging in samples with a wide range of pO2. Both the relaxation rates R1e and R2e show a decreased sensitivity to self-relaxation compared to OX071, with a negligible effect of the probe concentration on R1e. SOX71 was successfully applied to image oxygen in a tumor.

CONCLUSION

SOX71, a succinylated derivative of OX071 was synthesized, characterized, and applied for in vivo EPR tumor oxygen imaging. SOX71 is highly biocompatible, and shows decreased sensitivity to oxygen and self-relaxation. This first report suggests that SOX71 is superior to OX071 for absolute oxygen mapping under a broad range of pO2 values.

Oxygen Imaging of a Rabbit Tumor Using a Human-Sized Pulse Electron Paramagnetic Resonance Imager

PURPOSE

Spatial heterogeneity in tumor hypoxia is one of the most important factors regulating tumor growth, development, aggressiveness, metastasis, and affecting treatment outcome. Most solid tumors are known to have hypoxia or low oxygen levels (pO2 ≤10 torr). Electron paramagnetic resonance oxygen imaging (EPROI) is an emerging oxygen mapping technology. EPROI utilizes the linear relationship between the relaxation rates of the injectable OX071 trityl spin probe and the partial oxygen pressure (pO2). However, most of the EPROI studies have been limited to mouse models of solid tumors because of the instrument-size limitations. The purpose of this work was to develop a human-sized 9-mT (250 MHz resonance frequency, 60 cm bore size) pulse EPROI instrument and evaluate its performance with rabbit VX-2 tumor oxygen imaging.

METHODS

A New Zealand white rabbit with a 3.2-cm VX-2 tumor in the calf muscle was imaged using the human-sized EPROI instrument and a 2.25-in. ID volume coil. The animal received a ~8-min intravenous injection of OX071 (5.2 mL total volume at 72 mM concentration) and, after 75 min, an intratumoral injection (120 μL total at 5 mM OX071 concentration) and underwent EPROI. At the end of the experiments, MRI was performed using a preclinical 9.4-T MRI system to outline the tumor boundaries.

RESULTS

For the first time, a human-sized pulse EPROI instrument with a 60-cm bore size/250-MHz frequency was built and evaluated using rabbit tumor oxygen imaging. For the first time, the systemic IV injection of the oxygen-sensitive trityl OX071 spin probe was used for an animal of this size. The resulting EPROI image from the IV injection showed complete tumor coverage. The image obtained after intratumoral injection showed localized coverage in the upper lobe of the tumor, demonstrating the need for improved intratumoral injection protocol.

CONCLUSIONS

This study demonstrates the performance of the world's first human-sized pulse EPROI instrument. It also demonstrates that the EPROI of larger animals can be performed using the systemic injection of a manageable amount of the spin probe. This brings EPROI one step closer to clinical applications in cancer therapies. Oxygen imaging is a platform technology, and the instrument and techniques developed here will also be useful for other clinical applications.

Towards Characterization of Skin Melanoma in the Clinic by Electron Paramagnetic Resonance (EPR) Spectroscopy and Imaging of Melanin

The incidence of melanoma is continuously increasing over time. Melanoma is the most aggressive skin cancer, significantly reducing quality of life and survival rates of patients at advanced stages. Therefore, early diagnosis remains the key to change the prognosis of patients with melanoma. In this context, advanced technologies are under evaluation to increase the accuracy of the diagnostic, to better characterize the lesions and visualize their possible invasiveness in the epidermis. Among the innovative methods, because melanin is paramagnetic, clinical low frequency electron paramagnetic resonance (EPR) that characterizes the melanin content in the lesion has the potential to be an adjunct diagnostic method of melanoma. In this review, we first summarize the challenges faced by dermatologists and oncologists in melanoma diagnostic and management. We also provide a historical perspective on melanin detection with a focus on EPR spectroscopy/imaging of melanomas. We describe key elements that allow EPR to move from in vitro studies to in vivo and finally to patients for melanoma studies. Finally, we provide a critical view on challenges to meet to make EPR operational in the clinic to characterize pigmented lesions.

Biocompatible Monophosphonated Trityl Spin Probe, HOPE71, for In Vivo Measurement of pO2, pH, and [Pi] by Electron Paramagnetic Resonance Spectroscopy

Hypoxia, acidosis, and elevated inorganic phosphate concentration are characteristics of the tumor microenvironment in solid tumors. There are a number of methods for measuring each parameter individually in vivo, but the only method to date for noninvasive measurement of all three variables simultaneously in vivo is electron paramagnetic spectroscopy paired with a monophosphonated trityl radical, pTAM/HOPE. While HOPE has been successfully used for in vivo studies upon intratissue injection, it cannot be delivered intravenously due to systemic toxicity and albumin binding, which causes significant signal loss. Therefore, we present HOPE71, a monophosphonated trityl radical derived from the very biocompatible trityl probe, Ox071. Here, we describe a straightforward synthesis of HOPE71 starting with Ox071 and report its EPR sensitivities to pO2, pH, and [Pi] with X-band and L-band EPR spectroscopy. We also confirm that HOPE71 lacks albumin binding, shows low cytotoxicity, and has systemic tolerance. Finally, we demonstrate its ability to profile the tumor microenvironment in vivo in a mouse model of breast cancer.

EPR and Related Magnetic Resonance Imaging Techniques in Cancer Research

Imaging tumor microenvironments such as hypoxia, oxygenation, redox status, and/or glycolytic metabolism in tissues/cells is useful for diagnostic and prognostic purposes. New imaging modalities are under development for imaging various aspects of tumor microenvironments. Electron Paramagnetic Resonance Imaging (EPRI) though similar to NMR/MRI is unique in its ability to provide quantitative images of pO₂ in vivo. The short electron spin relaxation times have been posing formidable challenge to the technology development for clinical application. With the availability of the narrow line width trityl compounds, pulsed EPR imaging techniques were developed for pO₂ imaging. EPRI visualizes the exogenously administered spin probes/contrast agents and hence lacks the complementary morphological information. Dynamic nuclear polarization (DNP), a phenomenon that transfers the high electron spin polarization to the surrounding nuclear spins (¹H and ¹³C) opened new capabilities in molecular imaging. DNP of ¹³C nuclei is utilized in metabolic imaging of 13C-labeled compounds by imaging specific enzyme kinetics. In this article, imaging strategies mapping physiologic and metabolic aspects in vivo are reviewed within the framework of their application in cancer research, highlighting the potential and challenges of each of them.

Noninvasive detection of the endogenous free radical melanin in human skin melanomas using electron paramagnetic resonance (EPR)

We explored the capability of low-frequency Electron Paramagnetic Resonance (EPR) to noninvasively detect melanin (a stable semiquinone free radical) in the human skin. As previous in vitro studies on biopsies suggested that the EPR signal from melanin was different when measured in skin melanomas or benign nevi, we conducted a prospective first-in-man clinical EPR study in patients with skin lesions suspicious of melanoma. EPR spectra were obtained using a spectrometer operating at 1 GHz, with a surface coil placed over the area of interest. Two clinical studies were carried out: 1) healthy volunteers (n = 45) presenting different skin phototypes; 2) patients (n = 88) with skin lesions suspicious of melanoma (n = 100) requiring surgical resection. EPR data obtained before surgery were compared with histopathology results. The method was not sensitive enough to measure differences in melanin content due to changes in skin pigmentation. In patients, 92% of the spectra were analyzable. The EPR signal of melanin was significantly higher (p < 0.0001) in melanoma lesions (n = 26) than that in benign atypical nevi (n = 62). A trend toward a higher signal intensity (though not significant) was observed in high Breslow depth melanomas (a marker of skin invasion) than in low Breslow lesions. To date, no naturally occurring free radicals have been detected by low-frequency EPR systems adapted for clinical studies. Here, we demonstrated for the first time the ability of this technology to detect an endogenous free radical, opening new avenues for evaluating clinical EPR as a potential aid in the diagnosis of pigmented skin lesions.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

OxyChip embedded with radio-opaque gold nanoparticles for anatomic registration and oximetry in tissues

PURPOSE

Electron paramagnetic resonance oximetry using the OxyChip as an implantable oxygen sensor can directly and repeatedly measure tissue oxygen levels. A phase I, first-in-human clinical study has established the safety and feasibility of using OxyChip for reliable and repeated measurements of oxygen levels in a variety of tumors and treatment regimens. A limitation in these studies is the inability to easily locate and identify the implanted probes in the tissue, particularly in the long term, thus limiting spatial/anatomical registration of the implant for proper interpretation of the oxygen data. In this study, we have developed and evaluated an enhanced oxygen-sensing probe embedded with gold nanoparticles (GNP), called the OxyChip-GNP, to enable visualization of the sensor using routine clinical imaging modalities.

METHODS

In vitro characterization, imaging, and histopathology studies were carried out using tissue phantoms, excised tissues, and in vivo animal models (mice and rats).

RESULTS

The results demonstrated a substantial enhancement of ultrasound and CT contrast using the OxyChip-GNP without compromising its electron paramagnetic resonance and oxygen-sensing properties or biocompatibility.

CONCLUSIONS

The OxyChips embedded with gold nanoparticles (OxyChip-GNP) can be readily identified in soft tissues using standard clinical imaging modalities such as CT, cone beam-CT, or ultrasound imaging while maintaining its capability to make repeated in vivo measurements of tissue oxygen levels over the long term. This unique capability of the OxyChip-GNP facilitates precisely localized in vivo oxygen measurements in the clinical setting.