Estimation of pO2 histogram from a composite EPR Spectrum of multiple random implants

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

OBJECTIVE

The overall objective of this clinical study was to validate an implantable oxygen sensor, called the 'OxyChip', as a clinically feasible technology that would allow individualized tumor-oxygen assessments in cancer patients prior to and during hypoxia-modification interventions such as hyperoxygen breathing.

METHODS

Patients with any solid tumor at ≤3-cm depth from the skin-surface scheduled to undergo surgical resection (with or without neoadjuvant therapy) were considered eligible for the study. The OxyChip was implanted in the tumor and subsequently removed during standard-of-care surgery. Partial pressure of oxygen (pO2) at the implant location was assessed using electron paramagnetic resonance (EPR) oximetry.

RESULTS

Twenty-three cancer patients underwent OxyChip implantation in their tumors. Six patients received neoadjuvant therapy while the OxyChip was implanted. Median implant duration was 30 days (range 4-128 days). Forty-five successful oxygen measurements were made in 15 patients. Baseline pO2 values were variable with overall median 15.7 mmHg (range 0.6-73.1 mmHg); 33% of the values were below 10 mmHg. After hyperoxygenation, the overall median pO2 was 31.8 mmHg (range 1.5-144.6 mmHg). In 83% of the measurements, there was a statistically significant (p ≤ 0.05) response to hyperoxygenation.

CONCLUSIONS

Measurement of baseline pO2 and response to hyperoxygenation using EPR oximetry with the OxyChip is clinically feasible in a variety of tumor types. Tumor oxygen at baseline differed significantly among patients. Although most tumors responded to a hyperoxygenation intervention, some were non-responders. These data demonstrated the need for individualized assessment of tumor oxygenation in the context of planned hyperoxygenation interventions to optimize clinical outcomes.

Oxygen-Sensing Paramagnetic Probes for Clinical Oximetry

Electron paramagnetic resonance (EPR) oximetry based on lithium naphthalocyanine paramagnetic crystals as oxygen sensors enables direct measurement of the partial pressure of oxygen (pO2) in tissues. The method uses topical or implantable forms of these oxygen-sensing crystals embedded in a biocompatible siloxane elastomer. This article presents a summary of these sensors for EPR oximetry and their applicability for tissue oxygen measurement in the clinic.

Implantable microchip containing oxygen-sensing paramagnetic crystals for long-term, repeated, and multisite in vivo oximetry

EPR oximetry is established as a viable method for measuring the tissue oxygen level (partial pressure of oxygen, pO₂) in animal models; however, it has not yet been established for measurements in humans. EPR oximetry requires an oxygen-sensing paramagnetic probe (molecular or particulate) to be placed at the site/organ of measurement, which may pose logistical and safety concerns, including invasiveness of the probe-placement procedure as well as lack of temporal stability and sensitivity for long-term (repeated) measurements, and possible toxicity in the short- and long-term. In the past, we have developed an implantable oxygen-sensing probe, called OxyChip, which we have successfully established for oximetry in pre-clinical animal models (Hou et al. Biomed. Microdevices 20, 29, 2018). Currently, OxyChip is being evaluated in a limited clinical trial in cancer patients. A major limitation of OxyChip is that it is a large (1.4 mm³) implant and hence not suitable for measuring oxygen heterogeneity that may be present in solid tumors, chronic wounds, etc. In this report, we describe the development of a substantially smaller version of OxyChip (0.07 mm³ or 70 cubic micron), called mChip, that can be placed in the tissue of interest using a 23G syringe-needle with minimal invasiveness. Using in vitro and in vivo models, we have shown that the microchip provides adequate EPR sensitivity, stability, and biocompatibility and thus enables robust, repeated, and simultaneous measurement from multiple implants providing mean and median pO₂ values in the implanted region. The mChips will be particularly useful for those applications that require repeated measurements of mean/median pO₂ in superficial tissues and malignancies.