Pharmacokinetics of a triarylmethyl‐type paramagnetic spin probe used in EPR oximetry

Abstract

The paramagnetic spin probe Oxo63 is used in oximetric imaging studies based on electron paramagnetic resonance (EPR) methods by monitoring the oxygen-dependent linewidth while minimizing the contributions from self-broadening seen at high probe concentrations. Therefore, it is necessary to determine a suitable dose of Oxo63 for EPR-based oxygen mapping where the self-broadening effects are minimized while signal intensity adequate for imaging can be realized. A constant tissue concentration of spin probe would be useful to image a subject and assess changes in pO 2 over time; accumulation or elimination of the compound in specific anatomical regions could translate to and be mistaken for changes in local pO 2 , especially in OMRI-based oximetry. The in vivo pharmacokinetics of the spin probe, Oxo63, after bolus and/or continuous intravenous infusion was investigated in mice using a novel approach with X-band EPR spectroscopy. The results show that the half-life in blood was 17-21 min and the clearance by excretion was 0. A number of techniques currently have the capability to assess tissue oxygen tension or are under development (1,2). Clarke-type oxygen electrodes are widely used experimentally and clinically to estimate pO 2 levels, although only a small portion of the tissue is sampled (3,4). However, results from such measurements over the past several years have shown a strong correlation between tumor pO 2 and treatment outcome, suggesting that tumor oxygenation status is a useful prognostic factor (5). Ideally, a method of oxygen measurement in tissue, e.g., a tumor, should be noninvasive, quantitative, and global with respect to the volume of interest. In addition, such techniques should allow for measurement in tissue before, during, and after treatment. Noninvasive imaging techniques such as blood oxygen level-dependent MRI (BOLD-MRI), single photon emission computed tomography (SPECT), and positron-emission tomography (PET) (6 -9) are now used to assess tumor oxygenation.BOLD-MRI examines differences in T 2 contrast between two images, based on the ratio of diamagnetic oxy-hemoglobin and paramagnetic deoxy-hemoglobin in the blood, and provides qualitative assessment of pO 2 (10,11). PET uses positron-emitting radioisotope-labeled molecular probes such as 18 F-fluoromisonidazole ( 18 F-MISO) (12-14) or 64 Cu-diacetyl-bis(N 4 -methylthiosemicarbazone) ( 64 Cu-ATSM) (15-17), which have high affinity for the hypoxic atmosphere and obtain spatial maps of hypoxic regions in a patient and/or experimental animals. Measurement of oxygen metabolism was clinically carried out by PET using 15 O gas as the probe (18 -22). SPECT is also a radiological imaging technique which has been used for measurement of blood perfusion volume in the brain employing a ␥-ray emitting radioisotope such as N-isopropyl-p- Noninvasive imaging techniques based on electron paramagnetic resonance (EPR) such as continuous wave (CW) and time-domain electron paramagnetic resonance imaging (EPRI) (11,33,34), or Overhauser-enhanced MRI (OMR...

Electron paramagnetic resonance oxygen imaging of a rabbit tumor using localized spin probe delivery

“…At such concentrations, the EPR line of trityl in animals has been reported to have substantial broadening due to spinspin interaction. 19 The CW image was able to reproduce these areas, while the ESE performance in these areas was found to be rather poor. Thus, for areas with high spin probe and pO 2 concentration ESE may emphasize the low concen- tration and pO 2 .…”

“…A similar protocol was published elsewhere. 19 For the rabbit, this scales to 6 g of OX063 for the initial bolus plus an additional 6 g infused per hour following the bolus. Over 12 g of trityl would be used for the sequence of images obtained in this study.…”

“…The most common spin probe used for oxygenation images in animals presently is the triarylmethyl radical OX063, referred also as trityl. 8,19 This spin probe is administered systemically in mice via tail vein. A short time after the spin probe is infused, it distributes into extracellular volume of tissues of the entire animal giving rise to an EPR signal.…”

“…After a single bolus spin probe infusion, there is a rapid decrease in the spin probe concentration in the animal body with a tissue half-life less than 5 min and tumor half-life of 30 min. 14,19 This alters the distribution of spin probe during the acquisition of image projections and affects image quality. This effect can be minimized using constant infusion of spin probe for the duration of the image after initial bolus as demonstrated in this work.…”

Measurement of pO2 in cultured mouse oocytes using electron paramagnetic resonance oximetry

“…After the injection of Ox063 solution, the oocytes were transferred through 10 washes with 50-μL drops of HEPES-CZB (experiment 1) or gas-equilibrated CZB medium (experiment 2) under paraffin oil. Approximately 1 h after the incubation under air (experiment 1) or (9). In the present study, we therefore examined the feasibility of measurement of pO 2 in cultured mouse oocytes using EPR oximetry with Ox063 as a preliminary study for monitoring the changes of pO 2 during ART procedures.…”