Southwestern In Vivo Cellular and Molecular Imaging Program

 


 

 

Tumor Oxygenation: Acute Modulation Detected by 19F NMR of Perfluorocarbons


R.P. Mason*, P.P. Antich*, A. Constantinescu*,
P.P. Peschke# & E.W. Hahn#


*Radiology, UT-Southwestern Medical Center at Dallas, TX 75235
and
#Forschungsschwerpunkt Pathophysiologie, DKFZ, Heidelberg, Germany.




Rationale

 Tumor response to therapy (radiotherapy, PDT or chemotherapy) may be influenced by tissue oxygen tension (pO2) and hypoxic tumors are generally refractory. We are developing a sensitive non-invasive method of monitoring tumor oxygen tension using 19F NMR of perfluorocarbon emulsion. It is widely reported that there is a linear relationship between the 19F NMR spin-lattice relaxation rate of perfluorocarbon emulsions and oxygen tension (pO2) [1]. We have previously, reported chronic changes in tumor pO2 during a longitudinal growth study in the Dunning prostate R3327-AT1 [2]. We have now investigated acute changes in the fast-growing metastatic MAT-Lu subline.


Objectives
  • To demonstrate the ability to monitor acute changes in regional tumor pO2.
  • To demonstrate radiobiologically significant precision.
  • To exploit a clinically pertinent perfluorocarbon emulsion.

Experimental

Dunning prostatic adenocarcinoma R3327-MAT-Lu (volume doubling time ~ 1.5 days) was implanted in a skin pedicle on the fore-back of a male Copenhagen rat as shown in cartoon (Fig. 1) [3]. Once the tumor had grown to 1 cm diameter, 4 x 2.5 ml of OxygentTM (90% w/v emulsion of perflubron [PFOB], Alliance Pharmaceuticals. Inc., CA) were infused into the tail vein over 2 days. Three days later the rat was anesthetized using gas (0.5 dm3/min N2O, 1 dm3/min O2 and 0.5% methoxyflurane) and placed in the bore of a 40 cm 4.7 Tesla magnet with a volume coil around the tumor. 1H and 19F magnetic resonance imaging (MRI) were used to assess the biodistribution of PFC emulsion within the tumor and 19F NMR spectroscopy was used to measure the relaxation rates of the CF3 and CF2Br resonances of perflubron. Each pO2 determination required 1 minute and generally three determinations were repeated to assess stability of pO2 and provide enhanced precision. To induce acute changes in pO2 the inspired gas was switched alternately to pure oxygen, or carbogen or air with a 10 minute equilibration period prior to measurement of pO2.


Results
  • 19F signal from the emulsion Oxygent was readily detected in the tumor with seven resolved resonances (Fig. 2).
  • Each resonance exhibits a linear response of R1 to pO2 (e.g. Fig. 3) and pO2 measurements were made on the basis of relaxation rates of the well resolved downfield CF3 and CF2Br resonances.
  • Distribution of Oxygent in the tumor is distinctly heterogeneous and intially corresponds to well perfused regions (Fig. 4)
  • The well perfused regions of the MAT-Lu tumor show relatively high pO2 when the rat breathes 66% oxygen (pO2=35.8+3.7 torr). Switching to pure oxygen or carbogen induced a significant increase in pO2.
  • When the tumor doubled in size pO2 of the regions interrogated by the PFC exhibited much lower pO2. However, elevating the inspired concentration of oxygen still caused an increase in tumor pO2.
  • Clamping the tumor produced hypoxia: pO2=-2.2+2.3 torr; i.e., not significantly different from zero.

Conclusions
  • Acute changes in tumor pO2 may be detected using 19F NMR relaxometry of perfluorocarbons.
  • Radiobiologically significant precision (< 5 torr) may be achieved.
  • Oxygent, a biocompatible emulsion undergoing clinical trials is a suitable NMR biosensor.

Discussion

This technique provides a non-invasive method of determining tumor pO2. pO2 values in the MAt-Lu were consideably lower than slower growing AT1 tumors under similar condiotions [4]. We are initiating comparative studies with the "gold standard" Eppendorf Histograph to determine whether pO2 values correlate with accepted measures of tumor hypoxia. Given the current use of perfluorocarbon emulsions in clinical trials we believe this new method may provide a non-invasive prognostic tool for evaluating individual patient tumors in the foreseeable future.


References
  1. "Non-Invasive Physiology: 19F NMR of Perfluorocarbons", R.P. Mason, Artif. Cells Blood Subst. Immob. Biotechnol., 22, 1141-1153 (1994).
  2. "Non-Invasive Determination of Tumor Oxygen Tension and Local Variation with Growth", R.P. Mason, P.P. Antich, E.E. Babcock, A. Constantinescu, P. Peschke & E.W. Hahn, Int. J. Radiat. Oncol. Biol. Phys., 29 (1), 95-103 (1994).
  3. "Isolated Tumor Growth in a Surgically Formed Skin Pedicle in the Rat: a New Tumor Model for NMR Studies", E.W. Hahn, P. Peschke, R.P. Mason, E.E. Babcock, & P.P. Antich, Magn. Reson. Imaging, 11, 1007-1017 (1993).
  4. "Tumor Oxygen Tension: Measurement using OxygentTM as a 19F NMR Probe at 4.7 T", R.P. Mason & P.P. Antich, Artif. Cells Blood Subst. Immob. Biotechnol., 22, 1361-1367 (1994)

Acknowledgments

This work was supported in part by the Whitaker Foundation and Alliance Pharmaceuticals Inc. NMR experiments were performed at the Mary Nell & Ralph B. Rogers MR Center, an NIH BRTP Facility #5-P41-RR02584. We thank ASTRO for a CaP/CURE grant to attend this meeting.

Figure Legends

Figure 1
Cartoon of pedicle tumor on foreback of rat.


Figure 2
19F NMR spectrum of OxygentTM in a Dunning prostate tumor at 4.7 Tesla


Figure 3
The linear relationship between spin lattice relaxation rate (R1) and pO2 for the CF3 resonance of perflubron. R1 (s-1) = 0.267 + 2.24x10-3 pO2 (torr).


Figure 4
Thin slice 1H and 19F images of MAT-Lu tumor indicating the heterogeneous distribution of PFC emulsion within the tumor. Slices have 500x1000 µm in plane resolution and are 4 mm thick. a)Transaxial slices ; b) perpendicular sagittal slice at level of line in "a".


Figure 5
a) Modulation of pO2 in well perfused regions of MAT-Lu tumor with inhaled gas.
b) Modulation of pO2 in hypoxic tumor regions.


Figure 6
As Fig. 4a, but two days later. Transaxial slices show that the tumor has grown, but the PFC has not redistributed.

 


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