OXYGENATION IN A HUMAN TUMOR XENOGRAFT: MANIPULATION THROUGH RESPIRATORY CHALLENGE AND ANTIBODY-DIRECTED INFARCT

Mason, Ralph; Constantinescu, Anca; Ran, Sophia+; Thorpe, Philip+

U.T. Southwestern Medical Center, Dallas, TX 75235 and +Maine Medial Center, South Portland, ME 04106

 

Introduction Tumor response to therapy is strongly influenced by physiological parameters. Quantitative measurements of tumor oxygen tension (pO2) could provide insight into tumor physiology and mechanisms of novel therapeutic approaches. We recently demonstrated the use of 19F NMR relaxometry of hexafluorobenzene (HFB) to monitor regional tumor oxygen tension in rats (1, 2) . We have now extended the application to human tumors implanted in immuno compromised (SCID) mice.

Methods Human lymphomas were implanted SQ in the flanks of CB17 SCID mice and allowed to grow to ~1.5 cm diameter (3-6 mm thick). Mice were anesthetized using ketamine/xylazine. HFB (~40 µl) was injected directly in both central and peripheral regions of the disc-like tumors . Each mouse was placed within a 2 cm single turn solenoid coil in a 4.7 T magnet with actively shielded gradients. Proton and 19F MR images were obtained to confirm the distribution of HFB. Tumor oxygenation was estimated on a voxel by voxel basis using 19F echo planar imaging (EPI) relaxometry of the HFB. Following a pulse burst saturation recovery preparation sequence with a variable recovery time, a single spin echo EPI sequence with blipped phase encoding was acquired, and pO2 estimated for each voxel. By using ARDVARC (Alternated Relaxation Delays with Variable Acquisitions to Reduce Clearance effects) (2) , we typically achieved a precision of 2-5 torr at each of 20 - 50 voxels within a tumor in 8 mins. The inspired gas could be manipulated via a nose cone from air to carbogen (95%O2/5%CO2; 1 dm3/min). In other cases the air breathing mice were injected i.v. with anti-VCAM1-tTF (truncated tissue factor) coaguligand (200 µl=40 µg protein) to produce tumor-specific infarct (3) .

Results Typical baseline oxygenation ranged from hypoxia to 50 torr with mean 9.7+2.5 torr and median 7.2 torr. Altering the inspired gas to carbogen produced significant changes within 8 mins showing elevation in mean and median pO2, and decreased hypoxic fraction. Following administration of the coaguligand there was a rapid reduction in pO2 in well oxygenated tumor regions (initial pO2 > 10 torr), but little change in initially poorly oxygenated regions (initial pO2 < 10 torr).

Discussion The precision of the measurements, together with the ability to simultaneously examine dynamic changes in multiple specified regions will provide a useful technique for investigating tumor hypoxia with respect to interventions. Successful observation of tumors implanted in mice now provides the opportunity to examine a variety of human tumors.

References:

1. D. Le, R. P. Mason, S. Hunjan, A. Constantinescu, B. R. Barker and P. P. Antich, Regional tumor oxygen dynamics: 19F PBSR EPI of hexafluorobenzene. Magn. Reson. Imaging, 15, 8, 971-81 (1997).
2. S. Hunjan, R. P. Mason, A. Constantinescu, P. Peschke, E. W. Hahn and P. P. Antich, Regional tumor oximetry: 19F NMR spectroscopy of hexafluorobenzene. Int. J. Radiat. Oncol. Biol. Phys. 40, 5, 161-71 (1998).
3. X. Huang, G. Molema, S. King, L. Watkins, T. S. Edington and P. E. Thorpe, Tumor infarction in mice by anti-body directed targeting of tissue factor to tumor vasculature. Science 275, 547-550 (1997).

Supported in part by The American Cancer Society and The Rogers NIH BRTP MR Center.