Southwestern In Vivo Cellular and Molecular Imaging Program

Prognostic Radiology Laboratory
(Lead Scientist: Ralph P. Mason, Ph.D.)

Tumor response to therapy is controlled by physiological parameters as well as genetic characteristics. Hypoxia may stimulate angiogenesis and metastasis and create genetic instability. Moreover, the closely related parameters pH (pHi and pHe), pO₂ and blood flow may influence therapeutic efficacy and these parameters are intimately related. Tumor physiology is critical from two perspectives: i) how the micro environmental milieu influences a specific therapeutic approach; ii) can measurements of physiological parameters provide a prognostic indication for therapy? Since the classic work of Gray over 40 years ago it has been appreciated that hypoxia produces cellular resistance to irradiation. Indeed, there is a critical threshold pO₂ ~15 torr (mmHg), below which cells exhibit increased resistance with an oxygen enhancement ratio of 2.3 between 0 and 15 torr. Since it was suspected that human tumors were hypoxic, many clinical trials attempted to manipulate tumor oxygenation (hyperbaric chambers, perfluorocarbons) for therapeutic advantage, but with disappointing results. It is now appreciated that lack of success may have resulted from the inability to differentiate a priori those tumors, which were hypoxic and likely to benefit. More recently, the Eppendorf Histograph oxygen electrode system has been used to demonstrate significantly better prognosis for patients with well oxygenated tumors, however, the method is highly invasive. Current clinical trials examine tumor hypoxia based on the histological markers EF5 and pimonidazole, but require biopsy to assess results. Such tests could indicate the need for adjuvant therapy and allow patient management to be individualized and optimized to the specific characteristics of a tumor. Indeed, given the success of the ARCON clinical trial and availability of hypoxia selective cytotoxic agents (e.g., Tirapazamine), there is a clear need to develop new less invasive approaches to evaluate tumor oxygenation.

Under the auspices of The American Cancer Society, we have been developing a technique using ¹⁹F MRI relaxometry of hexafluorobenzene (HFB) (Figure) to probe tumor oxygenation- FREDOM: Fluorocarbon Relaxometry for Dynamic Oxygen Mapping (Figure). This facilitates rapid quantitative mapping of tumor pO₂, and in particular, assessment of dynamic changes accompanying interventions. Our recent studies have shown that baseline pO₂ distributions observed in tumors are closely similar to measurements obtained with the Eppendorf Histograph (Figure). FREDOM has the advantage of facilitating dynamic measurements and we have also shown that measurements are similar to those obtained using the OxyLite^TM fiberoptic device or traditional static electrodes. Our latest studies are showing significant differences in tumor oxygen dynamics depending on tumor growth rate, aggressiveness and degree of differentiation. Given the complex nature of tumor physiology we are now funded to integrate measurements of tumor pO₂, transmembrane pH and blood flow (NIH R01: CA-79515). Indeed, there is extensive evidence that cellular pH plays a role in mitogenic stimulation and response to cytotoxic therapy. In particular, the pH gradient between the interstitial and intracellular compartments is involved in many regulatory processes, and strongly influences drug uptake. Thus, the measurement of pH (pHi and pHe) in tumors promises to provide insight into developmental processes and prognostic information regarding therapeutic outcome. In addition, low pH may indicate poor vascularity, and thus, indirectly limit the effective level of drugs in tissues.

Prior to translating the FREDOM approach, we propose to implement fMRI in clinical studies of tumors, since they are entirely non-invasive. Since the early observations by Ogawa et al. that deoxyhemoglobin is paramagnetic and induces transverse relaxation in blood, this approach has become popular for investigations of neuronal activity in humans based on the BOLD (Blood Oxygen Level Dependent) effect. At our Institution, McColl et al. have applied the technique to examine activation resulting from both motor and intellectual tasks. Dr. Yetkin has extensive prior experience and we may thus readily implement the methods for investigations of tumors. Although blood flow may complicate interpretation of images, and pO₂ is not quantitated, it is thought that the approach may be useful for examining tumor dynamics in an entirely non-invasive fashion.

BOLD/FLOOD (FLOw and Oxygen Dependent) methods are at a preliminary stage in tumors, but they will be combined with pharmacokinetic curves of contrast agent uptake and clearance in tumors which is thought to be indicative of vascular integrity and angiogenesis. This approach is being evaluated in many Centers to stage disease, including NIH sponsored clinical trials for breast cancer evaluation led by Dr. Weatherall at this Institution. We anticipate that dynamic contrast characteristics will correlate with genetic signatures (Research Area 2) determined by Minna et al. and may provide a non-invasive imaging technique to characterize tumors.

While our tumor physiology work has focused on MR methods it is clear that many complementary nuclear medicine techniques are available (e.g., hypoxia indicators, blood flow and blood volume and comparative studies will be undertaken.  through the Program of Molecular and Cellular Imaging (Pathology at UT-Southwestern). We can integrate tissue histology and tumor window chamber models. We believe that such investigation will provide an intermediate stage to bridge the transition between traditional histology and tomographic imaging. This will be highly relevant in assessing the mechanism of action of genes, angiogenesis and anti angiogenesis agents at the capillary level within nascent tumors in window chambers.

References:

1. Mason, RP, Constantinescu A, Hunjan S, Le D, Hahn EW, Antich PP, Blum C, Peschke P. Regional tumor oxygenation and measurement of dynamic changes. Radiat. Res. 1999, 152, 239-249.

2. Mason, RP. Transmembrane pH gradients in vivo: measurements using fluorinated vitamin B6 derivatives. Curr. Med. Chem. 1999, 6, 481-499.

3. Mason, RP, Rodbumrung W, Antich PP. Hexafluorobenzene: a sensitive 19F NMR indicator of tumor oxygenation. NMR in Biomed. 1996, 9, 125-134.

4. Mason, RP, Hunjan S, Le D, Constantinescu C, Barker BR, Wong PS, Peschke P, Hahn EW, Antich PP. Regional Tumor Oxygen Tension: Fluorine Echo Planar Imaging of Hexafluorobenzene Reveals Heterogeneity of Dynamics. Int. J. Radiat. Oncol. Biol. Phys. 1998, 42, 747-50.

5. Hunjan, S, Zhao D, Constantinescu A, Hahn EW, Antich PP, Mason RP. Tumor Oximetry: an enhanced dynamic mapping procedure using fluorine-19 echo planar magnetic resonance imaging. Int. J. Radiat. Oncol. Biol. Phys. 2001, 49, 1097-1108.

For Further Information Contact: RALPH  P. MASON, Ph.D.
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