Evaluation and Immunohistochemical Qualification of Carbogen-Induced ΔR2☆ as a Noninvasive Imaging Biomarker of Improved Tumor Oxygenation

Purpose: To evaluate and histologically qualify carbogen-induced Delta R-2* as a noninvasive magnetic resonance imaging biomarker of improved tumor oxygenation using a double 2-nitroimidazole hypoxia marker approach. Methods and Materials: Multigradient echo images were acquired from mice bearing GH3 prolactinomas, preadministered with the hypoxia marker CCI-103F, to quantify tumor R-2* during air breathing. With the mouse remaining positioned within the magnet bore, the gas supply was switched to carbogen (95% O-2, 5% CO2), during which a second hypoxia marker, pimonidazole, was administered via an intraperitoneal line, and an additional set of identical multigradient echo images acquired to quantify any changes in tumor R-2*. Hypoxic fraction was quantified histologically using immunofluorescence detection of CCI-103F and pimonidazole adduct formation from the same whole tumor section. Carbogen-induced changes in tumor pO(2) were further validated using the Oxylite fiberoptic probe. Results: Carbogen challenge significantly reduced mean tumor R-2* from 116 +/- 13 s(-1) to 97 +/- 9 s(-1) (P<.05). This was associated with a significantly lower pimonidazole adduct area (2.3 +/- 1%), compared with CCI-103F (6.3 +/- 2%) (P<.05). A significant correlation was observed between Delta R-2* and Dhypoxic fraction (r=0.55, P<.01). Mean tumor pO(2) during carbogen breathing significantly increased from 6.3 +/- 2.2 mm Hg to 36.0 +/- 7.5 mm Hg (P<.01). Conclusions: The combined use of intrinsic susceptibility magnetic resonance imaging with a double hypoxia marker approach corroborates carbogen-induced Delta R-2* as a noninvasive imaging biomarker of increased tumor oxygenation. (C) 2013 Elsevier Inc.