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 ΔR2* 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 R2* during air breathing. With the mouse remaining positioned within the magnet bore, the gas supply was switched to carbogen (95% O2, 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 R2*. 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 pO2 were further validated using the Oxylite fiberoptic probe. Results: Carbogen challenge significantly reduced mean tumor R2* 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 ΔR2* and Δhypoxic fraction (r=0.55, P<.01). Mean tumor pO2 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 ΔR2* as a noninvasive imaging biomarker of increased tumor oxygenation