Imaging hypoxia using a nitroimidazole based T1 MR contrast agent

Introduction: Hypoxic regions in tumors are known to affect radiation sensitivity and promote development of metastases [1]. Noninvasive imaging based methods such as MRI are particularly suitable for longitudinal measurements and generation of three-dimensional spatial maps of tumor hypoxia [2]. An MRI method that could differentiate hypoxic versus normoxic tissues without relying on washout modeling would be clinically useful. Previous research demonstrated that 2nitroimidazole accumulated in hypoxic tissues due to an enzyme mediated reduction of the nitro group under hypoxic conditions [3]. In this work, we report both in vitro and in vivo evidence for accumulation of a T1 shortening agent, a GdDOTA monoamide conjugate of 2-nitroimidazole abbreviated as GdDO3NI (Fig 1b), in hypoxic tumor tissue. Materials and Methods: MR experiments were performed on a Varian 4.7T MR scanner. In vitro measurement of T1 relaxivity (r1) of GdDO3ABA (control agent, Fig 1a) and GdDO3NI (hypoxia targeting agent, Fig 1b) was performed at 37°C in a tissue simulating 1% agarose phantom. For the r1 measurements, a spin-echo sequence was used with several TR values (0.1-6s). Relaxivity was extracted as slope of linear fit to relaxation rates (R1) vs. concentration. In vivo imaging studies were performed on 10 Copenhagen rats bearing subcutaneous syngeneic R3327 AT1 prostate tumors (volume ~3cc), since they are known to have hypoxic regions in the core [4]. Following baseline T1 mapping (TR: 0.1-6 s and TE: 12 ms), T1-weighted images (TR/TE = 200/10 ms, FOV = 5 cm X 5 cm, matrix = 128 X 128, slice thk = 1 mm) were obtained pre and post injection of 0.1 mmole/kg body wt contrast agent (GdDO3ABA or GdDO3NI, n = 5 each) for 150 min. Data analysis was performed by segmenting the voxels in the tumor region into periphery and center, based on a criterion of 50% enhancement at 90 s post injection. Gd concentration in the tumor slices was calculated using the relation [CA] = (R1,post – R1,pre) / r1. For validation, ex vivo inductively coupled plasma mass spectroscopy (ICP-MS) analysis was performed to quantify the Gd concentration in the periphery and core regions of tumors (n=3).