Preclinical Evaluation of the Fluorinated 2-Nitroimidazole N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitro-1-imidazolyl) Acetamide (SR-4554) as a Probe for the Measurement of Tumor Hypoxia¹

A novel probe, N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitro-1-imidazolyl) acetamide (SR-4554), has been used to detect tumor hypoxia noninvasively by ¹⁹F magnetic resonance spectroscopy (¹⁹F MRS). The compound was designed to undergo a hypoxia-dependent, one-electron reduction to metabolites that are selectively retained in tumors and has attractive pharmacokinetic, toxicological, and detection sensitivity properties. As a prelude to clinical studies, we report here for the first time on the ability to detect a MR signal following SR-4554 administration in various transplantable tumors and describe validation studies, consisting of a correlation between signal retention and radiobiological hypoxic fraction, and the effects of modulating the degree of hypoxia by hydralazine and carbogen breathing. SR-4554 was absorbed and then eliminated from EMT6 tumors with a half-life of 51 min following an injection of 180 mg/kg i.p. of SR-4554. Using a quantitative ¹⁹F MRS technique, the ¹⁹F retention index (¹⁹FRI; ¹⁹F signal level at 6 h/45 min) was determined for four commonly used murine tumors (EMT6, SCCVII, KHT, and RIF-1). The retention of high tumor concentrations of fluorinated probe at 6 h, despite the much lower (20-fold) concentration of parent SR-4554 detected by high-performance liquid chromatography, was consistent with the involvement of one or more nitroreduced metabolites and suggested that ¹⁹F MRS might give a quantitative measure of tumor hypoxia. In these murine tumors, ¹⁹FRI correlated with the reported radiobiological hypoxic fraction of the tumors (r = 0.988; P = 0.01). In addition, changes in tumor microenvironment were detected by ¹⁹F MRS. An increase in hypoxia induced by hydralazine treatment of RIF-1 tumor-bearing mice was associated with a 2.4-fold increase in ¹⁹FRI compared to untreated controls. In contrast, carbogen breathing by C3H mammary tumor-bearing mice produced a 6-fold decrease in the ¹⁹FRI compared to air-breathing mice. The data presented support the preclinical and clinical development of SR-4554 as a noninvasive probe for tumor hypoxia.