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.
This work was sponsored by the Cancer Research Campaign (United Kingdom), Karen Elise Jensens Fund (Denmark), the Danish Cancer Society, an Overseas Research Scholarship (United Kingdom; to E. O. A.), and a Life Fellowship from the Cancer Research Campaign (to P. W.).