Preclinical development and current status of the fluorinated 2-nitroimidazole hypoxia probe N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitro-1-imidazolyl) acetamide (SR 4554, CRC 94/17): a non-invasive diagnostic probe for the measurement of tumor hypoxia by magnetic resonance spectroscopy and imaging, and by positron emission tomography

Hypoxia occurs to a variable extent in a vast majority of rodent and human solid tumors. It results from an inadequate and disorganized tumor vasculat ure, and hence an impaired oxygen delivery. A probe for the non-invasive de tection of tumor hypoxia could find important utility in the selection of p atients for therapy with bioreductive agents, anti-angiogenic/anti-vascular therapies and hypoxia-targeted gene therapy In addition, tumor hypoxia has been shown to predict for treatment outcome following radio- or chemothera py in human cancers, the underlying mechanism for which may involve hypoxia driving genetic instability and resulting tumor progression. Beyond oncolo gy, utility can also be envisaged in stroke, ischemic heart disease, periph eral vascular disease, arthritis and other disorders. Design, validation, p reclinical development and current status of a fluorinated 2-nitroimidazole , N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitro-1-imidazolyl) acetamide (S R 4554, CRC 94/17), which has been rationally designed for the measurement of tumor hypoxia by magnetic resonance spectroscopy (MRS) and imaging (MRT) , are reviewed. Application in positron emission tomography (PET) detection is also proposed. Design goals were: (i) a nitro group with appropriate re dox potential for selective reduction and binding in hypoxic tumor cells; ( ii) hydrophilic/hydrogen bonding character in the side chain to limit nervo us tissue penetration and prevent neurotoxicity; and (iii)three equivalent fluorine atoms to enhance MRS/MRI detection, located in a metabolically sta ble position. Reduction of SR 4554 by mouse liver microsomes was dependent on oxygen content, with a half-maximal inhibition at 0.48 +/- 0.06%. SR 455 4 underwent nitroreduction by hypoxic but not oxic tumor cells in vitro and electron energy loss spectroscopic analysis showed selective retention in the hypoxic regions of multicellular tumor spheroids. Pharmacokinetic desig n goals were met. In particular, low brain tissue concentrations were seen in contrast to excellent tumor levels, as measured by high performance liqu id chromatography. The extent of this restricted entry to brain tumor was s urprising given the overall octanol/water partition coefficient and was att ributed to the hydrophilic/ hydrogen bonding character of the side chain. Q uantitative MRS was used to assess the retention of F-19 signal in murine t umors and human tumor xenografts The 19F retention index (FRI; ratio of F-1 9 signal levels at 6 h relative to that at 45 min) ranged from 0.5 to 1.0 a nd 0.2 to 0.9 for murine tumors and human xenografts respectively. The corr elation between SR 4554 retention and pO(2) was not a linear one, but when FRI was >0.5, the % pO(2) less than or equal to 5 mmHg was always >60%, ind icating that high FRI was associated with low levels of oxygenation. Finall y, whole body F-19-MRI in mice demonstrated that SR 4554 and related metabo lites localized mainly in tumor, liver and bladder regions A selective MRS signal was readily detectable in tumors at doses at least 7-fold lower than those likely to cause toxicity in mice. We conclude that proof of principl e is established for the use of SR 4554 as a non-invasive MRS/MRI probe for the detection of tumor hypoxia. Based on these promising studies, SR 4554 has been selected for clinical development.