Targeting hypoxia in tumors using 2-nitroimidazoles with peptidic chelators for technetium-99m: Effect of lipophilicity

Tumor hypoxia is an important prognostic factor for response to therapy. Ra diolabeled 2-nitroimidazoles have been used for imaging hypoxia, and the oc tanol/water partition coefficient (P) of these compounds appears to play a crucial role in their suitability for imaging. A series of 11 2-nitroimidaz oles coupled to peptidic chelators for Tc-99m with divergent P was develope d and evaluated in an in vitro system. Two classes of N3S chelators were us ed: dialkyl-Gly-Ser-Cys-linker-2-nitroimidazole (Class I) and dialkyl-Gly-l ys(2-nitroimidazole)-Cys (Class II). The chelators were prepared by automat ed solid phase peptide synthesis. Xanthine oxidase was able to reduce the 2 -nitroimidiazole moiety on the ligands, but the rate of reduction varied 5- fold among the different chelators. The chelators were labeled by transchel ation from [Tc-99m]gluconate at temperatures between 22 and 100 degrees C. The reaction mixtures were analyzed by HPLC and their P values determined. The accumulation of each complex in suspension cultures of Chinese hamster ovary cells incubated under aerobic or extremely hypoxic conditions was det ermined. Radiochemical yields ranged from 5 to 80% for the II compounds. HP LC showed that some of the compounds formed two complexes with 99mTc, possi bly syn and anti conformations with respect to the Tc=O bond. In general, t he Class I chelators labeled more readily than the class II chelators. The P values of the Tc-99m complexes varied from 0.0002 to 5 and were generally in accordance with predictions based on structure. There were also differe nces in P as a function of pH; the free acids had a lower P at pH 7.4 than at pH 2.0 due to ionization, whereas the amides did not show this effect. A ccumulation levels in aerobic cells were related to P but varied over a nar row range. Four of the II compounds showed selective accumulation in hypoxi c cells. The peptidic class of 2-nitroimidazoles, with flexible design and convenient solid-phase synthesis, deserves further study as agents for imag ing hypoxia in tumors.