Targeting Hypoxia in Tumors Using 2-Nitroimidazoles with Peptidic Chelators for Technetium-99m:  Effect of Lipophilicity

• Published in: Bioconjugate chemistry 2000-05, Vol.11 (3), p.401-407
• Main Authors: Zhang, Xiuguo, Su, Zi-Fen, Ballinger, James R, Rauth, A. M, Pollak, Alfred, Thornback, John R
• Format: Article
• Language: English
• Subjects: Animals; Cell Hypoxia; Chelating Agents; Chemical Phenomena; Chemistry, Physical; CHO Cells; Chromatography, High Pressure Liquid; Cricetinae; HeLa Cells; Humans; Hydrogen-Ion Concentration; Hypoxia - complications; Hypoxia - diagnosis; Hypoxia - metabolism; Lipids - chemistry; Lysine - analogs & derivatives; Lysine - chemical synthesis; Lysine - metabolism; Magnetic Resonance Spectroscopy; Neoplasms - complications; Nitroimidazoles - chemical synthesis; Nitroimidazoles - chemistry; Nitroimidazoles - metabolism; Oxidation-Reduction; Structure-Activity Relationship; Technetium; Xanthine Oxidase - metabolism
• ISSN: 1043-1802; 1520-4812
• DOI: 10.1021/bc9901595

Tumor hypoxia is an important prognostic factor for response to therapy. Radiolabeled 2-nitroimidazoles have been used for imaging hypoxia, and the octanol/water partition coefficient (P) of these compounds appears to play a crucial role in their suitability for imaging. A series of 11 2-nitroimidazoles coupled to peptidic chelators for 99mTc with divergent P was developed and evaluated in an in vitro system. Two classes of N3S chelators were used:  dialkyl-Gly-Ser-Cys-linker-2-nitroimidazole (Class I) and dialkyl-Gly-Lys(2-nitroimidazole)-Cys (Class II). The chelators were prepared by automated 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 transchelation from [99mTc]gluconate at temperatures between 22 and 100 °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 determined. Radiochemical yields ranged from 5 to 80% for the 11 compounds. HPLC showed that some of the compounds formed two complexes with 99mTc, possibly syn and anti conformations with respect to the TcO bond. In general, the Class I chelators labeled more readily than the class II chelators. The P values of the 99mTc complexes varied from 0.0002 to 5 and were generally in accordance with predictions based on structure. There were also differences 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. Accumulation levels in aerobic cells were related to P but varied over a narrow range. Four of the 11 compounds showed selective accumulation in hypoxic cells. The peptidic class of 2-nitroimidazoles, with flexible design and convenient solid-phase synthesis, deserves further study as agents for imaging hypoxia in tumors.