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Nitroimidazole-Containing H2dedpa and H2CHXdedpa Derivatives as Potential PET Imaging Agents of Hypoxia with 68Ga

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posted on 2015-05-18, 00:00 authored by Caterina F. Ramogida, Jinhe Pan, Cara L. Ferreira, Brian O. Patrick, Karla Rebullar, Donald T. T. Yapp, Kuo-Shyan Lin, Michael J. Adam, Chris Orvig
68Ga is an attractive radiometal for use in positron emission tomography (PET) imaging. The success of 68Ga-based agents is dependent on a chelator that exhibits rapid radiometal incorporation, and strong kinetic inertness to prevent transchelation of 68Ga in vivo. The linear chelating agents H2dedpa (1,2-[[6-carboxypyridin-2-yl]­methylamino]­ethane) and H2CHXdedpa (CHX = cyclohexyl/cyclohexane) (N4O2) have recently been developed that bind Ga3+ quickly and under mild conditions, ideal properties to be incorporated into a 68Ga PET imaging agent. Herein, nitroimidazole (NI) derivatives of H2dedpa and H2CHXdedpa to investigate specific targeting of hypoxic tumor cells are investigated, given that NI can be reduced and retained exclusively in hypoxic cells. Nine N,N′-bis-alkylated derivatives of H2dedpa and H2CHXdedpa have been synthesized; they have been screened for their ability to bind gallium, and cyclic voltammetry of nonradioactive complexes was performed to probe the redox cycling mechanism of NI. The compounds were radiolabeled with 67Ga and 68Ga and show promising radiolabeling efficiencies (>99%) when labeled at 10–5 M for 10 min at room temperature. Moreover, stability studies (via apo-transferrin challenge, 37 °C) show that the 67Ga complexes exhibit exceptional stability (86–99% intact) after 2 h. In vitro uptake studies under hypoxic (0.5% O2) and normoxic (21% O2) conditions in three cancerous cell lines [HT-29 (colon), LCC6HER‑2 (breast), and CHO (Chinese hamster ovarian)] were performed. Of the four H2dedpa or H2CHXdedpa NI derivatives tested, all showed preferential uptake in hypoxic cells compared to normoxic cells with hypoxic/normoxic ratios as high as 7.9 ± 2.7 after 120 min. The results suggest that these novel bis-alkylated NI-containing H2dedpa and H2CHXdedpa ligands would be ideal candidates for further testing in vivo for PET imaging of hypoxia with 68Ga.

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