Abstract
1018
Objectives: The expression of carbonic anhydrase-IX (CA-IX) is markedly upregulated by hypoxia through the hypoxia-inducible factor-1 cascade, and is found in many types of tumors; therefore, CA-IX is considered to be an attractive target for cancer diagnosis. We previously reported an 111In-labeled compound based on bivalent ureidosulfonamide (US) as the CA-IX-targeting moiety ([111In]US2), which showed in vivo specific detection of CA-IX high-expressing tumor in mice with SPECT (Iikuni S et al., Theranostics. 8, 2992-3006, 2018). Based on this encouraging result, in the present study, we designed [68Ga]US2 as the PET imaging agent targeting CA-IX in order to conduct higher sensitivity and resolution imaging. We conducted fundamental evaluations with [67Ga]US2 for in vivo CA-IX imaging due to the longer half-life of 67Ga (78 h) than that of 68Ga (68 min).
Methods: [67Ga]US2 was synthesized with 67GaCl3 in 2-(N-morpholino)ethanesulfonic acid buffer (0.1 M, pH 5.5) using the same precursor as that for [111In]US2. An in vitro stability of [67Ga]US2 was evaluated by incubating it in murine plasma. An in vitro targeting ability of [67Ga]US2 was assessed by a cell binding study using CA-IX high-expressing (HT-29) and low-expressing (MDA-MB-231) cells under normoxic and hypoxic conditions. An in vivo biodistribution study of [67Ga]US2 was performed in HT-29 and MDA-MB-231 tumor-bearing mice. To elucidate a potential for in vivo specific imaging of CA-IX, SPECT with [67Ga]US2 was conducted using HT-29 and MDA-MB-231 tumor-bearing mice.
Results: [67Ga]US2 was obtained in 40% radiochemical yield and over 99% radiochemical purity. In the stability assay, almost all [67Ga]US2 existed as an intact form (> 99%) in murine plasma at 37 °C until 24 h, indicating the marked stability of [67Ga]US2. In the in vitro cell binding study, [67Ga]US2 highly and selectively bound to the HT-29 cells as compared with the MDA-MB-231 cells under normoxic and hypoxic conditions. After intravenous administration of [67Ga]US2 in HT-29 and MDA-MB-231 tumor-bearing mice, selective and specific accumulation within the HT-29 tumor (3.81 ± 0.75% injected dose/g tumor at 1 h postinjection) was observed. Moreover, the tumor/blood ratio exceeded 1.7 from 1 h postinjection, indicating favorable pharmacokinetics for the in vivo imaging. [67Ga]US2 clearly and selectively visualized the HT-29 tumors in the mice with SPECT, while almost no radioactivity signal was observed from the MDA-MB-231 tumors.
Conclusions: Our study revealed that [67Ga]US2 may serve as a useful tumor imaging probe targeting CA-IX, suggesting the potential of [68Ga]US2 for PET imaging of the CA-IX-expressing tumor.