RT Journal Article SR Electronic T1 FMISO-PET imaging may be affected by inhibitors of multidrug-resistant protein 1 independent of the hypoxic state JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1251 OP 1251 VO 59 IS supplement 1 A1 Yoichi Shimizu A1 Yukihiro Nakai A1 Hiroyuki Watanabe A1 Masahiro Ono A1 Hideo Saji A1 Yuji Kuge A1 Yuji Nakamoto A1 Tsuneo Saga A1 Kaori Togashi YR 2018 UL http://jnm.snmjournals.org/content/59/supplement_1/1251.abstract AB 1251Objectives: [18F]fluoromisonidazole ([18F]FMISO) is a PET imaging probe widely used for detection of hypoxia. To date, we revealed that [18F]FMISO was accumulated in hypoxic cells mainly by the process of glutathione conjugation following reductive metabolism (Sci Rep., 2015, 5, 16802). We also found that the FMISO accumulation level in the hypoxic cells depended on the cell types, and this feature was presumed to be correlated with the ability of the glutathione conjugation and its efflux (Ann Nucl Med. 2017, 31, 596-604). Multi-drug resistant protein 1 (MRP1) is an ATP-binding cassette transporter, and is known to mediate the ATP-dependent export of glutathione conjugates out of cells. In our previous study, we found that the cells expressing MRP1 highly showed low [18F]FMISO accumulation, which indicated that MRP1 enhanced the efflux of the [18F]FMISO metabolite from the cells, which was independent of the hypoxic state of the cells. In this study, to reveal this phenomenon in detail, we evaluated whether MRP1 inhibitors affect the [18F]FMISO accumulation in hypoxic cells. METHODS: In the MRP1 inhibition assay, MK-571 (0-100 μM), lapatinib (0-25 μM) or cyclosporin A (0-500 μM) was added to FaDu human pharyngeal squamous-cell carcinoma cells. After one hour incubation, the cells were treated with calcein AM, and the fluorescence intensity of the cells was measured after one hour additional incubation. In the in vitro study, FaDu cells were pretreated with MK-571 (100 μM), lapatinib (25 μM) or cyclosporin A (50 μM) in hypoxic condition (5% CO2, 1% O2) for 1 h. Then, the cells were treated with [18F]FMISO (5 MBq), and incubated in the same hypoxic condition for 4 h. After the incubation, the cells were dissolved by 1M NaOH, and the radioactivity of the cell lysates was measured. In the in vivo study, MK-571 (80 mg/kg), lapatinib (100 mg/kg) or cyclosporin A (25 mg/kg) was intraperitoneally injected to Balb/c nu/nu mice xenografted FaDu cells. At 1 h after injection of the compounds, [18F]FMISO (10 MBq) was administrated via tail vein. The tumors were excised at 4 h after injection of [18F]FMISO, and then the radioactivity of the tumor tissues was measured. RESULTS: The fluorescence intensity of the cells treated with MK-571, lapatinib or cyclosporin A increased with the concentration of the compounds, and was significantly higher than that of the non-treated cells. This result suggests that those compounds had the abilities as MRP1 inhibitors. In the in vitro study, the cells pretreated with MK-571, lapatinib or cyclosporin A showed significantly higher radioactivities compared with the cells without the pretreatment of those compounds (8.37 ± 0.75 (MK-571), 7.90 ± 0.56 (lapatinib), 6.91 ± 0.27 (cyclosporin A) vs. 3.62 ± 0.22 (non-treated group) %dose/mg protein) in the same hypoxic condition (oxygen concentration: 1%). This result suggests that MRP1 inhibitors inhibited the efflux of [18F]FMISO metabolites, which led to enhance the cellular [18F]FMISO accumulation levels independent of the hypoxic state. In the in vivo study, the radioactivity of the tumor tissues of the mice pretreated with MK-571, lapatinib or cyclosporin A was significantly higher than that of the mice without pretreatment (3.78 ± 1.19 (MK-571), 2.56 ± 1.33 (lapatinib), 4.73 ± 0.80 (cyclosporin A) vs. 1.32 ± 0.34 (non-treated group) %ID/g), though the radioactivity of the muscle of the mice pretreated with the compounds were also higher than those of the mice without pretreatment (0.73 ± 0.43 (MK-571), 0.43 ± 0.28 (lapatinib), 0.62 ± 0.21 (Cyclosporin A) vs. 0.12 ± 0.02 (non-treated group) %ID/g). These results indicate that MRP1 inhibitors prolonged the [18F]FMISO distribution in the mice. Conclusions: In this study, we revealed that MRP1 inhibitors enhanced the [18F]FMISO accumulation in the hypoxic cells. These results suggest that [18F]FMISO-PET imaging is affected by MRP1 inhibitors independent of the hypoxic state.