Imaging H₂S in hypoxia using [^99mTc]Tc-gluconate

Objectives: Hydrogen sulfide is the third gasotransmitter that is known to be produced endogenously in hypoxia, various cancers and inflammations. H₂S can be produced through both nonenzymatic and enzymatic pathways. It is reported that cystathione γ-lyase (CSE) localized in the cytosol in normoxia is translocated into mitochondria in hypoxia, and then metabolized to L-cysteine to produce H₂S (1). Thus, production of H₂S increases in hypoxic condition. Furthermore, it was reported that the endogenous H₂S could be imaged by [^99mTc]Tc-gluconate (2). In the president study, we tried to detect H2S generated by hypoxic cells in vitro, and tried to image H2S generated by hypoxic tissue in vivo using [^99mTc]Tc-gluconate.

Methods: Gluconate (0.3 M, 0.1 mL) was labeled with ^99mTc in the presence of stannous chloride as a reducing agent. Radiochemical purity of [^99mTc]Tc-gluconate was checked by radio TLC using two instant thin-layer chromatography strips (eluted with acetone and normal saline). In vitro cell uptake study of [^99mTc]Tc-gluconate was performed in hypoxic and normoxic conditions using colon carcinoma cell line CT26. Hydrogen sulfide level was measured by a modified methylene blue method (absorbance was measured at 670 nm). For in vivo imaging study, acute hindlimb ischemia-reperfusion model was established in BALB/c mice after 3 h of ischemia and 3 h of reperfusion. [^99mTc]Tc-gluconate (12.5 MBq) was intravenously injected into mice through tail vein and the lower limb uptake was imaged with SPECT/CT.

Results: The labeling yield of [^99mTc]Tc-gluconate was almost quantitative (99.8±0.1%). In vitro, [^99mTc]Tc-gluconate uptake to hypoxic cell was higher than normoxic cell in all the time. At 120 min of incubation, radioactivity in hypoxic and normoxic CT26 cells reached 109,772 ± 6,889 and 25,587 ± 1,884 CPM/mg, respectively, representing the H₂S level in hypoxia was 40% higher than normoxia. In vivo, SPECT/CT imaging of [^99mTc]gluconate showed five times higher uptake in ischemic limb than normal limb (Fig.1). The SUVmean of ischemic limb were 0.39 ± 0.03, and 0.07 ± 0.01 in normal limb.

Conclusions: We demonstrated that [^99mTc]Tc-gluconate is a novel imaging agent for endogenous hydrogen sulfide that is generated in hypoxic cells and tissues. Thus, it might be used for imaging various diseases related to hypoxia in clinical field. References (1) Wallace JL, Wang R. Nature Reviews Drug Discovery. (2015) 14:329. (2) Park JY, Kim YJ, Lee JY, Lee Y-S, Jeong JM. Nucl Med Biol (2019) 76-77:28.

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