Imaging of the third gasotransmitter hydrogen sulfide using ^99mTc-labeled alpha-hydroxy acids

Hydrogen sulfide (H₂S) defined as the third gasotransmitter after nitric oxide (NO) and carbon monoxide (CO) is an important mediator of various physiological functions. Although various H₂S-imaging techniques using fluorescence and luminescence have been developed, only one radionuclide imaging using ⁶⁴Cu-labeled cyclen complex was reported. Thus, we tried to develop ^99mTc-labeled H₂S imaging agents.

Various α-hydroxy acids such as glycolate, L-lactate, D-lactate, D-gluconate, D-glucoheptonate, D-glucuronate, D-glucarate, and citrate were labeled with ^99mTc in the presence of stannous chloride. The labeled compounds were incubated with 0.2 mM of NaHS, and reactive sulfur species and then analyzed by ITLC/normal saline to detect the formation of insoluble complex. Matrigels containing various concentrations of NaHS were xenografted on the shoulder of normal mice, and an imaging study was performed after intravenous injection of [^99mTc]Tc-gluconate. We also obtained autoradiography image of a rat brain with a temporary brain ischemia after intravenous injection of [^99mTc]Tc-gluconate.

[^99mTc]Tc-gluconate showed the highest formation of insoluble complex (87.8 ± 3.6%) after incubation with 0.2 mM NaHS. The other reactive species such as glutathione, cysteine, sulfite, sulfate, thiosulfate, and NO did not form insoluble complex representing the reaction being specific to H₂S. The Matrigel containing 2 μmol NaHS showed uptake of [^99mTc]Tc-gluconate, which proved the feasibility as the specific H₂S imaging agent in vivo. Temporary ischemic lesion of rat brain showed high radioactivity accumulation representing the feasibility as endogenous H₂S imaging agents.

We proved that ^99mTc-labeled α-hydroxy acid especially [^99mTc]Tc-gluconate is a novel endogenous H₂S imaging agent, which might contribute to study and diagnosis of various diseases related with inflammation and hypoxia.