Dimethylarsinothioyl Glutathione as a Metabolite in Human Multiple Myeloma Cell Lines upon Exposure to Darinaparsin

Here, we report the identification of dimethylarsinothioyl glutathione (DMMTA^V(GS)) as a metabolite in cellular extracts of dimethyarsinous glutathione (Darinaparsin, DMA^III(GS)) treated human multiple myeloma (MM) cell lines. Co-elution of sulfur and arsenic on the inductively coupled plasma mass spectrometer (ICP-MS) indicated the presence of sulfur along with arsenic in the newly observed unidentified molecule on the speciation chromatograms of cell lines treated with DMA^III(GS). Liquid chromatography–electrospray ionization–mass spectrometry of the unknown peak in the MS and tandem MS modes revealed molecular ion peaks at <i>m</i>/<i>z</i> = 443.9 and 466.0, corresponding to [DMMTA^V(GS) + H]⁺ and [DMMTA^V(GS) + Na]⁺, as well as peaks at 314.8 for the loss of glutamic acid and 231.1 for the loss of glycine. In addition, peaks were observed at 176.9 corresponding to cysteine and glycine adducts and at 137.1 for the [C₂H₆AsS]⁺ ion. An increase in the peak area of the unidentified peak was observed upon spiking the cell extracts with a standard of DMMTA^V(GS). Heat deactivation of MM cells prevented the formation of DMMTA^V(GS) raising the possibility of its formation via an enzymatic reaction. Formation studies in DMA^III(GS) treated MM cells revealed the dependence of DMMTA^V(GS) formation on the depletion of DMA^III(GS). The presence of 5 mM glutathione prevented its formation, indicating that DMA^III, a dissociation product of DMA^III(GS), is likely a precursor for the formation of DMMTA^V(GS). DMMTA^V(GS) was observed to form under acidic and neutral pH conditions (pH 3.0–7.4). In addition, DMMTA^V(GS) was found to be stable in cell extracts at both acidic and neutral pH conditions. When assessing the toxicity by exposing multiple myeloma cells to arsenicals externally, DMMTA^V(GS) was found to be much less toxic than DMA^III(GS) and DMMTA^V, potentially due to its limited uptake in the cells (10 and 16% of the uptakes of DMA^III(GS) and DMMTA^V, respectively)