N-Acetyl-DL-cysteine → cobalt-dichloride-N-acetyl cysteine

Cobalt chloride hexahydrate (CoCl2-6H2O (mw=237.9 g/mol, available from Acros)) and NAC (N-Acetyl-L-Cysteine, HSCH2CH(NHCOCH3)CO2H (mw=163.19 g/mol, available from Sigma)) was dissolved in water, resulting in the formation of cobalt-dichloride-N-acetyl cysteine (CoCl2—NAC), referred to herein as the Co—NAC contrast agent. All chemicals were purchased from commercial sources and were used without any further purification. For toxicity testing, two different Co—NAC solutions with low (1 wt. %) concentration of cobalt chloride and with high (10%) concentration were prepared. In both cases, the concentration of NAC in the solution was 2 wt. %. The Co—NAC solution with low cobalt chloride concentration was prepared by dissolving 100 mg (0.42 mmol) of CoCl2-6H2O and 200 mg (1.22 mmol) of NAC in 9.7 mL distillated water. The solution with high cobalt chloride concentration was prepared by dissolving 1000 mg (4.2 mmol) of CoCl2-6H2O and 200 mg (1.22 mmol) of NAC in 8.8 mL distilled water. In both cases, the solutions were sonicated at the room temperature for 15 minutes to completely dissolve the components. To avoid any contaminations and bacteria grow, the fresh Co—NAC solutions were prepared every morning for 6 days and immediately delivered for toxicity testing (see, Example 2 below).