Photodegradation of Sodium Nitroprusside

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Abstract
Because the belief that cyanide is released from nitroprusside in vivo recently was challenged, a series of experiments that examined the conditions under which nitroprusside is degraded were performed. These experiments include an examination of the release of cyanide and nitric oxide from nitroprusside in vitro, the release of cyanide in vivo and a comparison of the biologic activity of intact and degraded nitroprusside. Nitroprusside in aqueous solution degraded when exposed to white or blue light but not to red light. Light at 20 .mu.W .cntdot. cm-2 produced 40% apparent photodegradation after 6 h exposure while white light at 220 .mu.W .cntdot. cm-2 produced 100% apparent photodegradation after 2 h exposure. At 100% apparent photodegradation 10% of the nitrosyl ligand was recovered as free nitric oxide and 0.4% of the cyanide ligand was recovered as free cyanide. Following a 2 h infusion of light-protected nitroprusside in 7 patients, cyanide concentrations ranged from 1.4-45.5 .mu.M in blood and 0.09-3.2 .mu.M in plasma. These values were not changed by exposing the samples to white light for 4 h. Intact and photodegraded nitroprusside produced identicl hypotensive responses in rats as would be expected, since the nitrosyl ligand was detected in solution following degradation and it mediates this action. Cyanide was released from nitroprusside both on its exposure to light in vitro and also in vivo. The latter was not an artifact of the assay for cyanide. Nitroprusside releases cyanide in vivo. Cyanide toxicity is a true complication of its use.