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Comparison of methylene blue, riboflavin, and N‐acetylcysteine for the reduction of nitric oxide‐induced methemoglobinemia

Objective: To investigate the treatment of nitric oxide (NO)‐induced methemoglobinemia by methylene blue (MB), riboflavin, and N‐acetylcysteine (NAC) in vitro. Design: Prospective, controlled in vitro study. Setting: Research laboratory in a university hospital. Participants: Five healthy volunteers. Interventions: Generation of 16% to 18% of methemoglobin in red blood cells by NO and subsequent addition of MB, riboflavin, or NAC. Simultaneous NO (32 ppm) and MB or riboflavin exposure of red blood cells. Induction of 14% to 18% of methemoglobin in red blood cells by NO, subsequent addition of MB or riboflavin, and further incubation with NO (80 ppm). Measurements and Main Results: After discontinuation of NO, mean half‐life for methemoglobin was significantly reduced by MB from 356 mins (controls) to 5 mins (10 μM) in a dosedependent manner (p < .001). NAC did not alter the half‐life for methemoglobin, and a reduction from 356 to 168 mins was seen for 120 μM riboflavin (p < .001). Methemoglobin formation after 3 hrs of NO exposure was 4.3% ± 0.7% in controls and 0.3% ± 0.1% with 10 μM MB (p < .001); 1 μM MB attenuated methemoglobin formation to 1.9% ± 0.1% (p < .01). With riboflavin (120 μM), methemoglobin was 2.2% ± 0.5% vs. 3.2% ± 0.6% in controls (p < .001). In the presence of high methemoglobin concentrations, further methemoglobin formation was inhibited by 1 and 10 μM MB (p < .001) and attenuated by 0.1 μM MB (p < .001) but not by riboflavin. Conclusions: In vitro, NO‐induced methemoglobin formation is significantly decreased by medium (1 μM) and high (10 μM) concentrations of MB and partially by high riboflavin concentrations (120 μM). NAC and low concentrations of riboflavin do not alter methemoglobin formation.