Safety of hydroxocobalamin in healthy volunteers in a randomized, placebo-controlled study.

Introduction: This randomized, double-blind, placebo-controlled, ascending-dose study was conducted in healthy volunteers to evaluate the safety of the investigational cyanide antidote hydroxocobalamin.

Methods: Four ascending dosing groups received intravenous doses of 2.5, 5, 7.5 or 10 g hydroxocobalamin over 7.5 to 30 minutes at a constant infusion rate. Volunteers (n = 136) randomized 3:1 to receive hydroxocobalamin or placebo underwent a 4-day in-house observation after infusion on Day 1 and follow-up visits on Days 8, 15, and 28.

Results: The most common drug-related adverse events were asymptomatic and self-limiting chromaturia and reddening of the skin, which are attributed to the red color of hydroxocobalamin. Other adverse events included pustular/papular rash, headache, erythema at the injection site, decrease in lymphocyte percentage, nausea, pruritus, chest discomfort, and dysphagia. Hydroxocobalamin was associated with an increase in blood pressure in some volunteers. Blood pressure changes peaked toward the end of hydroxocobalamin infusion and typically returned to baseline levels by 4 hours postinfusion. Maximum mean changes from baseline in systolic blood pressure ranged from 22.6 to 27.0 mmHg across hydroxocobalamin doses compared with 0.2 to 6.7 mmHg in the corresponding placebo groups. Maximum mean change from baseline in diastolic blood pressure ranged from 14.3 to 25.4 mmHg across hydroxocobalamin doses compared with -3.0 to 3.8 mmHg in the corresponding placebo groups. Two allergic reactions that occurred within minutes after start of the 5- and 10-g hydroxocobalamin infusions were successfully managed with dexamethasone and/or dimethindene maleate.

Conclusion: Timely intervention for acute cyanide poisoning could entail administration of an antidote in the prehospital setting based on a presumptive diagnosis. Results of this placebo-controlled study in healthy volunteers corroborate previous studies and French postmarketing experience in cyanide-exposed patients in suggesting that the safety profile of hydroxocobalamin is consistent with prehospital or hospital use.