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INTRODUCTION

  1. Pharmacology. Hydroxocobalamin is an analog of vitamin B12 that is used for the treatment of vitamin B12 deficiency syndromes in small doses and as an antidote for human cyanide poisoning in large doses. Hydroxocobalamin rapidly exchanges its hydroxyl group with free cyanide to produce nontoxic, stable cyanocobalamin. Since the molar binding ratio of hydroxocobalamin to cyanide is 1:1, 5 g of hydroxocobalamin will neutralize 97 mg of cyanide. Independent of its cyanide binding, hydroxocobalamin scavenges nitric oxide, a mediator of vasodilation. When administered to patients with cyanide poisoning, it rapidly improves the heart rate, systolic blood pressure, and acidemia. In humans, outcome is best when hydroxocobalamin is administered before cardiopulmonary arrest occurs. In normal individuals given 5- and 10-g hydroxocobalamin doses, the plasma half-life values for cobalamin compounds average 26 and 31 hours, respectively. Oral absorption is poor; absorption by intranasal route provides only small nontherapeutic doses for cyanide poisoning; large fluid volumes (200 mL) required for each 5 g dose preclude IM use; and intraosseous administration is under investigation.

  2. Indications

    1. Treatment of acute cyanide poisoning or symptomatic patients suspected to be at high risk for cyanide poisoning (eg, smoke inhalation victims?).

    2. Prophylaxis or treatment of cyanide poisoning during nitroprusside infusion has been proposed.

  3. Contraindications. Use caution when managing patients with known hypersensitivity to hydroxocobalamin or cyanocobalamin and consider alternative treatments.

  4. Adverse effects

    1. Adverse reactions in healthy volunteers include chromaturia (red-colored urine) in 100%, erythema in 94-100%, rash in 20-44%, high blood pressure in 18-28%, nausea in 6-11%, headache in 6-33%, decreased lymphocyte percentage in 8-17%, and infusion site reaction in 6-39%. Although red-colored body fluids usually normalize within 2–7 days, erythema can last up to 2 weeks and chromaturia up to 35 days. A self-limiting acneiform rash may occur 7–28 days after infusion.

    2. Allergic reactions have not been reported with acute intravenous therapy for cyanide poisoning. However, allergic reactions have been reported in patients using chronic IM therapy and in healthy volunteers unexposed to cyanide who were given IV hydroxocobalamin while participating in clinical safety trials.

    3. Use in pregnancy. FDA Category C. The acute, short-term use of hydroxocobalamin for a seriously symptomatic, cyanide-poisoned patient (Introduction) is not precluded in pregnancy and is preferred over nitrite administration. Cobalamin compounds cross the placenta and have been detected in human newborn urine samples.

  5. Drug or laboratory interactions

    1. Coloration of bodily fluids caused by cobalamins can interfere with colorimetric laboratory tests for periods ranging from 12 to 48 hours for blood and serum and up to 8 days for urine. Sampling and storing specimens before antidote administration is recommended, if possible. Test interferences are variable depending on which brand of analyzer is used. Test results that are commonly affected include:

      1. Falsely decreased ALT and amylase.

      2. Falsely increased AST, serum creatinine, glucose, alkaline phosphatase, albumin, total protein, bilirubin, triglycerides, cholesterol, hemoglobin, MCH, MCHC, basophils, and most urine chemistry parameters.

      3. Unpredictable effects for carbon monoxide, lactate, CK, CKMB, and PT/INR.

      4. Currently, interference ...

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