Comparison of cobinamide to hydroxocobalamin in reversing cyanide physiologic effects in rabbits using diffuse optical spectroscopy monitoring

Abstract

Our purpose is to compare cobinamide to hydroxocobalamin in reversing cyanide (CN)-induced physiologic effects in an animal model using diffuse optical spectroscopy (DOS). Cyanide poisoning is a major threat worldwide. Cobinamide is a novel molecule that can bind two molecules of cyanide, has a much higher binding affinity than hydroxocobalamin, and is more water soluble. We investigated the ability of equimolar doses of cobinamide and hydroxocobalamin to reverse the effects of cyanide exposure in an animal model monitored continuously by DOS. Cyanide toxicity was induced in 16 New Zealand white rabbits by intravenous infusion. Animals were divided into three groups: controls (n=5) received saline following cyanide, hydroxocobalamin (N=6) following cyanide, and cobinamide (N=5) following cyanide. Cobinamide caused significantly faster and more complete recovery of oxy- and deoxyhemoglobin concentrations in cyanide-exposed animals than hydroxocobalamin- or saline-treated animals, with a recovery time constant of 13.8+/-7.1 min compared to 75.4+/-25.1 and 76.4+/-42.7 min, for hydroxocobalamin- and saline-treated animals, respectively (p<0.0001). This study indicates that cobinamide more rapidly and completely reverses the physiologic effects of cyanide than equimolar doses of cobalamin at the dose used in this study, and CN effects and response can be followed noninvasively using DOS.

Figure 1

Comparison of the molecular structure of (a) hydroxocobalamin to (b) cobinamide (lower panel). Cobinamide lacks the dimethyl-benzimidazole ribonucleotide tail coordinated to the cobalt atom in the lower axial position. Whereas cobalamin has only an upper ligand binding site, cobinamide has both an upper and a lower ligand binding site. The dimethylbenzimidazole group on cobalamin has a negative trans-effect on the upper binding site, thereby reducing cobalamin’s affinity for ligands. The combined effect is that each cobinamide molecule can bind two cyanide molecules and that cobinamide has a much greater affinity for cyanide than cobalamin.

Figure 2

Effects of cobinamide and hydroxocobalamin on tissue oxyhemoglobin, deoxyhemoglobin, and total hemoglobin concentrations in representative cyanide-poisoned rabbits. Changes in tissue oxyhemoglobin (OHb), deoxyhemoglobin (RHb), and total hemoglobin (THb) as measured by DOS are shown during and following cyanide infusion in individual rabbits. The decrease in deoxyhemoglobin concentration during 60 min of cyanide infusion is due to the inability of tissues to remove oxygen from circulating blood, leaving more hemoglobin in the oxygenated state. At 60 min, when the cyanide infusion was stopped, the animals received either 5 cc of saline (a), 5 cc of 16.3 mM cobalamin in saline (b), or 5 cc of 16.3 mM cobinamide in saline (c). In the control animal, oxy- and deoxyhemoglobin concentrations gradually returned toward baseline after the cyanide infusion was discontinued but never fully returned to preinfusion values. In the hydroxocobalamin-treated animal, oxy- and deoxyhemoglobin concentrations returned to baseline at ∼130 min, i.e., 70 min following drug administration. In contrast, oxy- and deoxyhemoglobin concentrations returned to baseline within 10 min following cobinamide administration.

Figure 3

Effect of cobinamide and hydroxocobalamin on tissue oxy- and deoxyhemoglobin concentrations in all cyanide-poisoned rabbits. The mean and standard error of percent change in (a) oxyhemoglobin (OHb) and (b) deoxyhemoglobin (RHb) concentrations from baseline are shown during cyanide infusion and treatment for control animals and for hydroxocobalamin-treated (IV OHCob) or cobinamide-treated (IV Cob) animals. (a) Tissue deoxyhemoglobin concentrations rise rapidly from their levels at the end of the cyanide infusion in cobinamide-treated animals compared to controls and hydroxocobalamin animal groups. (b) Tissue oxyhemoglobin concentrations fall significantly faster from their peak value at the end of cyanide infusion following treatment with cobinamide compared to hydroxocobalamin or controls.

Figure 4

RBC cyanide concentrations. The concentration of cyanide in RBCs is shown at the end of the cyanide infusion (zero time) and for 60 min thereafter. At zero time, the animals received saline (controls), hydroxocobalamin, or cobinamide. Values are expressed as percent of the peak cyanide levels achieved at the completion of cyanide infusion and shown as mean and standardized error. Cobinamide-treated animals had significantly faster reduction in blood cyanide levels compared to hydroxocobalamin and control groups.