Efficacy of Liposome-Encapsulated Hemoglobin in a Rat Model of Cerebral Ischemia

Use of liposome-encapsulated hemoglobin (LEH) for oxygen delivery in the treatment of cerebral ischemia has been studied previously and its expected benefits confirmed. However, the relationship between the timing of administration and the efficacy of LEH in cerebral ischemia has not been studied in detail. We therefore investigated the therapeutic time window of LEH by using a rat model of cerebral ischemia, as well as evaluating the contribution of oxygen delivery to the efficacy of LEH. Dose-dependent effects and the therapeutic time window of LEH were studied using models of transient and permanent middle cerebral artery occlusion (MCAO), respectively, in SD rats. LEH was intravenously administered at 0.5 h after the onset of ischemia in the transient MCAO model and at 0.5, 2, 4, or 6 h in the permanent MCAO model. Efficacy of LEH treatment was evaluated using the infarct volume, which was examined with 2,3,5-triphenyltetrazolium chloride staining and estimated by integrating the unstained areas in serial sections of cerebral tissue. Effects of oxygen delivery by LEH were examined immunohistochemically with pimonidazole to stain for areas of low oxygen tension in the tissue. LEH treatment dose-dependently reduced the cerebral infarct volume, which was especially significant in the cortical region at doses of over 60 mg hemoglobin (Hb)/kg. In rats with permanent MCAO, LEH administration at a dose of 300 mg Hb/kg at 0.5 h and 2 h after the onset of cerebral ischemia significantly reduced cerebral infarct volume. Furthermore, immunohistochemical staining with pimonidazole showed that the areas of cerebral tissue that were hypoxic and had abnormal histological structure were reduced after LEH treatment. These results indicated that LEH is efficacious in the treatment of cerebral infarction secondary to MCAO and that oxygen delivery to ischemic cerebral tissues by LEH administered early after the onset of cerebral ischemia contributes to this effect.