Beagle puppy model of perinatal asphyxia: alterations in cerebral blood flow and metabolism

Development of a neonatal Göttingen Minipig model for dose precision in perinatal asphyxia: technical opportunities, challenges, and potential further steps

Animal models provide useful information on mechanisms in human disease conditions, but also on exploring (patho)physiological factors affecting pharmacokinetics, safety, and efficacy of drugs in development. Also, in pediatric patients, nonclinical data can be critical for better understanding the disease conditions and developing new drug therapies in this age category. For perinatal asphyxia (PA), a condition defined by oxygen deprivation in the perinatal period and possibly resulting in hypoxic ischemic encephalopathy (HIE) or even death, therapeutic hypothermia (TH) together with symptomatic drug therapy, is the standard approach to reduce death and permanent brain damage in these patients. The impact of the systemic hypoxia during PA and/or TH on drug disposition is largely unknown and an animal model can provide useful information on these covariates that cannot be assessed separately in patients. The conventional pig is proven to be a good translational model for PA, but pharmaceutical companies do not use it to develop new drug therapies. As the Göttingen Minipig is the commonly used pig strain in nonclinical drug development, the aim of this project was to develop this animal model for dose precision in PA. This experiment consisted of the instrumentation of 24 healthy male Göttingen Minipigs, within 24 h of partus, weighing approximately 600 g, to allow the mechanical ventilation and the multiple vascular catheters inserted for maintenance infusion, drug administration and blood sampling. After premedication and induction of anesthesia, an experimental protocol of hypoxia was performed, by decreasing the inspiratory oxygen fraction (FiO₂) at 15%, using nitrogen gas. Blood gas analysis was used as an essential tool to evaluate oxygenation and to determine the duration of the systemic hypoxic insult to approximately 1 h. The human clinical situation was mimicked for the first 24 h after birth in case of PA, by administering four compounds (midazolam, phenobarbital, topiramate and fentanyl), frequently used in a neonatal intensive care unit (NICU). This project aimed to develop the first neonatal Göttingen Minipig model for dose precision in PA, allowing to separately study the effect of systemic hypoxia versus TH on drug disposition. Furthermore, this study showed that several techniques that were thought to be challenging or even impossible in these very small animals, such as endotracheal intubation and catheterization of several veins, are feasible by trained personnel. This is relevant information for laboratories using the neonatal Göttingen Minipig for other disease conditions or drug safety testing.

Pathophysiology of Perinatal Asphyxia in Humans and Animal Models

Perinatal asphyxia is caused by lack of oxygen delivery (hypoxia) to end organs due to an hypoxemic or ischemic insult occurring in temporal proximity to labor (peripartum) or delivery (intrapartum). Hypoxic–ischemic encephalopathy is the clinical manifestation of hypoxic injury to the brain and is usually graded as mild, moderate, or severe. The search for useful biomarkers to precisely predict the severity of lesions in perinatal asphyxia and hypoxic–ischemic encephalopathy (HIE) is a field of increasing interest. As pathophysiology is not fully comprehended, the gold standard for treatment remains an active area of research. Hypothermia has proven to be an effective neuroprotective strategy and has been implemented in clinical routine. Current studies are exploring various add-on therapies, including erythropoietin, xenon, topiramate, melatonin, and stem cells. This review aims to perform an updated integration of the pathophysiological processes after perinatal asphyxia in humans and animal models to allow us to answer some questions and provide an interim update on progress in this field.

Does thrombin play a role in the pathogenesis of brain damage after periventricular hemorrhage?

Neonatal periventricular hemorrhage (PVH) is a devastating complication of prematurity in the human infant. Based upon observations made primarily in adult rodents and the fact that the immature brain uses proteolytic systems for cell migration and growth, we hypothesized that thrombin and plasmin enzyme activities contribute to the brain damage after PVH. The viability of mixed brain cells derived from newborn rat periventricular region was suppressed by whole blood and thrombin, but not plasmin. Following injection of autologous blood into the periventricular region of newborn rat brain, proteolytic activity was detected in a halo around the hematoma using membrane overlays impregnated with thrombin and plasmin fluorogenic substrates. Two-day old rats received periventricular injection of blood, thrombin, and plasminogen. After 2 days, thrombin and blood were associated with significantly greater damage than saline or plasminogen. Two-day old mice received intracerebral injections of blood in combination with saline or the proteolytic inhibitors hirudin, alpha2macroglobulin, or plasminogen activator inhibitor-1. After 2 days, hirudin significantly reduced brain cell death and inflammation. Two-day-old mice then received low and high doses of hirudin mixed with blood after which behavioral testing was conducted repeatedly. At 10 weeks there was no statistically significant evidence for behavioral or structural brain protection. These results indicate that thrombin likely plays a role in neonatal periventricular brain damage following PVH. However, additional factors are likely important in the recovery from this result.

High-dose phenobarbital therapy in term newborn infants with severe perinatal asphyxia: a randomized, prospective study with three-year follow-up

OBJECTIVE

To determine whether 40 mg/kg phenobarbital given to term infants with severe asphyxia would result in a lower incidence of seizures in the newborn period and an improved neurologic outcome.

METHODS

We conducted a randomized, controlled, prospective study. Entry criteria included (1) an initial arterial pH less than or equal to 7.0 with a base deficit 15 mEq/L or more, (2) Apgar score less than or equal to 3 at 5 minutes of age, or (3) failure to initiate spontaneous respiration by 10 minutes of age. Sample size was calculated to detect a 50% reduction in the incidence of neonatal seizures.

RESULTS

No differences were present between treatment and control groups with respect to severity of asphyxia assessed by initial arterial pH, base excess, cerebrospinal fluid lactate dehydrogenase concentration or detection of CSF creatine kinase of its BB isoenzyme. Seizures occurred in 9 of 15 infants in the treatment group and 14 of 16 infants in the control group (p = 0.11). No adverse effects were observed from phenobarbital on heart rate, respiratory rate, blood pressure, or arterial blood gas values. Three-year follow-up revealed normal outcome in 11 of 15 infants in the treatment group and 3 of 16 in the control group (p = 0.003).

CONCLUSION

Phenobarbital, when administered in a dose of 40 mg/kg intravenously over 1 hour in term, severely asphyxiated newborn infants appeared to be safe and was associated with a 27% reduction in the incidence of seizures and a significant improvement in neurologic outcome at 3 years of age.

Blood flow distribution in the normal human preterm brain

Disturbances in cerebral blood flow (CBF) are a major factor in the etiology and pathogenesis of cerebral damage in the neonate. As most animals are more mature at birth than man, extrapolation from animal studies to the human is questionable. Therefore, we have measured regional CBF (rCBF) in preterm infants. rCBF flow was measured in 12 normotensive and normoxic preterm infants [mean birth weight 915 g (range 550 to 2680 g), mean gestational age 27.7 wk (25 to 32 wk)]. All infants had a normal cerebral ultrasound examination. rCBF was measured using a mobile brain dedicated fast-rotating four-head multidetector system specially designed for neonatal studies. The tracer was 99mTc-labeled D,L-hexamethylpropylenamine oxime in a dose of 4 Mbq/kg. rCBF of the subcortical white matter was 0.53 (0.48-0.58) of the global CBF. After correction for scattered radiation, the estimate of rCBF to the white matter was reduced to 0.39 (0.36-0.42). The flow to the basal ganglia was 2.33 (2.08-2.59) times the global CBF. After correction for partial volume effect, the cortical flow was higher than the flow to the basal ganglia and highest in the frontotemporal cortex (motor cortex). The flow to the cerebellum was of the same magnitude as the flow to the basal ganglia, but with a significantly higher variation. rCBF in 12 preterm infants showed a flow distribution similar to flow in other newborn mammals. The gray-white matter contrast, however, was greater. This new information, combined with existing data showing low global CBF, suggests that blood flow to the white matter in the preterm human neonate is extremely low.

Animal models of perinatal asphyxia: contributions, contradictions, clinical relevance

Animal models have contributed immensely to our understanding of hypoxic ischemic encephalopathy in the newborn. A number of animal models have been used, including both primate and subprimate species. Although the Rhesus monkey model has a dramatically similar pathological distribution of brain injury when compared with the human, other pathologic processes secondary to asphyxia may be more appropriately assessed in other species. The maxim that because primates are closer on the phylogenetic tree to humans than are subprimates all observations in the primate are applicable to the human is simply not true. Understanding of the neurochemical consequences of asphyxia in the past decade have arisen from experiments primarily in the neonatal rat. We have come to understand that not only is the hypoxic event of major significance, but that, once reperfused, reoxygenation causes further injury. Free-radical generation following reperfusion may be massive and may further contribute to cell membrane injury. These observations have lead to rational theoretic approaches to the treatment of hypoxic ischemic brain injury. On the other hand, previously used treatments such as osmotic agents and glucocorticoids would appear to be not only inefficacious but hazardous in the treatment of hypoxic ischemic brain injury. The role of nitric oxide (NO) in the pathogenesis of brain injury is yet uncertain, but there is little doubt that it plays a significant role. Although survival of the immature animal subjected to hypoxic environment is longer than in the mature animal, the central nervous system of the immature animal is more sensitive to glutamate and N-Methyl-D-aspartate (NMDA) receptor-mediated injury.

Carbohydrate and energy metabolism during the evolution of hypoxic-ischemic brain damage in the immature rat

The brain damage that evolves from perinatal cerebral hypoxia-ischemia may involve lingering disturbances in metabolic activity that proceed into the recovery period. To clarify this issue, we determined the carbohydrate and energy status of cerebral tissue using enzymatic, fluorometric techniques in an experimental model of perinatal hypoxic-ischemic brain damage. Seven-day postnatal rats were subjected to unilateral common carotid artery ligation followed by 3 h of hypoxia with 8% oxygen at 37 degrees C. This insult is known to produce tissue injury (selective neuronal necrosis or infarction) predominantly in the cerebral hemisphere ipsilateral to the carotid artery occlusion in 92% of the animals. Rat pups were quick-frozen in liquid nitrogen at 0, 1, 4, 12, 24, or 72 h of recovery; littermate controls underwent neither ligation nor hypoxia. Glucose in both cerebral hemispheres was nearly completely exhausted during hypoxia-ischemia, with concurrent increases in lactate to 10 mmol/kg. During recovery, glucose promptly increased above control values, suggesting an inhibition of glycolytic flux, as documented in the ipsilateral cerebral hemisphere by measurement of glucose utilization (CMRglc) at 24 h. Tissue lactate declined rapidly during recovery but remained slightly elevated in the ipsilateral hemisphere for 12 h. Phosphocreatine (P approximately Cr) and ATP in the ipsilateral cerebral hemisphere were 14 and 26% of control (p less than 0.001) at the end of hypoxia-ischemia; total adenine nucleotides (ATP + ADP + AMP) also were partially depleted (-46%).(ABSTRACT TRUNCATED AT 250 WORDS)

Beagle puppy model of perinatal asphyxia: blockade of excitatory neurotransmitters

The N-methyl-D-aspartate receptor antagonist MK801 has been reported to prevent neuronal change in models of ischemia in adult animal systems. We studied the hypothesis that blockade of the N-methyl-D-aspartate receptor would prevent the depression of cerebral high-energy phosphates found in perinatal asphyxia without producing alterations in cerebral blood flow, and thus prevent neuropathologic damage. Newborn beagle puppies were anesthetized, tracheotomized, ventilated, and randomized to asphyxial insult (I = discontinuation of ventilatory support for 5 min) or no insult (NI) and drug treatment with MK801 (10 mg/kg intravenously) or an equal volume of saline (S). Puppies received MK801 or saline 15 min prior to I/NI. In S/I pups during insult, blood flow increased to brainstem structures but decreased elsewhere. MK801 had no effect on cerebral blood flow in either control or insulted puppies. 1H NMR studies demonstrated no effect of the MK801 on NI brains. Phosphocreatine levels were 1.7 +/- 0.1, 0.6 +/- 0.1, and 0.9 +/- 0.1 mmole/kg (mean: +/- S.D.) for the S/NI, S/I, and MK801/I pups, respectively. Cerebral lactate was 1.3 +/- 0.2, 3.0 +/- 0.7, and 2.0 +/- 0.4, respectively. The pH fell 0.8 units in the S/I puppies, compared to 0.4 units in the MK801/I puppies. We conclude that pretreatment with the N-methyl-D-aspartate receptor antagonist MK801 in part protects the developing brain against severe metabolic insult.

Thromboxane synthesis inhibitor in a beagle pup model of perinatal asphyxia

During perinatal asphyxia, cerebral blood flow is markedly reduced in the gray and white matter of the telencephalon. Since previous work has implicated prostaglandins in the control of blood flow, we tested the hypothesis that a thromboxane synthesis inhibitor would improve cerebral blood flow and blunt the metabolic alterations that accompany asphyxia. Forty-three newborn beagles 2-7 days old were anesthetized, ventilated, and randomized to insult (5 minutes of asphyxia) or no insult and received treatment with either the thromboxane synthesis inhibitor CGS 13080 (CIBA-GEIGY Corp.) (0.06 mg/kg/hr i.v. infusion) or saline. Cerebral blood flow was measured in 25 pups. Pups received treatment 30 minutes before insult or no insult. In pups randomized to insult and receiving saline, cerebral blood flow increased during insult in the medulla but decreased elsewhere. Pups randomized to insult and treated with thromboxane synthesis inhibitor had increased cerebral blood flow during insult in all cerebral regions studied. In addition, these pups experienced a significantly higher incidence of intraventricular hemorrhage than did pups randomized to insult and receiving saline. In other experiments with 18 pups, brain extracts were prepared for proton nuclear magnetic resonance spectral analysis of high-energy phosphorylated compounds and lactate levels. In pups exposed to insult and receiving saline, mean +/- SD phosphocreatine concentration fell from 1.9 +/- 0.1 to 0.4 +/- 0.1 mmol/kg, lactate concentration increased from 2.0 +/- 0.5 to 3.3 +/- 0.4 mmol/kg, and the calculated pH fell 0.8 units. There were no differences between groups.(ABSTRACT TRUNCATED AT 250 WORDS)