Atorvastatin in the Treatment of Traumatic Brain Injury

Introduction

Statins are a widely used class of medications which are inhibitory analogs of the enzyme β-hydroxy-β-methylglutaryl coenzyme A (HMG CoA) reductase. While very extensively used for treatment of hyperlipidemia and in the prevention of cardiovascular disease, their application to traumatic brain injury as neuroprotective agents attenuating secondary neurological injury is still limited. In this chapter, we will review current understanding of the role of statins in preclinical studies, their pharmacokinetics, and adverse effect profiles, and conclude with a discussion of clinical investigations involving statins as they relate to neuroprotection.

Pharmacodynamics

The most well-understood mechanism of action of statin medications is the inhibition of the enzyme HMG CoA reductase. Inhibition of this enzyme, which is involved in the biosynthesis of cholesterol, results in a decreased in the fraction of low-density lipoprotein (LDL) cholesterol in plasma and is responsible for many of the cardiovascular benefits of statins. However, statins have also been found to have many other noncholesterol-mediated mechanisms of action (ie, pleiotropic), which may play more important roles in neuroprotection than cholesterol-based mechanisms. These include upregulation of expression of endothelial nitric oxide synthase (eNOS), antiapoptotic effects, increased angiogenesis, various antioxidant and antiinflammatory mechanisms, and augmentation of neurogenesis and synpatogenesis ( ).

Statins have been demonstrated in multiple studies to enhance expression of eNOS via upregulation of the protein kinase Akt pathway ( ). Kureishi et al. investigated the effect of simvastatin on cultured human umbilical vein endothelial cells (HUVEC) and found that simvastatin exposure of the cultured cells at 1.0 μM resulted in the phosphorylation of Akt, resulting in the activation of eNOS. Interestingly, simvastatin-induced Akt activation was terminated when cultures were incubated with l -mevalonate (a metabolite of HMG-CoA reductase), suggesting the importance of statin’s inhibition of this enzyme in their effect. As traumatic brain injury is well known to result in deficiencies in cerebral autoregulation and ischemia is understood to be a potential mechanism for secondary brain injury, it has been hypothesized that statins could contribute to improvements in cerebral perfusion postinjury and thus potentially improved clinical outcomes. The NO-induced vasodilation has been proposed to limit neurological loss, especially in ischemic penumbra locations in stroke literature ( ).

Antiapoptotic effects of statins have primarily been investigated as they relate to attenuation of glutamate-NMDA-mediated excitotoxicity. Studies have demonstrated that via a cholesterol-mediated mechanism, statins interfere with cellular cholesterol synthesis, and in particular, oxygen and glucose deprivation (OGD)/reoxygenation-evoked neuronal death by reducing formation of 4-hydroxy-2-nonenal (4-HNE) ( ). In a study by Lim et al., cortical neuronal cell cultures from Sprague–Dawley rats were treated with simvastatin and found to have significantly lower rates of 4-HNE compared to untreated rats. Moreover, statins have been demonstrated to promote expression of prosurvival proteins such as Bcl-2 and suppress activation of proapoptotic proteases like caspase-3 ( ). In a study of primary cortical neuronal cultures from E16–E17 fetal C57BL6 mice, simvastatin treatment significantly increased Bcl-2 mRNA production and chronic treatment resulted in significantly attenuated levels of Aβ _1–42 -induced caspase 3 activation. Preclinical studies in numerous animal models have confirmed that the biochemical and genetic antiapoptotic effects of statins do translate into improved neuronal survival in TBI models and even improved cognitive function ( ). For instance, used a controlled cortical impact (CCI) model of TBI in male Wistar rats and treated an experimental group with 1 mg/kg sim-vastatin or atorvastatin. Morris Water Maze tests demonstrated statin-treated mice had improved spatial learning at 31–35 days after TBI, and had reduced neuronal loss in the CA3 region of the hippocampus. While clinical investigations have not been conducted to demonstrate similar neuronal and biochemical processes in human patients with traumatic brain injury, preclinical investigations have been largely positive for such antiapoptotic effects.

Statins have also been demonstrated to improve angiogenesis. By both increased nitric oxide production, as well as upregulation of vascular endothelial growth factor (VEGF), statins have been demonstrated to augment endothelial cell proliferation and migration ( ). In a study investigating statins in a TBI rat model, statin-mediated upregulation of VEGF was demonstrated posttraumatic brain injury, and recovery of spatial learning was improved as measured by a modified Morris Water Maze task ( ). Applications of statins in human studies, however, have not thus far investigated VEGF upregulation and angiogenesis effects.

Mixed results have been demonstrated with regard to statin effects on acute lesional volume in traumatic brain injury animal models. In a study from 2007, rats pretreated with lovastatin before a CCI injury had a 20% reduction in contusion volume and 35% reduction in FJB-positive degenerating neurons ( ). However, in a similar study with simvastatin and atorvastatin, contusion volumes were not statistically different in treated mice despite similar significant improvements in inflammatory marker expression and improved cerebral hemodynamics ( ). Posttrauma administration of statins has also demonstrated mixed results in animal models on contusion volume, with one study by demonstrating no difference in contusional size in a rat TBI model with oral administration of atorvastatin for 7 days following injury. Interestingly, however, functional deficits were reduced, neuronal survival and synaptogenesis were increased, and angiogenesis was augmented. In contrast, the same group reported in the same year findings of reduced intracranial hematoma volumes in rats treated with atorvastatin at 8 days postinjury ( ). While contusion volumes, hematoma volumes, and hematoma absorption rates represent different anatomic injuries, statins have unclear effects in the acute phase of injury.

Some of the most powerful but least well-understood mechanisms of neuroprotection of statins involve their antioxidant and antiinflammatory effects. Stoll et al. reported in 2005 that statins, by interfering with NAD(P)H oxidase expression and activity, resulted in the inhibition of reactive oxygen species generation. Damaging effects of free radicals have also been demonstrated to be attenuated by statins by their augmentation of antioxidant enzymes, lipid peroxidation, LDL cholesterol oxidation, and the aforementioned increase in expression of NOS ( ). Additionally, statins have been demonstrated to exert antiinflammatory properties via their reduction in production of isoprenyl intermediates in cholesterol biosynthesis ( ).

With regards to inflammatory chemokines and mediators, statins have also been demonstrated in multiple preclinical animal models to reduce expression of such factors. In a study by Chen et al., simvastatin was administered to a TBI model of rats (weight drop model) and brain samples were analyzed 24 h after injury ( ). The researchers found that compared to untreated control rats, simvastatin-treated rats had significantly reduced expression of TLR4 and NF-kappaB, as well as downstream inflammatory mediators including interleukin-1beta (IL-1β), tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and intracellular adhesion molecule-1 (ICAM-1). Furthermore, treated rats had lower cortical apoptosis, brain edema, blood–brain barrier (BBB) impairment, and motor deficits than untreated rats. Additionally, cardiovascular literature has demonstrated that C-reactive protein plasma concentrations, a potent marker of systemic inflammation, are significantly reduced in myocardial infarction patients administered pravastatin, in an effect that was independent from the magnitude of lipid panel change ( ). While different from central nervous system effects, this supports the noncholesterol-mediated antiinflammatory effects of statins. Given that recent research has implicated some of the same inflammatory chemokines in human traumatic brain injury, in vitro and preclinical investigations of statin’s antiinflammatory properties have generally been supportive.

Preclinical investigations with spinal cord injury rat models have also demonstrated similar findings with regards to statins’ effects on inflammatory markers. Atorvastatin was shown to reduce expression of iNOS, TNF-α, and IL-1β, as well as reduce necrosis, neuron and oligodendrocyte apoptosis, reactive gliosis, and demyelination ( ). While spinal cord parenchyma differs significantly from cerebral parenchyma, the ensuing postinjury inflammatory processes are similar.

Other less well-understood effects of statins as they relate to immunomodulation and inflammation include stabilization of the BBB and reduction of microglial-related inflammation. An in vitro study published in 2006 investigated the effects of statins on human-derived BBB endothelial cells ( ). Statin administration resulted in a 50–60% reduction in diffusion of bovine serum albumin as well as [14(C)] sucrose across the in vitro BBB via reduction in the aforementioned isoprenylation processes. Furthermore, when multiple sclerosis-derived monocytes and lymphocytes were introduced to the in vitro model, statin treatment significantly restricted cross-BBB migration of these cell lines by reducing monocyte chemotactic protein-1/CCL2 and interferon-gamma-inducible protein-10/CXCL10. In another study ( ), microglial markers were reduced in a murine TBI model with statin administration, with important resultant reduction in hippocampal neuronal degeneration and reduced functional neurological deficits.

Finally, statins have been implicated in improving neuronal generation and synapse generation in multiple different models of brain injury. Previously mentioned studies investigating effects of statins on angiogenesis have also demonstrated enhanced neuronal migration, neurogenesis, and axonal sprouting in cell cultures with statin administration ( ). These effects are believed to be related to upregulation of brain-derived neurotrophic factor (BDNF), as well as the previously discussed augmentation of VEGF. Moreover, synapse generation has also been demonstrated to be improved with statin administration, with one study demonstrating increased synaptophysin histological staining in TBI-model rats treated with statins in both pericontusional cortex as well as in the ipsilateral hippocampus ( ).

In summary, a number of cholesterol-mediated and noncholesterol-mediated pharmacodynamic effects of statins have been demonstrated in preclinical investigations ( Table 5.1 ). These include upregulation of expression of eNOS, antiapoptotic effects, increased angiogenesis, various antioxidant and antiinflammatory mechanisms, and augmentation of neurogenesis and synaptogenesis. Nearly all of these studies have been supportive of the potential role of statins in neuroprotection, with a few studies being specifically related to statin effects in traumatic brain injury models. However, as is discussed in the clinical investigation section, many preclinical investigations of seemingly efficacious neuroprotective agents have had difficulty in translating such efficacy in true clinical investigations of traumatic brain injury.