The Application of Glibenclamide in Traumatic Brain Injury

Abstract

Sulfonylurea receptor 1 (SUR1) is a known component of ATP-sensitive potassium channels in the pancreas but more recently has been identified in association with a nonselective cation channel, transient receptor potential melastatin 4 (TRPM4). SUR1-TRPM4 channels are upregulated in all members of the neurovascular unit in response to mechanical trauma. Channel opening results in sodium influx, oncotic swelling, and cell death and plays a critical role in microvascular dysfunction, edema formation, and progressive secondary hemorrhage following traumatic brain injury. Sulfonylureas, including glibenclamide, bind to SUR1 and inhibit opening of the SUR1-TRPM4 channel and have the potential to minimize posttraumatic secondary injury. Preclinical studies confirm the ability of glibenclamide to reduce hemorrhage volumes, prevent capillary fragmentation, reduce edema, and improve cognitive outcomes. A Phase II trial recently completed recruitment and was designed to provide information about drug efficacy and safety.

Keywords

Glibenclamide, Hemorrhage, Secondary injury, Sulfonylurea, Traumatic brain injury

Acknowledgments

The work in this chapter was supported in part by grants to J. Marc Simard from the Department of Veterans Affairs (Baltimore, MD), the Department of the Army (W81XWH 1010898), the National Heart, Lung and Blood Institute (HL082517), and the National Institute of Neurological Disorders and Stroke (NINDS) (NS061808 and NS060801), as well as to Howard Eisenberg from the Department of Defense (Research Subaward Agreement PO #10291742 and Clinical Trial Subaward Agreement PO #10321073).

Conflicts of Interest

J. Marc Simard holds a US patent (7,285,574), “A novel non-selective cation channel in neural cells and methods for treating brain swelling.” J. Marc Simard is a member of the scientific advisory board and holds shares in Remedy Pharmaceuticals. No support, direct or indirect, was provided to J. Marc Simard, or for this project by Remedy Pharmaceuticals. The other authors declare no conflict of interest.

Sulfonylurea Receptor 1 and Its Role in Central Nervous System Injury

Sulfonylurea receptor 1 (SUR1) is a member of the ATP-binding cassette (ABC) transporter superfamily of transmembrane proteins. The SUR1 protein (which is encoded by the Abcc8 gene) is the regulatory subunit associated with pore-forming ion channels. For decades, attention has centered on the association between SUR1 and the Kir6.2 subunit. The combination forms ATP-sensitive potassium (K _ATP) channels, which are found in pancreatic islet cells and some (but not all) neurons. ATP depletion causes opening of that channel and potassium efflux, resulting in hyperpolarization ( Fig. 6.1 ). In pancreatic beta cells, K _ATPchannels mediate insulin secretion in response to glucose levels ( ).

In , Chen and Simard identified a second pore-forming subunit that associates with SUR1. The subunit was initially described as an ATP- and calcium-sensitive nonselective cation channel (NC _Ca-ATP) and was later identified as transient receptor potential melastatin 4 (TRPM4). The SUR1-TRPM4 channel transports inorganic, monovalent cations (and is impermeable to divalent cations such as Ca ²⁺and Mg ²⁺), with an equivalent pore radius of 0.41 nm. Nanomolar concentrations of cytoplasmic calcium trigger channel opening, which is blocked by intracellular ATP ( ).

Although not constitutively expressed, the SUR1-TRPM4 channel is upregulated in all members of the neurovascular unit—neurons, astrocytes, and endothelial cells—in response to mechanical trauma and hypoxia. ATP depletion that accompanies injury causes opening of the SUR1-TRPM4 channel as it does in the K _ATPchannel, but in contrast allows sodium influx, resulting in depolarization, oncotic cell swelling, and ultimately oncotic cell death ( ). SUR1 upregulation and subsequent SUR1-TRPM4 opening has been demonstrated in various models of central nervous system (CNS) injury, including ischemic stroke ( ), subarachnoid hemorrhage ( ), traumatic brain injury (TBI; ), spinal cord injury ( ), encephalopathy of prematurity ( ), and the edema associated with metastatic brain tumors ( ).

Glibenclamide—A Potent Inhibitor of Sulfonylurea Receptor 1

Given that opening of the SUR1-TRPM4 channel is shown to play a prominent role in mediating CNS injury in animal models, pharmacologic agents that block this process are promising therapeutic tools. To date, antagonists that specifically target the TRPM4 pore-forming subunit have not been identified. However, the SUR1 regulatory subunit contains binding sites for sulfonylureas, a class of drugs that include glibenclamide (known in the United States as glyburide) and repaglinide. Glibenclamide binds to SUR1 with high affinity and specificity (with an EC ₅₀of 48 nM at pH 7.4) and inhibits opening of the corresponding ion channel ( ). Specifically, glibenclamide prolongs the closed state of the channel, without affecting the open channel dwell times or channel conductance ( ).

Sulfonylureas have a long history of use in humans and have been prescribed for the treatment of diabetes mellitus type II for more than 25 years. Through their effects on the SUR1 component of the K _ATPchannel in pancreatic islet cells, sulfonylureas result in increased insulin release, and have therefore proven useful as oral hypoglycemic agents. The experience with sulfonylureas in diabetic patients has demonstrated that these drugs are not only effective but are also safe when used in humans ( ). This was confirmed by a Phase I study of the injectable form of glibenclamide, in which 34 healthy volunteers were administered a bolus dose of glibenclamide equivalent to the dose that had been used in animal models, followed by a 3 day continuous infusion ( ). These doses of glibenclamide were found to be safe and well tolerated, with minimal effects on blood glucose levels or insulin release.

In recent years, glibenclamide has also been shown to inhibit the SUR1 component of the SUR1-TRPM4 channel in the CNS. Under normal conditions, sulfonylureas are excluded from CNS tissue due to a combination of low lipid solubility at a neutral pH and active transport from endothelial cells into the blood ( ). Following CNS injury and/or hypoxia, however, glibenclamide accumulates within the injured CNS tissue. Transport into the brain is facilitated by areas of blood–brain barrier breakdown after injury, which allows passive movement of glibenclamide into the brain. Secondly, the injured tissue tends to exhibit higher levels of glycolytic activity and lactic acidosis, resulting in a lower extracellular pH. Ion trapping therefore causes glibenclamide, which is a weak acid, to accumulate in the relatively alkaline intracellular compartment of endothelial cells and neurons ( ). Studies that tracked fluorescently tagged glibenclamide as well as [ ³H]glibenclamide confirmed that the drug is selectively taken up by ischemic CNS tissue ( ). Additionally, the glibenclamide binding site on SUR1 is accessed via the lipid layer. Accordingly, the active form of the drug is its nonionized state, which predominates at a lower pH. Drug potency is therefore enhanced by the acidic environment of injured CNS tissue ( ). As a result, low doses of glibenclamide are sufficient to inhibit SUR1-TRPM4 channel opening in injured CNS tissue, while minimizing the side effects associated with insulin secretion in the pancreas.

Sulfonylurea Receptor 1 and Glibenclamide in the Setting of Traumatic Brain Injury

SUR1-TRPM4 opening and its inhibition by glibenclamide are being studied in various models of traumatic brain injury. Trauma produces CNS tissue damage through two critical mechanisms: primary and secondary injury. Primary injury refers to the physical damage that immediately takes place as a direct result of the trauma. The kinetic energy produced at the moment of impact causes rupture of neurons, astrocytes, and microvessels, resulting in necrotic cell death and hemorrhage. A variety of secondary injury responses follow, including the release of excitotoxic substances, free radical damage, inflammation (involving microglia, neutrophils, and macrophages), and microvascular dysfunction leading to ischemia, edema, and progressive secondary hemorrhage (PSH; ). These all contribute to further tissue injury, thereby compounding the initial insult. While the primary injury that accompanies a trauma cannot be ameliorated pharmacologically, glibenclamide’s potential rests in its ability to modify the secondary injury response in TBI—in particular, with respect to edema formation and PSH.

Sulfonylurea Receptor 1-Related Edema and Progressive Secondary Hemorrhage After Trauma

In a large proportion of patients with traumatic cerebral contusions, serial scans demonstrate expansion of the primary hemorrhage during the acute phase following the injury. Often described as “blossoming” of the contusion, this process is also referred to as PSH and results from microvascular dysfunction ( ). Capillary failure produces petechial hemorrhages in the tissues surrounding the site of primary injury; over time, these hemorrhages coalesce, resulting in expansion of the initial contusion as well as the delayed appearance of noncontiguous hemorrhages. These areas of secondary hemorrhage compound the initial injury by increasing the mass effect on the surrounding brain; furthermore, the blood itself produces free radicals and is highly toxic to neural tissue, particularly white matter. As a result, PSH is a highly destructive mechanism of secondary injury ( ).

Historically, PSH was thought to arise from microvessels that were “fractured” at the time of injury, resulting in persistent bleeding, especially in the setting of coagulopathy. TBI patients commonly experience coagulopathy, possibly due to the release of tissue factor from injured cerebral tissue, resulting in activation of the extrinsic coagulation pathway and disseminated intravascular coagulation (DIC). However, this relationship is not absolute. A substantial number of TBI patients with PSH are not coagulopathic, and many coagulopathic patients do not demonstrate PSH ( ). Furthermore, when Factor VII is used in rare circumstances to correct the coagulopathy, it is only modestly effective in preventing PSH ( ). Other mechanisms must also play a role in PSH.

Recent data indicate that mechanical trauma triggers a molecular cascade starting with transcription factors that lead to SUR1 upregulation and ultimately culminating in edema formation and PSH ( Fig. 6.2 ). In this model of injury, the kinetic energy imparted by the initial trauma fractures microvessels and shears tissues at the focus of the impact, immediately producing the primary contusion. In the surrounding area (ie, the “penumbra”), the amount of kinetic energy that is deposited is insufficient to fracture microvessels. However, the kinetic energy activates two mechanosensitive transcription factors, specificity protein 1 (Sp1) and nuclear factor-κB (NF-κB) ( ). Immediately following the trauma, these transcription factors undergo nuclear translocation in the penumbral endothelial cells (as well as glia and neurons) and produce upregulation of SUR1 transcription. Then, when ATP is depleted (which is consistently seen in the setting of injury), the channel opens, resulting in sodium influx. Sodium is followed by water, resulting in oncotic swelling (ie, cytotoxic edema). In the penumbral endothelial cells, this process initially results in capillary lumen narrowing, producing ischemia. The ischemic environment in the penumbra further contributes to SUR1 upregulation, via the effects of hypoxia inducible factor 1 (Hif1) on Sp1. As the endothelial cells accumulate intracellular fluid and undergo oncotic swelling, the cytoskeleton is rearranged, producing gaps in the tight junctions that join the endothelial cells and form the blood–brain barrier. This allows the paracellular flow of protein-rich fluid into the extracellular space of the brain (ie, vasogenic edema). As these processes continue, the endothelial cells ultimately undergo oncotic/necrotic cell death, resulting in a loss of capillary wall integrity (ie, “capillary fragmentation”). The resulting microvascular failure allows extravasation of blood from the capillaries, producing petechial hemorrhages that may eventually coalesce, causing expansion of the primary hemorrhage ( ).