5-Lipoxygenase-Activating Protein Inhibitors: Promising Drugs for Treating Acute and Chronic Neuroinflammation Following Brain Injury

Leukotrienes

The 5-lipoxygenase (5-LO) pathway mediates the production of leukotrienes, a class of potent inflammatory lipid mediators made in leuko cytes that contain a conjugated triene as part of their structure. The biosynthesis of leukotrienes is under strict control (for review see ). Upon infection or tissue injury, cells of the innate immune system (neutrophils, monocytes, basophils, eosinophils, and tissue specific macrophages, ie, brain microglia) recognize pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) and become activated. Upon activation, the cytosolic concentration of calcium increases, leading to the activation and translocation of cytosolic phospholipase A2α (cPLA2α) to the nuclear membrane where it cleaves arachidonic acid (AA) from glycerolphospholipids ( ). The release of AA is the first critical step in leukotriene formation. Like cPLA ₂ , the enzyme 5-LO translocates to the nuclear membrane in response to elevated intracellular calcium levels. There it interacts with the 5-LO-activating protein (FLAP) and oxidizes AA to its metabolites. The first enzymatic step is the abstraction of a hydrogen atom from C-7 of AA followed by the addition of molecular oxygen to form 5-hydroperoxyeicosatetraenoic acid (5-HpETE). A second enzymatic step is removal of a hydrogen atom from C-10, resulting in the formation of the conjugated triene epoxide leukotriene A ₄ (LTA ₄ ) which is enzymatically converted into either leukotriene B ₄ (LTB ₄ ) by LTA ₄ hydrolase or leukotriene C ₄ (LTC ₄ ) by LTC ₄ synthase. Metabolism of LTC ₄ occurs by sequential peptide cleavage reactions involving a γ-glutamyl transpeptidase that forms leukotriene D ₄ (LTD ₄ ) and a membrane-bound dipeptidase that converts LTD ₄ into leukotriene E ₄ (LTE ₄ ) before ω-oxidation ( Fig. 12.1 ).

An important regulatory step in leukotriene biosynthesis involves the transport of the intermediate LTA ₄ from an immune cell to a nonimmune cell that expresses LTC ₄ synthase. For example, infiltrated neutrophils in the brain contain 5-LO and FLAP and are capable of making LTB ₄ and the intermediate LTA ₄ . The LTA ₄ is released from these cells and taken up by neighboring astrocytes that have LTC ₄ synthase but silenced genes for 5-LO and FLAP. Astrocytes convert LTA ₄ to LTC ₄ and then subsequently in a stepwise fashion to LTD ₄ and LTE ₄ ( ). This mechanism of leukotriene biosynthesis, that requires the interaction of two distinct cell types, is termed transcellular biosynthesis.

The biological activities of LTB ₄ include neutrophil chemotaxis and sequential activation of downstream inflammatory responses. LTC ₄ , LTD ₄ , and LTE ₄ , collectively known as the cysteinyl leukotrienes, are known to mediate vascular permeability, cytokine and chemokine production, and smooth muscle contractility. Leukotrienes exert their biological activities through G protein-coupled receptors (GPCRs); BLT-1 and BLT-2 receptors for LTB ₄ and Cys-LT1, Cys-LT2, and Cys-LT3 receptors for the cysteinyl leukotrienes. The rank order of potency for the Cys-LT1 receptor is LTD ₄ ≫ LTC ₄ > LTE ₄ ( ) and for the Cys-LT2 receptor is LTD ₄ = LTC ₄ > LTE ₄ ( ). The recently discovered Cys-LT3 receptor (also referred to as GPR17) has the highest affinity for LTE ₄ and also binds purinergic ligands ( ). The Cys-LT1 receptor is mostly expressed in lung smooth muscle cells, interstitial lung macrophages, and spleen, and it is known to mediate airway inflammation and asthma. It is the molecular target of the receptor antagonists (montelukast, zafirlukast, and pranlukast) used for treating asthma ( ). The Cys-LT2 receptor is predominately expressed in the heart, brain, and adrenal glands. Until recently, the only pharmacological inhibitor for Cys-LT2 receptor has been the nonselective dual antagonist/partial agonist Bay-u9773. However, a selective Cys-LT2 receptor antagonist, HAMI3379, and a selective Cys-LT2 receptor agonist, N -methyl LTC ₄ (NMLTC ₄ ), have been reported ( ). These drugs will help to identify the precise physiological and pathophysiological roles of the Cys-LT2 receptor. The receptor that binds both LTE ₄ and purinergic ligands (Cys-LT3 or GPR17) is predominantly expressed in the brain and binds two receptor antagonists (AR-C69931MX and MR52179) selectively ( ) ( Table 12.1 ).

Approved Antileukotriene Drugs

The pathophysiological role of leukotrienes in asthma and allergic rhinitis is well documented ( ). Over the past 25 years, considerable effort has focused on identifying and developing drugs that block the actions of leukotrienes to improve asthma management and limit its morbidity. Currently, the leukotriene receptor antagonists (zafirlukast (Accolate), montelukast (Singulair), and pranlukast (Onon)) and the 5-LO inhibitor, zileuton (Zyflo), are approved drugs for treating asthma. The receptor antagonists primarily block the actions of Cys-LT1, only one of the three receptors for leukotrienes and do not block the actions of LTB ₄ . The 5-LO inhibitor zileuton has suboptimal efficacy and an associate risk of liver toxicity. These limitations have led to the ongoing exploration of FLAP inhibitors as alternative therapeutics for asthma as well as other inflammatory disorders.