Within the thrombus there is heterogeneity

Within the thrombus, there is heterogeneity in the activation of ski or skii based on the concentration gradients of soluble agonists emanating from the site of vascular injury, which has been excellently reviewed by Tomaiuolo et al.[1]. As part of the coagulation cascade, active thrombin is generated at the site of vascular injury, which is essential for the conversion of fibrinogen to fibrin and the activation of platelets through protease-activated receptors (PARs). Stimulation of platelets by thrombin results in the formation of a dense core of activated platelets that generate or release several soluble platelet agonists including ADP, a purinergic receptor agonist (P2Y1 and P2Y12), and thromboxane (TX)A2, a thromboxane receptor agonist [thromboxane prostanoid receptor alpha isoform (TPα)] generated by cyclo-oxygenase (COX). Surrounding the core of the thrombus is a shell of loosely packed platelets whose formation is driven chiefly by the secondary mediators, ADP and TXA2. In order to stay firmly adherent in flowing vessel, the thrombus must contract and stablize. The thrombus is stablized by the generation of a fibrin mesh and persistent αIIbβ3 signaling resulting in clot retraction.
Current Antiplatelet Therapy Inhibitors to most of the major receptors involved in thrombus formation including adhesion receptors (GPIb, GPVI, and αIIbβ3) and G protein-coupled receptors (GPCRs; TPα, P2Y12, P2Y1, PAR1, and PAR4), have been developed as antiplatelet therapies that are either currently approved or under investigation in ongoing clinical trials, comprehensively reviewed by Metharom et al. and Yeung et al.6, 7. The most commonly used prophylactic antiplatelet therapy, however, is aspirin, which inhibits platelet function by directly acetylating COX-1. Inhibition of COX-1 results in full inhibition of the formation of the prostaglandins in the platelet such as TXA2; a known ligand for activation of the platelet through the GPCR TPα receptor on the platelet surface. The success of aspirin has led investigators to focus on targeting the formation of other prothrombotic oxylipins. Oxylipins, which are primarily classified as potent bioactive lipid mediators with short half-lives, are synthesized de novo from polyunsaturated fatty acids (PUFAs) by oxygenases [8]. Platelets are known to express two oxygenases, the well-studied COX-1 and 12-LOX, whose potential as an antiplatelet target is less well understood. While aspirin has been shown to be effective in reducing the risk for platelet activation and occlusive thrombosis, several studies have shown that a significant percentage of cardiovascular patients are resistant to aspirin treatment. The causes of aspirin resistance are controversial and have not been fully delineated. However, alternative therapy such as 12-LOX inhibitors may be a viable option for antiplatelet treatment in these individuals.
 12-LOX Regulation of Platelet Function and Thrombosis LOXs are a family of nonheme iron-containing enzymes that catalyze the stereoselective dioxygenation of PUFAs containing a cis,cis-1,4-pentadiene moiety [9]. While LOX isozymes have broad substrate specificity, they are classified by the carbon of arachidonic acid (AA; C20:Δ4, n−6) they oxygenate 9, 10, 11. For example, 12-LOX oxygenates position C-12 of AA to form 12(S)-hydroperoxyicosa-5,8,10,14-tetraenoic acid (12-HpETE), which is quickly reduced by glutathione peroxidase in the cell to form 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) 12, 13, 14. In instances where multiple LOX isoforms are expressed within the same organism and oxygenate the same carbon of AA, they are further characterized by their stereospecificity (S or R) and their primary tissue or cell expression, as is the case with platelet-type 12-(S)-LOX, (which is encoded by the ALOX12 gene and referred to in this review as 12-LOX) and epithelial 12-(R)-LOX (ALOX12B) 9, 15. Humans express six functional LOX isoforms (ALOXE3, ALOX5, ALOX12, ALOX12B, ALOX15, and ALOX15B) that share a general structure featuring two domains: an N-terminal PLAT (polycystin-1, lipoxygenase, α toxin) domain, which is important for membrane localization and substrate acquisition, and a C-terminal catalytic domain 9, 15, 16.