Abstract Oxidative transformation of arachidonic acid by cyclooxygenases, lipoxygenases, and cytochromes P450 gives rise to endogenous lipid mediators that regulate cellular processes in homeostasis and disease. These lipid mediators form an evolving and expanding family referred to as eicosanoids. This application comprises two projects that both are concerned with novel transformations in eicosanoid biochemistry and present important ramifications for the use of non-steroidal anti-inflammatory drugs (NSAIDs) that inhibit their biosynthesis. The first project is centered around the 5-LOX/COX-2 cross-over biosynthetic pathway while the second project is concerned with novel metabolic transformations of eicosanoids, and what these mean for the use of plasma and urinary prostanoid metabolites as markers of drug response. Today, novel members of the eicosanoid family are often discovered in lipidomics approaches by their similarity with known eicosanoids. We have employed an approach based on understanding the structure-function of the biosynthetic pathways, enzymes, and substrates that allowed us to make predictions of novel transformations. This has led to the discovery of the 5-LOX/COX-2 cross-over biosynthetic pathway forming hemiketal eicosanoids (HKE2 and HKD2) and 5- hydroxy-prostaglandins, the identification of 15R-prostaglandins formed by aspirin-acetylated COX-2, and the identification of the Baeyer-Villiger oxidative pathway underlying the metabolism of PGD2 to 11-dehydro- thromboxanes. Underscoring the relevance of our approach is the fact that the novel eicosanoids we have shown to be formed in vitro and in vivo have not been identified in lipidomics analyses, likely due to their unusual properties that make them difficult to detect in standard analyses. We have designed analytical procedures that allow to detect and quantify the novel eicosanoids in vitro and in vivo. We plan to continue the identification of novel eicosanoids and to establish their cellular targets and biological role in homeostasis and in models of inflammatory disease. Our ongoing investigation into the biological effects of the cross-over eicosanoids has identified the receptor tyrosine kinase (RTK) VEGFR2 as a target for the pro-angiogenic activity of HKE2, as well as other RTK and an unknown target that mediates inhibition of platelet activation that are to be further analyzed. For the 5-hydroxy-prostaglandins we plan to employ screening approaches as well as targeted testing of prostanoid receptors in order to identify their cellular targets and determine biological effects. We aim to identify compounds that can be used to manipulate biosynthesis of 5-LOX/COX-2 cross- over eicosanoids independent from the formation of prostaglandins and leukotrienes. We will continue to characterize novel metabolic pathways of prostanoids and establish the relevance of these pathways in vivo. Identification and characterization of novel eicosanoids, their biological effects, and...