Quantitative GC–MS/MS in the clinical analysis of eicosanoids: A critical review and discussion, a 40-years historical retrospect, and possible implications for LC-MS/MS

Mass spectrometry was a protagonist in the discovery of prostaglandins, thromboxane, leukotrienes and other arachidonic acid-derived molecules, collectively known as the eicosanoids. Mass spectrometry has played a significant role in exploration of their metabolic pathways in humans and animals, in health and disease, and in pharmacotherapy. Clinical researchers in the United States of America and in Europe, in close cooperation with chemist analysts, were the pioneers in the application of gas chromatography-mass spectrometry (GC-MS) and gas chromatography-tandem mass spectrometry (GC-MS/MS) to quantitate eicosanoids and index metabolites in plasma, serum and urine samples from clinical trials by using stable-isotope labeled analogs as internal standards. In the present article, the application of the stable-isotope dilution GC-MS/MS methodology in the quantitative clinical analysis is reviewed. The focus is on prostaglandins, thromboxane, leukotrienes, and their so-called index metabolites for renal and whole-body synthesis of certain eicosanoids such as PGE₂ and its major urinary metabolite (PGE-MUM), respectively. Nowadays, LC-MS/MS, which evolved later than GC-MS/MS, is increasingly used in numerous areas of research, including the eicosanoids in clinical studies. The present work critically discusses the current practice of LC-MS/MS users in the quantitative analysis of eicosanoids in biological samples. While the LC-MS/MS technology offers rapidity and high-throughput analysis, especially due to the renunciation of time-consuming analytical derivatization steps that are required in GC-MS/MS, LC-MS/MS seems to lack sufficient analytical sensitivity, i.e., lower limit of quantitation, for many eicosanoids such as thromboxane B₂ and leukotriene B₄. Reported data on basal concentrations of certain eicosanoids in plasma and urine samples from healthy humans as determined by LC-MS/MS are several orders of magnitude higher than originally reported by pioneering eicosanoid researchers, who developed, validated and used sophisticated, tailored GC-MS- and GC-MS/MS-based analytical methods for individual eicosanoids. Modern eicosanoids researchers would greatly benefit from the milestones and signposts set previously eicosanoids researchers from the very start. A key milestone and signpost is the concentration of primary eicosanoids and their metabolites in plasma, serum and urine samples of healthy humans. Issues for consideration in the GC-MS/MS and LC-MS/MS analysis of eicosanoids are discussed.