# Untargeted Urinary Volatilomics Reveals Hexadecanal as a Potential Biomarker for Preeclampsia
## 1 Introduction  
Preeclampsia is a complex multisystem disease, diagnosed by the sudden onset of hypertension (>20 weeks of gestation) and at least one other associated complication, including proteinuria, maternal organ dysfunction or uteroplacental dysfunction [1]. The pathogenesis of PE is complex and involves abnormal placentation, leading to altered uteroplacental perfusion characterized by ischemia-reperfusion injury to the syncytiotrophoblast, which ultimately leads to placental dysfunction. This ischemia leads to oxidative stress (OS) in the placenta, which triggers a cascade of systemic inflammatory responses in the mother characterized by endothelial dysfunction. These processes share similarities with other pregnancy complications, including fetal growth restriction (FGR) and miscarriage [2,3]. Oxidative stress-induced endothelial dysfunction is a key factor in the progression of PE and contributes to maternal symptoms such as hypertension and proteinuria. The imbalance between pro- and anti-angiogenic factors, especially the increased levels of soluble FMS-like tyrosine kinase-1 (sFlt-1) and decreased placental growth factor (PIGF), further exacerbate the condition by impairing vascular remodeling and blood flow in the placenta [4]. Oxidative stress reflects a disturbance in the balance between reactive oxygen species (ROS) and the body's antioxidant defenses. While normal pregnancies have low levels of oxidative stress, PE is associated with a significant reduction in antioxidant capacity, resulting in increased OS [5]. A recent meta-analysis identified several biomarkers of oxidative stress,including ischemia-modified albumin (IMA), uric acid (UA) and malondialdehydedehyde (MDA), which are useful for the early detection of PE [2]. These findings emphasize the critical role of oxidative stress in the pathophysiology of PE and the importance of identifying reliable biomarkers for early diagnosis and intervention. Metabolomics has become a powerful tool for studying the comprehensive metabolic profile of biological systems, providing insights into the biochemical changes associated with disease states [6]. In particular, untargeted metabolomics allows for an unbiased analysis of metabolites without prior assumptions about the metabolic profile of the sample [7]. This approach has contributed to the identification of potential biomarkers for various diseases, including pregnancy-related disorders such as PE [8]. Advanced analytical techniques such as nuclear magnetic resonance (NMR) and mass spectrometry (MS) are frequently used in metabolomics [9]. While NMR offers high reproducibility and minimal sample preparation, MS is characterized by its higher sensitivity and specificity, making it the preferred method for metabolomic studies in complex biological samples [10]. Hexadecanal (palmitaldehyde, C\({}_{16}\)H\({}_{32}\)O) is a long-chain fatty aldehyde that is involved in the biosynthesis of sphingolipids and fatty alcohols, essential components of cell membranes [11]. Hexadecanal has been detected not only in human biological systems but also in a variety of other species, including fungi (Saccharomyces cerevisiae), mice and plants. Hexadecanal is a reactive carbonyl species formed by the lipid peroxidation of polyunsaturated fatty acids (PUFAs) in triacylglycerols, phospholipids and cholesteryl esters [12]. Oxidative stress-induced lipid peroxidation has been linked to numerous pathological conditions, including inflammation, atherosclerosis, neurodegenerative diseases and cancer. Although hexadecanal has been less studied than other lipid peroxidation products, such as 4-hydroxy-2-tonenal (HNE) and malondialdehydedehyde (MDA), its role in oxidative damage makes it a potentially important biomarker for PE [13]. Due to its high sensitivity and reproducibility, gas chromatography-mass spectrometry (GC-MS) is a widely used technique in metabolomics for the analysis of volatile organic compounds (VOCs) [14].