Oxytocin is a peptide hormone consisting of 9 amino acids, with a mass of 1007 Da. It is synthesised in the hypothalamus and secreted from the posterior pituitary. It has well known roles in lactation and uterine contraction, however it is also thought to act within the brain to influence complex social behaviours such as bonding, empathy, and trust. Recently there has been increasing interest in the potential role of oxytocin in the pathophysiology of depression and autism amongst other conditions.
Oxytocin measurement in human plasma is challenging due to low circulating concentrations. Immunoassay kits for the measurement of oxytocin are readily available, however questions remain regarding their specificity and sensitivity. We aimed to develop an LC-MS/MS method for the measurement of plasma oxytocin that would be sensitive enough to detect low ng/L concentrations of oxytocin while still being suited to routine clinical use.
An LC-MS/MS method was established using a Waters Acquity UPLC system and Xevo TQS mass spectrometer, with a BEH130 C18 column. The method was linear to at least 5000 ng/L and had a short run time of 2.1 minutes. Two methods for sample clean up prior to analysis were tested. Using solid-phase extraction we were able to detect oxytocin concentrations of 50 ng/L in spiked human plasma. Improved sensitivity was provided by using antibody-coupled magnetic beads to extract oxytocin from plasma; 25 ng/L could be detected. For comparison, samples were also analysed with a Waters ionKey MS system with a flow rate of 2.5 μL/min, however this did not further improve sensitivity.
In summary we have developed a sensitive LC-MS/MS method for analysis of oxytocin which would be suitable for clinical use. Further optimisation of the extraction protocol may enhance sensitivity. Additional testing would also be required to establish reference ranges and to determine stability of oxytocin post-sample collection.