Used primarily as a reference standard in pharmaceutical quality control to ensure the purity and consistency of pemetrexed active pharmaceutical ingredient (API) during manufacturing. It helps in validating analytical methods such as HPLC and LC-MS for detecting and quantifying impurities in pemetrexed formulations. Monitoring this impurity is critical to meet regulatory requirements and maintain drug safety, as excessive levels can impact efficacy and patient tolerance. It is also used in stability studies to assess degradation pathways of pemetrexed under various storage conditions.