(S)-6-Amino-2-(((S)-1-carboxy-3-phenylpropyl)amino)hexanoic acid is a stereoisomer of lisinopril. It is a known impurity and degradation product of the widely prescribed angiotensin-converting enzyme (ACE) inhibitor, lisinopril. This compound is structurally characterized as a dipeptide analog containing lysine and phenylalanine moieties. This substance is primarily used as a reference standard in pharmaceutical quality control. Its main application is for the identification, quantification, and monitoring of this specific impurity during the manufacturing process and stability studies of lisinopril drug substance and finished dosage forms. It is critical for ensuring the purity, safety, and efficacy of the final cardiovascular drug product. As a certified reference material, it is essential for analytical method development and validation in compliance with regulatory guidelines (e.g., ICH). Handling requires standard laboratory precautions for fine chemicals.