Non-steroidal anti-inflammatory drugs (NSAIDs) are an important class of medications; however, they have some drawbacks. We are developing a new NSAID with pronounced anti-inflammatory and analgesic activities and a very low toxicity—(Z)-3-(2-oxo-2-(p-tolyl)ethylidene)piperazin-2-one (piron). In this work, we describe the synthesis of the main metabolite of piron—(Z)-4-(2-(3-oxopiperazin-2-ylidene)acetyl)benzoic acid. The anti-inflammatory activity of the synthesized metabolite was determined in vivo.