Pirfenidone-d5 is intended for use as an internal standard for the quantification of pirfenidone by GC- or LC-MS. Pirfenidone is an orally bioavailable pyridone derivative with antifibrotic, anti-inflammatory, and antioxidant activities. It inhibits TGF-β1-stimulated increases in collagen type I, fibronectin, and Hsp47 expression in A549 lung cancer cells in a concentration-dependent manner. Pirfenidone (300 mg/kg per day) inhibits fibrosis and inhibits increases in collagen content, fibrocyte pool size, and the levels of chemokines CCL2 and CCL12 in lung in a mouse model of pulmonary fibrosis induced by bleomycin . In a mouse model of non-alcoholic steatohepatitis (NASH), pirfenidone inhibits fibrosis and increases in hepatocyte apoptosis, lobular inflammation, and hepatic expression of the fibrosis-related genes Col1a1, Timp1, Tgfb1, Pdgfb, and Fgf2 when administered at a dose of 280 mg/kg but has no effect on steatosis. Pirfenidone also inhibits NADPH-dependent lipid peroxidation in sheep liver microsomes and scavenges hydroxyl radicals (IC50s = ~6 and ~2.5 mM, respectively). Formulations containing pirfenidone have been used in the treatment of idiopathic pulmonary fibrosis.