Menadione 500MG. Menadione, An IDO inhibitor, CAS Number: 58-27-5, Molecular Formula: C11H8O2, Molecular Weight: 172. 18, Purity: greater than or equal to 98% by HPLC, Appearance: Off-white to yellow solid, Synonym: 2-Methyl-1, 4-naphthoquinone, Vitamin K3, Size: 500MG