Waldemar Placek

1. Introduction Isotretinoin (ISO), a highly effective oral prescription synthetic retinoid derived from vitamin A, is the only approved medication to treat severe, resistant, nodular, phlegmonosa, conglobata, and unresponsive to systemic treatment with tetracyclines and macrolides acne. ISO is metabolized in the liver by the cytochrome P-450 (CYP) microsomal enzyme system to three main metabolites: 4-oxo-isotretinoin, retinoic acid (tretinoin), and 4-oxo-retinoic acid (4-oxo-tretinoin) [1, 2].