Methotrexate EP Impurity C Catalogue G101333 CAS 2410-93-7 M.W 455.42 M.F C20H21N7O6 Specifications FAQs (S)-2-(4-(((2-amino-4-oxo-1,4-dihydropteridin-6-yl)methyl)(methyl)amino)benzamido)pentanedioic acid