Desmethyl Erlotinib (OSI-420 free base) is an active metabolite of Erlotinib. Erlotinib is a potent EGFR tyrosin kinase inhibitor. Desmethyl Erlotinib is a click chemistry reagent, it contains an Alkyne group and can undergo copper-catalyzed azide-alkyne cycloaddition (CuAAc) with molecules containing Azide groups.