Abstract: The present application is directed to compounds of Formula (I): compounds of Formula (II): compounds of Formula (III): and compounds of Formula (IV): compositions comprising these compounds and their uses, for example as medicaments and/or diagnostics. Specifically claimed are: (1) compounds containing reactive functional groups (compound of formula I), compounds containing compounds to be linked together (compound of formula II) and compounds having a reactive functional group/a compound to be linked (compound of formula IV), (2) an antibody-drug conjugate (compound of formula III), wherein the antibody is covalently attached by a linker to one or more drugs, (3) pharmaceutical compositions comprising compounds of Formula (II) or Formula (III), (4) treatment/diagnosis of disease comprising compounds of Formula (II) or Formula (III), and (5) methods of preparing an ADC of Formula III.