It provides
linker compounds, or stereoisomers or salts thereof, including (a) a
linker unit having from 1 to 4 attachment sites for a
drug unit and having one of the following structures (i) or (ii): (b) at least one polar group containing a
polymer unit, optionally a
sugar unit, optionally a carboxyl unit, and combinations thereof; and (c) optionally a stretcher group having an
attachment site for a targeting group, wherein a—is an
attachment site to an
enzyme-cleavable group; (3—is an
attachment site to the at least one polar group; 6—is H, an attachment site to at least one of the
drug unit, or an attachment site to a linking group attached to the at least one of the
drug unit. It also provides drug-
linker compounds and conjugates formed from linker compounds, as well as related pharmaceutical compositions and methods.