The present disclosure relates to a therapeutic agent of the formula: Z—C(═O)—(CH2)n-Φ-S—S—(CRR′)m-(CH2)p-C(═O)—NH—(CH2)q-NH—Y[NH—(CH2)r-X-T-W][NH—(CH2—CH—O)t(CH2)s-NH—V]  Formula I or a pharmaceutically acceptable salt thereof, useful for treating tumors, including cancers. Where the compound of Formula I also contains a radionuclide or an imaging agent or both, the compound of formula I is a theranostic agent useful for treating and diagnosing tumors, including cancers.