Compounds comprising two human GlxI inhibitors covalently linked via a chemical linker are provided, wherein each of said two human GlxI inhibitors, which may be the same or different, is an S-substituted glutathione or an S-substituted glutathione prodrug, wherein said GlxI inhibitors each have a γ-glutamyl amino group, wherein said chemical linker is covalently bound to each GlxI inhibitor via said γ-glutamyl amino group, and wherein said chemical linker has a length of at least 50 Angstroms. Monovalent irreversible inactivators of human GlxI are also provided. An antineoplastic composition is provided, which comprises a compound described above and a pharmaceutically acceptable carrier. In vitro and in vivo methods of preventing or inhibiting the growth and proliferation of neoplastic cells and/or tumors are also provided.