An immunoconjugate consisting of the humanized monoclonal antibody M9346A against folate receptor 1 (FOLR1) conjugated, via the disulfide-containing cleavable linker sulfo-SPDB, to the cytotoxic maytansinoid DM4, with potential antineoplastic activity. The anti-FOLR1 monoclonal antibody moiety of mirvetuximab soravtansine targets and binds to the cell surface antigen FOLR1. After antibody-antigen interaction and internalization, the immunoconjugate releases DM4, which binds to tubulin and disrupts microtubule assembly/disassembly dynamics, thereby inhibiting cell division and cell growth of FOLR1-expressing tumor cells. FOLR1, a member of the folate receptor family is overexpressed on a variety of epithelial-derived cancer cells. The sulfo-SPDB linker prevents cleavage in the bloodstream and may improve this agent's efficacy in multidrug resistant tumor cells.