An immunoconjugate of a humanized murine monoclonal antibody (huN-901) and DMI, a semi-synthetic derivative of the plant-derived ansa macrolide maytansine. The antibody moiety of lorvotuzumab mertansine selectively attaches to CD56 antigen, a neural cell adhesion molecule (NCAM)) expressed on the surface of cells of small cell lung cancer (SCLC) and other neuroendocrine (NE) tumors. Thus, the DMI conjugate is targeted specifically to CD56-expressing tumor cells, where it inhibits tubulin polymerization and assembly, resulting in inhibition of mitosis and cell cycle arrest in the S phase. Check foractive clinical trials using this agent.