In recent years, a great deal of research has been directed toward the use of antibodies to deliver cell proliferation inhibitors and cytotoxic agents to specific cells that are targeted for elimination, by forming antibody-drug conjugates (ADCs).  The ADCs contain an antibody selected for its ability to bind to a cell targeted for therapeutic intervention, linked to a drug selected for its cytostatic or cytotoxic activity.  Binding of the antibody to the targeted cell delivers the drug to the site where its therapeutic effect is needed. 
Many antibodies that recognize and selectively bind to targeted cells, like cancer cells, have been disclosed for use in ADCs, and many methods for attaching payload (drug) compounds such as cytotoxins to antibodies have also been described.  In spite of the extensive work on ADCs, though, only a few classes of cell proliferation inhibitors have been used extensively as ADC payloads.  Even though the first ADC approved for use in humans in the U.S. was launched in 2000 (and later withdrawn from the market), a decade later only a few chemical classes of drug compounds (maytansinoids, auristatins, calicheamycins and duocarmycins) had reached clinical trials as payloads for ADCs.  Antibody-Drug Conjugates: the Next Generation of Moving Parts, A. Lash, Start-Up, December 2011, 1-6.  This suggests how difficult it is to identify a suitable class of drug compounds that make effective ADC payloads.  Given the widely acknowledged value of ADCs as therapeutics, particularly for treating cancer, there thus remains a need for novel cell proliferation inhibitors for use as payloads in ADCs.