Factors Involved in the Design of Cytotoxic Payloads for Antibody–Drug Conjugates

Selective targeting of cancer can be accomplished by the use of monoclonal antibodies (mAbs) that bind to tumor-associated antigens expressed preferentially on the surface of cancer cells. Several of these non-derivatized antibodies, or naked antibodies, have been approved for the treatment of various cancer types. However, compelling single agent activity has only been demonstrated in the treatment of hematological malignancies. For example, the anti-CD20 mAb rituximab (Rituxan) is widely used in the treatment of B-cell lymphomas. Also, the anti-CD52 antibody alemtuzumab (Campath) is used in the treatment of some leukemias. However, antibodies targeting solid tumors, such as trastuzumab (Herceptin) for breast cancer, anti-EGF receptor antibodies cetuximab (Erbitux) and panitumumab (Vectibix) for head and neck and colon cancers, and the anti-angiogenic agent bevacizumab (Avastin), display only modest antitumor activity as single agents. Thus, these antibodies are most often used in combination with conventional anticancer drugs, thus retaining the high systemic toxicity of standard chemotherapy. In an alternative approach, the efficacy of a naked antibody can be greatly enhanced by attachment of a cytotoxic molecule to give an antibody–drug conjugate (ADC). The selection of the optimal cytotoxic effector, linker, and antibody component of the ADC can be facilitated by an understanding of the environments to which the ADC will be exposed to once it is administered to a patient and also through the knowledge gained from the development of previous ADCs. Most of the current ADCs under development are for the treatment of cancer; however, ADCs are also being investigated for the treatment of autoimmune diseases [1]. This chapter will deal with the selection of an appropriate cytotoxic effector for the preparation of anticancer ADCs, but much of the discussion is relevant to ADCs for the treatment of autoimmune diseases as well.