The concept of arming antibodies by conjugation to protein toxins dates back to 1970, and was followed a few years later by antibody conjugates with cytotoxic drugs. Antibody therapeutics have come of age in the intervening decades with the advent of hybridoma technology to develop murine monoclonal antibodies (MAb), chimerization and humanization to address the shortcomings of murine MAb as therapeutics, and more recently with direct routes to human antibodies using phage display or transgenic mice.
Clinically useful, unconjugated monoclonal antibodies (mAbs) selectively recognize antigens that are preferentially expressed on or near tumor cells and exert their cytotoxic effects through mechanisms such as cell signaling, antibody-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis, and complement-dependent cytotoxicity. However, the majority of these mAbs are used in combination with chemotherapy, and many others have demonstrated insufficient clinical activity. Therefore, significant effort has been devoted to empowering mAbs through various modifications. One approach by which the activities of these mAbs have been enhanced is through conjugation with cytotoxic drugs, generating antibody-drug conjugates (ADCs) capable of antigen-specific delivery of highly potent cytotoxic drugs to tumor cells.
ADCs comprise an antibody, usually in IgG format, conjugated to a cytotoxic drug via a chemical linker. The therapeutic concept of ADCs is to use an antibody as a vehicle to deliver a cytotoxic drug to a tumor cell by means of binding to a target cell surface antigen. ADCs are prodrugs requiring drug release for activation, commonly after ADC internalization into the target cell. Numerous preclinical efficacy studies show that ADCs have significant potential for enhancing the antitumor activity of “naked” antibodies and reducing the systemic toxicity of the conjugated drugs.
In order to improve efficacy without compromising safety, an ADC must preferentially deliver the cytotoxic payload to tumor cells expressing the target antigen rather than to healthy tissue. This is accomplished by exploiting the specificity of a mAb targeting an antigen that is highly expressed on the surface of malignant cells. After binding the target antigen, the ADC-antigen complex is typically internalized and transported to intracellular organelles where release of the attached drug can occur. Upon release, the cytotoxic drug can interfere with various cellular mechanisms, leading to cell death. The development of ADCs has relied on several key areas of research including the choice of an appropriate antigen target, development of novel highly potent cytotoxic drugs, development of stable linkers that can release the cytotoxic payload upon internalization, and conjugation technology.
Select an Appropriate Target
Target selection from a candidate pool is a critical first step in generating ADCs and the most important contributor to the antitumor activity and tolerability of an ADC. As a general rule, cancer cells do not display novel immunogenic antigens because such behavior would lead to rapid clearance by the immune system. However, cancer cells occasionally express normal cell surface antigens at levels that are distinguishable from their healthy normal cellular counterparts. Ideally, the target antigen should be expressed with substantially higher copy numbers on tumor cells, as the efficacy and tolerability of an ADC appear to depend, in part, on its relative expression on normal tissue compared to tumor cells.
The linker that connects the cytotoxic drug to the mAb is a key determinant of ADC activity, including the selectivity, pharmacokinetics, therapeutic index, and overall success of the ADC. These linkers covalently couple the cytotoxic drug to the antibody, producing an ADC that should be relatively stable in circulation.
Connection to the antibody
The specific method of attachment of the cytotoxic drug and linker to the mAb has been shown to play a key role in ADC activity and tolerability. The two most common naturally occurring amino acids that are used to attach the linker drug to the antibody are cysteines and lysines. The most common methods of antibody-drug conjugation are alkylation of reduced interchain disulfides, acylation of lysine residues, and alkylation of genetically engineered cysteine residues.
Carter P J, Senter P D. Antibody-drug conjugates for cancer therapy[J]. The Cancer Journal, 2008, 14(3): 154-169.
Sievers E L, Senter P D. Antibody-drug conjugates in cancer therapy[J]. Annual review of medicine, 2013, 64: 15-29.
Flygare J A, Pillow T H, Aristoff P. Antibody-drug conjugates for the treatment of cancer[J]. Chemical biology & drug design, 2013, 81(1): 113-121.