There has been much excitement over antibody-drug conjugates (ADCs) in recent months. ADCs are currently showing great results when used as a type of targeted cancer therapy composed of a tumor-specific monoclonal antibody (mAb) linked via a chemical bond or “linker” to a cytotoxic agent or “payload” with a validated mechanism of action such as a highly potent microtubule disrupting agent or a DNA modifying agent. SAFC is one of the few companies with in-depth knowledge and capabilities of providing the linkers to create the ADC treatments.
Here, Grant Boldt, Ph.D, Director of Business Development at SAFC, provides an overview of what the technology is and why it is gaining popularity for use in cancer treatments.
While single-agent cytotoxic chemotherapeutic drugs (designed to eliminate fast growing cancer cells) may be effective, they have also proven to be harmful to healthy cells in patients taking the treatments. Antibody-drug conjugates (ADCs) address this by utilizing readily available, biologically compatible, and minimally immunogenic tumor-seeking monoclonal antibodies (mAb) that are able to distinguish between cancer cells and normal cells, while, at the same time, increasing the benefit of the cytotoxic agent. ADCs offer a more effective way to deliver the cytotoxic anticancer agent to the targeted cancer cell, so the antineoplastic or chemotherapeutic agent incorporated may be up to 1,000-fold more potent than currently used in antineoplastic drugs. This allows normally intolerable doses of cytotoxic chemotherapeutic drugs to be used in anticancer therapies.
Though mAbs may circulate in the body for extended periods of time, an unconjugated mAb does generally have a longer circulating half-life than an ADC. Still, the circulating half life of ADCs is measured in days which allow the drug more time to build up in cancer cells, thus further increasing the potential benefit of ADCs. Because of their favorable activity and increased tolerability, ADCs are now considered the next big advancement in anticancer drug development, offering a paradigm shift in cancer therapy.
The principle idea of ADCs dates back more than 100 years when German physician and scientist, Paul Ehrlich, a pioneer in the fields of chemotherapy, hematology and immunology conceptualized the idea of a medicinal compound that could selectively target and kill disease-causing cells. His idea of attaching a toxin to antibodies for use against human disease connected medicine and biology with chemistry for the first time, and inspired generations of scientists eager to discover a “magic bullet” for human diseases.
The development of targeted therapy using mAbs in the 1980s and 1990 revolutionized cancer therapy. As the “next big thing,” ADCs realize Ehrlich’s original idea to some extent. When the mAb finds and binds to the intended target, the entire ADC/antigen complex (mAb, linker and the cytotoxic drug) is internalized via a process called receptor-mediated endocytosis. The ADC then elicits apoptosis, or specific tumor cell killing, and targeted cancer cells are killed by the selectively delivered cytotoxic anticancer drug is delivered to the cancerous tissues, while normal tissues is spared from chemotherapeutic damage,
Successful ADC development depends on a number of factors, including the selection of a proper target, the stability of the linker as well as the optimization of the entire complex of mAb, linker and cytotoxic agent. While antibodies have the ability to precisely target cancer cells, the principle requires the uptake of the entire ADC complex.
The complexity of ADCs is, in part, caused by the fact that each element of an ADC has its own characteristics and constraints and, when linked, influences each other. The expression profiles of cancer tissue versus normal, noncancerous tissues, the potency of the cytotoxic agent, the chemical linker used to attach the drug and place it on the monoclonal antibody, the pharmacokinetic, and stability profiles all need to be taken into account when designing ADCs. The stable linkers must be powerful enough to keep the cytotoxic agent from detaching from the mAb before being delivered to the targeted cancer cell so that it would not render it ineffective or create unwanted toxicity.
Linker Chemistry is Crucial
Many of the clinical failures of ADCs in the past decades, including issues with immunogenicity, lack of potency, premature release, and insufficient target selectivity, are thought to be the result of inadequate or faulty linker technology. In most cases, these linkers released their toxic payload prematurely which caused severe adverse events similar to those experienced with the use of non-specific chemotherapeutic agents. A stable link between the mAb and cytotoxic agent is a crucial aspect of an ADC. Linkers are based on chemical motifs including disulfides, hydrazones or peptides (cleavable), or thioethers (non-cleavable) and control the distribution and delivery of the cytotoxic agent to the target cell.
Cleavable and non-cleavable types of linkers are included in the most promising ADCs. Both types of technologies have been proven to be safe in preclinical and clinical trials. Brentuximab vedotin (SGN-35; Adcetris™, Seattle Genetics), for example, includes an enzyme-sensitive cleavable linker that delivers the potent and highly toxic anti-microtubule agent Monomethyl Auristatin E (MMAE), a synthetic antineoplastic agent, to human specific CD30-positive malignant cells. Because of its high toxicity MMAE, which inhibits cell division by blocking the polymerization of tubulin, cannot be used as a single-agent chemotherapeutic drug. However, the combination of MMAE linked to an anti-CD30 monoclonal antibody (cAC10, a cell membrane protein of the tumor necrosis factor[TNF]-receptor) proved to be stable in extracellular fluid, cleavable by cathepsin and safe for therapy. The drug, approved by the FDA, is expected to be an effective treatment for relapsed or refractory systemic large-cell lymphoma (ALCL), an aggressive subtype of T-cell lymphoma characterized by the uniform expression of CD30. Another ADC, trastuzumab emtansine (T-DM1, Genentech/Roche/ImmunoGen), which has been studied in patients with HER2-positive cancers, is a combination of the microtubule-formation inhibitor mertansine (DM-1), a derivative of the maytansine, and antibody trastuzumab (Herceptin®/ Genentech/Roche) attached by a stable, non-cleavable linker. Trastuzumab emtansine, which has been studied in patients with HER2-positive cancers, is designed to target and inhibit HER2 signaling.
Better, more stable linkers
The availability of better and more stable linkers has changed the function of the chemical bond. The type of linker, cleavable or non-cleavable, lends specific properties to the cytotoxic drug. A non-cleavable linker, for example, keeps the drug within the cell. In this case, the entire antibody, linker and cytotoxic agent enter the targeted cancer cell where the antibody is degraded to the level of an amino acid. The resulting complex – amino acid, linker and cytotoxic agent – now becomes the active drug. In contrast, cleavable linkers are catalyzed by enzymes in the cancer cell where it releases the cytotoxic agent. The difference is that the cytotoxic payload delivered via a cleavable linker can escape from the targeted cell and, in a process called “bystander killing,” attack neighboring cancer cells.
Another type of cleavable linker, currently in development, adds an extra molecule between the cytotoxic drug and the cleavage site. This allows researchers involved in the development of ADCs with more flexibility without worrying about changing cleavage kinetics. Future direction in the development of ADCs may include the development of site-specific conjugation (TDCs) to further improve stability and therapeutic index and α emitting immunoconjugates and antibody-conjugated nanoparticles.
While the concept seems simple, translating the efficacy of ADCs into a safe and effective drug has proven to be a complicated and challenging process requiring both biologics (i.e. mammalian cell culture) and small molecule manufacturing capabilities, including synthetic chemistry, necessary for the linker and cytotoxic agent. Depending on the actual chemistry of linkage, the sites of attachment, the selected cancer target, and the cytotoxic agent, significant chemistry, manufacturing, and control issues become involved in their development. One of the complicating factors in the production of ADCs is that the payload is a highly potent cytotoxic agent. Manufacturing therefore requires a high containment area with occupational exposure levels in the 10-9 gram/M3range.
Today, the two primary linkage technologies used in ADC technologies are proprietary to the companies that developed them. SAFC is one of the few companies providing clients with most parts of the production process. SAFC is also one of the few CMOs with the ability to participate in the linker technology. However, as biopharmaceutical companies around the world are gearing up in the development of more advanced ADCs with new linker technologies, this limitation is expected to change in just a few years.