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Creative BioLabs is a contract research organization based in New York that specializes in antibody discovery and engineering. They are interested in the development of antibody drug conjugates (ADCs), which are drugs that target disease-causing agents without harming the body. ADCs consist of a target, antibody, cytotoxic warhead, and linker. The goal of using ADCs is to effectively target diseases while minimizing damage to normal tissue. However, there are challenges in developing ADCs, such as balancing efficacy and safety. Scientists are exploring bispecific antibody technology to improve the therapeutic index of ADCs. Bispecific antibodies have two specific antigen binding sites and can block two different targets or retarget therapeutic effectors. There are more than 100 different bispecific formats available, and selecting the right format is crucial for developing bispecific ADCs. Strategies for bispecific ADCs include maximizing internalization and trafficking to lysosomes to Welcome to Creative BioLabs. 100% of the effort, 100% of the service. As a dynamic contract research organization, we are based in New York and serve the whole world. Our seasoned scientists are skilled in antibody discovery, antibody engineering, and biomanufacturing solutions. We are glad to have you here with us today to share your knowledge on the development of ADCs. Can you first give us a brief introduction of ADCs? Thanks for inviting me. The conceptual framework for antibody drug conjugates came up at the same time with the discovery of antibodies, with some scientists proposing in the early 1900s the concept of a magic bullet, an ideal therapeutic that would specifically target a disease-causing agent without causing harm to the body. There are four key elements comprising an antibody drug conjugate, including the target, antibody, cytotoxic warhead, and a linker connecting the warhead to the antibody. To develop an ADC successfully, we need to consider of all of these parameters. There has been a great deal of progress made in understanding the relationship between these various components today. So what therapeutic effects can we expect to get with ADC, or in other words, what do you think is the goal when we use ADC as a treatment reagent? Well, when we use ADC as a treatment reagent, the goal is to produce drugs that have a broad therapeutic index by effectively targeting the disease while causing minimal damage to the normal tissue. It is also the overarching goal of biopharmaceutical development today. What would you consider as challenges in the development of ADC? There are definitely challenges. Although the concept of ADC is simple, and over the past few years, numerous improvements have been made in the chemical properties of the warheads, the linkers, and the means of conjugation to the antibody. Achieving the ideal combination of properties has proven challenging, as reflected by the limited number of ADCs that have demonstrated success in the clinic to date. Despite the successes to date, and the prospect of new ADCs reaching patients in the coming years, many challenges remain, and there is substantial room for improvement, most notably in improving the therapeutic index. But ultimately, the most challenging part I could think of in developing any ADC is balancing its efficacy and safety. Right, it definitely sounds very challenging. At present, are there any technologies that can balance the efficacy and safety of ADC in order to improve the therapeutic index of ADC? Yes, recently many scientists focus on ways to capitalize on bispecific antibody technology to improve the therapeutic index of ADC, in pursuit of the magic bullet ideal. How exciting! Can you tell us what is the bispecific antibody? Bispecific antibody is an artificial antibody, containing two specific antigen binding sites. The proof of concept for bispecific antibodies was first demonstrated more than half a century ago, initially by chemical conjugation of two antibodies to form bispecific FAB2 molecules, and later by fusing two different hybridoma cells, enabled by the hybridoma technology in 1975. Advances in protein engineering technologies have enabled the generation of recombinant bispecific antibodies, with defined architecture and the desired biochemical, functional, and pharmacological properties. The ability to select among different bispecific formats to tailor these properties for the specific application, provides opportunities to extend the potential of therapeutic antibodies. So has any group used bispecific antibody to treat cancer? Yes, of course. To date, the majority of the bispecific antibody approaches to treat cancer have fallen into one of two broad functional categories. Firstly, simultaneous blockade of two cancer-associated targets, such as oncogenic receptors, growth factor ligands, or cytokines. And second, redirection of a therapeutic effector, such as engaging immune effector cells or molecules, pre-targeting a therapeutic toxin or radionuclidy. Why is the mechanism when bispecific antibodies work in vivo? Oh, there are a number of mechanisms. Bispecific antibodies can simultaneously block two different targets or mediators, that have a primary role in the disease pathogenesis. They can retarget to mediate effector functions, such as antibody-dependent cell-mediated cytotoxicity. They are able to avoid or delay the development of resistance. They can also induce more potent anti-proliferative effects. And finally, activating cytotoxic T and NK cells to induce tumor lysis, such as bispecific T cell engagers and bispecific killer cell engagers. I know there are many bispecific antibody formats. Do you know how many are actually there? There are now more than 100 different bispecific formats, enabling researchers to select the ideal parameters including size, half-life, stability, flexibility, orientation, and developability to achieve the desired therapeutic outcome. But basically, bispecific antibody formats can be classified into five distinct structural groups, monovalent bispecific IgG, appended IgG, BS-AB fragments, bispecific fusion proteins, and BS-ABs generated by chemical conjugations. That's very interesting. Now that I have a better understanding of the bispecific antibody, just curious how it works in ADC. Yes, good question. This ability of bispecific antibodies to simultaneously engage two targets presents some creative possibilities to address both the efficacy and safety aspects of ADC. Several strategies currently in development employ bispecific targeting to enhance ADC internalization and lysosomal delivery with improving efficacy as the goal. Another emerging area of research seeks to use the dual targeting capability of bispecific antibodies to improve selectivity toward the tumor relative to normal tissue, an approach that could impact both the safety and efficacy of ADCs. Wow, that is good to hear. You mentioned before there are more than 100 different bispecific formats. Is it important to select the bispecific formats and binding modalities for ADC? Yes, it is very important. Identifying the right bispecific antibody formats with the desired functionality is critical to develop bispecific antibody drug conjugates. And this is also a challenge, since there are so many different bispecific formats to choose from. Typically, we choose one to match the proposed mechanisms of action and the specific clinical application. Ideally, several alternative bispecific formats would be constructed, and the final lead candidate would be chosen after in vitro and in vivo functional characterization. Bipartopic antibodies, these are subset of bispecific antibodies, have demonstrated the superior ability to promote receptor clustering for improved receptor internalization, lysosomal trafficking, and receptor downregulation, hence lead to improved drug potency. Capitalizing on disability of bipartopic antibodies to increase lysosomal trafficking is a promising strategy to enhance delivery of ADC to target cells. And additionally, although the mechanisms of ADC toxicity are complex, target expression in normal tissue can lead to on-target toxicity. So strategies for increasing tumor selectivity, and thus the therapeutic index of ADC are needed to limit toxicity resulting from target engagement in normal tissue. For example, monovalent bispecific IgGs are the preferred format for increasing target selectivity by altering antibody affinity to maximize killing of cancer cells while sparing normal cells. And from our discussion so far, I'm sure we all believe that bispecific antibody has an important application in ADC. So can you tell us the current common strategies for bispecific ADC? Sure. At present, there are two strategies for bispecific ADC. The first one is maximizing internalization and trafficking to lysosomes. The second one is enhancing selectivity. Both strategies are based on the mechanism of ADC action. Can you explain in more details why maximizing the internalization and trafficking to lysosomes can be one of the bispecific ADC strategies? Because almost all of the ADC payloads to date require not only binding of the target at the tumor cell surface, but also uptake into the cell and subsequent delivery to the lysosome in order to effectively release the active cytotoxic warhead. For example, there are a number of targets showing promising tumor expression profiles, but poor lysosomal trafficking limits their full potential as effective ADC targets. In addition, many potential ADC targets either internalize poorly or undergo a high rate of endocytic recycling. This will cause the ADC to return to the cell surface intact without delivering the payload to the lysosome. Under this background, several bispecific ADC approaches have emerged recently that seek to enhance internalization and trafficking to the lysosome, thus maximizing the amount of drug that can be effectively delivered to tumor cells at a given dose. Early work suggested that targeting a single receptor with bispecific antibodies that recognize distinct epitopes could lead to increased avidity or overall affinity toward the target and greater potency. Subsequent studies demonstrated that non-overlapping antibody pairs and bipartotopic antibodies, or non-antibody scaffolds, could drive receptor clustering and crosslinking. This can promote enhanced internalization, trafficking to the lysosome, and degradation of the target. Enhanced lysosomal trafficking can first result in more effective tumor cell killing. The tubulison warhead employed in the ADC then possesses bisander killing activity, which means that, once liberated from target-expressing tumor cells, the cytotoxic warhead can enter and kill nearby non-target-expressing tumor cells. Do you think these enhanced lysosomal targeting strategies will improve the therapeutic index? I think while these strategies could, in principle, introduce an increased risk for on-target toxicity, the majority of ADC toxicities observed in the clinic are target-independent. Proliferative index and regenerative potential of a target organ will also play a role in the toxicity profile of an ADC. For example, many ADC warheads have mechanisms that are designed to differentially affect rapidly dividing cells. So if a normal tissue expresses the target antigen, but proliferates slowly, it is likely to be less sensitive to the ADC, compared to a rapidly dividing tumor that expresses the target. There are some ongoing and pending clinical trials that are going to provide the key proof of concept for bipartotopic ADCs, but, we have already seen preclinical evidence suggesting that, they represent a promising strategy to enhance lysosomal trafficking and delivery, thus turning poorly internalizing tumor-associated antigens into tractable ADC targets. And why can enhancing selectivity be one of bispecific ADC strategies, and how does it work? According to some of the recent works to improving ADC targeting and selectivity, bispecific antibodies use dual targeting to extend the reach of an ADC, or to create a two-in-one ADC. So that either target is sufficient to deliver the ADC into the tumor cell, so the therapeutic benefit can be broadened, when the targets are heterogeneously expressed within the tumor. Also in some other studies, the properties of each binding arm was adjusted to suit the particular targets and to improve tumor selectivity. Some said that dual targeting alone is not sufficient to achieve tumor selectivity. To produce a bispecific antibody that can tell the difference between tumors expressing, both targets from normal tissue and non-transformed cells that express only one of the targets, a couple of other factors, like the affinity of the individual arms, density of the target, the overall avidity, and the valency of the bispecific format, we all have to pay attention to. Some scientists propose that selecting the appropriate combination of affinity-optimized bispecific ADC variants could lead to higher selectivity for tumor versus normal tissue, which could broaden the therapeutic index. Thanks again for sharing your insight today on bispecific ADCs. We have reviewed the conception of ADC and can now understand the bispecific antibody and new development in bispecific and bipartotopic ADC strategies. And to wrap up, could you give us a quick summary? Sure, and I'm very glad to have this opportunity to share with our audience this wonderful new technology. As we move forward, the types of bispecific and bipartotopic technologies will likely start to be used more frequently, for poorly internalizing tumor antigens, where optimization of cytotoxic warhead delivery requires greater tumor selectivity, increased ADC uptake and enhanced lysosomal trafficking. With the current progress in antibody engineering, especially the ADC development, I really think this is the right time to go ahead and do more with the bispecific and bipartotopic ADCs, like improving its activity and therapeutic index. There will be many more challenges, but of course, there will also be more breakthroughs in the near future.
