This year Cambridge Healthtech Institute’s 14th Annual PepTalk, the Protein Science Week, being held in San Diego, California January 19 – 23, 2015, welcomed more than 300 high-caliber speakers to the Town and Country Resort & Convention Center to share case studies, unpublished data, breakthroughs and solutions as well as a great number of quality posters supporting and enhancing medical and scientific research. Among the may topics discussed was the latest ongoing news about antibody-drug conjugates.
Despite the relatively simple concept, in practice antibody-drug conjugates are extremely complicated. First, the underlying antibody-drug conjugate discovery and development process is complex. This process poses significant challenges in that systematic design uses a combination of optimal conjugation strategy (e.g. lysines, cysteines, Fc glycans, non-natural amino acids), linkers (e.g. cleavable, non-cleavable), and small-molecule toxophores (e.g. MMAE, DM1, Calicheamicin, PBD dimers). Second, in addition to the complexity of unconjugated monoclonal antibodies, antibody-drug conjugates exhibit unique properties, which are derived from the linkage of a biologically produced antibody to a small molecule drug.
Therefore, when designing antibody-drug conjugates, a number of parameters need to be considered, including appropriate antigen target and the conjugation method. This complexity also requires an increased level of control and characterization to achieve the right formulation, stability and consistency for effective scale up and manufacturing that meets the regulatory requirements. Furthermore, to fully realize the goal of improved efficiency and tolerability, each of the components that make up an antibody-drug conjugate (antibody, linker and anti-cancer drug) needs to be optimized.
With more and more antibody-drug conjugates coming down the pipeline, it becomes imperative for drug developers and pharmaceutical companies to consider the manufacturability of their ADCs. This requires incorporating process design and CMC (Chemistry, Manufacturing and Control) strategies early on in the drug development stage.
In addition, engineering antibody-drug conjugates for successful development also requires a thorough developability analysis which, together with cell-line productivity and cost-of-goods analysis are key in determining manufacturing feasibility. 
In the key-note presentation Wen Jin Wu, MD, PhD, a senior investigator working for the division of monoclonal antibodies at the Office of biotechnology Products (OPS-CDER) of the U.S. Food and Drug Administration (U.S. FDA), showed how the unique properties of antibody-drug conjugates or ADCs create a number of technical challenges that require careful CMC considerations. With the increase in Investigational New Drug submissions for antibody-drug conjugates (IND) and recent approval of ADC products such as brentuximab vedotin (Adcetris®; Seatlle Genetics) and ado-trastuzumab emtansine (Kadcyla®; Genentech/Roche) the FDA has gained much in-depth knowledge regarding ADC development programs. In his presentation Jin Wu discussed the FDA experience with ADC development and the regulatory challenges, with special focus on CMC. He also discussed a strategy for the development of the next generation of ADCs. 
Engineering and lead optimization
Among the exhibitors, Sebastian Schlicker, PhD (Genedata AG; Basel, Switzerland) described how an enterprise workflow platform that supports a full ADC discovery process, including antibody screening and engineering, antibody expression and purification, drug conjugation and reporting of ADC-specific analytics (e.g. DAR, drug distribution, homogeneity), can be instrumental in drug development. The workflow platform, called Genedata Biologics, focuses on antibody screening, protein engineering and lead optimization, and biologics production, enables the registration and tracking of large panels of potential antibody-drug conjugate candidates, and integrates results from analytics and functional assays in one integrated system, thereby substantially increasing throughput and efficiency of the antibody-drug discovery process. 
Light and ADCs
Scientists have long known and confirmed that chemical and physical stability problems may arise when antibody-drug conjugates are exposed to light. These stability problems may be caused when the conjugated drugs act as photo-sensitizer (e.g., CBI, duocarmycin or an anthraquinone moiety), and/or the conjugation of drug moieties to antibodies changes the sensitivity towards light exposure. In his talk Christian Schöneich, PhD, Takeru Higuchi Distinguished Professor and Chair, Departments of Pharmaceutical Chemistry, School of Pharmacy, University of Kansas, focused on light-induced photo-degradation of ADC mimics, designed to evaluate the light-sensitivity and degradation mechanisms of ADCs.  Understanding degradation pathways and the mechanisms involved is important because the degradation of the ADC may interfere with the intended biological activity. In turn, this understanding has enabled pharmaceutical scientists to propose various stabilization strategies.
Crossing the Blood-Brain Barrier
Among the very interesting posters at PepTalk was one presented by Xiaocheng Chen, PhD. Her team at Genentech has been involved in the research of receptor-mediated transcytosis or RMT as a promising route to facilitate delivery of therapeutics across the blood-brain barrier or BBB. The blood-brain barrier is a highly selective barrier preventing molecules from freely entering the brain. While it is designed as a gatekeeper to protect the brain and central nervous system from harm by keeping foreign substances out while permitting access only to certain metabolically essential molecules, such as glucose, insulin, and growth hormone, it also limits the access of protein therapeutics entering the brain for the treatment of disease.
Receptor-mediated transcytosis may to facilitate delivery of therapeutics across the blood-brain barrier. However, there are only a very limited number of identified receptor-mediated transcytosis targets. Furthermore, key characteristics of an ideal RMT targets remain unclear. To address these issues, Chen and her team utilized multiple approaches. They reviewed RNAseq and proteomics methods and performed a detailed screening against potential candidates. Furthermore the researchers also looked at a number of target candidates reported in the literature. As part of their findings, the researchers reported that two novel RMT targets have been identified. In the therapeutic dosing study, these novel targets showed significant brain uptake. 
In a separate presentation, Jean Lachowicz, PhD, Chief Scientific Officer at Angiochem, Inc. discussed how, using a peptide recognized by the brain capillary endothelial cell receptor LRP1, peptide-mAb conjugates can cross the BBB in therapeutic concentrations. Using this strategy in the development of ADCs to target brain tumors has resulted in significant decrease in tumor size and prolonged survival in mice. 
Homogeneity: Necessary but not sufficient
While homogeneity of antibody-drug conjugates is necessary, it is insufficient to ensure ‘best-in-class conjugation. In his presentation Trevor Hallam, PhD, Cief Scientific Office of Sutro Biopharma, Inc. who was instrumental in shaping his company’s biochemical synthesis technology to generate a disruptive discovery and manufacturing platform for novel bispecific and antibody-drug conjugates protein therapeutics, showed that conjugation positional analysis is fundamental. Hallam demonstrated that combining two or more mechanistically diverse warheads precisely in a single ADC is possible and that novel, next-generation antibody-drug conjugates have the potential to efficiently kill tumors while limiting their ability to develop resistance to the cytotoxic conjugates. He explained that a systematic analysis of linker ‘warhead’ positioning reveals the key differences in the in vivo killing potency of solid tumors that are not predicted by internalization rates, binding affinity, stability or PK, and may show differences in the tumor drug disposition. 
Building on the success of established ADC platforms, approved for the treatment of patients with cancer, means looking beyond currently approved agents. The vast majority of these agents utilize derivatives of maytansine and auristatin. While these agents have proven to be successful, John Labmert, PhD, Executive Vice President and Chief Scientific Officer at ImmunoGen, Inc., in his presentation described exciting advances in the development of payloads with different killing mechanisms. Building on this knowledge, Vishal Verma, PhD, a scientist, medical chemistry, at Genentech, Inc., who’s current work focuses on incorporating medicinal chemistry aspects of drug discovery to linker and drug design for antibody-drug conjugates, discussed how the identification and evaluation of novel payloads for the next-generation of ADCs will be instrumental in addressing the current shortcomings.
Adeela Kamal, PhD, Associate Director, Oncology Research at MedImmune, responsible for leading ADC efforts as wel as managing other preclinical oncology research, discussed her company’s strategies in identifying high value tumor targets, applying state-of-the-art ADC technology with novel payloads, site-specifuc conjugation, and, finally, how to advance ADC programs in the clinic.
With leaders from around the world, the organizers of PepTalk – The Protein Week, one of the largest gatherings of protein science researchers in the United States, demonstrated again, beyond doubt, their ability to present valuable, information-rich, content to more than 1,200 attendees.