In September 2019, a number of articles discussing antibody-drug conjugates were published in a variety of journals. Here is an overview of some of the articles that make up ‘the must read’ articles this month.
In late September, in an article published in JAMA Oncology Christine M. Walko, PharmD, at the H. Lee Moffitt Cancer Center and Research Institute, Tampa, Florida and Howard (Jack) West, MD, at the City of Hope Comprehensive Cancer Center, Duarte, California, published a brief overview of antibody-drug conjugates for cancer treatment 
Other article published in September:
- HER2-Targeting ADCs Beyond trastuzumab emtansine
Over the last decades, targeted drug delivery has significantly improved the treatment of cancer. While different approaches have been developed to delivery drugs more specific, than, for example, standard chemotherapy one option stands out. Antibody-drug conjugate or ADCs provide a potentially ideal solution to meet the challenge. 
Still considered a relatively new model, ADCs consist of three key components: payload, monoclonal antibody, and linker. The monoclonal antibody targets the antigen-expressing tumor cells and internalizes the payload linked by the linker to the target cells to reduce the side effects of the traditional chemotherapy drugs. Overall, the off-target effect has an excellent therapeutic prospect.
Today five antibody-drug conjugates have been approved in the united states and are commercially available. Among the approved agents is (ado-) trastuzumab emtansine (T-DM1; Kadcyla®; Genentech/Roche), a successful example of targeting human epidermal growth factor receptor-2 (HER2).
Trastuzumab emtansine consists of a humanized monoclonal antibody trastuzumab covalently linked to the cytotoxic agent DM1, which, upon in the internalization, binds to tubulin, thereby disrupting microtubule assembly/disassembly dynamics, inhibiting cell division and the proliferation of cancer cells that overexpress HER2.
The agent’s antibody, trastuzumab (Herceptin®; Genentech/Roche), which has excellent targeting and specific anti-tumor activity against HER2, generates an annual sales of more than U.S. $ 6 billion.
The linker used in this ADC is not cleavable and releases the Lys-linker-payload to kill the cells.
In a review paper published in the European Journal of medical Chemistry in early September 2019, Xu, Guo, Jiang, et al, described two antibody-drug conjugates using trastuzumab, the same antibody used in (ado-) trastuzumab emtansine. In contrast to the approved agents, the two investigational agents include a cleavable linker and a more cytoxic payload allowing them to have the ‘same’ targetability’ as the ‘reference drug, but also reduce resistance to (ado-) trastuzumab emtansine and improve efficacy in heterogeneous tumors.
The authors of the article describe the mechanism of action and the biochemical characteristics of different parts and preclinical and clinical progress of trastuzumab deruxtecan (DS-8201a), being developed by Daiichi Sankyo and (vic-) trastuzumab duocarmazine (SYD985), being developed by Synthon Biopharmaceuticals.
- In Vivo Immunofluorescence Localization
Antibodies are important tools in cancer detection, diagnosis, and treatment. They are used to unravel the role of proteins in tumorigenesis, can be directed to cancer biomarkers enabling tumor detection and characterization, and can be used for the treatment of patients with cancer as both a monoclonal antibody or antibody-drug conjugates. 
Antibodies can activate immune effector cells, inhibit signaling pathways, or directly kill cells carrying the specific antigen.
Despite clinical advancements in the development and production of novel and highly specific antibodies, diagnostic and therapeutic applications can be impaired by the complexity and heterogeneity of the tumor microenvironment.
For the development of efficient antibody-based therapies and diagnostics, it is crucial to assess the biodistribution and interaction of the antibody-based conjugate with the living tumor microenvironment.
In the Journal of Visualized Experiments, a peer-reviewed scientific journal that publishes experimental methods in video format, Jennifer Wischhusen at the Apoptosis, Cancer and Development Laboratory, Equipe labellisée ‘La Ligue’, LabEx DEVweCAN, Centre de Cancérologie de Lyon, INSERM U1052-CNRS UMR5286, Centre Léon Bérard and Katheryne Wilson at the Department of Radiology/Molecular Imaging Program at Stanford, School of Medicine, Stanford University, describe In Vivo Immunofluorescence Localization (IVIL) as a new approach to study interactions of antibody-based therapeutics and diagnostics in the in vivo physiological and pathological conditions.
The technique, a therapeutic or diagnostic antigen-specific antibody is intravenously injected in vivo and localized ex vivo with a secondary antibody in isolated tumors. As a result, IVIL reflects the in vivo biodistribution of antibody-based drugs and targeting agents.
The authors describe two IVIL applications assessing the biodistribution and accessibility of antibody-based contrast agents for molecular imaging of breast cancer. This protocol will allow future users to adapt the IVIL method for their own antibody-based research applications.
- Kinetic reaction modeling
With the specificity of antibodies and the efficacy of cytotoxic drugs in one molecule, antibody-drug conjugates form a promising class of anti-cancer therapeutics. This is emphasized by the large number of molecules in clinical trials and five marketed agents. 
The conjugation reaction of antibodies with small-molecule drugs is a central step during production of antibody-drug conjugates.
A detailed kinetic model for the conjugation reaction grants enhanced process understanding and can be profitably applied to process optimization. One example is the identification of the optimal amount of drug excess, which should be minimized due to its high toxicity and high cost.
In an article published the Journal of Biotechnology, Sebastian Andris, Jonathan Seidel and Jürgen Hubbuch, describe how they set up six different kinetic model structures for the conjugation of a cysteine-engineered antibodies with a maleimide-functionalized surrogate drug.
All models consisted of a set of differential equations. These models were fit to an experimental data set, and the best model was selected based on cross-validation.
The selected model was successfully validated with an external validation dataset (R² of prediction: 0.978).
The authors describe that based on the modeling results, process understanding was improved. The model shows that the binding of the second drug to the antibody is influenced by the attachment of the first drug molecule.
Additionally, an increase in reaction rate was observed for the addition of different salts to the reaction. The authors also describe that, in a next step, the model was applied to an in silico screening and optimization which illustrates its potential for making the process development of antibody-drug conjugates more efficient. Finally, the combination of the kinetic model with a PAT tool* for reaction monitoring was demonstrated.
The authors conclude that the proposed modeling approach provides a powerful tool for the investigation of ADC conjugation reactions and establishes a valuable in silico decision support for process development.
* Process analytical technology (PAT) has been defined by the United States Food and Drug Administration (FDA) as a mechanism to design, analyze, and control pharmaceutical manufacturing processes through the measurement of Critical Process Parameters (CPP) which affect Critical Quality Attributes (CQA).
 Walko CM, West HJ.Antibody Drug Conjugates for Cancer Treatment.JAMA Oncol. 2019 Sep 26. doi: 10.1001/jamaoncol.2019.3552. [Article]
 Xu Z, Guo D, Jiang Z, Tong R, Jiang P, Bai L, Chen L, Zhu Y, et al. Novel HER2-Targeting Antibody-Drug Conjugates of Trastuzumab Beyond T-DM1 in Breast Cancer: Trastuzumab Deruxtecan(DS-8201a) and (Vic-)Trastuzumab Duocarmazine (SYD985).Eur J Med Chem. 2019 Sep 6;183:111682. doi: 10.1016/j.ejmech.2019.111682. [Pubmed][Article]
 Wischhusen JC, Wilson KE. In Vivo Immunofluorescence Localization for Assessment of Therapeutic and Diagnostic Antibody Biodistribution in Cancer Research. J Vis Exp. 2019 Sep 16;(151). doi: 10.3791/59810. [Pubmed][Journal]
 Andris S, Seidel J, Hubbuch J. Kinetic reaction modeling for antibody-drug conjugate process development. J Biotechnol. 2019 Sep 23. pii: S0168-1656(19)30861-2. doi: 10.1016/j.jbiotec.2019.09.013.[Pubmed][Article]