The development of novel novel antibody-drug conjugates or ADCs, which combine the targeted binding specificity and pharmacological advantages of antibodies with the potency advantages of small molecule chemotherapeutic payloads via conjugation with a chemical linker, designed for the treatment of cancer and hematological disease, rely not only on the science – the proper understanding of the underlying mechanism of action (MOA) of an antibody-drug conjugates – but also the expert ‘art’ of combining the antibody, linker and the cytotoxic drug.
With four antibody-drug conjugates commercially available in the United States, these new agents hold the power to revolutionize medicine – potentially beyond oncology and hematology.
However, the technology to develop these novel drugs has its challenges. Antibody-drug conjugates suffer, to name just a few problems, from low therapeutic window, tumor non-specificity, off-target toxicity and lack of efficacy. In addition, it is often difficult to create conjugates with a uniform and optimal drug-to-antibody ratio or DAR for each type of cytotoxic payload.
This year, at the 8th Annual Engineering Antibody-Drug Conjugates conference (part of PEGS Boston), being held May 2 – 4 2018 at the Seaport World Trade Center, scientists will present their research and explorations on using alternative scaffolds and new payloads to improve half-lives and specificity, optimizing linker-payload chemistry to enhance efficacy. They will also present new strategies to expand therapeutic window and improve stability of antibody-drug conjugates.
Site-specifically conjugated ADCs with Potent Anthracycline Payloads
During the upcomming meeting, Roger Beerli, Ph.D, CSO, NBE-Therapeutics, is expected to present a novel antibody-drug conjugates format based on the site-specific conjugation of a derivative of the anthracycline PNU-159682 using the transpeptidase Sortase A.
While anthracyclines are very effective in NHL, antibody-drug conjugates containing doxorubicin are generally not clinically efficacious. This is probably due to the low drug potency and inadequate linker technology used. The anthracycline analogue PNU-159682 (3′-deamino-3”,4′-anhydro-[2”(S)-methoxy-3”(R)-oxy-4”-morpholinyl]doxorubicin), thousands of times more cytotoxic than doxorubicin, linked to a non-cleavable peptide linker, provides exquisite stability in vivo, whereas the anthracycline payload endows the ADC with superior potency combined with attractive immune-oncology properties intrinsic to this class of compounds. 
Beerli and a team of researchers at NBE-Therapeutics generated homogeneous PNU-ADCs directed against HER2 and ROR1. These novel ADCs showed to have very high anti-tumor efficacy in vivo, both in PDX as well as in syngeneic solid tumor models. In the case of HER2, the PNU-ADC exceeded the efficacy of T-DM1 (ado-trastuzumab emtansine; Kadcyla®; Genentech/Roche) used as a benchmark ADC.
The researchers also noted that that in syngeneic breast cancer models, both HER2, as well as ROR1 ADCs resulted in the induction of a long-lasting tumor-selective anti-tumor immunity involving activated CD8 T cells. Importantly, a repeated-dose non-GLP toxicology study in cynomolgus monkeys did not reveal significant toxin-related pathology of PNU-ADCs at any of the evaluated dose-levels.
Based on these results, the researchers believe that this strongly supports further preclinical and clinical development of a promising new ADC format.
Attachment Site Cysteine Thiol pKa is a Key Driver for Site-Dependent Stability
In her presentation, Breanna S. Vollmar, Ph.D, Senior Scientific Researcher, Protein Chemistry at Genentech, desceribes how she, and a team of scientists, set out to understand the underlying mechanisms of site-dependent stability for the company’s Thiomab™ antibody-drug conjugate (TDC) platform utilizing engineered reactive cysteines residues at specific sites on the antibody.
Based on their observations the researchers believe that cysteine thiol pKa is a significant driver of the circulation stability of TDCs utilizing disulfide or maleimide attachment chemistry. She concludes that this represents a new parameter for the optimization of next generation ADCs utilizing engineered cysteines. 
Catalent’s SMARTag™ technology platform, which couples a proprietary aldehyde-based conjugation chemistry with proprietary linker chemistries, enables precise, programmable, site-selective chemical protein modification and uses only naturally occurring modifications to proteins requiring minimal cell-line engineering, helps drug developers to precisely control conjugate configuration, generating ADCs with optimal efficacy, safety and stability.
SMARTag has demonstrated compatibility with clinically validated antibody-drug conjugate payloads including maytansine, auristatin, pyrrolobenzodiazepine (PBD) and duocarmycin. Researchers at Catalent have shown that the technology can be optimized for use with various proprietary bioconjugate payloads, including cytotoxics, targeted small molecules, peptides, oligonucleotides, and other therapeutic modalities.
During the 8th Annual Engineering Antibody-Drug Conjugates conference, David Rabuka, Global Head, Research & Development, Chemical Biology, Catalent Biologics, Catalent Pharma Solutions will present recent data on their novel protein modification platform and its application to generating novel bioconjugates, including ADCs, utilizing the company’s new conjugation chemistries and linkers.
Additionally, Rabuka will highlight progress in developing conjugates with a focus on preclinical studies as well as highlight the company’s progress in cell line development and manufacturing of using this chemoenzymatic approach.
Yelena Kovtun, Associate Director, Pipeline R&D, ImmunoGen, is expected to present the recent data of IMGN632, is a novel conjugate of anti-CD123 antibody, G4723A, linked to a mono-imine containing Indolinobenzodiazepine pseudodimer (IGN) class of cytotoxic compounds.
Unlike other conjugates (including ADCs with proven clinical success), the investigational IMGN632 is the only antibody-drug conjugate to demonstrated potent anti-leukemic activity at the concentrations 100-fold lower the levels that impact normal human cells. The strategy to select an optimal antibody, payload, linker and conjugation method to achieve this uniquely wide therapeutic window will be covered in Kovtun’s presentation. 
Two noteworthy poster presentation will be presented during this year’s conference. Chun-Chung Lee, PhD, Research Fellow, Institute of Biologics, Development Center for Biotechnology, shows the results of a study in which a researchers developed a novel site-specific tri-mannosyl core antibody-drug conjugate platform.
The researchers explain that the novelty of this platform is that it offers drug developers to control the antibody conjugating single payload (DAR=2 or 4) or dual payloads (DAR=2A&2B) by one step or stepwise of MGAT-1 and MGAT-2 treatments, respectively.
Through this tri-mannosyl core ADC platform, the researchers were able to control the different toxin dosages (DAR=2 or 4) conjugating to the same antibody. As a result, the correlation between conjugating dosages of toxin, safety and efficacy will be methodically evaluated.
The researchers show that this platform will also be available for designing the therapeutic antibody conjugated with dual payloads, which will target the same molecule in cancer cells and induce different drug response. Finally, Chun-Chung Lee confirms that the platform is also available for use as companion diagnostics and therapy for cancers when the antibody conjugating both image dye and cancer drugs.
A second poster will highlight the development of concisely produced homogeneous antibody-drug conjugates by a tryptophan-selective protein bioconjugation.
Kounosuke Oisaki, PhD, Lecturer, Graduate School of Pharmaceutical Sciences, University of Tokyo, discusses transition metal-free method for tryptophan (Trp)-selective bioconjugation of proteins. Oisaki is expected to show that this method exhbits low levels of cross-reactivity and leaves higher-order structures of the protein and various functional groups therein unaffected. Oisaki will present recent progress following studies aiming at concise production of a homogeneous antibody-drug conjugate (ADC) using the Trp-selective bioconjugation are ongoing.
Results from reseasearch with antigen-targeted Amanitin-conjugates (ATACs), representing a new class of ADCs using the payload Amanitin, will be presented. Andreas Pahl, PhD, CSO, Heidelberg Pharma is expected to share the latest update of Amanitin. which introduces a novel mode of action into oncology therapy, designed to inhibit RNA polymerase II. The technology platform includes Amanitin supply, site-specific conjugation, demonstrated safety profile and biomarker. Improvements of the technology and an update of the development of HDP-101 will be presented. HDP-101 is the first ATAC directed against BCMA entering Phase I trials by the end of 2018.
Finally, Kurt Gehlsen, PhD, Vice President & CSO, Therapeutics, Research Corporation Technologies, is expected to present updated results of research with abdurins, a novel antibody-like scaffold that can be engineered to bind to targets of interest and due to an FcRn binding motif.
Abdurins have a circulating half-life longer than any other protein scaffold of similar size and can be fused with protein toxins, other binding domains and engineered to carry payloads. Abdurin-drug conjugates retained high affinity binding and had in vivo half-life up to 70 hours for certain conjugates.