Over the years there have been major challenges in the development of antibody-drug conjugates (ADC), including: choosing the right target, right antibody, right linker, right drug, and the right patient. With multiple variables, the possible combinations can be seemingly endless.

Here are some of the challenges we face today:

  • Hydrophobic drug payloads are more easily recognized and cleared by the immune system, often leading to dose-limiting toxicities, including neutropenia, liver toxicity and eye toxicity.
  • High drug loading of hydrophobic drug can lead to such toxicity. There has been much progress in the last 5 years in the development of novel methods to control the drug-to-antibody ratio (DAR) loading using site-specific conjugation technology. Now it is routine to produce single peak purity ADCs with only two drugs per antibody with a very tight distribution, and no detectible 0 or 4 drug loaded species.
  • Unstable attachment of the drug linker to the protein can cause potential free drug exchange or release leading to dose-limiting toxicity.
  • Cleavable drug linkers need to be stable in serum, with a stable DAR profile after lengthy incubation with serum enzymes.
  • Payload needs to be appropriate, whether membrane permeable for bystander killing, or not. The mechanism of action needs to be appropriate for the cancer cell type in particular.

There are some common learnings that can help overcome these challenges. Modern ADC design incorporates these design features into the attachment, linker and payload components of a successful ADC molecule.

Modern payloads have been made more easily soluble, either by either modifying payload functional groups themselves, providing a solubilizing linker, or using a hydrophilic polymer backbone approach.

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Unstable maleimide attachment has been mostly solved through a variety of means including base-catalyzed hydrolysis, as well as switching to the bromo or iodoacetamide versions of the same payload linker. The plethora of alternative chemistries available allow for a stable covalent attachment of the drug linker to the protein.

The variety and diversity of payloads available today is truly amazing. There are many various mechanisms of action available beyond the simple tubulin inhibitors or DNA damagers. This allows for the modern development of dual-payload ADCs using orthogonal mechanism of action payloads.

It can be argued that single ADCs applied together might be a better approach clinically, but a dual labeled ADC is a scientifically interesting concept that should be further developed. Various stoichiometries can be used to balance varied potencies of chosen payloads.

Recent improvements in linker solubility, as well as the choice of a less potent TOP-1 class of payload, have led to the development of several modern, hydrophilic DAR 8 ADCs, including the Diiachi-Sankyo’s trastuzumab deruxtecan (DS-8201) and the Immunomedics’ sacituzumab govitecan (IMMU-132).

Seattle Genetics has pushed the boundaries of the possible with their SGN-CD48A molecule, with has all in one molecule: stable attachment, solubilizing PEG side chain, glucuronide sugar solubilizing and cleavage, and DAR 8 loading of MMAE.

Mersana’s Dolaflexin platform uses a hydrophilic polymer to achieve a high DAR loading of 12-15 of an auristatin payload (auristatin F – hydroxypropylamide or AF-HPA). Despite the analytical and manufacturing challenges, this continues to be an interesting approach, and their new Dolasynthen platform may bring further improvements.

Important improvements in the development of DNA-interactive payloads have taken place, including Femtogenix’s pyrridinobenzodiazepine (PDD) platform. This platform allows a toolbox approach to payload selection through the availability of payloads with multiple mechanisms of action (i.e., DNA mono-alkylating and cross-linking) and multiple potencies (i.e., from nM to pM), thereby allowing the production of ADCs with a very favorable therapeutic window.

LegoChem Biosciences has a novel technology that uses an improved glucuronide sugar as a solubilizing protective group on the PBD payload. Their pro-drug approach creates a payload that is more than ~100x more safe as a free drug than the de-protected payload. This may allow for a much better safety profile for their LCB-17 drug candidate compared with standard pyrrolobenzodiazepine (PBD) payload ADCs.

Another important industry trend is the increase in the number Contract Development and Manufacturing Organizations (CDMOs). Ten years ago there were but a handful of places that could manufacture antibody-drug conjugates (such as Lonza and others). Today there are more than 40 CDMOs of all shapes and sizes.

Furthermore, the technical expertise exhibited these by CDMOs is truly impressive, with many companies offering their own proprietary, site specific, conjugation technology to clients.

New horizons for future ADCs may include bi-functional antibody scaffolds that can allow for either bi-paratopic antigen recognition or immune-cell recruitment. New payloads are being implemented that are immunomodulatory, with up-regulating or down regulating local immune function.

One thing is clear, the ADC industry is moving quickly, is learning from past lessons, and is poised for great developments in the near future. My job is to help clients achieve their ADC development goals quickly and efficiently by pointing the way to the latest and best technologies available for advanced ADC design.

This is an excerpt from Rick Powers course: Workshop A – Advanced Design of ADCs: Principles and Applications with Next Generation Linkers and Site-Specific Technology, which he taught at the Hanson-Wade’s World ADC meetings held in the United States, Asia, and Europe.

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[2] Purcell JW, Tanlimco SG, Hickson J, Fox M, Sho M, Durkin L, Uziel T, Powers R, et al. LRRC15 Is a Novel Mesenchymal Protein and Stromal Target for Antibody-Drug Conjugates. Cancer Res. 2018 Jul 15;78(14):4059-4072. doi: 10.1158/0008-5472.CAN-18-0327. Epub 2018 May 15.[Pubmed][Article]
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[4] Buecheler JW, Winzer M, Tonillo J, Weber C, Gieseler H. Impact of Payload Hydrophobicity on the Stability of Antibody–Drug Conjugates. Mol. Pharmaceutics20181572656-2664 / May 29, 2018
[5] Hu Y, Hou Y, Wang H, Lu H. Polysarcosine as an Alternative to PEG for Therapeutic Protein Conjugation. Bioconjugate Chem.20182972232-2238 / June 4, 2018
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[7] Li X, Nelson CG, Nair RR, Hymel D, Burke Jr. TR, Rader C. et al. Stable and Potent Selenomab-Drug Conjugates. Cell Chemical Biology 24, 433–442 April 20, 2017 [PDF]
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[9] Vollmar, et al, Bio Conj Chem 2017 Oct 18;28(10):2528-2548
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Article DOI: 10.14229/jadc.2019.10.23.001

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Rick Powers
Rick Powers' 35 years in Biotech and 20 years in ADCs have given him a unique perspective on the industry. After UC Davis, Rick joined Hybritech in Torrey Pines (1985) as an early conjugation pioneer with hetero-bifunctional chemistry. Later work in Irv Weissman’s SyStemix led him to Coulter Pharmaceutal in SSF, CA, (1999) where we worked with King, Bebbington, and Yarrington (of Celltech Mylotarg fame) to conjugate early cc-1065 payloads. Rick moved across the street to Eos Biotechnology (2000) to help Dave Martin with his very early Seattle Genetics ADC R&D agreement. Rick received one of the first deliveries of vc-MMAE from Damon Meyer and Peter Senter in 2001. Eos Biotech was bought by Protein Design Labs which later morphed into Facet Biotech to be sold to Abbott Laboratories (2010), which spun out Abbvie as a $22B start up company in 2013. Rick’s last 8 years at Abbvie/Abbott gave great experience with two drugs in humans trials, including StemCentrx ADC process development work. Rick managed the entire Abbvie Bioconjugation lab in Redwood City and operated as an “internal CMO” and trainer to the entire Abbvie organization, including North Chicago, IL and Worcester, MA. Rick is currently consulting to select clients, and was an invited expert speaker at the World ADC San Diego 2018, Shanghai, China 2019, and San Diego 2019. He will be speaking at the WADC London 2020 conference. He gives the three-hour “Workshop A- Advanced Design of ADCs: Principles and Applications using next generation linkers and site specific technology”. With extensive knowledge of the latest industry trends, Rick has extensive experience in antibody drug conjugate (ADC) technology, including novel drug linker screening, analytical testing methods, bi-functional ADC constructs, bioconjugation process development and manufacturing scale-up. Rick lives and works in Palo Alto, California