5 min read
By Dr. John Kenney on 10/18/2019
After a couple weeks of tackling the tsunami of emails, meetings and project updates that greeted me after my two-week “conference marathon” in Boston last month, here is a summary of Discovery on Target 2019. And, like the CAR-TCR Summit the week before, this year’s DOT (#17 in the series) offered a wide range of useful and interesting updates on the latest in antibody research.
Discovery on Target has seven tracks focusing on novel drug targets and programs. In particular, I attended sessions on Antibodies Against Membrane Protein Targets, GPCR-based Drug Discovery and an Antibody Technology Forum.
GPRs, Ion Channels, and Transporters together represent the largest group of potential membrane antigen targets for therapeutic antibody discovery. However, generation of antibodies to these targets is technically very challenging. For example, Amgen’s Erenumab, the first GPCR-targeted antibody, was only recently approved in May 2018. By contrast, there are more than 100 small molecules or peptides approved as drugs targeting GPCRs. Nevertheless, there is great interest in pursuing antibodies at therapeutics due to their improved PK/PD, selectivity, multiple formats (ADC, bi-specifics) relative to peptides and small molecules.
Talks by Catherine Hutchings and Trevor Wilkinson, among others, described the challenges of generating potential therapeutic Abs to GPRs, Ion Channels, and Transporters. Some of these challenges include:
- The kinetics of activation - GPCR activation occurs in seconds while ion channel activation occurs in milliseconds. Abs may reasonably be assumed to exhibit slower binding.
- GPCRs and Ion Channels have diverse ligands.
- Structurally complex, with multiple conformational states.
- Small extracellular target and/ or low surface expression.
- Highly conserved (especially, ion channels).
- Difficult to obtain good sources of protein for use immunogens and screening Ags.
- Successful programs typically require a multi-pronged approach to immunization (e.g., peptides, whole cells, DNA, detergent solubilized proteins, lipoparticles, nano-discs, and combinations of the above).
- Deep mining is needed – screen lots of clones or binders to find the few that will be effective.
- Functional Screening needed to show blocking of a biological effect, not just binding to target. Often this is difficult to do in a high-throughput manner.
Common problems encountered in GPCR / Ion Channel / Transporter Ab discovery were:
- Lack of target validation and understanding of the biology of the target.
- Obtaining Ags that were fit for purpose.
- Poor assessment of target epitope (e.g., not hitting the right place on the target).
- Target-binding Abs not capable of full inhibition or had slow onset of inhibition.
The Antibody Technology Forum had presentations from a number of groups that used various technology platforms or combinations of platforms for drug discovery. In general, antibodies obtained from in vivo animal-based discovery platforms, versus display libraries, were favored as having the fewest problems with developability of the candidate antibody to a therapeutic.
A keynote presentation by Dr. Peter Tessier of the University of Michigan’s Pharmaceutical Sciences and Chemical Engineering department explored chemical and physical determinants of drug-like monoclonal antibodies. His team is developing bioinformatics methods to predict the overall specificity of antibodies in terms of their relative risk for displaying high levels of polyspecificity (e.g., non-specific binding) and self-interactions (e.g., aggregation).
Citing a prior study that found that clearance rates in monkeys were linked to self-interaction and off-target binding, Dr. Tessier said their study explored molecular descriptors that could define problems with the assumption that CDR regions likely contribute most to non-specific interactions. The team evaluated rules based on the number of AAs for solvent exposure and found, among other things, that:
- Arg is the most important single residue in promoting self- and non-specific interactions (in addition, His, Glut, Pro and Tyro were found to be important).
- Lysine was not identified as important in promoting self- and non-specific interactions.
- Aspartic Acid is most important in reducing self- and non-specific interactions (Glut and Asp were also important).
- Positively charged AA’s can be good for affinity, but they can be bad for polyspecificity and self interactions.
Dr. Tessier recommended this article, which summarizes an improved AC-SINS method that can improve selection of mAbs with excellent biophysical properties during early antibody discovery.
Also of special note was a presentation by Steven Rutherford from Genentech on their efforts to discover Abs to outer membrane proteins of Gram-negative bacteria that will inhibit growth. He pointed out that in spite of the increase in antibiotic-resistant bacteria, most pharma companies have left this research space. The essential membrane proteins targeted were BAM-A (inserts OMPs) and LptD (inserts LPS).
The envelope of Gram negative bacteria presents unique access challenges:
- BAM-A and LptD act more like chaperones than enzymes and they function by undergoing conformational changes.
- Large cohorts of mice and rats needed to be immunized with multiple antigens (e.g., protein, peptides and cells) to raise a serum response.
- Hybridomas were FACS sorted with protein to improve frequency of binders to the target antigens.
- Found anti-BAM-A Abs that were direct-acting – seemed to do more than just binding.
- Obtained a large library of anti-LptD Abs that did not inhibit growth.
- Found that LPS is a barrier to whole-cell binding by anti-BAM and anti-PptD antibodies (LPS keeps out both small and large molecules).
While inhibiting bacterial growth with an antibody remains elusive, their studies have pointed the way to critical residues in BAM-A and LptD that are important for function of the membrane proteins.
There were, of course, a number of other intriguing presentations, and the hallway conversations were quite productive (it was a really good crowd this year).
If you’re interested in learning more about our perspectives on the content shared at either the CAR-TCR Summit or DOT 2019, please feel free to give me a ring +1 650.938.4300 or +1 888.843.1069 (toll-free), or via email at email@example.com. Even better, if you’d like to have a one-on-one conversation over a cup of good coffee, please come visit Antibody Solutions’ new facility in Santa Clara, Calif. (and be sure to allot time for a tour and meet-and-greet with our team).
Thank you again for being a part of the Antibody Solutions family, and we look forward to hearing from you soon.