Two other important areas also benefit from a robust antibody discovery platform. The first, target validation, allows us to reveal the biology of a given target (such as its localization or possible signaling pathways). The second deals with the generation of so-called anti-idiotope antibodies to support clinical-stage programs in need of detecting and quantifying a given drug IgG in human serum samples (such as for pharmacokinetics or immunogenicity evaluation purposes).
Once we’ve identified an appropriate lead antibody, optimization typically follows. This step is necessary to improve the lead’s safety, biophysical, and functional properties, or potency.
- A decrease of the antibody lead immunogenicity potential can be achieved by humanization or germlining approaches. The goal is to minimize the number of non-human amino acids in the final molecule. Humanization consists of grafting the antibody’s complementary determining regions (CDRs) onto carefully selected full-length human frameworks. CDRs can also be grafted onto banks of individual human frameworks and selected for beneficial characteristics such as affinity, expression, and stability (our proprietary “framework shuffling” platform). In all cases, state-of-the-art molecular biology techniques are essential to successfully germline and humanize the antibody lead.
- At MedImmune, we can often achieve optimization of the lead antibody's specific activity by using an affinity maturation process where we increase the binding affinity to its cognate antigen. Typically, we introduce and select certain amino acid mutations in the antibody’s CDRs, using various in vitro platforms such as phage display, phage expression, and ribosome display. The latter involves the use of ribosomes (the molecular complex responsible for protein synthesis within cells) to display functional proteins and allows access to unprecedentedly large protein libraries.
- MedImmune also introduces various function-altering mutations during optimization as needed. These may include modifications to modulate the antibody's effector functions or enhance its serum persistence. We use Fc engineering strategies to improve our lead molecule’s antibody-dependent cell-mediated cytotoxicity (ADCC) and enhance its potency towards select targets, such as cancerous cells. We have also developed a novel and groundbreaking strategy to improve the serum half-life of human IgG. We identified a particular triple mutation known as YTE which, when introduced into the Fc portion of human IgGs, increases their binding to the neonatal Fc receptor (FcRn). This results in a 3- to 4-fold increase in their serum half-life. Such long-lasting molecules have the potential to revolutionize the field of antibody-based therapy.
Structural biology efforts are crucial to expose an antibody’s mechanism of action, understand its general functional or biophysical properties, inform future strategies for lead optimization, or strengthen our intellectual property positions. For these purposes, MedImmune often uses three major approaches:
- Epitope mapping – helps pinpoint the binding site of an antibody of interest to its cognate antigen at the amino acid level.
- X-ray crystallography – provides unparalleled access to the three-dimensional structure of an antibody alone or in complex with its antigen.
- Computational modeling – an important tool for understanding and predicting our molecule’s structure, function, and properties.
We routinely apply these proprietary strategies to our lead molecules as appropriate. Their large scale expression is a crucial step that allows MedImmune to develop bona fide drugs. We use high-expressing, stably transfected mammalian cell lines for this purpose, which produces large (multi gram) amounts of antibodies for pre-clinical, developmental, and clinical studies. Here again, we are at the leading edge of biotechnology by developing various proprietary strategies to optimize production.