The VNAR drug discovery platform
Sharks are the most evolutionarily ancient animal species to possess an adaptive immune system similar to humans, including the production of antibodies against invading pathogens. These ancestral antibodies called IgNARs for Immunoglobulin New Antigen Receptors have a number of remarkable and unique properties. IgNARs evolved as heavy chain only antibodies without the associated light chains found in human antibodies. Their individual variable domains (known as VNARs) are highly soluble compared to the human variable domains, which on their own are generally insoluble and aggregate. Additionally, VNARs are particularly stable and bind antigens under conditions that match, which would disrupt the integrity of most mammalian antibodies. Furthermore, shark antibodies are believed to be particularly stable since they evolved under the high osmolarity of shark blood, which is maintained by the protein denaturant urea.
Only a very small region at the tip of an antibody directly binds to an antigen and sharks have evolved a unique way of engaging target antigens. The diversity of human antibodies is generated by a complex of two variable domains (VH and VL) each of which has three sites of contact called complementarity-determining regions (CDR 1-3). Thus, antibodies rely on contributions from up to 6 CDRs, which create a relatively flat surface of for antigen binding. By contrast, VNAR single domains of shark antibodies lack a CDR2 and concentrate diversity in an extended CDR3 loop supported by a smaller CDR1 that preferentially seeks out cavities and buried epitopes. Additionally, the position of disulfide bridges that affect the stability, flexibility and orientation of the CDR3 loop create diverse isoforms (called type I, II and IV) that can recognize a wide array of antigens.
VNARs are the smallest known immunoglobulin-based antigen binding domains that can have agonistic or antagonistic effects on their own. Their unique combination of structural and biophysical properties makes them attractive building blocks for drug discovery applications, particularly in situations where monoclonal antibodies have proven to be less than ideal. Such examples include perturbation of protein-protein interactions and for targeting difficult but important target classes such as transporters, ion channels and G protein-coupled receptors and cell-surface carbohydrates. A new generation of modular therapeutic agents can be custom built with innovative functions beyond the reach of classical antibodies for the treatment of a broad range of human diseases.
- Ossianix VNAR Phage Display Libraries: Ossianix has developed an in-house suite of proprietary and patented semi-synthetic phage display libraries based on specific VNAR isoforms. These large and structurally diverse libraries (greater than1010) have produced potent and selective antibodies to a broad range of target classes. To complement this approach, Ossianix also builds VNAR phage display libraries derived from nurse shark lymphocytes.
- Our semi-synthetic libraries combine naturally occurring mutations in the VNAR backbone with fully randomised CDR3 binding regions of multiple lengths to more closely mimic the range of antibody diversity found in the natural immune response. Specific, high-affinity VNARs are typically isolated directly from the libraries without in vitro affinity maturation and without the delay required for immunization, thus significantly decreasing the discovery cycle time.
- The libraries are also biased towards larger CDR3 regions to capture the natural predisposition of VNARs to have longer CDR3 loops than are typically found in human antibodies. The long CDR3 loops with a small binding footprint provide a finer level of antigen discrimination, which is advantageous for accessing hidden epitopes or small functional domains.
- Ossianix Blood-Brain Barrier Technology: A major limitation associated with CNS Biotherapeutics is their very low penetrance into the CNS. Many CNS Biologics have failed in the clinic due to insufficient brain exposure and poor PK. A variety of approaches have been tried to ferry large molecules across the blood-brain barrier, but the most promising to date utilize endogenous transporters in brain capillaries, the so-called “Trojan Horse” approach. Due to their small size and stability, VNARs are ideally suited as carriers and Ossianix has developed a panel of species cross-reactive modules that can be fused either to therapeutic VNARs, antibodies, enzymes, peptides or small molecules to treat a wide variety of CNS disorders.
- Flexible Formatting Platform: The VNAR platform is very versatile and a wide array of novel therapeutic products with different valency and specificity options can be readily produced. Mono- or bivalent antigen binding molecules can be developed with different half-lives, depending on the therapeutic indication. Half-life extension of products can be readily achieved by using either a VNAR reactive with human serum albumin or the Fc domain from human IgG. Effector functions can also be engineered for reduced or enhanced cytotoxicity as required.