Artificially boosting the body's immune responses against cancer is the most exciting advance in the treatment of tumours in recent years. Our new case study demonstrates the use of Affimer technology for the rapid generation of candidate biotherapeutic molecules targeting a cancer immune checkpoint.
The coming of age of cancer immunotherapy
Scientists have spent decades attempting to train the immune system against cancer. Most of the time the body works hard to suppress immune reactions, to prevent autoimmune reactions destroying healthy tissue. This is achieved by immune checkpoints that quell the immune response upon activation and allow the immune system to recognise our own body’s cells. Recognising that cancer cells use our own immune checkpoints to evade attack provided a leap forward in targeting these tumours. One example is the PD-1/PD-L1 immune checkpoint, where PD-1 on the surface of T cells interacts with PD-L1 on antigen presenting cells to prevent the immune system targeting these cells. Blocking this checkpoint awakens the immune system to the presence of cancer cells and its use in the clinic is now showing improved and incredibly durable patient responses.
Identifying PD-L1 Affimer inhibitors
Our unique Affimer technology utilises a high throughput platform to identify and characterise a panel of PD-L1 inhibitors as potential biotherapeutics.
Affimer binders are selected from a phage library screen against functional proteins. From an initial panel of over 750 Affimer binders to the functional PD-L1 target, the best candidates were selected based upon 1) their inhibition potential of the PD-1/PD-L1 immune checkpoint, 2) specificity of binding to the PD-L1 target protein and 3) their protein expression characteristics. A range of fully-automated, high-throughput, multiplex assays, requiring minimal amounts of precious target proteins, were used to characterise the PD-L1 Affimer inhibitors according to these criteria.
The use of multiplex reactions that can explore up to 35 different target proteins simultaneously, to characterise Affimer protein binding, allows us to ensure a high degree of target specificity. Equally, the high throughput nature of our technology platform means that quality inhibitor candidates can be identified in a matter of weeks.
The optimum format for PD-L1 Affimer inhibitors as biotherapeutics was explored with the generation of various Affimer multimers. These were assessed for their increased affinity, through avidity effects and potential production characteristics in large scale bioreactors. In addition to offering increased target affinity the generation of multi-target Affimer binders expands the therapeutic potential of these molecules.
Why Affimer biotherapeutics?
Within the clinic monoclonal antibody therapies targeting PD-L1 are already showing remarkable results. More patients with a greater range of cancers are showing positive responses to these therapies than with previous cancer immunotherapies. The durability of the patient response has been one of the most notable features of this treatment, with patients living much longer than with other standard cancer treatments, such as chemotherapy. Yet, repeated administration of monoclonal antibodies have been associated with increased immunogenicity and studies show significantly more anti-tumour activity is possible using a small-protein therapeutic compared with a conventional antibody.
The drawbacks associated with antibodies include their large size, which limits their ability to diffuse into solid tumours, and their ability to activate antibody dependent cell-mediated cytotoxicity, through their Fc-region. While Fc-mediated effects are an important part of the efficacy of many antibody therapeutics, in the case of PD-1 / PD-L1 inhibition this may be counterproductive. As PD-1 and PD-L1 are expressed on the surface of T cells targeting these proteins could result in an unwanted reduction of these anti-tumorigenic cells. In the clinic, treatment with these immune checkpoint inhibiting antibodies has been associated with lower T cell numbers in patients.
At 12-14kDa Affimer proteins are only a tenth of the size of a typical antibody. Even in a multimeric format for potential therapeutic application, the size of these proteins can be optimised to achieve the greatest benefit of both target accessibility and physiological retention to allow it to exert its therapeutic effect. Furthermore, as they do not share any structure with antibodies, undesirable Fc-mediated effects are not a consideration for Affimer biotherapeutics.
For a greater insight into our unique Affimer technology and how we are utilising this platform to rapidly generate highly specific Affimer biotherapeutics, download our case study below - ‘Identification of specific PD-L1 Affimer inhibitors with therapeutic potential’.