Abstract The fibronectin type III (FN3) monobody domain is a promising non-antibody scaffold which features a less complex architecture than an antibody while maintaining analogous binding loops. We previously developed FN3Con, a hyper-stable monobody derivative with diagnostic and therapeutic potential. Pre-stabilization of the scaffold mitigates the stability-function trade-off commonly associated with evolving a protein domain towards biological activity. Here, we aimed to examine if the FN3Con monobody could take on antibody-like binding to therapeutic targets, while retaining its extreme stability. We targeted the first of the Adnectin derivative of monobodies to reach clinical trials, which was engineered by directed evolution for binding to the therapeutic target VEGFR2; however, this function was gained at the expense of large losses in thermostability and increased oligomerisation. In order to mitigate these losses, we grafted the binding loops from Adnectin-anti-VEGFR2 (CT-322) onto the pre-stabilized FN3Con scaffold to produce a domain that successfully bound with high affinity to the therapeutic target VEGFR2. This FN3Con-anti-VEGFR2 construct also maintains high thermostability, including remarkable long-term stability, retaining binding activity after 2 years of storage at 36 °C. Further investigations into buffer excipients doubled the presence of monomeric monobody in accelerated stability trials. These data suggest that loop grafting onto a pre-stabilized scaffold is a viable strategy for the development of monobody domains with desirable biophysical characteristics, and is therefore well-suited to applications such as the evolution of multiple paratopes or shelf-stable diagnostics and therapeutics.

Mutational and biophysical robustness in a pre-stabilized monobody