Thermostable designed ankyrin repeat proteins (DARPins) as building blocks for innovative drugs

Designed Ankyrin Repeat Proteins (DARPins) are a class of antibody mimetics with a high and mostly unexplored potential in drug development. They are clinically validated and thus represent a true alternative to classical immunoglobulin formats. In contrast to immunoglobulins, they are built from solenoid protein domains comprising an N-terminal capping repeat, one or more internal repeats and a C-terminal capping repeat. By using in silico analysis and a rationally guided Ala-Scan, we identified position 17 of the N-terminal capping repeat to play a key role for the overall protein thermostability. The melting temperature of a DARPin domain with a single full-consensus internal repeat was increased by about 8°C to 10°C when the original Asp17 was replaced by Leu, Val, Ile, Met, Ala or Thr, as shown by high-temperature unfolding experiments at equilibrium. We then transferred the Asp17Leu mutation to various backgrounds, including different N- and C-terminal capping repeats and clinically validated DARPin domains, such as the VEGF-binding ankyrin repeat domain of abicipar pegol. In all cases, the proteins remained monomeric and showed improvements in the thermostability of about 8°C to 16°C. Thus, the replacement of Asp17 seems to be generically applicable to this drug class. Molecular dynamics simulations show that the Asp17Leu mutation reduces electrostatic repulsion and improves van-der-Waals packing, rendering the DARPin domain less flexible and more stable. Interestingly, such a beneficial Asp17Leu mutation is present in the N-terminal caps of three of the five DARPin domains of ensovibep, a SARS-CoV-2 entry inhibitor currently in clinical development. This mutation is likely responsible, at least in part, for the very high melting temperature (>90°C) of this promising anti-Covid-19 drug. Overall, such N-terminal capping repeats with increased thermostability seem to be beneficial for the development of innovative drugs based on DARPins.