DARPin

DARPins (an

In addition, DARPins can be used as crystallization chaperones for soluble and membrane proteins, including G protein-coupled receptors (GPCRs), either as binding partners or as rigid fusions to the target protein, a concept now being extended to structure determination by cryoEM.[3]^[4][5][6]

Origin, structure and generation

The DARPin platform was discovered and developed in the laboratory of

DARPin libraries were designed via sequence alignments of several thousand natural ankyrin repeat motifs (of about 33

Due to the high specificity, stability, potency and affinity, as well as their flexible architecture, DARPins have a rigid body-binding mode.^[1][9] Multi-specific or multivalent constructs made by genetic fusion suggest that fused DARPins have similar binding properties as single-domain DARPins.^[1] The absence of cysteines in the scaffold enables engineering of site-specific cysteines, allowing site-directed coupling of chemicals to the molecule. Non-natural amino acids can be introduced for the same purpose.^[15]

Potentially, DARPins can provide clinical benefit by overcoming the limitations of conventional therapeutic approaches, which typically target a single disease pathway and thus may compromise efficacy. In many cases, the complexity of a disease results from the dysregulation of multiple pathways. DARPin technology can be leveraged to rapidly generate thousands of different "multi-DARPins" where the binding domains are connected (i.e., by linkers), thereby enabling the targeting of several disease pathways. DARPins and multi-DARPins can also be fused to non-DARPin elements, such as a toxin,^[16] to generate targeted therapeutics, and their manufacture is facilitated by the resistance of DARPins against aggregation. The diversity of formats and robustness of multi-DARPins facilitates an empirical approach (such as through outcome-based screening) to efficiently identify DARPins with potential activity in specific disease pathways.

The potential benefits of DARPins are largely due to their structural and biophysical characteristics. Their small size (14-18 kDa) is thought to enable increased tissue penetration, and their high potency (<5-100 pM) makes DARPins active at low concentrations.^[17] DARPins are soluble at >100 g/L, and their high stability and solubility are considered desirable properties for drug compounds. DARPins can be produced rapidly and cost-efficiently (i.e., from E. coli). Their pharmacokinetic (PK) properties can be adjusted by fusion to half-life extending molecules, such as polyethylene glycol (PEG), or to DARPins binding to human serum albumin. Because of their favorable biophysical properties,^[1] DARPins are considered highly developable using standard processes, potentially exhibiting robust class behavior.

Clinical development and applications

DARPins have been used as research tools,

Currently, MP0112 is being investigated in three different clinical trials. The first two trials are safety and efficacy studies of abicipar in patients with wet AMD to establish comparability between Japanese and non-Japanese patients.[18]^[20] The third study is to test the safety and efficacy of abicipar in patients with DME.^[19]

In July 2014, Molecular Partners initiated a first-in-human study to investigate the safety, tolerability and blood levels of MP0250, a second DARPin candidate, in patients with cancer.^[21]

Molecular Partners AG has several additional DARPins in preclinical development with potential indications in various disease areas, including ophthalmology, oncology, immuno-oncology and immunology.

References