The complement system plays a crucial role in innate immunity as a first line of defense against microbial intruders. However, its dysregulation can cause or exacerbate numerous inflammatory disorders when its natural microbial elimination mechanisms “spill over” and cause damage to human cells. Therefore, therapeutic modulation of complement responses is a rapidly emerging clinical strategy, with 11 of the 12 approved drugs introduced since 2018, which covers a broad spectrum of indications - from ocular to renal and hemolytic diseases. However, various medical needs remain unaddressed. The cascading nature of the complement system, featuring an array of classically “undruggable” targets within its >30 components, favors biologics as therapeutic modalities – with restricted distribution and application profiles. This is particularly evident in neuroinflammatory diseases that, despite demonstrated associations with complement overactivation, have largely remained inaccessible for complement-targeted biologics. Moreover, many important targets within the complement cascade have proven challenging to modulate with common modalities.
Drug-like macrocyclic peptides offer potential to address this gap – with their small, rigid-ring topology, they can engage challenging targets such as protein-protein interactions, while retaining the potential for drug-like characteristics beyond Lipinski’s “rule of 5” (bRo5), including oral bioavailability and brain penetration. Complement-targeted macrocycles may thus not only afford attractive alternatives to existing therapies by improving administration options, but may also enable complement modulation in the brain. Based on strong preliminary data, and initially focusing on established-yet-relevant targets, this proposal aims at facilitating the discovery and development of drug-like macrocyclic peptides as modulators of complement and other host defense pathways.
Upon detection of danger signals, the initiation of complement responses critically relies on the activation of serine proteases, thus serving as a bottleneck of downstream processes. While considered druggable targets, their high structural homology with other circulating serine proteases presents a challenge for small molecule development. To date, only one initiation inhibitor, the monoclonal antibody sutimlimab that inhibits the C1s protease of the classical pathway, has been approved. We previously developed small macrocyclic peptide inhibitors of C1s which serve as promising alternatives. In aim 1, we will implement high-throughput SAR campaigns to optimize these into drug-like macrocycles, with enhanced potency and desirable bRo5 properties.
In contrast, most of the approved therapeutics are targeting central protein-protein interactions within the complement cascade, which could potentially be addressed by drug-like macrocycles. However, de novo discovery of drug-like macrocyclic peptides that can target such undefined binding pockets is challenging; fully synthetic methods that enable screening within the bRo5 space suffer from low diversity, while genetically-encoded methods deliver high diversity at the expense of the chemical flexibility needed to explore the drug-like space. In aim 2, we develop a new peptide nucleic acid (PNA)-encoded macrocyclic peptide (PeMP) screening platform. The use of PNA as the genetic barcode enables full orthogonality with modern peptide chemistry, facilitating broad chemical exploration of the bRo5 drug-like space, whilst maintaining high library diversities associated with genetic barcoding. In aim 3, we aim to apply the PeMP screening platform for the discovery of new, drug-like macrocyclic inhibitors of complement activation, although the platform is anticipated to be broadly applicable to an array of drug targets and may significantly advance de novo discovery campaigns.
Collectively, the proposal not only aims to deliver new drug-like macrocyclic complement modulators but also aims to develop a new screening platform for discovery of drug-like macrocycles directly in hit identification – benefiting the wider research and drug discovery communities.
